A new polymer that "heals itself" has been developed – it automatically patches holes 3cm wide, 100 times bigger than before. Inspired by the human blood clotting system, it contains a network of capillaries that deliver healing chemicals to damaged areas. The new material has been created by engineers at the University of Illinois at Urbana-Champaign. The polymer mimics biological healing by first rapidly forming a gel scaffold over the hole that is then sealed by another polymer, restoring most of the material’s strength.

 Potential applications range from prosthetic skin to more robust battery anodes. Various self-healing mechanisms have been developed. Earlier, Scott White's group at the University of Illinois at Urbana-Champaign created a polymer containing capsules that crack open and release a healing agent. But while this method and others work well if the cut surfaces are in contact, they can't repair a hole, as healing fluids simply drop out before they can solidify. Now, White and colleagues solve this problem by embedding two channels into a polymer. They then put in either a polyethylene glycol or a tris [(4-formylphenoxy)methyl]ethane)) into each channel. The monomers of the repair polymer were also divided between two channels, while the polymerisation initiator was confined to one channel and the promoter to the other. They demonstrated the process using two different repair polymers – polyhydroxyethylmethacrylate and two liquid thiol-enes. In the undamaged epoxy, the contents of each channel are kept separate, and both mixtures remain stable, low viscosity liquids. When a hole opens up in the polymer, both channels are ruptured and the liquids mix. Within seconds, the two gel components begin to set, with the gel stiffening as it spreads. ‘It's like a little dynamic scaffold that's able to support its own weight against gravity,’ explains co-author Nancy Sottos. ‘The normal types of healing agents that we've used in the past just wouldn't be able to do that.’ Once this gel scaffold is in place, the two monomers in the other channel can combine and polymerise, sealing the hole tight and restoring the polymer to close to 62% of its original strength. Testing the seal with nitrogen gas at 345kPa, for hole diameters up to 6.3 mm, they consistently found no leakage. Sealing was less consistent if they punched the hole in the polymer rather than cutting it, as the gel did not always penetrate all the radial cracks emanating from the impact site. This design is only a proof of concept, as the polymer contained only two channels, and the fluids had to be pumped in artificially. The team now hopes to design a polymer impregnated with a vascular network of microchannels containing pressurised healing fluids, allowing them to flow to an arbitrary impact site and seal the hole.

University of Illinois researchers have developed self-healing materials that do not require extra chemicals or catalysts. “The key advantage of using this material is that it is catalyst-free and low-temperature, and can be healed multiple times,” said U. of I. materials science and engineering professor Jianjun Cheng. “This can heal the crack before it causes major problems by propagating.” Other self-healing material systems have focused on solid, strong materials. The new study uses softer elastic materials made of polyurea, one of the most widely used classes of polymers in consumer goods such as paints, coatings, elastics and plastics. If the polymer is cut or torn, the researchers can simply press the two pieces back together and let the sample sit for about a day to heal — no extra chemicals or catalysts required. The materials can heal at room temperature, but the process can be sped up by curing at slightly higher temperatures (37° Celsius, or about body temperature).The “hindered urea bond” polymer bonds back together on the molecular level nearly as strongly as before it was cut. In fact, tests found that some healed samples, stretched to their limits, tore in a new place rather than the healed spot, evidence that the samples had healed completely. The researchers use commercially available ingredients to create their polymer. By slightly tweaking the structure of the molecules that join up to make the polymer, they can make the bonds between the molecules longer so that they can more easily pull apart and stick back together — the key for healing. This molecular-level re-bonding is called “dynamic chemistry.” Now that they’ve established the chemistry required, the researchers are exploring how dynamic polyurea could bolster different applications. For example, they could fine-tune the mixture so that a polyurethane coating or paint could be removable. “In some areas, when it’s not necessary for the coating to be permanent and you want it to be removable, this chemistry may be applied to existing coating materials to make it reversible,” Cheng said. “In general, polyurea and polyurethane are widely used. This chemistry could modify existing materials to make them more dynamic, healable.” The National Science Foundation and the National Institutes of Health supported this research.
An abstract of Nature Communications paper summarises as follows: Polymers bearing dynamic covalent bonds may exhibit dynamic properties, such as self-healing, shape memory and environmental adaptation. However, most dynamic covalent chemistries developed so far require either catalyst or change of environmental conditions to facilitate bond reversion and dynamic property change in bulk materials. Here we report the rational design of hindered urea bonds (urea with bulky substituent attached to its nitrogen) and the use of them to make polyureas and poly(urethane-urea)s capable of catalyst-free dynamic property change and autonomous repairing at low temperature. Given the simplicity of the hindered urea bond chemistry (reaction of a bulky amine with an isocyanate), incorporation of the catalyst-free dynamic covalent urea bonds to conventional polyurea or urea-containing polymers that typically have stable bulk properties may further broaden the scope of applications of these widely used materials.