We use light to program the folding mechanics of a flat, two-dimensional material.  Mixtures of the photoresist (SU-8 50) and polydimethylsiloxane (PDMS) are irradiated with ultraviolet light through a photomask.  The exposed regions of the material are less elastic than the unexposed regions due to photoresist cross-linking in the exposed regions.  Because PDMS absorbs nonpolar solvent, the gradients of decreasing cross-link density from exposed to unexposed regions cause programmable swelling, and therefore folding, of the material.  During investigation and optimization of the system, we observed an interesting temporal and biomimetic folding phenomenon.  Specific fabrication parameters cause the material to fold first in one direction, then flatten, and then fold in the perpendicular direction, all in less than thirty seconds.    The figure above is a chronological series of eight frames from a video clip showing that the material assumes a final cylindrical position (g & h) folded on a perpendicular axis and in opposite direction of the initial cylindrical fold (b & c) (a: 1.72 s; b: 2.20 s; c: 4.30 s; d: 10.05 s; e: 12.22 s; f: 14.33 s; g: 14.74 s; h: 18.75 s).  Further exploration and mechanical simulations have led to a detailed understanding of swelling and temporally based strain relationships that cause this double folding.  Insight into these mechanics advances the understanding of strain-induced folding in the field of soft materials.

Mechanical perspective on programmable strain-induced folding in polydimethylsiloxane

248th National Meeting (2014)

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