A process is described for testing a biomedical property of an internal tissue of a patient. Optical energy emitted by an external source is transferred through a nail of the patient to an instrument device implanted beneath the nail. A portion of the transferred optical energy is converted to electrical power for driving components of the implanted instrument. Using the electrical power, a characteristic of the internal tissue associated with the measurement of the biomedical property is sensed and an optical signal based on the sensed characteristic is transmitted through the nail to an external data reader.