LONG-TERM BIOCOMPATIBILITY OF A MINIATURE STIMULATOR IMPLANTED IN FELINE HIND-LIMB MUSCLES

Chronic foreign-body responses and muscular changes were examined foll owing the implantation of active miniature stimulators into the hind l imb muscles of cats for periods of up to three months. The radio-frequ ency (RF)-powered stimulators were injected into muscles through a 12- gauge hypodermic needle, The tissue responses around the active stimul ators were compared histologically to those provoked by passive device s, broken glass, silicone tubing, polyester suture material coated wit h polybutylate, and two of the internal components of the stimulator ( ferrite, integrated circuit chip). Active and passive stimulators prod uced similar, benign foreign-body reactions that resulted in an essent ially identical fibrous capsule over time. The responses were similar to those produced by the internal components and the suture material, and were more modest than those produced by the broken glass. The caps ule did not appear to interfere with the functionality of active de,ic es because thresholds measured during the post-implantation survival p eriod did not change significantly over time. Unexpectedly, the severi ty of the reaction differed significantly amongst the various target m uscles, Medial gastrocnemius exhibited the most severe response, where as tibialis anterior had the least reaction.