The material, which is 80 per cent water, has potential uses in multiple sectors ranging from healthcare to robotics. Although it looks and feels like a squishy jelly, it behaves like an ultra-hard shatterproof glass when compressed, despite the high water content. Other applications listed by the researchers include bioelectronics and even cartilage replacement for biomedical use.
Described in Nature Materials, the non-water portion of the material is a network of polymers held together by reversible on/off interactions that control the material’s mechanical properties. The way materials behave — whether they’re soft or firm, brittle or strong — is dependent upon their molecular structure. Stretchy, rubber-like hydrogels have many properties that make them a popular research subject, such as toughness and self-healing capabilities, but making hydrogels that can withstand compression without getting crushed is challenging.
“In order to make materials with the mechanical properties we want, we use crosslinkers, where two molecules are joined through a chemical bond,” said first author Dr Zehuan Huang, from the Yusuf Hamied Department of Chemistry at Cambridge.
“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.”
According to the team, they used barrel-shaped molecules called cucurbiturils to make a hydrogel that can withstand compression. The cucurbituril is the crosslinking molecule which holds two guest molecules in its cavity, like a molecular handcuff. The researchers designed guest molecules that prefer to stay inside the cavity for longer than normal, which keeps the polymer network tightly linked allowing it to withstand compression.
To make the hydrogels, the team chose specific guest molecules for the handcuff. Altering the guest molecules’ molecular structure within the handcuff allowed the dynamics of the material to ‘slow down’ considerably, with the final hydrogel’s mechanical performance ranging from rubber-like to glass-like states.
According to the researchers, they’ve now used the material to make a hydrogel pressure sensor for real-time monitoring of human motions including standing, walking and jumping. Currently the team is working further to develop the materials toward biomedical and bioelectronic applications, in collaboration with experts from engineering and materials science.
“To the best of our knowledge, this is the first time that glass-like hydrogels have been made. We’re not just writing something new into the textbooks, which is really exciting, but we’re opening a new chapter in the area of high-performance soft materials,” said Huang.