To improve soft robotics, skin-integrated electronics and biomedical devices, Penn State researchers have developed a 3D-printed material that is soft and stretchy — properties needed to mimic the properties of tissues and organs — and that self-assembles. Their approach uses a process that eliminates many drawbacks of previous fabrication methods, such as reduced conductivity or device failure, the team said.
They published their results in Advanced materials.
“People have been developing soft and stretchable conductors for nearly a decade, but the conductivity is typically not very high,” said corresponding author Tao Zhou, assistant professor of engineering and mechanical and biomedical engineering in the Department of Engineering and Materials Science and Engineering in the Department of Earth and Mineral Sciences at Penn State. “Researchers realized that they could achieve high conductivity with liquid metal conductors, but the major limitation is that a secondary method is needed to activate the material before it can achieve high conductivity.”
Liquid metal-based stretchable conductors suffer from inherent complexity and challenges caused by the post-fabrication activation process, the researchers said. The secondary activation methods include stretching, compression, 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 secondary activation to make the material conductive,” said 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 the bottom surface highly conductive and the top surface self-insulating.”
In the new method, the researchers combine liquid metal, a conductive polymer blend called PEDOT:PSS, and hydrophilic polyurethane that allows the liquid metal to transform into particles. When the soft composite material is printed and heated, the liquid metal particles on the bottom surface self-assemble into a conductive path. The particles in the top layer are exposed to an oxygen-rich environment and oxidize, creating an insulated top layer. The conductive layer is crucial for transmitting information to the sensor, such as muscle activity recordings and strain detection on the body, while the insulated layer helps prevent signal leakage that can result in less accurate data collection.
“Our innovation here is a material innovation,” Zhou said. “Normally, when liquid metal mixes with polymers, they are not conductive and require secondary activation to achieve conductivity. But these three components enable 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 for people with disabilities.
The papers’ other authors are Salahuddin Ahmed, Marzia Momin and Jiashu Ren, all doctoral students in Penn State’s department of engineering science and mechanics, and Hyunjin Lee, a doctoral student in Penn State’s department of biomedical engineering. This work was supported by the National Taipei University of Technology-Penn State Collaborative Seed Grant Program and by Penn State’s Department of Engineering Science and Mechanics, the Materials Research Institute and the Huck Institutes of the Life Sciences.
