A slowly penetrating interfascicular nerve electrode for selective activation of peripheral nerves.

To meet the future needs of functional electrical stimulation (FES) applications, peripheral nerve electrodes must be able to safely, selectively, and independently stimulate small subpopulations of the axons within a common nerve trunk. A new electrode has been designed to place contacts outside of the perineurium, but within the epineurium of the nerve. This slowly penetrating interfascicular nerve electrode (SPINE) combines the safety and simplicity of extraneural cuff electrodes with the intimate interface of intrafascicular wire and probe electrodes. We briefly discuss a mathematical method of quantifying performance of nerve electrodes based on the functional output of the intact neuromuscular system. The quantification involves three variables: 1) the functional recruitment trajectory (FRT), 2) functional overlap (O), and 3) overall functional selectivity (lambda). Second, we present results from six acute SPINE implants on the feline sciatic nerve. Quantification of stimulation results demonstrate interfascicular stimulation is functionally different than extraneural stimulation in 32 of 38 trials. In 19 of 28 trials, interfascicular stimulation is functionally selective based on depth of penetration and 52 of 58 trials demonstrate selectivity based on the side of the penetrating element. Third, tissue sections show that the SPINE electrode penetrates into the nerve within 24 h without evidence of edema or damage to the perineurium.