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Electrode Material Testing

Electrode Material Testing - "Criteria for Selection of Implanted Electrode Materials"

Table 1 Warburg capacitance "fcm2" of metal in contact with 0.9% brine


Table 1 shows the capacitance/cm2 of many metals in contact with 0.9% saline at room temperature. Figure 1 is a log-log plot of low current density Warburg capacitance versus frequency for the materials shown in Table 1. The closer to the top of the graph, the lower the impedance of the electrodes. Note that platinum black represents the lowest impedance. Some of the materials shown in Table 1 cannot yet be used as implanted electrodes.



Figure 1 Warburg capacitance "fcm2" of metals in contact with various electrolytes. "Electrode Redrawing and Measurement of Bioelectric Events".


Table 2 Properties and equivalents of materials


Table 2 lists the atomic numbers of electrode materials that are candidates for implanted electrodes.


Table 3 Properties of insulating materials


When applying hard insulation to an electrode, attention must be paid to the expansion coefficient of the material constituting the electrode and the expansion coefficient of the insulation applied. Table 3 lists the thermal expansion coefficients of many electrode materials and insulating materials.

Table 3 lists the dielectric strength, dielectric constant, coefficient of expansion, and melting point of many insulating materials used in implanted electrodes. Polyimide appears to produce the least tissue response. If glass or fused silica is used, the coefficient of expansion must match that of the electrode it covers; otherwise, cracking can result when cooling occurs. Metals such as tantalum can be anodized to create a thin oxide coating that acts as an insulator. Tantalum produces the oxide coating with the highest capacitance per unit area.


Figure 2 Stimulating electrodes

When using small-area electrodes to stimulate expandable tissue, despite the low current, the current density is high, so two new considerations arise: (1) stimulation waveform distortion and (2) electrochemical decomposition. Burg capacitance properties of the electrode-electrolyte interface and type of stimulator output circuit (constant voltage or constant current). Electrolytic decomposition stimulation occurs when there is a DC component in the battery.