(Invited) Fabriations and Applications Using Liquid Metal Towards Stretchable Electronics
Authors: Ota Hiroki 
Published: February 2024

Highly deformable devices have many potential applications, including wearable electronics, robotics and health monitoring. Mechanically deformable devices and sensors can conformally cover electronic systems on curved or soft surfaces. Liquids deform more easily than solids, so sensors and actuators that utilise liquids trapped in soft templates as sensing components are ideal platforms for such applications. Such highly deformable electronics using ultra-flexible conductive materials are called stretchable electronics. Liquid metals (LMs) are one of the leading materials. In the last few years, liquid metals have steadily gained interest, especially in the field of flexible soft electronics and related applications. The term liquid metal has several definitions and many research efforts are underway to utilise liquid metals in various fields. The simplest definition can be given as 'a composite of low-melting alloys that retain their liquid state at room temperature, are easily deformable and have high electrical conductivity'. Here, an overview of liquid metals, new processing methods, sensors and actuators, batteries and smart devices are reported in focus.

To date, we have adopted a patterned method using microfluidic channels with regard to processing methods for liquid metals such as EGaIn and Galinstan. Microfluidic channels are one of the most accurate and reliable method of producing electrode patterns among the three processing methods, although they are limited by the substrate material and pattern design for pattern fabrication. The creation of microfluidic channels follows the method used in Micro Electro Mechanical Systems (MEMS) and micro Total Analysis Systems (microTAS). In practice, the molds are fabricated by photolithography using SU-8 photoresist. Rubber materials such as polydimethylsiloxane (PDMS) are poured into the mold and allowed to cure. Microfluidic channels are fabricated by chemically bonding the flat PDMS with the PDMS to which the pattern has been transferred. We have succeeded in fabricating a two-dimensional pattern of liquid metal by the method described above. Furthermore, we have successfully fabricated microfluidic channels by 3D printing of rubber materials and constructed 3D circuits by using liquid metal. As liquid metal is a liquid material, processing methods centred on "printing" processing can be used, and the most suitable processing method can be selected according to the device and application.

Physical sensors such as strain sensors and chemical sensors such as oxygen gas sensors have been reported using liquid metal wiring. By optimising the wiring of the liquid metal, capacitor structures and resistors can be fabricated. By encapsulating these in a flexible material, PDMS, it is possible to realize sensors that can measure displacements such as strain. Indeed, by optimising three sensor structures, we have reported a physical sensor capable of sensing three sensing modes: two-dimensional tension and vertical compression. However, taking into account the actual application in wearable devices for healthcare applications such as pulse rate, a detection sensitivity below 1 k Pa is required. The authors realized a high-sensitivity pressure sensor by using a Wheatstone bridge circuit structure for the liquid metal wiring in the PDMS to form a Diaphragm sensor. In fact, a pulse rate sensing device was realized based on the developed device. Furthermore, a PDMS glove capable of measuring 16 points simultaneously was realized by attaching the developed pressure sensor to a hand mold using 3D printing5).

Such physical sensors are very easy to construct and can be realized with one type of liquid metal. While there are many different types of liquid metals as described above, liquid metals easily mix when they come into contact with each other. Therefore, the authors proposed a "liquid" state heterojunction using ionic liquids and liquid metals with extremely low vapor pressure. By using ionic liquids [EMIM][Otf], [EMIM][PF6] and [EMPYR][NTf2], sensors for temperature, humidity and oxygen gas were realized. Furthermore, with regard to their sensitivity, the detection sensitivity was 10 times higher than that of normal metal temperature and humidity sensors for temperature and humidity sensors. In this study, heterojunctions were realized using microfluidics. Due to the low Reynolds number of the microfluidics, the robustness of the liquid state heterojunction with respect to deformation was also very high due to the high viscosity effect, making the configuration suitable for stretchable sensors. In addition to electrodes and sensors, development of batteries and smart devices has also been initiated and will be reported in this presentation.