(1) Sal-gel in its first, melted state, is soft and transparent. (2) Secondary nucleation is initiated by touching the gel with salt crystals. (3) Sal-gel in its secondary, frozen state, is rigid and opaque. (4) The gel is heated at ~90 C to revert it to the initial melted state.
Nature Materials (1) Sal-gel in its first, melted state, is soft and transparent. (2) Secondary nucleation is initiated by touching the gel with salt crystals. (3) Sal-gel in its secondary, frozen state, is rigid and opaque. (4) The gel is heated at ~90 C to revert it to the initial melted state.

A team of engineering researchers at the University of Waterloo in Waterloo, Ontario have created a material that can change states from a soft gel into a hard solid without any change in temperature, and back to a soft gel under temporary heat.

The “sal-gel” is presented as a solution to a long-standing need for a synthetic material with strength and shape adaptability. Researchers anticipate its use as a smart material component in soft robotics, adhesion, aeronautics, and a range of other technologies.

In soft robotics, the sal-gel could be used to shift the composition of robotic arms to suit different tasks, such as gripping and picking up delicate objects. In adhesives, the material could fit the contours of a surface, then instantly harden to create a bond.

“It should be particularly effective for applications that require a gentle touch, but a firm grip,” says Aleksander Cholewinski, a PhD student at the University of Waterloo and co-leader of the sal-gel research. “It is a two-in-one solid that meets both of those needs.”

The gel is engineered from a mixture of polymers with supercooled melted salt. When it is touched with a crystal of the same salt, it turns instantly into a hard, opaque solid. Briefly applying heat (at about 90 degrees Celsius) reverts the material to its gel state.

“The underlying concept opens up a new avenue in material science in creating materials with two stable and reversible solid states,” says Kuo Yang, a PhD student and research co-leader who first came up with the sal-gel idea.

Cholewinski and Yang collaborated with postdoctoral fellows Geoffrey Rivers and Li Yu under the direction of chemical engineering professor Boxin Zhao in Zhao’s lab at the University of Waterloo.