(Invited) Stretchable Liquid-State Electronics Using Liquid Metal

Abstract

Stretchable devices have many potential applications in wearable electronics, robotics, health monitoring, and more. Mechanically deformable devices and sensors enable conformal coverage of electronic systems on curved and soft surfaces. Sensors and actuators 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. A new approach utilizing the concept of “liquid-state electronics” with conductive liquids for stretchable sensors has been proposed. Liquid metals(LMs) is one of the promising materials. Over the past few years, LMs have been steadily attracting more interest, particularly in the flexible and soft electronics field and the related applications. There are several definitions for the term “liquid metal” and numerous research efforts have been underway to use liquid metals in various fields. The simplest definition for this term can be given as “a composite of alloys with low melting points having high electrical conductivity that allows retention of a liquid state at room temperature and good deformability”. Here, we report outline of soft sensors and actuators using liquid metals, highly sensitive pressure sensor based on the Wheatstone bridge, an temperature, humidity, oxygen, light sensors with liquid-liquid “heterojunction” and gel actuators using liquid metalIn terms of a pressure sensor, current liquid-metal based pressure 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. The developed capable of resolving sub-50 Pa changes in pressure with sub-100 Pa detection limits and a response time of 90 ms is demonstrated in this report. As examples of potential applications, a polydimethylsiloxane (PDMS) wristband with an embedded microfluidic diaphragm pressure sensor capable of real time pulse monitoring.Liquid-based devices such as pressure sensors have been limited to metal lines based on a single liquid (e. g. Liquid metal) component given the difficulty in the fabrication of liquid-based junctions due to intermixing. We demonstrate state-of-art microfluidic environmental sensors using liquid metal, temperature, humidity, oxygen and light sensors with mechanically robust liquid-liquid “heterojunction”. This report will present an important advancement towards the realization of liquid-state electronic systems.Gel actuators are composed of gels which have functionality of actuation according to environmental situation such as temperature change and pH. However, in terms of gels reacting environmental changes, they require the change in entire chamber, which makes control of an individual actuator difficult. Poly(N-isopropylacrylamide) [PNIPAM] that has thermo responsive actuation is one of the conventional gels for actuators. PNIPAM gel exposed to an environment at higher than lower critical solution temperature(LCST) changes hydrophilic to hydrophobic. Eventually, its volume decreases. In order to actuate the gels, they are required to temperature change inside a whole chamber. It is difficult to control the actuation individually. In this presentation, a gel actuators that combine a heater using ultra-soft coil with liquid metal, and a thermo responsive gel is proposed. The resistive heat by liquid metal changes temperature only around an actuator, which achieves the electrical control of an individual actuator.