KR20200116294A - Conductive ink and stretchable electrode using the same - Google Patents

Abstract

The present invention relates to a conductive ink for a stretchable electrode and to a stretchable electrode using the same. The conductive ink for a stretchable electrode comprises: a metal conductor; a stretchable polymer; a hydrophobic solvent; a surfactant; and water, and thus forms a conductive ink layer having a porous structure, thereby exhibiting high elasticity as well as excellent conductivity.

Description

Conductive ink and stretchable electrode using the same

The present invention relates to a conductive ink showing excellent conductivity as well as high elasticity by forming a conductive ink layer having a porous structure, and a stretchable electrode using the same.

In order to satisfy consumers' desires for a variety of multifunctional electronic devices such as microelectronic devices, medical devices, and wearable electrical materials, much attention has been focused on developing smaller, thinner, lighter, and elastic materials.

Elastic electronic devices are well attached to curved surfaces such as cloth, organs, or skin, and can be widely used in epidermis and wearable devices, soft robots, and implants.

Recently, there are increasing cases of electrode development for acquiring bio-signals using electrically conductive fibers (threads, fabrics, etc.) with excellent durability.

According to Korean Patent Registration No. 1010810, a fabric-electrode capable of being in close contact with the skin is disclosed. The technology implements a stretchable electrode by weaving a conductive thread in a ribbed knit structure, and if the electrode is placed inside the elastic clothing, the electrode adheres well to the wearer's skin when the wearer wears the clothing. . On the other hand, there is a problem that it is not suitable for the elderly and patients who have to purchase clothes that fit each body shape such as women, men, and children, and are reluctant to wear tight clothing.

In order to solve this problem, recently, a stretchable device is widely used, and most of the stretchable device includes two parts, that is, an active (conductive) layer and a substrate. The substrate is composed of silicone rubber, polyethylene naphthalate, polyurethane polymer, and fluororubber consisting of modulus in the range of several hundred millipascals or megapascals.

Wearing comfort of the stretchable device can be increased by lowering the elastic modulus of the substrate.

Organic based flexible conductors (poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate; poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate) (PEDOT: PSS) or for incorporating carbon-based materials such as hydrogels. Many attempts have been made in recent years, and several groups have proposed promising scientific methods for flexible conductive materials.

Specifically, Electron Device Lett. In 2011, 32 , 1424, a technique showing stretchability of up to 600% at 2.4 Ωcm is disclosed using an Ag flake-polyurethane ink on a polyurethane substrate.

Also, Nat. Mater. 2017, 16 , 834 discloses a printable conductor with a high conductivity of 935 S cm-1 at 400% strain through in situ formation of silver nanoparticles from Ag flakes in fluororubber.

Korean Patent Registration No. 1010810

Electron Device Lett. 2011, 32, 1424 Nat. Mater. 2017, 16, 834

It is an object of the present invention to provide a conductive ink showing excellent conductivity as well as high elasticity by forming a conductive ink layer having a porous structure.

In addition, another object of the present invention is to provide a stretchable electrode using the conductive ink.

In addition, another object of the present invention is to provide an electronic device including a stretchable electrode.

In addition, another object of the present invention is to provide a method of manufacturing the conductive ink.

In addition, another object of the present invention is to provide a method of manufacturing the stretchable electrode.

The conductive ink for a stretchable electrode of the present invention for achieving the above object may include a metal conductor, a stretchable polymer, a hydrophobic solvent, a surfactant, and water.

The conductive ink may be contained in an amount of 10 to 35 parts by weight of a stretchable polymer, 15 to 50 parts by weight of a hydrophobic solvent, 1 to 20 parts by weight of a surfactant, and 1 to 10 parts by weight of water based on 100 parts by weight of the metal conductor.

The metal conductor may have a flake shape having a thickness and a length of 1 to 4 μm, respectively.

The flakes may have a horizontal length and a vertical length of 1: 1-3.

The metal conductor may be at least one selected from the group consisting of silver flakes, gold flakes, copper flakes, and iron flakes.

The stretchable polymer may be at least one selected from the group consisting of ecoflex, polydimethylsiloxane (PDMS), and polyurethane (polyurethane, PU).

The hydrophobic solvent may be at least one selected from the group consisting of methyl isobutyl ketone, propanol, butanol, pentanol, hexanol, ethylene glycol, and propylene glycol.

The surfactant is poly(3,4-ethylene dioxythiophene): poly(4-styrenesulfonate) (poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate), PEDOT:PSS), polyacetylene ), polyparaphenylene, polypyrole, and polyaniline may be at least one selected from the group consisting of.

In addition, the flexible electrode of the present invention comprises a hydrogel layer; An elastic layer formed on the upper surface of the hydrogel layer; And a conductive ink layer on which the conductive ink is formed on an upper surface of the elastic layer.

The thickness of the elastic layer may be 10 to 40 μm.

In addition, the electronic device of the present invention for achieving the another object may include the stretchable electrode.

The electronic device may be a wearable device, a skin device, or a robot.

In addition, the method for producing a conductive ink for a stretchable electrode of the present invention for achieving the another object includes: (A) preparing a first mixture by mixing a metal conductor, a stretchable polymer, and a hydrophobic solvent; (B) preparing a second mixture by mixing a surfactant and water; And (C) preparing a conductive ink by mixing the first mixture and the second mixture; may include.

In addition, a method of manufacturing a stretchable electrode of the present invention for achieving the above another object includes (i) treating the surface by immersing the elastic layer in a benzophenone solution; (Ii) applying a hydrogel to the surface-treated elastic layer and curing it to form an elastic layer/hydrogel layer; (Iii) forming a conductive ink layer by applying the conductive ink to a Teflon substrate; (Iv) separating the conductive ink layer from the Teflon substrate with a tape; (V) attaching the conductive ink layer attached to the tape to the upper surface of the elastic layer of the elastic layer/hydrogel layer cured in (ii); And (vi) removing the tape from the conductive ink layer attached to the tape formed on the upper surface of the elastic layer of the elastic layer/hydrogel layer.

In the step (ii), curing may be performed with UV/O3.

After applying the conductive ink in step (iii), the first heat treatment is performed at 50 to 70° C. for 30 to 90 minutes, and then secondary heat treatment at 100 to 120° C. for 80 to 120 minutes, and then at 130 to 140° C. The third heat treatment may be performed for 120 minutes to form a porous conductive ink layer.

In the present invention, unlike a flat structure for forming a conductive path, by forming a conductive ink layer having a porous structure over the entire stretchable electrode, it is possible to provide a stretchable electrode having high stretchability and conductivity.

1A is a schematic diagram showing a stretchable electrode of the present invention; 1B shows the components of the conductive ink of the present invention; Fig. 1C shows the conductive ink of the present invention; 1D is a photograph of the conductive ink of the present invention transferred onto a substrate; 1E is a photograph of a hybrid substrate of the present invention consisting of a thin elastic layer (indicated by a red dotted box) and a hard hydrogel layer (indicated by a blue dotted box); 1F is a SEM image of the cross-section of the hybrid substrate; 1G is a photograph of the stretchable electrode of the present invention elongated to 1780%.
2A to 2C are views showing a process of manufacturing the stretchable electrode of Example 1; FIG. 2D is a diagram showing an extended substrate of Example 1, Comparative Example 1 (ECO) and Comparative Example 2 (DNHG); 2E is a graph showing a strain-stress curve for the substrates of Example 1, Comparative Example 1 (ECO) and Comparative Example 2 (DNHG); 2F is a graph showing the hysteresis characteristics of the substrates of Example 1, Comparative Example 1 (ECO) and Comparative Example 2 (DNHG); 2G and 2H are views showing the degree of stretching of the conductive ink layer in the stretchable electrodes of Example 1, Comparative Example 1 (ECO) and Comparative Example 2 (DNHG); 2I is a SEM photograph of a cross section of the stretchable electrode of Example 1. In Fig. 2i, the Ag ink layer is strongly attached to the surface of the hybrid film.
3A is a photograph of IPL-treated conductive ink (evaporation of water contained in the hydrogel causes foam formation and cracking of the film surface, see the inset shown by the red dotted line); 3B is a photograph of a sintered conductive ink printed on a hybrid substrate through heat treatment according to the present invention.
4A is a photograph of the conductive ink of the present invention printed on the surface of Teflon; 4B is a graph showing the viscosity of the conductive ink used in FIG. 4A; 4C is a photograph of controlling the flow characteristics of the conductive ink to optimize the resolution (blue dotted box is the conductive ink of the present invention); 4D is a photograph of a conductive ink layer being peeled off from the Teflon surface by a water-soluble tape.
5A is a graph showing the resistance of the conductive ink layer of the stretchable film prepared according to Example 1 (Hybrid), Comparative Example 1 (ECO-T) and Comparative Example 3 (ECO-P); 5B is a graph showing the recovery of the standardized resistance for the conductive ink layer of the three stretchable films; 5C is a graph showing the change in standardized resistance to the conductive ink layer of the stretchable film of Example 1 (Hybrid) during a periodic test at a strain rate of 50%; 5D-F are optical microscopic images of conductive ink layers of stretchable films prepared according to Example 1 (Hybrid), Comparative Example 1 (ECO-T) and Comparative Example 3 (ECO-P); Figure 5g-i is an initial state of the conductive ink layer of the stretchable film prepared according to Comparative Example 3 (ECO-P) (Figure 5g), 400% strain (Figure 5h) and the inside of the red dotted box at high magnification (Figure 5i) Is an SEM image showing; Figures 5j-l show the inside of the red dotted box (Figure 5l) in the initial state (Figure 5j), 400% strain (Figure 5k) and high magnification of the conductive ink layer of the stretchable film prepared according to Example 1 (Hybrid). This is an SEM image.
6 is a photograph (above) in which the elastic conductive ink layer transferred onto the hybrid film of Example 1 was epoxy-laminated on the PET film at a fixed distance of 4 mm (above), and the hybrid film with the elastic conductive ink layer formed up to 75.2 mm This is a photo (bottom) that is stretched (1780%).
7A is a graph showing the change in the standardized resistance for the conductive ink layer of the stretchable film of Comparative Example 3 (ECO-P) at 50% strain; 7B is a graph showing a change in standardized resistance for a conductive ink layer of the stretchable film of Comparative Example 1 (ECO-T).
8A is a photograph of the stretchable film of Example 1 (formation of a conductive ink layer of a KIST pattern on a hybrid film) connected to a commercial green LED; FIG. 8B is a photograph of an LED continuously shining brightly when the stretchable film of Example 1 is stretched; 8C is a schematic diagram showing an image and pressure sensor structure of a smart skin patch (using the stretchable film of Example 1); 8D is a photograph showing a comfortable state and a bending state of a smart skin patch attached to a human hand; 8E is a graph showing the performance of a pressure sensor on a smart skin patch while a finger is moving.
9 is a flowchart showing a process of manufacturing a skin patch.

The present invention relates to a conductive ink showing excellent conductivity as well as high elasticity by forming a conductive ink layer having a porous structure, and a stretchable electrode using the same.

The stretchable electrode of the present invention has a structure in which a conductive ink layer having excellent stretchability is provided on a low modulus hybrid substrate, and exhibits an elongation of 1700 to 1900%.

Hereinafter, the present invention will be described in detail.

The conductive ink for a stretchable electrode of the present invention includes a metal conductor, a stretchable polymer, a hydrophobic solvent, a surfactant, and water.

The metal conductor according to the present invention is a material that imparts conductivity to the stretchable electrode, and is superior to the organic conductor in terms of conductivity, and has a flake shape having a thickness and length of 1 to 4 µm, preferably 1.5 to 2.5 µm, respectively. It is preferable in terms of maintaining excellent conductivity after elongation compared to other shapes such as spherical.

The length of the flakes means a horizontal length and a vertical length, respectively, the length of each of the horizontal and vertical lengths is 1 to 4 ㎛, preferably 1.5 to 2.5 ㎛, while the length ratio of the horizontal length and vertical length is 1: 1 It is -3.

When the thickness and length of the flakes are less than the lower limit of the preferred range, it may be difficult to have conductivity after stretching, and when the thickness and length of the flakes exceed the upper limit of the preferred range, stretching may be hindered.

In addition, if the length ratio of the vertical length based on the horizontal length of the flake is less than the lower limit of the preferred range, it may be difficult to have conductivity after stretching, and if it exceeds the upper limit of the preferred range, it will interfere with the stretching and the conductivity will decrease after stretching. I can.

The metal conductor is not particularly limited as long as it is a flake-shaped conductive material, but preferably, one selected from the group consisting of silver flakes, gold flakes, copper flakes, and iron flakes may be mentioned.

In addition, the stretchable polymer is a material that allows the conductive ink to be excellently stretched, specifically, 1 selected from the group consisting of ecoflex, polydimethylsiloxane (PDMS), and polyurethane (polyurethane, PU). More than a species can be mentioned.

The content of the stretchable polymer is 10 to 35 parts by weight, preferably 20 to 30 parts by weight based on 100 parts by weight of the metal conductor. When the content of the stretchable polymer is less than the lower limit of the preferred range, the conductive ink may not have stretchability, and if it exceeds the upper limit of the preferred range, the hardness may decrease.

The conductive ink of the present invention forms a reverse micelle in the conductive ink by using a stretchable polymer, a hydrophobicity of a solvent, a polarity of a surfactant, and water. Even if the elastic polymer is cured when performing the first heat treatment to cure the conductive ink, water and solvent do not evaporate by the reverse micelle, and the water and solvent evaporate during the second heat treatment to form porosity, and the porous form Since sintering is performed through the tertiary heat treatment while maintaining the sinter, a porous conductive ink can be provided.

The hydrophobic solvent may include at least one selected from the group consisting of methyl isobutyl ketone, propanol, butanol, pentanol, hexanol, ethylene glycol and propylene glycol, and 15 to 50 parts by weight based on 100 parts by weight of the metal conductor , Preferably 20 to 35 parts by weight.

When the content of the hydrophobic solvent is less than the lower limit of the preferred range, a reverse micelle cannot be formed inside the conductive ink, and thus a porous conductive ink cannot be provided, and when the content of the hydrophobic solvent exceeds the upper limit of the preferred range, a large amount of porosity is formed. Hardness may decrease.

In addition, as the surfactant, poly(3,4-ethylene dioxythiophene): poly(4-styrenesulfonate) (poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate), PEDOT:PSS), poly Acetylene (polyacetylene), polyparaphenylene (polyparaphenylene), polypyrole (polypyrole), and one or more selected from the group consisting of polyaniline (polyaniline), 1 to 20 parts by weight based on 100 parts by weight of the metal conductor, preferably Is used in 3 to 8 parts by weight.

In addition, water is used in an amount of 1 to 10 parts by weight, preferably 1 to 5 parts by weight, based on 100 parts by weight of the metal conductor.

When the contents of the surfactant and water are out of the above preferred range, the reverse micelle may not be formed in the conductive ink.

In addition, the present invention can provide a method of manufacturing the conductive ink.

The method for preparing a conductive ink of the present invention comprises the steps of: (A) preparing a first mixture by mixing a metal conductor, a stretchable polymer, and a hydrophobic solvent at normal pressure and room temperature; (B) preparing a second mixture by mixing a surfactant and water at normal pressure and room temperature; And (C) preparing a conductive ink by mixing the first mixture and the second mixture at normal pressure and room temperature.

In the step (A), a metal conductor, a stretchable polymer, and a hydrophobic solvent are mixed to prepare a first mixture.When a surfactant and water are added to the step (A) and mixed together, reverse micelles are formed inside the conductive ink. Therefore, it is preferable to mix the first and second mixtures after mixing the surfactant and water (second mixture) in step (B) separately from step (A).

In addition, the present invention provides a stretchable electrode.

The flexible electrode of the present invention includes a hydrogel layer; An elastic layer formed on the upper surface of the hydrogel layer; And a conductive ink layer on which the conductive ink is formed on an upper surface of the elastic layer.

The conductive ink of the present invention is laminated on a hybrid substrate including a hard hydrogel layer and a thin elastic layer (FIG. 1A). The hard hydrogel is composed of a polymer containing double reticulated moisture, and has an elastic modulus similar to that of human skin and elasticity of 2000% or more, so it is a suitable material for electronic products such as skin.

For interfacial compatibility with the metal conductor of the conductive ink (FIGS. 1B and 1C), an elastic layer (eg, Ecoflex) is coated on the upper surface of the hard hydrogel layer using benzophenone. The elastic layer is formed as a very thin coating layer having a thickness of 10 to 40 µm, preferably 20 to 35 µm on the surface of the hydrogel layer (Fig. 1e, 1f), and has excellent compatibility with the hydrophobic solvent component of the conductive ink, as well as 700%. To show high elasticity.

The stretchable electrode to which the conductive ink is transferred (FIG. 1D) onto the hybrid substrate (hydrogel layer + elastic layer) may have high elasticity of 1700 to 1900% (FIG. 1G).

The stretchable electrode of the present invention may be used in an electronic device, and the electronic device is not particularly limited as long as it is an electronic device capable of using a stretchable electrode, but preferably, a wearable device, a skin device, or a robot may be mentioned.

In addition, the present invention provides a method of manufacturing a stretchable electrode.

The method of manufacturing the stretchable electrode of the present invention comprises the steps of: (i) treating the surface by immersing the elastic layer in a benzophenone solution; (Ii) applying a hydrogel to the surface-treated elastic layer and curing it to form an elastic layer/hydrogel layer; (Iii) forming a conductive ink layer by applying the conductive ink of the present invention to a Teflon substrate; (Iv) separating the conductive ink layer from the Teflon substrate with a tape; (V) attaching the conductive ink layer attached to the tape to the upper surface of the elastic layer of the elastic layer/hydrogel layer cured in (ii); And (vi) removing the tape from the conductive ink layer attached to the tape formed on the upper surface of the elastic layer of the elastic layer/hydrogel layer (FIGS. 2a-c).

First, a material for forming an elastic layer is formed on a thin sheet of polyethylene terephthalate (PET) by spin coating, and then surface treatment is performed by immersing in a benzophenone solution.

Thereafter, a hydrogel solution is dropped on the surface-treated elastic layer, and then cured with ultraviolet (UV)/O3 to crosslink the hydrogel and the elastic layer (FIG. 2A).

Conventionally, the conductive ink is sintered by thermal annealing or intense pulsed light, but when the hybrid film including the hydrogel layer containing moisture is dried by the above two methods, the elasticity is reduced (FIG. 3).

Instead of the conventional method, when the adhesive force of the tape to the conductive ink is stronger than that of the conductive ink to the original substrate by using a transfer method using a tape, preferably a water-soluble tape, the printed conductive ink is applied to the hybrid substrate. Can be attached.

Specifically, the conductive ink of the present invention is first printed on a Teflon substrate (FIG. 4A), and then sintered through thermal annealing, and then the sintered conductive ink layer is separated from the Teflon substrate (FIG. 2B). For example, the sintered conductive ink layer is easily separated from the Teflon substrate by a tape (Fig. 4D), and after the conductive ink layer is adhered to the hybrid substrate, an aqueous solution is dropped on the surface of the water-soluble tape, and the water-soluble tape is removed for several minutes. A conductive ink layer is provided on the hybrid substrate by rapidly dissolving in it (Fig. 2C).

Since the Teflon substrate has a superhydrophobic surface, the printed conductive ink spreads along the edge of the pattern to reduce the resolution. The above problem can be solved by changing the viscosity of the conductive ink. Reducing the amount of the solvent in the conductive ink increases the pattern resolution, and the influence of the viscosity begins to dominate over the ink interface properties within the appropriate viscosity range for printing (Fig. 4b-c).

The conductive ink of the present invention is sintered through three heat treatments. Specifically, the conductive ink applied to the Teflon substrate is first heat treated at 50 to 70° C. for 30 to 90 minutes to evaporate the solvent first, while the elastic polymer Is cured, secondary heat treatment at 100 to 120 °C for 80 to 120 minutes to evaporate water and residual solvent, and third heat treatment at 130 to 140 °C for 80 to 120 minutes to sinter the conductive ink An ink layer is formed.

Hereinafter, a preferred embodiment is presented to aid the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention, It is natural that such modifications and modifications fall within the appended claims.

Example 1.

Conductive ink layer

22.2 parts by weight of Ecoflex's prepolymer (Ecoflex 00-30, Smooth-On), a stretchable polymer, and a crosslinking agent are mixed in a 1:1 weight ratio for 5 minutes, and then, over 10 minutes, a hydrophobic solvent, methyl isobutyl ketone (popular chemical), 33.3 parts by weight Part was added, and then 100 parts by weight of Ag flakes (DSF-500MWZ-S, Daejoo Electronics) were added and stirred for 5 hours to prepare a first mixture. To the first mixture, a second mixture in which 4.4 parts by weight of poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and 2.2 parts by weight of water were mixed was added and stirred for 1 hour. Thus, a conductive ink was prepared.

After printing the conductive ink on a Teflon substrate, a mild heat treatment was performed at 60° C. for 1 hour, heat treatment at 110° C. for 2 hours, and then heat treatment at 130° C. for 2 hours to prepare a sintered conductive ink layer. .

Flexible electrode

A mixture of Ecoflex's prepolymer forming an elastic layer and a crosslinking agent (1:1 weight ratio) for 5 minutes was spin-coated on a PET film (250 μm) at 3000 rpm and then cured overnight for planarization. Subsequently, after surface treatment by immersing Ecoflex/PET in an ethanol solution containing 2% by weight of benzophenone (Sigma Aldrich B9300), the hydrogel solution was poured onto the rectangular Ecoflex layer of the Ecoflex/PET, followed by UV for 5 minutes. Cured with /O3 and left in a moisture box for 1 day. After curing, the Ecoflex layer/hydrogel layer (hybrid substrate) was peeled from the PET. The obtained conductive ink layer was separated from Teflon using a water-soluble tape (3M WST5414), attached to the hybrid substrate, and then removed by dissolving the water-soluble tape in deionized water to remove the elastic electrode (hydrogel layer/elastic layer/conductive ink. Layer) was prepared.

The hydrogel solution used for the stretchable electrode was used as a monomer crosslinked with calcium sulfate (Sigma-Aldrich 31221) by ionic bonds, in order to make a hard hydrogel network, and acrylamide (Sigma-Aldrich W201502). -Aldrich A8887) was used as a monomer to construct a polyacrylamide network. The sodium alginate and acrylamide: deionized water were mixed in a mass ratio of 9:1, and the resulting mixture was stirred for 1 day. Ammonium sulfate (Sigma-Aldrich A9164; 0.0059 X acrylamide weight) was added as a photoinitiator for acrylamide polymerization, and N,N-methylenebisacrylamide (Sigma-Aldrich M7279, 0.0006 X acrylamide weight) was added as a crosslinking agent. I did. After stirring for 1 hour, calcium sulfate (0.06 g) was added to 10 mL of water to crosslink the alginic acid salt, and N,N,N',N'-tetramethylethylenediamine (Sigma-Aldrich T7024, 0.003 x acrylic Amide weight) was used as a crosslinking accelerator of polyacrylamide.

Comparative Example 1. Using only the elastic layer_Omitted the hydrogel layer

In the same manner as in Example 1, the process of forming the hydrogel layer on the elastic layer was omitted to prepare a stretchable film composed of an elastic layer/conductive ink layer.

Comparative Example 2. Only the hydrogel layer is used_elastic layer is omitted

In the same manner as in Example 1, except for the elastic layer, a stretchable film consisting of a hydrogel layer/conductive ink layer was prepared.

Comparative Example 3. Comparative Example 1. Using only the elastic layer_Omitting the hydrogel layer_Direct printing of conductive ink

In the same manner as in Comparative Example 1, a conductive ink layer was prepared separately and not attached to the elastic layer using a tape, but a conductive ink was printed on the upper surface of the elastic layer and then sintered to prepare a stretchable film.

<Test Example>

Test Example 1. Electrode characterization

2A to 2C are views showing a process of manufacturing the stretchable electrode of Example 1; FIG. 2D is a diagram showing an extended substrate of Example 1, Comparative Example 1 (ECO) and Comparative Example 2 (DNHG); 2E is a graph showing a strain-stress curve for the substrates of Example 1, Comparative Example 1 (ECO) and Comparative Example 2 (DNHG); 2F is a graph showing the hysteresis characteristics of the substrates of Example 1, Comparative Example 1 (ECO) and Comparative Example 2 (DNHG); 2G and 2H are views showing the degree of stretching of the conductive ink layer in the stretchable electrodes of Example 1, Comparative Example 1 (ECO) and Comparative Example 2 (DNHG); 2I is a SEM photograph of a cross section of the stretchable electrode of Example 1. In Fig. 2i, the Ag ink layer is strongly attached to the surface of the hybrid film.

As shown in Fig. 2, the degree of stretchability of the hybrid substrate prepared in Example 1 is 400%, which is very similar to the pure Ecoflex and hydrogel of Comparative Examples 1 and 2 (Fig. 2D).

However, the substrates exhibit different elastic modulus and break points (Fig. 2e). Because the elastic layer coated on the surface of the hydrogel layer is very thin, the tensile modulus of the hydrogel layer (0.004 Nmm-2) is almost the same as that of the hybrid substrate (hydrogel layer/elastic layer) (0.005 Nmm-2). . The tensile modulus (0.69 N·mm-2) of the Ecoflex layer of Comparative Example 1 was 170 times higher than that of the hybrid substrate of Example 1 and the hydrogel layer of Comparative Example 2. The lower the elastic modulus of the substrate, the more conformal contact with the curved surface of the skin is ensured, and skin comfort during exercise is guaranteed.

The hysteresis phenomenon of the substrate occurs slowly after stretching at an elongation of 300%. As a result, the recovery characteristics of the hybrid substrate (Example 1) and the hydrogel layer (Comparative Example 2) were found to be good compared to the recovery characteristics of the Ecoflex layer (Comparative Example 1) (Fig. 2f).

In addition, since the Ag conductive material contains hydrophobic components such as Ag flakes and stretchable polymers, it exhibits hydrophobicity, but the wet surface of the hydrogel maintains hydrophilicity.

The thin elastic layer (Ecoflex layer) of the present invention plays a very important role in the processed super-decomposable conductive material (Fig. 2g, h). In general, the elastic layer is used and compatible with a hydrophobic conductive material that increases in proportion to the elongation of the substrate (ECD in Figs. 2g and h). After attaching the hydrogel layer to the elastic layer, the conductive ink did not exhibit elongation during stretching due to a slip motion on the substrate surface (DNHG in FIGS. 2G and h). Therefore, it was confirmed that the thin elastic layer coated on the hydrogel layer increased the slip resistance of the conductive ink layer (Hybrid of FIGS. 2g and h).

A cross-sectional SEM image of the stretchable electrode manufactured according to Example 1 including all three components is shown in FIG. 2I. It is shown that a thin elastic layer positioned between the Ag conductive ink layer and the hard hydrogel layer is in good contact with both layers.

Test Example 2. Elasticity test

5A is a graph showing the resistance of the conductive ink layer of the stretchable film prepared according to Example 1 (Hybrid), Comparative Example 1 (ECO-T) and Comparative Example 3 (ECO-P); 5B is a graph showing the recovery of the standardized resistance for the conductive ink layer of the three stretchable films; 5C is a graph showing the change in standardized resistance to the conductive ink layer of the stretchable film of Example 1 (Hybrid) during a periodic test at a strain rate of 50%; 5D-F are optical microscopic images of conductive ink layers of stretchable films prepared according to Example 1 (Hybrid), Comparative Example 1 (ECO-T) and Comparative Example 3 (ECO-P); Figure 5g-i is an initial state of the conductive ink layer of the stretchable film prepared according to Comparative Example 3 (ECO-P) (Figure 5g), 400% strain (Figure 5h) and the inside of the red dotted box at high magnification (Figure 5i) Is an SEM image showing; Figures 5j-l show the inside of the red dotted box (Figure 5l) in the initial state (Figure 5j), 400% strain (Figure 5k) and high magnification of the conductive ink layer of the stretchable film prepared according to Example 1 (Hybrid). This is an SEM image.

The conductive ink layer (FIGS. 5E and 5F) attached by the transfer method has a smaller amount of residual ink at the edge of the pattern compared to the conductive ink layer (FIG. 5D) printed directly.

6 is a photograph (above) in which the elastic conductive ink layer transferred onto the hybrid film of Example 1 was epoxy-laminated on the PET film at a fixed distance of 4 mm (above), and the hybrid film with the elastic conductive ink layer formed up to 75.2 mm. This is a photo (bottom) that is stretched (1780%).

7A is a graph showing the change in the standardized resistance for the conductive ink layer of the stretchable film of Comparative Example 3 (ECO-P) at 50% strain; 7B is a graph showing a change in standardized resistance for a conductive ink layer of the stretchable film of Comparative Example 1 (ECO-T).

As shown in FIG. 5, it was confirmed that the elongation of the conductive ink layer of Comparative Example 3 (ECO-P) by stretching reached 500%, while its relative resistance (ΔR/R0) was about 19.2. In addition, the maximum elongation of the conductive ink layer of Comparative Example 1 (ECO-T) reached 600%, which is a maximum of 100% higher than that of the conductive ink layer of Comparative Example 3 (ECO-P). The difference observed at the elongation point of the two conductive ink layers is due to the difference in their manufacturing method. That is, in Comparative Example 3 (ECO-P), since the conductive ink is printed on the Ecoflex substrate and then sintered, the Ecoflex substrate is in an overcured state, whereas in Comparative Example 1 (ECO-T), the Ecoflex substrate is not affected by the heat treatment. . The substrate on which the conductive ink layer was formed was damaged faster than the conductive ink layer. Therefore, the maximum stretchability of the elastic Ag conductive ink layer could be investigated.

In the present invention, a hybrid substrate (Example 1) was selected to determine the maximum stretchability of the elastic Ag conductive ink layer.

Since the elastic modulus and elongation point of the hybrid substrate are almost the same as the elastic modulus and elongation point of the hydrogel, the conductive ink layer could be elongated to the breaking point of 1780% in the initial state (Table 1), and the resistivity thereof was shown in Comparative Examples 1 and 3 Compared to the 400% strain suffered the smallest change.

The elastic Ag conductive ink layer on the hybrid substrate was stretched to 1780% while attached to the PET film (Fig. 6). As shown in Figure 6, the position of the PET film increased from 4 mm (top) to 75.2 mm (bottom) during the stretch test. It should also be noted that the recovery of resistance is an important property of conductive materials.

As shown in Fig. 5B, the conductive ink layers of the three different stretchable electrodes recovered slowly as a result of monitoring the shape of the resistance change observed after 100% stretching. The resistance recovery of Example 1 (hybrid substrate) was more reliable than that of Comparative Example 1 (ECO-T) and Comparative Example 3 (ECO-P), and it was confirmed that the form of resistance recovery was different.

This phenomenon was assumed using a different deformation mechanism.

In the case of Comparative Example 3 (ECO-P), the conductive ink layer follows the deformation releasing operation of the elastic layer Ecoflex, but the transferred conductive ink layer is inconsistent with the substrate due to physical contact.

The results of the cycle test performed at a strain rate of 50% confirmed that excellent reliability was exhibited in the conductive ink layer of Example 1 for 1000 cycles (FIG. 5C). It was confirmed that the cycle characteristics of the conductive ink layers of Comparative Example 1 (ECO-T) and Comparative Example 3 (ECO-P) tested under the same conditions were not excellent in reliability during 1000 cycles, as shown in FIGS. 7A and 7B. I did.

5D-F shows an optical microscope image to determine the resolution of the pattern obtained with the conductive ink layer, and the conductive ink layer of Comparative Example 3 (ECO-P) printed directly was due to diffusion of ink during screen printing. Residues form on both sides of the pattern. Compared with the conductive ink layer of Comparative Example 3 (ECO-P), the current degree was low due to removal during the transfer process as in the transferred conductive ink layer (FIGS. 5E and 5F). The conductive path is formed by the penetration of Ag flakes inside the matrix.

Therefore, the influence of the Ag flake content on the penetration properties of the system was investigated in detail in the elastic layer.

The conductive ink layer of Example 1 exhibited electrical performance at a high strain rate of 600%. However, when the content of Ag flakes is lower than that in Example 1, the degree of penetration is weakened during stretching, and when deformed by 250%, the standardized resistance of the conductive ink layer rapidly increases (but the film remains as it is), and Ag flakes When the content of is higher than that of Example 1, the electrical performance reached the maximum, but due to the relatively high rigidity of the conductive ink layer due to aggregation of Ag flakes, it could be stretched only up to 400% elongation.

As shown in Fig. 5G-i, the conductive ink layer of Comparative Example 3 (ECO-P) reached an elongation of 400%, and then a cavity was formed inside the conductive ink layer while maintaining a constant conductivity.

The rearrangement property of the elastic conductive ink layer formed on the Ecoflex substrate of Comparative Example 3 (ECO-P) was smaller than that of the hybrid substrate of Example 1. Therefore, the average distance between the Ag flakes was greater on the Ecoflex substrate (Fig. 5G, H) than the hybrid substrate (Fig. 5J, 5K) at the same strain of 400% (magnified SEM images of the two conductive ink layers are Figs. 5I and 5L, respectively).

The Ag flakes in the conductive ink layer prevented repositioning the conduction path under the assumption that the resistance to stretch is greater in Ecoflex.

Test Example 3. Use of flexible electrode

8 is a diagram to which an LED wiring and an elastic electrode manufactured for a smart skin patch are applied.

8A is a photograph of the stretchable film of Example 1 (formation of a conductive ink layer of a KIST pattern on a hybrid film) connected to a commercial green LED; FIG. 8B is a photograph of an LED continuously shining brightly when the stretchable film of Example 1 is stretched; Figure 8c is a schematic diagram showing an image of a smart skin patch (using the stretchable film of Example 1) and a pressure sensor structure (the sensor is composed of a microstructured Au thin film and a PEDOT ink film connected to the Ag conductive ink layer wire on the surface of the hybrid substrate) Is; 8D is a photograph showing a comfortable state and a bending state of a smart skin patch attached to a human hand (the conductive ink layer is deformed while being bent); 8E is a graph showing the performance of the pressure sensor on the smart skin patch while the finger is moving (blue arrows indicate the pressure sensor).

As shown in Fig. 8, in order to demonstrate the applicability of the conductive ink layer, it was used to manufacture the LED wiring in the shape of a "KIST" pattern (Fig. 8A). It was confirmed that the green LED emits bright light even after stretching the patterned conductive ink layer to 200% strain (FIG. 8B).

In addition, the elastic conductive ink is deposited on a biocompatible skin-like patch comprising a simple compression sensor (a structure consisting of a microstructured gold electrode transferred onto a hybrid substrate and an ink-based PEDOT electrode is shown in Fig. 8C).

A conductive ink layer was applied by selecting a finger that can deform as much as possible on the body, and the conductive ink layer was completely unfolded when the bent finger returned to a circle (FIG. 8D). Nevertheless, in this position the pressure sensor remained fully operational.

These results show that the fabricated sensor can be easily and comfortably placed on human skin because of the relatively low elastic modulus of the patch platform.

According to Fig. 8E, good reliability of the stretchable device is shown in real time because force is applied to the patch before and after deformation of the finger.

In the present invention, the excellent elasticity of the conductive ink layer was demonstrated in the hybrid low modulus substrate composed of a thin elastic layer and a hydrogel layer. The conductive ink containing a metal conductor and a stretchable polymer could be stretched to a high elongation of 1780%.

Therefore, the conductive ink layer having excellent elasticity was successfully attached to the low modulus hydrogel layer through a thin elastic layer.

The hybrid substrate of the present invention ensures comfortable movement of the fabricated patch, and thus can be attached to human skin. The stretchable conductive ink layer on the hybrid substrate is expected to be used in various fields requiring a low elastic modulus platform such as a skin patch sensor network and implantable medical devices.

-Characterization and stretching test of conductive ink-

Stretching and cycling tests were performed using a self-made machine with the settings shown in FIG. 6. The conductive ink layer was fixed on the PET film with an epoxy resin to prevent slipping.

The components of the resistance probe and motor fixture were designed by Rhinoceros (Robert McNeel & Associates) and printed on a 3D printer (Anartz Engine). The existing stepper motor was operated by the Arduino Uno, and the custom transform program was written using the C programming language.

To measure the electrical resistance, a round tip test pin (P75-D), a digital multimeter (NI USB-4065) and LabVIEW (National Instrument) were used, and a draw speed of 8 mm·min-1 was used.

The rheological viscosity and strain-stress curves were measured at the Korea Polymer Testing Institute.

-Smart skin patch production-

9 is a flowchart showing a process of manufacturing a skin patch.

The linear elastic conductive ink was transferred to the glove-type hybrid substrate using a water-soluble tape, and the PEDOT ink was deposited on the substrate using the transfer method of the elastic conductive ink.

The microstructured poly(dimethylsiloxane) (PDMS) (Dow Corning Sylgard 184) film was fabricated using a pyramid-patterned silicon mold, and Au/Cr was deposited on the PDMS film using a thermal evaporator.

Claims (16)

A conductive ink for a stretchable electrode comprising a metal conductor, a stretchable polymer, a hydrophobic solvent, a surfactant, and water. The method of claim 1, wherein the conductive ink is contained in an amount of 10 to 35 parts by weight of a stretchable polymer, 15 to 50 parts by weight of a hydrophobic solvent, 1 to 20 parts by weight of a surfactant, and 1 to 10 parts by weight of water based on 100 parts by weight of the metal conductor. Conductive ink for flexible electrodes, characterized in that. The conductive ink for a stretchable electrode according to claim 1, wherein the metal conductor has a flake shape having a thickness and a length of 1 to 4 μm, respectively. The conductive ink for a stretchable electrode according to claim 3, wherein the flakes have a horizontal length and a vertical length in a length ratio of 1:1. The conductive ink for a stretchable electrode according to claim 1, wherein the metal conductor is at least one selected from the group consisting of silver flakes, gold flakes, copper flakes, and iron flakes. The conductive ink for a stretchable electrode according to claim 1, wherein the stretchable polymer is at least one selected from the group consisting of ecoflex, polydimethylsiloxane (PDMS), and polyurethane (polyurethane, PU). The conductive ink for a stretchable electrode according to claim 1, wherein the hydrophobic solvent is at least one selected from the group consisting of methyl isobutyl ketone, propanol, butanol, pentanol, hexanol, ethylene glycol and propylene glycol. The method of claim 1, wherein the surfactant is poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate):poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate), PEDOT:PSS ), polyacetylene (polyacetylene), polyparaphenylene (polyparaphenylene), polypyrole (polypyrole) and polyaniline (polyaniline) at least one selected from the group consisting of a conductive ink for a stretchable electrode, characterized in that. Hydrogel layer;
An elastic layer formed on the upper surface of the hydrogel layer; And
A conductive ink layer comprising the conductive ink of any one of claims 1 to 8 on the upper surface of the elastic layer. The stretchable electrode of claim 9, wherein the elastic layer has a thickness of 10 to 40 µm. An electronic device comprising the stretchable electrode of claim 9. The electronic device of claim 11, wherein the electronic device is a wearable device, a skin device, or a robot. (A) preparing a first mixture by mixing a metal conductor, a stretchable polymer, and a hydrophobic solvent;
(B) preparing a second mixture by mixing a surfactant and water; And
(C) preparing a conductive ink by mixing the first mixture and the second mixture; a method for producing a conductive ink for a stretchable electrode comprising: (i) treating the surface by immersing the elastic layer in a benzophenone solution;
(Ii) applying a hydrogel to the surface-treated elastic layer and curing it to form an elastic layer/hydrogel layer;
(Iii) forming a conductive ink layer by applying the conductive ink of any one of claims 1 to 8 to a Teflon substrate;
(Iv) separating the conductive ink layer from the Teflon substrate with a tape;
(V) attaching the conductive ink layer attached to the tape to the upper surface of the elastic layer of the elastic layer/hydrogel layer cured in (ii); And
(Vi) removing the tape from the conductive ink layer attached to the tape formed on the upper surface of the elastic layer of the elastic layer/hydrogel layer; and a method of manufacturing a stretchable electrode comprising: a. The method of claim 14, wherein the curing in step (ii) is performed with UV/O3. The method of claim 14, wherein after applying the conductive ink of any one of claims 1 to 8 in step (iii), the first heat treatment is performed at 50 to 70 °C for 30 to 90 minutes, and then 80 After secondary heat treatment for 120 to 120 minutes, a third heat treatment is performed at 130 to 140° C. for 80 to 120 minutes to form a porous conductive ink layer.

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