PolyU develops a highly permeable superelastic conductor which can be used for wearable electronic applications

PolyU has developed a highly permeable and superelastic conductor which can be used for wearable electronic devices that can withstand long-time wearing and be employed in various applications, including health monitoring devices, soft robotics and on-skin electronics. This research was recently published in Nature Materials (link).

Currently, 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, and comprised of interdisciplinary academics from the Department of Applied Physics and the Department of Biomedical Engineering at PolyU, developed a new type of highly permeable superelastic conductor. 

The conductor, called "liquid-metal fibre mat" (LMFM), enables the fabrication of biocompatible and multifunctional monolithic stretchable electronics. It 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.

Professor Zheng explained that eutectic gallium-indium alloy (EGaIn), a type of liquid metal commonly used in soft electronics such as flexible printed circuit boards, is used 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. The moisture permeability of the sample is 22 times higher than that of the medical patch. In vivo animal experiments on rabbit skin, it showed the excellent biocompatibility of the LMFM without causing any irritation, Professor Zheng added.

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 rapidly on the surface of EGaIn upon exposure to air offering soft and stretchable features. 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 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.

This research project received a total funding of HK$1.8 million, mainly from the Hong Kong Scholars Program and the Research Grants Council of Hong Kong. Professor Zheng said they will apply for another funding in hope of converting the research into application. The team's priority is to develop wearable electrocardiographic monitoring equipment. Further research may take three to five years, and development of a prototype may require a few million dollars.

理大研發高透氣超彈性導電材料 適用於穿戴式電子設備 (Chinese)