Entangled polymers and nanosheets create skin-like, self-healing hydrogel

We all encounter gels in daily life—from the soft, sticky substances you put in your hair to the jelly-like components in various foodstuffs. While human skin shares gel-like characteristics, it has unique qualities that are very hard to replicate. It combines high stiffness with flexibility, and it has remarkable self-healing capabilities, often healing completely within 24 hours of an injury.

Until now, artificial gels have either managed to replicate this high stiffness or natural skin’s self-healing properties, but not both. Now, a team of researchers from Aalto University and the University of Bayreuth are the first to develop a hydrogel with a unique structure that overcomes earlier limitations, opening the door to applications such as drug delivery, wound healing, soft robotics sensors and artificial skin.

In the study, the researchers added exceptionally large and ultra-thin specific clay nanosheets to hydrogels, which are typically soft and squishy. The result is a highly ordered structure with densely entangled polymers between nanosheets, not only improving the mechanical properties of the hydrogel but also allowing the material to self-heal.

The research is published in Nature Materials.

Healing via ‘entanglement’

The secret of the material lies not only in the organized arrangement of the nanosheets, but also in the polymers that are entangled between them—and a process that’s as simple as baking. Postdoctoral researcher Chen Liang mixed a powder of monomers with water that contains nanosheets. The mixture was then placed under a UV lamp—similar to that used to set gel nail polish.

“The UV-radiation from the lamp causes the individual molecules to bind together so that everything becomes an elastic solid—a gel,” Liang explains.

“Entanglement means that the thin polymer layers start to twist around each other like tiny wool yarns, but in a random order,” adds Hang Zhang, from Aalto University. “When the polymers are fully entangled, they are indistinguishable from each other. They are very dynamic and mobile at the molecular level, and when you cut them, they start to intertwine again.”

Four hours after being cut with a knife, the material is already 80 to 90% self-healed. After 24 hours, it is typically completely repaired. Furthermore, a one-millimeter-thick hydrogel contains 10,000 layers of nanosheets, which makes the material as stiff as human skin, and gives it a comparable degree of stretch and flexibility.

“Stiff, strong and self-healing hydrogels have long been a challenge. We have discovered a mechanism to strengthen the conventionally soft hydrogels. This could revolutionize the development of new materials with bio-inspired properties,” says Zhang.

Gaining inspiration from nature

“This work is an exciting example of how biological materials inspire us to look for new combinations of properties for synthetic materials. Imagine robots with robust, self-healing skins or synthetic tissues that autonomously repair,” says Olli Ikkala, from Aalto University.

Even though there may be some way to go before real-world application, the current results represent a pivotal leap. “It’s the kind of fundamental discovery that could renew the rules of material design,” says Prof. Ikkala.

The collaboration was led by Dr. Hang Zhang, Prof. Olli Ikkala and Prof. Josef Breu. The synthetic clay nanosheets were designed and manufactured by Prof. Josef Breu at the University of Bayreuth in Germany.