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Stretchable conductor for wearable electronics

Mar 26, 2021
Stretchable conductor for wearable electronics

The Hong Kong Polytechnic University (PolyU) has developed a highly permeable and superelastic conductor which can be used for wearable electronic devices that can withstand long-time wearing. This novel conductor is fabricated by coating or printing liquid-metal onto an electrospun elastomeric fibre mat, which offers high permeability, stretchability, conductivity and electrical stability so as to be employed in various applications including health monitoring devices, soft robotics and on-skin electronics. This research, titled “Permeable superelastic liquid-metal fibre mat enables biocompatible and monolithic stretchable electronics” was recently published in Nature Materials.

The permeability of the ‘liquid-metal fibre mat’ sample we fabricated is 22 times higher than that of the medical patch.

According to a study conducted by IDTechEx1  in 2016, wearable tech products will be the next big market after smartphones with the market size reaching over US$150 billion by 2026. Electronic devices and systems with high stretchability are essential in the fields of wearable electronics, on-skin electronics, soft robotics and bioelectronics. However, many stretchable electronics are fabricated with impermeable elastic thick films, the long-time wearing of which can cause health concerns including skin irritation and inflammation. Moreover, low permeability will limit the use of multi-layered devices and hinder the development of advanced functionality of stretchable electronics.

To overcome these limitations, the research team led by Professor Zijian ZHENG, Professor of PolyU’s Institute of Textiles and Clothing (ITC), comprised of interdisciplinary academics from the Department of Applied Physics and the Department of Biomedical Engineering, PolyU, developed a new type of highly permeable superelastic conductor. The conductor enables the fabrication of biocompatible and multifunctional monolithic stretchable electronics. This new conductor is called “liquid-metal fibre mat” (LMFM) and is fabricated by coating or printing liquid metal onto an electrospun elastomeric fibre mat followed by a mechanical activation process in which the liquid metal self-organises into a laterally porous and vertically buckled film hanging among the fibres. The LMFM possesses excellent permeability, retains super elasticity and ultrahigh conductivity in tensile testing. In addition, it shows excellent biocompatibility when directly applied to the human skin.

“We selected eutectic gallium-indium alloy (EGaIn), a type of liquid metal commonly used in soft electronics such as flexible printed circuit boards, as the conductive component for printing on the stretchable poly(styrene-block-butadiene-block-styrene) (SBS) mat, a material that is usually used for rubber products like gloves or balloons as an elastomer, to fabricate the LMFM. By repeatedly stretching the sample to a strain of 1,800 per cent (i.e. 18 times) for 12 cycles, porous morphologies will be formed to provide excellent permeability. We fabricated an LMFM sample with a 320µm thick SBS mat and a 0.8mg cm-2 mass loading of EGaIn. The moisture permeability of the sample (724g·m-2·day) is 22 times higher than that of the medical patch (31g·m-2·day). In vivo animal experiments on rabbit skin, it shows the excellent biocompatibility of the LMFM without causing any irritation,” Professor Zheng explained.

EGaIn is a metal that can be maintained in a liquid state under room temperature. It has low viscosity, high conductivity and low toxicity, and is also capable of forming a thin solid layer of oxide (Ga2O3) rapidly on the surface of EGaIn upon exposure to air offering soft and stretchable features. After stretching, the oxide formed on the surface of EGaIn buckles breaks up into holes providing an accordion-like structure for high stretchability and conductivity through the wrinkles.

Furthermore, the LMFM can be fabricated vertically and stacked in three layers of printed EGaIn electrical circuits on monolithic elastic mats – with one layer acting as an electrocardiography (ECG) sensor, another as a sweater sensor, and the final layer as an electrothermal heater. The fabricated three-layer sample, with a total thickness of 1mm, performs well while maintaining high permeability; it implies that the stacked architecture of the LMFM can provide excellent wearing comfort and multifunctionality.

To conclude, LMFM is a new type of stretchable conductor that is easily made by coating or printing liquid metal onto an elastic electrospun fibre mat. With a simple pre-stretch process, it can offer high permeability as well as conductivity. LMFM is also able to be stacked in multiple layers while maintaining high permeability. This innovative permeable and stretchable conductor can be adopted as a user-friendly platform needed to fabricate monolithic stretchable electronics that provide high integration density, multifunctionality and long-time wearability.

This research project is mainly funded by the Hong Kong Scholars Program as well as the Research Grants Council of Hong Kong. The research team will continue to further enhance the performance of LMFM and develop various types of healthcare-related electronic devices and systems.

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