On the in vitro long-term stability of thin-film stimulation contacts in polyimide-based neural interfaces

Micro-machined peripheral nerve interfaces have entered translational research successfully. Polyimide-based interface substrates showed chemical inertness, mechanical flexibility and low water uptake. They allow neural interface with a thicknesses of ten micrometer reducing the probability of severe inflammatory reactions. To realize device thicknesses in this range, photolithographic processes are used to deposit thin-film metallization in the range of a few hundred nanometers. In order to prove long-term stability within chemical safe stimulation limits, in vitro stimulation of sputtered iridium oxide film stimulation contact sites was performed. After 4.5 billion pulses, first electrochemical changes occurred indicated by a decrease in impedance magnitude and simultaneously an increase of the cathodic charge storage capacity. Both scenarios indicated a change in the effective surface area. These contact sites were used for an optical in depth analysis using white light interferometry and scanning electron microscopy in combination with a focused ion beam, which confirmed crack formation and delamination. However, for all other contact sites further stimulation up to 6.5 billion pulses was applied. This amount was estimated about 6.6 years of stimulation in chronic human application with a conservative approach assuming daily work with each stimulation contact. The outcome of the long-term stimulation indicates stability for chronic stimulation studies in humans with life-times reasonably long for first translational studies on larger subject cohorts.