Like a water balloon bursting when squeezed, most soft hydrogels will squish into pieces when compressed. Now, for the first time, researchers have created a superjelly with properties that range from rubber-like and squishy at rest to glass-like, ultrahard, and shatterproof when compressed (Nat. Mater. 2021, DOI:10.1038/s41563-021-01124-x). Despite its 80% water content, the hydrogel can be run over by a car and bounce back to its original form. The team, led by Oren Scherman of the University of Cambridge, designed a synthetic polymer with a cross-linking molecule called a cucurbituril, which has a barrel-shaped middle chamber that holds two different guest molecules. Driven by their affinities for the curcubituril cavity and each other, the guest molecules continually go in and out of the chamber, dissipating the stress put on the material. After testing many kinds of electron-donating guest molecules, the researchers found that they could dramatically change the material’s properties by tuning the second guest molecule’s chemical identity to slow down the rate at which it enters and exits the chamber.“What’s really exciting is that we’ve actually understood things from a fundamental parameter scope,” Scherman says. That opens the door to applications such as cartilage replacement, bioelectronic skins, soft robotics, and prosthetics.