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New Hydrogels Mimic Body's Ability to Heal

After Being Cut, Gels Form Instant, Strong Bonds Thanks to Dangling Side Chains

Cut your finger chopping onions and you may wince (or cry), but with a little care, your wound will heal itself soon enough. Jacobs School bioengineers have invented a hydrogel that mimics this self-healing ability of biological tissue, opening up a wide range of applications including medical sutures, targeted drug delivery, self-healing plastics and industrial sealants. The self-healing hydrogel binds in seconds, as easily as Velcro, and forms a bond strong enough to withstand repeated stretching, professor Shyni Varghese's research team reported recently in the Proceedings of the National Academy of Sciences.

Hydrogels

Hydrogels are made of linked chains of polymer molecules that form a flexible, gelatin-like material similar to soft tissues. But their ability to mimic living tissue has stopped short of self-healing, limiting their potential applications. Until now, researchers have been unable to develop hydrogels that can sustain repeated damage and rapidly repair themselves. Varghese's team overcame this challenge with the use of “dangling side chain” molecules that extend like fingers on a hand from the primary structure of the hydrogel network and enable them to grasp one another. “Self-healing is one of the most fundamental properties of living tissues that allows them to sustain repeated damage,” says Varghese. The research team wondered whether it could mimic self-healing in synthetic, tissue-like materials such as hydrogels. “The benefits of creating such an aqueous self-healing material would be far-reaching in medicine and engineering,” Varghese said.

Shyni Varghese
Bioengineering professor Shyni Varghese at work in her lab.

To design the side chain molecules of the hydrogel that would enable rapid self-healing, Varghese turned to professor Gaurav Arya in the Department of NanoEngineering, who performed computer simulations of the hydrogel network. The simulations revealed that the ability of the hydrogel to self-heal depends critically on the length of the side chain molecules, or fingers. Hydrogels having an optimal length of side chain molecules exhibit the strongest self-healing. If the molecules are too short, they can't reach across two hydrogel surfaces facing each other. If the molecules are too long, they collapse back into the hydrogel through a phenomenon called hydrophobicity. Just as oil can't mix with water, long side chain molecules can't interact with water molecules, which means they can't reach across the hydrogel surface to form bonds.

When two cylindrical gel pieces featuring these optimized fingers were placed together in an acidic solution, they stuck together instantly. Varghese's lab further found that simply adjusting the solution's pH levels caused the gels to bind (low pH) and separate (high pH) very easily. Repeated pH fluctuations did not reduce bond strength.

The hydrogel's strength and flexibility in an acidic environment such as the stomach makes it ideal as an adhesive to heal stomach perforations or for controlled drug delivery to ulcers. Varghese said this new self-healing hydrogel could also be useful in industrial plants. The hydrogel, for example, could be applied like a spray paint to the inside of a hazardous materials container where it would act like a sealant when cracks form in the container. To test this theory, students cut a hole in the bottom of a plastic container, “healed” it by sealing the hole with the hydrogel and demonstrated that the gel prevented leakage.

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