CMUT-Based Force Sensing

Strain gauges are regarded as the gold standard in various force sensing applications, including minimally invasive surgical devices, due to their high sensitivity and ability for miniaturization at millimeter scale. However, further reduction in size and improvement concerning the limit of detection are desirable to enable seamless integration into highly constrained environments, such as needle tips, without compromising performance. Capacitive micromachined ultrasonic transducers (CMUTs), though not previously applied to direct force measurement, offer significant potential as high-sensitivity alternatives when their resonant characteristics are effectively harnessed. In this work, we present a piston-structured CMUT with a thinned back electrode, configured as a force-sensing element mounted on a deflecting cantilever. The system is evaluated using both numerical simulation and experimental analysis. The numerical simulation is verified analytically and validated by measuring the deflection of the top plate. As a proof of concept, the performance of the CMUT is characterized via impedance-based frequency tracking, avoiding full circuit integration. We demonstrate the feasibility of CMUTs as force-sensitive resonators, achieving sensitivities of 0.80 Hz/mN under tensile loading and 0.76 Hz/mN under compression, with a linearity error of 2.69% and an initial response threshold of 20.5 mN. These results establish the functionality of CMUTs as force-sensing elements and lay the groundwork for further optimization toward a new class of highly integrable force sensors.