Approaching the limits of piezoelectricity driven hot-electron injection for self-powered in vivo monitoring of micro-strain variations

In this paper we explore the limits of self-powering a piezoelectricity driven hot-electron injection (p-HEI) device used for monitoring mechanical activity in biomechanical implants and structures. Previously reported p-HEI devices operate by harvesting energy from a piezoelectric transducer to generate current an voltage references which are then used for initiating and controlling the process of hot-electron injection. As a result, the minimum energy required to activate the device is limited by the power requirements of the reference circuits. The p-HEI device presented in this paper operates by directly exploiting the self-limiting capability of an energy transducer when driving the process of hot-electron injection in a pMOS floating-gate transistor. As a result we show that the p-HEI device can activate itself at input power levels less than 10 nW. Using a prototype fabricated in a 0.5-μm bulk CMOS process we validate the functionality of the proposed injector and show that for a fixed input power, its dynamics is quasi-linear with respect to time. The paper also presents measurement results using a cadaver bone where the fabricated p-HEI device has been integrated with a piezoelectric transducer and is used for self-powered monitoring of mechanical activity.