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Abstract
Using mechanically adaptive neural interfaces has become a promising research strategy to improve on the chronic stability of biotic-abiotic interfaces. High charge-injection capacity electrode materials are incorporated onto shape memory polymer, mechanically adaptive substrates. For the first time, these processes enable integration of iridium and titanium nitride electrodes onto softening substrates using photolithography to define all features in the device. No electroplated layers are utilized leading to a highly scalable method for consistent device fabrication. The iridium and titanium nitride electrode are metallically bonded to the gold conductor layer, covalently bonded to the softening substrate via sulfur-based click chemistry. The SMP-based neural interfaces with iridium electrodes can deliver over 2 billion symmetric biphasic pulses (100µs/phase), with a charge of 200 µC/cm2 and geometric surface area (GSA) of 300 µm2. Electrical stability of iridium is tested under simulated physiological conditions in an accelerated electrical aging setup with periodic measurement of electrochemical impedance spectra (EIS) and charge storage capacity (CSC) at various intervals. Electrochemical characterization, and both optical and scanning electron microscopy suggest significant breakdown of the 600 nm thick parylene-C insulation, although no delamination of the conductors or of the final electrode interface was observed. Minor cracking at the edges of the thin film iridium electrodes was occasionally observed. SMP neural interfaces with TiN electrodes are tuned for improved electrochemical properties; roughness as a function thickness and stoichiometry are controlled to maximize the electrochemical surface area (ESA) of the interface. A 200 nm film thickness was estimated as the safe thickness limit for deposition and fabrication; above that thickness film-stress develops cracks across the stack. Stoichiometric TiN was found to develop a more well-defined columnar structure. This was traditionally believed to be only possible on films with thicknesses near 1 µm. Electrochemical characterization comparisons between standard TiN and ultra-high vacuum (UHV) TiN, measured a one order-of-magnitude improvement on cathodal CSC (CSCc) and impedance. The resulting devices will provide electrical recording and stimulation of the nervous system to better understand neural wiring and timing, target treatments for debilitating diseases and give neuroscientists spatially selective and specific tools to interact with the body.
