Programming matter through strain

We describe the use of light in a lithographic form of grayscale patterning as a means to program the properties and folding mechanics of flat, thin-film-polymeric materials. In this process, a finely dispersed (phase-separated) mixture of photoresist (SU-8 50) in polydimethylsiloxane (PDMS) is irradiated with ultraviolet light through a photomask. The subsequent photoresist cross-linking in the exposed regions causes changes in the material’s chemo-mechanical properties (notably making it stiffer and more resistant to solvent-induced swelling in the area of exposure). Light scattering due to the dispersed, non-index-matched photoresist domains leads to an intrinsic grayscale profile of the pattern width through the depth of the exposure field, a feature bringing significant and previously unexplored consequences for strain-induced folding dynamics. Solvent induced swelling, where PDMS absorbs a nonpolar solvent, is used to actuate folding mechanics with complex temporal and spatial profiles that explicitly follow the design rules established by the gradient cross-link density, features that elicit a programmable swelling (and therefore, folding) of the two-dimensional sheets. During investigation and optimization of the system, we observed an interesting temporal and biomimetic folding phenomenon that distinguishes the current results from other forms of strain induced folding reported in the literature. Under specific fabrication parameters, an evolution through an intermediate metastable state is observed, one in which the material will fold in one direction, then flatten and fold in the opposite direction. Mechanics modeling and finite element simulations have led to a detailed understanding of the system and the dynamics that allow a temporal evolution of 3-D structure through a double mode of folding. Insights into these mechanisms provide an advanced understanding of strain-induced folding in the field of soft materials.