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Electroconductive Polymer Augments Signal Transduction from a Regenerative Peripheral Nerve Interface for Control of a Neuroprosthetic Limb
Theodore A. Kung, MD1, Paul S. Cederna, MD1, Nicholas B. Langhals, PhD1, David C. Martin, PhD2, Melanie G. Urbanchek, PhD1.
1University of Michigan, Ann Arbor, MI, USA, 2University of Delaware, Newark, DE, USA.

PURPOSE: A major obstacle to the use of closed-loop neuroprosthetic limbs is the lack of a stable, high-fidelity interface for control of the device. The regenerative peripheral nerve interface (RPNI) consists of a unit of free muscle that has been neurotized by a peripheral nerve. In conjunction with a biocompatible electrode on the surface of the muscle, the RPNI permits signal transduction from a peripheral nerve and offers enormous potential for both motor function and sensory feedback from neuroprosthetic limbs. 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 (n = 11), the left extensor digitorum longus (EDL) muscle was removed as a nonvascularized free tissue transfer and neurotized by the divided ipsilateral common peroneal nerve (Figure 1). The RPNI was interfaced to either a stainless steel pad electrode (SS group, n = 6) or a pad electrode coated with poly(3,4-ethylenedioxythiophene) conductive polymer (CP group, n = 5). The contralateral intact EDL muscle of each rat served as control. Thin-film acellular extracellular matrix was used to secure the electrode against the muscle and segregate the RPNI from surrounding tissues. Monthly electrodiagnostic testing was performed by percutaneous nerve stimulation with recording from the indwelling electrode. Insertional electromyography (EMG) electrical activity was used to evaluate reinnervation of the EDL muscle within the RPNI.

RESULTS:
The free EDL muscle transfer undergoes successful revascularization and reinnervation, as evidenced by the transduction of compound muscle action potentials (CMAP) from the RPNI. The EDL muscle remains healthy and demonstrates attributes comparable to normal muscle. The addition of conductive polymer to the SS electrode yields a 68% increase in the mean maximum CMAP amplitude at 1 month and a 52% increase at 2 months (Figure 2). The CP group also displayed a 95% increase in mean CMAP area at 2 months compared to the SS group. Histologic examination confirms axonal sprouting, elongation, and synaptogenesis within the RPNI. There was minimal fibrous encapsulation of the electrode.

CONCLUSION:
The novel RPNI in combination with modern electrode technology bridges the critical signaling gap between a living peripheral nerve and a prosthetic device. The application of free muscle to a transected peripheral nerve controls neuroma formation and amplifies bioelectric signals. Electroconductive polymer augments recorded signals through the implanted electrode from the RPNI. Translation of the RPNI to the clinical setting will allow conceptualization of highly sophisticated neuroprosthetic limbs which can replicate the intricate function of the native human upper extremity.


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