Sensitivity and selectivity of intraneural stimulation using a silicon electrode array

Artificial electrical stimulation of peripheral nerves needs the development of multielectrode devices which stimulate individual fibers or small groups in a selective and sensitive way. To this end, a multielectrode array in silicon technology has been developed, as well as experimental paradigms and model calculations for sensitivity and selectivity measures. The array consists of twelve platinum electrode sites (10 x 50 microns at 50 microns interdistance) on a 45 microns thick tip-shaped silicon substrate and a Si3N4 insulating glass cover layer. The tip is inserted in the peroneal nerve of the rat during acute experiments to stimulate alpha motor fibers of 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 microns, which was estimated also on theoretical grounds. The study provides clues for future designs of two- and three-dimensional devices.