Liquid-State Environment Sensors Using Liquid Metal
Authors: Ota Hiroki 
Published: February 2020

Flexible devices have many potential applications in wearable electronics, robotics, health monitoring, and more. Mechanically deformable devices and sensors such as flexible devices enable conformal coverage of electronic systems on curved and soft surfaces. Sensors utilizing liquids confined in soft templates as the sensing component present the ideal platform for such applications, as liquids are inherently more deformable than solids. However, to date, current liquid-metal based strain sensors are incapable of resolving small pressure changes in the few kPa range, making them unsuitable for applications such as heart-rate monitoring which require much lower pressure detection resolution. In addition, liquid-based devices have been limited to metal lines based on a single liquid component given the difficulty in the fabrication of liquid-based junctions due to intermixing. Here we demonstrate state-of-art microfluidic environmental sensors using liquid metal, highly sensitive pressure sensor based on tactile diaphragm pressure sensor, and temperature/humidity/oxygen sensors with mechanically robust liquid-liquid “heterojunction”. This report will present an important advancement towards the realization of liquid-state electronic systems.
Introduction
Electronic devices and sensors which exhibit large amounts of mechanical deformability have many applications such as in smart wallpapers and human-machine interfaces for prosthetics. In this regard, tremendous advancements have been made in engineering solid-state electronic materials and devices on elastic substrates. Recently, sensors based on using liquid active components embedded within soft elastomeric substrates have shown much promise for such applications as liquids present the ultimate limit in deformability. In this paper, we report four microfluidic sensors using liquid metals, highly sensitive pressure sensor based on tactile diaphragm pressure sensor, and temperature/humidity/oxygen sensors with liquid-liquid “heterojunction”. The work presents an important step towards the potential realization of liquid-state electronic systems that offer new form factors and functionality.
Current Result
< Microfluidic diaphragm pressure sensor> In both sensors, the devices were fabricated by photolithography techniques using poly(dimethylsiloxane) (PDMS) as the substrate, and eutectic GaInSn (Galinstan) as electrodes. A microfluidic tactile diaphragm pressure sensor based on embedded Galinstan microchannels (70 µm width 70 µm height) capable of resolving sub-50 Pa changes in pressure with sub-100 Pa detection limits and a response time of 90 ms is demonstrated(Fig. 1). An embedded equivalent Wheatstone bridge circuit makes the most of tangential and radial strain fields, leading to high sensitivities of 0.0835 kPa-1 change in output voltage. The Wheatstone bridge also provides temperature self-compensation, allowing for operation in the range of 20-50 °C. As examples of potential applications, a polydimethylsiloxane (PDMS) wristband with an embedded microfluidic diaphragm pressure sensor capable of real time pulse monitoring(Fig. 2) and a PDMS glove with multiple embedded sensors to provide comprehensive tactile feedback of a human hand when touching or holding objects are demonstrated(Fig. 3).
<Temperature/humidity/oxygen sensors with liquid-liquid “heterojunction”> In termes of temperature/humidity/oxygen sensors with liquid-liquid “heterojunction”, liquid metal was used as electrode, and ionic liquids as a sensing material(Fig. 4 and 5). As shown in Fig. 6, equivalent circuit of the sensors were established based on Nyquist plot The sensor showed stable sensitivity to temperature without hysteresis as shown in Fig. 7 with a 0.039/°C increase in conductivity, which is quite high compared to other reports. We also show proof of concept for humidity and oxygen gas sensing using three kinds of ionic liquids, 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][Otf]), 1-Butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), and 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPYR][NTf2]). As shown in Fig. 7, the sensitivity of each ionic liquid to humidity and oxygen differed depending on the ionic liquid. For each type of stimuli, the sensing can be optimized by choosing the proper ionic liquid. In the future, multiple liquid heterojunction sensors, each with a different ionic liquid may be multiplexed into a fully integrated system to sense different stimuli with calibrated response.

Figure 1