Professor Chang-Soo Han's team develops next-generation stretchable device core technology

Expected to be utilized in various fields such as displays, sensors, and touch panels

▲ Professor Chang-Soo Han, School of Mechanical Engineering, College of Engineering

A 'stretchable device' is an advanced form of a 'flexible device', and the choice of material used in its development is very important. The shape must be maintained even after long-term use and should not be greatly affected by changes in temperature and humidity.

Professor Chang-Soo Han's research team at the School of Mechanical Engineering at the College of Engineering has developed the core technology of the 'next-generation stretchable device' that uses a non-volatile ion gel to overcome the disadvantages of hydrogels that have been mainly used for conventional stretchable devices.

The ion gel used does not react sensitively to changes in the ambient temperature and humidity and maintains its shape and form for a longer time compared to existing hydrogels or other organic conductive gels. Due to these characteristics, once an ion gel is produced, it can be recycled, thereby reducing both the cost and time spent on production. In addition, thanks to the stable properties of the ion gel, it can be directly used to prototype stretchable devices such as stretchable sensors, stretchable touch panels, and artificial skin materials.

The method of synthesizing the ion gel is the original technology for the synthesis of stretchable materials, and it is expected to greatly influence other researchers in their quests to produce stretchable materials. If the polymer, dispersion medium, or ionic solution of ion gel were to be replaced with better materials, a stretchable material with higher utilization value can be produced.

▲ Ion gel composition and structure

Further advanced from a flexible device, a stretchable device is widely used as a stretchable health monitoring device. Previous flexible devices were limited in monitoring accurately because constant contact with the skin could not be maintained in its original rigid form (smart watch, smart strap, etc.).

One of the materials that is frequently used to fabricate stretchable devices is a transparent gel-type material composed of a structure in which a high proportion of liquid is dispersed in a polymer network that can be stretched more than 10 times its initial length. Among them, hydrogel is the most studied because with water as its main constituent, it has high material accessibility, biocompatibility, conductivity and is transparent at the same time. However, as the water in the gel evaporates, there is a fatal disadvantage that it can not be used for a long time.

In order to compensate for the disadvantages of hydrogels, this study uses polyvinyl chloride (PVC) as a polymer network and uses dibutyl adipate (DBA) as a dispersing material and [EMIM +] [TFSI-] (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) was added as a conductive material to prepare an ion gel with increased conductivity to test its utilization as a strain sensor. The constituents of the gel, PVC, DBA, and [EMIM +] [TFSI-] are hydrophobic materials and have the advantage of being less affected by humidity changes and not evaporating.

▲ Ion gel can maintain its shape without being affected by changes in temperature and humidity.

The ion gel can be stretched up to 300% and it retained its elasticity after 30 repeated tensile tests. The performance as a strain sensor is similar to that of a conventional metal strain sensor, but its usage value is very high because it is transparent and stretches well.

Even as the temperature and humidity change, its performance as a strain sensor did not change significantly, and even if the ion gel was left in the air for 3 weeks, the DBA or ionic solution in the gel remained with little to no evaporation.

An ion gel was used to produce a stress sensor and when evaluated, it exhibited excellent performance as a sensor. It was then fabricated in the form of motion gloves to implement a device that can input the alphabet using sign language and control the motion of a mouse pointer.

▲ You can enter the alphabet in sign language by using ion gel gloves

Prof. Han said, “This study has developed core materials that are essential for stretchable devices that are still difficult to implement. It is expected to be used in various fields such as stretchable displays, stretchable sensors, stretchable input devices, stretchable touch panels, and transparent sign language input devices.

This research was conducted with the support of the National Research Foundation of Korea, and was published in the online edition of the prestigious journal in the field of materials science, Journal of Materials Chemistry A.

* Journal link: https://pubs.rsc.org/en/content/articlelanding/2020/TA/D0TA00090F#!divAbstract

* Title of paper: Humidity-resistive, elastic, transparent ion gel and its use in a wearable, strain-sensing device

* Author information: Young-Jun Son (co-first author, Korea University doctoral program), Prof. Jin-Woo Bae (co-first author, Korea University of Technology & Education), Ho-Jung Lee (co-author, Korea University master’s program), SeongHyun Bae (Sungkyunkwan University doctoral program), Prof. SeungHyun Baik (Sungkyunkwan University) Kyoung-Yong Chun (corresponding author, Korea University research professor), Prof. Chang-Soo Han (corresponding author, Korea University)