Iridium-Bismuth-Oxide Coatings for Use in Neural Stimulating Electrodes: The Influence of Ir/Bi Ratio

Implantable neural prosthetics with stimulating electrodes are increasingly employed in medical practices to treat neural disabilities. The electrode material is expected to provide high charge storage and injection capacity (CSC/CIC) and low impedance for safe, efficient, and precise neural stimulation, while at the same time, being small. To improve the current state-of-the-art neural-electrode material, iridium oxide (IrOx), Ir_mBi_1-mOx coatings of various compositions (m = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) produced by thermal deposition were evaluated. The Ir_0.8Bi_0.2Ox yielded a CSC of 17.7 ± 1.1 mC/cm², which is four-fold higher than that of IrOx. At the same time, the impedance of Ir_0.8Bi_0.2Ox at 1 kHz was measured to be half of that of IrOx. The superior performance of Ir_0.8Bi_0.2Ox was explained by forming amorphous structures that facilitate the intercalation of H⁺ and OH^- ions into deeper oxide structures that contribute to faradaic charge storage. The Ir_0.8Bi_0.2Ox electrode also showed good stability and biocompatibility, which makes it potentially a good candidate for neural stimulating electrodes.