The material looks and feels like a jelly, with a 'squishy' texture, but when compressed it acts like an ultra-hard, shatterproof glass. Never before has such resistance to compression been incorporated into a soft material. This is even more remarkable considering that it is mostly composed of water.
The 'super jelly' could be used for a range of potential applications, including soft robotics, bioelectronics, or even as a cartilage replacement for biomedical use.
The behaviour of materials is dependent on their molecular structure. Stretchy, rubber-like hydrogels have many useful properties (such as toughness, flexibility, and self-healing capabilities) that make them a popular research subject, particularly as a potential material for surgical applications. However, making hydrogels that can withstand compression without being crushed is a challenge.
The 'super jelly' is 80 per cent water. The rest of the material is a network of polymers held together by reversible on-off interactions that control the mechanical properties of the material.
"In order to make materials with the mechanical properties we want, we used crosslinkers, where two molecules are joined through a chemical bond," said Dr Zehuan Huang, first author of the Nature Materials paper describing its development. "We use reversible crosslinkers to make soft and stretchy hydrogels, but making a hard and compressible hydrogel is difficult and designing a material with these properties is completely counterintuitive."
Working in the laboratory of Professor Oren Scherman, the researchers used barrel-used molecules called cucurbiturils to create the new hydrogel. The cucurbituril is the crosslinking molecule which holds two guest molecules in its cavity "like a molecular handcuff". For this material, they designed guest molecules that tend to stay inside the cavity for longer than normal, keeping the polymer network tightly linked and allowing it to withstand considerable compression.
"At 80 per cent water content, you'd think it would burst apart like a water balloon, but it doesn't; it stays intact and withstands huge compressive forces," said Professor Scherman, director of the University of Cambridge's Melville Laboratory for Polymer Synthesis. "The properties of the hydrogel are seemingly at odds with each other."
Co-author Dr Jade McCune added: "The way the hydrogel can withstand compression was surprising. It wasn't like anything we've seen in hydrogels. We also found that the compressive strength could be easily controlled through simply changing the chemical structure of the guest molecule inside the handcuff."
To make their glass-like hydrogels, the team chose specific guest molecules for the handcuff. Altering the molecular structure of guest molecules allowed the dynamics of the material to "slow down" considerably, with the mechanical performance of the final hydrogel ranging from rubber-like to glass-like states.
Scherman continued: "People have spent years making rubber-like hydrogels, but that's just half of the picture. We've revisited traditional polymer physics and created a new class of materials that span the whole range of material properties from rubber-like to glass-like, completing the full picture."
The researchers used the material to make a hydrogel pressure sensor for real-time monitoring of human motions, including standing, walking and jumping. They are now working to further develop the 'super jelly' towards biomedical and bioelectronic applications.
