Soft robot actuators heal on their own

Repeated activity is used in soft robotic actuators, but the moving parts of this machine must be reliable and easy to repair. Now, a team of researchers has a biosynthetic polymer, designed according to the teeth of the squid ring, which is self-healing and biodegradable, creating a material not only good for actuators, but also for hazardous material suits and other applications where small holes can cause a danger.

“Today’s self-healing materials have deficiencies that limit their practical application, such as low cure resistance and long cure times (hours),” the researcher reported in today’s issue of Natural materials.

The researchers produced high-resistance synthetic proteins that mimic those found in nature. Like the creatures they are designed with, proteins can heal themselves on both tiny and visible damage.

“Our goal is to create programmable self-healing materials with unprecedented control over their physical properties through synthetic biology,” said Melik Demirel, professor of engineering and mechanics and holder of the Lloyd and Dorothy Foehr Huck Chair in Biomimetic Materials.

Industrial robotic arm and prosthetic leg robotic machines have joints that move and require a soft material that adapts to this movement. So do fans and personal protective equipment of various kinds. But, all materials under continuous repetitive motion develop small breaks and cracks and eventually break. Using a self-healing material, small initial defects are repairable before a catastrophic failure occurs.

Demirel’s team creates the self-healing polymer by using a series of DNA tandem repeats made up of amino acids produced by gene duplication. Tandem repeats are usually short series of molecules arranged to repeat multiple times. The researchers manufacture the polymer in standard bacterial bioreactors.

“We were able to reduce a typical healing period from 24 hours to one second so that our soft protein-based robots can now repair themselves immediately,” said Abdon Pena-Francelsch, lead author of the paper and a former doctoral student at the Demirel’s laboratory. “In nature, self-healing takes a long time. In this sense, our technology surpasses nature.”

The self-healing polymer cures with the application of water and heat, although Demirel said it could also cure with light.

“If you cut this polymer in half, when it heals it regains 100 percent of its resistance,” Demirel said.

Metin Sitti, director of the Department of Physical Intelligence at the Max Planck Institute for Intelligent Systems, Stuttgart, Germany, and his team were working with the polymer, creating holes and healing them. They then created soft actuators that, through use, cracked and then cured in real time, about a second.

“Self-healing, physically smart soft materials are essential for building robust and fault-tolerant soft robots and actuators in the near future,” Sitti said.

By adjusting the number of tandem repeats, Demirel’s team created a soft polymer that quickly cured and retained its original strength, but also created a polymer that is 100% biodegradable and 100% recyclable in the same original polymer.

“We want to minimize the use of petroleum-based polymers for many reasons,” said Demirel. “Sooner or later we will run out of oil and it is also polluting and causing global warming. We cannot compete with really inexpensive plastics. The only way to compete is to supply something that petroleum based polymers cannot deliver and -healing provides the required performance. “

Demirel explained that while many petroleum-based polymers can be recycled, they are recycled into something different. For example, polyester shirts can be recycled in bottles, but not polyester fibers again.

Just as the squid polymer mimics biodegrades in the ocean, the biomimetic polymer biodegrades. With the addition of an acid such as vinegar, the polymer will also be recycled into a powder that can again be made from the same self-healing, soft polymer.

“This research illuminates the picture of the properties of materials that are made accessible by going beyond the proteins that exist in nature using synthetic biology approaches,” said Stephanie McElhinny, manager of the biochemistry program, Office of Army Research , an element of the US Army Army Combat Capability Development Command’s Research Army Laboratory. “The fast, high-resistance self-healing of these synthetic proteins demonstrates the potential of this approach to deliver novel materials for future Army applications, such as personal protective equipment or flexible robots that could maneuver in confined spaces.”