Sensitivity and selectivity of intraneural stimulation using a silicon electrode array

A multielectrode array in silicon technology, as well as experimental paradigms and model calculations for sensitivity and selectivity measures, have been developed. The array consists of twelve platinum electrode sites (10*50 mu m and 50 mu m interdistance) on a 45- mu m thick tip-shaped silicon substrate and a Si₃N₄ insulating glass cover layer. The tip is inserted in the peroneal nerve of a rat during acute experiments to stimulate alpha motor fibers or the extensor digitorum longus muscle. Sensitivity calculations and experiments show a cubic dependence of the number of stimulated motor units on current amplitude of the stimulatory pulse (recruitment curves), starting at single motor level. Selectivity was tested by a method based on the refractory properties of neurons. At the lowest stimulus levels (for one motor unit) selectivity is maximal when two electrodes are separated by 200-250 mu m, which was also estimated on theoretical grounds. The study provides clues for future designs of two- and three-dimensional devices.