Soft and stretchable material shows multi-role potential

Their approach employs a process that eliminates many drawbacks of previous fabrication methods, such as less conductivity or device failure, the team said. Their findings have been published in Advanced Materials.   

In a statement, Penn State Assistant Professor Tao Zhou said: “People have been developing soft and stretchable conductors for almost a decade, but the conductivity is not usually very high.

“Researchers realised they could reach high conductivity with liquid metal-based conductors, but the significant limitation of that is that it requires a secondary method to activate the material before it can reach a high conductivity.” 

Liquid metal-based stretchable conductors suffer from inherent complexity and challenges posed by the post-fabrication activation process, the researchers said.

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Secondary activation methods include stretching, compressing, shear friction, mechanical sintering and laser activation, all of which can lead to challenges in fabrication and can cause the liquid metal to leak, resulting in device failure.   

“Our method does not require any secondary activation to make the material conductive,” said Zhou. “The material can self-assemble to make its bottom surface be very conductive and its top surface self-insulated.” 

In the new method, the researchers combine liquid metal, a conductive polymer mixture called PEDOT:PSS and hydrophilic polyurethane that enables the liquid metal to transform into particles.

When the composite soft material is printed and heated, the liquid metal particles on its bottom surface self-assemble into a conductive pathway.

The particles in the top layer are exposed to an oxygen-rich environment and oxidise, forming an insulated top layer.

The conductive layer is critical for conveying information to the sensor — such as muscle activity recordings and strain sensing on the body — while the insulated layer helps prevent signal leakage that could lead to less accurate data collection. 

“Our innovation here is a materials one,” said Zhou. “Normally, when liquid metal mixes with polymers, they are not conductive and require secondary activation to achieve conductivity. But these three components allow for the self-assembly that produces the high conductivity of soft and stretchable material without a secondary activation method.”  

The material can also be 3D-printed, Zhou said, making it easier to fabricate wearable devices. The researchers are continuing to explore potential applications, with a focus on assistive technology.