JPH0334742B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION OBJECTS OF THE INVENTION [Industrial Field of Application] The present invention relates to a stimulation electrode for implantation in a living body used in an electrical stimulation device for compensating biological functions by nerve stimulation, muscle stimulation, etc. In particular, the present invention relates to an electrode that can provide electrical stimulation to living organisms without involving redox reactions of biological materials.

[Prior Art] It has been known since Galvani's experiments that muscles can be contracted by applying electrical stimulation to nerves. Based on this principle, attempts to compensate biological functions by applying controlled electrical stimulation to living organisms have become popular in recent years. For example, cardiac pacemakers, respiratory pacemakers that stimulate the diaphragmatic nerve, drive and control limb movements that stimulate the peripheral nerves and muscles of the limbs,
These include spinal nerve stimulation for patients with neuritic sclerosis and cochlear implants implanted in the cochlea. Some of these have reached the stage of practical application.

Although there is a method of applying electrical stimulation using skin electrodes placed outside the body, a method of implanting electrodes inside the body is better in order to selectively stimulate only the necessary stimulation areas. The electrode for this is
Since they are required to have biocompatibility and corrosion resistance, noble metal alloys such as platinum, iridium, and rhodium have been mainly used.

When current is passed using these noble metals as electrodes, the current is carried by electrons in the metal, but in living organisms the current is carried by ions, so charging and discharging of the double layer capacitance on the electrode surface is excluded. For example, exchange of electrons on the electrode surface cannot be avoided. That is, on the anode surface, the electrode receives electrons from the biological system to oxidize the biological material, and on the cathode surface, the electrode gives electrons to the biological system to reduce the biological material. At this time, the reaction product may cause an undesirable chemical reaction in the tissue of the living body, which may cause some damage. Also,
If hydrogen gas is generated on the cathode, it may cause a change in the pH of the surrounding area. These chemical side effects have been investigated in the past, and as countermeasures, (1) electric stimulation is given as constant current square wave pulses that continue in both forward and reverse directions as shown in Figure 1; , a method in which the same amount of electricity flows in both forward and reverse directions to cancel out the products of the redox reaction (hereinafter referred to as the constant current cancellation pulse method).

(2) A method in which an electrode is enclosed in a capsule filled with physiological saline and a current is passed through a small hole in the capsule (hereinafter referred to as the capsule electrode method).

(3) A method using a so-called capacitive electrode whose electrode surface is coated with a dielectric material such as barium titanate (hereinafter referred to as the capacitive electrode method).

methods have been proposed.

The constant current cancellation pulse method (1) is a stimulation method that is currently widely used as a current stimulation method, but many types of biological substances are involved in electrode reactions, and these electrode reactions are irreversible in most cases. Therefore, the substances oxidized in the anodic reaction cannot be completely reduced in the cathode reaction, and the side effects associated with the redox reaction at the electrode cannot be completely eliminated.

The capsule electrode method (2) is effective in preventing the side effects associated with the chemical reactions mentioned above, but it is difficult to put this into practical use as an implanted electrode because the capsule has to be large. There is.

In capacitive electrodes (3), instead of causing an electrode reaction on the electrode surface, ions are adsorbed and desorbed at the interface between the living body and the dielectric, and electrons or holes are accumulated at the interface between the metal and the dielectric. By doing so, it attempts to store the amount of electricity necessary for stimulation.
Since the contact between the biological material and the electrons in the metal is broken, no redox reaction of the biological material occurs. However, in this case, the amount of charge that can be accumulated on the electrode surface is limited to about several tens of μC/cm ² , and if you want to increase the amount of electricity delivered in the stimulation pulse, you have no choice but to use an electrode with a large surface area.
For practical use, there is a problem in miniaturization.

[Problems to be Solved by the Invention] The present invention eliminates the redox reaction of the biological material brought about by the biological material participating in the electrode reaction by cutting off the contact between the electrode having electronic conductivity and the biological material. The present invention aims to provide a biostimulation electrode that can provide electrical stimulation to a living body without the side effects caused by the product. In particular, the present invention aims to provide a biostimulation electrode that dramatically increases the amount of charge accumulated per unit surface area of the electrode and enables miniaturization of the electrode.

"Structure of the Invention" [Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventor has created a biological stimulation electrode using a chemical substance that participates in redox reactions (hereinafter referred to as electrode active material). An electrode characterized in that it is coated with a diaphragm (hereinafter referred to as a solid electrolyte membrane) having ionic conductivity was provided. That is, as schematically shown in FIG. 2, an electrode active material 2 is placed on a metal conductor substrate 1 for supplying current from an external power source, and this is placed on a solid electrolyte membrane 3.
This electrode has a structure covered with Although it is desirable that the metal conductor substrate be completely covered with the electrode active material, it may be partially in contact with the solid electrolyte membrane.

The electrode active material must be composed of both an oxidant and a reductant, and it is desirable that the electrode reaction with ions passing through the solid electrolyte membrane be reversible. When the electrode active material is composed of a mixture of an oxidant and a reductant, it is desirable that both of them be insoluble substances.If the reductant is a metal, the oxidant can be its oxide, hydroxide, or Possible examples include halides, and combinations of the same metal compounds with different valences.

When the electrode active material is composed of a single substance, a polymer electrode obtained by electrochemical beeping such as polypyrrole, a mixed valence compound, e.g.
Non-stoichiometric compounds such as so-called bronze oxides represented by the general formula A _x BO ₃ and Fe _3-x O ₄ can also be used.

As solid electrolyte membranes, in addition to ion exchange membranes and polymer electrolyte membranes such as polyethylene oxide-alkali halide compounds, NASICON
Ceramic solid electrolytes such as (Na ₃ Zr ₂ PSi ₂ O ₁₂ ) can also be used.

[Function] The solid electrolyte membrane of the present invention has the seemingly contradictory role of isolating biological substances from the electronically conductive electrode surface, which is the site of redox reactions, and at the same time providing ionic current to the living body. be.

This point will be explained in detail below. When a current pulse is applied to a stimulating electrode to cause a current to flow inside a living body, the current is carried by electrons from the conductor to the electrode. Inside the living body, current is carried primarily by sodium ions and chloride ions. Conversion from electronic current to ionic current takes place at the electrodes, at the anode the reactant is oxidized and the electrode receives electrons, and at the cathode the electrode donates electrons and the reactant is reduced. Oxidation-reduction reactions associated with energization are inevitable in electrode reactions, and biological substances such as ascorbic acid and NADH easily undergo oxidation-reduction when they come into contact with energized electrodes.

Therefore, it is possible to isolate the biological material from the oxidation-reduction field by covering the electrode surface with some kind of film and blocking the contact between the biological material and the electrode. However, it is necessary to pass a current through this film, and this The coating must not be electronically conductive. If an electronically conductive film is used, the surface of the film will provide a new site for electrode reactions.

In addition to the capacitive electrodes mentioned above, solid membranes having ionic conductivity, that is, solid electrolyte membranes, can be used for this purpose.

As is clear from the above, the electrode active material itself performs a redox reaction by giving and receiving electrons, and releases ions involved in the redox reaction.
Or take it in. At this time, it is desirable that the ionic species involved in the reaction be the same as the ionic species flowing in the solid electrolyte, but it is not necessary that they be the same.

The amount of charge that can be accumulated on the electrode surface is determined by the characteristics of the electrode reaction performed by the electrode active material and the amount of the electrode active material. For this purpose, mixtures of oxidants and reductants can be used, as in conventional chemical cells. For example, in the case of an electrode active material made of a fine powder mixture of metal M and its oxide MO, the electrode reaction is expressed by the following equation.

MO + H ₂ O + e ^- = M + 20H ^- It is well known that chemical batteries can store a much larger amount of electricity than capacitors that store charge electrostatically. The electrode active material, which is made of a mixture of the above oxidant and reductant, stores electricity in the form of a chemical substance like a normal chemical battery, and can store electricity until the oxidant or reductant is completely consumed. , the capsule electrode method,
It has a different effect of accumulating electricity at a higher density than capacitive electrodes.

As the electrode active material, in addition to a mixture of an oxidant and a reductant, a compound having a non-stoichiometric composition can also be used. As an electrode active material for lithium batteries
The use of a Li intercalation compound of MoS ₂ is being considered, and it is possible to use exactly the same type of electrode reaction as in this case. In other words, it is possible for matrix B with electronic conductivity to take in metal ions M through intercalation, and when generating a non-stoichiometric compound M _x B, an electrode is constructed using the following reaction. It is possible to do so.

xM ⁺ +M _y B+e ^- =M _x+y B In this case, the amount of electricity that can be stored is determined by the non-stoichiometric composition range and the size of the chemical diffusion coefficient. It is possible to store electricity at a high density compared to other methods.

[Example] Example 1 Figure 2 shows that iron powder and a ferrous hydroxide mixture were kneaded with a polyvinyl alcohol aqueous solution and coated as an electrode active material 2 on a platinum substrate 1, and then a cation exchange membrane was applied. This is an electrode coated as a solid electrolyte membrane 3.

The typical thickness of each layer is Electrode active material 20μm Cation exchange membrane 10μm It is.

A pair of electrodes shown in Figure 2 were placed facing each other in an electrolytic bath, and constant potential electrolysis was performed in physiological saline at an overvoltage of 0.6 V. The current value was observed and the amount of electricity that could be passed was measured, and the amount of electricity was 20 mC/cm ² . It was possible to conduct electricity at high density.

Example 2 Using the same configuration as in Example 1, tungsten bronze (Na _x WO ₃ ) was sputter-deposited on the platinum substrate 1 as an electrode active material, and an electrode was constructed that was entirely covered with a cation exchange membrane. did. Typical conditions for each layer were: electrode active material: tungsten bronze (x = 0.7), 500 nm thick, and cation exchange membrane: 10 μm thick. Cubic tungsten bronze is x=
It is an electronic conductor with a non-stoichiometric composition that is continuously variable from 0.4 to 1.0.

When the cation exchange membrane was subjected to Na ⁺ substitution in physiological saline and the amount of electricity that could be passed was measured in the same manner as in Example 1, it was found that electricity could be passed up to 50 mC/cm ² .

Example 3 Similar to Examples 1 and 2, amorphous tungsten trioxide was deposited on the platinum substrate 1 of FIG. 2 as the electrode active material 2.
Then, a polyethylene oxide-NaCl-based polymer solid electrolyte membrane 3 was vacuum-deposited and coated to form an electrode.

The typical thickness of each layer is Amorphous tungsten trioxide 500nm Polyethylene oxide 2μm It is.

It is widely known that amorphous tungsten trioxide causes an intercalation reaction of Na through the reaction xNa′+WO ₃ +e ^- →Na _x WO ₃ (x<0.3), and the electrode reaction is caused by the above reaction. proceed.

Using this electrode, the amount of electricity that could be passed was measured in the same manner as in Examples 1 and 2, and it was found that electricity could be passed up to 20 nC/cm ² .

"Effects of the Invention" With the biostimulation electrode of the present invention, (1) it is possible to provide an electrode that can isolate biomaterials from the electrode reaction field and provide electrical stimulation to a living body without involving redox reactions of the biomaterials; Was it.

(2) By dramatically increasing the amount of charge that can be stored per unit surface area of the electrode, we were able to provide a compact and high-performance electrode.

Remarkable technical effects were obtained.

[Brief explanation of drawings]

FIG. 1 shows the relationship between current and time in the constant current cancellation pulse method, and represents rectangular wave pulses that continue in both positive and negative directions. Figure 2,
FIG. 1 is a schematic cross-sectional view showing the structure of a biological stimulation electrode of the present invention. Explanation of symbols of main parts: 1...metal conductor substrate, 2...electrode active material, 3...solid electrolyte membrane.

Claims (1)

[Claims] 1 It has a structure in which the surface of an electrode active material consisting of a mixture of an oxidant and a reductant having electronic conductivity, or an electrode active material consisting of a non-stoichiometric compound having electronic conductivity, is covered with a solid electrolyte membrane. A biological stimulation electrode characterized by: