A piezo-powered floating-gate sensor array for long-term fatigue monitoring in biomechanical implants

Measurement of the cumulative loading statistics experienced by an implant is essential for prediction of long-term fatigue failure. However, the total power that can be harvested using typical in-vivo strain levels is less than 1 μW. In this paper, we present a novel method for long-term, battery-less fatigue monitoring by integrating piezoelectric transduction with hot-electron injection on a floating-gate transistor array. Measured results from a fabricated prototype in a 0.5-μ CMOS process demonstrate that the array can sense, compute, and store loading statistics for over 70000 stress-strain cycles which can be extended to beyond 10⁷ cycles. The measured response also shows excellent agreement with a theoretical model and the nominal power dissipation of the array has been measured to be less than 800 nW.