A Liquid-Metal Wearable Sensor for Respiration Monitoring: Biomechanical Requirements, Modeling, Design, and Characterization

Liquid metals are metals in the liquid state at room temperature; they mostly belong to the alloys of Ga and In. These alloys have attracted attention because of their capacity to conduct electricity and heat, coupled with unique mechanical properties, which allow them to undergo extreme deformations, and self-heal. Their many possible applications range from microfluidics to soft robotics. They are particularly interesting for the fabrication of wearable sensing devices. We design and develop a liquid-metal-based hybrid sensor to study the qualitative and semiquantitative aspects of respiratory mechanics, monitoring the expansion and contraction of chest movements, exploiting the coupling of eutectic Ga-In alloy (EGaIn) and polydimethylsiloxane (PDMS), which show optimal characteristics to build a sensor for this application. The liquid metal has a suitable ohmic response and PDMS, due to its hyperelastic behavior and low viscosity, can be deformed easily and can transmit, without distortion, the primary frequency components useful to characterize different respiratory patterns, from slow and deep breathing to fast and shallow breathing. The time course and spectrum of the electric resistance signal from the sensor correspond closely to a standard reference obtained simultaneously by measuring chest deformation via optoelectronic stereophotogrammetry. The fabrication process is easy, cheap, and robust and can be industrialized.