(Invited) Highly Deformable Photonic Devices Based on Liquid Metal Towards Wearables
Authors: Ota Hiroki 
Published: December 2020

Introduction

Stretchable electronics, the next step beyond flexible electronics, are driven by advanced organic materials and microfabrication techniques. They have been used for mechanically deformable devices and sensors which enable conformal coverage of electronic systems on curved and soft surfaces. Liquid-state electronics utilizing functional liquids confined in soft templates as the sensing and actuating component present the ideal stretchable electronic platform [1]. However, to date, photonic devices based on functional liquid materials as represented by photodetectors and optical memories and vital sensors still have not been proposed. Here, we report liquid-state photonic devices based on liquid metal and photo-switchable ionic liquid and vital sensing device. As a proof of concept, a liquid-state light sensor and an optical memory which is switched on and off by UV and blue light exposures, and pulse sensing device are demonstrated in this study. In addition, in order to realize these devices, a liquid-state heterojunction was used in interconnects between sensing ionic liquid and liquid metal that prevent the intermixing of the two liquid components when the completed devices undergo mechanical deformation[1]. This advancement is crucial to scaling up current liquid-state devices to a system level.

Device Fabrication and Experimental Results

A liquid-liquid heterojunction was obtained by microchannels [1]. The liquid-liquid heterojunction is critical to preventing intermixing of two liquid components when the completed devices undergo mechanical deformation. The photo-switchable ionic liquid used in this study is composed of azobenzene [2]. Azobenzene changes the molecular structure depending on the wavelength of the irradiated light. Using these technologies, liquid-state light sensor and optical memory were demonstrated. Light sensor composed of microchannels filled in liquid metal and ionic liquid, fabricated by soft lithography. Resistance of a light sensor was decreased by UV light with respect to the light intensity. Similarly, its resistance was decreased by blue light. The relationship between resistance change and electric power for UV light and blue light was proportional and showed that the sensitivity of the UV light was higher than blue light. Using same mechanism, pulse sensing device was demonstrated. Optical memory took advantage of a mixture of ionic liquid and PPE. The solution exhibited and kept low resistance when irradiated with blue light, and high resistance when irradiated with UV light. 2 bit optical memory composed of microchannels filled in liquid metal and a mixture of ionic liquid and PPG, fabricated by soft lithography and 3Dprinting [3]. Interference of light between bits was prevented by filling liquid metal into microchannels fabricated by 3D printing. The digital display controlled by two 1-bit liquid-state optical memories showed “Y” or “N” or “U”.

Acknowledgement

This work was supported by JST CREST Grant Number JPMJCR1905, Japan, by the Japan Science and Technology Agency, PRESTO Grant Number JPMJPR18J2, and by JSPS Grant-in-Aid for Challenging Exploratory Research.

References

(1) H. Ota et al., “Highly deformable liquid-state heterojunction sensors,” Nature Communications, vol. 5, 2014.
(2) C. Wang et al., “From Macromolecular to Small-Molecular Triggers: Facile Method toward Photoinduced LCST Phase Behavior of Thermoresponsive Polymers in Mixed Ionic Liquids Containing an Azobenzene Moiety,” Macromolecular Rapid Communications, vol. 37, no. 23, pp. 1960–1965, 2016.
(3) T. Ibi, E. Komada, T. Furukawa, and S. Maruo, “Multi-scale, multi-depth lithography using optical fibers for microfluidic applications,” Microfluidics and Nanofluidics, vol. 22, no. 6, pp. 1–8, 2018.