A team of engineers at Purdue University, West Lafayette, IN, say they have developed a technique that could produce “soft machines” made of elastic materials and liquid metals for potential applications in robotics, medical devices, and consumer electronics.

Purdue researchers have developed a technique to embed a liquid-alloy pattern inside a rubber-like polymer to form a network of sensors. (Credit: Rebecca Kramer, Purdue University)

This elastic technology could be used to create robots with sensory skin or stretchable garments that could be worn for therapeutic purposes.

However, they warn, new manufacturing techniques must be developed before soft machines become commercially practical. The fabrication technique, which uses a custom-built 3D printer, has been used to create strain gauges, commonly found in many commercial applications to measure how much something is stretching.

The researchers embedded liquid-alloy devices into a polymer called polydimethylsiloxane, or PDMS, a silicon-based elastomer. They used a liquid gallium-indium alloy to create patterns of lines to form a network of sensors.

Since gallium oxidizes quickly and forms a thick gallium-oxide skin, the Purdue researchers use the oxide skin for structural stability. This allows them to embed their electronics in elastomer without ruining or altering the printed structures during the processing steps.

Strain gauges measure how much a material stretches or deforms. Because conventional strain gauges are made of rigid metal film, they can't measure more than a 1-percent deformation before breaking, whereas a soft strain gauge could continue stretching with the material, measuring 100 percent of a material’s strain.

Their research has shown that the liquid alloy does not readily adhere to the PDMS, whereas the oxide skin does. This may enable the researchers to increase this adhesion, which could allow them to further develop the technique. The new process also can be used to fabricate pressure sensors, capacitors, and conductors, they say.