An implantable sensor chip, which is preferably small enough to be injected through a hypodermic needle, includes a base constructed of an electrically insulating material, and at least one sensor for generating an output signal in the presence of a constituent or the existence of a condition. The sensor includes an electrically conductive lead mounted on an upper surface of the base. One end of the lead includes a conducting plate. An insulating layer is mounted on the upper surface of the base over the electrically conductive lead and the conducting plate, sealing the electrically conductive lead and conducting plate between the insulating layer and the base. The insulating layer has an aperture extending therethrough for exposing a portion of the conductive plate. At least one cell sealingly covers the exposed portion of the electrically conductive plate with the cell in electrical communication with the plate. The cell generates an electrical signal in response to the presence of the constituent or the existence of the condition. The electrical signal is conducted by the lead to an output end of the lead. The output signal is amplified by an amplifier, and may be processed further either at the skin surface, on the sensor chip, or by another implanted processing means. The sensor chip preferably includes a plurality of individual sensors constructed thereon.