Low-Cost Desktop Printed Sensors for Therapeutic Ultrasound Applications

High-intensity focused ultrasound (HIFU) is a noninvasive technology that uses acoustic waves to thermally ablate or mechanically disrupt solid tumors with high spatial precision. However, the focused ultrasound waves can cause a localized increase in temperature around the treated zone, often risking unsafe temperature elevation while treating tumors closer to the skin surface. Flexible sensors attached to the therapy transducer are a cost-effective approach to avoid skin burns and to get real-time feedback on changes in pressure and temperature during the HIFU treatment. Herein, we report low-cost inkjet-printed flexible sensors, integrated with the HIFU transducer to monitor pressure and temperature. The temperature sensors were fabricated as meandering structures using a desktop inkjet printer and nanosilver conductive ink on a polyethylene terephthalate (PET) flexible substrate designed for low-temperature applications. The temperature coefficient of resistance has been studied in the range of 30 °C–70 °C, showing linear characteristics with a mean sensitivity of $\sim 0.03~\Omega $ /°C. For the pressure/contact sensor, a carbon layer was printed over the silver interdigitated electrode layer, showing changes in resistance when deformed by uniaxial forces. Despite the defects in the printed films, the sensors showed a quick response (2–3 s) and recovery (5–6 s) times, with a mean sensitivity of about 24, 16, and 8 N $^{-{1}}$ for 1, 2, and 5g weights, respectively. These sensors have significant potential to be quickly prototyped without relying on the traditional requirements of cleanroom and photolithography facilities for HIFU applications.