Stability of electro-active materials and coatings for charge-injection electrodes

The stability of materials and coatings for charge injection in electrical stimulation with chronically implanted electrodes is reviewed. The focus of the review is iridium oxide, whose performance and charge-injection mechanisms are contrasted with other high-charge capacity materials such as high-surface-area titanium nitride and noble metal coatings, as well as traditional noble metals. The appropriateness of iridium oxide and these materials for applications in functional electrical stimulation is discussed with an emphasis on the stability of their electrochemical function in chronic implants. Gaps in our knowledge of the in vivo electrochemistry and chronic stability of iridium oxide are identified. The structure and electrochemical properties of iridium oxide that permit high charge injection capacities are described and thoughts on how these generally desirable features may also compromise the chronic in vivo stability of iridium oxide electrodes are presented.