Structural multi-colour invisible inks with submicron 4D printing of shape memory polymers

Abstract

Four-dimensional (4D) printing of shape memory polymer (SMP) imparts time responsive properties to 3D structures. Here, we explore 4D printing of a SMP in the submicron length scale, extending its applications to nanophononics. We report a new SMP photoresist based on Vero Clear achieving print features at a resolution of ~300 nm half pitch using two-photon polymerization lithography (TPL). Prints consisting of grids with size-tunable multi-colours enabled the study of shape memory effects to achieve large visual shifts through nanoscale structure deformation. As the nanostructures are flattened, the colours and printed information become invisible. Remarkably, the shape memory effect recovers the original surface morphology of the nanostructures along with its structural colour within seconds of heating above its glass transition temperature. The high-resolution printing and excellent reversibility in both microtopography and optical properties promises a platform for temperature-sensitive labels, information hiding for anti-counterfeiting, and tunable photonic devices.

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.

Fig. 2. Optical and scanning electron micrographs (SEM) of as-printed structures.

a Optical transmittance micrographs of a printed colour palette for a constant pitch of 2 μm but varying write speed and nominal height h₂ for a range of laser power (I–VI laser power 30–35 mW respectively). b SEM images of grid structures with different nominal height h₂ for constant write speed of 1 mm/s and laser power of 30 mW. c Measured grid linewidth w₁ and height h_2m as a function of nominal height; d SEM images for grid structure with different write speeds for fixed nominal height of 2.7 μm and laser power of 30 mW. e Measured grid linewidth and height as a function of write speed. Values in c and e represent mean and the error bars represent the standard deviation of the measured values (n = 5).

Fig. 3. Finite difference time domain (FDTD) analysis of the grid structure.

a Measured and FDTD simulated transmittance spectra of structures with different nominal height h₂ (from the black dashed rectangle in Fig. 2a ranging from 1.2 μm to 2.7 μm. 1st and 2nd represent the first and second order resonance dip respectively). Marked positions λ₁ = 490 nm and λ₂ = 710 nm are used for FDTD field analysis in Fig. 3b–e. b, c Cross section view of near-field normalised electric field phase and amplitude for a grid structure (laser power: 30 mW, write speed: 1 mm/s, nominal grid height: h₂ = 2.7 μm) at dip transmittance 490 nm and peak transmittance 710 nm wavelength respectively (|E/E_inc| represents the normalised electric field amplitude). d, e Top view of far-field normalised electric field amplitude for the above grid structure at dip transmittance 490 nm and peak transmittance 710 nm wavelength respectively; the white circle represents collection field for the microscope used in this work (NA = 0.2, CA = 11.5°). f Simulated transmittance spectra for structures with different linewidth w₁ (the colours of the spectrum lines were mapped from the corresponding spectra).

Fig. 4. Submicron scale shape memory effect.

a Different colours as printed, compressed and recovered respectively, observed by the objective lens (NA = 0.2, CA = 11.5°) at the transmittance mode (in the colour palette, the laser power was varying from 30 to 35 mW with a step of 0.5 mW in the transverse direction, and the write speed was varying from 0.6 to 1.1 mm/s with a step of 0.05 mm/s in the vertical direction). b Comparison of measured spectra of three different grid structures (marked as number 1, 2, 3 in Fig. 4a) as printed, after programming and after recovery. c Tilted (30° tilt angle) and top view of SEM images before and after programming and after recovery. d Measured spectra and e SEM images for a structure programmed into different degree of flatness. f Measured spectra of a grid structure for four programming cycles. g The painting as printed, compressed, and recovered, respectively.