Effects of stratum corneum and conductive gel properties on sensory afferents recruitment by 3D TENS computational modeling

Incorporation of tactile sensory feedback by non-invasive transcutaneous electrical nerve stimulation (TENS) is potentially helpful for a prosthetic hand to accomplish dexterous manipulation. As for TENS through surface electrode, the stimulating current flows directly through the conductive gel and the stratum corneum (SC) layers to activate the tactile afferent nerve fibers. In our study the specific modulation effect of recruiting afferent nerve fibers in the two layers was investigated by establishing a 3D finite element model of the forearm. The results showed that both decreasing the gel thickness and increasing the gel resistivity improved the electrical stimulation sensitivity for electrotactile feedback, which would be beneficial to lower the threshold current of TENS. Meanwhile, decreasing the SC resistivity was also beneficial to improve the electrical stimulation sensitivity, but the variations of the normal SC thickness had negligible influence on the sensitivity. On one hand, these results gave us a specific guidance for choosing appropriate property parameters for conductive gel. On the other hand, both the thickness and resistivity of SC were susceptible to age, gender, and physical conditions etc., so it was significant for us to comprehend specific modulation effect under variation of SC properties.