Iridium Oxide Redox Gradient Material: Operando X-ray Absorption of Ir Gradient Oxidation States during IrOx Bipolar Electrochemistry

Abstract

Electrodeposited iridium oxide (K1.7IrO0.8 (OH)2.2 × 1.8 H2O; also called IrOx) is among the best substrates for neural growth, decreasing impedance and stimulating cell growth, when used as a connected electrode. Without direct contact, it has been proven to stimulate neurons through a bipolar mechanism related to the conducting character of the material in the presence of remote electric fields. The remote wireless electrostimulation that arises from it is of large significance in clinical applications. Ionic intercalation simultaneous with iridium oxidation state changes at the induced IrOx cathode and the formation of a redox and ionic gradient at the IrOx substrate is envisaged as the most probable explanation for the observed effects on neural cell growth. This work shows the iridium state gradient using X-ray absorption spectroscopy (XAS) with significant electrochemical features and relaxation times that allow for a persistent effect in the material even after the electric field creating the induced dipole is switched off. It also shows correlated intercalated sodium gradients observed by semiquantitative energy-dispersive X-ray (EDX) analysis data. The bipolar effect is proven and yields new evidence for the behavior of other biocompatible neural growth substrates.