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In Vitro Electrode Stability Protocols for Use in a Regenerative Peripheral Nerve Interface
Stephanie A. Goretski, Jana D. Moon, BS, Melanie G. Urbanchek, PhD, Paul S. Cederna, MD, Nicholas B. Langhals, PhD.
University of Michigan, Ann Arbor, MI, USA.
PURPOSE: A Regenerative Peripheral Nerve Interface (RPNI) is designed to intuitively control prostheses utilizing an electrode to record and stimulate activity from a free muscle graft reinnervated by a peripheral nerve. In order to achieve long term stability of the interface, the electrode must remain stable and produce minimal foreign body responses. Electrode stability can be measured by long term electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Unstable electrodes will exhibit variations in impedance and decreases in cathodic charge storage capacity (CSCc). In this study, we examined readily available stainless steel pad electrodes as proposed materials for use within the RPNI.
METHODS: Stainless steel (SS) electrodes (n=4) were submerged in 37°C +/-0.5°C 1X phosphate buffered saline and incubated over ~200 hours during which EIS and CV were performed 3X daily using a three electrode system. During EIS, a sinusoidal stimulation with amplitude of 10 mV with respect to the open circuit potential was applied at frequencies from 1 Hz to 100 kHz. CV was completed by sweeping the potential on the working electrode from an initial voltage and upper limit of 0.8 V to a lower limit of -0.6 V using a scan rate of 1000 mV/s. The mean CSCc of the four experimental electrodes was then calculated and differences were quantified using a linear regression model. The initial 10 hours of incubation time during testing were excluded from models to focus on long term stability as opposed to initial electrode fluctuations.
RESULTS: A linear regression line with a 0.54 coefficient of determination and a -0.13 slope coefficient showed an overall decrease in CSCc of 15%. Average CSCc for all electrodes was 151.8 µC with standard deviation of 21.5 µC. Within the first five hours of testing, the impedance for three of the four electrodes was 2.5-7 times greater at 1 kHz than the average of the remaining measurements. During the remaining time points, impedance had a 10% standard deviation from the average impedance at 1 kHz (Figure 1).
CONCLUSION: Impedance and CSCc for SS pad electrodes were stable over time supporting the belief that SS electrodes may be reasonable candidates for use within a RPNI due to minimal degradation, retention of charge for stimulation, and preservation of non-faradaic charge transfer processes. Furthermore, we have developed instrumentation and a series of protocols for stability testing in vitro using EIS and CV. These developments will be used to explore other proposed electrode materials including various electrode metals as well as electroconductive polymer coatings for use in RPNIs.
This work was sponsored by the Defense Advanced Research Projects Agency (DARPA) MTO under the auspices of Dr. Jack Judy through the Space and Naval Warfare Systems Center, Pacific Grant/Contract No. N66001-11-C-4190 and the University of Michigan Undergraduate Research Opportunity Program.
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