Triggerless shape-shifting polymers

Shape-shifting materials can be used for a variety of applications, including drug delivery and biomedical devices. They have typically relied on external triggers to induce the desired activity. Andrey V. Dobrynin of the University of Akron and Sergei S. Sheiko and coworkers at the University of North Carolina, Chapel Hill, have now developed materials that can be programmed to change shape at predetermined rates without an external trigger (Nat. Commun. 2016, DOI: 10.1038/ncomms12919). The new materials are hydrogels with dual networks of cross-links made by the copolymerization of N,N-dimethylacrylamide and methacrylic acid. The first network has a low concentration of covalent connections that provide the memory of the original shape. The second network has a high concentration of reversible hydrogen bonds that temporarily lock in a new shape but allow the material to transform back to its original shape as they dissociate. For the transformation to take place over a useful timescale, whether that be seconds or hours, the lifetime of the hydrogen bonds needs to be longer than the time it takes for the strands that make up the covalent network to relax back to their original shape. The researchers used the shape-shifting hydrogels to assemble an artificial flower with individually programmed petals that open at different times over the course of about two hours to simulate blooming.