Hydrogels make programmed chemical origami

Israeli scientists have created elastic sheets which buckle into pre-programmed 3D shapes on command. 

Just as crisps crinkle up when fried, the millimetre-thick gels constructed by Yael Klein and co-workers from the Hebrew University of Jerusalem automatically crumple into flower-like structures when heated. They flatten out again when cooled in water.

A sheet can be programmed to fold into any particular shape, thanks to the researchers’ understanding of the gels’ chemistry; though the options available - domes, tubes and sombrero-like structures - won’t cause origami experts sleepless nights.  

The controllable folding occurs because the sheets are made from a cross-linked elastic hydrogel, consisting mainly of the polymer N-isopropylacrylamide (NIPA), which shrinks by driving out water above 33?C. Dilute NIPA gels shrink a lot because they contain more water. Gels with high NIPA concentrations shrink only a moderate amount. Klein’s team took advantage of this by changing the spatial concentration of NIPA across the material, creating a thin sheet that constricts a lot in some places and a little in others when heated. Because of these varying constrictive stresses, the gel deforms or buckles.  

That buckling, the researchers have found, is mathematically predictable, so long as the sheet is sufficiently thin that it will prefer to curl into a third dimension than stretch out in the same dimension. Given a three-dimensional structure, the team can prescribe the hydrogel concentration gradient required for a cross-linked gel to crumple into that shape when heated. Valves and nozzles are programmed to mix the hydrogel in the required concentrations. Adding a catalyst polymerises the lot into an elastic sheet, ready for folding  (see movie, below).

Randall Kamien, of the University of Pennsylvania, US, noted that possible applications could include ’smart’ materials which fold to particular shapes in response to changing temperature. In turn, these the shapes could release certain chemicals. The researchers suggest that other materials which shrink in response to a signal - not just heat, for example, but also light, pH, or glucose level - could be used to make self-folding 3D shapes.  

Similar smart responses are seen in nature: the Venus fly trap closes on its prey by pumping ions from one cell to another, which forces a leaf’s curvature to change in much the same way as the gels are forced to buckle. And, said co-author Eran Sharon, the same principles could play a role in the development of wrinkled natural structures, like flower petals. The physics of stress and curvature, not genetics, might be responsible for these 3D shapes.

Richard Van Noorden