Making mechanical skin

Soft, elastic materials that are also electrically conductive are hard to find. It is even more difficult to create a circuit that resists damage, going so far as to heal itself. For Carnegie Mellon University researchers, however, these kinds of innovations are just another day in the office.

In two recently published papers, CMU research teams have made great strides in wearable electronics and other soft machines. Carmel Majidi’s Soft Machines Lab (SML) has been working in this area for some time and these two documents reflect it.

Previously, in 2018, Majidi, an associate professor of mechanical engineering, and his team created a circuit that was able to heal electrically on its own, meaning it could continue to work even after the main paths were cut or damaged. They have now created a material made from liquid metal that is also capable of physically repairing damage. When two pieces of electric liquid metal composites are put together, they can fuse together in the same way that skin heals after a cut. This innovation allows circuits to resist more damage because they can simply repair it.

SML achieved these advances by using a liquid metal made from an alloy of gallium and indium. This metal is safer than other liquid metals such as mercury, the authors say. These discoveries allow the technology to be expanded to other polymers, including gels. This broadens the range – and effect – of their research. In fact, circuits made with liquid metals can be erased and redesigned, making them highly adaptable.

These new materials can also be 3-D printed. Sarah Bergbreiter, a mechanical engineering professor, worked with Majidi and SML to print these materials using a new manufacturing process. Creating 3-D structures of these self-healing and rewritable circuits will greatly expand the range of applications.

One such application is energy harvesting. Energy can be generated using electricity from the contact between two surfaces. Imagine, for example, the electricity that makes your hair stand on end when you rub a balloon against it. This same principle can be applied to wearable electronics, allowing them to harvest energy from human movement.

Although the theoretical basis is there, this is the first time anyone has been able to make it work. Additionally, liquid metals are highly conductive, so they can easily produce large amounts of energy. Also, because the electronics are soft and supple, they can be easily integrated into clothing.

When Majidi’s team added the material to a pair of sports shorts, they were able to gather enough energy from the wearer’s movement to power a digital display hygrothermometer sensor (a small wearable computing device).

The applications of this research are far-reaching. The authors say its uses could include bio-inspired robotics, human-machine interaction, the wearable computer, and solar cells. This soft robotics will be highly adaptable and durable, allowing for a wide range of applications.

The paper, titled “Controlled assembly of liquid metal inclusions as a general approach for multifunctional composites”, was published in Advanced material.