Structural Multi-Colour Invisible Inks with Submicron 4D Printing of Shape Memory Polymers

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.


Introduction
4D printing 1-3 brings together the design flexibility of 3D printing with stimuli responsive properties of its constituent materials. It continues to generate excitement in diverse fields, e.g., soft robotics 4-6 , drug delivery 7,8 , flexible electronics 9,10 and tissue engineering 11 . Commonly used printing methods for 4D printing include direct ink writing 3,12 , Polyjet 13,14 , Digital Light Processing (DLP) lithography 15,16 and Stereolithography (SLA) 17,18 . The material and lithographic challenges inherent to these methods limit the minimum feature size of printed structures to ~10 μm. 19 At an order of magnitude smaller, submicron scale features that interact strongly with light have yet to be systematically explored in 4D printing.
The motivation to improve print resolution is fuelled by applications in optics, e.g. structural colour generation 20,21 , temperature sensitive passive labels, and colorimetric pressure sensors, all of which require submicron resolution and precision. Traditionally, different structures such as gratings 22,23 , thin films and multilayers 24,25 , localized resonance structures 26,27 generate fixed colours without the use of pigments. Recently, dynamically reconfigurable colours have gained interest, where optical responses of nanostructures can be tuned either by changing the refractive index [28][29][30] or dimensions 31,32 of the structures. Of these methods, tuning the dimensions of the optical devices by shape memory polymers (SMPs) is of interest due to their relatively short response times (seconds to minutes depending on actuation temperature 33 ). Distinct from the pattering of SMPs through nanoimprinting [34][35][36] and self-assembly [37][38][39] , our use of 3D printing introduced here will lead to direct patterning of complex structures at will, bringing together fields of mechanical and optical metamaterials with local control of properties, e.g., colours 40 , phase, and Young's modulus.
To print finer 3D structures, we develop a new resist suited for two-photon polymerization lithography (TPL) 6,41,42 . Here, photo-initiators in a liquid resin are excited by a two-photon absorption process within the focal region of a femtosecond laser. Polymerization and crosslinking then ensue. Printed features as small as ~10 nm can be achieved under specific conditions. 43 Due to the high resolution it provides, TPL has been used to print different stimuli responsive materials such as hydrogels 6,44 , liquid crystal elastomers 41,45 , magnetic nanoparticles embedded resists 46,47 , silicon functionalized monomers 48 , and other examples printing 49 . Recently, hydrogel photoresists have been used for tunable photonic devices. Marc et al. 50 used a cholesteric liquid crystals (LC)-based hydrogel resist to change colours at microscale. In their work, colours were tuned within a limited range by changing the intrinsic periodicity of chiral LC. Tao et al. 51 demonstrated a hydrogel based reconfigurable photonic crystals exhibiting colour shifts in the presence of humidity. In contrast to previous works reporting colour shifts, our work investigates submicron 4D printing where large and rapid visual responses are achieved as nanostructures recover from a flattened (colourless) state to an upright (colourful) state. The visual effect is analogous to a letter written in multiple colours of invisible ink where secret information is revealed, e.g., with the application of heat. We tackled two key challenges in (1) developing and characterizing new stimuli responsive resists suitable for TPL, and (2) designing and fabricating robust 3D photonic structures capable of rapid recovery after being flattened.
In this work, we meet these challenges by additive manufacturing of SMP for programmable colour generation. We developed and characterized a SMP photoresist suited for TPL based on Vero Clear 13 , which is an optically transparent thermosetting polymer resin containing acrylate functional group. We performed resolution tests achieving ~300 nm half pitch gratings and measured the thermodynamic properties of the new resist to determine an optimal composition for robust mechanical performance. A range of structural colours were achieved by controlling the geometry of the crosslinked SMP structures at the submicron level.
We realized colour switching behaviour by heating and deforming (i.e. programming) the printed structure at 80 °C . Remarkably, the deformed nanostructures exhibited excellent recovery when heated above its composition-adjustable glass transition temperature (Tg). This reversibility in both structural features and optical responses demonstrate significant promise for submicron scale additive manufacturing of SMPs.

Two-photon polymerization lithography of shape memory polymer
We designed structures consisting of a base layer with submicron-scale grids on top of it, as shown in Figure 1. Due to the interaction of these nanostructures with light, i.e., scattering and interference, the 3D printed structures function as colour filters, preferentially transmitting certain wavelength ranges of an incident white light illumination. Colours depend sensitively on the geometric parameters of the grid, i.e., grid height ℎ 2 , and grid linewidth 1 , while it is less sensitive to pitch 2 and the thickness of the base layer ℎ 1 . By printing in SMP, we realize a 4D effect, with the ability to change its geometry and optical properties in response to temperature variation as a function of time.
To achieve the shape memory effect, the print is first deformed at a temperature Th higher than the SMP's glass transition temperature. While keeping the external load, the temperature is decreased to Tl (< Tg) as the print transitions from a soft rubbery state to a stiff glassy state. Here, in the altered flattened geometry the print loses its colour, rendering the print "invisible". The temporary configuration is achieved at Tl after releasing the external load as the polymer chains are "frozen" at its glass state. The print finally recovers to its original geometry and colour when heated back to Th where the polymer chains regain the entropic elasticity.
A Vero Clear 14 based SMP photoresist was developed (See Methods and Figure S1 for the preparation process). To test the resist, we placed a droplet of it onto a fused silica glass substrate and exposed using in the commercial TPL system Nanoscribe GmbH Photonic Professional GT using the "dip-in" configuration 40,52,53 . During exposure, photo-initiators at the focal point of the objective are activated by two-photon excitation from the femtosecond pulsed laser at 780 nm wavelength, leading to polymerization of the resist into solid structures.
After exposure, the uncured resist was removed using a development process (see Methods for details). The characterization of the photoresist is provided in Supplementary part 2. With the characterized resist ,we were able to print samples with linewidth of ~280 nm ( Figure S2e) and conservative minimum resolvable pitch of 600 nm (i.e. 300 nm half pitch, Figure S2f). To the best of our knowledge, these are the smallest feature sizes and highest print resolutions achieved via additive manufacturing of a SMP. We next investigate the different colours achieved by the grid structure shown in Figure   1 and how it depends on the various design parameters. As ℎ 1 only affects the phase of light and does not contribute to the change of colour, it is fixed at ~4 μm to raise the grids above the substrate making it easier to compress. The two parameters ℎ 2 and 1 can be varied by controlling the write speed, laser power and number of grid layers. Figure 2a shows a transmittance optical micrograph of a colour palette as a function of write speed 1 and nominal height ℎ 2 for a range of laser power (30-35 mW) and fixed pitch 2 (see Table S1 for the fabrication parameters). The corresponding transmittance spectra for Figure 2aI were measured using a CRAIC microspectrophotometer and mapped onto the CIE 1931 chromaticity diagram in Figure S3, demonstrating a reasonably wide range of colours. To study the effect of pitch 2 on colour, we fabricated structures of constant nominal height h2 of 1.8 μm and varied its pitch. The transmittance spectra for structures with different pitches are shown Figure S4a (see the corresponding SEM images in Figure S4b-h). When 2 is 1 μm, the adjacent lines are fused together during the polymerization process, and produce a nearly transparent patch. When 2 is 3 μm, the gaps are too wide, leading to low colour saturation.
Thus, a gap of 2 μm was chosen for the remainder of our studies. Here the minimum resolvable pitch 2 is larger than that in Figure S2f, which could be caused by the proximity effect while printing multilayer girds. It should be noted that the experimental obtained colour gamut in  Both the linewidth 1 and height ℎ 2 for the grid structure in the black box are plotted in Figure   2c. The height increases at an interval of 100 nm per layer, which is less than the nominal layer height (300nm as shown in Table S1) as a result of shrinkage during the writing and development (rinsing) process.
To study the influence of write speed on the structure, Figure 2d Figure   3b and c, respectively. The incident plane wave passes through the base layer without scattering.
After propagating through the submicron grids, the wave front is delayed compared to the wave fronts that pass through the air gap. As seen in the near field phase plot in Figure 3b, the regions within and directly above the grid lines appear to accumulate phase faster than the region in between. The interference of these two regions of transmitted light causes some focusing and redistribution of the energy flow of light (see corresponding electric field amplitude of the near field in Figure 3c). For the peak and dip positions, the constructive interferences occur at different parts above the structure. The far field energy distribution can be obtained by performing the near-field-to-far-field transform. Figure 3d and e present the normalized far field electric field amplitude within the objective collection angle (CA) for the dip and peak position respectively, corresponding to experimental observation with objective lens adopted in our experiment (the NA=0.2, CA=11.5°). The integration of the far field electric intensity within the collection angle results in the transmittance spectra dip and peak shown in Figure   3a.
As discussed in Figure 2, write speed and laser power affect both linewidth 1 and grid height ℎ 2 . To study the influence of 1 alone on colour, we simulated the spectra for a fixed height ℎ 2 of 0.9 μm and varied the linewidth 1 (Figure 3f). As the linewidth increases from 200 nm to 500 nm, the transmittance dip redshifts from 450 nm to 600 nm, resulting a shift of the transmitted colour from yellow to blue. This effect could be risen from the increase of effective refractive index as the increase of 1 . See Supplementary Information part 9 for a detailed discussion about this relation. The redshift effect of the measured spectra as the increasing of laser power and decreasing of write speed in Figure S8c could be explained by the increase of effective refractive index. This result suggests that one could also achieve large colour variation simply by varying the width of the structures.

Submicron scale shape memory effect
To study the shape memory effect of the submicron scale structure, we printed a colour palette (Figure 4aI), with a fixed nominal height of 1.8 μm, but varied the laser power and write speed from 30 mW to 35 mW horizontally with a step of 0.5 mW and 0.6 mm/s to 1.1 mm/s vertically with a step of 0.05 mm/s respectively. Doing so also generates a broad range of colours due to variation in both width and height of the structures. The structures were then heated above its Tg to 80 °C using a heat gun. Under the high temperature, a stress of ~500 kPa was applied using a metal block on the surface of the structure. Then the sample was cooled down to room temperature in air (in ~30 seconds) with the metal block maintained. Upon removal of the load, the deformed structures appear transparent as shown in Figure 4aII. All the colours were recovered when the sample was heated up again to 80 °C by the heat gun (Figure 4aIII). The recovery process occurs within seconds due to the rapid response of the SMP. The detailed setup of this programming process is provided in Figure S9. We compared the spectra of three different colours (marked as 1-3 in Figure 4a As the structures have been squeezed to the point of contact between surfaces, one would have predicted that this deformation was irreversible. Experience with collapsed nanostructures from capillary forces teaches us that the stiction Van der Waals forces will keep these nanostructures together 55 . Yet, once heated above Tg again, the grids recover to regular squares again due to the shape memory effect. The top view of structure 2 during the programming process is given in Figure 4c, in which the linewidth before deformation and after recovery matches well, indicating a good shape recovery effect. It should be noted that as there was no support on the edges of grids, the walls along the edge might have been too thin to overcome the stiction forces and could not recover, as shown in the tilted view SEM image in Figure 4cIII. This irreversible damage account for some of the dark corners and edges of the recovered structures in Figure 4aIII. To further understand the programming process, Figure 4d shows spectra for a structure programmed into different degree of flatness, and the corresponding SEM images are given in Figure 4e. When the structure is slightly flattened (Figure 4eII), the grids configuration is similar to the original one (Figure 4eI), and there are only gentle shifts of both colour and spectrum (Figure 4d). While the structure is further flattened, the gaps between the grids are filled (Figure 4eIII-IV), leading to high transmittance of light wavelength in the whole visible range and a transparent appearance (Figure 4d). The robustness of the programming process is checked in Figure 4f by programming a structure for 4 times, the spectra for recovered structures in different cycles match well, indicating a good repeatability of the shape memory effect.
To demonstrate the potential application, we printed an image of an art piece by one of the authors, depicting an octopus in foreground with a mountainous landscape in the background (Figure 4gI). This image comprised 52×52 pixels with each pixel designed as 10 μm. The print was then programmed into a transparent featureless image as shown in Figure   4gII, using the same process. Here both the octopus and its surrounding have become invisible.
Upon heating, the painting recovers to its original state again as shown in Figure 4gIII. Figure   S10 presents the SEM images of the top left corner of the original painting. In Figure S10a, different parts were printed by different nominal heights, resulting in different colours in Figure   4g. To make the whole structure stable, the lines along two adjacent write fields were written twice, leading to wider lines comparing to lines within one write field as shown in Figure S10b.
And the grids near the boundaries are stretched to be wider, which results in slightly different colour along the boundaries of different write fields. To overcome this issue, some more stable photoresist with higher stiffness and less shrinkage should be developed in the future. as printed, compressed and recovered, respectively.

Discussion
By fabricating of the characterized SMP photoresist, we printed 300 nm scale structures with good recoverability in both microtopography and optical properties. There is still room for improvement of resolution comparing to commercially available photoresist such as IP-Dip (~100 nm feature size 56,57 ). An ideal SMP for TPL should have high stiffness at the glassy state and low shrinkage during printing to avoid collapse and distortion so as to further bring down the feature size. Meanwhile it should have good stretchability at the rubbery state to avoid irreversible break during programming. Also, micron and submicron scale mechanical test should be implemented to understand the micromechanical behaviour of the printing. These issues should be investigated in the future works.
In conclusion, we demonstrated the concept of submicron scale 4D printing of shape memory polymer with the application for multi-colour invisible inks by two-photon polymerization lithography of the custom-tailored photoresist. Due to the flexible tunability of the design variables by additive manufacturing, different colours can be easily obtained by varying the printing parameters such as laser power, write speed and nominal height of grids.
The printed structures can switch colour stably and rapidly by the programming process.
Though there are some key challenges in this area, we believe that this approach opens up potential applications of 4D printing in high precision required fields such as optics and sensors.

Two-photon polymerization lithography
Before writing, a fused silica substrate (25×25×0.7 mm 3 , refractive index = 1.46) was cleaned with IPA solution in ultrasound for 2 mins. The substrate was baked at 120 °C for 10 mins on a hotplate, then cooled to room temperature. Then the substrate was spin-coated with TI PRIME adhesion promoter (MicroChemicals GmbH, Germany) at 2000 rpm for 20 s. The substrate was again baked at 120 °C for 2 mins on a hotplate, then cooled to room temperature.
A drop of photoresist was placed onto the coated side of the substrate, then the substrate was transferred to a two-photon lithography system (Photonic Professional GT, Nanoscribe GmbH, Germany). A 63×NA1.4 oil immersion objective lens in Dip-in Laser Lithography (DiLL) configuration was used. The laser power and write speed were set to 30-40 mW and 0.5-2 mm/s, respectively. After writing, the substrate was immersed in propylene glycol monomethyl ether acetate (PGMEA) for 5 mins, isopropyl alchohol (IPA) for 2 mins and deionized water for 1 min to remove the uncured resist. The substrate was blown with a N2 gun to remove the water, then cured with UV exposure of 365nm and ~1 J/cm 2 for 10 mins (UVP® CL-1000® Ultraviolet Crosslinkers, USA). Finally, the substrate was kept in a clean container for 2 days to release residual stress.

Materials characterization
The dynamic mechanical analysis tests were conducted on a DMA tester (TA Instruments, Q800 DMA, U.S.) in the tension film mode. After erasing thermal history at 80 °C for 5 min, DMA tests started from 80 to 0 °C at a cooling rate of 3 °C/min. For the test of pure elastomer, the test was started at 40 °C to avoid rupture. The dimensions of the testing samples were 6 mm × 15 mm × 0.5 mm and was prepared by curing the photoresist in a Teflon mould in the UV oven with a power of ~1 J/cm 2 for 10 mins.
The uniaxial tensile experiments were conducted on the DMA tester (TA Instruments, Q800 DMA, U.S.). The samples were prepared by the same method as above. The test was conducted using the stress control mode with a stress rate of 2 MPa/min. The temperature was controlled to be 20 °C higher than the glass transition temperature (Tg) for each composition.
The viscosity tests were conducted on a Discovery Hybrid Rheometer (DHR2, TA instruments Inc., UK) with an aluminium plate geometry (20 mm in diameter), with frequency ranging from 10 to 4000 Hz. The temperature was precisely controlled to be 22 °C by a Peltier system. The plate gap was set as 100 μm.

The SEM images was taken with a JSM-7600F Schottky Field Emission Scanning Electron
Microscope (JEOL, Japan) using a voltage of 5kV. Before the test, the samples were sputtered with gold in vacuum for 60 seconds with a current of 40 mA at the control gas manual mode.
The height of the structures was measured by a Profilometer KLA Tencor D-600 (KLA Inc., U.S.). The scan speed was 0.01mm/s and the stylus force was 1 mg.
The refractive index of the photoresist needed for FDTD simulation was measured by an EP4 Ellipsometer (ACCURION, Germany). A wafer substrate was cleaned with IPA and then baked on a hotplate at 130 °C for 10 mins. Then the resist was spin coated on the substrate with a speed of 7000 rpm for 1 min. Then the substate was baked again on the hotplate 130 °C for 3 mins. Afterwards, the sample was used for the measurement of the ellipsometry angles Δ and Ψ of the photoresist. The refractive index n and extinction coefficient k were fitted based on the measured Δ and Ψ using the Cauchy dispersion function as shown in Supplementary information Part 7.

Optical measurements
Transmittance spectra were measured using an objective lens (NA=0.2, CA=11.5°) on an optical microscope (Nikon Eclipse LV100ND) with a CRAIC 508 PV microspectrophotometer and a Nikon DS-Ri2 camera. Samples were illuminated with a halogen lamp. The spectra (transmittance mode) are normalized to the transmittance spectrum of fused silica glass, which is measured under the same conditions as for the sample.

Numerical Simulation
FDTD simulation to calculate the theorical spectra was conducted with a commercial software (FDTD, Lumerical Solutions). The dimension profile for the simulation was obtained from the SEM images and the Profilometer. In FDTD simulation, planewave was injected from bottom of grids and base layer, then electric field and corresponding power components were collected and followed by a near-to-far-field transform process. The final spectra were obtained by integration of the energy within the collecting angle of the objective lens used in measurement.

Data availability
All data are available from the corresponding author upon reasonable request.  Figure S1. Chemical structures of the components in the customized photoresist

Materials characterization
To develop a suitable resist for additive manufacturing of shape memory polymer, one needs to satisfy several criteria simultaneously: (1) sufficient viscosity for patterning with our TPL system, (2) a suitable Tg, e.g., above room temperature, and (3) sufficient deformability to avoid irreversible damage during the programming process as described in Figure 1. To meet these demands, we cured macroscopic films (0.5mm thick) of the resist in ultraviolet light to perform mechanical tests. Results are shown in Figure S2a-d. As the temperature increases, the storage modulus decreases for all concentrations due to the viscoelasticity of the SMP (Figure S2a). At the same temperature, the storage modulus decreases as the increase of the concentration of the elastomer, which has a much smaller elasticity modulus than Vero Clear. Figure S2b plots tan δ, i.e. the ratio of loss modulus (corresponding to energy dissipation) and storage modulus, as a function of temperature. The peaks of these plots correspond to the glass transition temperature of the particular composition of Vero Clear to elastomer ratio. A single tan delta peak indicates that the blend of elastomer and Vero Clear co-cured without phase separation.
The Tg for pure Vero Clear is ~65 °C and ~17 °C for pure elastomer. As the concentration of the elastomer increases from 10% to 50%, Tg decreases from 54 °C to 29 °C. Varying the ratio of Vero Clear to elastomer thus allows for prints that respond to a range of temperatures. For convenience, we chose a Tg of ~40 °C, i.e. above room temperature for the programmed structures to be maintainable without external load at room temperature. Figure S2c presents the stress-strain curves of the polymer blends above Tg. The temperature was set to 20 °C above the corresponding Tg for each composition. As the blend of Vero Clear and the elastomers is homogenous as shown by the tan delta results, expectedly, the strain at failure increases from 34% to 85% ( Figure S2c inset) with increasing elastomer content. For the interest of this study, the mass ratio of 7:3 of Vero Clear and elastomer was chosen which has a strain at failure of ~50%. It should be noted that the mechanical tests in Figure S2a-c were conducted by UV cured macroscopic films of 0.5 mm and the patterned film by the TPL process is in the submicrometer range. The mechanical properties of TPL patterned structures are likely to be similar to that of the bulk film.
In the Dip-in Laser Lithography (DiLL) configuration of Nanoscribe, the viscosity of the photoresist should be controlled: A viscosity that is too low would result in the resist flowing away (unless gaskets are used) as the sample stage moves during printing. A viscosity that is too high could reduce the print resolution by suppressing the diffusion of oxygen inside the resist, which acts as quencher of the photopolymerization process 1 . The developed SMP photoresists in our work have viscosities between 140 to 220 mPa•s (as shown in Figure S2d), which is about 10 times less viscous than the commercial photoresist IP-Dip (2420 mPa•s according to the official datasheet). Nonetheless, we were able to expose and pattern high resolution structures without problem, indicating a suitable viscosity range of the resist within the TPL system.
To examine the resolution of this resist, we exposed a series of single-pixel lines on a base layer (~2μm thick), used to aid adhesion. Figures S2e-f show scanning electron micrographs (SEM) of patterns formed in a resist with a 7:3 mass ratio composition of Vero Clear to elastomer. The linewidth decreases as expected with decreasing laser power and increasing write speed ( Figure S2e). The polymerization threshold of 30 mW was determined at a reasonable write speed of ~1.1mm/s resulting in a linewidth of ~280 nm. This write speed is an order of magnitude slower than that for the commercial TPL photoresist IP-Dip 2 , which may be attributed to lower reaction speeds of both the initiator and monomer. Though the linewidth can be further reduced by lowering the laser power, the resultant structures tend to collapse due to poor mechanical strength 3 . Hence, the lowest laser power used in this work was 30 mW. To determine the resolution limit, we fabricated lines with different pitches with a fixed write speed of 1.1mm/s while varying the laser power, as shown in Figure S2f. A conservative minimum resolvable pitch was determined to be 600 nm (i.e. 300 nm half pitch).
Gaps between lines at 400 nm pitch were not well defined.

Fabrication parameters
Different structures in the manuscript were fabricated with different parameters. See Table S1 for the fabrication parameters.

Determining of the refractive index used in the FDTD simulation
To measure the refractive index of the photoresist, a wafer substrate was first washed by IPA and then baked on a hotplate at 130 °C for 10 mins. Then the resist was spin coated on the substrate with a speed of 7000 rpm for 1 min. Then the substate was baked again on the hotplate 130 °C for 3 mins.
The ellipsometry angles Δ and Ψ were measured by an EP4 ellipsometer (ACCURION, Germany). The measured data were fitted by the Cauchy dispersion function as shown in Eq.
(S1) and Eq. (S2), where A, B, C, A1, B1, C1 are parameters for the Cauchy dispersion which can be determined by fitting the measured ellipsometric angles Δ and Ψ. n and k are refractive index and extinction coefficient of the material, respectively. The fitting process was done by the Levenberg-Marquardt algorithm in the EP4 model software.
( ) 2 4 B C n A λ λ λ = + + (S1) ( ) The fitted parameters are given in Table S2 After fitting the dispersion function, the refractive index n and k as functions of wavelength are obtained and plotted in Figure S6.  Figure S9. Schematic of the 4D programming process. The structure was first heat up to 80 • C by a heat gun. The temperature was monitored by a thermometer. Then a stress of ~500kPa * was applied through a metal block. A layer of Teflon film was put between the structure and the block to avoid contamination. The heat gun was removed afterward, and the structure was cooled down to room temperature with the stress maintained. Finally, the stress was removed, and the structure was heat up to 80 °C to recover. (*The stress was estimated by the following process: A force applied by thumb was estimated to be ~50 N based on Ref. 5 ; The contact area of the metal surface and the support was 1cm×1cm; The pressure 50N 500kPa 1cm 1cm P ≈ = × )