Fig. 1. Schematic of colour and shape change of a constituent nanostructured element of the “invisible ink” 3D printed in shape memory polymer (SMP).

The as-printed structures with upright grids (left) function as a structural colour filter that transmits only a limited wavelength range of visible light. Deformation of the structures at elevated temperature flattens the nanostructures (right) rendering it colourless, where it remains in an invisible state after cooling to room temperature. Heating recovers both the original geometry and colour of nanostructures, leading to a submicron demonstration of 4D printing.