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

4D printing combines the design flexibility of 3D printing with the stimuli-responsive properties of its materials, finding applications in diverse fields such as soft robotics, drug delivery, flexible electronics, and tissue engineering. Existing 4D printing methods, including direct ink writing, Polyjet, DLP lithography, and SLA, typically limit minimum feature size to ~10 µm. Submicron-scale features, which interact strongly with light, remain largely unexplored in 4D printing. This limitation hinders applications in optics, including structural color generation, temperature-sensitive labels, and colorimetric pressure sensors, all requiring submicron resolution. Dynamically reconfigurable colors, where optical responses are tuned by changing refractive index or dimensions of nanostructures, are of particular interest. Tuning dimensions using shape memory polymers (SMPs) is attractive due to their relatively short response times. This work utilizes 3D printing for direct patterning of complex SMP structures, combining mechanical and optical metamaterials with local control of properties like color, phase, and Young's modulus. To achieve finer 3D structures, a new photoresist suited for two-photon polymerization lithography (TPL) is developed. TPL, with its ability to create features as small as ~10 nm, has been used with various stimuli-responsive materials, but applications with SMPs at the submicron level for color generation remain largely unexplored. This research tackles the challenges of developing and characterizing new SMP resists suitable for TPL and designing robust 3D photonic structures capable of rapid recovery after deformation. The aim is to achieve additive manufacturing of SMPs for programmable color generation.

The generation of structural colors traditionally relies on structures like gratings, thin films, multilayers, and localized resonance structures. These methods produce fixed colors without pigments. Recently, dynamically reconfigurable colors have gained interest, allowing for tuning of optical responses by changing refractive index or dimensions of nanostructures. Shape memory polymers (SMPs) are attractive for tuning dimensions due to their relatively fast response times. Existing methods for patterning SMPs include nanoimprinting and self-assembly, but 3D printing offers advantages in direct patterning of complex structures. Prior work has explored using hydrogel photoresists for tunable photonic devices, with some achieving microscale color changes by altering the periodicity of chiral liquid crystals or demonstrating humidity-responsive color shifts in hydrogel-based reconfigurable photonic crystals. However, the submicron scale and large, rapid visual responses achieved in this research using SMPs represent a significant advancement.

A new SMP photoresist was developed based on Vero Clear, an optically transparent thermosetting polymer resin with acrylate functional groups. The photoresist's composition included 2-hydroxy-3-phenoxypropyl acrylate (HPPA), Bisphenol A ethoxylate dimethacrylate (BPA), and diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO). Resolution tests using two-photon polymerization lithography (TPL) achieved ~300 nm half-pitch gratings. Thermodynamic properties were measured to determine optimal composition for robust mechanical performance. Structural colors were achieved by controlling the geometry of submicron-scale crosslinked SMP structures. Color switching was achieved by heating and deforming (programming) the structures above their glass transition temperature (Tg). The structures were designed with a base layer and submicron-scale grids on top. The color is determined by the interaction of these nanostructures with light. The parameters affecting the color were grid height (h2), grid linewidth (w1), pitch (w2), and the thickness of the base layer (h1). The shape memory effect was achieved by deforming the structure at a temperature (Th) above Tg, then cooling to a temperature (T1) below Tg while maintaining the deformation. Upon releasing the load at T1, the structure remains deformed and colorless. Heating back to Th recovers the original geometry and color. The characterization involved optical microscopy with a microspectrophotometer for measuring transmittance spectra, scanning electron microscopy (SEM) for imaging, profilometry for height measurements, and dynamic mechanical analysis (DMA) for thermodynamic property determination. Finite Difference Time Domain (FDTD) simulations were used to model light interaction with the grid structures. The programming process involved heating the printed structure to 80°C, applying ~500 kPa of pressure using a metal block, cooling to room temperature while maintaining pressure, and then releasing the pressure. The recovery was achieved by heating again to 80°C. Additional experiments used a nanoimprint machine to control pressure more precisely.

The researchers successfully developed a new SMP photoresist suitable for two-photon polymerization lithography (TPL), achieving a resolution of approximately 300 nm half-pitch. This represents a significant improvement over conventional 4D printing methods. They demonstrated the ability to generate a wide range of colors by varying the geometric parameters of the submicron-scale grid structures, specifically grid height and linewidth. Finite-difference time-domain (FDTD) simulations confirmed the relationship between structure geometry and optical properties. Importantly, they demonstrated a reversible shape memory effect at the submicron scale. The printed structures, after being deformed and rendered colorless at a temperature below Tg, fully recovered their original shape and color upon heating above Tg within seconds. This shape memory effect was highly repeatable, with minimal color change even after multiple cycles of deformation and recovery. The study also investigated the impact of various printing parameters, such as laser power and write speed, on the resulting structure size and color. Finally, the practical application of the developed technique was illustrated by creating a multicolor image that could be rendered invisible and then recovered, showcasing the potential of this technology for applications such as anti-counterfeiting and temperature-sensitive labels.

The successful creation of a submicron-scale 4D printing process using a novel SMP photoresist addresses a significant gap in the existing literature. The ability to generate a wide range of colors with high resolution and rapid reversibility opens up new possibilities for applications requiring precise control over optical properties and mechanical behavior. The findings are significant for the development of advanced optical devices, anti-counterfeiting techniques, and temperature-sensitive labels. The rapid recovery time is particularly noteworthy, enhancing the practicality of these applications. The use of FDTD simulations validated the experimental results and provided a deeper understanding of the light-matter interaction at the nanoscale. The demonstrated ability to create complex multicolor images further strengthens the potential of this technology for diverse applications. However, the edge effects observed during recovery highlight potential avenues for future improvement by developing photoresists with increased stiffness and reduced shrinkage.

This work successfully demonstrated submicron-scale 4D printing of shape memory polymers using two-photon polymerization lithography. A novel photoresist enabled the creation of multi-color structures with ~300 nm half-pitch resolution. The reversible shape memory effect allows for the creation of 'invisible inks' with rapid color switching. Future research could focus on improving the photoresist to enhance resolution and reduce edge effects during recovery, exploring different stimuli for actuation, and expanding the range of possible applications.

While the study demonstrated high repeatability of the shape memory effect, edge effects during recovery resulted in some irreversible changes in the structure. Future work should aim to improve the photoresist to minimize these edge effects. The study focused primarily on thermal actuation; exploring other stimuli like light or magnetic fields could expand the range of potential applications. The study's focus on optical properties could be expanded to include a more comprehensive investigation of mechanical properties at the submicron scale. The influence of long-term stability and durability on color recovery and shape memory effects also requires further investigation.