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Percutaneous implants of sintered hydroxyapatite (HAp) and a microprocessor-based portable stimulator for functional neuromuscular stimulation (FNS) orthoses are discussed. First, a button-shaped HAp was percutaneously implanted up to 23 months in the skin of dogs, and 30 months in human subjects. The HAp was closely contacted with the skin tissue. Then, button-shaped HAp electrical connectors were percutaneously implanted in the skin of rabbits. Sufficient contractions of the calf muscle were obtained by electrical stimuli delivered to intramuscular electrodes through the connector. These results indicated that the HAp electrical connector could be clinically utilized as an implantable skin interface for FNS. The microprocessor-based stimulator is capable of processing control command signals and delivering 16-channels of electrical stimuli. The usefulness of the stimulator was demonstrated by applying it to flexor and extensor muscles of the thumb in a C5 quadriplegic patient.