Implanted Transmitters and Body Fluid Permeability

Low power radio transmitters implanted in the body to telemeter physiological information are caused to drift by the permeability of their cases to body fluids. Potting the circuit in epoxy resin delays the process only slightly, though filled epoxys are somewhat more effective. A wax coat is helpful, but apparently not sufficient; a layer one mm thick can suffice a few months. Coating the entire unit with silicone rubber only approximately doubles the time of uptake of moisture by the underlying epoxy. Tests on various materials have been published [1].