Spontaneous shape transformations of gels, particularly the self-folding of flat gel films into coiled tubes, has been of great interest in recent years. In studies to-date, a rectangular film usually folds along its short axis when forming a tube; folding along the long axis has been seen only in rare instances when the film is constrained. Here, we report a system in which the same gel film folds along its long or short axis depending on the concentration of a solute. Our gels are sandwiches (bilayers) of two layers: a passive layer of N,N¢-dimethylyacrylamide (DMAA) and an active layer of DMAA that also contains the biopolymer alginate. It is well-known that solutions of alginate can be cross-linked by multivalent cations like Ca2+, Sr2+, and Cu2+. These same multivalent cations induce our bilayer gels to fold into tight tubes. The folding occurs instantly (within seconds) when a flat film of the gel is introduced into a solution of these cations. The likely cause for folding is that the active layer shrinks when the alginate chains in it are cross-linked whereas the passive layer is unaffected (the resulting mismatch in swelling degree between the two layers creates internal stresses that drive folding). Cations that do not cross-link alginate such as Na+ and K+ do not induce gel folding. Gels also do not fold if the alginate concentration in the active layer is too low. Moreover, what is particularly striking is the direction of folding. When the concentration of Ca2+ is high (100 mM or higher), the gels fold along their long axis, whereas when the Ca2+ is low (40 to 80 mM), the gels fold along their short axis. We hypothesize that the folding axis is dictated by the inhomogeneous nature of alginate-cation cross-linking, i.e., that the edges get cross-linked before the faces of the gel. At high Ca2+, the stiffer edges present a significant constraint to folding; in turn, the gel folds such that the longer edges are deformed less, which explains the folding along the long axis. At low Ca2+, the edges and the faces of the gel are more similar in their degree of cross-linking; therefore, the gel folds along its short axis. An analogy can be made to natural structures (such as leaves and seed pods) where stiff elements provide the directionality for folding.

Folding of gel sheets bearing alginate upon exposure to multivalent cations

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.