Liquid-metal fibre mat is highly permeable, superelastic and has good conductivity.
Liquid-metal fibre mat is highly permeable, superelastic and has good conductivity.
Photo: 123

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 fiber 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.

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 Zheng Zijian, 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 fiber mat” (LMFM) and is fabricated by coating or printing liquid metal onto an electrospun elastomeric fiber mat followed by a mechanical activation process in which the liquid metal self-organizes into a laterally porous and vertically buckled film hanging among the fibers.

The LMFM possesses very good permeability, retains super elasticity and ultrahigh conductivity in tensile testing. In addition, it shows good 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,” introduced Professor Zheng Zijian.

By repeatedly stretching the sample to a strain of 18 times for 12 cycles, porous morphologies will be formed to provide good 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),” continued Professor Zheng Zijian. “In vivo animal experiments on rabbit skin, it shows the excellent biocompatibility of the LMFM without causing any irritation.”

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 very good wearing comfort and multi-functionality.

To conclude, LMFM is a new type of stretchable conductor that is easily made by coating or printing liquid metal onto an elastic electrospun fiber 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, multi-functionality and long-time wearability.

This research, titled “Permeable superelastic liquid-metal fiber mat enables biocompatible and monolithic stretchable electronics”, has been recently published in Nature Materials.