Intertwined polymers and nanosheets create skin-like self-healing hydrogels

We all encounter gels in our daily lives, from the soft, sticky substances we put in your hair to jelly-like components of various food items. Human skin shares gel-like properties, but has unique qualities that are extremely difficult to replicate. It combines high stiffness and flexibility, and has amazing self-healing ability, and often completely heals within 24 hours of injury.

Until now, artificial gels have been able to replicate this high stiffness or natural skin self-healing properties, but not both. Now, a team of researchers from Aalto and Bailos University will first develop hydrogels with unique structures that overcome early limitations, opening the door to applications such as drug delivery, wound healing, soft robotic sensors and artificial skin.

In this study, the researchers added a specific clay nanosheet that was very large and very thin to the hydrogel. The result is a highly ordered structure with a polymer tightly entangled between the nanosheets, which not only improves the mechanical properties of the hydrogel, but also allows the material to self-heal.

This study is published in Nature Materials.

Healing through entanglement

The secret of the material lies not only in the organized arrangement of nanosheets, but also in the process as simple as the polymer and baking intertwined between them. Postdoctoral researcher Chen Liang mixed it with monomer powder and water containing nanosheets. The mixture was then placed under the UV lamp. This is similar to what is used to set up a ger manicure.

“UV radiation from the lamp causes individual molecules to join together, making everything a resilient solid.

“Entanglement means that thin polymer layers begin to twist each other like small wool threads, but in a random order.” “If the polymers are completely 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 entangle again.”

Four hours after being cut with a knife, the material is already 80-90% self-healing. After 24 hours, it will usually be fully repaired. Additionally, the 1 millimeter-thick hydrogel contains 10,000 layers of nanosheets, making the material as hard as human skin, giving it an equal degree of stretch and flexibility.

“Hard, strong, self-healing hydrogels have been a challenge for a long time. We have discovered a mechanism to strengthen traditional soft hydrogels, which could revolutionize the development of new materials with bio-inspired properties,” says Zhang.

Get inspiration from nature

“This work is an exciting example of how biological materials stimulate us to look for new combinations of properties of synthetic materials. Imagine a robot with robust, self-repairing skin or synthetic tissue that autonomously repairs.”

There may be ways to go ahead with real-world applications, but the current results represent a pivotal leap. “It’s a fundamental discovery that allows you to update the rules of material design,” says Professor Ikkara.

The collaboration was led by Dr. Hang Chan, Professor Oli Ikkara and Professor Joseph Blue. Synthetic clay nanosheets were designed and manufactured by Professor Joseph Bleu of Bailos University, Germany.