Researchers from the Eelkema Lab at TU Delft discovered a reaction cycle that allows them to make hydrogels grow and shrink on command. 

Chemist Rienk Eelkema and his group are developing chemical processes in which they mimic nature, with a focus on reactions in living cells that provide the fuel to power the cell. Only a few reactions are known that allow you to drive non-living materials in a similar way, and they have several drawbacks. The rate is difficult to control and they only work on a limited set of molecules.