Soft And Strong: A Self-Healing Gel Supports Skin and Bones

Using water, clay, and a special compound, researchers from the University of Tokyo have created a hydrogel that could be used as tissue scaffolding, cartilage replacement, or as a matrix for targeted drug delivery. Because of its softness, the gel could be injected via syringe.

The material boasts simple ingredients, strength, and a self-healing characteristic. It can be broken up and remolded because it does not rely on covalent bonding. Hydrogen bonding and electrostatic forces are employed to ensure that the material can be molded back together again.

According to the researchers, the moldable gel is 50 times stronger than comparable materials. And it is easy to make in about 3 minutes. Tiny, thin clay disks (2–3 % by mass) are added to water. The edges of the clay disks bear a slight positive charge, but their flat surfaces bear a slight negative charge. To prevent the disks from clumping, sodium polyacrylate is added, which disperses the disks by wrapping around the positively charged edges. After shaking the solution for a few minutes, a compound that researchers call the G binder is added and the solution is agitated again.

The G binder is built from long chains of polyethylene glycol. The ends of the chains are multiple branches tipped with an amino acid, guanidinium. The positively charged branches bond with the negatively charged surface of the clay. The researchers believe these telechelic dendrites in the G binder are responsible for the material’s self-healing mechanism. The arms seek and snag nearby clay disks, researcher Justin Mynar explains.

Unlike other gels held together by relatively weak noncovalent forces, this gel keeps its shape. In an experiment, the researchers put the gel in the solvent tetrahydrofuran for six hours. The tetrahydrofuran replaced almost all of the water in the gel, but the gel retained its identity, Mynar says.

The hydrogel still requires toxicity testing for all of its elements, researchers say, but early tests are encouraging. The scientists prepared the gel from a solution containing the protein myoglobin, which carries oxygen to muscle cells, and the protein kept its shape for a week. When enclosed with the gel, the myoglobin retained 7% of its activity compared with free myoglobin.

A letter about the material appears in the January 21 issue of the journal Nature.