Rubber composition and bioelectrode

Abstract

The present invention relates to a rubber composition containing a fluorosilicone rubber and silver particles, and a bioelectrode containing the rubber composition.

Description

Rubber composition and bioelectrode

[0001] The present invention relates to a rubber composition and a bioelectrode.

[0002] Rubber compositions have been used in a variety of applications. Examples of rubber compositions that come into contact with human skin include bioelectrodes, rubber bands, hoses, gloves, boots, tents, toys, etc. Among these, there are rubber compositions that are imparted with electrical conductivity by containing a conductive material.

[0003] A particularly useful application of conductive rubber compositions is a bioelectrode that acquires electrical signals from a living body by directly contacting the body. Examples of such bioelectrodes include those in which the portion that comes into contact with the skin of the living body is formed from a conductive rubber composition. Bioelectrodes formed from conductive rubber compositions are softer than metal electrodes, allowing electrical signals to be acquired from the living body without causing pain to the living body. Examples of conductive rubber composition materials include those containing vinyl methyl silicone rubber and silver particles. However, because vinyl methyl silicone rubber is wetted by sebum components such as squalene and oleic acid, the penetration and deposition of sebum components derived from the living body on the surface of the rubber composition can cause the bioelectrode to become insulated, making it impossible to acquire biosignals.

[0004] Patent Document 1 (JP 2018-061778 A) discloses a conductive rubber composition for measuring biosignals, which contains at least one selected from the group consisting of synthetic rubber and elastomer, a rubber component having a cross-linked structure, conductive metal particles, an adhesive, and a polar solvent. However, as described above, sebum components derived from the living body penetrate and accumulate on the surface of the bioelectrode, making it impossible to use the bioelectrode in the desired state.

[0005] As exemplified by the above-mentioned bioelectrode, conventional conductive rubber compositions are required to maintain stable and excellent conductivity without allowing sebum components derived from the skin of the living body to penetrate and accumulate in the rubber composition, even when used in contact with the living body for a long period of time.

[0006] JP 2018-061778 A

[0007] The present invention has been made in view of the above circumstances, and aims to provide a rubber composition that stably maintains excellent conductivity without allowing sebum components derived from the skin of a living body to penetrate or accumulate in the rubber composition even when used in contact with a living body for a long period of time, and a bioelectrode made using the rubber composition.

[0008] The gist of the present invention is as follows: [1] A rubber composition comprising fluorosilicone rubber and silver particles. [2] The rubber composition according to [1] above, wherein R2 / R1 is 10 or less, where R1 is the surface resistance of the rubber composition measured in accordance with JIS K7194:1994 after folding a sheet of the rubber composition 30 mm long, 30 mm wide, and 2 mm thick 30 times, and R2 is the surface resistance of the rubber composition measured in accordance with JIS K7194:1994 after folding a sheet of the rubber composition 30 mm long, 30 mm wide, and 2 mm thick 30 times and then immersed in squalene at 50°C for 300 hours. [3] A bioelectrode comprising the rubber composition according to [1] or [2] above. [4] The bioelectrode according to [3] above, which is an earpiece-type bioelectrode to be inserted into the external auditory canal. [5] The bioelectrode according to [3] above, which is a bioelectrode used in contact with the scalp.

[0009] It is possible to provide a rubber composition that stably maintains excellent conductivity even when used in contact with a living body for a long period of time, without sebum components derived from the skin of the living body penetrating or accumulating in the rubber composition, and a bioelectrode made using the same.

[0010] Fig. 1 is a diagram schematically showing an earpiece-type bioelectrode according to an embodiment of the present invention. Fig. 2 is a cross-sectional view schematically showing a bioelectrode according to an embodiment of the present invention. Fig. 3 is a perspective view schematically showing a bioelectrode according to an embodiment of the present invention. Fig. 4 is a graph showing the results of a durability test of Examples 1 to 3 and Comparative Example 1.

[0011] The rubber composition of the present invention contains a fluorosilicone rubber and silver particles. The fluorosilicone rubber is a rubber represented by the following formula (1), which is obtained by copolymerizing a methyltrifluoropropylsiloxane represented by the following formula (A) with a methylvinylsiloxane represented by the following formula (B), and is also represented as FVMQ (fluorovinyl methyl siloxane).

[0012] Fluorosilicone rubber has the property of being resistant to wetting by biological sebum components such as squalene and oleic acid. Therefore, even when the rubber composition is brought into contact with the skin of a living body, the biological sebum components do not penetrate or accumulate in the rubber composition. Furthermore, because the rubber composition contains silver particles, it can be made into a conductive rubber composition that stably maintains the desired conductivity. In particular, when the rubber composition of the present invention is used as a component of a bioelectrode that comes into contact with a living body, penetration and accumulation of sebum components into the rubber composition can be suppressed over a long period of time, thereby suppressing a decrease in surface resistance over a long period of time and enabling stable measurement of biological electrical signals. The rubber composition may be a rubber composition before or after vulcanization, but a vulcanized rubber composition is preferred.

[0013] A rubber composition measuring 30 mm in length, 30 mm in width, and 2 mm in height was bent 30 times, and the surface resistance (mΩ) of the rubber composition measured in accordance with JIS K7194:1994 before immersion in squalene was defined as R1. A rubber composition measuring 30 mm in length, 30 mm in width, and 2 mm in height was bent 30 times, and the rubber composition was immersed in squalene at 50°C for 300 hours. After wiping off any liquid remaining on the surface, the rubber composition was measured in accordance with JIS K7194:1994. R2 is defined as the surface resistance (mΩ). R2 / R1 is preferably 10 or less, and more preferably 2 or less. The surface resistance R1 is preferably 3.0 mΩ or less. The surface resistance R2 is preferably 30.0 mΩ or less, and more preferably 6 mΩ or less. The surface resistance measured as described above is an index representing the oil resistance of the rubber composition. A rubber composition having a surface resistance within the above range can have excellent oil resistance. The volume change rate of the rubber composition is preferably 0 to 5%, more preferably 0 to 3%, and even more preferably 0 to 1%. By having a volume change rate within the above range, the rubber composition is less susceptible to sebum penetration and can maintain stable conductivity over a long period of time. The volume change rate of the rubber composition can be measured using a hydrometer according to the method specified in JIS K6258:2016.

[0014] Each component constituting the rubber composition of the present invention will be described in detail below. (Fluorosilicone Rubber) The fluorosilicone rubber is not particularly limited as long as it is a copolymer of methyltrifluoropropylsiloxane and methylvinylsiloxane. Such fluorosilicone rubber may be commercially available, and examples include commercially available fluorosilicone rubbers such as FE-221-U (trade name), FE-251-U (trade name), and FE-351-U (trade name).

[0015] (Silver Particles) The properties of the silver particles are not particularly limited, but a form containing both agglomerated silver powder and flake-shaped silver powder can be used. Agglomerated silver powder is a three-dimensional aggregation of multiple particulate primary particles. Flake-shaped silver powder is scaly in shape. The average particle diameters of the agglomerated silver powder and flake-shaped silver powder are not limited to specific values. For example, the agglomerated silver powder preferably has an average particle diameter in the range of 4 μm to 8 μm, and the flake-shaped silver powder preferably has an average particle diameter in the range of 5 μm to 15 μm. The average particle diameter of the silver particles is the average diameter measured using an electron microscope photograph and calculated by the arithmetic mean. The content ratio of the agglomerated silver powder to the flake-shaped silver powder is preferably 70:30 to 30:70 (agglomerated silver powder: flake-shaped silver powder = 70:30 to 30:70), and it is more preferable to have the agglomerated silver powder and flake-shaped silver powder in the same ratio.

[0016] Examples of flake silver powder include those sold under the trade name "327077" (manufactured by Sigma-Aldrich Corporation), those sold under the trade name "FA-D-3" (manufactured by Dowa Electronics Co., Ltd.), and those sold under the trade name "FA-2-3" (manufactured by Dowa Electronics Co., Ltd.). Of these, the flake silver particles sold under the trade name "FA-2-3" (manufactured by Dowa Electronics Co., Ltd.) are preferred. The aggregated silver particles sold under the trade name "G-35" (manufactured by Dowa Electronics Co., Ltd.) are preferred.

[0017] The content of silver powder in the rubber composition can be set as appropriate within a range that can impart conductivity, but is preferably in the range of 80 to 300 parts by weight, more preferably 100 to 250 parts by weight, and even more preferably 150 to 200 parts by weight, per 100 parts by weight of fluorosilicone rubber. By keeping the content (parts by weight) of silver powder in the rubber composition within the above range, the rubber composition can achieve both excellent flexibility and conductivity.

[0018] (Other Additives) The rubber composition of the present invention contains fluorosilicone rubber and silver particles, and may further contain other additives such as colorants such as carbon black and pigments; vulcanizing agents; processing aids such as wax, metallic soap and carnauba wax; acid acceptors such as calcium hydroxide, magnesium oxide, zinc oxide and hydrotalcite; antioxidants; thermoplastic resins; plasticizers; softeners; colorants; dispersants; flame retardants; tackifiers; mold release agents; various metal powders, etc.

[0019] (Method for Producing Rubber Composition and Molded Article) The method for producing the rubber composition of the present invention is not particularly limited. The rubber composition can be produced by adding fluorosilicone rubber, silver particles, and, if necessary, additives, and kneading them using a kneader, roll, or the like. Alternatively, a vulcanizing agent can be added to the rubber composition as needed, and the composition can be molded into a desired shape by a desired method such as compression molding, transfer molding, injection molding, extrusion molding, or calendar molding, and then vulcanized at 165 to 220°C for 1 to 30 minutes to produce a molded article. The molded article can then be optionally immersed in salt water of a predetermined concentration and heated in an autoclave at 120 to 140°C to ionize the silver constituting the silver particles. If necessary, the resulting molded article can be subjected to secondary vulcanization at 200 to 230°C for 1 to 5 hours.

[0020] The type of molded article is not particularly limited, but examples include bioelectrodes. Bioelectrodes are not particularly limited, but include electrodes that acquire electrical signals from a living body by contacting the skin of the living body. Bioelectrodes manufactured from the rubber composition of the present invention and containing the rubber composition contain fluorosilicone rubber, making them resistant to wetting by sebum components of the living body, such as squalene and oleic acid. Therefore, when measuring electrical signals from a living body using the bioelectrode, sebum components derived from the living body's skin do not penetrate or accumulate in the bioelectrode. Furthermore, because the bioelectrode contains silver particles, it is possible to stably measure accurate electrical signals (e.g., surface resistance) from the living body with minimal potential fluctuations. The living body whose electrical signals are measured using the bioelectrode is not particularly limited, but examples include humans, non-human mammals, birds, and the like. Furthermore, the site on the living body to which the bioelectrode is contacted is not particularly limited, and examples include the ear canal, the scalp, and other exposed skin sites.

[0021] (Bioelectrode) The uses of the bioelectrode are not particularly limited, and it can be used as a medical device, a treatment device, a wearable information device, a health monitoring device, etc. in fields such as medicine, nursing care, and welfare. Below, the bioelectrode is described by specifically listing embodiments of the bioelectrode. Note that the bioelectrode shown below is an example, and the bioelectrode of the present invention is not limited to the bioelectrode shown below. Figures 1 to 3 are each a diagram showing the bioelectrode of the present invention. Below, the bioelectrodes of Figures 1 to 3 are described in detail.

[0022] FIG. 1 is a schematic diagram illustrating an earpiece-type bioelectrode 1 of the present invention, which is inserted into the external auditory canal of a living body. As shown in FIG. 1 , the earpiece-type bioelectrode 1 is composed of a tip portion 2, a conductive layer 3 containing the rubber composition of the present invention, and wiring 4. When the earpiece-type bioelectrode 1 of FIG. 1 is inserted into the external auditory canal of a living body, the conductive layer 3 comes into contact with the skin inside the external auditory canal. The conductive layer 3 acquires electrical signals from the living body through this contact with the skin inside the external auditory canal, and the electrical signals are transmitted to a measuring device (not shown) via the wiring 4. Sebum components such as earwax are secreted from the skin in contact with the conductive layer 3 inside the external auditory canal. However, because the conductive layer 3 of the earpiece-type bioelectrode 1 of the present invention contains fluorosilicone rubber, these sebum components do not penetrate or accumulate in the conductive layer 3. Therefore, the earpiece-type bioelectrode 1 can stably and accurately measure electrical signals from the living body over a long period of time with little potential fluctuation.

[0023] FIG. 2 is a cross-sectional view of an example of a bioelectrode of the present invention, and FIG. 3 is a perspective view of the bioelectrode of FIG. 2. As shown in FIG. 2, the bioelectrode 10 is used by directly contacting the skin 14 of a living body. It comprises a conductive substrate 11 made of conductive silicone rubber or the like and a conductive layer 12 disposed on the substrate 11, the conductive layer 12 containing the rubber composition of the present invention. A covered wire 13 is connected to the surface of the substrate 11. As shown in FIG. 3, the covered wire 13 comprises a core 21 and a covering material 22, and the core 21 is fixed to the surface of the substrate 11 with adhesive tape 23 or the like. When the conductive layer 12 of the bioelectrode 10 is brought into contact with the skin 14 of a living body, an electrical signal from the skin 14 of the living body is transmitted to a measuring device (not shown) via the conductive layer 12, the substrate 11, and the core 21. Sebum components are secreted from the skin of a living body, but because the conductive layer 12 of the bioelectrode 10 of the present invention contains fluorosilicone rubber, the sebum components do not penetrate or accumulate in the conductive layer 12. Therefore, the bioelectrode 10 can stably measure accurate electrical signals from a living body with little potential fluctuation. The part of the living body that the bioelectrode 10 comes into contact with and the electrical signal are not particularly limited, but examples include the scalp as the part and brain waves as the electrical signal. Although sebum is secreted from the scalp, this sebum does not penetrate or accumulate in the conductive layer 12, so the bioelectrode 10 can stably measure accurate electrical signals from a living body over a long period of time.

[0024] (Examples 1 to 3, Comparative Example 1) The rubbers shown in Table 1 below were wound around an open roll, silver particles were added, and the mixture was kneaded using the open roll to form a sheet. The sheet was then press-vulcanized at 180°C for 10 minutes, and immersed in 10% by mass salt water in an autoclave and boiled at 121°C for 60 minutes to produce a rubber sheet with a thickness of 2 mm.

[0025]

[0026] The names of the materials used in each example are as follows: FVMQ: fluorosilicone rubber, FE-221-U (trade name) manufactured by Shin-Etsu Chemical Co., Ltd. VMQ: vinylmethylsilicone rubber, LIVEO QP1-25 BASE JPN (trade name) manufactured by Dow Corning Toray Co., Ltd. Silver particles: FA-2-3 (trade name) manufactured by Dowa Electronics Co., Ltd. Silver particles: G-35 (trade name) manufactured by Dowa Electronics Co., Ltd. Crosslinking agent: peroxide-based crosslinking agent (2,5-dimethyl-2,5-ditertiarybutylperoxyhexane); Perhexa 25B (trade name) manufactured by Nippon Oil & Fats Co., Ltd.

[0027] The rubber composition sheets thus produced were subjected to measurements of hardness, volume resistivity, durability test, and volume change rate. The methods for measuring each of the above properties are as follows.

[0028] (1) Hardness (Durometer A Hardness, Instantaneous) (-) In accordance with JIS K6253-3:2023, the instantaneous hardness of a durometer A sheet of the rubber composition having a length of 50 mm, a width of 20 mm and a thickness of 2 mm was measured using an automatic rubber hardness tester Model P2-A (device name) manufactured by Kobunshi Keiki Co., Ltd.

[0029] (2) Volume Resistivity (Ω cm) According to the four-probe method of JIS K7194:1994, the volume resistivity of a rubber composition sheet having a length of 50 mm, a width of 20 mm and a thickness of 2 mm was measured using a low resistivity meter Loresta GX MCP-T700 (trade name) manufactured by Mitsubishi Chemical Analytech Co., Ltd. as a measuring device and an ASP Probe (trade name) manufactured by Mitsubishi Chemical Analytech Co., Ltd. as a terminal.

[0030] (3) Durability Test (Time) A rubber composition sheet measuring 30 mm in length, 30 mm in width, and 2 mm in thickness was bent 30 times to apply strain. Next, the surface resistance value R1 of the rubber composition sheet was measured according to the four-point probe method of JIS K7194:1994 using a Loresta GP MCP-T610 (trade name) manufactured by Mitsubishi Chemical Analytech Co., Ltd. and a terminal PSP probe (trade name) manufactured by Mitsubishi Chemical Analytech Co., Ltd. Thereafter, the rubber composition sheet was immersed in squalene at 50°C for 300 hours. While the rubber composition sheet was immersed in squalene, the rubber composition sheet was removed from the squalene over time, and the liquid on the surface of the rubber composition sheet was wiped off. Next, the surface resistance of the rubber composition sheet was measured in accordance with the four-probe method of JIS K7194:1994 using a Loresta GP MCP-T610 (trade name) manufactured by Mitsubishi Chemical Analytech Co., Ltd. and a Terminal PSP Probe (trade name) manufactured by Mitsubishi Chemical Analytech Co., Ltd. From the start of the durability test until 300 hours later, the surface resistance was measured to see if it was 1×10 ７ The time until the surface resistance reached 10 mΩ (threshold value) was measured. The surface resistance of the rubber composition sheet 300 hours after the start of the durability test was taken as R2. Note that if the surface resistance was 1×10 even after 300 hours had passed from the start of the durability test, ７ When it was less than mΩ (threshold value), it was evaluated as "more than 300 hours."

[0031] (4) Volume Change Rate (%) A rubber composition sheet measuring 50 mm in length, 25 mm in width, and 2 mm in thickness was immersed in squalene at room temperature (25°C) for 70 hours and then removed. The volume of the rubber composition sheet before and after immersion in squalene was measured using an automatic hydrometer DSG-1 (trade name) manufactured by Toyo Seiki Seisaku-sho, Ltd. The volume change rate was calculated according to the following formula based on the method specified in JIS K6258:2016: Volume Change Rate (%) = {(Volume of rubber composition sheet after immersion in squalene) - (Volume of rubber composition sheet before immersion in squalene)} / (Volume of rubber composition sheet before immersion in squalene) × 100

[0032] The results of the hardness, volume resistivity, durability test, and volume change rate measured for each example as described above are shown in Table 1 above. The change in surface resistance over time from 0 to 300 hours in the durability test is shown in FIG. 4. As shown in Table 1 and FIG. 4, in Examples 1 and 2, a low surface resistance of 3.0 mΩ or less was maintained over the duration of the durability test from 0 to 300 hours, R2 / R1 was 1.1 or less, and the volume change rate was also low at 0%. In Example 3, a low surface resistance of 200 mΩ or less was maintained over the duration of the durability test from 0 to 300 hours, R2 / R1 was 1.05 or less, and the volume change rate was also low at 0%. In contrast, in Comparative Example 1, the surface resistance was 1×10 24 hours after the start of the durability test. ７ mΩ (threshold value), R2 / R1 was significantly greater than 10, and the volume change rate was also high at 10%. From the above, it was confirmed that the rubber composition of the present invention can stably maintain excellent conductivity for a long period of time without penetration or deposition of sebum components.

[0033] REFERENCE SIGNS LIST 1 Earpiece-type bioelectrode 2 Tip portion 3, 12 Conductive layer 4 Wiring 10 Bioelectrode 11 Base material 13 Covered wire 14 Living body skin 21 Core wire 22 Covering material 23 Adhesive tape

Claims

1. A rubber composition comprising a fluorosilicone rubber and silver particles.

2. The rubber composition according to claim 1, wherein R2 / R1 is 10 or less, where R1 is the surface resistance of the rubber composition measured in accordance with JIS K7194:1994 after folding a sheet of the rubber composition 30 mm long, 30 mm wide, and 2 mm thick 30 times, and R2 is the surface resistance of the rubber composition measured in accordance with JIS K7194:1994 after folding a sheet of the rubber composition 30 mm long, 30 mm wide, and 2 mm thick 30 times and then immersed in squalene at 50°C for 300 hours.

3. A bioelectrode containing the rubber composition according to claim 1 or 2.

4. The bioelectrode according to claim 3, which is an earpiece-type bioelectrode to be inserted into the ear canal.

5. The bioelectrode according to claim 3, which is used by contacting it with the scalp.

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