Shape-changing machines have long been a staple of science fiction—for good reason. Consider the power of the villainous killing machine in the 1991 film Terminator 2: Judgment Day. When the liquid-metal T-1000 arrives, the heroes quickly realize they have two big problems: First, their foe can morph, turning human-looking appendages into deadly blades. Second, blowing holes in the machine barely slows it down; it can heal itself!
Self-healing machines are among us already. Of course, the reality doesn’t quite match up to the T-1000, but scientists have found that the fictional machine’s two capabilities are closely related. “The fundamental science that gives rise to self-healing materials is the same behavior that allows them to change shape,” says Zhenan Bao, a chemical engineer at Stanford University. And in recent months, scientists have developed a new variety of materials with the abilities to heal and shift their shape—among other skills. The researchers have used these substances to construct new types of electronics with applications in robotics, bioelectronic interfaces, wearable devices and advanced displays. These machines could also be more environmentally friendly than those made from traditional materials such as silicon and metal.
The science of self-healing materials goes back almost two centuries, but it really took off in the 1970s. That’s when researchers began studying the self-healing potential of polymers—large molecules consisting of repeated parts, the way a chain is made up of links. The composition of a molecule’s main polymer chain, or “backbone,” determines a range of properties, including the molecule’s toughness or elasticity. Some healable polymers require a trigger, such as exposure to a particular temperature, light or pressure, to reknit their broken bonds. Others heal spontaneously. These “dynamic” polymers use weaker molecular bonds than those in most stable molecules. For instance, many dynamic materials are held together with hydrogen bonds, in which positively charged hydrogen atoms attract other, negatively charged atoms. “What’s nice about hydrogen bonds is it’s spontaneous,” says Carmel Majidi, a mechanical engineer at Carnegie Mellon University. “You don't have to melt or heat the materials; they just form these bonds upon contact.”
Weak bonds give such materials interesting properties. “It looks like a solid. And if you stretch it very quickly, it breaks like a solid. But if you hold it, it drips like a liquid,” Bao says. “The molecules are not fixed in place, so these polymer networks are continuously forming and dissociating.” This promiscuity is what allows for self-healing. “When we damage the material, the bonds break. But when you put the pieces together, these hydrogen bonds form very readily, and the material recovers its mechanical properties,” Bao says.
The same principle underlies stretchable electronics. “These dynamic bonds allow us to stretch the material to several hundred times its original size because the bonds can break and reform,” Bao says. Using multiple types of bonds, with different strengths, produces materials that are both pliable and tough.
Materials need other properties for use in electronics, though. First, they need to be good conductors. Most polymers are insulators, however. One solution is to add metal particles, nanowires or carbon nanotubes to a polymer in order to make the stretchable material conductive. Bao and her colleagues have used such approaches to construct self-healing “electronic skins” that conform to the body and are capable of sensing pressure and strain and measuring heart rate.
Another solution is liquid metals. In a study published earlier this year, Majidi and his colleagues introduced liquid-alloy microdroplets into a polymer gel dotted with silver flakes. The resulting material was stretchable, self-healing and conductive enough to power the motor of a soft robot. “The ultimate goal is to build electronic and robotic systems that encompass all the properties of biological tissues,” Majidi says, “not just for functionality but also resilience and self-healing.”
Those adaptive materials are simple conductors. Researchers are also developing similarly stretchy materials with other electronic properties. They include semiconductors in which conductivity increases with temperature, as well as dielectrics, which are insulators that change their charge properties, or “polarize,” in electric fields. Researchers have successfully combined these different materials to make healable transistors, capacitors and other electronic components. “There’s all sorts of material functionality that could be useful in soft robotics or wearable electronics,” Majidi says. “We work with thermoelectrics for converting heat into electricity, so it would be great to have a thermoelectric garment that could recover its energy-harvesting capabilities if it’s damaged.” Finding such real-world applications is Majidi’s current focus. “Now that we’ve overcome a lot of the bottlenecks, that’s the next big step,” he says.
Benjamin Tee, an engineer at the National University of Singapore, thinks self-healing electronics will be a boon for the environment. “Self-repair has a lot of implications for reducing electronic waste,” he says. “Is there a future where if you drop your phone, it can repair itself?” In a study published in 2020, Tee and his colleagues developed a stretchy, transparent dielectric material for use in light-emitting capacitors. They employed this material to construct a device capable of producing bright illumination with much less power than previous stretchable optoelectronics required. As a result, it was longer-lived and safer for use in human-machine interfaces. It also self-healed after damage. “It can recover close to 100 percent of its original brightness,” Tee says. The team demonstrated the device in a soft robotic gripper that sensed objects in darkness by detecting reflected light. Other potential applications include near-invulnerable flexible screens, wearable devices, and more.
In time, more components will acquire self-healing capabilities. “The holy grail is having an entire electronic system that can self-repair,” Tee says. This vision comes closer to the T-1000, but a major barrier is that complex electronics require multiple layers. When such devices are damaged, the layers often no longer line up, causing circuits to malfunction.
In a study published this spring, Bao and her colleagues presented a potential solution to this problem. They used two different polymers that have backbones that don’t mix but also identical hydrogen bonds that enable the layers to cling together. “They don’t like to mix, like oil and water,” Bao says. “But we have molecules on each side that allow them to stick together at the interface.” The researchers stacked 11 alternating layers, creating a film 70 microns thick (slightly more than half the thickness of a dollar bill). To test its abilities, they cut the film in half, causing the layers to misalign. They then heated the material to 70 degrees Celsius, and the layers realigned by themselves.
The team demonstrated the technique in a self-realigning pressure sensor and a soft robot whose components were roughly assembled magnetically and then microscopically aligned by heating. The researchers have not yet demonstrated it in complex electronics—but the study brings that application closer. Team members are already working on different functional materials, thinner layers and more complex layered structures, Bao says.
As often happens, science fiction is slowly becoming reality. But hopefully the shape-shifting, self-healing machines of tomorrow will be less aggressive than the T-1000.