Poke a hole in a human and something remarkable happens. First of all, you go to jail. But meanwhile, the wound heals itself, filling in the missing tissue and protecting itself from infection. Poke a hole in a robot, however, and prepare for a long night of repairs. The machines may be stronger than us, but they’re missing out on a vital superpower.
Until now. Researchers at Belgium’s Vrije Universiteit Brussel report this week in Science Robotics that they’ve developed a squishy, self-healing robot. Cut it open, apply heat, let it cool down again, and the wound heals itself. While self-healing materials are nothing new, their application in so-called soft robotics—a relatively new kind of pliable machine that uses pneumatics or hydraulics to move—could be big. Think Terminator-style robots that automatically heal bullet wounds. OK, maybe don’t think of that.
To build their squishbot, the researchers crafted an elastomer, a elastic variety of polymer. Its network of microscopic chains are held together by something called a Diels-Alder reaction, which is temperature-sensitive. So these bonds break when you heat them and reform as they cool. “On the microscopic level, there is enough mobility to seal the gap,” says electromechanical engineer Seppe Terryn, lead author on the paper. “And then if we decrease the temperature again the entire network will be formed again.” Think of melting down a cube of Jell-O, then putting it back in the fridge—the difference being that this polymer goes back to its original shape and strength after injury. Also, it’s more expensive and less tasty.
Now, of course it’d be ideal if the soft robot could heal itself without the application of heat, but in a way there’s an advantage here. “This means also that we can do the healing in a controlled way,” says Terryn. “So in the long term, the robots can decide when is the best time to start the healing and start heating up.”
That, though, would require that the robot knows it’s injured. So what the team is working on next is a material loaded with sensors that could tell exactly where a wound opens up, then deploy targeted heat to the area to heal it. The robot could even start preemptively healing if it detects microcuts from normal wear and tear.
This system, then, very much mimics the way an animal seals up a wound. That’s opposed to other self-healing materials already out there which, for instance, use embedded microcapsules to release healing agents. (These are better for rigid structures like glass, not floppy robots. That and they don’t need temperature changes to work.) What Terryn and his team are doing instead is adapting an existing technology. “They’re taking these Diels-Alder polymers that have been shown before to have reversible covalent bonds and making use of them in these very biomimetic applications,” says North Dakota State University’s Michael Kessler, who also works in self-healing materials.
In addition to this system needing heat to work, another downside is that the healing isn’t wildly efficient. “The main concern with the material proposed in this paper is the time and the heating required for ‘healing,’” says roboticist Pietro Valdastri of the University of Leeds. “Depending on the application, 40 minutes at 80 degrees centigrade plus cooling time can be too long to wait.”
But that’s now. Self-healing will only get better from here, and surely will be essential for soft robots, which today are typically made of fabrics like polyester. After all, the whole point of a robot soft is it can interact with humans without killing them and pick up squishy objects like tomatoes.
That and they pack well: A four-foot-long soft robot arm can deflate and ship in far less space than a traditional robot arm. And that’s important because soft robots are going places. “Having a robot that doesn’t need to be pulled out for repair,” says roboticist Jon Pompa of soft robot outfit Pneubotics, “if you could identify some failure modes and have the materials of the robot do some kind of self-repairing stuff, that would be a really excellent argument why to use them in extreme environments.”
For instance, if you pack a soft robot in a rocket and fire it to Mars to do some construction ahead of human habitation, you’re screwed if it springs a leak and deflates mid-mission. But what Terryn’s team has shown is that you could theoretically have an injured soft robot deflate itself and heat up to repair the wound. That would save you a lot of money and heartache.
So get ready to see a lot more soft robots and, at some point, soft robots you can stab without getting in trouble. Sorry, I’m still thinking about Terminator.