Liquid metal-based soft pressure sensor for wearable health monitoring applications

Abstract

Wearable health monitoring is a promising technology that monitors various health indicators such as pulse, blood pressure, body pressure etc. with wearable sensors and provides valuable information for early diagnosis and prevention of disease for people. Conventional wearable devices are so rigid and bulky that it is difficult to wear conformally on the skin, leading uncomfortableness, and inaccuracy of data. Recently, researchers try to develop soft sensors which can be attached on skin without any restriction of human movement and deliver reliable data. Wearable pressure sensors capable of sensitive, precise, and continuous measurement of physiological and physical signals have great potential for the determination of human health status and early diagnosis of disease. Especially wearable soft pressure sensor has a lot of potential because it can detect various bio-signal and body pressure with simple sensor attachment. Liquid metal-based pressure sensor has been highlighted for its intrinsic stretchability and electromechanical stability as wearable electronic component. However, high sensitivity and reliable structure for liquid metal-based pressure sensor are still challenging obstacles to be solved. This work introduces a 3D-printed rigid microbump-integrated liquid metal-based soft pressure sensor (MLP) for wearable health monitoring applications. Using multi-material fused deposition modeling (FDM) 3D-printed master mold, a fabrication of liquid metal microchannel and integration of microbump array above the microchannel were directly achieved. A rigid (Polylactic acid, PLA) and a water-soluble thermoplastic polymer filament (Polyvinyl alcohol, PVA) were chosen for fabrication of liquid metal microchannel and the integration of microbump. By changing the ratio between the thickness of top elastomer and that of bump ($k_t$) and the ratio between the width of microchannel and that of microbump ($k_w$), the correction factor χ that is the ratio the pressure transmitted to the microchannel and the effective mechanical modulus could be designed. The microbump integration leads to enhancing the sensitivity of pressure sensor ($0.16 kPa^{-1}$) due to locally concentrated deformation of the microchannel. Also, MLP could have a low limit of detection (~ 16 Pa), and fast response ($\tau_{80} ~ 77 ms$) with negligible signal drifting (< 0.5 %) and stable signal response under cyclic loading. In addition, MLP shows excellent robustness against 10,000 cycles of multidirectional stretching/bending, temperature change, and water immersion. Finally, these characteristics enable a wide range of applications as demonstrated in health monitoring systems with the help of reliable packaging system. Wearable pulse monitoring system measured the pulse wave and monitored the changes in the pulse rate during exercise. Also, a cuff-less blood pressure estimation system was developed using pulse transit time (PTT). Through wireless and wearable body pressure monitoring system, it was confirmed that pressure is applied to each body part differently according to various lying postures and floor conditions. In addition, through the development of performance and design of the system for practical use in hospitals, wireless and wearable bedsore prevention system was developed and pre-clinical test of 6-hour monitoring was conducted. The pressure and temperature values were recorded continuously and their changes with respect to the changes of lying postures could be monitored. It is expected that the proposed sensor and health monitoring systems can monitor blood pressure and body pressure in real-time for the prevention of cardiovascular-based diseases and pressure-based skin diseases such as bedsores in the future.