Revolutionary self-healing hydrogel regenerates like human skin

Hydrogels, soft and water-rich materials, have long fascinated researchers because of their potential in medical and technological applications. However, despite their promise, most artificial hydrogels have lacked the ability to combine high stiffness with self-healing properties.

Now, researchers from Aalto University and the University of Bayreuth have developed a hydrogel that overcomes this limitation, offering new possibilities for artificial skin, wound healing, drug delivery, and even soft robotics.

Natural biological tissues, like human skin, possess a unique combination of properties that synthetic materials struggle to replicate. Skin is strong yet flexible and, most impressively, capable of self-repair. Until now, scientists have only been able to replicate either the stiffness of biological tissues or their self-healing ability—but never both at once.

Hydrogels have many advantages, such as biocompatibility, nutrient transport, and ionic conductivity. These features make them promising materials for biomedical applications, but their mechanical limitations have kept them from reaching their full potential.

Most self-healing hydrogels are too soft, with a Young’s modulus below 100 kilopascals (kPa). Others that achieve stiffness above 100 megapascals (MPa) typically lose their ability to heal.

The new hydrogel, developed by researchers at Aalto and Bayreuth, achieves both: a modulus of 50 MPa, tensile strength up to 4.2 MPa, and near-complete self-healing ability.

Postdoctoral researcher Chen Liang and his team achieved this by incorporating ultra-thin nanosheets of synthetic clay into the hydrogel structure. This allowed the hydrogel’s polymers to form dense entanglements between the nanosheets, reinforcing the material while still allowing dynamic molecular movement for healing.

The breakthrough lies in the way polymer chains become entangled within nanosheets. The researchers mixed a powdered monomer solution with water containing these nanosheets. The mixture was then exposed to UV light, similar to the process used for curing gel nail polish. The UV radiation caused the molecules to bond together, forming a solid but elastic gel.

"Entanglement means that the thin polymer layers start to twist around each other like tiny wool yarns, but in a random order," explains 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."

Within four hours of being cut, the material is already 80–90% healed. After 24 hours, it is completely restored. The hydrogel, only one millimeter thick, contains 10,000 layers of nanosheets, giving it mechanical properties comparable to human skin. It is both stiff and stretchable, a feat that was previously difficult to achieve in synthetic materials.

This discovery solves a fundamental challenge in materials science. "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.

The key to the hydrogel’s success lies in the nanoscale confinement of polymer chains. Traditionally, self-healing in hydrogels relies on dynamic molecular interactions such as hydrogen bonding, electrostatic forces, or reversible covalent bonds. However, these interactions typically function best in softer materials. Stiffer materials require more permanent bonding structures, which tend to hinder self-healing.

By utilizing nanosheets of synthetic clay known as hectorite, the researchers created a co-planar nanoconfinement that organizes the polymer chains at the molecular level. This structure not only increases stiffness but also preserves the dynamic movement necessary for self-healing. The nanosheets prevent polymer strands from slipping too far apart after being cut, allowing them to reconnect more easily.

Previous studies have shown that nanoconfinement can enhance the mechanical properties of soft materials, improving stretchability, toughness, and durability. However, this is the first time nanoconfinement has been successfully applied to achieve both high stiffness and self-healing in a hydrogel.

This innovation opens the door to a wide range of applications. Soft robotics, a field that seeks to develop flexible and adaptive robots, could greatly benefit from self-healing hydrogels. Robots with artificial skins made from this material could sustain damage and repair themselves, improving durability and longevity.

In biomedical fields, the hydrogel could be used for wound dressings that adapt to movement while remaining strong and intact. Drug delivery systems could take advantage of the hydrogel’s ability to maintain structural integrity while allowing controlled diffusion of medication. The material could even be integrated into wearable medical devices that require both flexibility and resilience.

Olli Ikkala from Aalto University sees even broader potential. "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. It’s the kind of fundamental discovery that could renew the rules of material design."

Although further research is needed before these hydrogels reach real-world applications, this study, published in the journal Nature Materials, represents a major leap forward in materials science. The synthetic clay nanosheets used in the study were designed and manufactured by Prof. Josef Breu at the University of Bayreuth, and the research team was led by Dr. Hang Zhang, Prof. Olli Ikkala, and Prof. Josef Breu.

This new class of hydrogels demonstrates that synthetic materials can take cues from nature to achieve previously unattainable properties. With continued exploration, materials like these could soon become part of everyday life, shaping the future of medicine, robotics, and engineering.

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