Glassy gel is hard as plastic and stretches 7 times its length

When you think of gel, you might imagine goo – but a new gel-like material has been engineered to be soft enough to stretch to almost seven times its original length while still being strong and clear, like glass.

Michael Dickey at North Carolina State University says his team discovered these “glassy gels” when his student, Meixiang Wang, was experimenting with ionic liquids and kept finding unexpected mechanical properties. The materials they devised are more than 50 per cent liquid, but as strong as the plastics used for water bottles, while also being very stretchy and sticky. “There are a bunch of cool things about them,” he says.

Each glassy gel consists of long molecules called polymers mixed with an ionic liquid, a fluid that is essentially a salt in liquid form. The gel is a transparent solid that can withstand up to 400 times atmospheric pressure, but also stretch very easily up to 670 per cent. Dickey says that this could make it well-suited for building soft robotic grippers or 3D printing deformable materials.

He and his colleagues made glassy gels from several different mixtures of polymers and liquid salts and found that their strength and stretch depended on the precise ratio used.

“Just by changing the ratio of two ingredients, you can go from something very stretchy like a rubber band, to something almost as hard as glass,” says Dickey.

This is because the materials get their stretchiness from the ionic liquid settling into spaces between the stiffer polymer molecules and pushing them apart, while their strength comes from the electrostatic attraction between the liquid’s charged particles and the polymers, which prevents them from fully breaking away from each other.

The glassy gels can also self-heal – a cut or break can be repaired by applying heat, which makes molecules on the broken edges reconnect. Richard Hoogenboom at Ghent University in Belgium says this could make them useful in some instances when conventional plastics are used, but the formula may have to be tweaked so that it only softens at temperatures high enough so this doesn’t happen accidentally.