Mechanically robust micro-fabricated strain gauges for use on bones

We present a new micro-fabrication process to improve the yield of thin film metal strain gauges that are embedded in a poly-dimethyl-siloxane (PDMS) membrane. This work is directed towards the realization of a flexible, implantable sensor array for measuring surface strain on live bone. Although previous lift-off and peeling processes have been demonstrated to be feasible for fabricating the strain gauges in PDMS, they resulted in a very low yield, approximately 5%. Hence, we implemented a different process to improve the mechanical robustness and fabrication yield. One of the key improvements was the use of wet etching to pattern the thin metal film instead of conventional lift-off. The results demonstrated both an increase in yield to 50% and further miniaturization of the devices. Electromechanical testing results revealed that our metal thin-film gauges have greater gauge factors than commercially available ones, which will result in potentially more precise strain sensing. Various biocompatible adhesives were tested for fixing the micro strain sensor to imitation bone materials. Ultimately this strain gauge will be part of an implantable, wireless array for real-time in vivo monitoring of bone strain in the presence of disease and bone remodeling.