Toward breath sensors that are self-powered by design*

Piezoelectric materials are widely used to generate electric charge from mechanical deformation or vice versa. These strategies are increasingly common in implantable medical devices, where sensing must be done on small scales. In the case of a flow rate sensor, a sensor's energy harvesting rate could be mapped to that flow rate, making it "Self-Powered by Design (SPD)". Prior fluids-based SPD work has focused on turbulence-driven resonance and has been largely empirical. Here we explore sub-resonance SPD sensing via a case study of human breathing. We present a model of self-powered piezoelectric sensing/harvesting and validate that model against experimental results. We evaluate how sensor size scaling with Reynolds number impacts its performance as we approach implantable scale, at which we are conducting in-vivo testing in a rabbit model and have detected signs of a methylcholine-induced asthma attack from within the airway. We offer a form of SPD sensing that scales down to micro- or nano-scales, where flows are locally laminar and wake-driven resonance is not an option, and a model-based roadmap for future SPD sensing solutions, including detecting broadband flow information.