As soft as jelly, as hard as glass

Researchers from the University of Cambridge have invented a ‘super jelly’ so strong it can hold its shape even with the equivalent of an elephant treading on it, despite being 80% water.

Jelly-like materials, or hydrogels, have many applications in soft robotics, tissue engineering and wearable tech, but it is difficult to make strong jellies that don’t break apart when under pressure.

According to this study, published in Nature Materials, the new hydrogel material is soft like a jelly, but acts like an ultra-hard, shatterproof glass when compressed.

The way a material behaves depends on its molecular structure. Some molecules bond rigidly so they can’t move at all, leading to tough, solid materials such as glass. Others can slide around each other and keep the material – such as rubber – flexible. Most hydrogels have networks of polymers linked together in a shape that keeps them flexible.

“In order to make materials with the mechanical properties we want, we use crosslinkers, where two molecules are joined through a chemical bond,” says Dr Zehuan Huang, the study’s first author.

“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.”

The new hydrogel follows the principle of cross-linked polymers, but has special barrel-shaped molecules called cucurbiturils that hold the polymers together – almost like handcuffs.

Cucurbiturils clutch the polymers tightly together like glass when the hydrogel is under pressure, but still allows for flexible movement at other times. The glass-like state was so strong it could be run over by a car.

“At 80% 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,” says Oren Scherman, director of the university’s Melville Laboratory for Polymer Synthesis. “The properties of the hydrogel are seemingly at odds with each other.”

Co-author Dr Jade McCune says that the way the hydrogel can withstand compression is surprising.

“It wasn’t like anything we’ve seen in hydrogels,” she says. “We also found that the compressive strength could be easily controlled through simply changing the chemical structure of the guest molecule inside the handcuff.”

The researchers used the material to make a hydrogel pressure sensor for real-time monitoring of human movement, and they hope to investigate uses in other biomedical or bioelectric technology.

“To the best of our knowledge, this is the first time that glass-like hydrogels have been made,” says Huang. “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.”