Self-assembling, highly conductive sensors could improve wearable devices

To advance soft robotics, skin-integrated electronics, and biomedical devices, researchers at Penn State developed a 3D-printed material that is soft and stretchable – traits needed for matching the properties of tissues and organs – and that self-assembles. Their approach employs a process eliminating drawbacks of previous fabrication methods, such as less conductivity or device failure.

“People have been developing soft and stretchable conductors for almost a decade, but the conductivity is not usually very high,” says corresponding author Tao Zhou, Penn State assistant professor of engineering science and mechanics and of biomedical engineering in the College of Engineering and of materials science and engineering in the College of Earth and Mineral Sciences. “Researchers realized they could reach high conductivity with liquid metal-based conductors, but the significant limitation of that is 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 say. The secondary activation methods include stretching, compressing, shear friction, mechanical sintering, and laser activation, leading to challenges in fabrication and causing the liquid metal to leak, resulting in device failure.

“Our method doesn’t require any secondary activation to make the material conductive,” says Zhou, who also has affiliations with the Huck Institutes of the Life Sciences and the Materials Research Institute. “The material can self-assemble to make its bottom surface be very conductive and its top surface self-insulated.”

In the new method, researchers combine liquid metal, a conductive polymer mixture called PEDOT:PSS, and hydrophilic polyurethane enabling 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 oxidize, 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 leading to less accurate data collection.

“Our innovation here is a materials one,” Zhou says. “Normally, when liquid metal mixes with polymers, they aren’t 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 says, making it easier to fabricate wearable devices. The researchers are continuing to explore potential applications, with a focus on assistive technology for people with disabilities.

Also authoring this work are Penn State engineering science and mechanics doctoral students Salahuddin Ahmed, Marzia Momin, and Jiashu Ren, and biomedical engineering doctoral student Hyunjin Lee. This work was supported by the National Taipei University of Technology-Penn State Collaborative Seed Grant Program and by the Department of Engineering Science and Mechanics, the Materials Research Institute, and the Huck Institutes of the Life Sciences at Penn State.

Penn State College of Engineering
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