Chronic neural stimulation with thin-film, iridium oxide electrodes

Experiments were conducted to assess the effect of chronic stimulation on t he electrical properties of the electrode-tissue system, as measured using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). S ilicon, micromachined probes with multiple iridium oxide stimulating electr odes (400-1600 mu m(2)) were implanted in guinea pig cortex, A 10-17 day po st-operative recovery period was followed by five days of monopolar stimula tion, two hours/electrode each day using biphasic, constant current stimula tion (5-100 mu A, 100 mu s/phase). EIS and CV-data were taken before and af ter stimulation. The post-stimulation impedance [at mid-range frequencies ( 100 Hz-100 kHz)] consistently and significantly decreased relative to prest imulation levels. Impedance magnitude increased permanently at low frequenc ies (<100 Hz), correlating to a change in the charge storage capacity (the area under a cyclic voltammagram). Impedance magnitude significantly increa sed during the recovery period, though this increase could be mostly revers ed by applying small currents, A mathematical model of the electrode-tissue system impedance was used to analyze in vivo behavior, The data and modeli ng results shows that applying charge to the electrode can consistently red uce the impedance of the electrode-tissue system, Analysis of explanted pro bes suggests that the interaction between the tissue and electrode is depen dent on whether chronic pulses were applied. It is hypothesized that the in terface between the tissue and metal is altered by current pulsing, resulti ng in a temporary impedance shift.