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| A polymer material can heal itself without the use of elevated temperatures or chemicals. |
Scientists' efforts to develop self-mending materials, from thermoset resins to self-healing rubbers, could have vast implications for a host of industrial, consumer, and medical applications. However, many self-repairing systems require the use of heat to reform bonds and fix cracks, making them commercially unviable. Taking a different approach, scientists at Carnegie Mellon University (CMU; Pittsburgh and Kyushu University (Fukuoka, Japan) are developing a polymer that self-heals at room temperature when exposed to UV light.
"Most self-mending techniques rely either on one-time healing by using an encapsulating agent that cannot be regenerated or on weak noncovalent bonding," explains CMU chemistry professor Krzysztof Matyjaszewski. "Our concept differs from these approaches. It produces covalent bonds that can be self-mended repeatedly."
The scientists' photoinduction method relies on covalently cross-linking polymers with trithiocarbonate units using a process known as reversible addition-fragmentation chain transfer. "Our material uses a network of chemical bonds that fracture at lower energy than the remaining links in the network," Matyjaszewski comments. "This phenomenon protects the majority of the network, and since the fractured bonds can reform new bonds with adjacent complementary function groups, the complete network is reformed."
Because this noncontact polymer can mend itself under room-temperature conditions, it is also easy to acquire and handle, Matyjaszewski says. In addition, it is capable of targeting specific areas for repair and does not require the use of chemicals.
While an early iteration of this covalent-bonding technique required close contact and pressure between the two surfaces of the severed material, Matyjaszewski does not consider this to be an inherent property of the material. "We believe that the system can absorb more energy than a system without breakable and healable bonds, thus presenting a tougher structure than the parent material," he remarks. "Any bonds that break during a noncatastrophic event will be absorbed by a fraction of the breakable/healable bonds and undergo self-repair to regenerate the material." As a result, the regenerated polymer will resemble the original material rather than a material containing multiple microfractures, eliminating the need for material surfaces to remain in proximity to each other or to be subjected to high pressure.
Despite progress in developing his self-healing material, Matyjaszewski cautions that it has not yet been evaluated to determine its suitability for medical device applications--including implantable devices. "Further work is required to develop implantable materials that can withstand repeated self-repair reactions autonomously in the biological environment," he says. However, he envisions that this concept could potentially prolong the lifetime of any material that is subjected to numerous stress-related events. "In the medical field, many potential applications can be imagined, including orthopedic implants."
