Elephant-proof jelly: The amazing soft-yet-strong material created by University of Cambridge scientists
A ‘super jelly’ that can withstand the equivalent of an elephant standing on it and completely recover to its original shape has been developed at the University of Cambridge.
Remarkably, the soft-yet-strong material is 80 per cent water. While it looks and feels like a squishy jelly, it acts like an ultra-hard, shatterproof glass when compressed.
It could be used in a host of applications, including soft robotics, bioelectronics or even as a cartilage replacement in humans.
The material’s non-water portion is a network of polymers held together by reversible on/off interactions that control its properties.
“In order to make materials with the mechanical properties we want, we use crosslinkers, where two molecules are joined through a chemical bond,” said Dr Zehuan Huang from the Yusuf Hamied Department of Chemistry, first author of the study published in Nature Materials. “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.”
Led by Professor Oren A Scherman, the research team used barrel-shaped molecules called cucurbiturils to make a hydrogel that can withstand compression.
The cucurbituril is described as a crosslinking molecule that holds two guest molecules in its cavity – like a molecular handcuff.
By designing guest molecules that favour staying inside the cavity for longer than normal, the polymer network is kept tightly linked, allowing it to withstand compression.
Prof Scherman, director of the university’s Melville Laboratory for Polymer Synthesis, said: “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 force. The properties of the hydrogel are seemingly at odds with each other.”
And they proved it by running over the material repeatedly with a car - and watched it spring back into shape.
Co-author Dr Jade McCune, also from the Department of Chemistry, 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.”
Altering the molecular structure of guest molecules in the handcuff allowed the dynamics of the material to be varied, with the mechanical performance of the final hydrogel ranging from rubber-like to glass-like states.
“People have spent years making rubber-like hydrogels, but that’s just half of the picture,” said Prof Scherman. “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.”
They made a hydrogel pressure sensor for real-time monitoring of human motions, including standing, walking and jumping.
Dr Huang said: “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.”
Researchers in the lab are working to develop these glass-like materials towards biomedical and bioelectronic applications in collaboration with experts from engineering and materials science. The research was funded in part by the Leverhulme Trust and a Marie Skłodowska-Curie Fellowship. Oren Scherman is a Fellow of Jesus College.
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