Intraneural electrical stimulation of median nerve: a simulation study on sensory and motor fascicles

Neuroprostheses can be an innovative solution to improve quality of life of upper limb amputees. In this framework, the recovery of sensory feedback is a property widely requested by amputee subjects. Neural prostheses are based on neural interfaces that allow delivering direct current stimuli to the nerve fibers. The study of the interaction between the nerve and the electrode is fundamental to investigate activation properties in the nerve. Furthermore, the results could provide useful insight into improve the design of the electrodes and to advance and ameliorate tactile sensations, elicited by these interfaces, obtaining tactile feedback more like natural sensations. This work aims at studying, by means of a FEM Neuron computational model, the axon fibers activation by means of neural stimulation provided through the intraneural electrodes DS-file. Three different types of stimulation waveforms (i.e. biphasic charge balanced stimulus with inter-pulse delay, biphasic charge balanced stimulus without inter-pulse delay, biphasic charge unbalanced stimulus with inter-pulse delay), three different nerve fascicles, i.e. two sensory and one motor fascicle, and ten distances from the electrode in the fascicles, are considered. The efficacy of the stimulation expressed as the percentage of activation of the fibers, and the safety, in terms of current intensity and used waveform, are studied in the previously described different conditions and the results are compared. The obtained results show that: i. stimulating a sensory fascicle with implanted active sites can activate a fascicle close to it, but not all the fascicles belonging to the same nerve. In fact, in the nerve considered in this study, a motor fascicle cannot be activated due to the values of the electrical potential which are too low to activate the fibers; ii. the current intensity necessary to activate fibers increases according to the distance from the source of the stimulus; iii. by using a biphasic charge unbalanced stimulus, the threshold to activate the fibers is lower than using the other tested waveforms. It is an important result because the stimulation is efficient and safer since current intensity is lower than the one used for the other two waveforms.