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Abstract-A new type of newe cuff electrode consisting of conductive segments embedded within a self-curling sheath of biocompatihle insulation has been developed. This spiral nerve cuff is biased to selfwrap around peripheral nerves and possesses a "self-sizing" property, presenting an alternative to present commercially available, fixed-size nerve cuffs that are manually wrapped around nerves and sutured shut ("split-cylinder" cuffs). Spiral cuff design and manufacture are described.We hypothesize that unlike traditional cuffs, the spiral cuff potentially can be implanted safely when sized to fit peripheral nerves snugly. Theoretical pressure analyses of traditional and spiral cuffs that support this hypothesis are presented. These analyses are designed to predict the minimum CNR (cuff diameter/nerve diameter ratio) at which there is no interference with intraneural blood flow. A safe CNR of 0.997 is predicted for a 1 mm split-cylinder cuff, while a safe CNR of 0.83 is predicted for a 1 mm spiral cuff. The quantitative relationships between nerve size and safe cuff size are detailed. Results of a preliminary experiment in which snug spiral cuffs were implanted on feline peripheral nerve support the prediction that they may be safe.

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Acute experiments were performed on adult cats to study selective activation of medial gastrocnemius, soleus, tibialis anterior, and extensor digitorum longus with a cuff electrode. A spiral nerve cuff containing twelve "dot" electrodes was implanted around the sciatic nerve and evoked muscle twitch forces were recorded in six experiments. Spatially isolated "dot" electrodes in four geometries: monopolar, longitudinal tripolar, tripolar with four common anodes, and two parallel tripoles, were combined with transverse field steering current(s) from an anode(s) located 180 degrees around from the cathode(s) to activate different regions of the nerve trunk. To quantify the degree of selectivity, a selectivity index was defined as the ratio of the force in one muscle to the force in all four muscles in response to a particular stimulus. The selectivity index was used to construct recruitment curves for a muscle with the optimal degree of selectivity. Physiological responses were correlated with the anatomical structure of the sciatic nerve by identifying the nerve fascicles innervating the four muscles, and by determining the relative positions of the electrodes and the nerve fascicles. The results indicated that the use of transverse field steering current improved selectivity. We also found that tripoles with individual dot anodes were more selective than tripoles with four common dot anodes. Stimulation with two parallel tripoles was effective in activating selectively fascicles that could not be activated selectively with only a single tripole. The multipolar cuff proved an effective method to control selectively and progressively the force in muscles innervated by fascicles that were well defined at the level of the cuff.