Described as "super jelly," the novel hydrogel uses a unique internal structure likened to molecular handcuffs to respond to compression by entering a glass-like state, enabling it to be run over by a car without being crushed.
Hydrogels are of much interest to materials scientists. Their high water content makes them suitable for use in the human body, and affords them stretchy and self-healing properties that can be put to use in robotics, advanced contact lenses, artificial tissues and wound healing. The authors of this new study sought to expand on these capabilities by tweaking the underlying molecular structure.
"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."
To achieve this, the scientists turned to barrel-shaped molecules called cucurbiturils, which "handcuff" pairs of other molecules together inside its cavity. They then used molecules designed especially to remain inside this cavity for longer than usual, which has the effect of keeping the network tightly linked and enabling it to vary from a rubber-like state to one resembling ultra-hard, shatterproof glass.
According to the team, this enables the "super jelly" to withstand forces equivalent to an elephant standing on top of it without being crushed. With no elephants on hand, the team put the material to the test by running over it in a car instead, demonstrating how it could revert to its original shape.