Chronic Wireless Stimulation in Rat Sciatic Nerve for Selective Control of Hindlimb Movements

Stroke is one of the leading causes of long-term impairment in the United States, and as many as half of all stroke survivors with difficulty walking will continue to have walking dysfunction after rehabilitation therapy (Hendricks et al., 2002) (Little et al., 2020). In particular, weakness and spasticity in the muscles controlling dorsiflexion (pointing the toes towards the shin) and plantar flexion (pointing the toes towards the heel) of the ankle are common causes of post-stroke gait dysfunction (Perry and Burnfield, 2010). Despite the success of devices for improving dorsiflexion, there still are no devices that adequately assist plantar flexion of the ankle during walking. Implanted devices for peroneal nerve stimulation are commonly used to treat foot drop after stroke. And so, this work investigates the potential use of a wireless nerve stimulator for achieving selective modulation of both dorsiflexion and plantar flexion. Six female Sprague Dawley rats were implanted with a wireless floating microelectrode array (WFMA) in the left sciatic nerve. Device performance was then tracked over a period of 38 weeks to determine the selectivity and chronic stability of the device-tissue interface. Four behavioral tests were used to assess nerve damage caused by implantation of the WFMA. Response to heat and pressure stimuli applied to the paw was used as an indicator of changes in sensory function in the limb. Performance on ladder and treadmill walking tasks were used as an indicator of changes in motor function in the limb. Stimulation tests were performed under anesthesia throughout the 38-week study to determine (1) the motor recruitment threshold, (2) the type of movement (direction of ankle rotation and toe flexion or extension), and (3) changes in recruitment thresholds over time. We found that WFMA devices implanted in rat sciatic nerve for 38 weeks do not cause a functional deficit in the limb. Animals’ response to thermal and pressure stimuli before and after device implantation were not significantly different, nor was animals’ performance on ladder and treadmill walking tasks. Motor recruitment thresholds as low as 4.1 µA were able to generate visible motion in the limb, and more than 57 % of all implanted electrodes (96 in total) had thresholds at or below 20 µA throughout the 38-week study. Each implanted device (n=6) was able to evoke the targeted movements of plantar flexion and dorsiflexion at the ankle, and a variety of other evoked movements were observed throughout the study as well. Both the type of movement and the threshold value remained stable for the duration of the 38-week study, where 72 electrodes (75 %) were able to evoke motion in the limb at week 1 and 79 electrodes (82 %) were able to evoke motion at week 38. In summary, the WFMA provides a chronically-stable neural interface that can be used for highly selective motor recruitment in the hindlimb using wireless power and communication.