<em>(Invited) </em>Stretchable Sensing Devices Combining Ionic Liquids and Soft Electrodes

Ota, Hiroki

In recent years, a variety of ultra-flexible devices have been proposed. Their applications include wearable devices and soft robots. Among ultra-flexible devices, the devices with stretchablity is attracting attention as next-generation sensing devices. In such devices, ionic liquids can be used as a sensing material. Ionic liquids are polymers in a liquid state and their structure can be easily altered, For example, ionic liquids can be developed reactive to temperature, humidity, light, gases, and many other factors. Furthermore, since they are in a liquid state, they are durable against device stretching. In this study, we report on a stretchable sensing device using ionic liquids. In particular, by using liquid metal and CNT(Carbon nanotube) as electrode materials, we propose a super-stretchable device and a super-flexible device with transparency and high breathability.

<Temperature/humidity/oxygen/optical sensors using liquid metal electrodes> liquid metal was used as electrode, and ionic liquids as a sensing material. Equivalent circuit of the sensors were established based on Nyquist plot The sensor showed stable sensitivity to temperature without hysteresis as shown with a 0.039/°C increase in conductivity, which is quite high compared to other reports. We also show proof of concept for humidity, oxygen gas, and optical sensings using four kinds of ionic liquids, 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][Otf]), 1-Butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPYR][NTf2]) and 1-butyl-3-(4-phenylazobenzyl)imidazolium bis(trifluoromethanesulfonyl)amide ([Azo][NTf2]) for optical sensing. 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 addition, using [Azo][NTf2], we demonstrate optical sensing and memory in this study.

< Transparent and Breathable Ion Gel-based Sensors using CNT> highly transparent, ultra-flexible, and gas-permeable polymer thin-film sensors using ion gels as the sensing material, which demonstrated the capacity for selective detections, were proposed. Particularly, simultaneous and independent sensing of temperature and humidity was demonstrated in this study. The sensors were fabricated using a simple spray coating method on a thin silicone rubber film (around 25 µm thickness). Owing to their thin-film shape, they showed more than 80% visible light transmittance and a higher gas permeability of 58.7 g/m² h than the human transepidermal water loss. Simultaneous and independent detection of temperature and humidity was achieved with a high sensitivity of 15.9%/°C and 2.5%/percentage of relative humidity, respectively, using two types of gels with ionic liquids.

These results suggest that the easily modifiable nature of ionic liquids contribute to the development of stretchable electronics.

(Invited) Stretchable Sensing Devices Combining Ionic Liquids and Soft Electrodes