Electrophysiological Effects of Biomaterial Implantation in a Regenerative Peripheral Nerve Interface (2013)

INTRODUCTION: We are developing a regenerative peripheral nerve interface (RPNI) to achieve high fidelity, intuitive prosthetic control.1,2 Multiaxial control of individual actuators within advanced neuroprostheses requires implantation of multiple adjacent yet electrically independent RPNIs, necessitating effective signal isolation.3–5 This study investigates the acute and long-term efficacy of silicone insulators to reduce extraneous signal in an RPNI. METHODS: Using a rat model, acute and long term evaluations were performed: 1) acute evaluations were performed using non-transferred extensor digitorum longus (EDL) muscle with an intact neurovascular supply (n=8); and 2) chronic RPNI implantations (n=3) were performed using EDL free-muscle transfer to the ipsilateral thigh. RPNI neurotization was performed by transecting and implanting the common peroneal nerve into the transferred muscle. A recording electrode with an overlying 7x5x1mm silicone layer (experimental), and an uninsulated recording electrode (control), were affixed to the EDL muscle surface. The entire construct was then encircled with small intestinal submucosa. The peroneal nerve was stimulated to activate the muscle of interest, whereas the tibial nerve was stimulated to emit extraneous signal from the posterior compartment. Electromyography was performed immediately for acute evaluations, and 6 weeks postoperatively for chronic implantations (Figure 1). Data were analyzed using paired samples T-test.Figure 1: Dorsal drawing of a rat shows experimental setup for acute and chronic RPNI evaluations. (Left) Acute evaluation: the recording electrodes included the experimental electrode (R1), and control electrode (R2) on the non-stimulated extensor digitorum longus muscle, as well as the stimulated reference electrode (R3) on the stimulated soleus muscle. The stimulating electrode (S) is a bipolar electrode and is stimulating the tibial nerve, activating the posterior compartment. The peroneal nerve has been transected (x), preventing activation of the anterior and lateral compartments. (Right) Chronic RPNI evaluation: electrodes included the chronically implanted experimental (R1) and control (R2) electrodes with the RPNI, and the stimulated reference (R3). The stimulating electrode (S) is stimulating the peroneal nerve to directly activate the RPNI, followed by the tibial nerve to activate the posterior compartment.RESULTS: Acute evaluation: At stimulation threshold, the experimental electrode recorded 47.5% extraneous signal whereas the control electrode recorded 55.9%, revealing an 8.4% improvement in signal isolation by utilizing the silicone insulator (p=0.030). At maximum stimulation, the experimental electrode recorded 53.2% extraneous signal versus the 62.7% recorded from the control electrode, demonstrating an improvement of 9.5% (p=0.001). Chronic RPNI: At both threshold and maximum stimulation of the peroneal nerve, the experimental electrode recorded signals which were similar to the control electrode for latency and noise, but demonstrated marginally reduced compound muscle action potential (CMAP) amplitude and area. Tibial nerve stimulation revealed that the isolation of extraneously derived EMG signals was improved in the experimental electrode, reducing 4%-11% of the extraneous signal compared to control. CONCLUSION: While signal acquisition is marginally impaired by silicone insulation, electrophysiological characteristics follow a pattern of early regeneration similar to uninsulated muscle. Although additional strategies will likely be required to optimize signal separation, utilization of a silicone insulator results in improved signal isolation and is a feasible option for incorporation into an RPNI.