Prospects for Implantable Sensors Powered by near Infrared Rechargeable Batteries

is charged and is able to automatically power the device when the photodiode array is not illuminated. The current flow backward from the battery to the photodiode array is shut off by inserting a diode between them. Of course, sunlight is not always available. One light source easily able to recharge the battery in the absence of sunlight is a NIR laser diode (e.g., a Coherent S-81–1000C-100-H at 810 nm). The photodiode array to convert light into electricity comprises eight Si PIN photodiodes (Hamamatsu, S6775) connected in series to obtain sufficient voltage to the charge the battery. The detection area for every photodiode is 5.5 × 4.8 mm. The dimensions of the complete photodiode array prototype, including the packages, are 28 × 20 × 3 mm. The battery is a composite dimensional manganese oxide (CDMO) lithium secondary battery (Sanyo, ML-2430, 100 mAh). The power conversion efficiency of the photodiode array varies with the voltage across it. The photodiode array sustains the best power conversion efficiency throughout the battery charge because the voltage across the photodiode array is regulated by the battery. Enhanced power conversion efficiency can be used to provide shorter charge time or reduced radiant power, or both. Animal studies have been conducted with the photodiode array completely implanted under the shaved abdominal skin of an anesthetised rat at 10 weeks of age. The pathlength of the rat skin over the photodiode array was 0.8 mm