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. The project was recently featured in the journal Nature Materials.

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, and 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 produced 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, and retains super elasticity and ultrahigh conductivity in tensile testing. In addition, it shows excellent biocompatibility when directly applied to the human skin.

To fabricate the LMFM, the research team 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.

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.

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.