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.