Electrochemical characterization and chronic stability of Utah electrode arrays for intracortical microstimulation

This study presents methods for evaluating the long-term electrochemical stability of Utah electrode arrays (UEAs) encapsulated with amorphous silicon carbide (a-SiC) or Parylene-C for intracortical microstimulation (ICMS). UEAs were implanted in rat motor cortex and monitored for 25 weeks using electrochemical impedance spectroscopy (EIS) to observe device impedance, cyclic voltammetry (CV) to obtain cathodic charge-storage capacity (Q _stor,c), and voltage transient (VT) measurements to derive maximum charge-injection capacity (Q _inj). Observed shifts in the open-circuit potential of the return electrode during stimulation, from in vitro to in vivo conditions, highlight the need to adjust potential limits when using a quasi-reference electrode. Both encapsulation materials exhibited stable impedance and maximum (Q _inj) trends in vivo, becoming similar after approximately 16 weeks post-implant. High-scan rate CV (up to 500,000 mV/s) was used to assess similarities between Q _stor,c and maximum Q _inj and determine if it can be used to estimate the maximum Q _inj. These methods can be used to evaluate the encapsulation and stimulation properties of chronically implanted neural electrodes.