Fabrication of a Novel Fluorescein Embedded Photocomposite Based on Interpenetrating Polymer Network (IPN) and Its Application in 4D Printing

With the rapid development of the 3D printing technique, the concept of 4D has emerged, defining time as the fourth dimension. This concept allows for changes in the appearance, function, or other properties of 3D printed objects in response to certain controlled external stimuli over time. In this work, a new photocomposite containing fluorescein based is designed on an interpenetrating polymer network (IPN) system, this system can be generated by the simultaneous photopolymerization of acrylate and epoxide monomers under visible LED@405 nm irradiation. Unexpectedly, combining the reversible fluorometric turn‐on property of fluorescein and the deformation of the IPN in the presence of water, the 3D flower structure printed from this photocomposite would demonstrate a novel dual responsive behavior with both fluorescence and shape changes when it is exposed to water for a short time. This work would present a novel functional photocomposite material and also expand the application prospect of 4D printing.


Introduction
Additive manufacturing, also known as 3D printing, has received extensive research since the first description in the 1980s by Charles Hull. [1]7][8] Photopolymerization plays a crucial role in the field of 3D printing, specific techniques, such as stereolithography (SL) and digital light procession (DLP) have attracted huge attention from scientists to prepare stereoscopic objects with wellcomplicated structures. [9,10]Especially, photocomposites constructed of at least two different kinds of components through the photopolymerization process could show some new and even superior properties when compared with a single polymer component. [11,12]For example, a 4D printing behavior with reversible deformation of zeolite reinforced poly(ethylene glycol) diacrylate (PEGDA) composite was once reported when it was exposed to water, considering the intrinsic hydrophilic nature of PEGDA. [13]However, to the best of our knowledge until now the 4D printing of PEGDA based polymers induced by water is still limited to deformation only.It remains a huge challenge of great significance to further expand the 4D responsive behavior of the photocured polymers.
Fluorescein is a very interesting small molecule dye, existing in either ring-closed spirolactone form or ring-opened fluorone form.Although the ring-closed fluorescein is nonfluorescent, the ring-opened form can emit strong fluoresce.16] It is envisioned that a composite system containing PEGDA and fluorescein at the same time would show both fluorescence and shape changes when it is under exposure to water.To validate such a hypothesis, in this work an interpenetrating polymer network (IPN) system composed of PEGDA and epoxy resin was proposed, which can be polymerized under visible LED@405 nm irradiation.Then upon the addition of fluorescein dye into the system, a novel photocomposite material can be designed for the first time.Further, a truly 3D flower structure fabricated from this composite by DLP was exposed to water to demonstrate the possibly simultaneous dual responsive behaviors with both shape changes and fluorescence changes, thus providing a better understanding of 4D printing as well as expanding its application prospect.

Results and Discussion
[19][20] Herein PEGDA (Mw = 726 g mol −1 , SR610) and (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) were selected as the representative resins to construct the composite, and their weights percent were 30 and 70 wt%, respectively (Scheme 1).In this formulation, the hydrophilic PEGDA provides the composite with deformation properties via water absorption, while EPOX tends to result in strong hardness as well as a high degree of crosslinking.The ratio of PEGDA to EPOX was optimized to facilitate the 3D printing in the subsequent steps.Besides, Bis-(4-tert-butylphenyl)iodonium hexafluorophosphate (Iod), ethyl 4-(dimethylamino)benzoate (EDB), and isopropylthioxanthone (ITX) were used as an efficient type-II photoinitiating system in this work, and the weight percent of the three-component photoinitiating system, i.e., 1 wt%/1 wt%/0.5 wt%, was calculated from the composite content (Scheme S1, Supporting Information).Moreover, upon the addition of fluorescein dyes in a small amount (0.1 wt%), the homogeneous composite changed from colorless to light yellow, as shown in Figure S1 (Supporting Information).
][27] Here, a near UV/visible light source, LED@405 nm, was utilized to irradiate the formulation, and the polymerization kinetics of PEGDA and EPOX were continuously monitored by real-time Fourier transform infrared spectroscopy (RT-FTIR).The fluid composite could be transformed readily into a yellow solid state after the photocuring process.Figure 1 depicts the photopolymerization profiles of PEGDA and EPOX in thin film conditions when the composite was under exposure to LED@405 nm irradiation at room temperature, where rapid polymerization kinetics can be observed within the first 50 s.And the final reactive groups conversion of PEGDA and EPOX eventually reached to 70% and 48%, respectively, after LED@405 nm irradiation for 200 s.Such rapid polymerization rates and high degrees of polymerization in the presence of fluorescein ensured an excellent photocurability of the designed photocomposite material during the following 3D printing process.In addition, the IR absorption spectra of the composite before and after LED irradiation were also recorded and compared.The C═C stretching band of PEGDA at 1635 cm −1 showed a dramatic intensity decrement after polymerization (Figure S2a, Supporting Information).At the same time an obvious intensity decrement of the peak at 795 cm −1 , which corresponded to the epoxy group of EPOX, was also observed.Moreover, a new peak at 1081 cm −1 appeared, suggesting the formation of a polyether network (Figure S2b, Supporting Information). [28,29]All the above results gave convincing evidence that initiated by the photoinitiating system (ITX/EDB/Iod) under LED@405 nm irradiation, both free radical photopolymerization of PEGDA and cationic photopolymerization of EPOX could take place simultaneously and efficiently, generating an interesting IPN photocomposite, where fluorescein dye was embedded inside.No apparent phase separation was found in agreement with the good mechanical properties obtained for the obtained IPNs.
Based on the above results, a mixture of fluorescein dye, acrylate, and epoxy resins has been developed and is referred to as AE formulation (AE for acrylate-epoxy) in this study.According to the different mass percentages of embedded fluorescein (0, 0.1, and 0.5 wt%), these formulations were named AE-1, AE-2, and AE-3, respectively, which were designed to explore the effect of the fluorescein's amount on the mechanical property of the resulting photocomposite (Table S1, Supporting Information).Tensile testing experiments of these three formulations were carried out in dumbbell-like samples.And linear elastic stress-strain behaviors of the three formulations are illustrated in Figure 2. From the figure, it can be seen that the tensile ratio of the AE-1 sample without fluorescein is the lowest among all the samples, while AE-2 has a higher tensile ratio than the AE-3 sample.It can be seen that the tensile ratio is in the order of AE-2 > AE-3 > AE-1.Meanwhile, the tensile stresses that the samples of different formulations can withstand are in the order of AE-2 is approximately equal to that of AE-1 and greater than that of AE-3.Based on this, it can be concluded that AE-2 with 0.1 wt% fluorescein formulation was able to withstand almost constant tensile stress with a significant increase in tensile ratio and an increase in modulus of elasticity.However, 0.5 wt% fluoresceins caused the least tensile stress endurance for the AE-3 sample.Therefore, it could be concluded that the addition of fluorescein contributed to the improvement in the elasticity modulus of the composite material and that the sample with 0.1 wt% fluorescein, i.e., AE-2, proved to be the best formulation.To demonstrate this, RT-IR experiments were performed on each of the three formulations and the results are shown in Figure S3 (Supporting Information).It can be seen that the conversion of both PEGDA and EPOX in the AE-2 formulation containing 0.1 wt% fluorescein is higher than the other two formulations.Therefore, it can be assumed that since fluorescein also absorbs 405 nm wavelength light and can interact with the initiation system (Figure S9, Supporting Information), the addition of a small amount of fluorescein can promote the photopolymerization reaction, however, when more fluorescein is added the internal filter effect occurs, which prevents the light from penetrating and affects the photopolymerization reaction. [30]And homogeneous dispersion of fluorescein dyes in the photocomposite after photocuring was observed in Figure S4 (Supporting Information).[33] Therefore the water absorption property of the photocomposite based on the AE-2 formulation was studied to investigate the potential application in 4D printing.It is anticipated that the hydrophilic nature of PEGDA would promise the photocomposite with good water absorption and the EPOX in the IPN could improve the mechanical strength.After being immersed in water for only 30 min, it was observed that the composite material had undergone obvious water absorption and swelling, and the weight of the composite material after water absorption was 1.27 times the initial weight (without a change of the dry extract).Therefore, it can be seen that the AE-2 formulation has a good water absorption property, which would favor the ring-opening reaction of fluorescein.To investigate the application potential of the designed composite in 3D and 4D printing, the AE-2 formulation was fabricated into the shape of a reticulated flower using ANYCUBIC PHOTON D2 3D printer by the DLP method (Figure 3a).As shown in Figure 3b and Figure S5 (Supporting Information), the macroscopic mesh-like flower structure indicated a pretty high spatial resolution in the color of yellow, due to the presence of fluorescein dye.At first, the printed 3D flower was dried in an oven at 40°C overnight to minimize the influence of possible moisture inside the IPN composite.Figure 3c depicts the fluorescence image of the flower structure before immersion, which was almost non-fluorescent.Remarkably, after dropping some water on the surface of the flower for just 1 min, a strong green fluorescence could be detected clearly from the printed mesh-like flower (Figure 3d).And the turn-on and turn-off of the fluorescence signal could be repeated for many times by wetting or drying the photocomposite.To elucidate the mechanism for such fluorescent changes in the composite, the fluorescein dye was subjected to fluorescence characterization.As revealed in Figure S6 (Supporting Information), the fluorescein could not emit fluorescence in the dry state.After the treatment of a few drops of water, the fluorescein could emit a very strong fluorescence signal, whose peak was located at about 555 nm.Then if the fluorescein was dried again, its fluorescence property would also return to be silent.In a control experiment, the 3D mesh-like flower printed from the AE-1 formulation remained nonfluorescent even after water treatment for 24 h, which could be ascribed to the absence of fluorescein dye in the AE-1 formulation (Figure S7, Supporting Information).It disclosed that the IPN photocomposite of AE-2 formulation would absorb water inside, which could facilitate the ring opening process of embedded fluorescein along with significantly enhanced fluorescence.
Inspired by the outstanding fluorescent enhancement phenomenon of the fluorescein embedded photocomposites upon the dropping of water, the deformation behavior of the 3D printed composite was further investigated in this work.In the beginning, the mesh-like flower based on AE-2 formulation was flat produced by the DLP method.Then it was placed into a beaker filled with water, and changes in the shape of the flower were inspected over time.It was found in Figure 4 that the petals of the mesh-like flower started to bend upwards and close toward the top along with the increment of time from 0 to 180 s.After 180 s it can be seen that the petals reached their final angle of closure since the IPN material constructed by PEGDA and EPOX could bend by absorbing water.The complete deformation process of the printed flower is available in video S1 (Supporting Information).The framework of the printed flower consisted of a mesh structure, which was capable of spatially controlling the movement of the petals by expansion and bending.The deformed mesh-like flower was then removed from the beaker and dried in an oven at 40°C.Significantly, as the water evaporated, the petals slowly unfolded from the top downwards and eventually returned to their previous flat state, and the fluorescence property was also recovered (Figure S8, Supporting Information).It seems that the 3D photocomposite could absorb water and swell to deform, and after drying it could be stretched again.Such water induced deformation behavior of the 3D flower could also repeat several times without obvious structural fatigue.Based on the above experimental results, the composite prepared from the AE-2 formulation had an excellent reversible 4D printing property reflected in both fluorescence and deformation aspects.It can therefore be confirmed that a new photocomposite containing fluorescein has been successfully proposed and prepared, which is suitable for 4D printing and demonstrates fluorescence/shape changes when in contact with water.

Conclusion
In summary, in this work, organic fluorescein dye was homogeneously dispersed into an IPN system based on PEGDA and EPOX resin to result in a novel photocomposite, which could be photocured through the simultaneous free radical polymerization and cationic polymerization under visible LED@405 nm irradiation at room temperature.And macroscopic 3D flower structure could be fabricated from the above composite by the DLP method with high spatial resolution.Moreover, 4D printing of the photocomposite was investigated by exposing the 3D flower to water.Notably, both the deformation and fluorescence enhancement phenomena of the composite could be observed clearly at the same time after exposure to water for a short time, and such a process could be repeated reversibly several times.For the first time, this work has reported a specially dual responsive behavior of IPN based polymer (PEGDA/epoxy monomer) composite in the presence of water, paving a broader way for the further exploration of 4D printing in the future.

Scheme 1 .
Scheme 1.The chemical structure of the monomers and the interconversion equilibrium process of fluorescein are studied in this work.

Figure 2 .
Figure 2. Tensile testing experiments of the designed photocomposites containing different contents of fluorescein: i) 0 wt%, ii) 0.1 wt%, and iii) 0.5 wt%, respectively.The inserted dumbbell-like sample corresponds to the composite formulation with 0.1 wt% fluorescein.

Figure 3 .
Figure 3. a) 3D model of the mesh-like flower; b) In situ photograph of the mesh-like flower printed from the AE-2 formulation; contrast-enhanced fluorescence images of the printed mesh-like flower (c) before and (d) after exposure to water, respectively.The scale bar is 1 cm.

Figure 4 .
Figure 4.In situ photographs of the 3D printed mesh-like flower from AE-2 formulation upon immersion in water for a) 0 s, b) 30 s, c) 60 s, d) 90 s, e) 120 s, and f) 180 s, respectively.