Bioelectrodes and Their Manufacturing Methods

Abstract

The present invention aims to provide a bio-electrode and a method for manufacturing the same. The bio-electrode is fabric-shaped, soft, versatile, and has excellent durability, and suppresses the decrease in conductivity caused by washing. The main objective of the present invention is a bio-electrode and a method for manufacturing the same. The bio-electrode has a laminated structure comprising a fibrous substrate containing non-conductive fibers and a conductive material. The conductive material contains carbon black and a urethane resin. The carbon black content is 15% to 35% by mass relative to the conductive material. The carbon black is dispersed in particles at least on the surface of the conductive material. On the surface of the conductive material, the ratio of the longest distance to the shortest distance (longest distance / shortest distance) between the carbon black particles and adjacent carbon black particles is 1 to 20, and the wet friction fastness of the conductive material surface is grade 4 or higher.

Description

Bioelectrodes and Their Manufacturing Methods This invention relates to a bioelectrode and a method for manufacturing the same. Bioelectrodes used to measure electrical signals of the human body or animal, such as electroencephalograms, electrocardiograms, and electromyograms, or to apply electrical stimulation to the organism, include various types such as gel electrodes, rubber electrodes, electrodes using thin metal plates, and electrodes using conductive fiber materials. The gel electrode and rubber electrode have the advantages of being flexible, closely contacting the body surface of the organism, and being able to stably acquire biological signals. However, on the other hand, they have problems such as poor air permeability and causing rashes due to stuffiness at the contact point. Furthermore, electrodes utilizing thin metal plates are hydrophobic and rigid, which presents a problem of low adaptability in applications involving contact with the soft, moist surfaces of living organisms. High contact pressure or the use of conductive pastes (jelly) is required to achieve tight contact with the surface. As electrodes that can be directly attached to the surface of a living organism without the use of conductive pastes, conductive fabric-shaped electrodes are considered effective, and various proposals have been put forward. Fabric electrodes have strong resistance to bending and can be shaped to fit the unevenness of the body surface. In fabric-shaped electrodes, to impart conductivity, electrodes coated with conductive polymers, those using metal fibers, or those using carbon materials have been proposed. As an electrode using conductive polymers, an electrode impregnated with a conductive polymer (poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate, PEDOT-PSS) and an olefin binder in a polyester fiber substrate has been proposed (Patent Document 1). As electrodes using metal fibers or carbon materials, the following electrodes have been proposed: an electrode formed by distributing carbon black and silicone rubber on a substrate using stainless steel threads (Patent Document 2); an electrode formed by impregnating a substrate with carbon black (Patent Document 3); and an electrode formed by printing a conductive ink containing carbon black on a substrate (Patent Document 4). [Prior Art Documents] [Patent Documents] Patent Document 1: International Publication No. 2017 / 183463; Patent Document 2: Japanese Patent Publication No. 2006-512128; Patent Document 3: Japanese Patent Application Publication No. 2020-180406; Patent Document 4: Japanese Patent Publication No. 2017-512542. [Problems to be Solved by the Invention] The bioelectrode described in Patent Document 1 is made by impregnating PEDOT-PSS and an olefin binder into a polyester fiber substrate with a single yarn fineness (below 500 nm). By carrying a conductive polymer and binder between the fine single fibers, the conductivity of the electrode cross-section is improved. The olefin material contained in the bioelectrode has excellent release properties and weak adhesion to the fiber substrate, thus allowing for improvement in wash durability. In addition, PEDOT-PSS used as a conductive polymer is expensive, posing a problem in terms of versatility. The bioelectrode described in Patent Document 2 has a substrate containing conductive yarns such as stainless steel mesh or a substrate containing conductive wires. In addition, these bioelectrodes exemplify the use of carbon black and silicone rubber (silicone gum). Compared with substrates containing non-conductive fibers, substrates containing conductive yarns such as stainless steel mesh and substrates containing conductive wires tend to have poor flexibility. Furthermore, the silicone material contained in these bioelectrodes has excellent interfacial properties for easy release. Therefore, these bioelectrodes are easily separated from the skin during use, and the position of the bioelectrodes may shift due to the wearer's movements during biosignal acquisition, potentially causing artifacts in bioelectrodes that acquire or input weak signals. On the other hand, the bioelectrode described in Patent Document 3 has a structure formed by impregnating inexpensive carbon black and a urethane adhesive together in a fiber substrate. Since the conductive resin is impregnated in the fiber substrate, there is a problem with the stability of conductivity on the electrode surface in contact with the skin; the difference in impedance between the electrodes before and after washing increases, and there is room for improvement. The bioelectrode described in Patent Document 4 has a structure formed by printing conductive ink containing carbon black and a urethane adhesive onto an elastic material. Because a soft conductive ink is used, the flexibility is sufficient; however, the conductive material of these bioelectrodes contains a large number of conductive particles, resulting in poor friction resistance and a tendency for color migration. In view of the above, the objective of this invention is to provide a bio-electrode and a method for manufacturing the same, wherein the bio-electrode is in the shape of a fabric, is soft and highly versatile, has excellent durability, and suppresses the decrease in conductivity caused by washing. [Means for Solving the Problem] The structure of the invention used to solve the aforementioned problem and achieve the objective is any one of the following. (1) A bioelectrode having a laminated structure comprising a fiber substrate containing non-conductive fibers and a conductor, the conductor containing carbon black and a urethane resin, the carbon black content being 15% to 35% by mass relative to the conductor, the carbon black being dispersed in particulate form at least on the surface of the conductor, the ratio of the longest distance to the shortest distance (longest distance / shortest distance) of the carbon black particles to adjacent carbon black particles on the surface of the conductor being 1 to 20, and the wet friction fastness of the surface of the conductor being grade 4 or higher. (2) The bioelectrode as described in (1), wherein the area ratio of the surface of the conductor with an in-plane resistivity of 4 logΩ to 6 logΩ as measured by scanning spreading resistance microscopy (SSRM) is 8% to 18%. (3) The bioelectrode as described in (1) or (2), wherein the difference in inter-electrode impedance before and after 20 washes based on Japanese Industrial Standards (JIS) L0217 (1995) 103 is less than 0.2 kΩ. (4) A method for manufacturing a bioelectrode, comprising the bioelectrode as described in any one of (1) to (3) by the following steps: mixing and degassing a solution comprising carbon black and urethane resin stirred using a homogenizer using a planetary ball mill to obtain a coating solution; coating the coating solution onto one side of a fiber substrate comprising non-conductive multifilament fibers and drying it. [Effects of the Invention] The present invention provides a bio-electrode that is fabric-shaped, soft and highly versatile, with excellent durability, and suppresses the decrease in conductivity caused by washing. The following provides a detailed description of embodiments of the bioelectrode of the present invention. However, the present invention is not limited to these embodiments. <Bioelectrode> The bioelectrode of the present invention has a laminated structure comprising a fiber substrate containing non-conductive fibers and a conductor, wherein the conductor serves as the contact surface with the organism, thereby acquiring and transmitting biological signals. Here, the laminated structure may be a laminated structure formed by directly laminating the fiber substrate and the conductor, or it may be a laminated structure of three or more layers, including a mixed layer in which fibers and conductors coexist between the fiber substrate and the conductor. <Conductor> In this invention, a conductor is a material that exhibits conductivity. The conductor mentioned herein is a formulation used to impart conductivity to a bio-electrode, comprising (A) carbon black and (B) a urethane resin. Furthermore, the formulation and conductor may contain other components besides (A) carbon black and (B) a urethane resin. Examples of other components include (C) a water-based thickener, (D) a conductivity enhancer, (E) a softening agent, (F) a surfactant and / or leveling agent, a crosslinking agent, a catalyst, and an antifoaming agent, but are not limited thereto. The conductivity mentioned herein is defined as having the property that, when the surface resistance of the bio-electrode is measured by the method described later, it is less than 1 × 10⁻⁶. 10 Ω is the surface resistivity value. <(A) Carbon Black> The carbon black used in this invention (hereinafter sometimes referred to as CB) is a conductive material. When using CB to impart conductivity, it is important to consider the particle size of the matrix forming CB (the particle size of the particles constituting the structure, hereinafter also referred to as primary particles and the particle size as primary particle size), the chain structure of the primary particles, and the particle surface properties. In the case of preparing a solution containing CB and a binder, it is particularly important to increase the specific surface area by modifying the particle surface properties. A large specific surface area means that there are many pores on the particle surface, and the binder can enter the pores and reduce the interparticle distance of CB dispersed in the conductor in a particle-like manner (described later), which helps to obtain high conductivity. Specific surface area can be determined by the Brunauer-Emmett-Teller (BET) method. The optimal BET specific surface area for carbon black is 400 m². 2 / g~2000 m 2 / g or more. The BET specific surface area of ​​carbon black is even better at 600 m². 2 / g~1600 m 2 / g. The BET specific surface area of ​​carbon black is 400 m². 2 Above / g, the interparticle distance becomes sufficiently small, reaching a range where sufficient conductivity can be imparted to biological electrodes, at 600 m 2 Higher conductivity can be obtained at concentrations above / g. This is achieved by setting the upper limit to 2000 m. 2Below a certain value (e.g.), the structure can be maintained, enabling stable conductivity in bioelectrodes. The BET specific surface area of ​​carbon black is 1600 m². 2 The effect becomes more pronounced below a certain value (e.g.). Commercially available carbon black products with the aforementioned BET surface area include: Lion paste W-310A, Lion paste W-311N, Lion paste W-376R, Lion paste W-370C, etc. Furthermore, to achieve high conductivity, it is ideal for the primary particles of the bioelectrode (CB) to be small. The average particle size of the primary particles of the CB is preferably in the range of 1 nm to 200 nm, more preferably in the range of 5 nm to 100 nm. By setting it to 1 nm or more, the bioelectrode's stability during friction becomes significant; at 5 nm or more, high stability can be achieved. By setting it to 200 nm or less, sufficient conductivity is imparted to the bioelectrode, and further, by setting it to 100 nm or less, high conductivity can be achieved. The primary particle size is obtained by observing an extremely thin slice cut from the bioelectrode using a transmission electron microscope (TEM) at any magnification where the primary particle size can be observed, calculating the diameter of the circumscribed circle for any selected number of primary particles, and then averaging these values. Commercially available carbon black products having the aforementioned average particle size include: Lion paste W-310A, Lion paste W-311N, Lion paste W-376R, Lion paste W-370C, etc. In the bio-electrode of the present invention, the content of (A) carbon black, relative to 100% by mass of the solid component of the conductor, needs to be 15% to 35% by mass. Preferably, the content of (A) carbon black, relative to 100% by mass of the solid component of the conductor, is 20% to 30% by mass. The higher the carbon black content, the better the conductivity; however, if the content increases, the adhesion due to friction decreases, leading to contamination and reduced performance in actual use. By setting it to 15% by mass or more, the continuity of carbon black allows the bio-electrode to maintain conductivity even after washing; when it is 20% by mass or more, the conductivity is further improved. Furthermore, by setting the content of CB to 35% by mass or less, and further to 30% by mass or less, the conductivity of the conductor under friction can be obtained. The term "solid component" as used herein refers to the components used to form the conductor, excluding the solvent. Furthermore, the carbon black is typically dispersed in a particle-like manner within the conductor. To achieve high conductivity, the carbon black is dispersed in a particle-like manner at least on the surface of the conductor. When evaluating the interparticle distance between particles (I) of a CB on the conductor surface and particles of adjacent CBs, the ratio of the longest distance to the shortest distance (longest distance / shortest distance) needs to be between 1 and 20. Preferably, the ratio of the longest distance to the shortest distance is between 1 and 18. As mentioned herein, the carbon black can be dispersed as structural units or as secondary particles formed by structural aggregation. The dispersed structure or a group of secondary particles is considered in evaluating the shortest and longest distances. The shortest and longest distances are defined as the distance within a 360-degree radius around any CB particle on the conductor surface that is closest to an adjacent CB particle, and the farthest distance that is furthest. A smaller ratio (longest distance / shortest distance) means a smaller difference between the shortest and longest distances, and indicates that the CBs are uniformly dispersed. When the shortest distance and the longest distance are equal, the ratio becomes 1, and the closer to this, the better. By setting the ratio of the longest distance to the shortest distance (longest distance / shortest distance) to 20 or less, carbon black is uniformly dispersed on the conductive surface, thereby efficiently forming conductive paths, thus improving conductivity, reducing the amount of carbon black used, and achieving a balance between friction resistance and conductivity. Furthermore, by setting the ratio to 18 or less, the continuity of carbon black can be maintained after washing, and conductivity suitable for use as a bioelectrode can be obtained. When the ratio of the longest distance to the shortest distance is within the aforementioned range, it can be achieved by the manufacturing method described later. Moreover, to obtain good conductivity, it is preferable that the shortest distance between carbon black particles is in the range of 10 nm to 25 nm, and the longest distance is in the range of 10 nm to 500 nm. <(B) Carbamate Resin> The conductor contains (A) carbon black and (B) carbamate resin. Here, (B) carbamate resin acts as an adhesive to hold the formulation constituting the conductor onto the fibrous substrate. The adhesive is highly relevant to the softness of the bioelectrode, therefore, it is important that the adhesive is a carbamate resin. Compared to resins other than carbamate resins, carbamate resins have superior softness; therefore, the bioelectrode of the present invention is soft and provides excellent wearing comfort. (B) The urethane resin prevents the formulations constituting the conductor in the bioelectrode from detaching from the fiber substrate, thereby reducing the separation of the bioelectrode from the skin and improving biosignal acquisition. In this respect, the inclusion of urethane resin in the conductor is one of the features of this invention. In a conductor, the adhesive may consist solely of a urethane resin or may contain one or more other resins. Preferred examples of other resins include: olefin resins, polyester resins, urethane resins, epoxy resins, silicone resins, vinyl chloride resins, nylon resins, and acrylic resins. Uraffinate resins are classified according to the polyols in their raw materials as ether-based, ester-based, carbonate-based, modified polyols, or ester / carbonate-based, or polymers combining these, etc., but all of these are acceptable. From the viewpoint of hydrolysis resistance, it is particularly preferred to include at least one from the group consisting of ether-based and carbonate-based urethane resins. The following are examples of commercially available products that can be used as urethane resins. Specifically, examples of ether-based urethane resins include "Resamine (registered trademark) D-2040" (Dai Nippon Seika), "SuperFlex (registered trademark) E-4800" (Daiichi Kogyo Pharmaceutical), examples of carbonate-based urethane resins include "EVAFANOL (registered trademark) HA-107C" (Nichika Chemical), "Resamine (registered trademark) D-6300" (manufactured by Dai Nippon Seika), "Resamine (registered trademark) D-6065NP" (Dai Nippon Seika), and "SuperFlex (registered trademark) 460" (manufactured by Daiichi Kogyo Pharmaceutical), and examples of polyether / carbonate-based urethane resins include "Resamine (registered trademark) D-4080" (Dai Nippon Seika) and "Resamine (registered trademark) D-4200" (Dai Nippon Seika). Carbamate resins include those derived from aromatic and aliphatic isocyanates. However, from the perspective of toxicity and yellowing, carbamate resins containing aliphatic isocyanates are ideal for bioelectrodes that come into contact with the skin and require durability. Furthermore, among carbamate resins containing aliphatic isocyanates, aliphatic isocyanate carbonate-based carbamates exhibit higher hydrolysis resistance and flexibility. In the bioelectrode of the present invention, from the viewpoint of the flexibility of the obtained bioelectrode, the glass transition temperature (Tg) of the urethane resin is preferably -60°C to ±0°C. The glass transition temperature (Tg) is determined based on the following method: First, the resin solution is poured into a stainless steel bath and dried at 60°C for 1 hour, and then dried at 120°C for 2 hours to prepare a film with a thickness of approximately 0.3 mm. Using this film, the dynamic viscoelasticity from -100°C to 200°C is measured using a viscoelasticity measuring device DMS6100 (manufactured by Seiko Instruments, Ltd.), and the peak temperature (T°C) of the obtained loss viscoelastic coefficient is taken as the glass transition temperature (Tg). Furthermore, the measurement is performed at a heating rate of 5°C / minute and a measurement frequency of 1 Hz. Furthermore, from the viewpoint of the abrasion resistance of the obtained bioelectrode, the tensile strength of the urethane resin monomer is preferably 5 MPa to 50 MPa. By having a tensile strength of 5 MPa or more for the urethane resin monomer, the bioelectrode can be firmly resisted by friction, while by having a tensile strength of 50 MPa or less for the urethane resin monomer, the fabric-like softness of the bioelectrode can be maintained more reliably. In the bioelectrode of the present invention, the content of the carbamate resin is not particularly limited, but is preferably 65% ​​to 85% by mass, and more preferably 70% to 80% by mass, relative to 100% by mass of the solid content of the conductor. By having a carbamate resin content of 65% by mass or more, the (A) carbon black contained in the bioelectrode is less likely to detach. On the other hand, if the carbamate resin content is 85% by mass or more, the distance between the carbon black particles in the bioelectrode becomes longer, making it impossible to ensure stable conductivity. <Fiber Substrate> The fibers used in the fiber substrate constituting the bioelectrode of the present invention are non-conductive fibers, not metal fibers such as stainless steel, aluminum, aluminum alloy, or copper, nor metal film fibers coated with metals such as silver, nor conductive fibers spun from resins mixed with metals or carbon. The term "non-conductive" here refers to the property that, when the surface resistivity of the fiber substrate is measured by the method described in the embodiments described later, it exhibits a value of 1 × 10⁻⁶. 10 The surface resistivity is above Ω. Non-conductive fibers are easier to obtain than conductive fibers, resulting in excellent manufacturability of the bioelectrode of this invention. The non-conductive fibers constituting the fiber substrate used in this invention can be natural fibers or chemical fibers. Examples of natural fibers include cellulose fibers such as cotton or linen, and protein fibers such as wool or silk. Examples of chemical fibers include regenerated fibers such as rayon, semi-synthetic fibers such as acetate, and synthetic fibers. From a processability point of view, the fibers constituting the fiber substrate used in this invention are preferably synthetic fibers. Examples of synthetic fibers include: nylon, polyamide fibers such as polyaramid, polyester fibers such as polyethylene terephthalate, acrylic fibers such as polyacrylonitrile, polyolefin fibers such as polyethylene or polypropylene, polyvinyl alcohol fibers, polyvinyl chloride fibers, polyurethane fibers, and heterocyclic polymer fibers such as polyimide fibers. From the viewpoint of imparting greater flexibility to bioelectrodes, it is particularly preferable to use one or more of polyamide fibers, polyester fibers, and polyolefin fibers. In the bioelectrode of the present invention, the cross-sectional shape of the fiber constituting the fiber substrate can be a circular cross-section, a triangular cross-section, a flat cross-section, a polygonal cross-section, a hollow type, or other irregular cross-sections with high degree of irregularity, and there is no particular limitation. Examples of fiber substrates for this invention include: netting, papermaking, textiles, woven fabrics, nonwoven fabrics, ribbons, ropes, etc. Any form appropriate to the intended use is acceptable and is not particularly limited; however, textiles are ideal from the viewpoint of conductivity and processability. In the case of textiles, ideally, the combined cover factor (calculated by multiplying the square root of the fineness by the yarn density) for both warp and weft yarns should be between 1500 and 3000. If the cover factor is low, there is a risk that the coating agent may reach the back side without the fiber substrate functioning as an insulating layer, and the coating liquid reaching the back side may adhere to the manufacturing equipment, resulting in inconsistent coating coverage. On the other hand, fabrics with a cover factor exceeding 3000 not only have reduced weaveability but also increased stiffness and decreased softness. The fiber can be any of the following forms: monofilament yarn, multifilament yarn, or cotton-like yarn. There are no particular restrictions on the fineness of a single fiber; for example, it can range from about 0.0001 dtex to about 300 dtex. For fiber substrates, multiple treatments such as fiber winding, napping, shrinkage, shape fixing, compression, dyeing finishing, oil imparting, heat fixing, solvent removal, shape fixing agent removal, combing, polishing, flat (roller) pressing, and high-performance short fiber pleating (nailing cutting) can be implemented in appropriate combinations in each step. However, the implementation is not limited as long as it does not impair the performance of the electrode. Furthermore, the surface resistivity of the fiber substrate is preferably 1 × 10⁻⁶ when the surface resistivity of the fiber substrate monomer is measured by the method described later. 10 Ω or higher. If conductive yarns or wires such as stainless steel mesh are used to reduce the resistivity of the fiber substrate monomers, so that the surface resistivity is 1×10⁻⁶, then... 10 Below Ω, when a conductor is applied, the electrodes become rigid, potentially hindering clothing comfort. Furthermore, if the fiber substrate itself is conductive, it cannot be flexibly used as an insulating layer. (The surface resistivity is 1×10⁻⁶.) 18 For fiber substrates with a resistivity of Ω or higher, fluoropolymers can be used, but there are concerns about reduced coating properties of processing agents. Therefore, the surface resistivity of the fiber substrate is preferably 1×10⁻⁶. 18 Below Ω. Furthermore, the surface resistivity of the fiber substrate monomer can be controlled by selecting the polymer forming the fiber substrate and by limiting the application of conductive materials during or after substrate formation. <Wet Friction Resistance of Conductor Surface> The wet friction resistance of the conductive surface of the bioelectric electrode of the present invention needs to be level 4 or higher. By setting it to level 4 or higher, carbon black shedding due to friction during wearing or washing can be suppressed. This wet friction resistance can be achieved by controlling the carbon black content relative to the conductor within the aforementioned range and uniformly dispersing the carbon black in the adhesive. <Area Ratio of In-Plane Resistivity of Conductive Surface> The area ratio of the highly conductive region with an in-plane resistivity of 4 logΩ to 6 logΩ, obtained by measuring the conductive surface of the bioelectric electrode of the present invention using scanning spreading resistance microscopy (SSRM), is preferably 8% to 18%. By setting the area ratio to 8% or higher, electrical signals from the bio-electrode can be obtained stably and sufficiently without the need for measures that increase wearing pressure or compromise comfort. By setting it to 18% or lower, the carbon black, as the outermost conductive component, is uniformly dispersed, resulting in excellent wet rubbing fastness and minimal changes in electrical properties before and after washing. The area ratio can be controlled by using a specified amount of carbon and dispersing it uniformly. <Interelectrode Impedance> Preferably, the difference in interelectrode impedance before and after 20 washes according to JIS L0217 (1995) 103 is 0.2 kΩ or less. While resistance or resistivity depends on the amount of conductive component remaining in the overall conductor, interelectrode impedance depends on the amount of conductive component remaining in the outermost layer of the conductor; therefore, the effect of shedding caused by washing becomes more significant. In obtaining detailed biological information using medical bioelectrodes, it is important not only to maintain a low interelectrode impedance but also to minimize changes in interelectrode impedance before and after washing. By ensuring the difference in interelectrode impedance before and after 20 washes is 0.2 kΩ or less, accuracy during repeated wear can be maintained. Furthermore, by setting the difference in interelectrode impedance before and after 50 washes according to JIS L1930 (2014) C4G to 0.2 kΩ or less, accuracy during repeated wear can be further maintained. The impedance difference between the electrodes can be controlled by using a specified amount of carbon and dispersing it evenly. <Other formulations> As described above, in the bioelectrode of the present invention, in addition to (A) carbon black and (B) urethane resin, the conductor may also contain (C) aqueous thickener, (D) conductivity enhancer, (E) softening agent, (F) surfactant and / or leveling agent, crosslinking agent, catalyst, defoamer and other components. Among them, (C) the aqueous thickener can be used to control the coating thickness and the degree of penetration of the coating liquid into the fiber substrate when applying a coating liquid for forming a conductor onto a fiber substrate. By imparting viscous properties to the solution (coating liquid) formed from the formulation constituting the conductor, the thickness of the conductor can be controlled, and a bioelectrode with superior conductivity can be obtained. From the perspective of reducing volatile organic compounds (VOCs) emitted during manufacturing, water-based thickeners are typically used after being dissolved in water. One type of water-based thickener may be used, or two or more may be used in combination. Examples of water-based thickeners include: inorganic thickeners such as silicates and montmorillonite; organic thickeners such as: cellulose-based thickeners such as carboxymethyl cellulose, methyl cellulose, and hydroxyethyl cellulose; vinyl-based thickeners such as polyvinyl alcohol, polyvinylpyrrolidone, and polyvinyl benzyl ether copolymers; polyacrylic-based thickeners such as polyacrylic acid or polyacrylate, poly(meth)acrylic acid-(meth)acrylate copolymers, and cross-linked nucleocarboxylic acid emulsions; polyurethane-based thickeners such as polyether-modified urethane compounds and hydrophobically modified polyoxyethylene polyurethane copolymers; and urea-based thickeners such as urethane-urea compounds. Among them, polyacrylic acid compounds can impart both thickening and thixotropic properties to coating liquids, making them suitable for use. Coating liquids endowed with thickening and thixotropic properties by polyacrylic acid compounds can achieve excellent properties such as preventing sedimentation during storage and improving workability by reducing viscosity during application. In the bioelectrode of the present invention, the content of the aqueous thickener is not particularly limited, but is preferably 0.1% to 10% by mass, more preferably 0.5% to 5% by mass, relative to 100% by mass of the solid content of the conductor. By having an aqueous thickener content of 0.1% by mass or more, the coating solution of the conductor forming the bioelectrode can be given thickening and thixotropic properties, thereby improving the coating properties. By setting it to 10% by mass or less, poor coating caused by excessive thickening can be suppressed, which is therefore preferred. <Manufacturing Method of Bioelectrode> The bioelectrode of the present invention can be manufactured, for example, according to the following process. First, a fiber substrate containing non-conductive fibers is prepared using non-conductive multifilament fibers or the like. A coating solution is obtained by mixing and degassing a solution containing a carbon black dispersion stirred in a homogenizer and a carbamate resin using a planetary ball mill; and by coating the coating solution onto one side of the fiber substrate containing the non-conductive fibers and drying it. This process produces a bioelectrode having a multilayer structure of a fiber substrate containing non-conductive fibers and a conductor. More preferably, after stirring the carbon black dispersion in a homogenizer, the carbon black dispersion is degassed using a vacuum dryer, and then mixed and degassed with the carbamate resin using a planetary ball mill. This further improves the dispersibility of the carbon black. If the uniformity of the carbon black is insufficient when preparing the bioelectrode, the stirring and mixing conditions are adjusted to improve dispersibility. Furthermore, as the coating liquid, it is preferable to use a dispersion or solution containing (A) carbon black and urethane resin, and apply it to the fiber substrate. Furthermore, in this specification, without specifically distinguishing between those that completely dissolve all components contained in the conductor (i.e., "solvent") and those that disperse insoluble components (i.e., "dispersion medium"), all are referred to as "solvent". The solvent will be described below. The solvent mentioned is not particularly limited, but may include: water; alcohols such as methanol, ethanol, 2-propanol, 1-propanol, and glycerol; ethylene glycols such as ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol; glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, diethylene glycol monomethyl ether, ethylene glycol diethyl ether, and diethylene glycol dimethyl ether; and glycol ethers such as ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, and diethylene glycol monobutyl ether acetate. Esters and esters; propylene glycol derivatives such as propylene glycol, dipropylene glycol, and tripropylene glycol; propylene glycol ether derivatives such as propylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, propylene glycol dimethyl ether, dipropylene glycol diethyl ether, and dipropylene glycol diethyl ether; propylene glycol ether acetate derivatives such as propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, dipropylene glycol monomethyl ether acetate, and dipropylene glycol monoethyl ether acetate; tetrahydrofuran; acetone; acetonitrile, etc. These solvents can be used alone or in combination of two or more. The solvent is preferably water, or a mixture of water and an organic solvent. When water is used as the solvent, the water content is not particularly limited, but is preferably 10% to 1000% by mass relative to 100% by mass of the solid content of the conductor, and more preferably 20% to 500% by mass. By setting the water content to 10% by mass or more, the fluidity of the formulation in the solution is ensured, resulting in good operability; by setting it to 1000% by mass or less, the concentration of the formulation will not become too low, and the amount of coating solution used will not increase excessively. Ideally, carbon black should be uniformly dispersed in the solvent beforehand. Methods for uniformly dispersing carbon black in the solvent include, for example, mechanical stirring using ball mills, bead mills, planetary ball mills, vibratory ball mills, sand mills, colloid mills, grinding mills, roller mills, high-speed impeller dispersers, dispersers, homogenizers, high-speed impact mills, ultrasonic dispersers, stirring blades, agitators, etc., with stirring using a homogenizer being preferred. When using commercially available carbon black dispersions, it is also preferable to stir the carbon black using a homogenizer before use to achieve a dispersion suitable for the present invention. Furthermore, in the coating solution prepared by mixing carbon black dispersion and carbamate resin, it is ideal to maintain the dispersibility of carbon black and to eliminate air bubbles. A planetary ball mill is preferred as a method for mixing and degassing the carbon black dispersion and carbamate resin. After uniformly dispersing the carbon black using a homogenizer, mixing and degassing it with the carbamate resin using a planetary ball mill yields a coating solution with uniformly dispersed carbon black. By using this coating solution, highly conductive bioelectrodes can be obtained even with reduced carbon black usage. In this invention, when the coating liquid is applied to the fiber substrate, a conventional method that allows for single-sided coating, such as a pipe coater or a blade coater, is sufficient. A pipe coater is preferred for achieving a uniform coating thickness. After application, drying is preferably performed by evaporating the solvent using heat. After drying, washing or high-temperature treatment can be performed to remove impurities. By applying the coating liquid to the fiber substrate and then drying it, a bioelectrode with a laminated structure comprising a non-conductive fiber substrate and a conductive material can be obtained. The resulting bioelectrodes are soft and exhibit minimal changes in conductivity due to washing. Therefore, they not only provide excellent acquisition of bioelectrical signals but are also comfortable to wear, making them suitable for measuring brain waves or bioelectrical signals such as electrocardiograms and electromyograms. [Example] Next, the bioelectrode of the present invention will be described in detail with reference to examples. The bioelectrode of the present invention is not limited to these examples. The measured values ​​in the examples and comparative examples were obtained by the following methods. <Determination of the distance between carbon black particles and calculation of the ratio of the shortest to the longest distance> The distance between carbon black particles was determined by measuring an extremely thin section cut from the conductive surface of a bio-electrode using a transmission electron microscope (TEM). The ultrathin slices are cut with a thickness twice that of the primary particle size of the CB. Furthermore, when the primary particle size of the CB is unknown, ultrathin slices are preliminarily cut with multiple thicknesses for surface observation. The largest identifiable particle diameter is set as a temporary primary particle size, and ultrathin slices are cut with thicknesses of 2, 3, and 4 times this temporary primary particle size for surface observation. If the difference in the largest identifiable particle diameter in each image is less than 10 nm, the average of these diameters is set as the primary particle size to determine the thickness of the ultrathin slice to be cut and the field of view to be observed. If the difference in diameter between the images exceeds 10 nm, the following operation is repeated: the largest particle diameter is set as a temporary primary particle size, and ultrathin slices are cut with thicknesses of 2, 3, and 4 times this particle size for surface observation. In an image acquired using a TEM with a field of view 40 times the primary diameter of a particle (CB), three CB particles are randomly selected from a region centered at the intersection of the image's diagonals and with a radius three times the primary diameter of the particle. For each selected particle, the distance from its outer edge to the outer edge of adjacent CB particles is measured within a 360-degree radius. The distance to the nearest particle is defined as the shortest distance, and the distance to the farthest particle as the longest distance. For the three selected particles, the average of the obtained shortest distances is defined as the average shortest distance, and the average of the obtained longest distances is defined as the average longest distance. The ratio of the average longest distance to the average shortest distance is defined as the ratio of the shortest distance to the longest distance (longest distance / shortest distance). Furthermore, the "adjacent CB particles" are CB particles whose outer edges are at least partially contained within the image. <Wet Friction Resistance> For bioelectrodes, according to JIS L 0849 (1996) (wherein, the standard of 100 rounds of friction was changed to 30 rounds of friction), a wet test was conducted using a friction tester type II (electro-vibration type) (the test piece was placed under standard conditions for more than 4 hours, and the white cotton cloth used for friction was moistened with water to a state of about 100% wetness). Regarding the determination of coloration, a grade of 1 to 5 was determined by the gray scale determination of contamination. <Area Ratio with In-Plane Resistance of 4 logΩ to 6 logΩ> The area ratio of the conductive surface of the bioelectric electrode of the present invention with an in-plane resistance of 4 logΩ to 6 logΩ was determined as follows. Using a scanning stretching resistance microscope (SSRM), a voltage was applied to the conductive surface of the bioelectric electrode from the back side of the test sample, and the conductivity of a 60 μm × 60 μm region on the surface of the sample was observed using a conductive probe. Using image processing software (GIMP 2.8 portable), the area ratio of the portion with an in-plane resistance of 4 logΩ to 6 logΩ in the measured region was determined. At this time, measurements were performed on three randomly selected parts of the surface, depending on the number of observations. The average of the area ratios obtained for each of the three parts was calculated and set as "the area ratio of the in-plane resistance of the conductive surface with an in-plane resistance of 4 logΩ to 6 logΩ". Observation Apparatus: NanoScope Iva AFM (atomic force microscope) manufactured by Bruker AXS Digital Instruments. Dimension: 3100-stage AFM system + SSRM option. SSRM Scanning Mode: Contact mode with simultaneous measurement of stretching resistance. SSRM Probe (Tip): Diamond-coated silicon cantilever probe. Model: DDESP-FM (manufactured by Bruker AXS). Ar Ion Beam Processing Apparatus: IM-4000 manufactured by Hitachi High-Tech Co., Ltd. Accelerating Voltage: 3 kV. <Washing Method> Washing is performed using the following two methods: 1. Wash 20 times using JIS L0217 (1995) 103 method, then hang dry. 2. Wash 50 times using JIS L1930 (2014) C4G method, then hang dry. <Interelectrode Impedance> The conductive parts of the bio-electrodes are brought into contact with each other, and a pressure of 31.3 g per square centimeter is applied. The interelectrode impedance (kΩ) is measured according to American National Standards Institute (ANSI) 4.2.2.1. Three measurements are performed, and the average value is taken as the interelectrode impedance. <Surface Resistance of Fiber Substrate> Regarding the fiber substrate of the bioelectrode, the surface resistance (Ω) was measured at 500 V for 60 seconds using a surface resistivity meter (Megaresta H0709, manufactured by SHISHIDO electrostatics) at 20°C and 40%RH. Measurements were taken at three points and averaged. [Example 1] Plain weave fabric was woven using 84dT-72F polyester filament (polyethylene terephthalate, PET) for both warp and weft yarns. The obtained fabric was scouring and washing (at 50°C for 10 minutes) with an aqueous solution of 0.5 g / L surfactant and 0.7 g / L sodium hydroxide (at 80°C for 20 minutes), followed by heat setting (at 180°C for 1 minute) using a dry heat treatment machine to obtain a fiber substrate with a fabric density of 180 warp threads / 2.54 cm and 94 weft threads / 2.54 cm. The carbon black dispersion "Lion Paste (registered trademark) W-376R" (manufactured by Lion Specialty Chemicals, 12.5% ​​solids) was stirred for 15 minutes using a homogenizer. After degassing using a vacuum dryer, 408 g / L of the carbon black dispersion was mixed with 389 g / L of the aqueous carbamate resin "SuperFlex (registered trademark) 460" (an aliphatic isocyanate carbonate carbamate manufactured by Daiichi Kogyo Pharmaceutical Co., Ltd., 38% solids), 11 g / L of the aqueous thickener "Printgen NFV" (manufactured by Matsui Shiki Chemical Co., Ltd., 45% solids), and 1 L of ion-exchanged water as a solvent. The mixture was stirred and degassed using a planetary ball mill for 90 seconds to obtain the coating solution. The obtained coating solution was applied to the fiber substrate using a tubular coating machine, heated at 130°C, washed with water, and then set at 170°C to obtain a conductive coating weight of 70 g / m. 2 Bioelectrodes. The components used and the characteristics of the resulting bioelectrodes are shown in Table 1. [Example 2] Except for changing the amount of carbon black dispersion to 326 g / L and aqueous urethane resin to 416 g / L, the same treatment as in Example 1 was performed to produce a conductive coating of 70 g / m. 2 Bioelectrodes. The components used and the characteristics of the resulting bioelectrodes are shown in Table 1. [Example 3] Except for changing the usage to 538 g / L carbon black dispersion and 346 g / L aqueous urethane resin, the same treatment as in Example 1 was performed to produce a conductive coating of 70 g / m. 2 Bioelectrodes. The components used and the characteristics of the resulting bioelectrodes are shown in Table 1. [Example 4] Except for changing the dosage to 277 g / L carbon black dispersion and 432 g / L aqueous urethane resin, and changing the stirring and degassing process using a planetary ball mill to 45 seconds, the same treatment as in Example 1 was performed to produce a conductive coating of 70 g / m. 2 Bioelectrodes. The components used and the characteristics of the resulting bioelectrodes are shown in Table 1. [Example 5] Except for changing the amount of carbon black dispersion to 571 g / L and aqueous urethane resin to 335 g / L, the same treatment as in Example 1 was performed to produce a conductive coating of 70 g / m. 2 Bioelectrodes. The components used and the characteristics of the resulting bioelectrodes are shown in Table 1. [Comparative Example 1] Except for changing the amount of carbon black dispersion to 212 g / L and aqueous urethane resin to 454 g / L, the same treatment as in Example 1 was performed to produce a conductive coating of 70 g / m. 2 Bioelectrodes. The components used and the characteristics of the resulting bioelectrodes are shown in Table 1. [Comparative Example 2] Except for changing the amount of carbon black dispersion to 652 g / L and aqueous urethane resin to 309 g / L, the same treatment as in Example 1 was performed to produce a conductive coating of 70 g / m. 2 Bioelectrodes. The components used and the characteristics of the resulting bioelectrodes are shown in Table 1. [Comparative Example 3] A stirrer was used instead of a homogenizer to stir the carbon black dispersion for 15 minutes. The carbon black dispersion, aqueous urethane resin, and aqueous thickener were mixed for 90 seconds without degassing. Otherwise, the same treatment as in Example 1 was performed to produce a conductive coating of 70 g / m. 2 Bioelectrodes. The components used and the characteristics of the resulting bioelectrodes are shown in Table 1. [Table 1] In the bioelectrode of the embodiment, carbon black is dispersed in particles on the surface of the conductive material. The ratio of the longest distance to the shortest distance between adjacent carbon black particles (hereinafter referred to as the "particle distance ratio") is small, resulting in a large area ratio of highly conductive regions in terms of in-plane resistivity. The change in inter-electrode impedance before and after washing is also small, thus providing high biosignal acquisition as a bioelectrode. Despite its high conductivity, it exhibits excellent wet rubbing fastness. On the other hand, in Comparative Example 1, the amount of carbon black as a conductive component is small, the ratio of carbon black particle distances is large, the highly conductive region is small, the inter-electrode impedance is high, and the biosignal acquisition is poor. In Comparative Example 2, the amount of carbon black as a conductive component is large, resulting in excellent conductivity, but low wet rubbing fastness and a large change in inter-electrode impedance before and after washing, making it impossible to acquire a stable biosignal before and after washing. In Comparative Example 3, the carbon black was not sufficiently dispersed, resulting in a large interparticle distance ratio, fewer highly conductive regions, a significant change in inter-electrode impedance before and after washing, poor biosignal acquisition, and low wet friction fastness. [Industrial Applicability] This invention enables the fabric-shaped bioelectrode to be manufactured that is soft, highly comfortable to wear, exhibits minimal performance differences before and after washing, and is reusable. Furthermore, the bioelectrode of this invention can be suitably used to measure bioelectrical signals such as brain waves, electrocardiograms, and electromyograms in humans or animals, or to apply electrical stimulation to organisms. none none

Claims

1. A bioelectrode having a laminated structure comprising a fiber substrate containing non-conductive fibers and a conductor, the conductor containing carbon black and a urethane resin, the carbon black content being 15% to 35% by mass relative to the conductor, the carbon black being dispersed in particulate form at least on the surface of the conductor, the ratio of the longest distance to the shortest distance between adjacent carbon black particles on the surface of the conductor being 1 to 20, and the wet friction hardness of the surface of the conductor being grade 4 or higher.

2. The bioelectrode as claimed in claim 1, wherein, The area ratio of the conductive material whose in-plane resistivity measured by scanning stretching resistance microscopy is 4logΩ to 6logΩ is 8% to 18%.

3. The bioelectrode as claimed in claim 1 or 2, wherein, The difference in inter-electrode impedance before and after 20 washes, based on the Japanese Industrial Standard L0217(1995)103, is less than 0.2kΩ.

4. A method for manufacturing a bioelectrode, comprising the following steps: mixing and degassing a solution comprising carbon black and urethane resin stirred using a homogenizer using a planetary ball mill to obtain a coating solution; applying the coating solution to one side of a fiber substrate comprising non-conductive multifilament fibers and drying it.