Advancing soft electronics and wearable technology

5th July 2024
Sheryl Miles

To push the boundaries of soft robotics, skin-integrated electronics, and biomedical devices, researchers at Penn State have developed a 3D-printed material that is both soft and stretchable.

As reported in Advanced Materials, this innovative material, called Asymmetric Self-Insulated Stretchable Conductor (aSISC), overcomes many of the limitations of previous fabrication methods. It holds the potential to transform the field of soft and stretchable electronics, which is vital for on-skin and implantable devices.

Image credit Penn State College of Engineering / Marzia Momin

Solving the challenge of conductivity and stretchability

Traditional liquid metal-based conductors, despite their high conductivity, often face issues like surface oxidation, hindering the formation of continuous conductive pathways. The aSISC material, however, integrates liquid metal with a conductive polymer to overcome these obstacles. It achieves a conductivity of 2,089 S cm⁻¹ on the bottom surface while maintaining insulation on the top surface, ensuring high performance and safety.

Traditional liquid metal-based stretchable conductors face significant challenges due to the need for post-fabrication activation processes like stretching, compressing, or laser activation. These methods complicate fabrication and can cause the liquid metal to leak, leading to device failure.

Novel self-assembly process

The Penn State team’s approach involves combining liquid metal with a conductive polymer mixture called PEDOT and hydrophilic polyurethane. This mixture allows the liquid metal to transform into particles. When the composite material is printed and heated, the liquid metal particles on the bottom surface self-assemble into a conductive pathway. Simultaneously, the particles on the top surface, exposed to an oxygen-rich environment, oxidise and form an insulated layer.

The conductive layer is crucial for transmitting information to sensors, such as recording muscle activity or strain sensing on the body. The insulated layer prevents signal leakage, which can result in less accurate data collection. This innovation in material composition allows for self-assembly that produces high conductivity in a soft and stretchable material without the need for secondary activation methods.

Mechanical properties and printability

One of the standout features of aSISC is its remarkable stretchability, exceeding 800%, and a modulus similar to human skin, making it ideal for wearable and implantable electronics. Even after 500 cycles of 25% tensile strain, the material retains its high conductivity, making it reliable for long-term use.

Furthermore, the aSISC material can be 3D-printed, simplifying the fabrication of wearable devices. The material's printability allows for the creation of intricate structures and devices using 3D printing technology. The researchers have demonstrated the material’s potential by printing various detailed and complex shapes, including wearable devices that can stretch over 250%. These devices can monitor muscle activity and track movements, highlighting the material's potential for health monitoring and therapeutic applications.

Future prospects

This breakthrough in material science at Penn State opens new possibilities for creating advanced, reliable, and high-performance wearable electronics that integrate seamlessly with the human body. The research team continues to explore potential applications, with a particular focus on assistive technology for people with disabilities, aiming to enhance comfort, adaptability, and functionality in next-generation electronic devices.

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