Augmented Signal Transduction from Regenerative Peripheral Nerve Interfaces (2013)

INTRODUCTION: Intuitive control of neuroprosthetic limbs requires a stable, high-fidelity interface between the patient and the device. The Regenerative Peripheral Nerve Interface (RPNI) consists of a unit of freely grafted muscle that is neurotized by a transected peripheral nerve. In conjunction with an electrode on the surface of the grafted muscle, the RPNI facilitates signal transduction from the residual peripheral nerve to an artificial limb. Electrodes can be coated with conductive polymer to enhance conductivity. This study examines the augmentation of signal strength and fidelity from the RPNI when conductive polymer is applied to implanted stainless steel electrodes. METHODS: In a rat model, the left extensor digitorum longus (EDL) muscle was moved as a nonvascularized free tissue transfer and neurotized by the divided ipsilateral common peroneal nerve (Figure 1). The RPNI was interfaced with either a stainless steel pad electrode (SS, n=8) or a pad electrode coated with poly(3,4-ethylenedioxy-thiophene) conductive polymer (PEDOT, n=8). Acellular extracellular matrix was used to secure the electrode on the surface of the muscle and separate the RPNI from surrounding tissues. The contralateral EDL muscle of each rat served as control. Monthly electrophysiological testing was performed using percutaneous nerve stimulation while recording from the implanted electrode.Figure 1: This representative RPNI consist of a free extensor digitorum longus (EDL) muscle which has been neurotized by the common peroneal nerve. A pad electrode affixed to the muscle surface facilitates bioelectric signal transduction.RESULTS: The free EDL muscle transfer remained healthy with successful revascularization and reinnervation as evidenced by robust transduction of compound muscle action potentials (CMAPs) through the RPNI. Histologic examination confirms axonal sprouting, elongation, and synaptogenesis within RPNIs. The PEDOT coating resulted in significant increases in elicited signal amplitude at all time points compared to uncoated SS electrodes. This signal augmentation with PEDOT-coated electrodes was observed throughout the course of the study (Figure 2). At 5 months, the PEDOT group displayed a 78% increase in mean maximum CMAP amplitude (PEDOT=7.94±0.82 mV, SS=4.46±1.36 mV).Figure 2: Adding a conductive polymer coating (PEDOT) to stainless steel (SS) electrodes augments compound muscle action potential (CMAP) amplitude recorded from the RPNI at all times points over a 5 month study period (*) indicates significance compared to control groups (Ctrl-SS and Ctrl-P), (†) indicates significance compared to SS, p< 0.05.CONCLUSION: Implanted electrodes with and without conductive polymer can be used with the RPNI to bridge the critical signaling gap between a living peripheral nerve and a prosthetic device. All RPNIs in this study demonstrated reinnervation and transduction of bioelectric signals which can be used to control a prosthetic device. Conductive polymer increases RPNI signal amplitude and displays reliability over 5 months. Continued serial evaluations are underway to examine the long-term effects of tissue integration and recording performance of the RPNI.