Shape morphing systems may find potential application in smart textiles, autonomous robotics, biomedical devices, drug delivery and tissue engineering. Lewis and Mahadevan at the Harvard MRSEC have developed 4-D printing by creating a hydrogel-cellulose fibril ink that could be printed to induce a programmable shape change as recently reported in Nature Materials. The printing process defines the alignment of the stiff cellulose fibrils, and the printed bilayers are composed of printed filaments, whose stiffness—and hence swelling behavior—are anisotropic. Schematic images (top row) of the fibril alignment that the ink experiences during extrusion (left). Time lapse image sequence of a printed 5-petal flower closing after immersion in water (right). Fluorescence microscopy (bottom row) showing the isotropic and undirectional cellulose fibril alignments observed in bulk cast and printed samples, respectively. Corresponding plot of swelling strain as a function of nozzle diameter (right).