Diamond encapsulated photovoltaics for transdermal power delivery

A safe, compact and robust means of wireless energy transfer across the skin barrier is a key requirement for implantable electronic devices. One possible approach is photovoltaic (PV) energy delivery using optical illumination at near infrared (NIR) wavelengths, to which the skin is highly transparent. In the work presented here, a subcutaneously implantable silicon PV cell, operated in conjunction with an external NIR laser diode, is developed as a power delivery system. The biocompatibility and long-term biostability of the implantable PV is ensured through the use of an hermetic container, comprising a transparent diamond capsule and platinum wire feedthroughs. A wavelength of 980 nm is identified as the optimum operating point based on the PV cell's external quantum efficiency, the skin's transmission spectrum, and the wavelength dependent safe exposure limit of the skin. In bench-top experiments using an external illumination intensity of 0.7 W/cm2, a peak output power of 2.7 mW is delivered to the implant with an active PV cell dimension of 1.5×1.5×0.06 mm3. This corresponds to a volumetric power output density of ∼20 mW/mm3, significantly higher than power densities achievable using inductively coupled coil-based approaches used in other medical implant systems. This approach paves the way for further ministration of bionic implants