Researchers at Northwestern University have developed a family of soft materials that imitates living creatures. As they report in a paper in Nature Materials, the film-thin materials come alive when hit with light – bending, rotating and even crawling on surfaces.

Called ‘robotic soft matter’ by the Northwestern team, the materials move without complex hardware, hydraulics or electricity. The researchers believe the lifelike materials could carry out many tasks, with potential applications in energy, environmental remediation and advanced medicine.

"We live in an era in which increasingly smarter devices are constantly being developed to help us manage our everyday lives," said Samuel Stupp, professor of materials science and engineering, chemistry, medicine and biomedical engineering at Northwestern, who led the experimental studies. "The next frontier is in the development of new science that will bring inert materials to life for our benefit – by designing them to acquire capabilities of living creatures."

Although the moving material seems miraculous, sophisticated science is at play. Its structure comprises nanoscale peptide assemblies that drain water molecules out of the material. An expert in materials chemistry, Stupp linked these peptide arrays to polymer networks designed to be chemically responsive to blue light.

When light hits the material, the polymer network chemically shifts from hydrophilic (attracts water) to hydrophobic (resists water). As the material expels the water through its peptide ‘pipes’, it contracts – and comes to life. When the light is turned off, water re-enters the material, which expands as it reverts to a hydrophilic structure.

This is reminiscent of the reversible contraction of muscles, which inspired Stupp and his team to design the new materials. "From biological systems, we learned that the magic of muscles is based on the connection between assemblies of small proteins and giant protein polymers that expand and contract," Stupp said. "Muscles do this using a chemical fuel rather than light to generate mechanical energy."

For Northwestern's bio-inspired material, localized light can trigger directional motion. In other words, bending can occur in different directions, depending on where the light is located. And changing the direction of the light also can force the object to turn as it crawls on a surface.

Stupp and his team believe there are endless possible applications for this new family of materials. With the ability to be designed in different shapes, the materials could play a role in a variety of tasks, ranging from environmental clean-up to brain surgery.

"These materials could augment the function of soft robots needed to pick up fragile objects and then release them in a precise location," he said. "In medicine, for example, soft materials with 'living' characteristics could bend or change shape to retrieve blood clots in the brain after a stroke. They also could swim to clean water supplies and sea water or even undertake healing tasks to repair defects in batteries, membranes and chemical reactors."

https://www.nature.com/articles/s41563-020-0707-7