Breakup doesn’t keep hydrogel down

New material is strong, soft and self-heals in seconds

Pulling yourself back together after a breakup can be tough to do. But a new hydrogel has no trouble. Using little more than water, clay and a new, designer compound, scientists have created a moldable gel that is both strong and can heal itself in seconds when split in two. The gel may advance efforts in tissue engineering and environmentally friendly chemistry.

BEND ME, BREAK ME Using simple techniques, scientists have created a moldable gel that has superior strength and could lead to new routes for making artificial tissues. T. Aida

BACK STORY: HYDROGELS FOR THE BODY From left: Kathy F. Atkinson/Univ. of Delaware; Georgia Tech Photo: Gary Meek; Molecular Bioengineering Lab/Cornell Univ.; Janeps/istockphoto

The new hydrogel is more than 50 times stronger than comparable squishy self-healing materials, researchers led by Takuzo Aida of the University of Tokyo report in the Jan. 21 Nature. Such substances are well suited for the body; they are 95 percent water. Hydrogels may one day serve as scaffolding for growing new tissue, as matrices for keeping drugs in their targeted area or as replacements for damaged cartilage. The new gel, unlike similar materials, is quick and relatively simple to make.

The work adds to a “growing field of materials with exceptional properties that really could not be imagined” before, comments chemical engineer J. Zach Hilt of the University of Kentucky in Lexington.

Ideally, hydrogels would be strong and self-healing, but scientists face a trade-off when trying to engineer a material with both properties.

Strong materials typically have covalent bonds, but these bonds make materials brittle. When cut or fractured, the edges can’t easily recombine. This means that strong materials that can self-heal, such as paints that fix their own scratches, have to rely on embedded capsules or other tricks to mend. A polymer, for example, might be laced with thin tubes that break open and “bleed” when the substance is damaged, healing the wound. These techniques work for things like countertops, but aren’t ideal for the body.

Softer self-healing materials usually aren’t strong. These materials tend to contain hydrogen bonds, which are weaker than covalent bonds.

Researchers have taken a step closer to something tough, yet tender enough to self-repair with the new hydrogel. Its ingredients are held together by noncovalent forces — hydrogen bonding and electrostatic forces, which are also relatively weak — yet is surprisingly durable, Hilt says.

“It’s not just a noncovalent gel,” he says, “but one with a mechanical strength that hasn’t been seen before.”

The secret to the gel’s success is the material that binds it all together — a specially designed compound that study coauthor Justin Mynar and his colleagues call the G binder.

To make the hydrogel, the team adds tiny, thin clay disks to water. The edges of the clay disks bear a slight positive charge, but their flat surfaces bear a slight negative charge, explains Mynar, now at the University of California, Berkeley. To prevent the disks from clumping, the team added a bit of sodium polyacrylate, which disperses the disks by wrapping around the positively charged edges. After shaking the solution for a few minutes, the team added the G binder, and shook some more.

Typically, fiberlike molecules are added at this stage because they tangle together and give a hydrogel added support. But these fibers aren’t designed to bond with the gel’s ingredients. The G binder does actively bond, via hydrogen bonds and electrostatic forces, and at multiple sites. It’s designed with branched ends that are positively charged and glom onto the negatively charged surface of the clay. And ta-da! — there’s a stiff gel.

“It’s stronger than very tough Jell-O,” Mynar says, “very hard to break with your fingers.”

The G binder is built from long chains of polyethylene glycol, a chemical cousin of antifreeze that has low toxicity and is used in skin creams, lubricants and laxatives. On the ends of the chains, Mynar created multiple branches tipped with guanidinium, a chemical relative of arginine, an amino acid.

Not only does the G binder impart strength, but it also gives the gel the ability to bounce back from stress and helps it heal. This quick recovery is likely due to the binder’s branching arms, which can quickly seek and snag nearby clay disks, Mynar says. “It’s like an octopus — maybe you can get away from the first arm but the others are going to get you.” 

While all of the hydrogel’s ingredients haven’t been tested for toxicity, the gel appears to be friendly to biologically important molecules. When the scientists prepared the gel from a solution containing the protein myoglobin, which carries oxygen to muscle cells, the protein kept its shape for a week. And when enclosed with the gel, the myoglobin retained 70 percent of its activity compared with free myoglobin, the researchers report. Because of its softness, the gel could also be injected into the body with a syringe.

Chemists may also find the gel useful in the laboratory. Freshly cut surfaces of the gel stick together, a re-bonding that allows the researchers to slice out blocks or other shapes and build larger structures. For example, Mynar and colleagues created blocks of the gel in ice cube trays and then connected the cubes end-to-end, forming a bridge. This work suggests that the gel might serve as a good tool for compartmentalizing biologically important molecules or setting up a cascade of reactions. Cubes of gel could be laced with different reactants of interest and lined up in sequence.

Unlike other gels held together by relatively weak noncovalent forces, the new gel kept its shape even after sitting in the solvent tetrahydrofuran for six hours. By then, the tetrahydrofuran has replaced almost all the water in the gel, Mynar says. Yet the gel retains its identity. This not only speaks to the gel’s durability, but also suggests a way to swap water for a solvent to perform a desired reaction within the gel.

“This is a combination of properties that hasn’t been seen,” Hilt says.

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