Electro-chemical stimulation of neuromuscular systems using ion-selective membranes : flexible device fabrication and motor unit recruitment order

Abstract

Spinal Cord injury (SCI) leads to paralysis, decrease in quality of life and high lifetime medical costs. Direct nerve Functional Electrical Stimulation (FES) induces muscles to contract by electrically stimulating nerves, which shows promise for clinical applications in restoring muscle function in SC. However, Functional Electrical Stimulation is limited by the lack of graded response in muscle contraction and by high fatigability due to the reversal of recruitment order of motor units. Previous work showed that ion-selective membranes can be used to modulate Ca 2 ions in situ, decreasing the current threshold for nerve stimulation and eliciting a more graded muscle contraction response. This work developed polyimide-based cuff ion-selective electrodes to enable the future application of this technique in vivo. The developed electrodes were flexible, elastic and conductive. In vitro tests of the electrodes by stimulation of frog sciatic nerve reproduced the decrease in stimulation current threshold, which had been observed in planar glass-based electrodes, in the flexible polyimide-based electrodes. Additionally, cuffing the stimulated nerves with ion-selective electrodes was more effective at decreasing current threshold than planar stimulation. This work also analyzed data on twitch width, contraction time and relaxation time to infer effects of ion-selective electrodes on recruitment order. Stimulation with the ion-selective electrodes had higher twitch width, contraction time and relaxation time than traditional electrical stimulation at all force levels. The difference was particularly high at low force levels, indicating an effect of Calcium ion depletion on recruitment order.