GB2581145A

GB2581145A - Method of manufacturing an elastic conductor, elastic conductor and electronic device comprising the elastic conductor

Info

Publication number: GB2581145A
Application number: GB1901521.3A
Authority: GB
Inventors: Konrad Krawczyk Krzysztof; Griesser Thomas
Original assignee: Montanuniversitaet Leoben
Current assignee: Montanuniversitaet Leoben
Priority date: 2019-02-04
Filing date: 2019-02-04
Publication date: 2020-08-12
Also published as: GB201901521D0; WO2020161077A1

Abstract

The method comprises the steps of: (a) forming a solid metal layer on a surface of an elastomer substrate; (b) optionally, activating a surface of the solid metal layer by removing a passivation layer from the surface of the solid metal layer; (c) applying a liquid metal onto an exposed surface of the solid metal layer; (d) forming an alloy from the solid metal layer and the liquid metal such that the alloy comprises at least one element originating from the solid metal layer and at least one element originating from the liquid metal, such that the alloy is composed of at least 5 wt.-% of material originating from the solid metal layer and at least 15 wt.-% of material originating from the liquid metal; (e) removing un-alloyed liquid metal from a surface of the alloy formed in step (d); and (f) optionally, applying a protective elastomeric film. The elastic conductor finds use in a wearable and/or stretchable electronic device.

Description

Method of manufacturing an elastic conductor, elastic conductor and electronic device comprising the elastic conductor

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing an elastic conductor, an elastic conductor obtainable by the method and an electronic device, in particular a wearable and/or stretchable electronic device, comprising the elastic conductor.

BACKGROUND

The field of flexible or elastic electronics has rapidly grown over the past decade as a result of the increasing demand for real-time health monitoring, light weight mobile electronics, wearable displays etc. In the proximate future multifunctional electronic devices are going to be incorporated on the human body or clothing and a stable performance under conditions of high strain and extreme body motion such as folding, twisting and stretching will be required. The growing demand for such devices has triggered research in the design and manufacturing of stretchable elastomers and elastomer composites. Although good conductivity and high stretchability seem to be mutually exclusive features, a few approaches to combine stretchability and good electrical conductivity have been proposed to date.

A common strategy is to obtain stretchable conductors from stiff, non-stretchable materials: (a) coiled metal wires, (b) meanders of metal foil, (c) thin layers of metal sputtered on a prestretched or buckled/microporous substrates. These interconnects are stretchable within the boundaries imposed by geometrical features of the conductive film due to the microporosity of the -2 -surface, pattern design (e.g. meanders), pre-stretching and/or grain boundary lithography. Within these limits, excellent conductivity, speed and only little fatigue are observed. Nevertheless, above critical strain values (i.e. approx. 40% for meanders) irreversible destruction of the circuitry occurs.

Additionally, the fabrication of conductive layers on prestretched substrates poses some technical challenges, which significantly increase the cost of manufacturing.

Well resolved and highly conductive stretchable interconnects were manufactured by infiltration of an elastomeric substrate with Ag+ ions, followed by chemical reduction. In this approach the high solubility of silver trifluoroacetate in organic solvents was exploited. The reduction is carried out by means of hydrazine or formaldehyde treatment. These substances are toxic or require a harsh environment, so the produced stretchable wiring boards need to be extensively rinsed. The preparation requires a multistep procedure and works well only on selected substrates, which limits its applicability in industrial processes.

Despite the advantages of the systems described above, the vast majority of stretchable conductive systems are based on conductive composites, in which the polymeric part is responsible for the stretchability, while percolated conductive fillers allow efficient charge transfer. Conductive fillers may be carbon based (e.g. graphite, amorphous carbon, carbon nanotubes (CNTs), graphene, pyrolyzed bacterial cellulose) or metallic (e.g. metal nanowires, microflakes, micropowders, microflowers and nanoparticles). Combinations of different kinds of fillers were also reported.

One method of fabricating conductive composites is the infiltration of a percolated filler-network with a liquid elastomer resin, which is subsequently cured. Thus obtained composites show excellent conductivity, since the filler network is per se highly percolated. The drawback is the relatively complex, -3 -multistep manufacturing process and the difficulty in the precise deposition of the filler.

In another approach, an ink contains both the elastomeric resin and the filler in a single component, which can be structured for instance by means of screen printing. The latter is the preferred deposition method for industrial applications because of the simplicity of the printing process, easy automatization, applicability on different substrates and good resolution. Common inks giving access to conductive stretchable composites comprise CNTs and/or microscopic metallic particles. Particularly good conductivities under strain can be achieved with silver microflakes, which are the filler of choice in most commercially available systems. These inks result in percolated networks with excellent conductivity, but the inks have generally high R/Ro values and significant fatigue, since many connections between individual filler nanoparticles are broken when strain is applied. The particles of the filler may also undergo encapsulation in the insulating polymer, which decreases the effective percolation. Thus, relatively thick layers of the composite are necessary to meet the required conductivity and fatigue resistance.

High stretchability with low R/Ro was reported for stretchable inks based on PEDOT:PSS. The conductive polymer is typically used together with a fluorosurfactant and can be applied via screen printing. The advantage of the system lies inter alia in high optical transparency, which makes it a stretchable analogue to ITO films. However, for applications in stretchable interconnects, the limited conductivity of PEDOT:PSS is a disadvantage.

Liquid metal conductors on elastomers show optimal R/Ro at almost any strain values but are difficult to manufacture industrially. In a recent publication (Hirsch, A., Michaud, H.O., Gerratt, A.P., Mulatier, S. de & Lacour, S.P.

Intrinsically stretchable biphasic (solid-liquid) thin metal films. Advanced Materials 28,4507-4512 (2016)), a thin layer of noble metal (i.e. Au) was evaporated onto an elastomer surface, followed by the evaporation of a layer -4 -of Ga. This resulted in a biphasic AuGa2/Ga layer, which showed excellent conductivity and was virtually fatigue free. The practical applicability for large scale production is however limited by the thermal evaporation steps, which require high vacuum.

OBJECT OF THE INVENTION

The present invention aims at overcoming the above described problems and drawbacks. Thus, an object of the present invention is to provide a method which enables the manufacturing of an elastic (flexible, stretchable) conductor exhibiting high stretchability along with excellent electrical conductivity (preserved even at high strain values) and excellent fatigue resistance (e.g. only minimum changes in conductivity after multiple stretching and release cycles) in a cost efficient manner and suitable for high throughput manufacturing.

SUMMARY OF THE INVENTION

This object may be solved by a method of manufacturing an elastic conductor as described in claim 1.

The present invention in particular relates to a method of manufacturing an elastic (flexible, stretchable) conductor (e.g. conductive trace), the method comprising the steps of (a) forming a solid metal layer on a surface of an elastomer substrate; (b) optionally, activating a surface of the solid metal layer by removing a passivation layer from the surface of the solid metal layer; (c) applying a liquid metal onto an exposed surface of the solid metal layer; (d) forming an (electrically conductive (in particular electrically conductive under strain) and/or biphasic (e.g. solid-liquid)) alloy from the solid metal layer and the liquid metal such that the alloy comprises at least one element (or metal) originating from the solid metal layer and at least one element (or metal) originating from the liquid metal, such that the alloy is composed of at least 5 wt.-% of material (i.e. one or more elements or metals) originating from the solid metal layer and at least 15 wt.-% of material (i.e. one or more elements or metals) originating from the liquid metal; (e) removing un-alloyed liquid metal from a surface of the alloy formed in step (d); (f) optionally, applying a protective elastomeric film.

The present invention further relates to an elastic (flexible, stretchable) conductor (e.g. conductive trace) obtainable by the method as described 10 herein.

In addition, the present invention relates to an electronic device, in particular a wearable and/or stretchable electronic device, comprising the elastic (flexible, stretchable) conductor as described herein.

Other objects and many of the attendant advantages of embodiments of the present invention will be readily appreciated and become better understood by reference to the following detailed description of embodiments and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates an exemplary set-up suitable for manufacturing an elastic conductor as described herein.

Figure 2 shows experimental results of an XPS measurement of the depth profile of two exemplary conductors manufactured according to the present invention.

DETAILLED DESCRIPTION OF THE INVENTION -6 -

Hereinafter, details of the present invention and other features and advantages thereof will be described. However, the present invention is not limited to the following specific descriptions, but they are for illustrative purposes only.

It should be noted that features described in connection with one exemplary embodiment or exemplary aspect may be combined with any other exemplary embodiment or exemplary aspect, in particular features described with any exemplary embodiment of the method of manufacturing an elastic conductor may be combined with any exemplary embodiment of an elastic conductor or with any exemplary embodiment of an electronic device and vice versa, unless specifically stated otherwise.

Where an indefinite or definite article is used when referring to a singular term, such as "a", "an" or "the", a plural of that term is also included and vice versa, unless specifically stated otherwise, whereas the word "one" or the number "1", as used herein, typically means "just one" or "exactly one".

The expressions "comprising", "including" or "containing" do not exclude other elements or steps and, as used herein, include not only the meaning of "comprising", "including" or "containing", but may also encompass "consisting essentially of" and "consisting or.

Unless specifically stated otherwise, the expression "at least partially", "at least a partial" or "at least a part of", as used herein, may mean at least 5 % thereof, in particular at least 10 % thereof, in particular at least 15 % thereof, in particular at least 20 Wo there of, in particular at least 25 Wo thereof, in particular at least 30 % thereof, in particular at least 35 °A° thereof, in particular at least 40 0/0 thereof, in particular at least 45 °A° thereof, in particular at least 50 % thereof, in particular at least 55 % thereof, in particular at least 60 % thereof, in particular at least 65 % thereof, in particular at least 70 Wo thereof, in particular at least 75 Wo thereof, in -7 -particular at least 80 0/0 thereof, in particular at least 85 % thereof, in particular at least 90 0/0 thereof, in particular at least 95 % thereof, in particular at least 98 % thereof, and may also mean 100 % thereof.

In a first aspect, the present invention relates to a method of manufacturing an elastic (flexible, stretchable) conductor (e.g. conductive trace), the method comprising the steps of (a) forming a solid metal layer on a surface of an elastomer substrate; (b) optionally, activating a surface of the solid metal layer by removing a passivation layer from the surface of the solid metal layer; (c) applying a liquid metal onto an exposed surface of the solid metal layer; (d) forming an (electrically conductive (in particular electrically conductive under strain) and/or biphasic (e.g. solid-liquid)) alloy from the solid metal layer and the liquid metal such that the alloy comprises at least one element (or metal) originating from the solid metal layer and at least one element (or metal) originating from the liquid metal, such that the alloy is composed of at least 5 wt.-% of material (i.e. one or more elements or metals) originating from the solid metal layer and at least 15 wt.-% of material (i.e. one or more elements or metals) originating from the liquid metal; (e) removing un-alloyed liquid metal from a surface of the alloy formed in step (d); (f) optionally, applying a protective elastomeric film.

The terms "elastic", "flexible" and "stretchable" are substantially interchangeable herein. These terms may particularly denote a material property of reversibly deforming under stress.

The term "conductor", as used herein, may in particular denote a material that is capable of conducting electric current, i.e. that is electrically conductive. A conductor within the meaning of the present invention may be capable of conducting electric current even under strain, such as under tensile strain or stress and/or under a flexural load, and preferably may substantially revert to its initial conductivity after release of the strain. In an embodiment, a conductor as well as its conductivity may be substantially fatigue resistant with regard to repeated appliances of stress/strain and release thereof. The conductor may for instance represent a conductive trace, such as a conductive trace formed or arranged on a substrate, such as an elastomer or elastic substrate.

The term "alloy", as used herein, may in particular denote a combination of at least two components, such as at least two metals, exhibiting a metallic bonding characteristics. It may be a solid solution of metal elements (forming a single phase) or a mixture of metallic phases (forming two or more solutions).

The term "liquid metal", as used herein, may in particular denote a metal or an alloy, which is in a liquid state. In particular, a liquid metal may be composed of metal ions and free electrons, such as mercury or fused metal. In addition, a liquid metal may have a high electrical conductivity due to the action of free electrons.

In an embodiment, the step (a) of forming a solid metal layer on a surface of an elastomer substrate comprises applying at least one metal or metal precursor (such as a salt or a complex thereof). For instance, the metal or metal precursor may be applied in the form of a solution or a suspension containing the metal or metal precursor. The metal or metal precursor may be applied directly on a surface of the elastomer, i.e. without any intervening structure or layer.

In an embodiment, the step (a) of forming a solid metal layer on a surface of an elastomer substrate comprises applying at least one metal or metal precursor by means of at least one selected from the group consisting of sputtering, evaporation, printing (such as inkjet printing, screen printing, flexographic printing or gravure printing), spray-coating and dip-coating. It might be advantageous if the solid metal layer may be formed in a continuous process, for instance in a reel-to-reel process, which may be efficiently achieved for instance by means of spray-coating.

In some embodiments, the solid metal layer may be formed as a solid metal layer having a substantially continuous surface. This may be achieved for instance by means of at least one selected from the group consisting of sputtering, evaporation, spray-coating and dip-coating.

In alternative embodiments, the solid metal layer may be formed as a patterned (or structured) metal layer, which may be for instance efficiently achieved by means of a printing technique, such as inkjet printing, screen printing, flexographic printing or gravure printing.

In an embodiment, a metal precursor is applied on a surface of the elastomer substrate. The metal precursor may comprise a salt of a metal or a complex of a metal. In particular, the solid metal layer may be formed by applying a solution of a metal precursor, which may be advantageous in terms of cost efficiency and in terms of appropriately adjusting the amount of applied solid metal and/or thickness of the solid metal layer.

If a metal precursor is applied on a surface of the elastomer substrate, the method may further comprise a step of reducing the metal precursor to the corresponding metal (or element).

In an embodiment, reducing the metal precursor includes a physical and/or chemical treatment of the metal precursor, in particular by at least one selected from the group consisting of thermal reduction, photochemical reduction, and chemical reduction.

In an embodiment, the step (a) of forming a solid metal layer may be carried out in an inert gas atmosphere (such as under a N2, CO2, or a noble gas, in particular Ar, atmosphere). By taking this measure, it may be avoided that a -10 -passivation layer, such as an oxide layer, is formed on a surface of the solid metal layer, which should otherwise be removed before applying a liquid metal. Alternatively, the step (a) of forming a solid metal layer may be carried out in a normal gas atmosphere, such as air. This might be advantageous for reducing the process costs, in particular if the solid metal layer basically consists of noble metals that are not prone to form a passivation layer, such as an oxide layer, in the presence of oxygen.

In an embodiment, the step (a) of forming a solid metal layer on a surface of 10 an elastomer substrate comprises (al) depositing, in particular spray-coating or dip-coating, a solution or suspension containing at least one metal or metal salt on the surface of the elastomer substrate; (a2) optionally reducing the metal salt to the corresponding metal (element); and (a3) drying the solution or suspension.

In an embodiment, the solid metal layer comprises an adhesion layer and/or an alloying layer composed of a metal or a combination thereof. The term "adhesion layer", as used herein, may in particular denote a (metal) layer configured for providing adhesion (or an adhesive connection) between the substrate and the later formed alloy. The term "alloying layer", as used herein, may in particular denote a (metal) layer configured for providing a part of the raw material for the later formed alloy. As will be discussed in further detail below, the alloy formed in step (d) comprises at least one element or metal originating from the solid metal layer, more specifically from an alloying layer forming part or constituting the solid metal layer.

In an embodiment, the solid metal layer may comprise, in particular consist of, one layer, which may function as an adhesion layer and/or an alloying layer. In this embodiment, the solid metal layer preferably comprises only one metal, such as silver or copper, or one alloy.

In alternative embodiments, the solid metal layer may comprise, in particular consist of, more than one layers (such as a plurality of sublayers). For example, the solid metal layer may comprise at least one adhesion layer and at least one alloying layer. The more than one layers may be composed of the same metal (or the same alloy) or of different metals (or different alloys).

The solid metal layer may comprise more than one metals or elements, which may be applied sequentially or as a mixture.

In an embodiment, the solid metal layer comprises at least one element (or metal) selected from the group consisting of copper, silver, gold and platinum. Preferably, the solid metal layer comprises at least one element (or metal) selected from the group consisting of copper and silver.

In an embodiment, a thickness of the solid metal layer is from 50 nm to 10 pm, in particular 100 nm to 5 pm, in particular 250 nm to 1 pm, for example 500 nm.

The material of the elastomer substrate is not particularly limited, as long as it is elastic (or flexible) and is capable of bearing a solid metal layer (or the later formed alloy) on its surface. For instance, the material of the elastomer substrate may comprise at least one polymer material. Suitable examples of the material of the elastomer substrate may be in particular thermoplastics, thermosets and composite materials. In particular, suitable examples of the material of the elastomer substrate include polyurethanes, polyurethane (meth)acrylates, PEG-(meth)acrylates; polyester, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC); polysulfones, such as polyethersulfone (PES); polyarylates (PAR); polycyclic olefins (PC0); polyimides (PI); polyolefins, such as polyethylene (PE), polypropylene (PP); vinyl polymers, such as polystyrene (PS), polyvinyl chloride (PVC), polymethyl methacrylate (PMMA); polyamides; polyethers; -12 -polyketones, such as aromatic polyetherketones (e.g. PEEK); polysulfides (e.g. PPS); fluoropolymers, such as polyvinylidene fluoride (P(VDF), such as P(VDFTrFE), which may be particularly suitable when used for a piezoelectric sensor), polytetrafluoroethylene (such as PTFE), fluorinated ethylene propylene (FEP); liquid crystal polymers; polyepoxides; polysiloxanes (e.g. PDMS); rubber materials, such as natural rubber (NR), synthetic natural rubber (IR), nitrile butadiene rubber (NBR), carboxylated nitrile butadiene rubber (XNBR), styrene butadiene rubber (SBR) and other rubber materials derived from polymer dispersions and caoutchouc or synthetic rubber latexes; biopolymers or combinations, copolymers and/or blends thereof. In particular, the material of the elastomer substrate may include a thermoplastic polyurethane.

In an embodiment, the elastomer substrate may have a tensile modulus of not more than 250 MPa, in particular of not more than 200 MPa. The lower limit of the tensile modulus of the elastomer substrate is not particularly limited, as long as the elastomer substrate is capable bearing a solid metal layer (or the later formed alloy) on its surface. In particular, the elastomer substrate may have a tensile modulus of not less than 25 MPa, in particular of not less than 50 MPa. The tensile modulus of the elastomer substrate can be for instance determined in accordance with ISO 527-1 and 527-3.

In an embodiment, the method further comprises a step of structuring (or patterning) the solid metal layer after step (a) and prior to step (c), for instance, if a solid metal layer having a substantially continuous surface has been formed in step (a), such as by means of at least one selected from the group consisting of sputtering, evaporation, spray-coating and dip-coating.

In an embodiment, the step of structuring the solid metal layer includes at least one selected from the group consisting of cutting, selective wetting, etching or a photolithography process.

-13 -In an embodiment, the method comprises the step (b) of activating a surface of the solid metal layer by removing a passivation layer, such as an oxide layer, from the surface of the solid metal layer. By doing so, the wettability of the surface of the solid metal layer may be improved, in particular its wettability for a liquid metal. This process step may be necessary if the solid metal layer does not basically consist of noble metals, such as silver, gold or platinum, and/or if the solid metal layer has been formed in the presence of oxygen.

In an embodiment, the activating of a surface of the solid metal layer includes a physical treatment, such as an irradiation with (e.g. intense pulsed) light, and/or a chemical treatment, such as a(n) (etching) treatment (for instance by spray coating) with an acid (for instance NCI), a base (for instance an amine compound, e.g. diaminooctane), or a reducing agent (for instance a polyol, aldehyde, hydride, e.g. lithium borohydride, hydrazine and its derivatives, e.g. hydrazine carboxylate) of the surface of the solid metal layer.

In an embodiment, prior to step (c), the liquid metal is subjected to an activation treatment. In particular, the activation treatment of the liquid metal may comprise a removal of a passivation layer, such as an oxide layer, from the surface of the liquid metal. By taking this measure, the wettability of the liquid metal with regard to a surface of the solid metal layer may be improved. This process step may be necessary if the process step (c) is carried out in a atmosphere comprising oxygen.

In an embodiment, the activation treatment of the liquid metal includes a physical treatment, such as an irradiation with (e.g. intense pulsed) light, and/or a chemical treatment, such as a(n) (etching) treatment (for instance by spray coating) with an acid (for instance HCI),a base (for instance an amine compound, e.g. diaminooctane), or a chemical reducing agent, of the surface of the liquid metal.

-14 -It might be advantageous to carry out both the activating of a surface of the solid metal layer (if any) and the activation treatment of the liquid metal (if any) by means of the same technique, for instance by an etching treatment by spray coating with an acid or any other of the techniques described in the foregoing. By doing so, the same equipment may be used for both activation treatments, thereby reducing equipment requirements and process costs.

In an embodiment, the step (c) of applying a liquid metal onto an exposed surface of the solid metal layer comprises a dip coating of an exposed surface of the solid metal layer into the liquid metal. By taking this measure, it might be possible to perform step (c) continuously, for instance in a reel-to-reel process, which may be advantageous in terms of cost efficiency and may allow high throughput manufacturing.

In an embodiment, the liquid metal comprises at least one element (or metal) selected from the group consisting of gallium, indium and tin. For example, the liquid metal may consist of one element (or metal) or of an alloy of two or more elements (or metals).

In an embodiment, the liquid metal comprises an (eutectic) alloy of gallium, indium and tin. For example, the liquid metal may be an alloy comprising a larger amount of gallium than an amount of any of indium and tin. For example, the liquid metal may be an alloy comprising from 65 to 86 wt.-% of gallium, from 5 to 22 wt.-% of indium and from 1 to 11 wt.-% of tin. In particular, the liquid metal may be Galinstan.

In the step (d) of forming an alloy from the solid metal layer and the liquid metal, a new material is formed (i.e. the alloy), which has different properties than both the solid metal and the liquid metal. As a result of the alloying process, a volume expansion may occur due to an increase of the lattice constant of the metal or the alloy. The alloy formed in step (d) may be solid. In addition, the alloy formed in step (d) is typically electrically conductive, and -15 -advantageously may remain its electrical conductivity under strain or stress (e.g. it may be electrically conductive under strain). The alloy formed in step (d) may be a biphasic alloy, such as a solid-liquid alloy.

In an embodiment, the alloy represents the (main) conductive material of the conductor. The alloy may even represent the only (electrically) conductive material of the conductor.

In an embodiment, the alloy formed in step (d) comprises at least 10 wt.-%, in particular at least 15 wt.-%, in particular at least 20 wt.-%, of material (i.e. one or more elements or metals) originating from the solid metal layer.

In an embodiment, the alloy formed in step (d) comprises at least 20 wt.-%, in particular at least 25 wt.-%, in particular at least 30 wt.-%, of material (i.e. one or more elements or metals) originating from the liquid metal.

In an embodiment, the alloy formed in step (d) is entirely composed of material (i.e. one or more elements or metals) originating from the solid metal layer or from the liquid metal.

In an embodiment, the step (d) of forming an alloy from the solid metal layer and the liquid metal includes (directly) contacting the solid metal layer with the liquid metal for a period of time of from 1 second to 1 hour, in particular of from 30 seconds to 30 minutes, in particular of from 1 minute to 10 minutes and at a temperature of from 10 °C to 150 °C, in particular of from 15 °C to 130 °C, in particular of from 20 °C to 100 °C. The period of time for completing the alloying may in particular depend on the characteristics of the solid metal layer (more specifically the alloying layer), such as its elemental composition, thickness and structure (e.g. porous or non-porous), the type of the liquid metal and the temperature.

-16 -In an embodiment, the alloy formed in step (d) has a depth profile in thickness direction. In other words, it may be possible that the composition of the alloy is not homogenous in a thickness direction, but that there may be a concentration gradient in the thickness direction. For instance, one or more elements or metals originating from the solid metal layer may have a higher concentration at a position within the alloy close to the substrate than at a position within the alloy farther from to the substrate or vice versa.

In an embodiment, the alloy formed in step (d) has a shape with a flat (substantially rectangular) cross-section. For instance, the alloy may form a layer, for instance a continuous layer or a patterned or structured layer.

In an embodiment, the alloy formed in step (d), such as an alloy layer, has a thickness of from 75 nm to 20 pm, in particular 250 nm to 10 pm, in particular 15 500 nm to 5 pm.

In an embodiment, a thickness of the alloy formed in step (d), such as an alloy layer, is at least one and a half times, in particular at least twice, in particular at least three times, the thickness of the solid metal layer prior to step (c).

In an embodiment, the step (d) of forming an alloy from the solid metal layer and the liquid metal includes a substantially complete dissolution (or transformation) of the solid metal layer. Thus, the alloying step of (d) may be carried out until substantially all of the material from the solid metal layer is transformed together with the liquid metal into an alloy. A thus formed alloy may retain the adhesion to the substrate of the previous solid metal layer and may remain fully wetted by the liquid metal.

The method of manufacturing an elastic conductor according to the first aspect comprises a step (e) of removing un-alloyed (or excessive) liquid metal from a surface of the alloy formed in step (d). By taking this measure, a flat-cross- -17 - section of the conductor may be achieved because no meniscus of accumulated liquid metal is present. In addition, the surface of the conductor may be resistant to soft touch because the formed alloy may not readily wet skin and/or most polymeric surfaces. As a consequence of removing un-alloyed liquid metal from the surface of the alloy, a protective elastomeric film may be applied by means of lamination, which may not be possible if substantial amounts of un-alloyed (or excessive) liquid metal remained on the surface of the alloy.

In an embodiment, the step (e) of removing un-alloyed liquid metal from a surface of the alloy is carried out by means of at least one selected from the group consisting of squeegee, compressed air (such as an air blade), doctor blade, scraper, pressing roll and a roller mill.

In an embodiment, the step (e) of removing un-alloyed liquid metal from a surface of the alloy substantially completely removes any un-alloyed liquid metal from the surface of the alloy, in particular so that the conductor does substantially not comprise any mechanically extractable liquid metal. By taking this measure, leakage of liquid metal may be avoided, which is advantageous in terms of safety, in particular since leaking or extractable liquid metal may corrode aluminium or other metals, which restricts the application range. Such restrictions, for instance in airplanes, may be avoided by substantially completely removing any un-alloyed liquid metal.

In an embodiment, the un-alloyed liquid metal removed in step (e) may be recycled and used again for step (c) of applying a liquid metal. By taking this measure, the material costs may be reduced and resources may be efficiently utilized being advantageous from an ecological point of view.

In an embodiment, the method further comprises, after step (e), a step of structuring (or patterning) an alloy layer (or film) formed in step (d), for -18 -instance if no structuring (or patterning) of the solid metal layer has been made.

In an embodiment, the step of structuring the alloy layer comprises at least one selected from the group consisting of cutting (such as laser cutting), etching or a photolithography process.

In an embodiment, the method of manufacturing an elastic conductor comprises the step (f) of applying a protective elastomeric film. By taking this measure, the alloy representing the conductive material of the conductor may be protected from environmental influences. As a result, the reliability and/or the robustness of the conductor may be improved.

In an embodiment, the step (f) of applying a protective elastomeric film comprises at least one selected from the group consisting of a lamination process (e.g. under application of heat and pressure or under application of ultrasound (such as ultrasound sintering or ultrasound lamination, which may allow the selective provision of heat for lamination at the interfaces to be laminated)), in particular by means of roll-to-roll (or reel-to-reel) processing; screen printing, in particular by means of roll-to-roll (or reel-to-reel) processing, optionally followed by curing (such as photo-polymerization); offset printing; flexographic printing and gravure printing. It should be appreciated that these techniques are not compatible with conventional methods for manufacturing an elastic conductor using a liquid metal, but are only compatible with a method for manufacturing an elastic conductor according to the present invention. In addition, the protective elastomeric film may hereby be provided in a particular cost-efficient manner, enabling a continuous high throughput production of the conductor. In alternative embodiments, the step (f) of applying a protective elastomeric film comprises at least one selected from the group consisting of casting a liquid elastomer resin and subsequently curing the liquid elastomer resin, curtain coating, spray coating, dip coating, doctor-blading and roll-coating.

-19 -In an embodiment, the protective elastomeric film comprises at least one polymer material, in particular at least one selected from the group consisting of thermoplastic materials, thermosetting resins, UV-and thermally curable (polymerizable) resins, materials obtained from polymer (elastomer) solutions, polymer (elastomer) dispersions, and latexes (emulsions of elastomer droplets in a non-miscible solvent). In particular, suitable examples of the material of the protective elastomeric film include polyurethanes, polyurethane (meth)acrylates, PEG-(meth)acrylates; polyesters, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC); polysulfones, such as polyethersulfone (PES); polyarylates (PAR); polycyclic olefins (PC0); polyimides (PI); polyolefins, such as polyethylene (PE), polypropylene (PP); vinyl polymers, such as polystyrene (PS), polyvinyl chloride (PVC), polymethyl methacrylate (PMMA); polyamides; polyethers; polyketones, such as aromatic polyetherketones (e.g. PEEK); polysulfides (e.g. PPS); fluoropolymers, such as polyvinylidene fluoride (P(VDF), such as P(VDFTrFE), which may be particularly suitable when used for a piezoelectric sensor), polytetrafluoroethylene (such as PTFE), fluorinated ethylene propylene (FEP); liquid crystal polymers; polyepoxides; polysiloxanes (e.g. PDMS); rubber materials, such as natural rubber (NR), synthetic natural rubber (IR), nitrile butadiene rubber (NBR), carboxylated nitrile butadiene rubber (XNBR), styrene butadiene rubber (SBR) and other rubber materials derived from polymer (elastomer) dispersions, caoutchouc or synthetic rubber latexes; biopolymers or combinations, copolymers and/or blends thereof. In particular, the material of the protective elastomeric film may include a thermoplastic polyurethane.

Figure 1 illustrates an exemplary set-up suitable for manufacturing an elastic conductor as described herein. More specifically, a roll-to-roll setup is shown which may be suitable for continuously carrying out several process steps. An elastomer substrate with a solid metal layer thereon (which may have been provided in a separate module) is provided from a substrate roll. A surface of -20 -the solid metal layer is exposed to a spray nozzle dispensing for instance a mixture of ethanol and hydrochloric acid (HCI) to thereby activate the surface of the solid metal layer. A thus activated substrate is dipped into a bath of heated liquid metal, which in turn has also been activated by the same spray equipment. While passing the bath of liquid metal, the solid metal layer is wetted by liquid metal and an alloying of the solid metal and the liquid metal takes place until remaining un-alloyed liquid metal is removed from the a surface of the formed alloy by pressing rolls or squeegees. Subsequently, a laminating film (such as a protective elastomeric film) is applied on the alloy-substrate structure in a manner that the alloy (layer) is sandwiched between the protective elastomeric film and the substrate, and the thus obtained multi-layer structure is laminated by being passed through heated rolls.

In a second aspect, the present invention relates to an elastic (flexible, stretchable) conductor (e.g. conductive trace) obtainable by the method according to the first aspect.

The conductor may for instance represent a conductive trace, such as a conductive trace formed or arranged on a substrate, such as an elastomer or 20 elastic substrate. The conductor may also represent a conductor path, a wiring, or an electrical contact.

In a third aspect, the present invention relates to an electronic device, in particular a wearable and/or stretchable electronic device, comprising the elastic (flexible, stretchable) conductor according to the second aspect.

In an embodiment, the conductor may form a conductive trace, a conductor path, a wiring, or an electrical contact of the electronic device.

In an embodiment, the electronic device is selected from the group consisting of an adhesive patch, a wearable display, a printed circuit, a printed wiring board, a transistor, an antenna, a radio-frequency identification (RFID) tag, a -21 -semitransparent or transparent coating, a light-emitting diode (LED), in particular an organic light-emitting diode (OLED), a solar cell, a capacitor, a sensor, a battery electrode or an organic memory device.

The present invention is further described by the following examples, which are solely for the purpose of illustrating specific embodiments, and are not construed as limiting the scope of the invention in any way.

Examples

A thin silver layer (thickness 500 nm) was deposited on a substrate made of polyurethane by means of spraying a solution of a silver salt, followed by chemical reduction of the silver compound. After drying at 50 °C for 20 min, the substrate was dipped in activated Galinstan. The Galinstan has previously been activated by spraying with a solution of HCI in dry ethanol and remains activated in an inert or HCI saturated atmosphere.

After one second, the substrate was taken out from the bath of Galinstan. Galinstan remains adhered to those portions of the substrate coated with silver. After an alloying time of 1 minute (Sample A) and 10 minutes (Sample B), respectively, excessive Galinstan was removed by means of a squeegee. Sample A remained electrically conductive after stretching by 50%, whereas Sample B remained electrically conductive even after stretching by 200%.

Figure 2 shows experimental results of an XPS measurement of the depth profile of Sample A (1 min) and Sample B (10 min). As can be taken from Figure 2, the content of silver of the stretchable alloy and thus the maximum stretchability (while still being conductive) strongly depends on the alloying time.

While the present invention has been described in detail by way of specific embodiments and examples, the invention is not limited thereto and various -22 -alterations and modifications are possible, without departing from the scope of the invention.