Strong, self-healing soft robots

Soft robots made of squishy, flexible materials can squeeze into tight spaces and manipulate unusually shaped objects better than their rigid cousins. But their soft structures can sustain mechanical damage through repetitive tasks, such as walking and gripping. Now, researchers have developed a protein-based polymer that can be used to make components such as pneumatic actuators that repair themselves in seconds when heated.

Soft robots are typically based on synthetic plastics such as silicone and rubber. Self-healing versions of these materials typically take a long time to mend, upwards of days, and most systems never regain their initial strength. Melik Demirel, a materials scientist at Pennsylvania State University, found inspiration in a class of materials that’s very different from the usual synthetic polymers: a protein found in squid.

This protein forms a polymer that is genetically encoded in repeating blocks of genes. “We realized that we can actually take some of the building blocks and like Lego blocks, we can create different versions,” Demirel says. A research team led by Demirel and Mettin Sitti, an engineer at the Max Planck Institute for Intelligent Systems, used synthetic biology methods to systematically tinker with the protein design until they developed a material with an intrinsic repair mechanism. Using genetically engineered microbes, they synthesized a biopolymer with highly organized cross-linking sections alternating with amorphous sections (Nat. Mater. 2020, DOI: 10.1038/s41563-020-0736-2).

This material can be molded into parts or made into films by hydrating and heating it, which establishes hydrogen bonds between the different regions of the polymer strands. The researchers damaged the material by scratching, puncturing, and cutting it, then heated it to 50 °C, essentially mimicking its manufacturing conditions. The repaired junctions regain the full strength of pristine samples.

The team then used the material to make pneumatic actuators, a common component in soft machines that is prone to puncture. They tested the soft actuators in a robot that used them to grip and carry a small object. In a second robot, the actuator lifted a burden 3000 times its weight. In both tests, the self-healing actuator matched the performance of state-of-the-art soft actuators both before and after healing.

The self-healing material has another advantage: it can be broken down and reused, which makes it a sustainable alternative to existing plastics. “It’s beyond biodegradable—it’s 100 percent circular,” says Demirel.

In addition to the material’s impressive healing properties, says Carmel Majidi, a mechanical engineer at Carnegie Mellon University who was not involved in the study, “this is a very compelling example of using synthetic biology to engineer new classes of materials.”

Now, the researchers are collaborating with industry partners to test the material at larger scale.