Hydrogels that heal themselves
doi:10.1038/nindia.2012.84 Published online 6 June 2012
Many synthetic materials developed in labs across the world are capable of healing themselves upon damage. Such materials are much in demand because of their many potential applications. However, scientists had not been able to achieve self-healing in some very important materials — hydrogels — because of the water present in them. Hydrogels are synthetic materials that very closely mimic the structural properties of tissues in biological systems.
Now, that barrier has been crossed with a team of scientists demonstrating self-healing in hydrogels. This opens gates to an array of applications in medicine, environmental science and industry1.
"One of the questions that repeatedly appeared before us was whether we could achieve healing in hydrogels that mimic various structural aspects of tissues", says Shiny Varghese, currently a principal investigator at the Department of Bioengineering University of California, San Diego. Her team worked with Raghunath Anant Mashelkar's team at the National Chemical Laboratory in Pune to establish design principles that can be easily extended to achieve self-healing in other synthetic materials.
The scientists have demonstrated that permanently cross-linked hydrogels can be engineered to achieve self-healing in an aqueous environment. What they did was this: they armed the hydrogel network with flexible-pendant side chains. These chains carried an optimal balance of water-loving and water-hating parts. This allowed the side chains to make hydrogen bonds across the hydrogel interfaces and thus achieve self healing.
They observed some quick self-healing, occurring within seconds of the insertion of a crack into the hydrogel or upon juxtaposition of two separate hydrogel pieces.
Also, this healing was reversible and could be switched on or off by simply changing the pH. This allows external control over the healing process. The hydrogels can also sustain multiple cycles of healing and separation without compromising their mechanical properties and healing kinetics.
The scientists have demonstrated various potential applications of such easy-to-synthesize, smart, self-healing hydrogels. "These self-healing hydrogels remain healed over a wide range of temperatures, light conditions, and humidity," Varghese says.
They tested these hydrogels as self-repairing coatings and sealants. They coated various surfaces and then mechanically damaged the coatings with 300-μm-wide cracks. "The coatings healed the crack within seconds upon exposure to low-pH buffers," they report.
Because this healing only requires initial contact, one can achieve repair by simply spraying the cracks with a low-pH buffer. "We found that these hydrogels could adhere to various plastics like polypropylene and polystyrene even in their hydrated state; this is likely because of hydrophobic interactions," they further report.
The hydrogels could be used as sealants for vessels containing corrosive acids. Varghese and team made a hole in a polypropylene container, coated it with a hydrogel, and poured hydrochloric acid into it. The hydrogel instantly sealed the hole and prevented any leakage of the acid.
Another interesting use of the hydrogels, Varghese says, is as tissue adhesives, especially for gastric tissue exposed to low pH, an environment in which the hydrogels can heal easily. Using fresh gastric mucosa of rabbits, they found that the hydrogels adhered well and strongly to it suggesting that the material was suitable to take care of stomach perforations. They also tested drug delivery with tetracyline-loaded hydrogels in a simulated gastric acid environment and evaluated the drug-release profile. Tetracycline was released at a constant rate for 4 days after the initial release.
The ability of these hydrogels to fuse also allows for the development of soft structures with complex architectures that could find applications as soft actuators and in robotic devices, Varghese adds.