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Objective: Our group is developing a long-term regenerative peripheral nerve interface (RPNI) with motor and sensory capabilities to provide closed-loop neural control of an artificial limb. Our RPNI uses the highly conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) to reduce impedance, and limit biofouling (Egeland 2009). Peripheral nerve interfaces are often compromised by neuromas, which can be painful and cause signal interference. Our null hypothesis is that neuroma formation is unaffected by the presence of PEDOT at three and six months.
Methods: In a rat model, peroneal nerves were divided. The proximal stump was coapted to a scaffolding of 10mm acellularized small intestinal submucosa (SIS). PEDOT was polymerized on these scaffolds for two groups. Dry PEDOT was chemically deposited resulting in a firm compound. Wet PEDOT was electrochemically deposited resulting in a softer compound. The six study groups (n=8 per group) were: Sham, Primary Repair, Divided Nerve, SIS, SIS+Dry PEDOT, and SIS+Wet PEDOT. Tactile sensation was evaluated with von Frey filaments on POD 90 and 180. Final evaluations included histomorphometric quantification. A tensiometer measured stiffness (Young's modulus) of the three types of SIS. Statistical significance was determined by Kruskal-Wallis analysis with multiple comparisons as the model was significant.
Results: Lower von Frey thresholds reflect higher sensitivity (Chaplan 1994), implying more pain due to neuroma formation within the RPNI. As expected, the Sham group was less sensitive than the Divided Nerve group. Addition of Dry PEDOT to the SIS scaffold increased the sensitivity of the operative site compared to SIS alone (0.013±0.017 vs. 0.22±0.33, p<0.05) at 90 days, but this difference diminished at 180 days. Sensitivity for the Wet PEDOT group was not increased (power=0.61, α=0.1). The percent neural area of the Sham group was higher than the Divided Nerve group (50.2%±5.3% vs. 16.1%±5.7%, p<0.01), but no significant differences were found between the SIS group vs. either PEDOT group (Fig 1). Tensiometry indicated that the SIS with Dry PEDOT was significantly stiffer than the SIS with Wet PEDOT (11.9±5.9 vs. 3.48±2.1, p<0.05, Fig 2).
Conclusions: Both the von Frey system and the histologic analysis quantitatively stratify rats with and without neuromas. Neuroma formation in the presence of Wet PEDOT was not different compared to SIS scaffolding alone; the polymer's mechanical pliability may contribute to this phenomenon.
