Espoo [ Finland], March 10 : Gels are commonly used products that are soft and sticky. They can be applied to hair or 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 after injury.
Until now, artificial gels have either replicated 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 is 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 breakthrough 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, improving the hydrogel's mechanical properties and allowing it to self-heal.
The secret of the material lies not only in the organised 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 cutting it with a knife, the material is already 80 or 90 percent self-healed. After 24 hours, it is typically completely repaired. Furthermore, a one-millimetre-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 revolutionise the development of new materials with bio-inspired properties," says Zhang.
"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. And 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.'
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