ELECTROCHEMICAL PROPERTIES OF FLEXIBLE ELECTRODES FOR IMPLANTED NEUROMUSCULAR EXCITATION APPLICATIONS

Abstract

<p>This thesis investigates the electrochemical performance of flexible implanted electrodes for the purpose of neuromuscular electrical stimulation. Electrodes performance is validated through their conductivity, stability, and charge delivery capacity (CDC) to avoid irreversible faradaic reactions during stimulation. To study these requirements, electrodes were fabricated by depositing platinum (Pt), gold (Au) and titanium (Ti) thin films on the flexible liquid crystal polymer (LCP) substrate. Their electrochemical properties were then studied using surface and interface characterization techniques including atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and a theoretical model. The EIS results demonstrate that larger electrodes provide higher conductivity and double layer capacitance. In terms of material, Pt offers the best conductivity in neuromuscular stimulation frequencies of 1- 250 Hz, followed by Ti and Au, respectively. Above 250 Hz, similar values of conductivity is offered by the electrodes. This material dependence of impedance magnitude is related to the surface morphology, structural quality and deposition parameters of the electrodes and is explained using surface roughness measurements and interface model parameters. Electrode long-term stability is explored by regular EIS measurements through 42-day experiments. With progressing time, an increase in surface roughness, decrease in chargetransfer resistance (Rct) and capacitive quality (¯) are observed due to the change in capacitive and faradaic behaviors. However, in the comparative evaluations, Au electrode shows the most consistency in keeping its capacitive behavior to perform reversible charge transfer, followed by Pt and Ti, respectively. Further, cyclic voltammetry (CV) curves were used to understand the charge transfer reactions and calculate charge delivery capacities (CDC) of Pt, Ti and Au. Pt with highest CDC value suggests the best electrochemical reversibility followed by Au and Ti. In the case of deposition pressure, for Ti electrode, lower deposition pressure yields higher charge delivery capacity. These results may make lower pressure deposited Pt electrode with high conductivity and CDC the best material for the short term applications of neuromuscular electrical stimulation, while Au possessing improved stability but lower conductivity and CDC is suggested for long term applications. This result provides deeper insight into the design and miniaturization of electrochemical electrodes for the further development of neuromuscular prostheses.</p>