Bio-compatible wireless inductive thin-film strain sensor for monitoring the growth and strain response of bone in osseointegrated prostheses

Many benefits can be derived from in situ monitoring of the growth, load response, and condition of human bone. In particular, bone monitoring offers opportunity to advance understanding and designing of osseointegrated mechanical components fixated into bones such as artificial joints and more recently osseointegrated prosthetic limbs. In this study, a bio-compatible wireless inductive strain-sensing system is proposed, which is designed to monitor the growth and strain response of bone-hosting implants. Thin-film circuit fabrication methods based on lithography are adopted to develop a conformable wireless strain sensor designed as a passive resistive–inductive–capacitive circuit. Two forms of strain sensing are designed into the thin-film sensor. First, parallel-plate capacitors fabricated from metal electrodes and a polyimide dielectric layer are introduced to modulate bone strain onto a resonant frequency of the thin-film sensor. A second resonant frequency is introduced in the sensor design to measure circumferential bone growth using a highly nonlinear titanium-resistive element, whose resistance exponentially increases well after 1000 µε under monotonic increasing hoop strain. To ensure the possibility for implantation in animal subjects in future study, the thin-film sensing system is fabricated using mainly bio-compatible polymers (e.g. polyimide) and metals (e.g. titanium and gold). Fabricated prototypes inductively coupled using an impedance analyzer are experimentally tested. Results reveal linear response of the first resonant frequency to low levels of strain with a sensitivity of 4.555 Hz per unit microstrain. The second resonant frequency is sensitive to the resistive fuse with nonlinear fuse behavior initiated above 1000 µε and impedance phase increasing exponentially thereafter.