A Soft Solution to a Hard Problem

Electronic devices are traditionally rigid, with conductive traces that are laid out on a circuit board to connect the functional components. This is fine for many applications, but when getting into soft robotics where artificial tissues need to stretch like the skin and other organs of biological creatures, or wearable devices in which sensors and computing units must closely conform to the curves of the body, more flexibility is required.

A lot of work has gone into developing conductors that are both soft and flexible for use cases such as these, but they are still not meeting the needs of device developers. Existing soft conductors tend to either have low levels of conductivity, or require a complex activation process that frequently leads to device failure. Liquid metal-based conductors fall into this latter category — they can be very highly conductive, but only after being activated by lasers, sintering, stretching, compression, or some other fabrication process that all too often causes the conductor to leak or otherwise fail.

A better solution may be on the horizon, thanks to the work of a team of engineers at Pennsylvania State University. They have developed a 3D-printable material that is soft and flexible, yet also very highly conductive. And crucially, this material does not require an activation procedure to become conductive. Rather, after being formed, it self-assembles to create a highly conductive tissue-like substance that is electrically insulated from its environment.

The key to this innovation involved combining a unique blend of materials. The researchers combined a liquid metal with PEDOT:PSS — a conductive polymer — and a hydrophilic polyurethane. This mixture coaxes the liquid metal into forming particles. Then when the material is heated, which is a normal part of the 3D-printing process, the particles will self-assemble to form pathways for electricity to pass through. Simultaneously the outer layers of the material, which are exposed to oxygen, will begin to oxidize and form an electrically-insulating outer layer. The insulator prevents signal leakage, and also keeps stray electrical signals from being transmitted by the conductor.

Experiments revealed that the material stacks up very well against the competition. When compared with existing stretchable conductors that do not require activation processes, it was found to be more than one million times more conductive. Furthermore, the material can stretch over 800 percent more than human skin, making it very versatile for a wide range of applications.

Eliminating the need for activation not only makes the material more reliable, but also much easier to work with. Since it can also be 3D-printed, it greatly simplifies the manufacturing of soft electronic devices. The team envisions this material being widely used in the future to support the development of a new generation of comfortable wearable electronics for applications like health tracking and also as an assistive technology for those with disabilities.