Improvement of Diameter Selectivity in Nerve Recruitment Using Multi-cuff Electrodes

Since the conventional method of electrical stimulation delivered using an extracellular electrode has poor selectivity in terms of nerve fiber type, it is difficult to avoid recruiting antitarget nerve fibers. For example, recruitment of motor fibers by electrical stimulation frequently causes unpleasant symptoms such as pain sensation and numbness. Many electrical stimulation methods have been proposed to achieve nerve typespecific recruitment based on diameter dependence; however, experimental evaluations have suggested insufficient selectivity of these methods. In this study, we evaluated diameter-selective recruitment using triplecuff electrodes, which are composed of a single cathodic cuff electrode sandwiched between two anodic cuff electrodes with various anode-to-cathode distances, De. Using numerical simulations, we determined the stimulation-response characteristics as functions of stimulus intensity (IC) and fiber diameter D(1-20 µmϕ), when De values were set at infinite and 2-mm distances. We observed that thinner axons were recruited as IC increased and that the thresholds for cathodic excitation and anodal block were dependent on De. The dependence of threshold intensities for cathodic excitation and anodal block on various axon diameters at each De was analyzed. The results confirmed that thin axons had higher thresholds and that threshold intensities were nonmonotonically dependent on De. When De was set to infinite distance (corresponding to a single-cuff electrode) and IC was relatively low, only thick axons (≥10 µmϕ, e. g., Aα fiber group) were selectively recruited. By decreasing De to about 2 mm and increasing the stimulus intensity, thicker axons were suppressed by anodal block ; in contrast, thinner axons (≤10 µmϕ) became selectively recruited. It was possible to recruit either moderately thick (5-9 µmϕ, e. g., Aβ fiber group) or thin (2-4 µmϕ, e. g., Aδ fiber group) axons by scrutinizing the De and IC dependence of nerve recruitment. Axon response was robust against radial position in the nerve bundle. These results suggest that electrical stimulation using a triple-cuff electrode properly adjusted for both stimulus intensity and distance between the anodic and cathodic electrodes has the potential for nerve-specific recruitment within a nerve bundle.