WO2021255195A1

WO2021255195A1 - Process for production of metal scaffolds and foams

Info

Publication number: WO2021255195A1
Application number: PCT/EP2021/066480
Authority: WO
Inventors: Joseph Buhagiar; Christabelle TONNA; Arif Rochman; Maurice GRECH; Albert CURMI; Pierre SCHEMBRI WISMAYER
Original assignee: University Of Malta
Priority date: 2020-06-18
Filing date: 2021-06-17
Publication date: 2021-12-23
Also published as: GB202300516D0; GB202009324D0; GB2611690A

Abstract

The present invention relates to a process for preparing a metal scaffold or foam, for instance a metallic scaffold suitable for use as a bone implant, and in particular a biodegradable metallic implant. The process comprises the steps of preparing a template, coating the template with a solid composition comprising metallic powder, and then material removal of the polymer template. The present invention also relates to a metallic scaffold or foam that is obtainable by said process. The present invention also relates to the use of a solid composition comprising metallic powder in a process of coating an additive- manufactured polymer template to produce a metal particle-coated polymer template.

Description

PROCESS FOR PRODUCTION OF METAL SCAFFOLDS AND FOAMS

Field of the invention

Background information

The use of bone grafts is standard treatment to repair skeletal fractures, or to replace and regenerate lost bone, and over 2 million bone graft procedures are performed worldwide annually (see Greenwald et al, JBone Joint SurgAm, 2001, 83, 98-103). This is due to their ease of use and handling, good safety profiles, intraoperative cost and time advantages, and adaptability to a variety of clinical challenges.

The most common type of bone graft is an autograft, where donor bone tissue is taken from elsewhere in the same patient’s body. However, possible complications of autografts include pain, infection, scarring, blood loss, and donor-site morbidity. Alternatively, allografts may be used, where the donor material originates from a different patient from the same species. However, this has the difficulty that the donor material is not genetically identical to the patient’s native bone material, and so may invoke an immune response in the recipient.

The use of artificial bone grafts (“bone substitutes” or “alloplastic” bone grafts) has therefore grown in popularity in recent years. A bone substitute is a synthetic, inorganic or biologically organic combination which can be inserted for the treatment of a bone defect instead of autogenous or allogenous bone (see Schlickewie and Schlickewie, Macromol Symp., 2007, 253(1), 10-23.) Optimal bone substitutes should possess all or at least the majority of the following properties: they should be biocompatible, easily moulded into the bone defect in a short setting time, osteoconductive (i.e. the ability of bone-forming cells in the grafting area to move across the scaffold and slowly replace it with new bone over time), osteogenic (i.e. the ability to induce osteogenesis), osteoinductive (i.e. promote growth of the bone on the surface of the scaffold), biodegradable, not invoke an inflammatory response, possible to trace in vivo, and readily available at a reasonable cost. Materials that have been used in bone substitutes include ceramic materials such as hydroxyapatite (HA), tricalcium phosphate (TCP) and calcium sulfate, and metallic structures including stainless steel, titanium alloys (e.g. Ti6A14V alloy), and Co-Cr-Mo alloy. Non-biodegradable metallic materials (including stainless steel, titanium alloys and Co-Cr-Mo alloys) are commonly used for load-bearing implant applications due to high strength, good fatigue resistance and good machining characteristics (see Gao et al., IntJMol Sci, 2014, 15(3), 4714-4732). However, these materials may produce adverse effects such as the release of significant amounts of metal ions into tissues, which may result in complications such as inflammatory and immune reactions. The development of iron-based metallic implants, which are biodegradable, gives rise to a class of bone substitutes which provide high mechanical support during repair and regeneration of damaged or diseased bone, but without the disadvantages of non-degradable metallic structures (see, e.g., Bose etal, Trends in biotechnology, 2012, 30(10), 546-554). Such iron-based materials may also be biocompatible (i.e. able to support normal cellular activity, e.g. osteoconductivity, without any local and systematic toxic effects to the host tissue) and may degrade at a controlled resorption rate in vivo, creating space for the new bone tissue to grow.

Known methods of preparing such metallic scaffolds that are suitable for use as bone substitutes can broadly be grouped into three categories.

First, it is possible to prepare such scaffolds directly by using computer-aided design, and the subsequent direct additive manufacturing (i.e. 3D printing) of a metal scaffold. 3D printers capable of directly printing metallic structures are known in the art; such processes are described, for example, in CN 103769587. However, these printers are very expensive, which makes direct preparation of the desired metallic scaffolds by additive manufacturing impractical in many settings.

It is also possible to prepare such scaffolds via computer-aided design and manufacture of a negative template, or mould. A metal-containing slurry may then be poured into the mould, and allowed to dry. The mould itself can then be removed by an appropriate method (which will depend on the material from which the mould is formed), such as dissolution or thermal degradation. Processes for preparing metal templates in this way include those described in WO 2006/130935, WO 2013/113250 and WO 2013/113251. As an alternative to casting into the negative template using a metal -containing slurry, injection moulding of molten metal can also be employed. An example of this type of process is described in WO 2014/110182. Use of a negative template is inefficient, because it introduces the additional processing steps of having to generate a negative mould from a printed replica template (a “positive” template; see below) and then burning off that positive template before the metal scaffold or foam can be cast from the mould. A third class of methods for preparing such scaffolds comprises methods in which a sacrificial template is prepared having the same structure as the desired end scaffold (a “positive” template). The template is then coated with a metallic layer, before the underlying template is subsequently removed by an appropriate method. In these methods, the coating step typically involves treatment of the sacrificial template with a metal-containing slurry (see, for example, the process described in WO 02/066693). Other techniques that can be used to deposit a metal coating onto the template surface include plating (see, for example, the processes described in WO 2013/010108) and low temperature vapour deposition (see, for example, the process described in US 2004/0153981). In US 5,881,353, an adhesive is coated on to a bulk urethane foam in order to impart tackiness, before a metallic powder can be applied.

There are disadvantages associated with the known methods for coating the positive sacrificial templates in these methods. In particular, slurry coating methods are very inefficient, requiring a large amount of slurry (and hence a large amount of metal) and more than a single application to achieve an acceptable coating of the underlying template. Meanwhile, (electroless) plating and vapour deposition techniques are typically only able to coat planar surfaces. Complex 3D structures, such as the metallic scaffolds and foams described herein having a network of struts and pores, cannot efficiently be coated using these techniques. Further, vapour deposition techniques require the use of specialist equipment, which can be expensive.

Thus, there exists a need to improve upon the existing methods of preparing metallic scaffolds, such as those suitable for use as bone implants, which employ positive sacrificial templates in order to prepare the end scaffold. In particular, it would be desirable to develop a cheap and efficient coating method for coating the positive template with the required layer of metal, and in particular a positive template that has a complex 3D structure e.g. comprising a network of struts and pores.

Summary of the invention

The present inventors have surprisingly discovered a particularly advantageous method for preparing metal scaffolds or foams, such as metal scaffolds suitable for use as bone implants, using a positive sacrificial template. In particular, this method employs a particularly efficient and facile metal coating step, which can be carried out without the need for expensive equipment. This is due to the unexpected discovery that it is possible to prepare polymeric sacrificial templates which are somewhat “sticky” or “tacky” in nature, such that they are capable of adhering metallic particles to their surface. Such polymeric templates may be prepared, for example, by additive manufacturing methods. The “sticky” polymer templates can be coated with a solid composition comprising metallic particles via a simple contacting or “dry-coating” step, such as tumbling, spraying, or use of a fluidised bed, amongst others. In one aspect, the present invention therefore provides a process for preparing a metallic scaffold or foam, the process comprising:

(a) preparing a polymer template;

(b) coating the polymer template by contacting the template with a solid composition comprising metallic powder to provide a metal particle-coated polymer template; and

(c) effecting the material removal of the polymer template to provide the metallic scaffold or foam.

The present invention also provides a metallic scaffold or foam obtainable by the process of the invention.

The present invention also provides use of a solid composition comprising metallic powder in a process of coating an additive -manufactured polymer template to produce a metal particle-coated polymer template.

Brief description of the figures

Fig. 1 shows an general overview of the product at different stages of the present method (not to scale).

(a) The 3D-printed polymer template (left), with a cut-out view showing solid struts (right) (b) The metal -coated polymer template (left) with the metallic coating indicated by hatching, and a cross- sectional view (right) showing the template coated with metallic particles (c) An example of a possible one-step heat treatment (upper) and a possible two-step heat treatment (lower) for removing the underlying polymer template (d) The final metallic scaffold or foam (left) with the metallic coating represented by a different hatching pattern to (b), to acknowledge that some processing may have occurred to the surface particles following deposition on the template surface, with a cut-out view to show hollow struts (right).

Fig. 2 shows tested 3D prints of the polymer template (a) A simple cubic template printed on an edge.

(b) A gyroid template printed on a vertex (c) A gyroid template printed on an edge (d) An alternative illustration of a gyroid template printed on a vertex.

Fig. 3 shows box plots illustrating the measurement of pores at “supported” and “unsupported” sides in both “apex” and “edge” printing orientations. The box plots indicate the 10^th, 25^th, 50^th, 75^th and 90^th percentiles and mean. Fig. 4 shows box plots illustrating the measurement of struts at “supported” and “unsupported” sides in both “apex” and “edge” printing orientations. The box plots indicate the 10^th, 25^th, 50^th, 75^th and 90^th percentiles and mean.

Fig. 5 outlines the post-processing steps applied following 3D printing in the case of the gyroid template (above) and the cubic template (below).

Fig. 6 shows sample prints of (a) a cubic template and (b) a gyroid template.

Fig. 7 shows scanning electron micrograph (SEM) images of (a) coarse and (b) fine Fe particles used in the example methods.

Fig. 8 shows particle distribution graphs for (a) coarse and (b) fine Fe powders used in the example methods.

Fig. 9 shows radiographic images of cubic templates (a) coated with PVA-based slurry 3 times and (b) dry-coated once. The templates are cured after 3D printing, and no IPA was used during post processing.

Fig. 10 is a graph showing the difference in both total Fe powder uptake (bars) and Fe powder uptake per cm² of surface area (circles) by a cubic template with 500 pm struts and 900 pm pores, depending on post-processing steps used.

Fig. 11 shows the effect of cleaning time in IPA on the final dimensions of cubic 3D printed templates. “Centre” refers to measurements taken at the core section of the template whereas “Outer side” refers to the measurements taken at the outer layers of the template. The dashed box marked with an asterisk indicates the cleaning conditions that resulted in clogged pores in the template.

Fig. 12 shows a micrograph illustrating clogged pores when no IPA is used during post-processing of a cubic template.

Fig. 13 shows the effect of cleaning time in IPA on the final dimensions of gyroid 3D printed templates. “Centre” refers to measurements taken at the core section of the template whereas “Outer side” refers to the measurements taken at the outer layers of the template.

Fig. 14 shows the percentage weight uptake of Fe powder in a dry coating step by a 3D printed gyroid template which has previously been cleaned in IPA for 10 minutes with and without ultrasonication. The length of IPA cleaning is observed to inversely correlate to the weight uptake in subsequent coating of the templates with dry metallic powder, which is indicative of a reduction in “tackiness” of the surface.

Fig. 15 shows X-ray radiography images showing powder coating distribution on 3D-printed gyroid template in a dry coating step which has previously been cleaned in IPA for 10 minutes (A) without and (B) with ultrasonication.

Fig. 16 shows the effect of prior UV curing time on the percentage weight uptake of Fe powder in a dry coating step by a 3D printed cubic template. The length of curing time is observed to inversely correlate to the weight uptake in subsequent coating of the templates with dry metallic powder, which is indicative of a reduction in “tackiness” of the surface.

Fig. 17 shows scanning electron micrograph images of Fe powder coverage on 3D printed gyroid templates coated with (A-B) coarse powder and (C-D) finer powder. After 3D printing, the templates underwent 10 minutes of immersion in IPA with ultrasonication, followed by 2 hours of curing under UV.

Fig. 18 shows SEM images of heat-treated structures when using (a) coarse powders and (b) fine powders with a lower temperature dwell at 175°C.

Fig. 19 is a line graph showing the different heat treatments applied to the metal -coated polymer templates.

Fig. 20 shows SEM images of the resulting metallic scaffolds from heat treatments with different lower- temperature dwells at (a) 175°C, (b) 200°C and (c) 225 °C.

Fig. 21 shows low and high magnification SEM images of a sintered gyroid structure prepared according to Example 8.

Fig. 22 shows both “covered” and “uncovered” configurations for pressureless sintering of FeMn scaffolds in a tube furnace. The “covered” configuration is a modified configuration that results in reduced oxide formation on the surface of the FeMn scaffold.

Fig. 23 shows Fe35Mn scaffold surfaces (A) as-sintered with ‘Covered’ configuration of Fig. 22, (B) as- sintered with ‘Uncovered’ configuration of Fig. 22, (C) following ultrasonication of the ‘Covered’ scaffold for 5 minutes in 1 M HC1 with 3.5 g/L hexamethylenetetramine, and (D) following ultrasonication of the ‘Uncovered’ scaffold for 5 minutes in 1 M HC1 with 3.5 g/L hexamethylenetetramine .

Detailed description

Definitions

As used herein, the term “additive manufacturing” refers to the construction of a three-dimensional object from a computer-aided design (CAD) model or a digital 3D model. It can refer to a variety of processes in which material is joined or solidified under computer control to create a three-dimensional object. Typically, in such processes, material is added in a layer by layer fashion, whereby successive layers of material are laid down or formed at precise positions. Alternatively, a 3D model can be printed simultaneously, such as in tomographic volumetric additive manufacturing. Examples of materials that can be layered by an additive manufacturing process include liquids, which solidify after being added to the growing object, or solids, which can be fused onto the growing object as they are applied. The term “additive manufacturing” is used interchangeably with the term “3D printing” throughout this specification.

As used herein, the term “alloy” refers to a combination of metals or metals combined with one or more other elements. When an alloy “comprises” a number of specific different elements, the alloy is not limited to the presence of these elements, i.e. further unspecified elements may also be present in these alloys. When an alloy is said to be an alloy of a list of elements joined by hyphens (e.g. an “iron- manganese” alloy), this notation also defines that the alloy comprises these elements, but is not limited to the presence of these elements, i.e. further unspecified elements may also be present in these alloys.

As defined herein, the term “dry-coating” refers to a method of coating whereby a powdered coating material is directly coated onto a solid article (such as the polymer template or the metal particle-coated polymer template defined herein) without the use of any extraneous solvent and/or binder. In particular, a “dry-coating” method is one in which the coating material is not applied to the article to be coated in the form of a slurry. A “dry-coating” method may also be referred to as a “powder coating” method.

As defined herein, the term “layer” refers to one or more levels or of potentially patterned strata within an object (e.g. a template, a scaffold or a foam), and not necessarily to a continuous plane.

As defined herein, the term “necking” refers to a process whereby two adjacent, discrete particles in a solid article (e.g. discrete particles that are present in the coating layer of a solid article) coalesce by formation of a “neck” or bridge between the particles. This neck region is typically narrower in diameter than the diameter of either particle itself. Typically, this neck or bridge is formed as the result of the application of heat over a period of time to the solid article. The term “necking” is particularly used to refer to a process of coalescence whereby the discrete particles are discrete metallic particles.

As defined herein, the term “powder” refers to a bulk solid comprising fine, discrete, solid particles that may flow freely when shaken or tilted. Preferably, the powder comprises particles which are spherical, or substantially spherical, in shape. Alternatively, the powder consists of particles which are spherical, or substantially spherical, in shape. Typically the mean particle diameter of the solid particles in a powder is less than 100 pm.

As defined herein, the term “scaffold” refers to a three-dimensional structure to which biological material is able to adhere and grow around once the structure is implanted into a living body, in particular a human patient. A “bone scaffold” is such a structure that is capable of being implanted into a bone defect, e.g. a bone fracture, and around which new bone tissue may grow. Typically, the term “scaffold” refers to a porous three-dimensional structure comprising a network of struts and pores.

As defined herein, the term “foam” refers to a three-dimensional open-celled porous structure for a non- biological application. Examples of such applications include casts for jewellery.

As defined herein, the term “surface” refers to the outermost or uppermost layer or portion of a physical object or space.

As defined herein, the term “template” refers to a mould or intermediate that is employed in the preparation of a finished 3D article, in particular a scaffold or a foam. Typically such a template is a “sacrificial template”, i.e. a template which is materially removed during the manufacturing process of the finished 3D article. A “positive template” is a template that is a scale model of the desired 3D article. Thus, a positive template is a template that can be coated with the material from which the 3D article is to be composed of, before subsequently being removed to yield the finished 3D article. A “negative template” is a template that is an inverse mould of the desired 3D article. Thus, a negative template is a template that can be filled with a cast of the material from which the 3D article is to be composed of, before subsequently being removed to yield the finished 3D article.

As defined herein, the term “tacky” refers to the ability of a substrate, preferably a polymer template, to adhere to metallic particles. Without being bound by any particular theory, the tackiness of the polymer templates described herein is believed to derive from the inherent stickiness or adhesive properties of the bulk polymer of the polymer templates, by virtue of the presence of exposed, non-cross-linked polymer chains and/or unreacted monomer units exposed on the surface of the polymer template. Overview of the process

An overview of the process of the present invention is shown in Fig. 1. The process begins with the preparation of a sacrificial polymer template (1), typically via a method of additive manufacturing, i.e. 3D printing (see Fig. 1(a)). This template typically comprises a network of pores (2) and solid struts (3). The template then undergoes optional post-processing steps, such as curing with UV light and/or washing with an appropriate solvent, e.g. ethanol or IPA.

The resulting polymer template has an inherently tacky or sticky surface, to which metal particles are capable of adhering. In the next step of the process, the sticky polymer template is contacted with a solid composition comprising a metallic powder. This contacting step is a dry-coating step in which the metallic powder becomes stuck onto the surface of the polymer template. This results in the formation of a metal -coated polymer template (4) (see Fig. 1(b)). This comprises a layer of metallic particles (5) adhered to the surface of the underlying polymer template (6). Further processing steps may optionally be carried out on this metal particle-coated template.

Finally, the underlying sacrificial polymer template is materially removed from the structure. This is typically achieved through heating of the metal -coated polymer template (e.g. via a single-step heat treatment depicted in Fig. 1(c), upper, or alternatively via the two-step heat treatment depicted in Fig. 1(c), lower, which is discussed further below), and yields a metal scaffold or foam (7) (see Fig. 1(d)). The final metal scaffold or foam (7) is a scaled version of the original polymer template (1) with the same overall shape and network of pores and struts, except that the struts (8) are hollow rather than solid.

Each of the steps in the process are discussed below in greater detail.

Preparation of the polymer template

The sacrificial polymer template used in the present process can be manufactured by any method known to the skilled person. Preferably, however, the polymer template is manufactured by a method of additive manufacturing (i.e. 3D printing). Typically, a digital 3D model or a computer-aided design model of the template is first prepared, before the polymer template is subsequently printed. For example, in this way, it is possible to readily design a template (and thus end scaffold) having specific structural features (e.g. overall width, height and length, strut dimensions, pore sizes and so forth) to fit a particular bone defect in a particular patient. The required measurements are preferably obtained by or computed tomography (CT) scans, including e.g. micro X-ray CT, and subsequent aggregation of different tomography images. This enables the process of the invention to produce a scaffold that is adapted to meet the requirements of a specific patient.

The polymer template should be a scale model of the final desired metallic scaffold or foam to take into account shrinking of the final product that typically occurs during the later stages of the production process (in particular step (c) in which the underlying template is removed). Thus, when the polymer template comprises a 3D network of pores and struts, the pore size of the polymer template should typically be greater than the pore size of the final metallic scaffold or foam, preferably at least 1.2 times greater, more preferably at least 1.5 times greater, still more preferably at least 1.8 times greater, and most preferably at least 2 times greater, for example at least 2.5 times greater or 3 times greater.

Further, when the polymer template comprises a 3D network of pores and struts, the strut thickness of the polymer template should typically be greater than the strut thickness of the final metallic scaffold or foam, preferably at least 1.1 times greater, more preferably at least 1.2 times greater, still more preferably at least 1.3 times greater, and most preferably at least 1.5 times greater, for example at least 1.8 times greater, 2 times greater or 2.5 times greater.

Thus, in a preferable embodiment, prior to the additive manufacturing of the polymer template, a computer-aided design model or digital 3D model of the template is developed. More preferably, a computer-aided design model of the template is developed. Therefore, in a particularly preferred embodiment, step (a) of the process comprises:

(i) developing a computer-aided design model or digital 3D model of a polymer template, preferably a computer-aided design model of a polymer template; and

(ii) preparing the polymer template by a method of additive manufacturing.

In order to develop the computer-aided design model or digital 3D model of the template, the site where the eventual metallic scaffold or foam is to be located is typically measured, and said measurements are used as input data for generation of the computer-aided design model or digital 3D model, with necessary compensation to account for the fact that the polymer template is typically a scale model of the final metallic scaffold or foam. Where the final product is a metal scaffold for use as a medical implant, e.g. a bone substitute, the input data are typically obtained from a patient. Preferably, said input data are obtained from a patient using a computed tomography scan. Typically, multiple resonance or tomography scans are taken, each through a different cross-section of the affected bone tissue. Typically, the aggregate of these images constitute a data input for generation of the computer- aided design model or digital 3D model. The method of creating the computer-aided design model or digital 3D model is not particularly limited, but for example, the image slices can be laminated to construct the desired 3D shape. In one possible method for generating the computer-aided design model or digital 3D model of the template, the overall outer shape of the template is first defined, and subsequently the pores are created within the outer shape.

Thus, in a particularly preferable embodiment, the metallic scaffold is suitable for use as a medical implant, preferably a bone substitute, and step (a) of the process comprises:

(i) obtaining the desired measurements for the metallic scaffold, preferably from a patient, and more preferably using a computed tomography scan;

(ii) using the obtained measurements as input data to develop a computer-aided design model or digital 3D model of a polymer template, preferably a computer-aided design model of a polymer template; and

(iii) preparing the polymer template by a method of additive manufacturing.

Additive manufacturing methods are well known to one of skill in the art. Any suitable method of additive manufacturing can be used to produce the polymer template in the present process. Typically, the additive manufacturing method may be selected from selective laser sintering (SLS), hot melt extrusion fabrication, VAT polymerisation (such as stereolithography (SLA), micro-stereolithography, digital light processing (DLP), continuous liquid interface production (CLIP), daylight polymer printing (DPP), continuous digital light projection (CDLP) and low-force stereolithography (LFS)), laminated object manufacturing (LOM), fused deposition modelling (FDM), multijet modelling (MJM), and inkjet printing. Areas of overlap can exist between many of these methods, which can be chosen as necessary by one of skill in the art based on the materials, tolerances, size, quantity, accuracy, cost structure, critical dimensions, and other parameters of the template to be produced.

3D printers (i.e. devices for carrying out additive manufacturing) have the ability to print structures made of several materials with different mechanical and/or physical properties in a single build process, and typically operate by taking a 3D computer file (i.e. a computer-aided design image or a digital 3D image) and constructing from it a series of cross-sections taken in the build direction. Each cross- section layer is then printed one on top of the other to create the desired template. Thus, in one embodiment, the polymer template is constructed from more than one different type of material. Typically, though, the polymer template is constructed from one single type of material.

Alternatively, a 3D printer can create the template from extrusion of a liquid that is solidified by either a change in temperature or a chemical change, such as cooling below the melting point or polymerisation. Thus, while a layer-wise build may be used in some instances, a vector-based build where 3D motions both in and out of a plane are determined based on the part to be made, machine physics, material chemistry, and other manufacturing considerations can also be used. In-plane rasterisation and out-of- plane motion of a “print head” supplying a material are both methods by which the target template may be formed.

A number of different technologies are available for performing additive manufacturing. The main differences between the technologies are in the way layers are built to create the template. In some methods, the layers are produced using melting or reflow. One example of such a method is SLS, wherein one or more lasers selectively fuse solid particles to form a bed of particles after the particles have been deposited in the correct spatial location by the printer. After each cross-section is created, a new layer of powder is applied to the top and the process is repeated until the desired template is obtained. Each layer may be of the same powder or a different powder. Typically, each layer is of the same powder. Alternatively, each layer is of a different powder. Each powder layer may be uniform or the layer can be sectioned with differing materials, thus providing the capability of obtaining templates with differing characteristics and/or functionalities. Typically, each powder layer is uniform.

Another method of additive manufacturing is hot melt extrusion fabrication. In this process, the material from which the template is to be constructed (which can be supplied as pellets and/or filaments, or so forth) may be liquefied and deposited during the manufacturing process by one or more extrusion heads. The pellets and/or filaments may be of the same or a different material and may be melted and mixed prior to or during extrusion. Typically, though, the pellets and/or filaments are of the same material. Alternatively, the pellets and/or filaments are of different materials.

Other additive manufacturing methods include application of liquid materials that are cured with different technologies, including inkjet printing. In this method, the “inkjet” may deposit layers in a sequential fashion. Each layer can be very thin; for example, each layer can be from 1 pm to 1 mm or more in thickness, preferably from 1 pm to 100 pm in thickness, e.g. from 10 pm to 50 pm in thickness. Each layer may be cured as the process proceeds, for example, by drying of each layer after deposition from the inkjet to the desired spatial location. Alternatively, inkjet printing layers can be applied and cured one layer at a time. In other words, a single layer may be applied, and then external conditions applied to affect curing of that layer (e.g. application of UV light, or mechanical cooling), before the next layer is applied, and the process repeated until the desired template has been constructed. One or more printing heads, each emitting the same or different materials, can be used to create the template by this method. Typically, if more than one printing head is used, each printing head emits the same material. Alternatively, if more than one printing head is used, each printing head emits a different material.

A similar printing technique is known as binder jetting. In this method, a liquid binder is selectively deposited by the printer onto the surface of a bed of powder, which binds together the areas of powder onto which the binder was deposited to form a solid layer. Typical liquid binders include phosphoric acid, acrylic acid, a mixture of phosphoric acid and isopropyl alcohol, and acidic calcium sulfate, optionally including further additives e.g. to improve flow, lubricate and adjust the pH of the solution, among other functions. The substrate upon which the solid layer region was fabricated is then moved downwards through the powder bed to expose more powder on the surface. More binder is then selectively applied by the printer to the surface of the powder bed in the required spatial arrangement to construct the next layer of the desired template. This process is repeated layer by layer until the desired 3D template is obtained.

Another method of additive manufacturing using liquid materials is VAT polymerisation, also known as photopolymerisation. In this process, a liquid photopolymer is contained in a vat (or tank) and a layer of photopolymer is selectively cured by exposure to a light source. Typically, the exposed layer is the surface layer of photopolymer, but may in some cases be the layer of photopolymer at the bottom of the vat. The exposed regions of liquid photopolymer harden into solid layers (typically by cross-linking or degrading of the polymer chains). The substrate upon which the solid layer region was fabricated is then moved down or up into the vat by a small amount, exposing more liquid photopolymer on the surface or bottom of the vat respectively, and the surface layer or bottom layer of the liquid photopolymer, respectively, is again selectively exposed to light to construct the next layer. Thus, typically, the substrate upon which the solid layer region was fabricated is moved down into the vat by a small amount, exposing more liquid photopolymer on the surface of the vat, and the surface layer of the liquid photopolymer is again selectively exposed to light to construct the next layer. Alternatively, the substrate upon which the solid layer region was fabricated is moved up into the vat by a small amount, exposing more liquid photopolymer on the bottom of the vat, and the bottom layer of the liquid photopolymer is again selectively exposed to light to construct the next layer. This process is repeated in a layer by layer fashion until the template is built. Any remaining liquid polymer is then drained from the vat, leaving behind the solid polymer template.

This method also allows for replacing the liquid polymer with a different liquid polymer at various stages during the building process, thus allowing for the use of different materials for different applications and with different properties. Typically, a single liquid polymer is used during the VAT polymerisation process. Alternatively, two or more (e.g. three, four, five, six or more) different liquid polymers are used at different points during the VAT polymerisation process.

VAT polymerisation processes include stereolithography (SLA), digital light processing (DLA), continuous liquid interface production (CLIP), daylight polymer printing (DPP), continuous digital light projection (CDLP) and low-force stereolithography (LFS). These differ primarily by the way in which the liquid photopolymer is exposed to a light source. In SLA, a concentrated beam of UV light, or a laser, is focussed onto the surface of the vat filled with liquid photopolymer. The beam or laser is focussed, creating each layer of the desired 3D template by means of e.g. cross-linking or degrading a polymer. A variation of SLA is 2-photon SLA, in which the 3D template structure is obtained by first using one focussed laser in order to create a 3D gel from the liquid photopolymer, and subsequently using a second targeted laser that cures the desired sections of the gel into a solid, with the uncured sections of gel being washed away at the end of the process to leave behind the desired 3D solid object. In DLP, a digital projector screen is used to flash a single image of each layer across the entire vat at once. As the projector is a digital screen, the image of each layer is composed of square pixels, resulting in a layer formed from small rectangular bricks called voxels. DLP can achieve faster printing times in some cases, as each entire layer is exposed at once, rather than drawn out with a laser. In DPP, a liquid crystal display (LCD) is used in place of a laser or projector. DPP is also referred to as LCD 3D printing.

In CLIP, a tank of resin is employed as the base material, and part of the bottom of the vat is transparent to UV light (referred to as the “window”). A UV light beam shines through the window, illuminating the precise cross-section of the object. The light causes the resin to solidify (e.g. by cross-linking). The solidified layer is moved slowly up through the vat allowing resin to flow under and maintain contact with the bottom of the solidified layer, and then the cross-section of the next desired layer is illuminated by the UV light, and so forth. Unlike the other VAT polymerisation techniques, CLIP is a continuous process. An oxygen-permeable membrane lies below the resin, which creates a “dead zone” between the forming 3D template and the window at the bottom of the vat. This persistent liquid interface prevents the resin from attaching to the window, meaning that photopolymerisation between the window and the polymer is inhibited.

Thus, it is possible using VAT polymerisation methods to produce a 3D template using a “top down” approach or using a “bottom up” approach to construct the necessary layers. Preferably, in step (a) of the process of the present invention, the polymer template is prepared using an additive manufacturing method selected from selective laser sintering (SLS), hot melt extrusion fabrication, VAT polymerisation, laminated object manufacturing (LOM), fused deposition modelling (FDM), multijet modelling (MJM), inkjet printing and binder jetting. More preferably, the polymer template is prepared using VAT polymerisation or binder jetting. Yet more preferably, the method of VAT polymerisation is selected from stereolithography (SLA), digital light processing (DLA), continuous liquid interface production (CLIP) and daylight polymer printing (DPP), and is most preferably stereolithography.

Tomographic volumetric additive manufacturing is a technique in which an entire three-dimensional object is simultaneously solidified by irradiating a liquid photopolymer volume from multiple angles with dynamic light patterns. This technique has the potential to produce complex parts with a higher throughput and a wider range of printable materials than layer-by-layer additive manufacturing, and may be suitable for the ultrafast fabrication of advanced and functional constructs.

The polymer template formed in step (a) of the process of the present invention comprises a polymer to which particles of a metallic powder are capable of adhering. That is to say, that metallic particles are capable of adhering to the surface of the polymer template in a later dry-coating step. The polymer template may therefore be described as being “sticky” or “tacky”. Preferably, the polymer template is sufficiently tacky such that a solid composition comprising metallic powder as defined herein is capable of adhering to the surface of the polymer template, substantially coating the template. By “substantially coating” is meant that at least 50% of the surface area of the polymer template, preferably at least 60%, more preferably at least 70%, yet more preferably at least 75%, still more preferably at least 80%, even more preferably at least 85%, and most preferably at least 90% or more, e.g. at least 92% or more, at least 95% or more, at least 97% or more, at least 99% or more, or 100%, is capable of being coated by the adhesion of said solid composition to the surface of the polymer template. Preferably, the polymer template is sufficiently tacky such that the percentage weight uptake of a solid composition comprising metallic powder as defined herein that is subsequently contacted with the polymer template is greater than 20%, more preferably greater than 25%, yet more preferably greater than 30%, still more preferably greater than 35%, even more preferably greater than 40%, and most preferably greater than 45%, e.g. greater than 50%, greater than 55%, or greater than 60%.

Thus, typically, step (a) of the process of the present invention comprises the preparation of a polymer template that is sufficiently tacky such that a solid composition comprising metallic powder as defined herein is capable of adhering to the template in the absence of any external binder or solvent, substantially coating the template. Preferably, therefore, it is a property of the bulk polymer of the polymer template of the template that its surface is sufficiently tacky such that a solid composition comprising metallic powder as defined herein is capable of adhering to the template in the absence of any external binder or solvent, substantially coating the template. By “bulk polymer” of the polymer template is meant the polymer that comprises greater than 50% by weight, preferably greater than 75% by weight, more preferably greater than 85% by weight, yet more preferably greater than 90% by weight, still more preferably greater than 95% by weight, even more preferably greater than 98% by weight and most preferably greater than 90% by weight of the total polymer template. In one embodiment, the polymer template consists of a polymer that is sufficiently tacky such that a solid composition comprising metallic powder as defined herein is capable of adhering to the template in the absence of any external binder or solvent, substantially coating the template. Particularly preferably, it is a property of the bulk polymer of the polymer template of the template that its surface is sufficiently tacky such that the percentage weight uptake of a solid composition comprising metallic powder as defined herein that is subsequently contacted with the polymer template is greater than 20%, more preferably greater than 25%, yet more preferably greater than 30%, still more preferably greater than 35%, even more preferably greater than 40%, and most preferably greater than 45%, e.g. greater than 50%, greater than 55%, or greater than 60%.

The polymer template may be formed from any suitable polymeric material known to one skilled in the art. Typically, the polymer template comprises a poly(acrylate), a poly(methacrylate), a poly(urethane), poly(lactic acid), polyethylene terephthalate), a poly(carbonate), a poly( styrene), a poly(ether ether ketone) (PEEK), a poly(ether ketone ketone) (PEKK), a poly(etherimide) (PEI), a poly(aryl ether ketone) (PAEK), acrylonitrile butadiene styrene (ABS), acrylonitrile styrene acrylate (ASA), poly(vinyl acetate), a thermoplastic elastomer, cellulose, hydroxypropylcellulose, methylcellulose, carboxymethylcellulose, wax-based resins, or a mixture thereof, i.e. the polymer template is manufactured from one or more of the foregoing materials. Typically, the polymer template is manufactured from only one of these materials. Alternatively, the polymer template is manufactured from two or more (e.g. three, four, five or six) of these materials.

Preferably, the polymer template comprises a poly (acrylate), a poly(methacrylate), a poly(urethane), a wax-based resin, or a mixture thereof. Most preferably, the polymer template comprises a poly (methacrylate). Example of suitable poly(methacrylate) resins are Standard resin (RS-F2-GPGR- 04) and High Temperature resin (RS-F2-HTAM-02), available from Formlabs.

For use as a medical implant, e.g. a bone implant, the scaffold is typically a porous structure, so as to allow cells to grow inside the structure after transplantation. It is also important for essential nutrients, oxygen and carbon dioxide to be transported to/from the cells adjacent to the implant following transplantation. Thus, preferably, when used as a bone implant the scaffold is a porous structure comprising a network of struts and pores. Accordingly, the polymer template (which is a positive sacrificial template of the final metal scaffold) is also preferably a porous structure comprising a network of struts and pores. In that case, typically the struts of the polymer template are solid struts. Alternatively, however, the struts of the polymer template may be hollow struts.

In one embodiment, the polymer template comprises an orthogonally arranged network of pores. More preferably, in this embodiment, the template comprises a square -section porous cubic structure. In an alternative embodiment, the template has a triply periodic minimal surface (TPMS). In this embodiment, the template may have cubic, tetragonal, rhombohedral or orthorhombic symmetry. Preferably, in this embodiment, the template has a Gyroid or Schwartz D structure. The overall form of the polymer template can take any shape, e.g. a regular shape or an irregular shape. Typically, the polymer template has a mean strut thickness of at least 1 pm, preferably at least 10 pm, more preferably at least 25 pm, yet more preferably at least 50 pm, even more preferably at least 100 pm, still more preferably at least 150 pm, yet more preferably at least 200 pm, even more preferably at least 250 pm, still more preferably at least 280 pm, yet more preferably at least 350 pm, and most preferably at least 420 pm. Typically, the polymer template has a mean strut thickness of less than 5000 pm, preferably less than 3000 pm, more preferably less than 2000 pm, yet more preferably less than 1500 pm, even more preferably less than 1000 pm, still more preferably less than 800 pm, yet more preferably less than 750 pm, even more preferably less than 700 pm, still more preferably less than 600 pm, yet more preferably less than 550 pm, and most preferably less than 500 pm. Thus, typically, the polymer template has a mean strut thickness of from 1 to 5000 pm, preferably from 10 to 3000 pm, more preferably from 25 to 2000 pm, yet more preferably from 50 to 1500 pm, even more preferably from 100 to 1000 pm, still more preferably from 150 to 800 pm, yet more preferably from 200 to 750 pm, even more preferably from 250 to 700 pm, still more preferably from 280 to 600 pm, yet more preferably from 350 to 550 pm and most preferably from 420 to 500 pm.

Typically, the polymer template has a mean pore diameter of at least 1 pm, preferably at least 10 pm, more preferably at least 25 pm, yet more preferably at least 50 pm, even more preferably at least 100 pm, still more preferably at least 150 pm, yet more preferably at least 200 pm, even more preferably at least 250 pm, still more preferably at least 300 pm, yet more preferably at least 450 pm, and most preferably at least 600 pm. Typically, the polymer template has a mean pore diameter of less than 5000 pm, preferably less than 3000 pm, more preferably less than 2000 pm, yet more preferably less than 1500 pm, even more preferably less than 1200 pm, still more preferably less than 1100 pm, yet more preferably less than 1000 pm, even more preferably less than 900 pm, still more preferably less than 850 pm, yet more preferably less than 800 pm, and most preferably less than 750 pm. Thus, typically, the polymer template has a mean pore diameter of from 1 to 5000 pm, preferably from 10 to 3000 pm, more preferably from 25 to 2000 pm, yet more preferably from 50 to 1500 pm, even more preferably from 100 to 1200 pm, still more preferably from 150 to 1100 pm, yet more preferably from 200 to 1000 pm, even more preferably from 250 to 900 pm, still more preferably from 300 to 850 pm, yet more preferably from 450 to 800 pm and most preferably from 600 to 750 pm.

The strut thickness and pore diameter of a polymer template are typically determined using scanning electron microscopy. Alternatively, these parameters can be measured using a destructive technique in which the template is sectioned, the cross-section is embedded in a resin and examined under an optical microscope. The polymer template produced in step (a) is a scale model of the desired metallic scaffold or foam. In other words, the polymer template is a “positive” template of the desired metallic scaffold or foam. It is not a “negative”, i.e. inverse, template of the desired metallic scaffold or foam. Post-processing of the polymer template

After the polymer template has been obtained, the polymer template may optionally be subjected to one or more post-processing steps prior to the coating step (b).

In one embodiment, following step (a) but prior to step (b) of the process, the polymer template is subjected to cleaning with a solvent. Without being bound by any particular theory, it is thought that a solvent wash reduces the tackiness of the polymer template as it reduces the amount of unlinked monomers stuck to the surface of the template. However, in the absence of cleaning, it is thought that the pores in the polymer template may remain clogged with e.g. unlinked monomers on the surface, resulting in a less faithful replication of the template surface in a subsequent coating step with metal particles. Preferably in this embodiment, the solvent is selected from water, an alcohol, tripropylene glycol monomethyl ether, tetrahydrofuran, tetrafluoroethylene, dimethyl sulfoxide, dimethylformamide, acetone, chloroform, dichloromethane, toluene, acetonitrile or a mixture thereof. Preferred alcohols include methanol, ethanol, n-propanol and isopropanol, with ethanol and isopropanol being most preferred. More preferably, the solvent is selected from ethanol, isopropanol and tripropylene glycol monomethyl ether, or a mixture thereof. Typically in this embodiment, the polymer template is fully submerged in the solvent during the cleaning. Typically in this embodiment, the polymer template is subjected to cleaning for a period of from 30 seconds to 30 minutes, preferably from 1 to 20 minutes, more preferably from 2 to 10 minutes and most preferably from 3 to 5 minutes.

During cleaning with a solvent, the template may simultaneously be subjected to ultrasonication. Thus, in one embodiment, following step (a) but prior to step (b) of the process, the polymer template is subjected to cleaning with a solvent under ultrasonication. Alternatively, following step (a) but prior to step (b) of the process, the polymer template is subjected to cleaning with a solvent and no ultrasonication is applied. Preferably, following step (a) but prior to step (b) of the process, the polymer template is subjected to cleaning with a solvent under ultrasonication. Typically, ultrasonication is applied for the whole duration of the solvent wash. Alternatively, though, ultrasonication may be applied only for part of the duration of the solvent wash. Typically in this embodiment, ultrasonication is applied to the polymer template for a period of from 30 seconds to 30 minutes, preferably from 1 to 20 minutes, more preferably from 2 to 10 minutes and most preferably from 3 to 5 minutes.

Without being bound by any particular theory, it is thought ultrasonication improves removal of unlinked monomers stuck to the surface of the template. This reduces the tackiness of the polymer template, but also reduces clogging of the pores. It is therefore believed that there is a balance between removing too little unlinked monomer from the polymer template (which would result in unfaithful replication of the polymer template) and removing too much unlinked monomer from the polymer template (which would result in a reduced tackiness, decreasing the efficiency with which metallic particles can bind to the template). The balance can be achieved by adjusting the duration of the solvent wash and the application of ultrasonication.

In one embodiment, following step (a) but prior to step (b) of the process, the polymer template is subjected to cleaning and/or drying with pressurised air. Typically in this embodiment, a jet of pressurised air is applied to the template for a period of from 5 seconds to 20 minutes, preferably from 15 seconds to 10 minutes, more preferably from 30 seconds to 5 minutes and most preferably from 1 to 3 minutes.

In one embodiment, following step (a) but prior to step (b) of the process, the polymer template is subjected to curing. Preferably, the curing is affected with UV light. Optionally, heat may also be applied to aid the curing process. Following the curing step, the resultant polymer template is partially cured. In such a curing step, some of the polymer chains undergo cross-linking. This cross-linking increases the toughness or hardness of the polymer template, which assists with the subsequent coating step. However, the curing process also reduces the tackiness of the polymer template. Thus, a complete curing is undesirable, because dry coating of a completely cured template would be inefficient. By “partially cured” it is understood that the polymer template has been subjected to a degree of cross- linking but has retained its ability to adhere to the solid composition of metallic particles as defined herein. In particular, a “partially cured” polymer template is a polymer template in which at least 50% of the surface area of the polymer template, preferably at least 60%, more preferably at least 70%, yet more preferably at least 75%, still more preferably at least 80%, even more preferably at least 85%, and most preferably at least 90% or more, e.g. at least 92% or more, at least 95% or more, at least 97% or more, at least 99% or more, or 100%, is capable of being coated by the adhesion of said solid composition to the surface of the polymer template.

Typically, in order to achieve the desired level of curing of the polymer template, the template is subjected to curing with UV light for a period of from 15 minutes to 24 hours, preferably from 30 minutes to 8 hours, more preferably from 45 minutes to 4 hours, still more preferably from 1 hour to 3 hours, and most preferably about 2 hours. Typically, the UV light employed has a wavelength of from 370 to 420 nm, preferably from 385 to 405 nm, and most preferably is 385 nm or 405 nm. Optionally, during the period of curing with UV light, the polymer template is additionally heated to a temperature of from 45 to 180°C, preferably from 50 to 120°C, more preferably from 55 to 100°C, and most preferably from 60 to 80°C, from 45 to 180°C, more preferably irradiation with UV light at a wavelength of from 385 to 405 nm at a temperature of from 50 to 120°C, and most preferably irradiation with UV light at a wavelength of 385 nm or 405 nm at a temperature of from 60 to 80°C.

In one embodiment, following step (a) but prior to step (b) of the process, the polymer template is:

(i) subjected to cleaning with a solvent, optionally under ultrasonication; and

(ii) subsequently subjected to cleaning and/or drying with pressurised air.

(i) subjected to cleaning and/or drying with pressurised air; and

(ii) subsequently subjected to cleaning with a solvent, optionally under ultrasonication.

(i) subjected to cleaning with a solvent, optionally under ultrasonication; and

(ii) subsequently subjected to curing in the presence of UV light or both UV light and heat.

(i) subjected to cleaning and/or drying with pressurised air; and

(i) subjected to cleaning with a solvent, optionally under ultrasonication;

(ii) subsequently subjected to cleaning and/or drying with pressurised air; and

(iii) subsequently subjected to curing in the presence of UV light or both UV light and heat.

(i) subjected to cleaning and/or drying with pressurised air;

(ii) subsequently subjected to cleaning with a solvent, optionally under ultrasonication; and

(i) subjected to first cleaning with a solvent, optionally under ultrasonication;

(ii) subsequently subjected to cleaning and/or drying with pressurised air;

(iii) subjected to a second cleaning with a solvent, optionally under ultrasonication; and

(iv) subsequently subjected to curing in the presence of UV light or both UV light and heat. In this embodiment, the first and second cleaning steps with a solvent may employ the same solvent. Alternatively, the first and second cleaning steps with a solvent may employ a different solvent. Preferably, the first and second cleaning steps with a solvent employ the same solvent.

Typically in this embodiment, the first and second cleaning steps with a solvent both employ ultrasonication. Alternatively, the first cleaning step employs ultrasonication and the second cleaning step does not. Alternatively, the first cleaning step does not employ ultrasonication and the second cleaning step does. Alternatively, neither the first nor second cleaning steps employ ultrasonication.

In each of the preceding seven embodiments, the preferable features of each of the individual steps of cleaning with solvent, cleaning and/or drying with pressurised air, and curing, are as set out above.

In some embodiments, no curing with either UV light or heat is effected between steps (a) and (b).

In a particularly preferred embodiment, following step (a) but prior to step (b) of the process, the polymer template is:

(i) subjected to cleaning with a solvent for a period of from 30 seconds to 30 minutes, preferably under ultrasonication; and

(ii) subsequently subjected to curing in the presence of UV light or both UV light and heat for a period of from 0 to 24 hours.

In a more preferred embodiment, following step (a) but prior to step (b) of the process, the polymer template is:

(i) subjected to cleaning with a solvent for a period of from 1 to 20 minutes, preferably under ultrasonication; and

(ii) subsequently subjected to curing in the presence of UV light or both UV light and heat for a period of from 0 to 8 hours.

In a yet more preferred embodiment, following step (a) but prior to step (b) of the process, the polymer template is:

(i) subjected to cleaning with a solvent for a period of from 2 to 10 minutes, preferably under ultrasonication; and

(ii) subsequently subjected to curing in the presence of UV light or both UV light and heat for a period of from 0 to 4 hours.

In a most preferred embodiment, following step (a) but prior to step (b) of the process, the polymer template is: (ⁱ) subjected to cleaning with a solvent for a period of from 3 to 5 minutes, preferably under ultrasonication; and

(ii) subsequently subjected to curing in the presence of UV light or both UV light and heat for a period of from 0 to 2 hours.

Importantly, following step (a) and any post-processing steps, the surface of the polymer template is inherently sticky or “tacky”. This enables the solid composition comprising metallic powder that is subsequently contacted with the polymer template in step (b) to stick or adhere to the surface of the polymer template. The stickiness or tackiness is an inherent feature of the polymer template. In other words, the stickiness or tackiness is not imparted to the template by application of an adhesive layer between steps (a) and (b) of the process. Thus, typically, the template prepared in step (a) is not coated with an adhesive layer prior to step (b). Typically, the template prepared in step (a) is therefore not immersed in, or otherwise contacted with, an adhesive material, e.g. an adhesive solution such as an acrylic type adhesive solution.

Dry-coating of the polymer template

The polymer template, optionally following any of the post-processing steps described above, is coated by contacting the template with a solid composition comprising metallic powder, to provide a metal particle-coated polymer template. In this step, typically the surface of the polymer template is at least substantially coated with metallic particles. The coating is believed to occur via the adhesion of the metallic particles in the solid composition to the surface of the polymer template, owing to the tackiness (i.e. stickiness) of the template. By “substantially coated” it is meant that at least 50% of the surface area of the polymer template, preferably at least 60%, more preferably at least 70%, yet more preferably at least 75%, still more preferably at least 80%, even more preferably at least 85%, and most preferably at least 90% or more, e.g. at least 92% or more, at least 95% or more, at least 97% or more, at least 99% or more, or 100%, is coated with metallic particles by the adhesion of said solid composition to the surface of the polymer template. The degree of coating of the surface of the polymer template can be determined by scanning electron microscopy or radiography, preferably radiography.

The solid composition comprising metallic powder is a solid and comprises a powder. Thus, the composition is not liquid or gaseous. The composition is not a slurry. The composition may, however, comprise a residual amount of solvent. Typically, the composition comprises less than 20% by weight of solvent, preferably less than 10% by weight of solvent, more preferably less than 5% by weight of solvent, yet more preferably less than 3% by weight of solvent, still more preferably less than 2% by weight of solvent, even more preferably less than 1% by weight of solvent, yet more preferably less than 0.5% by weight of solvent, still more preferably less than 0.2% by weight of solvent, and most preferably less than 0.1% by weight of solvent. A powder is as defined herein. Preferably, the solid composition comprises powder particles which are spherical, or substantially spherical, in shape. Alternatively, the solid composition consists of, or consists essentially of, granular particles which are spherical, or substantially spherical, in shape.

Typically, the solid composition has a distinct particle size distribution such that at least 80% of the metallic particles in the solid composition by number have a diameter of less than 25 pm, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm. Preferably, at least 90% of the metallic particles in the solid composition by number a diameter of less than 25 pm, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm. More preferably, at least 95% of the metallic particles in the solid composition by number have a diameter of less than 25 pm, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm.

Further, typically, at least 80% of the metallic particles in the solid composition by number have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, more preferably greater than 0.1 pm, and most preferably greater than 0.2 pm. Preferably, at least 90% of the metallic particles in the solid composition by number have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, and most preferably greater than 0.1 pm. More preferably, at least 95% of the metallic particles in the solid composition by number have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, and most preferably greater than 0.1 pm.

Typically, at least 80% of the metallic particles in the solid composition by number have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.2 pm to 5 pm. Preferably, at least 90% of the metallic particles in the solid composition by number have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.1 pm to 5 pm. More preferably, at least 95% of the metallic particles in the solid composition by number have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.1 pm to 5 pm.

Typically, the solid composition has a distinct particle size distribution such that at least 80% of the metallic particles in the solid composition by volume have a diameter of less than 25 pm, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm. Preferably, at least 90% of the metallic particles in the solid composition by volume a diameter of less than 25 mih, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm. More preferably, at least 95% of the metallic particles in the solid composition by volume have a diameter of less than 25 pm, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm.

Further, typically, at least 80% of the metallic particles in the solid composition by volume have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, more preferably greater than 0.1 pm, and most preferably greater than 0.2 pm. Preferably, at least 90% of the metallic particles in the solid composition by volume have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, and most preferably greater than 0.1 pm. More preferably, at least 95% of the metallic particles in the solid composition by volume have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, and most preferably greater than 0.1 pm.

Typically, at least 80% of the metallic particles in the solid composition by volume have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.2 pm to 5 pm. Preferably, at least 90% of the metallic particles in the solid composition by volume have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.1 pm to 5 pm. More preferably, at least 95% of the metallic particles in the solid composition by number have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.1 pm to 5 pm.

Typically, the solid composition has a distinct particle size distribution such that at least 80% of the metallic particles in the solid composition by mass have a diameter of less than 25 pm, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm. Preferably, at least 90% of the metallic particles in the solid composition by mass a diameter of less than 25 pm, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm. More preferably, at least 95% of the metallic particles in the solid composition by mass have a diameter of less than 25 pm, preferably less than 20 pm, more preferably less than 15 pm, still more preferably less than 10 pm, and most preferably less than 5 pm.

Further, typically, at least 80% of the metallic particles in the solid composition by mass have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, more preferably greater than 0.1 pm, and most preferably greater than 0.2 pm. Preferably, at least 90% of the metallic particles in the solid composition by mass have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, and most preferably greater than 0.1 pm. More preferably, at least 95% of the metallic particles in the solid composition by mass have a diameter of greater than 0.001 pm, preferably greater than 0.01 pm, and most preferably greater than 0.1 pm.

Typically, at least 80% of the metallic particles in the solid composition by mass have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.2 pm to 5 pm. Preferably, at least 90% of the metallic particles in the solid composition by mass have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.1 pm to 5 pm. More preferably, at least 95% of the metallic particles in the solid composition by number have a diameter of from 0.001 pm to 25 pm, preferably from 0.01 pm to 20 pm, more preferably from 0.05 pm to 15 pm, still more preferably from 0.1 pm to 10 pm and most preferably from 0.1 pm to 5 pm.

The metallic powder can be purchased with the required size distribution, or prepared by any technique known in the art. In particular, the metallic powder can be prepared from larger particles of a metal (e.g. coarse granules) by any known means to reduce particle size, such as milling (e.g. planetary ball milling). This technique is capable not just of reducing the particle size of coarse granules, but also of formation of an alloy when coarse granules of two or more different metals are milled together. In a particularly preferable embodiment, coarse particles of iron and manganese are mixed together and subjected to planetary ball milling to produce a metallic powder comprising iron-manganese alloy.

Mechanical ball-milling is a solid-state process that may be used to grind, mix and optionally alloy powders of different materials to create homogenous powder mixtures with a desired powder size distribution. Depending on the type of ball mill used (e.g. a planetary ball mill), the rotation of the vessel containing the powder mixtures and grinding media (which are hardened balls of a specific material and size) cause the grinding media to impact the powders, at which point the powders either fracture or cold-weld due to the high temperatures attained from repeated impact. The balance between fracture and welding achieved depends on the process parameters set and the properties of the powders being processed. The high temperatures reached may also facilitate alloying of elements through solid- state diffusion, without the need to heat the composition in a furnace. This allows for the preparation of certain alloyed powders that cannot otherwise be achieved using equilibrium processing.

Typically, the metallic powder comprises iron, magnesium, zinc, titanium or aluminium. Preferably, the metallic powder comprises a mixture of at least one of iron, magnesium, zinc, titanium or aluminium, optionally in combination with one or more different elements selected from: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Fe, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. Thus, in one embodiment, the metallic powder comprises a mixture of iron with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In another embodiment, the metallic powder comprises a mixture of magnesium with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Fe, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In another embodiment, the metallic powder comprises a mixture of zinc with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Fe, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In another embodiment, the metallic powder comprises a mixture of titanium with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Fe, Mg, Zn, Ca,

Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In another embodiment, the metallic powder comprises a mixture of aluminium with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Fe, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. Typically, the metallic powder comprises iron. Preferably, the metallic powder comprises a mixture comprising iron. The aforementioned mixtures may be alloys. Thus, in a preferable embodiment, the metallic powder comprises an iron alloy.

Preferably, the metallic powder comprises a mixture comprising iron and manganese, more preferably an alloy comprising iron and manganese. Thus, in one embodiment, the metallic powder comprises an iron-manganese alloy and, optionally, one or more (e.g. one, two, three, four, five or more) of the following elements: Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In one embodiment, the metallic powder comprises an iron-manganese-carbon alloy and, optionally, one or more (e.g. one, two, three, four, five or more) of the following elements: Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, N, S, Mo and Ni. In one embodiment, the metallic powder comprises an iron-manganese-silicon alloy and, optionally, one or more (e.g. one, two, three, four, five or more) of the following elements: Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, C, P, Cr, V, B, Zr, Ta, N, S, Mo and Ni. In one embodiment, the metallic powder comprises an iron-manganese-carbon-silicon alloy and, optionally, one or more (e.g. one, two, three, four, five or more) of the following elements: Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, P, Cr, V, B, Zr, Ta, N, S, Mo and Ni.

In an embodiment, the metallic powder comprises silver. Without wishing to be bound by any particular theory, it is considered that the addition of silver imparts certain benefits on the final metallic scaffold or foam. In particular, if the polymer template is removed in step (c) by heating to a temperature above the melting point of silver (962° C), then the silver within the metal coating layer melts concomitantly with the degradation of the polymer template. The melting of the silver within the metal layer is believed to enable gaps between the metallic particles to be “filled in”, which increases the overall mechanical strength of the finished 3D article when the article is subsequently cooled to room temperature and the silver once again solidifies. Additionally, if silver is present in the final metal scaffold, the galvanic couple set up between the silver atoms and the bulk solid state iron structure (or iron alloy, e.g. solid state FeMn structure) may increase the degradation rate of the metallic scaffold in an in vivo setting. This is desirable in the case of bone scaffolds that are designed to biodegrade. Silver may also impart anti-bacterial properties on the metallic scaffold or foam.

Preferably, therefore, the metallic powder comprises a mixture of iron and silver, for example an alloy comprising iron and silver, or a mixture of iron, manganese and silver, for example an alloy comprising iron, manganese and silver. Thus, in one embodiment, the metallic powder comprises an iron-silver alloy. In another embodiment, the metallic powder comprises an iron-manganese-silver alloy. More preferably, the alloy additionally comprises carbon and/or silicon. Thus, in one embodiment, the metallic powder comprises an iron-manganese-silver-carbon alloy. In another embodiment, the metallic powder comprises an iron-manganese-silver-silicon alloy. In another embodiment, the metallic powder comprises an iron-manganese-silver-carbon-silicon alloy.

In a particularly preferred embodiment, the metallic powder typically comprises an alloy comprising from 30 to 90 wt% iron, preferably from 40 to 80 wt% iron, still more preferably from 50 to 70 wt% iron, and most preferably from 55 to 65 wt% iron. The metallic powder typically comprises an alloy comprising from 10 to 40 wt% manganese, preferably from 20 to 38 wt% manganese, and most preferably from 30 to 35 wt% manganese. The metallic powder typically comprises an alloy comprising from 0 to 20 wt% silver, preferably from 0.1 to 10 wt% silver, and most preferably from 1 to 5 wt% silver. The metallic powder typically comprises an alloy comprising from 0 to 2 wt% carbon, preferably from 0.5 to 1.5 wt% carbon, and most preferably from 0.7 to 1.2 wt% carbon. The metallic powder typically comprises an alloy comprising from 0 to 10 wt% silicon, preferably from 1 to 8 wt% silicon, and most preferably from 3 to 6 wt% silicon. Thus, in one embodiment, the metallic powder comprises an alloy comprising from 30 to 90 wt% iron, from 10 to 40 wt% manganese, from 0.1 to 20 wt% silver, from 0 to 2 wt% carbon, and from 0 to 10 wt% silicon. In a particularly preferable embodiment, the metallic powder comprises an alloy comprising from 55 to 65 wt% iron, from 30 to 35 wt% manganese, from 1 to 5 wt% silver, from 0.7 to 1.2 wt% carbon, and from 3 to 6 wt% silicon.

Any contacting method known to a person skilled in the art can be employed in step (b) of the present method. Typically, the contacting method is a dry-coating method. Thus, the contacting method does not involve the use of an extraneous solvent. In particular, the contacting method does not involve the formation of a slurry comprising the solid composition comprising metallic particles. The solid composition itself may, however, comprise a trace amount of solvent, as discussed above. Preferably, the dry-coating method is selected from: tumbling the polymer template in the presence of the solid composition; spraying of the solid composition onto the polymer template; and use of a fluidised bed to contact the solid composition with the polymer template. Alternatively, the dry-coating method comprises centrifugation of the polymer template in the presence of the solid composition.

In one embodiment, therefore, the polymer template is coated in step (b) by tumbling the polymer template in the presence of a solid composition comprising metallic powder. By “tumbling” it is understood that both the polymer template and the solid composition are placed inside a container (i.e. a vessel) and the vessel is rotated such that the solid composition moves relative to, and comes into contact with, the polymer template in a random manner. Preferably, the container is an enclosed container.

In another embodiment, the polymer template is coated in step (b) by spraying of the solid composition onto the polymer template. Any appropriate method of spraying known to a person skilled in the art may be employed in this embodiment. Typically, the solid composition is sprayed using a device that employs compressed gas, preferably air or nitrogen, to eject the metallic particles of the solid composition towards the polymer template. Preferably, the solid composition is ejected from the device as a continuous jet of particles. Typically, in this embodiment, the polymer template is rotated relative to the jet of particles during the spraying, such that the polymer template is evenly coated on all sides with the metallic particles. Further, this technique allows the internal surfaces of a complex 3D structure, e.g. a structure comprising a network of struts and pores, to also be evenly coated.

In another embodiment, the polymer template is coated in step (b) by use of a fluidised bed to contact the polymer template and the solid composition. A fluidised bed is a physical phenomenon occurring when a quantity of solid particulate substance is placed under appropriate conditions to cause a solid/fluid mixture to behave as a fluid. Thus, in this embodiment, the polymer template and the solid composition are contacted by placing both the polymer template and the solid composition within a vessel (e.g. a fluidised bed reactor) in the presence of a gas, preferably air, under conditions of temperature and pressure at which the solid composition and gas mixture behaves as a fluid. This fluid then contacts the polymer template, coating it with metallic particles that are present in the fluid.

Typically, contacting step (b) is performed only once in the process of the present invention.

Preferably, however, contacting step (b) is repeated at least once, for example once, twice, thrice, four times or more. If contacting step (b) is repeated at least once, the solid composition used when (b) is repeated may comprise the same or a different metallic powder to that used when step (b) was first carried out. Preferably, the solid composition comprises the same metallic powder to that used when step (b) was first carried out. Alternatively, the solid composition comprises a different metallic powder to that used when step (b) was first carried out. If contacting step (b) is repeated at least twice, the solid composition used each time step (b) is repeated may be the same as, or different to, the preceding time that step (b) was carried out. Thus, if step (b) is carried out a total of three times, each step may employ a solid composition comprising the same metallic powder, or it may be the case that two of the three steps employ a solid composition comprising the same metallic powder and one of the three steps employs a solid composition comprising a different metallic powder to the other two, or it may be the case that all three of the steps employ a solid composition comprising a different metallic powder to each another.

Optional post-processing of the metal particle-coated polymer template

After the metal particle -coated polymer template has been obtained, this template may optionally be subjected to one or more post-processing steps prior to the sacrificial template removal step (c).

In one embodiment, following step (b) but prior to step (c) of the process, the metal particle-coated polymer template is subjected to treatment with pressurised air such that excess metallic powder is separated from the template. Typically in this embodiment, a jet of pressurised gas is applied to the template for a period of from 5 seconds to 15 minutes, preferably from 10 seconds to 10 minutes, more preferably from 20 seconds to 5 minutes and most preferably from 30 seconds to 3 minutes. Typically in this embodiment, the excess metallic powder that is separated from the template comprises metallic particles which were not adhered to, or only very weakly adhered to, the surface of the polymer substrate following the coating step (b).

In one embodiment, following step (b) but prior to step (c) of the process, the metal particle-coated polymer template is subjected to curing. Preferably, the curing is effected by UV light. In such a curing step, some of the polymer chains undergo cross-linking. This cross-linking increases the toughness or hardness of the polymer template. This treatment may be particularly useful if a further coating is to be applied to the metal particle-coated polymer template (e.g. via application of a slurry, as discussed below). Optionally, heat is also applied to aid the curing process.

Typically, in this embodiment, the template is subjected to curing with UV light, preferably for a period of from 15 minutes to 24 hours, preferably from 30 minutes to 8 hours, more preferably from 45 minutes to 4 hours, still more preferably from 1 hour to 3 hours, and most preferably about 2 hours. Typically, the UV light employed has a wavelength of from 370 to 420 nm, preferably from 385 to 405 nm, and most preferably is 385 nm or 405 nm. Optionally, during the period of curing with UV light, the polymer template is additionally heated to a temperature of from 45 to 180°C, preferably from 50 to 120°C, more preferably from 55 to 100°C, and most preferably from 60 to 80°C. In one embodiment, the curing step involves irradiation with UV light at a wavelength of from 370 to 420 nm at a temperature of from 45 to 180 °C, more preferably irradiation with UV light at a wavelength of from 385 to 405 nm at a temperature of from 50 to 120°C, and most preferably irradiation with UV light at a wavelength of 385 nm or 405 nm at a temperature of from 60 to 80° C.

In one embodiment, following step (b) but prior to step (c) of the process, the metal particle-coated polymer template is further coated by application of a slurry comprising metallic powder. Thus, the dry-coated template is further coated using a slurry-based technique. Any appropriate slurry known to the skilled person may be employed in this embodiment. Preferably, the slurry is an alcohol-based slurry, more preferably a poly(vinyl alcohol)-based slurry, or an ether-based slurry, more preferably a polyethylene glycol)-based slurry. The alcohol or ether typically acts as a binder. The slurry may optionally comprise a solvent such as water, an alcohol (e.g isopropyl alcohol) or acetone in order to achieve the correct balance of stickiness and viscosity of the slurry. The metallic powder present in the slurry may be the same as or different to (any of) the metallic powder(s) employed in coating step (b). All of the typical and preferred features of the metallic powder employed in coating step (b) are also considered typical and preferred features of the metallic powder employed in the further coating step of this embodiment. In this embodiment, the metal particle-coated polymer template is partially or fully submerged, preferably fully submerged, in the slurry, typically for a period of from 1 to 30 minutes. During the application of the slurry, the slurry is preferably agitated, for example by tumbling or mechanical stirring. In some embodiments, a centrifugation step is employed following application of the slurry, in order to remove excess slurry from the template.

In this embodiment, it is particularly preferred that following coating step (b) but prior to the metal particle-coated polymer template being further coated by application of a slurry, the metal particle- coated polymer template is subjected to a heating step at a temperature of from 150 to 250°C, more preferably from 160 to 220°C, and most preferably from 175 to 200°C. Without wishing to be bound by any particular theory, it is believed that this temperature treatment causes the metallic particles adhered to the surface of the polymer template to “neck”, i.e. bridges or “necks” comprising metal atoms begin to form between adjacent metallic particles on the surface of the template. This process is believed to impart additional structural integrity to the dry-coated layer of metal particles on the surface of the polymer template, which helps to prevent the dry-coated metal particles from being washed away when the slurry is applied to the metal particle-coated polymer template.

Thus, in one embodiment, following step (b) but prior to step (c) of the process, the metal particle- coated polymer template is:

(i) optionally, subjected to treatment with pressurised air such that excess metallic powder is separated from the template;

(ii) subsequently subjected to curing in the presence of UV light, or both UV light and heat; and (iii) subsequently further coated by application of a slurry comprising metallic powder.

In another embodiment, step (b) but prior to step (c) of the process, the metal particle-coated polymer template is:

(ii) subsequently heated to a temperature of from 150 to 250°C; and

(iii) subsequently further coated by application of a slurry comprising metallic powder.

(ii) subsequently subjected to a curing step in the presence of UV light, or both UV light and heat;

(iii) subsequently heated to a temperature of from 150 to 250°C; and

(iv) subsequently further coated by application of a slurry comprising metallic powder.

In each of the preceding three embodiments, the preferable features of each of the individual steps of treatment with pressurised air, curing, and further coating with a slurry, are as set out above.

Removal of the polymer template

The metal particle-coated polymer template, optionally following any of the post-processing steps described above, is converted to a metallic scaffold or foam by the material removal of the sacrificial underlying polymer template. By “material removal” is meant that at least 75% by weight of the polymer template is removed from the metal particle-coated polymer template, preferably at least 80% by weight, more preferably at least 85% by weight, yet more preferably at least 90% by weight, still more preferably at least 95% by weight, even more preferably at least 97% by weight, yet more preferably at least 98% by weight, and most preferably at least 99% by weight, e.g. at least 99.5% by weight, or at least 99.9% by weight. In one embodiment, all, or substantially all, of the polymer template is removed in this step.

Any suitable technique for removing a sacrificial polymer template known to a person of skill in the art may be employed. Typically, the removal of the polymer template is affected by application of a solution that dissolves the polymer template but does not dissolve the metal coating to any appreciable extent, or by heating the metal particle-coated polymer template. Preferably, the removal of the polymer template is affected by heating the metal particle-coated polymer template.

In one embodiment, step (c) comprises a single heating step to achieve a sintering temperature. The metal particle-coated polymer template is held at the sintering temperature for a period of time before the resulting scaffold is cooled. Thus, in this embodiment, step (c) comprises heating the metal particle- coated polymer template to a sintering temperature that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal. Typically, the rate of heating is from 60 to 600°C/hour, and is preferably from 250 to 350°C/hour. Typically, following sintering, the resulting metal scaffold or foam is cooled to room temperature.

In an embodiment where the metal coating comprises iron or an iron alloy, e.g. an iron-manganese alloy, the sintering temperature is typically from 950 to 1300°C, preferably from 1050 to 1200°C, and more preferably from 1100 to 1150°C. Preferably, this sintering temperature is maintained for a period of from 1 to 5 hours, more preferably from 2 to 4 hours, and yet more preferably about 3 hours.

In an alternative embodiment, step (c) comprises the sub-stages of:

(i) heating the metal particle-coated polymer template to a temperature which is below the temperature at which the polymer will undergo complete thermal degradation; and

(ii) subsequently heating the metal particle-coated template to a sintering temperature that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal.

The temperature in step (i) may in some instances be a temperature that is sufficiently high to cause necking of the metal particles coating the template. In other instances, however, the temperature in step (i) is not a temperature that is sufficiently high enough to cause necking of the metal particles.

Preferably, therefore, step (c) comprises discrete “low temperature dwell” and “sintering” steps. Thus, step (c) comprises the sub-stages of:

(i) heating the metal particle-coated polymer template to a temperature which is below the temperature at which the polymer will undergo complete thermal degradation (the “dwelling temperature”), and dwelling at that temperature for a time period of from 15 minutes to 6 hours, preferably from 1 to 3 hours; and

(ii) subsequently heating the metal particle-coated template to a sintering temperature that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal. In this embodiment, step (i) may be referred to as the “low temperature dwell” step and step (ii) may be referred to as the “sintering” step. Typically, the polymer template degrades during the increase in temperature (the temperature “ramp”) up to the dwelling temperature and/or whilst at the dwelling temperature and/or during the increase in temperature between step (i) and step (ii), i.e. up to the sintering temperature. Thus, by the time the sintering temperature is reached, the underlying template has been materially removed. Typically, following heating step (ii), the resulting metal scaffold or foam is cooled to room temperature. Typically, the rate of heating in step (i) is from 60 to 600°C/hour, and is preferably from 250 to 350°C/hour. Typically, the rate of heating in step (ii) is from 60 to 600°C/hour, and is preferably from 250 to 350° C/hour. Typically, the rate of heating in step (i) is the same as the rate of heating in step (ii). Alternatively, the rate of heating in step (i) is less than the rate of heating in step (ii). Alternatively, the rate of heating in step (i) is greater than the rate of heating in step (ii).

The present inventors have discovered that this particular two-step heat treatment has a surprising benefit. Without wishing to be bound by any particular theory, it is believed that by employing a first, low-temperature heat treatment, the overall structural integrity of the metal coating layer of the template may be increased. In effect, it is thought that through this process, the metallic coating layer obtains sufficient structural integrity to retain the shape of the template underneath once the template is subsequently degraded in the ramp to the sintering temperature. The structural integrity of a metallic scaffold or foam product that has undergone a low temperature dwelling step may be improved compared to corresponding scaffolds or foams which did not undergo an initial low temperature dwelling step.

Typically, the low temperature dwelling step (i) is effected at a temperature of from 100 to 750°C, preferably from 125 to 500°C, more preferably from 150 to 250°C, and still more preferably from 175 to 200°C.

In an embodiment where the metal coating comprises iron or an iron alloy, e.g. an iron-manganese alloy, the temperature employed in step (i) is typically from 100 to 750°C, preferably from 125 to 500°C, more preferably from 150 to 250°C, still more preferably from 160 to 220°C, and more preferably from 175 to 200°C. Preferably, this temperature is maintained for a period of from 1 to 3 hours, more preferably about 2 hours.

Thus, preferably, in an embodiment where the metal coating comprises iron or an iron alloy, e.g. an iron-manganese alloy, the low temperature dwelling step (i) is typically effected at a temperature of from 100 to 750°C, preferably from 125 to 500°C, more preferably from 150 to 250°C, still more preferably from 160 to 220°C, and more preferably from 175 to 200°C. This temperature may also be referred to as the “dwelling temperature” or “low temperature dwelling temperature”. Preferably, this low temperature dwelling temperature is maintained for a period of from 1 to 3 hours, more preferably about 2 hours.

In an embodiment where the metal coating comprises iron or an iron alloy, e.g. an iron-manganese alloy, the temperature employed in step (ii) that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal, is typically from 950 to 1300°C, preferably from 1050 to 1200°C, and more preferably from 1100 to 1150°C. This temperature may also be referred to as the “sintering temperature” or the “thermal degradation temperature”. Preferably, this sintering temperature is maintained for a period of from 1 to 5 hours, more preferably from 2 to 4 hours, and yet more preferably about 3 hours.

Without wishing to be bound by any particular theory, a higher sintering temperature and a longer sintering time are thought to improve densification of the resulting metal scaffold or foam, but may have negative consequences e.g. extensive oxidation, depending on the nature of the metal present in the scaffold or foam. Thus, in some cases (e.g. where the metal particle-coated template comprises iron), it is preferable for the low temperature dwelling step (i) to be carried out in the presence of a reducing atmosphere to prevent oxidation. In some cases (e.g. where the metal particle-coated template comprises iron), it is preferable for the sintering step (which is step (ii) if the two-step heating process is used) to be carried out in the presence of a reducing atmosphere to prevent oxidation. Preferably, in some cases (e.g. where the metal particle-coated template comprises iron), if the two-step heating process is used it is preferable for both the low temperature dwelling step (i) and the sintering step (ii), as well as the temperature ramp between these two steps, to be carried out in the presence of a reducing atmosphere to prevent oxidation. An example of a preferred reducing atmosphere is an N2-H2 atmosphere. Preferably, any cooling of the metal scaffold or foam following the sintering step (which is step (ii) if the two-step heating process is used) is also carried out in the presence of a reducing atmosphere. Typically, the reducing atmosphere is obtained by applying a stream of reducing gas to the metal particle-coated polymer template, preferably wherein said reducing gas comprises nitrogen and hydrogen. Typically, the flow rate of the reducing gas is 50 to 650 L/hr, and is preferably from 100 to 200 L/hr.

Therefore, in an especially preferred embodiment, step (c) comprises the sub-stages of:

(i) heating the metal particle-coated polymer template to a temperature of 150 to 250°C, optionally for a period from 1 to 3 hours; and

(ii) subsequently heating the metal particle-coated polymer template to a sintering temperature of from 1050 to 1200°C, preferably under a reducing atmosphere, optionally for a period of from 1 to 5 hours. In another especially preferred embodiment, the low temperature dwelling step is omitted, and step (c) comprises heating the metal particle-coated polymer template to a sintering temperature of from 1050 to 1200° C, preferably under a reducing atmosphere, and optionally maintaining the sintering temperature for a period of from 1 to 5 hours.

In step (c) the metal particle-coated polymer template may be enclosed by a cover. Such a cover typically acts as a barrier to the flow of gas. Thus, the cover slows the flow of gas or retards the flow of gas. The cover is not necessarily, however, completely impermeable to gas. Thus, some gas exchange may still occur between the areas enclosed by and external to the cover, although typically this gas exchange is kept to a minimal level. The purpose of the cover is to create a microenvironment around the metal particle-coated polymer template which is poor in oxygen and/or enriched in reducing gases, i.e. to prevent or reduce oxidation of the surface of the metal particle-coated polymer template. This is particularly beneficial when the metal coating of the template comprises a metal that is prone to oxidation, e.g. manganese. The cover can be made from any material that does not release gas. Typically the cover should be made from a material that does not degrade at the chosen sintering temperature. Typical materials from which the cover can be made are metal (e.g. stainless steel) and ceramic. By “enclosed by a cover”, it is understood that the metal particle-coated polymer template will, for the duration of step (c), typically be positioned on a surface of a furnace or on a suitable base plate, e.g. an alumina plate. Thus, the cover will contact said surface of the furnace in such a way that the metal particle-coated polymer template is completely surrounded by said surface of the furnace and said cover.

In step (c), a mass of sacrificial metal may also be present alongside the metal particle-coated polymer template when the latter is heated. This sacrificial metal typically has a greater potential to be oxidized than the metal of the metal particle-coated polymer template. Thus, in the presence of oxygen, oxide will preferentially form on the surface of the sacrificial metal over the surface of the metal particle- coated polymer template. Preferably in this embodiment, both the metal particle-coated polymer template and the mass of sacrificial metal are enclosed by a cover as described above.

Optionally, once the final metallic scaffold or foam has been prepared, a finishing step may be applied. Typically, such a finishing step involves smoothing the surface of the metallic scaffold or foam and/or removing impurities from the surface of the metallic scaffold or foam. This can be achieved by any method known in the art, for example by sand-blasting, tumbling with abrasive particles, or using a vibratory polishing machine. Alternatively, the finishing step can be a chemical finishing step, which involves contacting the surface of the metallic scaffold or foam with a chemical cleaning agent. Typically, said chemical cleaning agent comprises an acid or an alkali, preferably an inorganic acid or alkali. Preferably, the chemical cleaning agent comprises hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid, trifluoroacetic acid, citric acid, formic acid, sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, sodium bicarbonate, sodium carbonate, potassium carbonate, calcium carbonate and ammonium hydroxide. Most preferably, the chemical cleaning agent comprises hydrochloric acid. The chemical cleaning agent may also comprise an organic compound, preferably an amine, e.g. a primary amine, a secondary amine, a tertiary amine or a quartemary ammonium salt. A particularly preferred amine is hexamethylenetetramine. Thus, a particularly preferred chemical cleaning agent comprises (i) an acid or an alkali and (ii) an organic amine. An exemplary chemical cleaning agent comprises hydrochloric acid and hexamethylenetetramine. Typically during the chemical cleaning step, ultrasonication may be simultaneously applied. Thus, in one embodiment, the metallic scaffold or foam is contacted with a chemical cleaning agent under ultrasonication. In an alternative embodiment, the metallic scaffold or foam is contacted with a chemical cleaning agent in the absence of externally applied ultrasonication.

Accordingly, a preferred method of preparing a metallic scaffold or foam comprises:

(a) preparing a polymer template by a method of additive manufacturing;

(c) effecting the material removal of the polymer template to provide the metallic scaffold or foam by heating the metal particle-coated polymer template to a sintering temperature that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal.

In one embodiment, the process of preparing a metallic scaffold or foam comprises:

(a) preparing a polymer template by a method of additive manufacturing;

(c) effecting the material removal of the polymer template to provide the metallic scaffold or foam by heating the metal particle-coated polymer template to a sintering temperature of from 1050 to 1200°C, preferably under a reducing atmosphere, optionally for a period of from 1 to 5 hours.

(a) preparing a polymer template by a method of additive manufacturing;

(al) optionally cleaning the polymer template using a solvent, further optionally under ultrasonication;

(a2) cleaning and/or drying the polymer template with pressurised gas; (a3) optionally curing the polymer template in the presence of UV light or both UV light and heat;

(b) coating the polymer template by contacting the template with a solid composition comprising metallic powder to provide a metal particle-coated polymer template;

(b 1) optionally, treating the metal particle-coated polymer template with pressurised gas; and

(a) preparing a polymer template by a method of additive manufacturing;

(al) cleaning and/or drying the polymer template with pressurised gas;

(a2) optionally cleaning the polymer template using a solvent, further optionally under ultrasonication;

(a3) optionally curing the polymer template in the presence of UV light or both UV light and heat;

(a) preparing a polymer template by a method of additive manufacturing;

(b 1) optionally, treating the metal particle-coated polymer template with pressurised gas;

(t>2) curing the metal particle-coated polymer template in the presence of UV light, or both UV light and heat;

(t>3) optionally, heating the metal particle-coated polymer template to a temperature of from 150 to 250°C; (t>4) further coating the metal particle-coated polymer template by application of a slurry comprising metallic powder; and

(a) preparing a polymer template by a method of additive manufacturing;

(t>2) heating the metal particle-coated polymer template to a temperature of from 150 to 250°C;

(t>3) further coating the metal particle-coated polymer template by application of a slurry comprising metallic powder; and

(a) preparing a polymer template by a method of additive manufacturing;

(a2) cleaning and/or drying the polymer template with pressurised gas;

(t>4) further coating the metal particle-coated polymer template by application of a slurry comprising metallic powder; and

(a) preparing a polymer template by a method of additive manufacturing;

(a2) cleaning and/or drying the polymer template with pressurised gas;

(b2) heating the metal particle-coated polymer template to a temperature of from 150 to 250°C;

(b3) further coating the metal particle-coated polymer template by application of a slurry comprising metallic powder; and

(a) preparing a polymer template by a method of additive manufacturing;

(al) optionally cleaning and/or drying the polymer template with pressurised gas;

(a2) cleaning the polymer template using a solvent, optionally under ultrasonication;

(b2) curing the metal particle-coated polymer template in the presence of UV light, or both UV light and heat;

(b3) optionally, heating the metal particle-coated polymer template to a temperature of from 150 to 250°C; (t>4) further coating the metal particle-coated polymer template by application of a slurry comprising metallic powder; and

(с) effecting the material removal of the polymer template to provide the metallic scaffold or foam by heating the metal particle-coated polymer template to a sintering temperature of from 1050 to 1200°C, preferably under a reducing atmosphere, optionally for a period of from 1 to 5 hours.

(a) preparing a polymer template by a method of additive manufacturing;

(al) cleaning and/or drying the polymer template with pressurised gas;

(аз) optionally curing the polymer template in the presence of UV light or both UV light and heat;

(a) preparing a polymer template by a method of additive manufacturing;

(al) cleaning the polymer template using a solvent for a period of from 30 seconds to 30 minutes, preferably from 1 to 20 minutes, more preferably from 2 to 10 minutes and most preferably from 3 to 5 minutes, and preferably under ultrasonication;

(a2) curing the polymer template in the presence of UV light or both UV light and heat for a period of from 0 minutes to 24 hours, preferably from 15 minutes to 8 hours, more preferably from 30 minutes to 4 hours, still more preferably from 1 hour to 3 hours, and most preferably about 2 hours; (b) coating the polymer template by contacting the template with a solid composition comprising metallic powder to provide a metal particle-coated polymer template; and

(c) effecting the material removal of the polymer template to provide the metallic scaffold or foam by heating the metal particle-coated polymer template to a sintering temperature that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal, preferably wherein the sintering temperature is from 1050 to 1200°C, preferably under a reducing atmosphere, and optionally wherein the sintering temperature is maintained for a period of from 1 to 5 hours.

Alternatively, a preferred method of preparing a metallic scaffold or foam comprises:

(a) preparing a polymer template by a method of additive manufacturing;

(c) effecting the material removal of the polymer template to provide the metallic scaffold or foam by:

(i) heating the metal particle-coated polymer template to a temperature which is below the temperature at which the polymer will undergo complete thermal degradation, and optionally dwelling at that temperature for a time period of from 15 minutes to 6 hours, preferably from 1 to 3 hours; and

(a) preparing a polymer template by a method of additive manufacturing;

(i) heating the metal particle-coated polymer template to a temperature which is below the temperature at which the polymer will undergo complete thermal degradation, and dwelling at that temperature for a time period of from 15 minutes to 6 hours, preferably from 1 to 3 hours; and (ii) subsequently heating the metal particle-coated template to a sintering temperature that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal.

(a) preparing a polymer template by a method of additive manufacturing;

(i) heating the metal particle-coated polymer template to a temperature of 100 to 750°C, preferably from 150 to 250°C, optionally for a period from 1 to 3 hours; and

(ii) subsequently heating the metal particle-coated polymer template to a sintering temperature of from 1050 to 1200°C, preferably under a reducing atmosphere, optionally for a period of from 1 to 5 hours.

(a) preparing a polymer template by a method of additive manufacturing;

(a2) cleaning and/or drying the polymer template with pressurised gas;

In one embodiment, the process of preparing a metallic scaffold or foam comprises: (a) preparing a polymer template by a method of additive manufacturing;

(al) cleaning and/or drying the polymer template with pressurised gas;

(a) preparing a polymer template by a method of additive manufacturing;

(b3) optionally, heating the metal particle-coated polymer template to a temperature of from 100 to 750°C, preferably from 150 to 250°C;

(b4) further coating the metal particle-coated polymer template by application of a slurry comprising metallic powder; and

(ii) subsequently heating the metal particle-coated polymer template to a sintering temperature of from 1050 to 1200°C, preferably under a reducing atmosphere, optionally for a period of from 1 to 5 hours. In one embodiment, the process of preparing a metallic scaffold or foam comprises:

(a) preparing a polymer template by a method of additive manufacturing;

(b 1) optionally, treating the metal particle-coated polymer template with pressurised gas; (b2) heating the metal particle-coated polymer template to a temperature of from 150 to 250°C;

(a) preparing a polymer template by a method of additive manufacturing;

(a2) cleaning and/or drying the polymer template with pressurised gas;

(b 1) optionally, treating the metal particle-coated polymer template with pressurised gas; (b2) curing the metal particle-coated polymer template in the presence of UV light, or both UV light and heat;

(a) preparing a polymer template by a method of additive manufacturing;

(a2) cleaning and/or drying the polymer template with pressurised gas;

(a) preparing a polymer template by a method of additive manufacturing;

(с) effecting the material removal of the polymer template to provide the metallic scaffold or foam by:

(a) preparing a polymer template by a method of additive manufacturing;

(al) cleaning and/or drying the polymer template with pressurised gas;

(a) preparing a polymer template by a method of additive manufacturing; (al) cleaning the polymer template using a solvent for a period of from 30 seconds to 30 minutes, preferably from 1 to 20 minutes, more preferably from 2 to 10 minutes and most preferably from 3 to 5 minutes, and preferably under ultrasonication;

(a2) curing the polymer template in the presence of UV light or both UV light and heat for a period of from 0 minutes to 24 hours, preferably from 15 minutes to 8 hours, more preferably from 30 minutes to 4 hours, still more preferably from 1 hour to 3 hours, and most preferably about 2 hours;

(i) heating the metal particle-coated polymer template to a temperature of 100 to 750°C, preferably from 150 to 250°C, and optionally maintaining this temperature for a period from 1 to 3 hours; and

(ii) subsequently heating the metal particle-coated polymer template to a sintering temperature that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal, preferably wherein the sintering temperature is from 1050 to 1200°C, preferably under a reducing atmosphere, and optionally wherein the sintering temperature is maintained for a period of from 1 to 5 hours.

Metallic scaffolds and foams

The present invention is also directed to metallic scaffolds or foams obtainable by the process described herein. The present invention is also directed to metallic scaffolds and foams obtained by the process described herein.

The metallic scaffolds or foams obtainable by the process of the present invention are a scaled replica of the polymer template. Thus, they possess many of the same structural features of said polymer template. However, whereas the template is typically comprised of a 3D network of pores and solid struts, the corresponding metallic scaffold or foam is typically comprises of a 3D network of pores and hollow struts. Typically, the end product of the process is a metallic scaffold for biological applications, such as a medical implant. Alternatively, the end product of the process is a metallic foam for non- biological applications, such as casting of jewellery. Typically, when intended for use as a medical implant (e.g. a bone implant), the scaffold is a porous structure, so as to allow cells to grow inside the structure after transplantation. It is also important for essential nutrients, oxygen and carbon dioxide to be transported to/from the cells adjacent to the implant following transplantation. Preferably, when intended for use as a bone implant the scaffold is a porous structure comprising a network of struts and pores.

In one embodiment, the metal scaffold or foam comprises an orthogonally arranged network of pores. More preferably, in this embodiment, the scaffold or foam comprises a square-section porous cubic structure. In an alternative embodiment, the scaffold or foam has a triply periodic minimal surface (TPMS). In this embodiment, the scaffold or foam may have cubic, tetragonal, rhombohedral or orthorhombic symmetry. Preferably, in this embodiment, the scaffold or foam has a Gyroid or Schwartz D structure.

Typically, the metal scaffold or foam has a mean strut thickness of at least 1 pm, preferably at least 10 pm, more preferably at least 25 pm, yet more preferably at least 50 pm, even more preferably at least 100 pm, still more preferably at least 150 pm, yet more preferably at least 200 pm, even more preferably at least 250 pm, still more preferably at least 280 pm, yet more preferably at least 350 pm, and most preferably at least 420 pm. Typically, the metal scaffold or foam has a mean strut thickness of less than 5000 pm, preferably less than 3000 pm, more preferably less than 2000 pm, yet more preferably less than 1500 pm, even more preferably less than 1000 pm, still more preferably less than 800 pm, yet more preferably less than 750 pm, even more preferably less than 700 pm, still more preferably less than 600 pm, yet more preferably less than 550 pm, and most preferably less than 500 pm. Thus, typically, the metal scaffold or foam has a mean strut thickness of from 1 to 5000 pm, preferably from 10 to 3000 pm, more preferably from 25 to 2000 pm, yet more preferably from 50 to 1500 pm, even more preferably from 100 to 1000 pm, still more preferably from 150 to 800 pm, yet more preferably from 200 to 750 pm, even more preferably from 250 to 700 pm, still more preferably from 280 to 600 pm, yet more preferably from 350 to 550 pm and most preferably from 420 to 500 pm.

Typically, the metal scaffold or foam has a mean pore diameter of at least 1 pm, preferably at least 10 pm, more preferably at least 25 pm, yet more preferably at least 50 pm, even more preferably at least 100 pm, still more preferably at least 150 pm, yet more preferably at least 200 pm, even more preferably at least 250 pm, still more preferably at least 300 pm, yet more preferably at least 450 pm, and most preferably at least 600 pm. Typically, the metal scaffold or foam has a mean pore diameter of less than 5000 pm, preferably less than 3000 pm, more preferably less than 2000 pm, yet more preferably less than 1500 pm, even more preferably less than 1200 pm, still more preferably less than 1100 pm, yet more preferably less than 1000 pm, even more preferably less than 900 pm, still more preferably less than 850 pm, yet more preferably less than 800 pm, and most preferably less than 750 pm. Thus, typically, the metal scaffold or foam has a mean pore diameter of from 1 to 5000 pm, preferably from 10 to 3000 pm, more preferably from 25 to 2000 pm, yet more preferably from 50 to 1500 mih, even more preferably from 100 to 1200 pm, still more preferably from 150 to 1100 pm, yet more preferably from 200 to 1000 pm, even more preferably from 250 to 900 pm, still more preferably from 300 to 850 pm, yet more preferably from 450 to 800 pm and most preferably from 600 to 750 pm.

The strut thickness and pore diameter of a metal scaffold or foam are determined using scanning electron microscopy.

Typically, the metal scaffold or foam has a narrower strut thickness than the corresponding polymer template from which it was manufactured. Typically, the metal scaffold or foam has a narrower pore diameter than the corresponding polymer template from which it was manufactured. Typically, the metal scaffold or foam has both a narrower strut thickness and a narrower pore diameter than the corresponding polymer template from which it was manufactured. That is because during the manufacturing process, in particular any sintering step to remove the polymer template, the outer metallic structure shrinks in size. Typically, the metal scaffold or foam comprises hollow struts.

Typically, the metal scaffold or foam is a scale replica of the polymer template, as shrinking of the final product typically occurs during the later stages of the production process (in particular step (c) in which the underlying template is removed). Thus, when the metal scaffold or foam comprises a 3D network of pores and struts, the pore size of the metal scaffold or foam is typically less than the pore size of the polymer template from which it was produced, preferably at least 1.2 times less, more preferably at least 1.5 times less, still more preferably at least 1.8 times less, and most preferably at least 2 times less, for example at least 2.5 times less or 3 times less. Further, when the metal scaffold or foam comprises a 3D network of pores and struts, the strut thickness of the metal scaffold or foam should typically be less than the strut thickness of the polymer template from which it is produced, preferably at least 1.1 times less, more preferably at least 1.2 times less, still more preferably at least 1.3 times less, and most preferably at least 1.5 times less, for example at least 1.8 times less, 2 times less or 2.5 times less.

Typically, the metal scaffold or foam comprises iron, magnesium, zinc, titanium or aluminium. Preferably, the metal scaffold or foam comprises a mixture of at least one of iron, magnesium, zinc, titanium or aluminium, optionally in combination with one or more different elements selected from:

Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Fe, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. Thus, in one embodiment, the metal scaffold or foam comprises a mixture of iron with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca,

Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In another embodiment, the metal scaffold or foam comprises a mixture of magnesium with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Fe, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In another embodiment, the metal scaffold or foam comprises a mixture of zinc with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Fe, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In another embodiment, the metal scaffold or foam comprises a mixture of titanium with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Fe, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. In another embodiment, the metal scaffold or foam comprises a mixture of aluminium with one or more (e.g. one, two, three, four, five or more) of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Fe, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni. Typically, the metal scaffold or foam comprises iron. Preferably, the metal scaffold or foam comprises a mixture comprising iron. The aforementioned mixtures may be alloys. Thus, in a preferable embodiment, the metal scaffold or foam comprises an iron alloy.

Preferably, the metal scaffold or foam comprises a mixture comprising iron and manganese, more preferably an alloy comprising iron and manganese. Thus, in one embodiment, the metal scaffold or foam comprises an iron-manganese alloy and, optionally, one or more (e.g. one, two, three, four, five or more) of the following elements: Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N,

S, Mo and Ni. In one embodiment, the metal scaffold or foam comprises an iron-manganese-carbon alloy and, optionally, one or more (e.g. one, two, three, four, five or more) of the following elements:

Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, N, S, Mo and Ni. In one embodiment, the metal scaffold or foam comprises an iron-manganese-silicon alloy and, optionally, one or more (e.g. one, two, three, four, five or more) of the following elements: Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, C, P, Cr, V, B, Zr, Ta, N, S, Mo and Ni. In one embodiment, the metal scaffold or foam comprises an iron- manganese-carbon-silicon alloy and, optionally, one or more (e.g. one, two, three, four, five or more) of the following elements: Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, P, Cr, V, B, Zr, Ta, N, S, Mo and Ni.

In an embodiment, the metal scaffold or foam comprises silver. Preferably, the metal scaffold or foam comprises a mixture comprising iron and silver, for example an alloy comprising iron and silver, or a mixture comprising iron, manganese and silver, for example an alloy comprising iron, manganese and silver. Thus, in one embodiment, the metal scaffold or foam comprises an iron-silver alloy. In another embodiment, the metal scaffold or foam comprises an iron-manganese-silver alloy. More preferably, the alloy additionally comprises carbon and/or silicon. Thus, in one embodiment, the metal scaffold or foam comprises an iron-manganese-silver-carbon alloy. In another embodiment, the metal scaffold or foam comprises an iron-manganese-silver-silicon alloy. In another embodiment, the metal scaffold or foam comprises an iron-manganese-silver-carbon-silicon alloy.

In a particularly preferred embodiment, the metal scaffold or foam typically comprises an alloy comprising from 30 to 90 wt% iron, preferably from 40 to 80 wt% iron, still more preferably from 50 to 70 wt% iron, and most preferably from 55 to 65 wt% iron. The metal scaffold or foam typically comprises an alloy comprising from 10 to 40 wt% manganese, preferably from 20 to 38 wt% manganese, and most preferably from 30 to 35 wt% manganese. The metal scaffold or foam typically comprises an alloy comprising from 0 to 20 wt% silver, preferably from 0.1 to 10 wt% silver, and most preferably from 1 to 5 wt% silver. The metal scaffold or foam typically comprises an alloy comprising from 0 to 2 wt% carbon, preferably from 0.5 to 1.5 wt% carbon, and most preferably from 0.7 to 1.2 wt% carbon. The metal scaffold or foam typically comprises an alloy comprising from 0 to 10 wt% silicon, preferably from 1 to 8 wt% silicon, and most preferably from 3 to 6 wt% silicon. Thus, in one embodiment, the metal scaffold or foam comprises an alloy comprising from 30 to 90 wt% iron, from 10 to 40 wt% manganese, from 0.1 to 20 wt% silver, from 0 to 2 wt% carbon, and from 0 to 10 wt% silicon. In a particularly preferable embodiment, the metal scaffold or foam comprises an alloy comprising from 55 to 65 wt% iron, from 30 to 35 wt% manganese, from 1 to 5 wt% silver, from 0.7 to 1.2 wt% carbon, and from 3 to 6 wt% silicon.

By virtue of the specific process by which it is produced, the metallic scaffold or foam possesses specific unique properties. In this way, the metallic scaffold or foam is distinguishable from a corresponding metallic scaffold or foam made via other methods, such as direct 3D printing or casting of a metal slurry or a molten metal into a negative template. In particular, the various heating steps that may be employed in the manufacturing process give rise to a specific mechanical strength profile, largely due to the necking between the deposited metallic particles that occurs during each heating step at low temperature.

The publications, patent publications and other patent documents cited herein are entirely incorporated by reference. Herein, any reference to a term in the singular also encompasses its plural. Where the term “comprising”, “comprise” or “comprises” is used, said term may substituted by “consisting of’, “consist of’ or “consists of’ respectively, or by “consisting essentially of’, “consist essentially of’ or “consists essentially of’ respectively. Any reference to a numerical range or single numerical value also includes values that are about that range or single value. Unless otherwise indicated, any % value is based on the relative weight of the component or components in question.

Examples

The following are examples that illustrate the present invention. However, these examples are in no way intended to limit the scope of the invention.

Example 1: 3D printing of sticky template

A 3D-printed polymer template was successfully produced using 3D prints fabricated in accordance with the following steps:

1. Prints were prepared using a Formlabs Form 2 printer. Templates were therefore fabricated using a vat polymerisation process called stereolithography, where the costs are relatively low, and the quality can reach very high levels.

2. The resin that has been tested is a Standard Grey resin (RS-F2-GPGR-04) from Formlabs which has a methacrylated base (see Formlabs, Grey Photoreactive Resin for Formlabs 3D printers - Safety Data Sheet, Formlabs, 2016).

3. Layer thickness was set to 100 pm.

4. Printing orientation has a significant effect on resulting dimensional variation between faces printed different orientations to the printing base. So far, tested template designs have been the simple cubic and gyroid designs, for which the preferred printing orientation for best replication of the Computer-Aided Design (CAD) model, is shown in Fig. 2.

As seen in Fig. 3 and Fig. 4, for TPMS structures with superior structural integrity, the dimensions of pores and struts measured at the “supported” sides (sides in contact with printing support structures) and “unsupported” sides (opposite to supported sides), do not vary significantly when investigating the effect of printing on “edge” and “apex” orientations (as illustrated in Fig. 2). Box plots indicate the 10^th, 25^th, 50^th, 75^th and 90^th percentiles and mean.

Post-processing on the 3D printed templates was carried out following the process outlined in Fig. 5, where IPA is isopropyl alcohol and the UV light source had an approximate wavelength of 405 nm.

Sample prints following this process may be seen in Fig. 6. Despite the use of IPA and post-curing with UV light, the polymer templates were left with a certain ‘tackiness’ or ‘stickiness’.

Example 2: Dry coating of sticky template with fine metallic powder

Coarse pure Fe powder (shown in the scanning electron micrograph of Fig. 7a) was used as-purchased. The processing method for the purchased powder yields irregularly shaped particles which are generally cheaper than gas atomised powder necessary for direct metal 3D printing as used in the study by Li et al. ( Acta Biomaterialia, 96, 2019, 646-661). The powder size was decreased to the desired size through the use of planetary ball milling (shown in the scanning electron micrograph of Fig. 7(b). The as- purchased powders were milled in a stainless steel vial with stainless steel balls (5 mm in diameter) added at a 30: 1 ball-to-powder ratio by weight, along with 2 mL (or 5 wt. %) of toluene as a process control agent. Milling was carried out for 12 hours at 270 rpm by alternating 10 minutes of milling and a 10 minute pause to prevent overheating.

The processed (milled) powder was then directly applied to the ‘tacky’ 3D printed template. This was achieved by tumbling the 3D printed templates in a tube along with the powders for an hour. The template is then lightly tapped to dislodge most of the excess, followed by blowing jets of pressurised air to remove loosely bound particles.

The particle size distributions of (a) the coarse Fe powder as-purchased and (b) the finer, milled Fe powder are shown in Fig. 8.

The traditional replication method for preparing a metallic scaffold or foam using a sacrificial polymer template generally makes use of a slurry for the application of the metallic powder coating. A typical binder used in the preparation of the slurry is polyvinyl alcohol (PVA) (see Hrubovcakova et al, Advances in Materials Science and Engineering, 2016, 6257368; Quadbeck et al, Materials Science Forum, 2007, 534-536, 1005-1008; Wang et al, Materials Science Engineering C Materials for Biological Applications, 2017, 70(2), 1192-1199). The dry powder coating was contrasted against coating with a PVA slurry prepared by adding 3 g/ml of Fe powder to a 5 wt.% PVA solution. In these control experiments, the polymer templates were added to the slurry in a tube and mixed for 15 minutes. Templates were then removed and left to dry in air. Three (i.e. multiple) coatings with slurry were applied.

The difference in powder uptake per square centimetre by the slurry-coating and dry-coating methods is displayed in Table 1. The results show that the dry coating method results in a significant improvement in coating uptake versus use of the slurry method, even when multiple coatings of slurry are applied.

The absolute coating uptake obtained by the dry method shows that full coverage of the metal template is easily obtainable via this method. Table 1: Indication of difference in powder uptake when using the traditional PVA-based slurry method against the dry-coating method of the present invention.

Coating uptake

Coating Notes (mg/cm²)

Cubic template

Slurry coated

9.52 500 pm struts (3 coats)

700 pm pores

Cubic template

Dry coated

66.3 500 pm struts 900 pm pores (1 coat)

(No IPA, no post-curing)

This conclusion is further emphasised by radiographic images showing the distribution of Fe powder on the template surfaces when using the slurry and the dry-coating approach (see Fig. 9). Moreover, variation in the post-processing procedure of the 3D printed polymer templates (i.e. the protocols shown in Fig. 5) were also observed to have an effect on the powder uptake (see Fig. 10). In particular, it was observed that application of IPA to the 3D printed template results in a small decrease in the uptake of Fe powder using the dry coating method, and that use of a 2-hour UV curing results in a more significant reduction in the uptake of Fe powder using the dry coating method. Effects of the post processing steps on subsequent powder uptake are discussed further in Examples 3 to 5.

Example 3: Effect of post-processing on dimensional changes of sticky templates

Cubic templates were 3D printed as described in Example 1, steps 1-4. The effect of immersion time in IPA during the cleaning step post-printing was investigated. The results are shown in the box plots in Fig. 11. An increase in immersion time was shown to lead to both larger pore sizes and thinner struts for this simple cubic structure, which is believed to be due to the removal of greater amounts of uncured printing resin. The length of IPA cleaning was also found to inversely correlate to the weight uptake in subsequent coating of the templates with dry metallic powder, which is indicative of a reduction in “tackiness” of the surface due to the reduction in amount of uncured resin on the template surface.

When no IPA is used at all, however, clogged pores were observed, as shown in the micrograph of Fig. 12.

The effect of immersion time in IPA during the cleaning step post-printing was investigated using a gyroid (TPMS) template. Gyroid templates were 3D printed as described in Example 1, steps 1-4. The results are shown in the box plots in Fig. 13. In this case, there is less of a clear trend on strut and pore size arising from variation of immersion time in IPA, indicating that the post-processing parameters for more complex structures may need be optimised for individual cases.

Example 4: Effect of ultrasonication during IPA cleaning on subsequent powder uptake

Gyroid templates were 3D printed as described in Example 1, steps 1-4. The effect of carrying out the IPA cleaning/immersion step with simultaneous ultrasonication was investigated. The addition of ultrasonication to the IPA cleaning step was observed to result in improved excess resin removal from the porous network. This is clear from Fig. 14, which shows the percentage weight uptake of iron powder (i.e. the percentage increase in weight relative to the weight of the uncoated template) in a subsequent dry coating step (as carried out according to the procedure in Example 2 above, with the exception that milling was carried out at 350 rpm for 2 hours in the presence of only 1 mL of toluene). Fig. 14 shows that the percentage weight uptake of iron powder decreased when ultrasonication was incorporated into the IPA immersion step. However, the X-ray radiography images in Fig. 15 show that significantly less clogging of the pores of the template with Fe powder were observed in the subsequent coating step if ultrasonication was incorporated into the IPA cleaning step. Fess clogging of the pores leads to a more faithful replication of the template surface in the coating step.

Example 5: Effect of UV curing time on subsequent powder uptake

Cubic templates were 3D printed as described in Example 1, steps 1-4. The effect of the UV curing time following template printing and prior to coating with a metallic powder was investigated. Fig. 16 shows that an increase in UV curing time leads to a decrease in the percentage weight uptake of iron powder in a subsequent dry coating step (as carried out according to the procedure in Example 2 above, with the exception that milling was carried out at 350 rpm for 2 hours in the presence of only 1 mF of toluene). Higher weight uptake in this step typically results in more faithful template replication. Without wishing to be bound by any particular theory, these results are believed to be due to a reduction in the “tackiness” of the polymer template surface during UV curing. These results show that the “partial” curing of a 3D printed polymer template prior to dry coating can provide a surprisingly good level of control in the amount of metallic powder that can be applied to the surface. On the one hand, curing is desirable because of an increase in the toughness or hardness of the template, but on the other hand, too long a cure can result in a template that has reduced tackiness and thus a reduced ability to uptake metallic powder in a subsequent dry coating step. Example 6: Effect of particle size on coating following post-processing of 3D printed polymer template

The effect of particle size on the coating step was also investigated for gyroid polymer templates which had undergone post-processing by (i) 10 minutes immersion in IPA with ultrasonication and (ii) 2 hours of UV curing. The products obtained when the coarse powder (as-purchased with the particle size distribution in Fig. 8a) was used and when the fine powder (following milling, with the particle size distribution in Fig. 8b) was used were studied using scanning electron microscopy. The results are shown in Fig. 17. Better coverage of the tacky gyroid templates was observed when using finer Fe powder (Fig. 17C and D) as compared to coarse Fe powder (Fig. 17 A and B).

Example 7: Use of two-step heat treatment

Once the polymer templates of Example 2 were dry-coated, a two-step heat treatment was then used in order to first allow a low-temperature dwell prior to template bum-off, and then to sinter the remaining metallic skeleton. The heat treatment was carried out under a reducing atmosphere as follows:

• The dry-coated template was heated to 175 °C at a rate of 180 °C/hour;

• The temperature was held constant at 175 °C for a period of 2 hours (the “low temperature dwell”);

• The template was then heated to 1120 °C at a rate of 180 °C/hour;

• The temperature was held constant at 1120 °C for a period of 3 hours (the “high temperature dwell” or “sintering temperature”); and

• The resulting scaffold was cooled for a period of approximately 6 hours.

Two-step heat treatments were initially carried out with templates dry-coated with coarse powders (i.e. an Fe powder having a particle size distribution as shown in Fig. 8(a)). None of the attempts made using these coarser powders were entirely successful in replicating the 3D printed template. Clearly improved replication was achieved with finer powders (i.e. an Fe powder having a particle size distribution as shown in Fig. 8(b)) when subjected to the same two-step heat treatment. Fig. 18 clearly shows the difference between (a) the scaffold generated after a two-step heat treatment when the coarse Fe powder was employed in the dry coating step, and (b) the scaffold generated after a two-step heat treatment when the fine Fe powder was employed in the dry coating step. Without wishing to be bound by any particular theory, it is believed that use of finer powders, which have a larger surface area:volume ratio, leads to greater structural integrity of the scaffold. Further investigation was also carried out on the choice of low-temperature dwell temperature when finer Fe powders were employed. Three different low-temperature dwell temperatures were tested: 175 °C, 200 °C and 225 °C. The low-temperature dwell temperature was maintained for 2 hours in all three cases. The sintering temperature was 1120 °C, the sintering time was 3 hours and the heating rate was 180 °C/hour (the maximum possible in the particular tube furnace being used). Both steps were carried out under a reducing atmosphere of 95% N₂/5% Ffi and the flow rate was 100 L/hr. Fig. 19 shows a schematic overview of the process conditions, and SEM images of the resulting scaffolds are shown in Fig. 20. The results indicate that in general, a lower dwell temperature is preferred to achieve a more accurate replication of the metallic scaffold, and the production of a scaffold having fewer defects/cracks. As only relatively minimal structural changes are anticipated to occur at the individual particle level at these low temperatures, these results suggest that the low-temperature dwell could in fact be dispensed with altogether, and a single heating-cooling cycle with a dwell at a high, sintering temperature would also provide scaffolds with optimal structural properties.

Example 8: Gyroid scaffold replicated using optimised parameters

An iron gyroid scaffold was prepared as follows:

• The polymer template of the scaffold was 3D printed following steps 1-4 outlined in Example 1.

• The template was immersed in IPA for 10 minutes with simultaneous ultrasonication.

• No UV curing step was applied.

• Fine Fe powder with a particle size distribution as shown in Fig. 8(b) was applied following the procedure outlined in Example 2.

• The dry-coated template was heated to a sintering temperature 1120°C at a rate of 300°C/hour. The sintering temperature was maintained for 3 hours before furnace cooling. The heat-sinter- cool cycle was carried out under a 120 L/hr flow of N2-5H2.

Fig. 21 shows SEM images of the resulting sintered Fe scaffold with gyroid structure.

Example 9: Preparation of an FeMn scaffold

An iron-manganese cubic scaffold was prepared as follows:

• The template was immersed in IPA for 1 minute without ultrasonication.

• The template was cleaned with compressed air. • The template was subjected to 2 hours of curing under UV light.

• Fine Fe35Mn powder mixture (i.e. containing 35% by weight of manganese) was applied following the procedure outlined in Example 2.

It was observed that in the sintering step, an oxide layer formed on the surface of the scaffold. This was thought to be due to the high affinity of Mn to oxygen at high temperatures. Thus, a modification was employed involving the covering of the powder-coated template with a stainless steel “cover” during the sintering step was used (see Fig. 22). This was done to limit contact with any oxidising agents introduced in the furnace by the gas. Fig. 23 A shows that the sintered structure obtained using the ‘Covered’ configuration during the sintering step was coated with less oxide than that obtained using the ‘Uncovered’ configuration (Fig. 23B).

When detrimental to the application of the scaffold, the oxide present on the surface may be removed by chemically post-treating the sintered structures. The structures presented in Figs. 23A and 23B were ultrasonicated for 5 minutes in 1 M HC1 with 3.5 g/L hexamethylenetetramine leading to complete removal of the oxide inclusions from the metal surface (see Figs. 23C and 23D, respectively).

Claims

1. A process for preparing a metallic scaffold or foam, the process comprising:

(a) preparing a polymer template;

2. A process according to claim 1, wherein at least 90% of the metallic particles by number in the solid composition have a diameter of less than 20 pm, preferably wherein at least 90% of the metallic particles by number in the solid composition have a diameter of less than 10 pm, and more preferably wherein at least 90% of the metallic particles by number in the solid composition have a diameter of less than 5 pm.

3. A process according to claim 1 or claim 2, wherein the metallic powder comprises iron, magnesium, zinc, titanium or aluminium, and preferably comprises a mixture of any of the foregoing elements with one or more different elements selected from: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Fe, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni.

4. A process according to claim 3, wherein the metallic powder comprises a mixture of iron with one or more of the following elements: Mn, Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni; preferably wherein the metallic powder comprises an iron- manganese alloy and, optionally, one or more of the following elements: Pd, Pt, Au, Ag, Cu, Al, Ti, Mg, Zn, Ca, Si, P, Cr, V, B, Zr, Ta, C, N, S, Mo and Ni.

5. A process according to claim 3 or claim 4, wherein the metallic powder further comprises silver, and is preferably an iron-silver alloy, an iron-manganese-silver alloy, an iron-manganese- carbon-silver alloy, or an iron-manganese-carbon-silicon-silver alloy.

6. A process according to any one of claims 1 to 5, wherein step (b) comprises a dry-coating method for coating the polymer template, preferably wherein the dry-coating method is selected from: tumbling the polymer template in the presence of the solid composition, preferably in an enclosed container; spraying of the solid composition onto the polymer template; and use of a fluidised bed to contact the solid composition with the polymer template.

7. A process according to any one of claims 1 to 6, wherein step (b) is repeated at least once, and wherein the solid composition used when step (b) is repeated may comprise the same or a different metallic powder to that used when step (b) was first carried out.

8. A process according to any one of claims 1 to 7, wherein following step (b) but prior to step (c), the metal particle-coated polymer template is subjected to treatment with pressurised air such that excess metallic powder is separated from the template.

9. A process according to any one of claims 1 to 8, wherein following step (b) but prior to step (c), the metal particle-coated polymer template is subjected to curing with UV light and/or heating to a temperature of from 45 to 180 °C.

10. A process according to any one of claims 1 to 9, wherein following step (b) but prior to step (c), the metal particle-coated polymer template is further coated by application of a slurry comprising metallic powder, preferably wherein the slurry is a poly(vinyl alcohol)-based slurry or a polyethylene glycol)-based slurry.

11. A process according to claim 10, wherein following step (b) but prior to the metal particle- coated polymer template being further coated by application of a slurry, the metal particle- coated polymer template is heated to a temperature of from 150 to 250 °C, preferably from 175 to 200 °C.

12. A process according to any one of claims 1 to 11, wherein step (c) comprises heating the metal particle-coated polymer template.

13. A process according to claim 12, wherein:

(A) step (c) comprises heating the metal particle-coated polymer template to a sintering temperature that is above the temperature at which the polymer template will undergo complete thermal degradation, but below the melting temperature of the metal; or

(B) step (c) comprises the sub-stages of:

(i) heating the metal particle-coated polymer template to a temperature which is below the temperature at which the polymer will undergo complete thermal degradation, and dwelling at that temperature for a time period of from 15 minutes to 6 hours, preferably from 1 to 3 hours; and

14. A process according to claim 13, wherein the temperature of step (B)(i) is from 100 to 750°C, preferably from 150 to 250°C, and more preferably from 175 to 200°C.

15. A process according to claim 13 or claim 14, wherein the temperature of step (A) or step (B)(ii) is from 1050 to 1200°C.

16. A process according to any one of claims 13 to 15, wherein the time period for which the sintering temperature is maintained is from 1 to 5 hours.

17. A process according to any one of claims 13 to 16, wherein the metal particle-coated polymer template is heated in step (A), step (B)(i) and/or step (B)(ii) at a rate of from 60 to 600°C/hour, preferably from 250 to 350°C/hour.

18. A process according to claim 17, wherein the metal particle-coated polymer template is heated at the same rate in step (B)(i) as in step (B)(ii).

19. A process according to any one of claims 13 to 18, wherein the heating takes place under a reducing atmosphere.

20. A process according to claim 19, wherein the reducing atmosphere is obtained by applying a stream of reducing gas to the metal particle-coated polymer template, preferably wherein said reducing gas comprises nitrogen and hydrogen.

21. A process according to claim 20, wherein the flow rate of the stream of reducing gas is from 50 to 650 L/hr, preferably from 100 to 200 L/hr.

22. A process according to any one of claims 13 to 21, wherein the metal particle-coated polymer template is enclosed by a cover during the heating process, wherein said cover acts as a barrier to the flow of gas, preferably wherein said cover comprises metal or ceramic.

23. A process according to any one of claims 13 to 22, wherein after step (c), the surface of the metallic scaffold or foam is contacted with a chemical cleaning agent.

24. A process according to any one of claims 1 to 23, wherein the polymer template is prepared in step (a) by a method of additive manufacturing, preferably wherein the method of additive manufacturing is a VAT polymerisation method, a liquid base polymerisation additive manufacturing method (e.g. stereolithography), or a binder jetting method.

25. A process according to claim 24, wherein the polymer template:

(i) comprises orthogonally arranged pores, and preferably has a square-section porous cubic structure; or

(ii) has a triply periodic minimal surface (TPMS), preferably a Gyroid or Schwartz D structure.

26. A process according to any one of claims 1 to 25, wherein following step (a) but prior to step (b), the polymer template is subjected to:

(i) cleaning with a solvent, preferably an alcohol or tripropylene glycol monomethyl ether; and/or

(ii) cleaning with pressurised air; and/or

(iii) curing with UV light and/or heating to a temperature of from 45 to 180°C.

27. A process according to claim 25 or claim 26, wherein following step (a) but prior to step (b), the polymer template is subjected to curing with UV light and the resultant polymer template is partially cured.

28. A process according to claim 26 or claim 27, wherein following step (a) but prior to step (b), the polymer template is subjected to both cleaning with a solvent, preferably an alcohol or tripropylene glycol monomethyl ether, and curing with UV light.

29. A process according to claim 25 or claim 26, wherein following step (a) but prior to step (b), the polymer template does not undergo curing with UV light.

30. A process according to any one of claims 25 to 29, wherein following step (a) but prior to step (b), the polymer template undergoes cleaning with a solvent, preferably an alcohol or tripropylene glycol monomethyl ether, with simultaneous ultrasonication.

31. A process according to any one of claims 25 to 30, wherein prior to step (b), at least 50% of the surface area of the polymer template is capable of being coated by the adhesion of said solid composition to the surface of the polymer template.

32. A process according to any one of claims 25 to 31, wherein following step (a) but prior to step (b), the polymer template is:

33. A process according to any one of claims 1 to 32, wherein the polymer template comprises a polymer to which particles of a metallic powder are capable of adhering.

34. A process according to claim 33, wherein the surface of the polymer template is sufficiently tacky such that a solid composition comprising metallic powder is capable of adhering to the template in the absence of any external binder or solvent, substantially coating the template.

35. A process according to claim 33 or claim 34, wherein the polymer is selected from poly(acrylates), poly(methacrylates), poly(urethanes) and wax-based resins.

36. A process according to any one of claims 1 to 35, wherein the polymer template comprises a strut thickness of at least 50 pm and a pore diameter of at least 100 pm, preferably a strut thickness of at least 280 pm and a pore diameter of at least 300 pm, preferably wherein the polymer template comprises a strut thickness of at least 420 pm and a pore diameter of at least 600 pm.

37. A metallic scaffold or foam obtainable by the process of any one of claims 1 to 36.

38. Use of a solid composition comprising metallic powder in a process of coating an additive- manufactured polymer template to produce a metal particle -coated polymer template.