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

Abstract

Electrodeposited iridium oxide (K1.7IrO0.8 (OH)2.2 × 1.8 H2O; also called IrO x ) 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 IrO x cathode and the formation of a redox and ionic gradient at the IrO x 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.

Induced Dipoles and Possible Modulation of Wireless Effects in Implanted Electrodes. Effects of Implanting Insulated Electrodes on an Animal Test to Screen Antidepressant Activity

“…That polarization does not occur when the electrode is protected with an insulating layer. The magnitude of endogenous electric fields in biosystems [7] is similar to those used in reported experiments [23,24] and therefore could induce the same polarization in the implanted material. Of course, endogenous fields possess a dynamic character which is not included in the previous experiments [23].…”

“…As previously reported for other materials [23][24][25], steel electrodes also get polarized when immersed in an ionic media, in the presence of external applied fields, yielding to electrochemical effects that involve oxidation in the induced anode pole and reduction in the opposite cathode pole, at certain external applied potentials. That polarization does not occur when the electrode is protected with an insulating layer.…”

“…In addition, such a dipolar effect modifies the redox potential of the material in an inhomogeneous way, creating a redox spatial gradient, and therefore a directional chemical gradient. It can also vary the concentration of ions near one of the poles of the implanted material, while the other pole has a different ionic or redox species [24]. Thus, neural cells grown on conductive materials show varying speeds and directions of growth if an external field is applied [23].…”

“…Thus, neural cells grown on conductive materials show varying speeds and directions of growth if an external field is applied [23]. Furthermore, it has been observed recently that the derived electrochemistry at the induced poles has effects during days and weeks [24], on a time scale similar to the antidepressant-like effect described in rats [21]. Such long-term effects on the materials are related to the gradient created due to oxidation, or intercalation of ions at the induced anode or cathode, respectively, and are described in detail in [24].…”

Oscillatory patterns in redox gradient materials through wireless bipolar electrochemistry. The dynamic wave-like case of copper bipolar oxidation

“…Those reactions may be of species in the electrolyte nearby or correspond to the material itself, depending on the conditions and materials chosen. Thus, intercalation of ions has been reported in induced cathodes in iridium oxide or conducting polymers, 13,24 asymmetric electrodeposition of metals, and solvent oxidation and reduction (forming O 2 and H 2 in the case of water, with corresponding pH changes). Thus, electron transfer occurs also at the induced poles of what can be described as a bipolar electrode, and impedance changes are observed in the cell.…”