WO2021006820A1

WO2021006820A1 - Immersion precipitation three-dimensional printing

Info

Publication number: WO2021006820A1
Application number: PCT/SG2020/050397
Authority: WO
Inventors: Rahul KARYAPPA; Michinao HASHIMOTO
Original assignee: Singapore University Of Technology And Design
Priority date: 2019-07-09
Filing date: 2020-07-09
Publication date: 2021-01-14

Abstract

A method of printing a porous and/or non-porous three-dimensional structure is disclosed herein. The method includes providing an ink composition including a polymer dissolved in a solvent, dispensing the ink composition into a non-solvent at a temperature ranging from 20°C to 30°C, wherein the non-solvent renders precipitation of the ink composition to form the porous and/or non-porous three-dimensional structure in the non-solvent, and removing the non-solvent from the porous and/or non-porous three-dimensional structure. Disclosed herein includes a system operable to print a porous and/or non-porous three-dimensional structure, the system includes a syringe operable to dispense an ink composition into a non-solvent at a temperature ranging from 20°C to 30°C, wherein the ink composition includes a polymer dissolved in a solvent, and a motion control module which controls vertical and horizontal positioning of the syringe.

Description

IMMERSION PRECIPITATION THREE-DIMENSIONAL PRINTING

Cross-Reference to Related Application

[0001] This application claims the benefit of priority of Singapore Patent Application No. 1020190635 IP, filed 9 July 2019, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] The present disclosure relates to a method, and a system, of printing a porous and/or non-porous three-dimensional structure, which involves immersion precipitation.

Background

[0003] Materials with controlled porosity have found diverse applications in separation, catalysis, energy storage, sensors and actuators, tissue engineering and drug delivery. Multiple methods have been developed to fabricate well-defined porous materials with the pore sizes ranging from nanometers to millimeters. For example, introduction of sacrificial template materials such as colloidal crystal, gas bubbles, droplets, salts, ice crystals, and self-assembled molecular aggregates, may impart porosity to the materials encapsulating them after the removal of the embedded sacrificial template materials. Alternatively, methods involving phase separation, direct templating and chemical reaction(s) have been studied for fabrication of hierarchical porous structures.

[0004] However, the methods mentioned above tend to require multiple steps and may be limited in terms of attaining complex fabricated structures. To this end, fabrication of porous models with arbitrary three-dimensional (3D) designs may have been actively investigation. For example, recent advance in digital fabrication, such as 3D printing, has enabled fabrication of porous 3D structures consisting of polymeric materials. Among the different methods of 3D printing, which may involve different mechanisms, fused deposition modeling (FDM), stereolithography (SLA) and solvent-cast 3D printing (SC3DP) have been explored for fabrication of 3D porous structures.

[0005] In FDM, thermoplastic filaments may be heated and extruded to fabricate 3D structures. Commercial filaments may be available for fabricating porous 3D structures by FDM. The filament contains sacrificial polyvinyl alcohol (PVA - a water-soluble polymer) that may be leached out in water after 3D printing to obtain the porous structure.

[0006] In SLA by digital light processing (DLP), photocurable resins may be mixed with sacrificial templates (e.g. salt and emulsions) to fabricate porous models, wherein the sacrificial templates may be removed by heating or leaching.

[0007] While FDM and SLA allow fabricating 3D objects with thermoplastics and photocurable polymers, such methods tend to be not compatible with other groups of functional polymers (such as cellulose esters and polyimides). In this regard, SC3DP provides another route for 3D prototyping of porous materials. In SC3DP, a viscous solution containing a polymer may be printed using a direct ink writing (DIW) 3D printer, wherein the printed ink solidifies in situ due to the evaporation of solvents. While the SC3DP may allow for fabrication of free-form 3D structures, the printed 3D objects is limited in that only dense, non-porous microstructures tend to be obtained as the ink required high concentration of polymers. The printing ink has to be sufficiently viscous to maintain the printed structures without spreading during the evaporation of the solvents. In addition, because SC3DP relies on solvent evaporation, the inks have to consist of solvents with high vapor pressures. Due to these constraints, SC3DP may be limited to dichloromethane (DCM) as the solvent for compatibility with a relatively high concentration (more than 27.5%) of the dissolved polymer (e.g. polylactic acid (PLA)). Fabrication of porous 3D structures by SC3DP unfortunately requires additional intervention to modify the rheological or thermodynamic properties of the printing inks.

[0008] There is thus a need to provide for a solution that addresses one or more of the limitations mentioned above. The solution should at least provide for a method of 3D printing of porous and/or non-porous structures compatibly usable with a wide variety of inks.

Summary

[0009] In a first aspect, there is provided for a method of printing a porous and/or non- porous three-dimensional structure including:

providing an ink composition including a polymer dissolved in a solvent; dispensing the ink composition into a non-solvent at a temperature ranging from 20°C to 30°C, wherein the non-solvent renders precipitation of the ink composition to form the porous and/or non-porous three-dimensional structure in the non-solvent; and removing the non-solvent from the porous and/or non-porous three-dimensional structure.

[0010] In another aspect, there is provided for a system operable to print a porous and/or non-porous three-dimensional structure, the system including:

a syringe operable to dispense an ink composition into a non-solvent at a temperature ranging from 20°C to 30°C, wherein the ink composition includes a polymer dissolved in a solvent; and

a motion control module which controls vertical and horizontal positioning of the syringe.

[0011] In various aspects, the method of printing a porous and/or non-porous three- dimensional structure and the system operable to print a porous and/or non-porous three-dimensional structure may include the use of a material to create porous structures. The material may be a sacrificial material, which means the material may be removed or sacrificed to form pores in the printed structures. The material may include a porogen or an additive, which may be added to the ink composition prior to dispensing the ink composition, wherein the porogen may include, for example, polyethylene glycol, polyvinylpyrrolidone, or sodium sulphate, wherein the additive may include, for example, conductive carbon black, polyvinyl alcohol, laponite clay, or a-cellulose.

Brief Description of the Drawings

[0012] The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the present disclosure. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[0013] FIG. 1A shows an overview of immersion precipitation three-dimensional (3D) printing (//GDP) of the present disclosure and structures printed by //GDP. Specifically, FIG. 1A depicts the method of //GDP. A polymer solution is printed by a direct ink writing (DIW) 3D printer in a non-solvent. The printed object is solidified via immersion precipitation, and porosity is imparted to the printed object. The scale bar denotes 2 pm.

[0014] FIG. IB shows a scattered plot illustrating the solvent vapor pressure ( P_vap ) and the polymer solution viscosity ( m_arr ) demonstrated for the present zp3DP. The parameters for conventional solvent-cast 3D printing (SC3DP) are also shown to highlight the versatility of zp3DP.

[0015] FIG. 1C shows the optical images of 3D printed models from different combinations of polymer-solvent-non-solvent (P-S-NS) ternary system. The names of the polymer, solvent and non-solvent used in the printing are presented in the form of P/S/NS, respectively, and are abbreviated, wherein the abbreviation is indicated in FIG. ID and IE, and. For example, in the top left image and referring to FIG. ID and IE, “ABS/Dc/E” represents for acrylonitrile butadiene/dichloromethane/ethanol. The scale bar denotes 4 mm.

[0016] FIG. ID indicates the abbreviation of polymers used in the present zp3DP.

[0017] FIG. IE indicates the solvents and additives used in the present zp3DP.

[0018] FIG. IF is a photograph of the DIW 3D printer and the pneumatic dispenser used in the present zp3DP.

[0019] FIG. 1G shows a polymer-solvent-non-solvent (P-S-NS) ternary phase diagram.

[0020] FIG. 1H shows the formulations of inks (in w/w%) and the different corresponding non-solvents for immersion precipitation used in the present method.

[0021] FIG. II is an image showing immersion precipitation of the present method. The plumes of solvent (S) leaving polymer (P) and diffusing into the surrounding non solvent (NS) are visualized and depicted.

[0022] FIG. 2A shows the parameters of the present zp3DP and characterization of polymer inks. Specifically, FIG. 2 A shows the parameters in the present zp3DP optimized for printing.

[0023] FIG. 2B shows the process -related apparent viscosity (m _app) characterized as a function of process-related shear rate (g).

[0024] FIG. 2C is a plot showing the printed filament width (w) with respect to the dispensing head velocity (v).

[0025] FIG. 3 shows a 3D structure of polyacrylate (PA) composite fabricated by the present ip3DP method (top left image, scale bar denotes 5 mm). The top middle image (scale bar denotes 200 mih) is a scanning electron microscopy (SEM) image of the surface of the 3D structure shown in the top left image. The top right shows a SEM image (scale bar denotes 2 pm) of the microstmctures of a fractured surface of the 3D structure shown in the top left image. The bottom three images (scale bar denotes 5mm) depicts use of the PA composite as a sacrificial material. The PA mold was printed by zp3DP, filled with PDMS, and dissolved in water.

[0026] FIG. 4 shows four images. The top left image (scale bar denotes 50 pm) is a SEM image of starch used in the reinforcement of cellulose acetate (CA). The top right image (scale bar denotes 5 mm) depicts a 3D model of starch-reinforced CA fabricated by zp3DP. The bottom images are SEM images of microstructures of a fractured surface of starch-reinforced CA (scale bar denotes 200 pm and 20 pm for bottom left and bottom right images, respectively).

[0027] FIG. 5A is an optical micrograph of conductive carbon (CC). Scale bar denotes 300 pm.

[0028] FIG. 5B is a SEM image of the interface of the 3D structure of CC-reinforced ABS45 printed in water, wherein 45 refers to the ABS concentration in w/w%. Scale bar denotes 100 pm.

[0029] FIG. 5C is an optical micrograph of polyvinyl alcohol (PVA). Scale bar denotes 300 pm.

[0030] FIG. 5D is a SEM image of the internal microstmcture of the fractured surface of the PVA-reinforced PLA25 printed in ethanol, wherein 25 refers to the PVA concentration in w/w%. Scale bar denotes 100 pm.

[0031] FIG. 5E is an optical micrograph of laponite clay (LC). Scale bar denotes 300 pm.

[0032] FIG. 5F is a SEM image of the internal micro structure of the fractured surface of the EC-reinforced ABS45 printed in ethanol, wherein 45 refers to the ABS concentration in w/w%. Scale bar denotes 100 pm.

[0033] FIG. 5G is an optical micrograph of a-cellulose (aC). Scale bar denotes 300 pm.

[0034] FIG. 5H is a SEM image of the internal microstmcture of the fractured surface of the aC-reinforced PCF40, wherein 40 refers to the PCF concentration in w/w%. Scale bar denotes 100 pm. [0035] FIG. 6 shows five 3D models printed consecutively by the present zp3DP method. ABS45 was printed in water (top image) and ethanol (bottom image). Scale bar denotes 1 cm.

[0036] FIG. 7 is a plot showing the process-related flow rate ( Q ) with respect to the applied pressure (P) for the polymer inks studied.

[0037] FIG. 8A demonstrates the effects of print velocity (v) on the printed structures, considering the width of the filament (w) and dispensed mass of the ink per unit length (, m=rri/v ). Scale bar denotes 5 mm.

[0038] FIG. 8B demonstrates the effects on coil instability. The increase in v resulted in coil instability, affecting the print fidelity. Scale bar denotes 5 mm.

[0039] FIG. 9 demonstrates the effect of print velocity (v) on the print fidelity of the present zp3DP method with slow demixing of the solvent and the non-solvent. At the high speed of the printing nozzle (v = 5 mm/s), the shear forced the printed structure tilted. Scale bar denotes 4 mm.

[0040] FIG. 10 is an optical image showing the printed overhangs of ABS and microscopic images of the printed filaments with the variations of nozzle diameters and dispensing head velocities. Scale bar denotes (top image) 10 mm, (bottom image) 100 pm.

[0041] FIG. 11A demonstrates the effect of surrounding media, air and ethanol, on the continuous filament formation when ABS30 (30 w/w% of ABS) was used. The ink was dispensed through a nozzle. Scale bar denotes 5 mm.

[0042] FIG. 11B demonstrates the effect of surrounding media, air and ethanol, on the continuous filament formation when ABS45 (45 w/w% of ABS) was used. The ink was dispensed through a nozzle. Scale bar denotes 5 mm.

[0043] FIG. l lC demonstrates the effect of surrounding media, air and ethanol, on the continuous filament formation when ABS60 (60 w/w% of ABS) was used. The ink was dispensed through a nozzle. Scale bar denotes 5 mm.

[0044] FIG. 11D demonstrates fabrication of straight lines of using ABS30 in ethanol at different dispensing head velocities (v). Scale bar denotes 5 mm.

[0045] FIG. 11E demonstrates fabrication of straight lines of using ABS45 in ethanol at different dispensing head velocities (v). Scale bar denotes 5 mm. [0046] FIG. 11F demonstrates fabrication of straight lines of using ABS60 in ethanol at different dispensing head velocities (v). Scale bar denotes 5 mm.

[0047] FIG. 12A is a SEM image of the internal micro structure of the fractured surface of ABS45 printed in water (m^ ~ 9 x 10⁴ Pa.s), exhibiting the micropores throughout the fractured surface. Scale bar denotes 20 pm.

[0048] FIG. 12B is a SEM image of the internal microstructure of the fractured surface of ABS45 printed in water containing 1 w/w% of carboxymethyl cellulose (CMC) (p,v_S ~ 2 Pa.s), exhibiting rather dense surface. Scale bar denotes 20 pm.

[0049] FIG. 13 A demonstrates thermogravimetric analysis (TGA) to investigate presence of residual solvent or non-solvent in the fabricated 3D structures by the present zp3DP. FIG. 13A shows the weight (%) and derivative of weight (%/°C) as a function of temperature of 3D filaments of ABS and PLA.

[0050] FIG. 13B demonstrates thermogravimetric analysis (TGA) to investigate presence of residual solvent or non-solvent in the fabricated 3D structures by the present zp3DP. FIG. 13B shows the weight (%) and derivative of weight (%/°C) as a function of temperature of ABS45 dissolved in acetone and printed in water with residence time of t_r = 30 min in water.

[0051] FIG. 13C demonstrates thermogravimetric analysis (TGA) to investigate presence of residual solvent or non-solvent in the fabricated 3D structures by the present zp3DP. FIG. 13C shows the weight (%) and derivative of weight (%/°C) as a function of temperature of ABS45 dissolved in DMF printed in ethanol with residence time of t_r = 30, 60 and 120 mins in ethanol.

[0052] FIG. 13D shows the weight (%) and derivative of weight (%/°C) as a function of temperature of PLA30 dissolved in DCM and printed in ethanol with residence time of t_r = 30 mins in water.

[0053] FIG. 14A is a schematic illustration of the 3D printed object showing dense non- porous interface and porous internal microstructure.

[0054] FIG. 14B is a SEM image of the microstructure of the fractured surface of ABS printed by the present zp3DP using ABS20 in acetone (ABS20 denotes 20 w/w% solution of ABS in acetone). Scale bar denotes 2 pm. [0055] FIG. 14C is a SEM image of the microstructure of the fractured surface of ABS printed by the present zp3DP using ABS30 in acetone (ABS30 denotes 30 w/w% solution of ABS in acetone). Scale bar denotes 2 pm.

[0056] FIG. 14D is a SEM image of the micro structure of the fractured surface of ABS printed by the present zp3DP using ABS40 in acetone (ABS40 denotes 40 w/w% solution of ABS in acetone). Scale bar denotes 2 pm.

[0057] FIG. 14E is a SEM image of the micro structure of the fractured surface of ABS printed by the present zp3DP using ABS50 in acetone (ABS20 denotes 50 w/w% solution of ABS in acetone). Scale bar denotes 2 pm.

[0058] FIG. 14F is a SEM image of the micro structure of the fractured surface of ABS printed by the present zp3DP using ABS60 in acetone (ABS60 denotes 60 w/w% solution of ABS in acetone). Scale bar denotes 2 pm.

[0059] FIG. 14G is a SEM image of the microstructure of ABS60 fabricated by SC3DP, exhibiting the dense internal structures. Scale bar denotes 2 pm.

[0060] FIG. 14H is a plot showing tensile stress-strain testing of the dogbone samples.

[0061] FIG. 15A depicts a 3D structure PLA30 printed by the present zp3DP method. Scale bar denotes 5 mm.

[0062] FIG. 15B is a SEM image of microstructures of the fractured surface. Scale bar denotes 200 pm.

[0063] FIG. 15C is a SEM image of microstructures of the fractured surface. Scale bar denotes 2 pm.

[0064] FIG. 16A demonstrates for the effect of residence time (/,) on the micropores of the printed 3D models by the present zp3DP method. FIG. 16A is a SEM image of the internal micro structure of the fractured surface of the ABS45 printed in water and immediately removed ( t_r = 0 min) and dried in air. Scale bar denotes 10 pm.

[0065] FIG. 16B is a SEM image of the internal microstructure of the fractured surface of the ABS45 printed in water and left for extraction of the solvents ( t_r = 30 min). Scale bar denotes 10 pm.

[0066] FIG. 17A demonstrates for the effect of porogens and solvents in the polymer ink on the micro structure. FIG. 17A is a schematic illustration of a 3D printed object showing the enhancement of porosity at the interface and the internal microstructures with the addition of PEG in the polymer ink. The SEM images of the interface of the 3D structure of ABS45 printed by the present zp3DP are also shown.

[0067] FIG. 17B demonstrates for the effect of porogens and solvents in the polymer ink on the micro structure. FIG. 17B is a SEM image that demonstrates for without addition of polyethylene glycol (PEG) (white arrows indicating nanometer- sized pores). The SEM image shows the internal microstmctures of the fractured surface of the printed ABS45. Scale bar denotes 10 pm.

[0068] FIG. 17C demonstrates for the effect of porogens and solvents in the polymer ink on the microstmcture. FIG. 17C is a SEM image that demonstrates for addition of PEG (40% of the weight of ABS). The SEM image shows the internal microstmctures of the fractured surface of the printed ABS45. Scale bar denotes 10 pm.

[0069] FIG. 17D demonstrates for the effect of porogens and solvents in the polymer ink on the micro structure. FIG. 17D is a SEM image that demonstrates ABS45 printed without PEG in water. The SEM image shows the internal microstmctures of the fractured surface of the printed ABS45. Scale bar denotes 10 pm.

[0070] FIG. 17E demonstrates for the effect of porogens and solvents in the polymer ink on the microstmcture. FIG. 17E is a SEM image that demonstrates ABS45 with PEG printed in water. The SEM image shows the internal microstmctures of the fractured surface of the printed ABS45. Scale bar denotes 10 pm.

[0071] FIG. 17F demonstrates for the effect of porogens and solvents in the polymer ink on the microstmcture. FIG. 17F is a SEM image that demonstrates ABS45 with PEG printed in ethanol. The SEM image shows the internal microstmctures of the fractured surface of the printed ABS45. Scale bar denotes 10 pm.

[0072] FIG. 17G demonstrates for effects of solvents in the ink dissolving the polymer on the microstmcture of the printed ABS45 (v/v% were adjusted to be the same in acetone and DMSO). Ethanol was used as non-solvent. FIG. 17G specifically demonstrates for acetone as the solvent to form the inks, and printed in ethanol. Scale bar denotes 10 pm.

[0073] FIG. 17H demonstrates for effects of solvents in the ink dissolving the polymer on the microstmcture of the printed ABS45 (v/v% were adjusted to be the same in acetone and DMSO). Ethanol was used as non-solvent. FIG. 17H specifically demonstrates for DMF as the solvent to form the inks, and printed in ethanol. Scale bar denotes 10 pm.

[0074] FIG. 171 demonstrates for effects of solvents in the ink dissolving the polymer on the micro structure of the printed ABS45 (v/v% were adjusted to be the same in acetone and DMSO). FIG. 17H specifically demonstrates for DMSO as the solvent to form the inks, and printed in ethanol. Scale bar denotes 10 pm.

[0075] FIG. 17J demonstrates for multi-material 3D printing with distinct porosities. FIG. 17J demonstrates for two inks (ABS45 and PCL50) printed sequentially in the common non-solvent. Scale bar denotes 5 mm.

[0076] FIG. 17K depicts SEM micrographs of three regions showing microstmctures of the fractured surface of the 3D object. Scale bar denotes 2 pm.

[0077] FIG. 18A demonstrates for effect of porogens in the polymer ink on the micro structure. FIG. 18A is an optical micrograph of polyvinylpyrrolidone (PVP) used as a porogen. Scale bar denotes 300 pm.

[0078] FIG. 18B is a SEM image of the internal microstructure of the fractured surface of the ABS45 with PVP (30% of the weight of ABS) printed in water. Scale bar denotes 10 pm.

[0079] FIG. 18C is an optical micrograph of sodium sulphate (SS) used as a porogen. Scale bar denotes 300 pm.

[0080] FIG. 18D is a SEM image of the internal micro structure of the fractured surface of the ABS45 with SS (10% of the weight of ABS) printed in water. Scale bar denotes 100 pm.

Detailed Description

[0081] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the present disclosure may be practised.

[0082] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0083] The present disclosure relates to a method of fabricating three-dimensional (3D) porous and/or 3D non-porous structures. The method is termed herein“immersion precipitation 3D printing (zp3DP)”. zp3DP is direct ink writing (DIW) 3D printing utilizing a polymer- solvent-non- solvent (P-S-NS) ternary system (FIG. IF and 1G). The polymer ink may be directly dispensed in a non- solvent from a motion-controlled syringe. Immersion precipitation of the dissolved polymers occurred in situ when the ink was in contact with the non-solvent. In the present method, solvent extraction is involved, which occurs much faster than solvent evaporation. For example, solvent-cast 3D printing (SC3DP) relies on evaporation of solvents from viscous inks having high vapor pressure to form the printed structure. However, the present zp3DP involving the use of solvents with low vapor pressure (e.g. water, DMF and DMSO), which aids in faster formation of the printed structure. The wider selection of solvents rendered by the present method provides for a wider selection of thermoplastics to be printed. To highlight this capability, the examples disclosed herein demonstrated fabrications of cm- scale models with 13 different polymers dissolved using 6 different solvents. It is demonstrated herein that inks with low viscosity (with low polymer concentrations) can impart internal porosity in the 3D printed object using the present method. This type of control of the porosity is not provided for by SC3DP which unfortunately requires high viscosity inks.

[0084] The present method is compatible with addition of pore-inducing agents for creating pores that extends into the printed object from the surface of the printed object. The present zp3DP method is advantageously straightforward, a one-step method that provides for tailoring of the porosity of the printed 3D objects. A system toolkit based on the present method for use in additive manufacturing to achieve fabrication of functional devices and structures is also disclosed herein. The system toolkit may be simply referred herein as a system.

[0085] Details of various embodiments of the present method, system, and advantages associated with the various embodiments are now described below.

[0086] In the present disclosure, there is provided a method of printing a porous and/or non-porous three-dimensional (3D) structure. The method may include providing an ink composition comprising a polymer dissolved in a solvent, dispensing the ink composition into a non-solvent at a temperature ranging from 20°C to 30°C, wherein the non-solvent renders precipitation of the ink composition to form the porous and/or non-porous three-dimensional structure in the non-solvent, and removing the non solvent from the porous and/or non-porous three-dimensional structure.

[0087] Advantageously, the present method is able to print 3D structures that are porous and/or non-porous without being limited by the type of polymers and solvents. For example, the present method is workable with solvents that may have high vapor pressure as the present method does not rely on evaporation of the solvent to print 3D structure. The present method is also workable with polymers and solvents that form ink compositions with higher viscosity, which means that the present method is workable with a wider range of ink compositions. The present method is able to print the 3D structure at room temperature (e.g. 20°C to 30°C, 20°C to 25°C, 25°C to 30°C) without the need for higher temperatures and temperature control.

[0088] In various aspects, providing the ink composition may include filling the ink composition into a syringe operable to dispense the ink composition into the non solvent. Prior to this, the ink composition may be prepared by dissolving a polymer in a solvent.

[0089] The polymer may include polystyrene, acrylonitrile butadiene styrene, acrylonitrile styrene acrylate, polyethylene, thermoplastic polyurethane, polylactic acid, polycaprolactone, polyvinyl alcohol, polyacrylate, cellulose acetate, cellulose acetate propionate, or cellulose acetate butyrate. Any suitable polymers that can precipitate via immersion precipitation may be used in the present method. The polymer may be dissolved in the solvent at a concentration ranging from 10 wt% to 60 wt%, 20 wt% to 60 wt%, 30 wt% to 60 wt%, 40 wt% to 60 wt%, 50 wt% to 60 wt%, etc. Such concentrations may help in controlling the porosity of the printed 3D structure. For example, at a concentration of 60 wt%, the printed 3D structure may be a dense non- porous 3D structure. Regarding the porous 3D structure printed, the pores may be present at the surface of the 3D structure. The pores may extend from the surface inward and/or connecting to form a network of porous channels therein.

[0090] In various aspects, the solvent may include acetone, chloroform, dichloromethane (DCM), dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), ethanol, N-methyl-2-pyrrolidone (NMP), or water. Other suitable solvents may be used as long as the solvent is compatible for the present method.

[0091] In various aspects, dispensing the ink composition into the non-solvent may include applying a pressure ranging from 10 kPa to 650 kPa, 50 kPa to 650 kPa, 100 kPa to 650 kPa, 150 kPa to 650 kPa, 200 kPa to 650 kPa, 250 kPa to 650 kPa, 300 kPa to 650 kPa, 350 kPa to 650 kPa, 400 kPa to 650 kPa, 450 kPa to 650 kPa, 500 kPa to 650 kPa, 550 kPa to 650 kPa, 600 kPa to 650 kPa, etc., to dispense the ink composition. Such pressures provide for adequate mass flow of ink composition dispensed into the non- solvent and helps in maintaining print fidelity of the 3D printed structure. The term “fidelity” herein refers to the accuracy of the printed 3D structure based on the inputs provided to a software that modulates/operates how the ink composition is dispensed to afford a completely or substantially identical printed 3D structure.

[0092] In various aspects, dispensing the ink composition into the non-solvent comprises having the ink composition immersed in the non-solvent for a time ranging from 30 minutes to 180 minutes, 60 minutes to 180 minutes, 120 minutes to 180 minutes, etc. The time in which the ink composition remains immersed in the non solvent may be referred herein as the residence time. Such residence time allows for sufficient and/or complete precipitation of the ink composition into the 3D structure. If residence time is too short, the solvent-non-solvent exchange may not be sufficient, which may compromise fidelity of the printed 3D structure.

[0093] The non-solvent may include water, ethanol or acetone. Other suitable non solvents may be used as long as the non-solvent is compatible for the present method, e.g. any non-solvent that can render the immersion precipitation of the polymer used. Advantageously, the present method which involves the non-solvent, circumvents the need for removal of solvent by evaporation. Evaporation of solvent from a printed 3D structure tends to depend on the characteristics of the solvent, e.g. vapor pressure, which is a drawback of 3D printing methods involving such a step. In various aspects, the solvent which can be used in the present method may have a vapor pressure 0.05 kPa to 60 kPa, 0.1 kPa to 60 kPa, 0.5 kPa to 60 kPa, 1 kPa to 60 kPa, 10 kPa to 60 kPa, 20 kPa to 60 kPa, 30 kPa to 60 kPa, 40 kPa to 60 kPa, 50 kPa to 60 kPa, etc. In other words, the present method is not limited by the vapor pressure of the solvents which some conventional methods may face. This allows for the wider selection of solvents, which in turn allows for wider selection of thermoplastics printable via the present method. Moreover, for solvents that do not vaporize under atmospheric pressure, such solvents are usable with the present 3D printing method.

[0094] In various aspects, the ink composition formed from the polymer and solvent may have a viscosity ranging from 1 Pa.s to 1000 Pa.s, 10 Pa.s to 1000 Pa.s, 100 Pa.s to 1000 Pa.s, 500 Pa.s to 1000 Pa.s, etc. Such viscosities render the ink composition printable. On the other hand, such low viscosity ink composition may not be compatible for use with conventional 3D printing methods, as the low viscosity ink composition may not be able to retain the printed structure even for a short amount of time. However, in the present method, such low viscosity ink composition may be used to facilitate printing of porous microstmctures.

[0095] In various aspects, removing the non-solvent may include (i) drying the porous and/or non-porous three-dimensional structure in the range of 20°C to 30°C, or (ii) removing the porous and/or non-porous three-dimensional structure from the non solvent, contacting the porous and/or non-porous three-dimensional structure with water, and drying the porous and/or non-porous three-dimensional structure. The printed 3D structure may be simply dried if the non-solvent used is volatile (i.e. has a low vapor pressure). The printed 3D structure may be contacted with water if the non solvent used is an organic non-solvent that is non-volatile. For example, if DMF is used as the non-solvent, the 3D structure precipitated in the non-solvent may be removed therefrom and immersed or gently rinsed with water to separate out the DMF, and the 3D structure contacted with water may be dried, e.g. placed in a heated oven (e.g. 60°C) to expedite drying.

[0096] In various aspects, the present method of printing a porous and/or non-porous three-dimensional structure may include the use of a material to create porous structures. The material may be a sacrificial material, which means the material may be removed or sacrificed to form pores in the printed structures. In various aspects, the present method may further include adding a porogen or an additive to the ink composition prior to dispensing the ink composition, wherein the porogen comprises polyethylene glycol, polyvinylpyrrolidone, or sodium sulphate, wherein the additive comprises conductive carbon black, polyvinyl alcohol, laponite clay, or a-cellulose. The porogen helps to enhance porosity of the printed 3D structure. The additive may be used to reinforce or enhance mechanical properties of the printed 3D structure.

[0097] Embodiments and advantages described for the first aspect can be analogously valid for the present system described herein, and vice versa. As the various embodiments and advantages have already been described above and examples demonstrated herein, they shall not be iterated for brevity. For example, the porogen and additive described above for the method of the first aspect and its various embodiments can be used in the present system.

[0098] The present disclosure also includes a system operable to print a porous and/or non-porous three-dimensional structure. The system may include a syringe operable to dispense an ink composition into a non-solvent at a temperature ranging from 20°C to 30°C, wherein the ink composition may include a polymer dissolved in a solvent, and a motion control module which controls vertical and horizontal positioning of the syringe. The system is operable for the method of the first aspect to be carried out and hence has the same advantages.

[0099] In various aspects, the present system has the syringe configured with a nozzle having a cross-sectional diameter ranging from 80 pm to 680 pm, 100 pm to 680 pm, 150 pm to 680 pm, 200 pm to 680 pm, 250 pm to 680 pm, 300 pm to 680 pm, 350 pm to 680 pm, 400 pm to 680 pm, 500 pm to 680 pm, 600 pm to 680 pm, etc. For example, nozzles with diameter in the range of 410 pm to 680 pm may be used. Such nozzle diameters may render lower pressure applied to dispense the ink composition from the syringe. Apart from the syringe and module, other types of dispenser module that is able to dispense ink composition and is compatible with the present method may be used.

[00100] In various aspects of the present system, the motion control module may be operable to move the syringe parallel to a surface of a substrate at a speed of 1 mm/s to 30 mm/s, 5 mm/s to 30 mm/s, 10 mm/s to 30 mm/s, 15 mm/s to 30 mm/s, 20 mm/s to 30 mm/s, 25 mm/s to 30 mm/s, wherein the ink composition may be dispensed onto the surface. For example, the syringe, and hence the nozzle, may be operated through the motion control module at a speed of 2 mm/s to 30 mm/s. Such movement speed of the syringe and nozzle helps to control the mass and/or width of the ink composition that is dispensed. The substrate may be positioned in the non-solvent to have the ink composition, which is dispensed, immersed in the non-solvent. Said differently, the substrate may be immersed entirely in the non-solvent such that the ink composition is also dispensed into the non-solvent entirely.

[00101] In various aspects of the present system, the motion control module may be operable to have the syringe dispense the ink composition to form one or more layers of the ink composition in the non-solvent, wherein the one or more layers of ink composition may be vertically arranged in the non-solvent to form the porous and/or non-porous three-dimensional structure. The one or more layers of ink composition vertically arranged in the non- solvent may be adhered together. The one or more layers of ink composition vertically arranged in the non-solvent may be spaced apart to form the porous and/or non-porous three-dimensional structure. For example, the one or more layers of ink composition arranged in the non-solvent may be spaced vertically apart at a distance ranging from 20 mhi to 0.5 mm, 50 mhi to 0.5 mm, 100 mhi to 0.5 mm, 200 mhi to 0.5 mm, 300 mhi to 0.5 mm, 400 mhi to 0.5 mm, etc. While the one or more layers may be vertically spaced apart, e.g. during printing, the printed layers may still be adhered together at one or more locations to form the porous and/or non-porous three-dimensional structure.

[00102] In various aspects, the present system may further include a pressure source operable to apply to a pressure ranging from 10 kPa to 650 kPa for dispensing the ink composition. Various embodiments of the pressure that can be applied are already disclosed in various aspects of the present method described above.

[00103] The word“substantially” does not exclude“completely” e.g. a composition which is“substantially free” from Y may be completely free from Y. Where necessary, the word“substantially” may be omitted from the definition of the present disclosure.

[00104] In the context of various embodiments, the articles“a”,“an” and“the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[00105] In the context of various embodiments, the term“about” or“approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

[00106] As used herein, the term“and/or” includes any and all combinations of one or more of the associated listed items. [00107] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

Examples

[00108] Fabrication of controlled porous materials has gained interests because of their broad range of applications in energy storage, catalysis, biotechnology, and life sciences. Recent development of three-dimensional (3D) printing has demonstrated fabrication of 3D porous materials, albeit with limitations in the printable materials and the simplicity of the methods. In particular, direct dispensing of polymer solutions, described herein as solvent casted 3D printing (SC3DP), requires the printing ink to possess high viscosity and high vapor pressure for 3D modelling. Herein, a unique and versatile method of 3D printing based on spatially controlled immersion precipitation, termed as“immersion precipitation 3D printing (zp3DP)” is introduced. zp3DP offers the capability to fabricate 3D porous models using inks with wide ranges of vapor pressure and viscosity. For example, using a model ink of acrylonitrile butadiene styrene (ABS) dissolved in acetone (20 - 60 w/w%), it was demonstrated that the concentration of the polymer in the ink allowed controlling the internal morphologies of the 3D printed structures ranging from complete porous microstmctures (with pore sizes ranging from 1 - 20 pm) to dense non-porous microstmctures. The addition of porogens to the printing inks demonstrated fabrication of microscale pores reaching the surface of the printed filament, allowing formation of interconnected pores. zp3DP offers a route to fabricate micro-to-centimeter structures with controlled internal porosity in thermoplastics and serves as a useful system toolkit in 3D printing of hierarchical structures and functional devices.

[00109] The present method, including a system operable for the present method, are described in further details, by way of non-limiting examples, as set forth below.

[00110] Example 1A: Experimental Section - Preparation of Printing Inks

[00111] The suppliers of the thermoplastics, solvents and other chemicals used are summarized in FIG. IF and 1G. [00112] The thermoplastics were dissolved in suitable solvents to prepare polymer inks with different concentrations. The compositions of these formulated inks are summarized FIG. 1H. After placing the thermoplastic in the solvent, the solution was stirred continuously to be homogeneous. The formulated inks were then stored in sealed bottles until used for printing. The inks were directly placed into the dispensing syringes immediately before printing.

[00113] Example IB: Experimental Section - Characterization of Printing Inks

[00114] The apparent viscosities of the polymer inks were determined using capillary flow analysis. Polymer solutions were extruded through a stainless steel nozzle with inner diameters of 510 pm and 410 pm (21 and 22 Gauge nozzle 12 size, respectively) with the capillary length of 2 cm and 3 cm, respectively. The dispensing pressure was set as 200 - 650 kPa. Once the extrusion reached steady state, the ink was deposited on a petri dish for 60 seconds under the set applied pressures. For volatile solvents such as acetone and DCM, the deposited filaments were dried for 24 hrs at room temperature (e.g. 20°C - 30°C) and then weighed on a high precision weighing balance to determine the mass flow rates. For DMF, water was added to the petri dish containing the extruded filaments for 2 hrs to separate DMF from the polymer. Petri dishes were then dried for 2 hrs at 60°C in an oven. The respective mass flow rates, calculated by weighing the dried filaments using a high precision balance, were converted to volumetric flow rates. The data was used to calculate apparent viscosities for the applied pressures.

[00115] Example 1C: Experimental Section - ¾?3DP Instrument and Software

[00116] A MuCAD V software (Musashi Engineering Inc., Japan) was used to generate the design and printed using a commercially available liquid dispensers (SHOTmini 200 Sx and IMAGE MASTER 350 PC Smart, Musashi Engineering Inc., Japan).

[00117] For the CAD designs, STL data was generated using a commercial CAD program and sliced using Slic3r software into 200 - 500 pm thick layers to generate the G-code instructions. The G-Code was then converted to the format readable by MuCAD V using a self-developed Python script.

[00118] Example ID: Experimental Section - ¾?3DP Method

[00119] The following printing protocol was used before each printing. The nozzle was attached to the cylindrical syringe and placed into its respective position in the liquid dispenser. For every nozzle attached, calibrations in the horizontal (x and y) and the vertical (z) directions (distance between the nozzle tip and the substrate) were performed. The pressure required for the extrusion, nozzle speed in the horizontal (x and y) directions, nozzle acceleration and deceleration times were calibrated according to the printing pattern and the viscosity of the ink. The base of the container (e.g. a petri dish and a polypropylene container) was covered with a double- sided tape when water was used as a non-solvent. After completion of printing and immersion precipitation, the printed structure was readily removed from the tape. When an organic solvent (e.g. ethanol, acetone) was used as a non-solvent, the base of the polypropylene container was made rough to hold the printed structure in place. All experiments were performed at room temperature (e.g. 20°C - 30°C).

[00120] Example IE: Experimental Section - Mechanical Testing, Thermogravimetric Analysis, and Imaging

[00121] Dogbone test coupons were fabricated by zp3DP with a width of 4 mm, 18 length of 2 cm, and a thickness of 0.6 mm. These samples were tested using Dynamic Mechanical Analyzer (DMA Q800, TA Instruments, USA). The tensile rate was 2% strain/min.

[00122] Pyrolysis characterization were performed in a differential thermogravimetric analyzer (Q50, TA Instruments, USA) with a precision of temperature measurement of ± 0.1°C and weight measurement of ± 0.01%. The sample weight loss and the rate of weight loss were recorded continuously as a function of time and temperature, in the range of 30°C - 1000°C. The experiments were performed at atmospheric pressure, under nitrogen atmosphere, with a flow rate of 30 ml/min at various linear heating rates of 5,10, 20, and 30°C/min.

[00123] Photographs were taken using a Nikon D5300 camera (Nikon, Japan) under white light illumination. Micrographs were taken using a Leica M125 Stereomicroscope with 10X objective and a Hirox digital microscope KH-8700 (Hirox Co Ltd., Japan). All image processing was done using ImageJ (National Institute of Health, USA). The microscopic morphologies of the 3D printed objects were observed using a field emission scanning electron microscope (JSM-7600F, JEOL, Japan) at 5 - 10 kV. The membranous structures were sampled in liquid nitrogen and then sputtered with gold for 30 - 60 seconds at 20 mA using an auto fine coater (JFC-1600, JEOL, Japan) before imaging. [00124] Example 2A: Discussion - Use of A Support Medium in 3D Printing

[00125] To enhance the capability of 3D printing, an embedding medium (i.e. support medium) may be used for printing of low viscosity polymer resins, hydrogel precursors, and solutions of polyanions and polycations. In conventional methods, the role of the support medium was either to offer physical support to the printed structures (e.g. high yield-stress bath). In the present method, the support medium provides for immersion precipitation in a ternary system to fabricate 3D porous objects. The present method involving immersion precipitation includes phase inversion for fabrication of porous polymer films and scaffolds. In immersion precipitation, a polymer solution is typically cast as a thin film on a solid support and then immersed in a bath filled with a non solvent. The polymer-solvent demixing in the P-S-NS system can be explained using a ternary phase diagram (FIG. 1G). The rate of diffusion between a solvent and a non solvent is shown to determine the morphologies of micropores, e.g. macrovoids, sponge-like structures, isolated pores, and completely dense structure. In addition, the present method combines immersion precipitation with robotic control to fabricate 3D objects to impart controlled porosity. The present zp3DP may thus be a spatially controlled, non- solvent- induced phase inversion by continuous immersion precipitation method for fabricating micro-to-centimeter scale models.

[00126] Example 2B: Polymer-Solvent-Non-solvent Ternary Phase Diagram

[00127] Ternary phase diagrams are used herein to determine suitability of a polymer solution in a solvent to understand the phase inversion by immersion precipitation. The corners in the phase diagram of FIG. 1G represent the pure components, and the three axes represent the binary combinations. Any point inside the triangle indicates a ternary composition. The phase inversion causes separation of the ternary system into polymer- rich (high polymer concentration) (point A on binodal curve) and polymer-lean (low polymer concentration) (point B on binodal curve). The boundary which delimits this liquid-liquid demixing is termed as a binodal curve (dashed curve). The binodal curve is the boundary between thermodynamically favorable set of conditions for a homogeneous polymer solution mixture (Region I) and the conditions favourable for phase separation (Region II, enclosed by the binodal curve). In the region II, the free energy of the mixture decreases by the phase separation. The bionodal curve can be obtained by the cloud points, representing the compositions at the bionodal curve, using rapid titration or turbidity measurement. When a polymer solution comes in contact with the non-solvent, it follows a certain composition path in the ternary phase diagram. The liquid-liquid demixing may be instantaneous or delayed depending upon the short (crossing the bionodal curve fast) or long (crossing the binodal curve slow) composition path. This rate of liquid-liquid demixing determines the final morphology.

[00128] Example 2C: Discussion - Printable Inks Usable with the Present Method

[00129] FIG. 1A provides the overview of the present zp3DP method. Polymers were dissolved in solvents to create printable polymer inks, and the polymer inks were dispensed layer-by-layer using a DIW 3D printer in a non-solvent bath. When the printed ink came in contact with the non- solvent, the ink rapidly solidifies via immersion precipitation to form rigid structures at the ambient condition (room temperature and atmospheric pressure) without any post-processing, wherein the atmospheric pressure may affect how easy solvents get removed, e.g. by evaporation. The rapid mass transfer of the solvent into the non-solvent (and vice versa) caused the dispensed polymer filaments to solidify in situ. The solvent diffused into the non solvent in the bath while the non-solvent diffused into the polymer ink (FIG. II). FIG. IB highlights the advantageous capability of the present zp3DP method in terms of (1) apparent viscosities of the printing ink ( m_arr = 1 - 1000 Pa.s) and (2) vapor pressures of the solvent (P_vap = 0.05 - 60 kPa). As described, a difference between conventional SC3DP and the present zp3DP is the rate of solidification of the extruded inks. The present zp3DP method is not limited to inks with high P_vap and high m_arr, both of which are needed for inks used in SC3DP. The 2D plot in FIG. IB compares the physical parameters of inks ( P_vap and m_arr) that were successfully printed in reported studies for SC3DP to those for the present zp3DP method. This analysis highlights that SC3DP has been demonstrated for solvents with high values of P_vap and m. To date, most works in SC3DP have been demonstrated only for polymers dissolved in DCM. However, in the present zp3DP, the use of non-solvent circumvents such constraints, wherein the present zp3DP allowed printing of inks with low viscosities (less than 80 Pa.s) and low vapor pressure (less than 20 kPa). Using appropriate combinations of the solvents and polymers (see FIG. ID, IE and 1H), polymer inks of PS, ABS, ASA, HIPS, PLA, PCL, PVA and TPU were successfully printed into 3D models (FIG. 1C). For example, ABS was dissolved in solvents with either high vapor pressure (e.g. acetone, with water as a non-solvent) or low vapor pressure (e.g. DMF, with ethanol as a non-solvent). Polymers were usable in any form as purchased (such as filaments and pellets) or in the form of commercial products (such as petri dish and Styrofoam) before they were dissolved in solvents. Interestingly, PA composites (aqueous pastes) were readily printed in acetone at room temperature (FIG. 1C), wherein extrusion of the aqueous PA composite in air typically cannot form 3D structures when water does not evaporate fast enough to maintain the physical structure of the printed paste for conventional methods that relied on such evaporation. The printed aqueous paste can be used for sacrificial mold that can be readily removed in water (FIG. 3).

[00130] To highlight the potential of the present zp3DP for performing 3D modeling of materials, which is not compatible if other methods of digital fabrication were used, it is demonstrated herein the printing of cellulose-ester derivatives (e.g. cellulose acetate (CA), cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB)), PLA and PCL mixed with liquid and solid additives. Addition of starch, polyethylene glycol (PEG) and polyvinyl pyrrolidone (PVP) to polymer matrices have been shown to improve mechanical properties, thermal properties and gas permeability of the composites, but it was challenging to perform 3D printing with such composites via FDM and SC3DP. The melting points of cellulose esters are close to their decomposition temperature, and hence it is difficult to perform FDM 3D printing. SC3DP was applied to print CA without additives albeit only with limited printing conditions. As described in examples below, such constraints limit the internal morphologies of the 3D printed structures. In contrast, the present zp3DP offers a route to pattern cellulose esters and their composites mixed in a solvent of low vapor pressure (such as DMF, DMSO and NMP) at room temperature (for example, starch reinforced cellulose acetate, FIG. 4). Conductive carbon black (CC), laponite clay (LP), PVA and a-cellulose (aC) were also used as additives in the polymer inks and printed by zp3DP (FIG. 5 A to 5H). The addition of CC, and, LP and aC can alter electrical and mechanical properties of the 3D printed objects, respectively. Similarly, the addition of PVA in the hydrophobic PLA can improve hydrophilicity of the 3D printed objects. Unlike FDM 3D printing, the printing materials are in the liquid form in the present zp3DP method, providing an easy and convenient route to functionalize the printing materials. Overall, the present zp3DP methods provides for a wide selection of printable materials to be used along with selection of the solvents applicable to the printing. In particular, the present zp3DP enabled 3D printing of copolymers and polymer composites that may not be possible with FDM and SC3DP methods.

[00131] To investigate the reproducibility of zp3DP, ABS was dissolved in acetone (45 w/w%) to form an ink. The polymer ink was dispensed in a bath (water and ethanol) to print five identical structures of a cm-scale model in a single, continuous experiment (FIG. 6). No noticeable difference was observed in the print fidelity and printed dimensions among the five models in either water or ethanol. The prolonged use of the same non-solvent may alter the diffusion rates between the solvent and non-solvent due to the accumulation of the solvents in the non-solvent. This effect depends on the volume of the non- solvent in the bath and the volume of the printed ink.

[00132] Example 2D: Discussion - Printing Parameters in ¾?3DP

[00133] Using a DIW 3D printer (FIG. IF), the present zp3DP method had four constituents that required consideration: (1) ink, (2) dispenser, (3) robot and (4) non solvent (FIG. 2A). Firstly, the viscosity of the polymer ink was identified, which was a parameter that had to be considered for printing. For non-Newtonian fluids, such as solutions of long-chain polymers, the viscosity varies in response to the shear rate, i.e. the measured (apparent) viscosity of the polymer inks through the nozzle depends on the applied pressure. The various polymer inks were prepared by dissolving thermoplastics of varying concentrations in solvents. The compositions of the polymers and the solvents used are summarized (FIG. ID, IE and 1H). The process-related viscosity of the printing inks under applied pressures of 200 - 650 kPa using capillary flow analysis was studied. The calculated process-related viscosities (m_arr) as a function of apparent process-related shear rates (y) is plotted (FIG. 2B). Over the investigated ranges of shear rates, most polymer solutions displayed a shear-thinning behavior, characterized by the decreasing viscosity over the increasing shear rate. The flow rate increased as the applied pressure increased for shear thinning polymer solutions (FIG. 7). In the capillary flow analysis, it is observed that the process-related viscosity (_ju_app) of the polymer solutions tested was 1 - 1000 Pa.s. In practice, the value of P was the most convenient to vary among the different parameters of printing because digitally controlled fluid dispensers was used for delivery of the fluid. [00134] To understand the effect of m on P, a value of P = 300 kPa was studied, the values of m_arr for ABS (45 w/w% in DMF) and PLA (30 w/w% in DCM) were 60 Pa.s and 180 Pa.s, respectively. For a good print fidelity of the PLA ink at PPLA = 600 kPa, the working pressure for the ABS ink is expected to be lower than PPLA for the same printing conditions. The experiment was performed at PABS = 150 kPa. Overall, the values of m_arr allowed estimating the required pressure to achieve adequate mass flow rates to model 3D structures by the present zp3DP method.

[00135] Secondly, the dispenser (consisting of the pressure source, syringe and nozzle) determined the rate of mass flow dispensed from the nozzle. The nozzle attached to the syringe exhibited high fluidic resistance to the viscous fluid, i.e. the applied pressure ( P ) and the diameter of the nozzle (d) were therefore parameters governing the rate of mass flow through the given nozzle ( m ). The polymer inks were extruded through nozzles of different diameters (d = 80 - 680 pm) by controlling the applied pressure ( P = 10 - 650 kPa).

[00136] For a capillary of uniform d, the relationship between Q and P for a power law fluid is given by

[00137] A simple analysis shows that for a Newtonian fluid (Poiseuille relationship), the fluidic resistance is inversely proportional to d⁴. This relationship suggests that, when the nozzle diameter is reduced to the half, the new value of the pressure is 16 times as high as the original value to maintain the same rate of flow. This restriction may pose practical challenges in handling high pressure. As an example, the applied pressure was limited to 650 kPa, and the use of the nozzles with large diameters ( d = 410 - 680 pm) were used.

[00138] Thirdly, the motion control robot (attached to the dispensing head consisting of the syringe and nozzle) offered the control over the movement of the syringe during the deposition of the ink. The velocity of the dispensing head (v) determined the mass of the ink dispensed per unit length (m=m /v) (FIG. 8A). The parameters considered are as follows.

[00139] To characterize the effect of the speed of the dispensing head on the printed filaments, straight lines of 3-cm of ABS45 (45 w/w% of ABS in acetone) was first printed in water and the dispensed mass per unit length (m) measured. For a given set of dispensing parameters, the increase in v resulted in the decrease in m and w (top image in FIG. 6), i.e. for the fixed mass flow rates, the faster motion of the syringe results in the smaller amount ink deposited per unit length. Importantly, it is observed that the head speed v was an important parameter affecting the fidelity of printing. For the present ip3DP of ABS30 (30 w/w% in acetone) in ethanol, with printing parameters of (P, D_z, d) = (10 kPa, 0.4 mm, 410 pm), the filaments in two adjacent layers (in the vertical direction) did not attach properly for v = 2 mm/s. The unattached layers underwent rope-coil instability and prevented adhesions between the deposited filaments (bottom image in FIG. 6). Proper attachment of the adjacent layers was observed for v = 1.5 mm/s (when all other parameters were maintained to be the same). The observation may be attributed to the difference in m and resulting difference in the cross-sectional dimensions of the printed filament, i.e. the extruded polymer ink has to spread on the printed layer to ensure printability. In this example, the attachment between adjacent vertical layers were compromised due to the smaller filament size resulting from the higher head speed. In such a case, adjustment of Az may be required to ensure the stability of the 3D printed structures.

[00140] Importantly, it was observed that the head speed v was a parameter that affected the fidelity of printing (FIG. 8B). When a 3D model was created layer by layer, the distance between adjacent layers (Az) and the nozzle-to-substrate distance (h) were adjusted to achieve good print fidelity.

[00141] Finally, the solvent-non- solvent diffusion rate for the selected combination of a solvent and a non-solvent played a role in the rate of solidification of polymers (FIG. 9). The described parameters for the present zp3DP method are summarized (FIG. 2A).

[00142] Regarding the effect of solvent-non- solvent diffusion rate, non-solvents is another variable that has to be considered for performing a 3D modelling in the present zp3DP. In the setup of zp3DP, non-solvents are the liquid medium surrounding the printing head. It is demonstrated that the print fidelity depended on the solvent-non- solvent diffusion rate. To highlight the effect of solvent-non-solvent diffusion rate, 3D structures of the PA composites (aqueous pastes) were printed in acetone at three different dispensing head velocities (v) with printing parameters of ( P , Az, d ) = (50 kPa, 0.2 mm, 620 pm) (FIG. 9). When the PA ink was printed at v = 5 mm/s and v = 2 mm/s, the obtained final 3D structure was not upright. The printed structure was upright only at v = 1 mm/s (FIG. 9). The diffusion coefficient (Do) between acetone and water is 1.16 x 10 ⁵ cm²/s for acetone (solute) and water (solvent), and 4.56 x 10 ⁵ cm²/s for water (solute) and acetone (solvent). The diffusion rate of water from the PA ink to acetone was lower than that of pure water to acetone. With slow diffusion between the solvent and non-solvent, the printed structure requires time to complete solidification. In such cases, the motion of the dispensing head exerted shear forces and the resulting 3D structures were tilted. Generally, for the combination of solvents and non-solvents with low diffusion rates (such as DCM and ethanol), v can be kept as low as possible to ensure sufficient time for the solidification of the printed filaments.

[00143] Details of the effect of nozzle speed (v) on the measured width of the printed filament (w) was investigated by printing the ABS ink (45 w/w% in acetone) in water for v = 2 - 30 mm/s (FIG. 10). A plot of w versus v is presented (FIG. 2C). For d = 410 pm (Gauge) and P = 100 kPa, it is observed four regimes showed different dependence of w on v. At the low velocity of dispensing head (v = 1 - 5 mm/s), the dispensed filament underwent rope-coil instability, i.e. the interface of the solidified filament was not straight and w was non-uniform. When v was increased (v = 5 - 10 mm/s), the solidified filament became straight and w decreased gradually. Once v passed certain threshold (v = 10 - 25 mm/s), w was nearly constant ( w = 260 pm). Further increase in the head velocity (v > 25 mm/s) resulted in an increase of the variance in w, where the increase in variance of w was attributed to the stretching of the dispensed filament by the moving nozzle. When P was increased to 200 kPa for the same diameter of the nozzle (d = 410 pm), similar trends were observed for the relationship between v and w. In the regime of v where w was constant (v = 15 - 25 mm/s), continuous increase in P from 100 kPa to 200 kPa did not affect the values of w. Importantly, the printed overhang filament did not deflect vertically, suggesting the rapid solidification of the ink via immersion precipitation. The effects of d on w was also investigated. For the nozzles with small diameters (d = 110, 160 and 210 pm), w was nearly constant for the wide range of v. The observation can be attributed to the decrease in mass flow rates (rii) due to the decrease in the nozzle diameter id). Due to the low values of m resulting from small d, the dispensed ink solidified immediately upon the contact with the non solvent. In these cases, the dimension of the obtained filaments depended largely on the nozzle diameters. The present zp3DP method achieved the width of the printed filament as small as w = 77 pm, which is smaller than those attainable by FDM 3D printers. The clogging of the nozzle was observed only when the following conditions were met simultaneously: (1) nozzles with small diameters were used (d = 60 and 110 pm), (2) solvents with high vapor pressure (such as DCM) were used and (3) the nozzle was on standby for some time after printing. Otherwise, clogging of the polymer ink in the nozzle did not interfere with the printing.

[00144] Example 2E: Discussion - Roles of Non-Solvents

[00145] The use of non-solvents in the present zp3DP method provides for capabilities to fabricate 3D models with polymer inks. Inks with a wide range of viscosity were printable by the present ;/;3DP. To verify that, three concentrations of ABS dissolved in acetone with apparent viscosities ranging over three orders of magnitudes (p _app ~ 1 - 1000 Pa.s) were dispensed in both air and ethanol (FIG. 11A to 1 IF). ABS30 (i.e. 30 w/w% of ABS) formed a droplet in air due to capillary effect, and was not suitable for printing (FIG. 11A). ABS45 formed a wet filament which was not suitable for layer- by-layer deposition in 3D modelling (FIG. 11B). ABS 60 formed a filament that solidified quickly by solvent evaporation, which can be used for 3D modelling in air via SC3DP (FIG. 11C). In contrast, all ABS30, ABS45 and ABS60 formed continuous filaments in ethanol (FIG. l lA to 11C). FIG. 11D to 11F show gradients of the width of the printed filaments in ethanol that depended on the nozzle speed, suggesting rapid solidification of ABS30, ABS45 and ABS60. These experiments verified the role of surrounding media in zp3DP, wherein the solvent extraction occurred much faster in non- solvent than air to maintain morphology of the dispensed filaments.

[00146] While the non-solvent used in zp3DP did not have a supporting effect on the dispensed filaments, it allowed layer-by-layer deposition of polymer inks via rapid solidification of the ink via immersion precipitation.

[00147] Another parameter, viscosity of the non-solvents, plays a role in the present zp3DP method for controlling the porosity of the micro structure of the 3D printed structures. Unlike embedded 3D printing where a supporting medium only provides physical support, the viscosity of the non-solvents does not affect the stability of morphology of the printed structure. However, the viscosity of the non-solvents affected the diffusion between the solvent and non-solvent in the present zp3DP method, which resulted in the different pore sizes. The increase in the viscosity of the non solvent can decrease the rate of diffusion between a solvent and a non-solvent, and the decrease in pore sizes of the 3D printed structure of the selected ink (FIG. 12A to 12B). Formation of micropores is discussed in the next example below.

[00148] After the completion of printing, the printed 3D structures were kept immersed in the non- solvent to ensure the extraction of the solvent from the polymer ink into the surrounding non-solvent. Complete extraction of the solvents in the printed structure was confirmed using the thermogravimetric analysis (TGA) (FIG. 13A to 13D). ABS dissolved in acetone (45 w/w%) and printed in water (denoted as ABS45/A/W) resulted in rapid extraction of the solvent. The printed object was immersed in water for 30 min (residence time, t_r = 30 min). The measurement by TGA suggested no residual acetone and water were present in the printed model, as apparent from the comparison with the TGA spectrum of pristine ABS (FIG. 13A and 13B). ABS dissolved in DMF and printed in ethanol (denoted as ABS45/Dm/E) required relatively long residence time (/, = 120 min) to complete the extraction of the solvent due to slow diffusion between the solvent and the non-solvent (FIG. 13C). PLA dissolved in dichloromethane and printed in ethanol (PLA30/Dc/E) was another representative combination for rapid extraction of the solvent (t_r = 30 min) (FIG. 13D). Overall, the adequate residence time depends on the diffusion rate between the solvent and the non-solvent, the concentration of polymers, and the size of the printed objects. For the combinations of materials and the sizes of objects investigated in the current study, the residence time of 30 - 180 mins was adequate to remove solvents from the printed model.

[00149] Example 2F: Formation of Microporous Structure by P

[00150] FIG. 15A to 15H summarize the porous structures patterned by the present zp3DP method. It is has been demonstrated herein that the present zp3DP method enabled one-step fabrication of 3D structures with internal porosity. The microstructures can be tailored from porous to non-porous by varying the polymer concentrations. To demonstrate the effect of concentration of polymers on the microstructure, the solutions of ABS in acetone (20 - 60 w/w%) were printed in ethanol at room temperature. SEM images of the cross-sections of the printed 3D objects revealed the microstructure that resulted from the present zp3DP. The interface of the printed structure was dense, while the internal microstructure was porous (FIG. 14A). The internal structures fabricated using the solution of 20 w/w% ABS (ABS20 - the same nomenclature was used for other concentrations of ABS) was completely porous, i.e. sponge-like configurations with an interconnected pore network of nearly uniform sizes were observed (FIG. 14B). ABS30 resulted in sponge-like microstructures with non-uniform pore sizes (FIG. 14C). ABS40 gave a few isolated pores surrounded by ABS (FIG. 14D). With further increase in the ABS concentration (ABS50), few isolated pores, smaller in sizes than ABS40, were observed (FIG. 14E). Finally, ABS60 gave dense microstructures without pores (FIG. 14G). These observations suggested that the present zp3DP permitted tuning of the microstmctures inside the printed objects from completely porous to dense microstructures by altering the concentration of the polymers in the ink. Similarly, a 3D object using PLA30 was printed (FIG. 15A to 15C). Tensile tests was performed to verify the mechanical strengths of the samples printed by conventional FDM and the present zp3DP method (FIG. 14H). The modulus and strength of the parts consisting of ABS60 and PLA30 printed by the present zp3DP method were comparable to those printed by FDM, given the representative values of FDM-printed ABS (E = 1.6 GPa, s = 22-34 MPa) and FDM-printed PLA (E = 1 .5 GPa, s = 39 MPa).

[00151] It is discussed herein that SC3DP allowed 3D printing of thermoplastics albeit with a limited range of the polymer concentration in the ink, which was confirmed using the same set-up. The high viscosity and vapor pressure of ABS60 ( m_arr ~ 400 Pa.s, P_vap ~ 25 kPa) indeed permitted sufficiently fast evaporation of the solvents to maintain the printed structures in air. 3D objects of ABS60 by SC3DP, however, exhibited only dense internal microstructures (FIG. 14G). As the concentration of the ABS was lowered, the inks were no longer printable without support medium, the printed inks spread before the evaporation of the solvent took place. This observation confirmed that fast removal of the solvent to the non-solvent in the present zp3DP permitted the use of inks with low polymer concentrations, enabling one-step fabrication of porous structures. The fast removal of the solvents in the present zp3DP also enhanced the rate of fabrication. Using the same ink of ABS60, 3D printing of ABS60 was successfully performed at v = 3 - 5 mm/s in zp3DP and at v = 0.5 - 1 mm/s in SC3DP. In general, the diffusion rate between two solvents is greater than the rate of evaporation of the solvent into the air. For example, diffusion coefficient (Do) between DCM and methanol is on the order of 10 ⁵ cm²/s. The evaporation rate of pure DCM (Do) is on the order of 10 ⁶ - 10 ⁵ cm²/s, while the evaporation rate of DCM in the polymer solution (D) is on the order of 10 ⁷ cm²/s due to increasing viscosity of the polymer solution. Overall, solidification of the inks is faster in the present zp3DP method than in SC3DP, offering two advantages: (1) use of printing inks with lower polymer concentrations imparting porosity, and (2) the higher rate of fabrication.

[00152] In addition, the residence time (t_r) played a role for successful formation of micropores by immersion precipitation. TGA showed that ABS45/A/W required the residence time of 30 min to complete the removal of acetone from the ink (FIG. 13B). Indeed, removing the 3D printed models from the non-solvent prematurely resulted in the insufficient formation of the micropores. To demonstrate the effect of t_r on the micro structure of the 3D printed structure, t_r = 0 min and t_r = 30 min were selected and formation of the micropores was observed. At t_r = 0 min, the extraction of acetone in water was incomplete. In that case, the residual acetone in the printed structure was evaporated in air before the analysis in SEM. Given that the diffusion rate between acetone and water is greater than the rate of evaporation of the acetone to the air, the sample fabricated with t_r = 0 min possessed smaller pore sizes than the samples with t_r = 30 min in water (FIG. 16A and 16B). This experiment highlighted the importance of resident time to control the pore sizes, wherein premature termination of immersion precipitation may affect the attainable pore sizes.

[00153] Example 2G: Effect of Additives and Solvents on Microporous Structures

[00154] In immersion precipitation, porous interfaces are obtained for rapid precipitation, while dense and non-porous interfaces are obtained for relatively slow precipitation. For the fabrication of 3D porous objects, the porosity through the outer surface of each layer of printed inks may have to be ensured. While it is in principle achieved by the adequate choice of the materials and their concentrations, the addition of porogens (e.g. dispersion of dissolution of co-solvents, non-solvents and polymeric additives) to the ink provides another way to ensure the formation of pores at the interface. To demonstrate the effect of porogens, polyethylene glycol (PEG, 40% of the weight of ABS) was added to ABS45. It was confirmed that the addition of PEG increased the porosity in the 3D printed object both at the interface and inside of the printed filament (FIG. 17A to 17F). The addition of the PEG to the ink reduced the solubility of the polymer to the solvent, making the dissolved polymer thermodynamically unstable. Simultaneously, PEG diffuses rapidly from the ink to the non-solvent bath, enhancing the demixing rate of the solvent and the non-solvent. The immersion precipitation was hence accelerated and promoted the formation of the pores at the interface (FIG. 17A to 17C). The pores inside of the printed structures were also larger with PEG than without PEG (FIG. 17D to 17F). The measured sizes of the pores were 1 pm without PEG. With the addition of PEG, the pore sizes were 2 - 18 pm when printed in water, and 2 - 5 pm when printed in ethanol. In addition to PEG, other molecular porogens, e.g. polyvinylpyrrolidone (PVP) and mesoscale sacrificial particulates, sodium sulphate (SS), were demonstrated to impart micropores to the printed object in zp3DP (FIG. 18A to 18D). Addition of PVP (10 - 40 % of the weight of ABS) to ABS45/A/W provided the pore sizes of 5 - 100 pm. The increase in the concentration of PVP resulted in the increased density of pores in the printed structures. Addition of SS (10 % of the weight of ABS) to ABS45/A/W resulted in the pore sizes of 100 - 300 pm to the printed objects. Here, SS served as sacrificial particulates that did not dissolved in the ink, which was removed by sonication of the 3D printed object in water for 30 min after the printing. Further increase in the concentration of SS resulted in the clogging of the nozzle (18G, d = 840 pm). A wide selection of porogens available in present zp3DP method promotes the controlled formation of micropores on the surface of the printed objects.

[00155] The rate of diffusion between the solvent and the non-solvent played a role to determine the resulting micropores, and the solvent used to formulate the ink also influenced the porosity of the printed objects, were discussed. Acetone, DMF and DMSO were used as solvents to prepare the polymer inks (45 w/w% in acetone, and equivalent volume of solvents in DMF and DMSO). The inks were printed in ethanol, and the internal morphology ranged from distinct pores to sponge-like structures (FIG. 17G to 171). The observed difference in internal morphology can be attributed to the rates of demixing that caused to precipitate ABS. The rates of demixing with the non solvents (ethanol and water in this experiment) increased in the order of DMSO, DMF and acetone. This order aligned with the decreasing order of the formed pore sizes (i.e. pores were the largest with DMSO as a solvent). Applying this to control the porosity using different combinations of materials and solvents, 3D structures consisting of multiple polymers with different porosity in space were fabricated. For example, ABS dissolved in acetone (ABS45) and PCL dissolved in DCM (PCL50) were printed in ethanol as a common non-solvent (FIG. 17J). ABS45 formed porous microstructures (FIG. 17K, Regions 1 and 3) while PCL50 formed dense microstructures (FIG. 17K, Region 2). This approach demonstrated unique capability of multi-material 3D printing with spatially controlled porosity. Overall, the present zp3DP allows controlling printed microstructures by changing the concentrations of the polymers, porogens and type of solvent in the ink. This one-step alteration of the printed microstructures would not be possible in other methods of 3D printing that requires post-processing.

[00156] Example 3: Commercial and Potential Applications

[00157] A versatile method to fabricate porous 3D structures by continuous immersion precipitation has been demonstrated in the present disclosure. The present method includes performing continuous immersion precipitation in combination with a motion- controlled robot to fabricate 3D models. The use of non-solvents and rapid demixing of solvents circumvents a requirement of conventional SC3DP, which is the need for polymer inks to have high vapor pressure and high viscosity for SC3DP of dense microstructures. The microstructure of printed filaments by the present zp3DP method can be readily controlled by varying polymer concentrations, porogens and solvents of the printing ink. The present zp3DP can provide for 3D printing of responsive materials (hydrogels, shape memory polymers and elastomers) and powder materials (metal, glass and ceramic) suspended in solvents. The present zp3DP method offers flexibility and pave avenues to fabricate biocompatible scaffolds, reinforced composites and functional devices by 3D printing. The materials usable with the present zp3DP method is not limited to those disclosed herein for the fabrication of 3D structures with controlled porosity, thereby envisaging broad applications in material processing and biomedical engineering.

[00158] While the present disclosure has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims. The scope of the present disclosure is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A method of printing a porous and/or non-porous three-dimensional structure comprising:

providing an ink composition comprising a polymer dissolved in a solvent; dispensing the ink composition into a non-solvent at a temperature ranging from 20°C to 30°C, wherein the non-solvent renders precipitation of the ink composition to form the porous and/or non-porous three-dimensional structure in the non-solvent; and removing the non-solvent from the porous and/or non-porous three-dimensional structure.

2. The method of claim 1 , wherein providing the ink composition comprises filling the ink composition into a syringe operable to dispense the ink composition into the non- solvent.

3. The method of claim 1 or 2, wherein the polymer comprises polystyrene, acrylonitrile butadiene styrene, acrylonitrile styrene acrylate, polyethylene, thermoplastic polyurethane, polylactic acid, polycaprolactone, polyvinyl alcohol, polyacrylate, cellulose acetate, cellulose acetate propionate, or cellulose acetate butyrate.

4. The method of any one of claims 1 to 3, wherein the polymer dissolved in the solvent has a concentration ranging from 10 wt% to 60 wt%.

5. The method of any one of claims 1 to 4, wherein the solvent comprises acetone, chloroform, dichloromethane, dimethyl formamide, dimethyl sulfoxide, N-methyl-2- pyrrolidone, or water.

6. The method of any one of claims 1 to 5, wherein dispensing the ink composition into the non-solvent comprises applying a pressure ranging from 10 kPa to 650 kPa to dispense the ink composition.

7. The method of any one of claims 1 to 6, wherein dispensing the ink composition into the non-solvent comprises having the ink composition immersed in the non-solvent for a time ranging from 30 minutes to 180 minutes.

8. The method of any one of claims 1 to 7, wherein the non- solvent comprises water, ethanol or acetone.

9. The method of any one of claims 1 to 8, wherein the ink composition has a viscosity ranging from 1 Pa.s to 1000 Pa.s.

10. The method of any one of claims 1 to 9, wherein the solvent has a vapor pressure 0.05 kPa to 60 kPa.

11. The method of any one of claims 1 to 10, wherein removing the non- solvent comprises

(i) drying the porous and/or non-porous three-dimensional structure in the range of 20°C to 30°C; or

(ii) removing the porous and/or non-porous three-dimensional structure from the non-solvent, contacting the porous and/or non-porous three-dimensional structure with water, and drying the porous and/or non-porous three-dimensional structure.

12. The method of any one of claims 1 to 11, further comprises adding a porogen or an additive to the ink composition prior to dispensing the ink composition, wherein the porogen comprises polyethylene glycol, polyvinylpyrrolidone, or sodium sulphate, wherein the additive comprises conductive carbon black, polyvinyl alcohol, laponite clay, or a-cellulose.

13. A system operable to print a porous and/or non-porous three-dimensional structure, the system comprising:

a syringe operable to dispense an ink composition into a non-solvent at a temperature ranging from 20°C to 30°C, wherein the ink composition comprises a polymer dissolved in a solvent; and a motion control module which controls vertical and horizontal positioning of the syringe.

14. The system of claim 13, wherein the syringe is configured with a nozzle having a cross-sectional diameter ranging from 80 pm to 680 pm.

15. The system of claim 13 or 14, wherein the motion control module is operable to move the syringe parallel to a surface of a substrate at a speed of 1 mm/s to 30 mm/s, wherein the ink composition is dispensed onto the surface.

16. The system of any one of claims 13 to 15, wherein the substrate is positioned in the non-solvent to have the ink composition, which is dispensed, immersed in the non solvent.

17. The system of any one of claims 13 to 16, wherein the motion control module is operable to have the syringe dispense the ink composition to form one or more layers of the ink composition in the non-solvent, wherein the one or more layers of ink composition are vertically arranged in the non-solvent to form the porous and/or non- porous three-dimensional structure.

18. The system of claim 17, wherein the one or more layers of ink composition vertically arranged in the non-solvent are adhered together.

19. The system of claim 17, wherein the one or more layers of ink composition vertically arranged in the non- solvent are spaced vertically apart at a distance ranging from 20 pm to 0.5 mm to form the porous and/or non-porous three-dimensional structure.

20. The system of any one of claims 13 to 19, further comprises a pressure source operable to apply to a pressure ranging from 10 kPa to 650 kPa for dispensing the ink composition.