Battery-Free, Wireless, Crack-Activated Pressure Sensor and Movable System for Pressure Injury Prevention

Battery-free, wireless, skin-mountable pressure sensing system capable of continuous, accurate measurement of pressure at skin interfaces have great potential for preventing development of pressure injuries. However, it is still challenging to continuously track the pressure with high sensitivity and stability over required pressure range on critical sites from patient sitting in wheelchair. Here this paper introduces a battery-free, wireless, crack-activated pressure sensor and a movable sensing system for continuous measurement of the pressure and temperature on locations of interest from human subjects while moving or stationary in wheelchair.<br/>The pressure sensor consists of a glass substrate with an opening cut, a Crack-Activated Metal Film (CAMF), a soft/rigid pad and a soft/rigid frame using an assembly process that provides easy control for pressure range and sensitivity using geometrical parameters. The CAMF has cracked Cr/Au traces on flexible PI film fabricated using a conventional micro-patterning technique and a pre-stretching method. The opening of these cracks reversibly generated from the bending-induced tensile strain against applied loadings facilitates the enhancement of sensitivity, compared with that obtained using conventionally evaporated metal-based device. A nanoscale thin-film of Parylene-C as an encapsulation layer protects the cracked Au film from serious damage, resulting in minimizing hysteresis and drift. For this reason, the encapsulated CAMF shows excellent sensitivity and high linearity at the low-pressure regime. Also, the encapsulated CAMF with different pre-strains by adjusting a thickness of rigid pad (Z) leads to dramatically enhancing sensitivity under the same applied loadings. The device shows the mechanical stability with negligible hysteresis and drifts against repeated loading/unloading, which is essentially required for continuous measurement of pressure for use in clinical study.<br/>Also, the battery-free, wireless platform and movable sensing system provide capabilities for stable, long-range communication at multiple mounting locations while moving or stationary. The operation of these devices and system, connected to a wireless reader, potable batteries and a multiplexer located over back rest of wheelchair, rely on continuous, simultaneous readings of pressure and temperature in a sequential readout scheme from four multiplexed antennas integrated under back rest, seat and two footrests, respectively. Experimental evaluation for sensing performances of the pressure sensor and movable system includes benchtop measurements, verified by numerical simulations to present the functionality of the system. Finally, clinical trials with spinal cord injured patient in wheelchair demonstrate the feasibility and stability of the sensor and movable system for prevention of sitting acquired pressure injuries.