Stimulation electrodes with higher levels of selectivity than currently available are required to restore limb function, especially in the upper extremity. One strategy to increase the selectivity of an electrode is to move it closer to the target axons. A balance between selectivity and invasiveness can be made with interfascicular electrodes. We hypothesized that through Finite Element Method (FEM) models we could show that directed interfascicular electrodes placed within the nerve can achieve levels of selectivity equal to that of intrafasciclular contact without penetrating the perineurium.
A simplified FEM model of a nerve was created of two cylindrical fascicles placed within a cube of epineurium. Using this model we showed that the contact-fascicle distance had a larger impact on the selectivity of a directed interfascicular contact than the relative diameter of the fascicles when the contact is less than 50 ¿¿m from the surface of the perineurium. When the contact is greater than 50 ¿¿m from the surface of the perineurium, the relative sizes of the fascicles have a more significant impact on the selectivity of the electrode. As the contact is placed closer to the surface of the perineurium, specifically within 50 ¿¿m, the surrounding fascicles have a reduced impact on the overall selectivity. Additionally, we have shown that interfascicular electrodes can achieve levels of subfasciclular selectivity greater than that of intrafascicular electrodes even when placed 50 ¿¿m off the surface of the perineurium.
A bio-inspired FEM model was created to compare the selectivity of interfascicular and intrafascicular electrodes in a more realistic environment. In each simulation, directed intrfascicular electrodes placed directly on the surface of the perineurium were able to achieve selectivity levels equal the intrafascicular electrodes. The results of the previous model were also corroborated, as the interfascicular contact-fascicle distance increased, the selectivity of the interfascicular contact decreased and the diameters of the neighboring fascicles had an increasingly significant effect.
These results suggest furthering the exploration and development of directed interfascicular contacts with the primary goal of placing the contacts within 50 ¿¿m of the target fascicle.