Gel-metal composite and production method for gel-metal composite

Abstract

A gel-metal composite (1) comprising a hydrogel (2) and a metal thin film (3) formed on one surface (2a) of the hydrogel (2), wherein the hydrogel (2) and the metal thin film (3) are directly bonded.

Description

Gel-metal composite and method for producing gel-metal composite

[0001] The present invention relates to a gel-metal composite and a method for producing the gel-metal composite.

[0002] Hydrogels are polymeric materials that contain large amounts of liquid, and have characteristics similar to biological tissues, such as high water content and flexibility. Metals have characteristics such as high conductivity, high rigidity, and catalytic activity. Therefore, gel-metal composites, which are composite materials of hydrogels and metals, possess the characteristics of both hydrogels and metals. Gel-metal composites are expected to be applied to soft electronics and materials engineering, such as the creation of electronic devices by metal wiring on hydrogels and the creation of anisotropic soft materials by increasing the local elastic modulus of the metal attachment area.

[0003] As a method for producing a gel-metal composite, for example, a method of introducing an adhesive into the composite interface between a hydrogel and a metal is known (see, for example, Non-Patent Document 1).

[0004] In the field of soft electronics, a method is known in which a composite material of an elastomer material and a metal is produced by transferring a metal thin film formed on a solid substrate onto an elastomer material without using an adhesive (see, for example, Non-Patent Document 2).

[0005] Shiyuan Wei, Rongkang Yin, Tao Tang, Yingxiao Wu, Yang Liu, Puxin Wang, Kai Wang, Ming Mei, Ruqiang Zou, Xiaojie Duan, “Gas-Permeable, Irritation-Free, Transparent Hydrogel Contact Lens Devices with Metal-Coated Nanofiber Mesh for Eye Interfacing”, ACS Nano 2019,13,7920-7929.MATTHEW A.MEITL,ZHENG-TAO ZHU,VIPAN KUMAR,KEON JAE LEE,XUE FENG,YONGGANG Y.HUANG,ILESANMI ADESIDA,RALPH G.NUZZO,JOHN A.ROGERS,“Transfer printing by kinetic control of adhesion to an elastomeric stamp”,Nature Materials,5(1),33-38,(2006).

[0006] To simplify the manufacturing process of gel-metal composites, it is desirable to directly bond the composite interface between the hydrogel and metal, but because the hydrogel contains water, it is difficult to directly bond the hydrogel and metal.

[0007] In view of the above circumstances, an object of the present invention is to provide a technique that enables a metal thin film to be directly attached to any desired portion of the surface of a hydrogel, regardless of the type of material.

[0008] One aspect of the present invention is a gel-metal composite comprising a hydrogel and a metal thin film formed on one surface of the hydrogel, the hydrogel and the metal thin film being directly bonded to each other.

[0009] One aspect of the present invention is a method for producing a gel-metal composite, comprising the steps of: providing a mask seal having an opening on one surface of a solid substrate; applying a metal coating to an exposed portion of the one surface of the solid substrate that is exposed in the opening, thereby forming a metal thin film on the exposed portion; removing the mask seal from the one surface of the solid substrate; and providing a hydrogel so as to cover the one surface of the solid substrate and the metal thin film; drying the hydrogel; and swelling the dried hydrogel in a liquid.

[0010] According to the present invention, it is possible to directly attach a metal thin film to any desired portion of the surface of a hydrogel, regardless of the type of material.

[0011] 1 is a cross-sectional view schematically showing a gel-metal composite according to one embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing a method for producing a gel-metal composite according to one embodiment of the present invention. FIG. 3 is a cross-sectional view schematically showing a method for producing a gel-metal composite according to one embodiment of the present invention. FIG. 4 is a cross-sectional view schematically showing a method for producing a gel-metal composite according to one embodiment of the present invention. FIG. 5 is a cross-sectional view schematically showing a method for producing a gel-metal composite according to one embodiment of the present invention. FIG. 6 is a cross-sectional view schematically showing a method for producing a gel-metal composite according to one embodiment of the present invention. FIG. 7 is a cross-sectional view schematically showing a method for producing a gel-metal composite according to one embodiment of the present invention. FIG. 8 is a cross-sectional view schematically showing a method for producing a gel-metal composite according to one embodiment of the present invention. FIG. 9 is a photograph of an acrylamide gel-gold composite obtained in an example. FIG. 10 is a photograph of an acrylamide gel-gold composite obtained in a comparative example. FIG. 11 shows the results of measuring the electrical resistance by the four-electrode method on the surface of the gold thin film and the surface of the acrylamide gel for the acrylamide gel-gold composite obtained in the example.

[0012] The present invention will be described below based on preferred embodiments with reference to the drawings, but the present invention is not limited to these embodiments. Note that the drawings used in the following description may show essential parts enlarged for the sake of convenience in order to make the features of the present invention easier to understand, and the dimensional proportions of each component may not necessarily be the same as those in reality.

[0013] [Gel-metal composite] Fig. 1 is a cross-sectional view showing a gel-metal composite according to one embodiment of the present invention. As shown in Fig. 1, the gel-metal composite 1 of this embodiment has a hydrogel 2 and a metal thin film 3 formed on one surface 2a of the hydrogel 2.

[0014] The hydrogel 2 and the metal thin film 3 are directly bonded at their composite interface without the use of an adhesive.

[0015] The hydrogel 2 is not particularly limited, but examples thereof include hydrogels composed of synthetic polymers such as polyacrylamide and polyvinyl alcohol, and biopolymers such as gelatin and alginic acid. Other examples of the hydrogel 2 include copolymer gels such as a combination of acrylamide and aminophenylboronic acid, and gels having an interpenetrating network structure such as a combination of polyvinyl alcohol and polyaniline. Other examples of the hydrogel 2 include functional gels in which gold nanoparticles or carbon nanotubes are supported on the aforementioned gels.

[0016] There are no particular limitations on the metal that constitutes the thin metal film 3, but metals with high chemical stability are preferred, such as gold and platinum.

[0017] The thin metal film 3 may be divided into small pieces on one surface 2a of the hydrogel 2. This increases the surface area of ​​the thin metal film 13.

[0018] Alternatively, another metal thin film 3 may be bonded via a metal directly bonded to the one surface 2 a of the hydrogel 2. That is, a metal such as gold or platinum may be directly bonded to the one surface 2 a of the hydrogel 2, and the metal thin film 3 may be bonded onto that metal.

[0019] According to the gel-metal composite 1 of this embodiment, by providing a metal thin film 3 on a portion of one surface 2a of the hydrogel 2, it becomes possible to apply electrical stimulation or receive electrical signals using the hydrogel 2, thereby enabling use as a soft electronics substrate. Furthermore, the gel-metal composite 1 of this embodiment can be used as an electrically operable cell culture substrate because it allows localized electrical stimulation or reception of electrical signals to the hydrogel 2.

[0020] The gel-metal composite 1 according to this embodiment is unlikely to experience peeling of the metal thin film 3 due to frictional forces on the surface or expansion and contraction of the hydrogel 2, and therefore can be treated as a highly reliable composite. Furthermore, because the thickness of the metal thin film 3 can be reduced to a few nanometers, the metal thin film 3 can be provided in a very small area on one surface 2a of the hydrogel 2. Furthermore, because the hydrogel 2 and the metal thin film 3 are directly bonded without the use of an adhesive, material design that takes advantage of the transparency of the hydrogel 2 and the metal thin film 3 becomes possible. The gel-metal composite 1 according to this embodiment has a metal thin film 13 (on the order of GPa) with a high elastic modulus provided on the surface of a hydrogel 14 (on the order of kPa) with a low elastic modulus, and therefore can be used as a soft material that undergoes characteristic deformation due to the difference in the mechanical properties of the metal thin film 13 and the hydrogel 14.

[0021] [Method for producing a gel-metal composite] A method for producing a gel-metal composite according to one embodiment of the present invention comprises the steps of providing a mask seal having an opening on one surface of a solid substrate (hereinafter referred to as the "first step"), applying a metal coating to an exposed portion of one surface of the solid substrate that is exposed in the opening, and forming a metal thin film on the exposed portion (hereinafter referred to as the "second step"), removing the mask seal from one surface of the solid substrate, and providing a hydrogel so as to cover one surface of the solid substrate and the metal thin film (hereinafter referred to as the "third step"), drying the hydrogel (hereinafter referred to as the "fourth step"), and swelling the dried hydrogel in a liquid (hereinafter referred to as the "fifth step").

[0022] 2 to 7 are cross-sectional views that schematically show a method for producing a gel-metal composite according to one embodiment of the present invention.

[0023] 2, in the first step, a solid substrate 11 is prepared to determine the surface shape and metal portion of the gel-metal composite. The solid substrate 11 is not particularly limited, but may be, for example, glass or a polymer film.

[0024] Next, as shown in FIG. 2, a mask seal 12 having an opening 12a is attached to the portion of one surface 11a of the solid substrate 11 where no metal thin film is to be provided.

[0025] 3, a metal coating is applied to the exposed portion 11b of the one surface 11a of the solid substrate 11 that is exposed in the opening 12a, thereby forming a metal thin film 13 on the exposed portion 11b. The metal thin film 13 can be formed on the exposed portion 11b by sputtering or vapor deposition.

[0026] "Third Step" As shown in FIG. 4, the mask seal 12 is removed from the one surface 11a of the solid substrate 11, and a hydrogel 14 is provided so as to cover the one surface 11a of the solid substrate 11 and the metal thin film 13.

[0027] The method for providing the hydrogel 14 is not particularly limited, but any of the following methods can be used: for example, a method in which a gel precursor solution is dropped onto the metal thin film 13 and the gel precursor solution is gelled on the spot by light irradiation or the like; or a method in which a gel prepared elsewhere is swelled to a desired swelling ratio and then placed on the metal thin film 13.

[0028] 5, the hydrogel 14 covering the surface 11a of the solid substrate 11 and the thin metal film 13 is dried. Drying increases the density of the polymer bond chains that make up the hydrogel 14, and the adhesive strength between the thin metal film 13 and the hydrogel 14 is strengthened by hydrophobic interactions. Drying of the hydrogel 14 can be accelerated by heating or reducing pressure.

[0029] "Fifth Step" As shown in Figure 6, the dried hydrogel 14 is swollen in a liquid. By swelling the hydrogel 14 in the liquid, the hydrogel 14 is changed to a shape suitable for practical use. Note that the swollen state of the hydrogel 14 is the practical shape. The above-mentioned hydrophobic interaction is further enhanced when the hydrogel 14 is swollen in the liquid.

[0030] Examples of the liquid used to swell the hydrogel 14 include water, a urea solution, a polymer solution, and an ionic liquid.

[0031] 7, when the hydrogel 14 is peeled off from the solid substrate 11, the metal thin film 13 is transferred to the hydrogel 14. In this way, the gel-metal composite 1 shown in FIG.

[0032] It is desirable that the surface of the hydrogel 14 in contact with the metal thin film 13 (the interface between the metal thin film 13 and the hydrogel 14) not be deformed during the drying of the hydrogel 14 in the fourth step and the swelling of the hydrogel 14 in the fifth step. In order to perform the fourth step and the fifth step without deforming the interface between the metal thin film 13 and the hydrogel 14, it is desirable to carry out the following treatment.

[0033] By applying a force perpendicular to the interface between the metal thin film 13 and the hydrogel 14 or by physically fixing the end of the hydrogel 14 in position, deformation in the direction of the interface between the metal thin film 13 and the hydrogel 14 can be suppressed.

[0034] An adhesion molecule layer is formed on the surface of a glass substrate as the solid substrate 11, and the hydrogel 14 is formed directly on the adhesion molecule layer, thereby suppressing deformation of the hydrogel 14. In this case, the glass substrate used as the solid substrate 11 can be removed by treating it with hydrofluoric acid or by severing the bonds within the adhesion molecules that make up the adhesion molecule layer.

[0035] By changing the components of the solution contained in the hydrogel 14 beforehand and the solution used in the swelling process after drying, the swelling rate of the hydrogel 14 can be controlled, and deformation due to drying can be suppressed.

[0036] The surface of the thin metal film 3 of the gel-metal composite 1 obtained by the method for producing a gel-metal composite of this embodiment is connected to an electrode and electrolytic plating is performed, thereby making it possible to increase the thickness of the thin metal film 3. Furthermore, the thickness of the thin metal film 3 can be increased by performing electroless plating on the surface of the thin metal film 3 of the gel-metal composite 1 obtained by the method for producing a gel-metal composite of this embodiment.

[0037] Furthermore, by arranging two gel-metal composites 1 so that their respective hydrogels 2 are in contact with each other, and sandwiching these gel-metal composites 1 in the thickness direction and performing dry adhesion, a film-like hydrogel having metal thin films 3 on both sides can be obtained.

[0038] Furthermore, by forming a hydrogel on the metal thin film 3 of the gel-metal composite 1, a hydrogel with the metal thin film 3 embedded therein can be obtained.

[0039] By selecting the material of the solid substrate 11 or by performing a surface treatment, the interaction between the solid substrate 11 and the hydrogel 14 can be reduced, thereby facilitating the peeling of the metal thin film 13 from the solid substrate 11 .

[0040] According to the method for producing a gel-metal composite of this embodiment, the metal thin film 13 and the hydrogel 14 are directly bonded to each other, which makes production easy, and the strong adhesion makes it possible to obtain a gel-metal composite 1 that is highly stable against external invasion.

[0041] According to the method for producing a gel-metal composite of this embodiment, by providing a thin metal film 13 on one surface of a swellable hydrogel 14 before it swells, the swelling of the hydrogel 14 causes the thin metal film 13 to be divided into small pieces, increasing the surface area of ​​the thin metal film 13. This improves the metal catalytic activity of the thin metal film 13 and maintains the permeability of the hydrogel 14, resulting in a gel-metal composite 1 that functions as a substrate surface having both the material properties of the thin metal film 13 and the hydrogel 14. On the other hand, if a metal is attached to the surface of the swollen hydrogel 14, electricity can be passed through the gel-metal composite 1 in liquid via the metal.

[0042] According to the method for producing a gel-metal composite of this embodiment, by providing a metal thin film 13 (on the order of GPa) with a high elastic modulus only at an arbitrary location on the surface of a hydrogel 14 (on the order of kPa) with a low elastic modulus, a gel-metal composite 1 is obtained that can be used as a soft material that undergoes characteristic deformation due to the difference in mechanical properties between the metal thin film 13 and the hydrogel 14.

[0043] According to the method for producing a gel-metal composite of the present embodiment, the method is composed of only steps that allow multiple processes to be performed simultaneously, such as forming the metal thin film 13 and drying the hydrogel 14, and therefore it is easy to scale up the production of the gel-metal composite 1.

[0044] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0045] [Example] As an example of a gel-metal composite, an acrylamide gel-gold composite having an acrylamide gel as the hydrogel and a gold thin film as the metal thin film will be described.

[0046] Example: "Preparation of an Acrylamide Gel-Gold Composite" A mask seal was attached to one surface of a solid glass substrate, exposing the metal-attached area (exposed glass area). Gold was then sputtered onto the metal-attached area to form a thin gold film on the entire surface of the glass substrate. Next, the mask seal was removed from the glass substrate, and an acrylamide gel that had been previously formed and stored in water was placed over the entire surface of the glass substrate and the thin gold film. The acrylamide gel was prepared by irradiating a gel precursor solution containing acrylamide as a monomer, bisacrylamide as a crosslinker, and lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate as a polymerization initiator with ultraviolet light in a low-oxygen environment. Next, the acrylamide gel covering the entire surface of the glass substrate and the thin gold film was dried. Next, the dried acrylamide gel was swelled in water, transferring the thin gold film from the glass substrate to the surface of the acrylamide gel (the surface in contact with the glass substrate), yielding an acrylamide gel-gold composite. A photograph of the resulting acrylamide gel-gold composite is shown in Figure 8. As shown in FIG. 8, the gold thin film formed on the exposed glass portion was transferred to the acrylamide gel.

[0047] Comparative Example "Preparation of an Acrylamide Gel-Gold Composite" Gold sputtering was performed without using a mask seal to form a thin gold film over the entire surface of a glass substrate. Next, an acrylamide gel similar to that used in the Examples, which had been previously formed and stored in water, was placed on the glass substrate so as to cover the thin gold film. The acrylamide gel covering the thin gold film was then dried. The dried acrylamide gel was then swelled in water, transferring the thin gold film from the glass substrate to one surface of the acrylamide gel (the surface in contact with the glass substrate), thereby obtaining an acrylamide gel-gold composite. A photograph of the resulting acrylamide gel-gold composite is shown in Figure 9. As shown in Figure 9, the thin gold film was transferred over the entire surface of the acrylamide gel.

[0048] [Evaluation] For the acrylamide gel-gold composite obtained in the examples, the electrical resistance was measured by the four-electrode method on both the surface of the gold thin film and the surface of the acrylamide gel. The results are shown in Figure 10. The results shown in Figure 10 indicate that the electrical resistance of the acrylamide gel surface was approximately 1,000 times higher than that of the surface of the gold thin film. This demonstrates that conductivity can be imparted to only a specific portion of the acrylamide gel surface.

[0049] The gel-metal composite according to the present invention is useful as a soft material that takes advantage of the high electrical conductivity, catalytic properties, and high rigidity of metals, and is widely applicable to fields such as soft electronics and materials engineering.

[0050] 1 Gel-metal composite 2 Hydrogel 3 Metal thin film

Claims

1. A gel-metal composite comprising a hydrogel and a thin metal film formed on one surface of the hydrogel, wherein the hydrogel and the thin metal film are directly bonded to each other.

2. A method for producing a gel-metal composite, comprising the steps of: providing a mask seal having an opening on one surface of a solid substrate; applying a metal coating to an exposed portion of the one surface of the solid substrate that is exposed in the opening, thereby forming a metal thin film on the exposed portion; removing the mask seal from the one surface of the solid substrate, and providing a hydrogel so as to cover the one surface of the solid substrate and the metal thin film; drying the hydrogel; and swelling the dried hydrogel in a liquid.

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