3D printing of photochromic and thermochromic shape memory polymers for multi-functional applications

The field of 3D printing is growing rapidly and offers a wide range of potential applications, particularly with the use of Shape Memory Polymers (SMPs). However, current studies on SMP-based 3D printing have mainly focused on structural design and deformation behavior. To enhance the practicality of 3D printed structures, the ability to change color is highly desirable, especially for applications such as anti-counterfeiting, encryption, and bioinspired camouflage. This paper presents the fabrication of a UV-curable PUA-based Shape Memory Polymer (SMP) system with thermochromic and photochromic capabilities. The system is acrylate-based, making it highly UV-curable and compatible with high-resolution 3D printing techniques. Thermo/photochromic effects are achieved by adding thermochromic microcapsules to the system, resulting in printed structures that can change color upon heating or UV) exposure. The printed parts with multi-color hidden information, such as QR codes and digital numbers, were successfully demonstrated through the printing of various anti-counterfeiting patterns.And the expriment results show the exceptional multi-functional performance including shape recovery and thermo/photochromic. The development of this UV-curable PUA-based SMP system represents a significant advancement in the application of SMP-based 3D printing for anti-counterfeiting and secure data recording.


Introduction
In recent years, there has been growing interest in 3D/4D printing smart materials [1,2]. The advancement in 3D/4D printing technologies has opened up new opportunities for various applications such as aerospace [3], soft robots [4][5][6] and biomedical [2,3,7],etc. One of the most promising materials for 4D printing is shape memory polymers (SMPs), which have a high stiffness and fast response rate. Despite their potential, current studies on SMP-based 4D printing have mainly focused on the deformation behavior and structural design of printed structures. The lack of additional functionalities, such as color changeability, has limited the practical applications of 4D printed structures.
One approach to generating 3D structures with new functionalities involves printing structures with soft active materials, such as shape memory polymers (SMPs) [8][9][10], hydrogels [11][12][13], and liquid crystal elastomers [14][15][16]. SMP-based 4D printing has gained remarkable attention due to its ability to switch material modulus in response to environmental stimuli, such as heat, moisture, magnetic field, or electricity, leading to structural changes over the fourth dimension 'time' [17][18][19]. Most studies on SMP-based 4D printing have focused on the deformation behavior and structural design of 4D printed structures. To broaden the potential applications of SMP-based 4D printing, researchers developed SMP systems with additional functions such as anti-counterfeiting, encryption, and bioinspired camouflage [20]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Emerging techniques enable effective information hiding in 3D printed objects through color change. For example, ColorMod developed by MIT Media Lab allows users to switch object colors using photochromic inks and a hardware/software system. This provides multi-color reprogrammability not limited to one-time changes [21].Copper-iodide photoluminescent clusters have desirable thermochromic and rigidochromic properties for functional and aesthetic color-changing 3D printed structures. These materials exhibit desirable thermochromic and rigidochromic characteristics, rendering them suitable for both utilitarian and visual applications within photocurable compositions [22]. A recent study introduced an ultraviolet-curable and thermochromic self-healing shape memory polymer (SMP) system, enabling color change in 3D printed structures. The effectiveness of this system is demonstrated by printing QR codes and anti-counterfeiting patterns [23]. Moreover, A novel 4D printing technology has been developed, capable of generating materials that exhibit simultaneous shape and color responses, similar to those observed in biomaterial systems. This technology offers a simple and effective method for manufacturing multi-responsive active composites through the utilization of shape-memory polymers and thermochromic pigments, enabling the materials to change both their shape and color in response to environmental stimuli [24]. While the current research primarily focuses on the integration of thermochromic dyes to induce color changes, the scope of investigation is limited, thereby restricting the exploration of concealed information possibilities. The application of a novel photoresponsive material, methyl methacrylate-triphenylethylene (TrPEF2-MA), has demonstrated significant progress in the field. This material has been successfully employed to fabricate intricate 3D structures with photoresponsive capabilities, showcasing reversible color switching triggered by ultraviolet/visible light irradiation [25]. In addition, the exhibit features a technology to create reprogrammable multi-color textures using photochromic dyes in a single material. CMY dyes were mixed to control each color channel separately. This allows for highresolution multi-color processes using single-material fabrication techniques. The fully reversible colorchanging process enables multiple recoloring of objects [26]. However, the structural deformation, as well as the thermochromic and photochromic properties, have not been studied yet.
Hence, we present an ultraviolet (UV)-curable and thermo/photochromic shape memory polymer (UTP-SMP) system that enables color changeability in 3D printed structures. By incorporating thermochromic and photochromic microcapsules into the UTP-SMP system, the printed structures can change colors from white to black, red, blue, or yellow upon heating/cooling and exhibit shape memory behavior. The UTP-SMP system consists of acrylate-based monomers and cross-linkers, making it highly UV-curable and compatible with LCD 3D printing for fabricating complex 3D structures with high 35 μm resolution. We conducted applications to invert the color and shape changes of printed UTP-SMP structures. In this work, the 3D printed UTP-SMP parts demonstrate excellent multi-responsive color-change capabilities that can be leveraged for encryption, decryption, and anti-counterfeiting applications.

3D printing resin fabrication Preparation of the liquid resin
The liquid resin used in this study was prepared by combining aliphatic urethane acrylate (FSP8282), isobornyl acrylate (IBOA), and photoinitiator-819 in three different weight ratios (table 1). The weight fraction of Photoinitiator-819 is 3 wt% of the total weight of the mixture materials of Aliphatic urethane acrylate and Isobornyl Acrylate.To obtain a homogeneous solution, the mixture was vigorously stirred at room temperature for 30 min. Once a uniform solution was obtained, it was filtered through a fine syringe filter to eliminate any impurities. After filtration, the solution was mixed for 30 min to ensure complete homogeneity. The solution was stored in a dark plastic bottle at 25°C until further use. The prepared resin was used for printing anticounterfeiting patterns, QR codes, and digital numbers with multi-color hidden functionalities.

3D Printing
The 3D printing process involves the use of a Mini 4K LCD 3D printer with a resolution of 35 μm, making it suitable for printing high-quality objects. The first step involves creating a 3D model of the desired object using computer-aided design (CAD) software( figure 1(a)). The model is saved as an .stl file, which is then imported into slicing software. The slicing software-CHITUBOX( figure 1(b)) generates a series of digital layers that correspond to the shape of the object.
The sliced file was then transferred to the control software of the 3D printer, which controlled the movements of the printer and LCD screen. The next step involved pouring the liquid resin into the printer's resin vat and leveling it using the built-in leveling system of the printer. Once the printer was started, the UV LED light source was turned on, emitting UV light that passed through the LCD screen and exposing the liquid resin to UV light in the desired pattern. The thickness of the printed layer is 50 μm, with an exposure time of 30 s for the bottom layer and 10 s for the other printer layers.The photopolymerization process is triggered by UV light, causing the liquid resin to solidify into the desired shape.
The printer was then moved to the next layer, and the process was repeated until the entire object was printed. This 3D printing process was used to print shape memory polymers, including anti-counterfeiting patterns such as QR codes, digital texts/numbers, and 3D structures, which demonstrated exceptional multifunctional performance, including shape recovery and thermo/photochromic effects.

Post-processing
After the printing process was complete, the object was carefully removed from the printer and placed in a bath of 99.8% ethanol. This was performed to remove any excess or uncured resin from the surface of the object. To  ensure the complete polymerization of the object, it was post-cured under UV light for 30 min. This step helped increase the strength and durability of the printed object. Figure 1(a) illustrates the fabrication process for high-resolution and complex 3D structures and devices using UTP-SMP and an LCD-based 3D printer. The process involves slicing a 3D CAD model into 2D digital images, which are then transmitted to an LCD screen to modulate the UV light that illuminates the surface of a UV-curable liquid resin. The process is repeated layer by layer until the entire structure is formed. After printing, the structure is cleaned and cured in a 405 nm UV LED curing chamber for 10 min. Some printed complex structures with the addition of thermochromic or photochromic microcapsules are shown in figure 1(c), demonstrating that the LCD 3D printer can be used to print 3D UTP-SMP structures.

Tensile stregnth test and DSC testing
To conduct the tensile testing, the dog bone specimens, measuring 10 mm in length, 4 mm in width, and 0.5 mm in thickness, were meticulously prepared. Additionally, the gauge length for these specimens was set at 4 mm. The universal testing machine (Zhiqu machine, China) was utilized to measure the tensile properties of the specimens at a strain rate of 0.1 s −1 . The mechanical properties of the specimens were determined by measuring their tensile strength, elongation at break, and toughness.
During a DSC test, a sample of the shape memory polymer is heated or cooled at a controlled rate using a TA Q600 instrument. The instrument measures the heat flow(Unit: mW m −2 ) into or out of the sample, generating a thermogram that represents the heat flow as a function of temperature. Various thermal transitions and associated properties can be determined from the thermogram. The glass transition temperature (T g ), melting temperature (T m ), and crystallization temperature (T cc ) of shape memory polymers were measured. Figure 2(a) shows the stress-strain curve for different material ratios, including IBFS-1,IBFS-2, and IBFS-3, and indicates that with an increase in the content of IBOA, there are obvious differences among the three different samples. As shown in figure 2(b) and figure 2(c), the tensile strength and modulus increased from IBFS-1 to IBFS-2 and then decreased dramatically for IBFS-3. The elongation and tensile toughness decreased with increasing IBOA content. IBFS-1 shows the best tensile toughness and elongation; hence, we select IBFS-1 to investigate the shape memory and color change features. figure 2(d) gives the thermal features including glass transmition temperature T g , T cc and T m ,which show a little change for IBFS-1,IBFS-2 and IBFS-3. The T g values of the for IBFS-1,IBFS-2, and IBFS-3 were below 60°.

Shape recovery properties
The shape-memory properties of a material are critical in applications where the material is required to undergo reversible deformation. The Tg value of a material, which is the temperature at which it transitions from a glassy state to a rubbery state, plays a vital role in determining its shape-memory properties. In this study, SMPs were found to have a Tg value of less than 60°C, indicating that they can undergo reversible deformation at temperatures lower than 60°C. To evaluate the shape memory properties of the SMPs, three different angles (80°C, 100°C, and 120°C) were selected for the investigation. The results of 180°shape memory testing experiments (as shown in figure 3(a)) demonstrated that the SMPs with a high content of aliphatic urethane acrylate (FSP8282) exhibited the fastest shape recovery time, with IBFS-1 performing the best compared to IBFS-2 and IBFS-3 (as shown in figure 3(b)). However, an increase in IBOA content led to a longer shape recovery time. These findings suggest that the shape-memory properties of SMPs can be tailored by adjusting the content of the constituent monomers, which is a crucial factor in designing shape-memory materials for specific applications.

Application of anti-counterfeiting
A solid black block with hidden text spelling 'IOHN' was used to investigate the communication of information through changes in temperature and UV exposure. As shown in figure 4(a), when the sample was heated to 35°C, the black color of the block turned transparent, except for the letter 'I,' while the letter 'H' returned to black when heated to 40°C. The letter 'N' returned to black at 60°C, and the letter 'O' changed from transparent to red when exposed to 365 nm UV light. This hybrid thermo-and photochromic technology can be used to hide multiple types of information and provide a way to prevent counterfeiting. Furthermore, this study investigated a color and deformation changing gecko, as shown in figure 4(b). The geckos underwent thermal/UV-induced color changes, turning dark blue under thermal changes and dark red under UV changes, providing multi-color information. Additionally, the tail of the gecko recovered its shape when heated algough the shape was fixed for two times. Hence, the example demonstrated that the presented printed SMP structures possess the abilities of both deformation and color change.
In addition, we investigated hidden information in the 3D QR code. Figure 5 shows that a 3D QR code was printed with varying heights, and colors were used to conceal the key corner information of the 3D QR code. Upon heating the 3D QR code to 60°, the color of the corner pixel of the 3D QR code changes to blue. Furthermore, the 3D QR code can be scanned at angles of 25°and 32°to the horizon line. The reason for this is that each individual square of the 3D QR code is assigned a specific height, creating a raised relief pattern. When light interacts with this relief pattern, it results in varying levels of reflection due to the differences in elevation.  This interplay between light and height is crucial for achieving the desired scanning effect. Overall, this demonstrates how colors and heat can be utilized to conceal and reveal information within a 3D QR code. The results of this study may hold implications for future research on information security and the protection of confidential data.
Finally, the structural deformation and color change of 3D printed parts were inverted. In figure 6(a), a QR code with black thermochromic microcapsules is studied. Initially, the colorless QR code was folded into a 3D shape and placed on a heating platform at 60°C. As it was heated, the QR code deformed and gradually changed to black. However, the deformed QR code could not be scanned when the heating time was below 268 s. After 268 s, the QR code could be scanned, indicating that the QR code could be scanned with some 3D deformation and color recovery. Figure 6(b) shows the deformation and color information of 3D printed digital numbers. Initially, the printed parts were bent into a three-dimensional (3D) structure. When heated to 35 o C , the digital number of '555' appeared. When heated to 40 o C, the digital number changed to '666' and the deformation recovered. Finally, exposure to 365 nm UV light caused the digital number to change from '666' to '888' due to both heating and UV exposure. The investigation of structural deformation and color change in 3D printed parts is an exciting area of research that has the potential to revolutionize the way we design and create objects with hidden information.In fact, UTP-SMP can change their shape in response to heating. They can return to their original shape even after being deformed or bent, which can be applied to anti-counterfeiting through structure design [25].

Conclusion
This study aims to develop a novel method for anti-counterfeiting applications by adding thermochromic microcapsules to PUA-based SMPs. A tensile strength test was performed on the SMPs and showed that the best toughness and fastest shape-recovery efficiency were achieved when the content of Aliphatic urethane acrylate (FSP8282) was 50%. Researchers then printed various structures, including the text 'I O U,' a gecko, and a 3D QR code, and investigated their multi-color changes when exposed to different temperatures or UV light. They found that the printed structures underwent both shape deformation and color changes simultaneously, making them promising candidates for multifunctional information hiding. Furthermore, the structural deformation and color information transfer of the QR code with black thermochromic microcapsules were investigated, and it was found that the QR code could be scanned with little 3D deformation and color recovery. Additionally, the deformation and color information for 3D printed digital numbers were examined, and it was observed that heating the printed parts caused the digital numbers to appear, with their deformation recovering upon cooling. Finally, UV exposure changed the digital numbers owing to both heating and UV exposure. Overall, this study presents a new method for multifunctional information hiding, with potential applications in anticounterfeiting measures.

Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is sufficiently accessible or reusable by other researchers. The data that support the findings of this study are available upon reasonable request from the authors.

Ethical compliance
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.