A preprogrammed force field generated in a confined space enables external-module-free anisotropic vat-photopolymerization 3D printing

doi:10.21203/rs.3.rs-4725730/v1

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A preprogrammed force field generated in a confined space enables external-module-free anisotropic vat-photopolymerization 3D printing

https://doi.org/10.21203/rs.3.rs-4725730/v1

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Additive Manufacturing, commonly known as 3D printing (3DP), continuously drives a paradigm shift in manufacturing processes from traditional mass production to individually optimized on-demand manufacturing. Vat-photopolymerization (VP) 3DP is a well-established methodology that enables highly precise high-speed modeling of intricate objects from photoresins with an inexpensive 3DP device. However, it remains a challenge to fabricate anisotropic objects without an external-field-applying module, which induces the alignment of molecular or particulate components in the photoresins. Herein, a simple, affordable, and easily accessible approach for fabricating precise and intricate anisotropic composite materials based on external-module-free anisotropic VP (AVP)-3DP. Preprogrammed force field, i.e., compressional stress in a confined space within a layer to be cured generated during the repetitive descending and ascending motion of a platform for anchoring printed objects in typical VP-3DP processes effectively stack mica flakes within a layer to be cured. While the AVP-3DP of composite resin containing mica flakes with the size of 3 μm, which is sufficiently smaller that the thickness of a layer to be cured, afforded a more isotropic material, those with the sizes down to 10 μm afforded highly anisotropic printed objects, suggesting tunability in anisotropy simply by changing the type of mica flakes. The proposed AVP-3DP methodology leads to anisotropy in the mechanical properties of the 3D-printed object by direction, allowing for up to a 4-fold differentiation for flexural strength and up to a 2-fold differentiation for flexural modulus (E). Moreover, with AVP-3DP of a composite resin containing specific mica flakes, exceptionally high average E of 13.6 GPa has been achieved, which is surprisingly comparable to that of metal lead (E = 13–15 GPa). The ability to create anisotropic composites through AVP-3DP paves the way for the development of next-generation highly precise functional anisotropic objects having intricate geometries with tailored mechanical performance for diverse applications.

Physical sciences/Materials science/Structural materials/Composites

Physical sciences/Chemistry/Materials chemistry/Mechanical properties

Since Kodama invented the first photopolymerization-based rapid prototyping system in 1981,^[1] additive manufacturing, also known as 3D printing (3DP), has revolutionized the landscape of materials science and engineering. This approach enables the on-demand fabrication of objects with intricate geometries directly from computer-aided design (CAD) data. Among the diverse 3DP techniques, vat photopolymerization (VP)-3DP has been established as a frontrunner due to its ability to achieve exceptionally precise high-speed modeling with affordable machines^[2]; however, the available materials are limited because only photopolymerizable resins and composite resins can be used^[3]. Moreover, it has also been, in principle, difficult to fabricate anisotropic materials since formulating a homogeneous solution or dispersion with an adequately low viscosity is a prerequisite for VP-3DP, even for composite resins containing fillers. Despite such homogeneity, the resins need to change to an inhomogeneous or anisotropic state at the moment of repetitive layer-by-layer photocuring operations during VP-3DP. As the anisotropy of materials enables direction-dependent enhancement in mechanical, electrical, and thermal properties^[4], they have attracted tremendous industrial interest in aerospace^[5], sensing^[6], biomedical^[7], and even robotics applications^[8]. In addition, it would also be significant if anisotropic materials could be applied to the fabrication of special parts in a variety of growing areas using 3DP as a prototyping tool^[9]. Consequently, significant efforts have been directed toward developing functional anisotropic materials based on VP-3DP by utilizing an external-field-applying module to induce the alignment of components in composite resins^[3c]. The external fields used to induce anisotropy include electric^[10], magnetic^[11], and acoustic fields^[12], and corresponding modules have been installed in or near a resin vat. As mentioned above, since composite resins are homogeneous and have a low viscosity enough to print, the organic and inorganic fillers added to the composite resins are mobile before photocuring and thus can align upon the application of an external field. Such field-assisted 3DP processes are highly effective for printing anisotropic materials and have been considered as one of the expanding fields, as systematically described in a very recent review^[3c]. However, such field-assisted 3DP requires an additional instrumental setup,^[13] which makes it less convenient to operate 3DP. In addition, the use of an external-field-applying module may increase the overall cost of 3DP. Nevertheless, likely due to the preconception that the formation of anisotropic structures is difficult without applying an external field, most interest has shifted to field-assisted 3DP processes.

Wherein, we decided to investigate the possibility of fabricating anisotropic materials from the principle of how a VP-3DP process cures resins and composite resins. Our search for conventional papers indicates that the construction of anisotropic materials via the genuine 3DP process itself has been limited to a series of studies originating from the use of two-photon polymerization (TPP) lithography reported by Xiong et al.,^[14] where multiwalled carbon nanotubes up to 0.2 wt% in a photoresin assembled into bundles upon photocuring. However, 3DP devices based on TPP are highly expensive and are not available for daily consumer use. How, then, can we fabricate anisotropic materials with consumer-use affordable 3DP devices while maintaining excellent printability and easy handleability? We noticed that there is a preprogrammed force field generated during the repetitive descending and ascending motion of the platform in the typical VP-3DP process, that is, the presence of compressional stress in a confined space within a layer to be cured. If the dimension parameters can be adjusted so that fillers can be aligned within this confined space upon compressional stress and cannot rotate, the production of anisotropic materials can be realized through the preprogrammed motion of VP-3DP without any special device or operation.

Herein, we report the fabrication of crosslinked polymer–mica composites (CPMCs) based on external-module-free anisotropic VP (AVP)-3DP of composite resins loaded with a series of mica flakes (Fig. 1a). We successfully formulated composite resins mixed with mica flakes with a loading of up to 40 wt% while maintaining flowability. Furthermore, we achieved high-degree control over the filler alignment within the printed object, exploiting the preprogrammed force field generated in a confined space within a layer to be cured upon common 3DP operation (Fig. 1b). The developed methodology allowed for the expression of the direction-specific mechanical properties of the printed objects, resulting in up to twofold differences in the flexural modulus (E); that is, an E of more than 11 GPa in one direction, while an E of approximately 5 GPa in another direction. The present methodology benefits the fabrication of highly precise and intricate 3D printed objects derived from crosslinked anisotropic composites without requiring any special or expensive devices or cumbersome operation.

Composite Resin Formulation and VP-3DP

In the present study, we selected a mixture of bisphenol A-glycidyl methacrylate (bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) with a fixed weight ratio (50 wt%/50 wt%) with diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as an initiator (Fig. 1a), which is a well-known monomer mixture for dental composites^[15], as a resin for adding fillers. The dependence of the mixing ratio on the mechanical properties after curing has recently been revisited^[16]. According to this report, changing the ratio of bis-GMA to TEGDMA in the mixture from 2:8 to 8:2 results in a minor shift in the mechanical properties, and in the case of the elastic modulus, only a change less than ca. 0.5 GPa can be observed, while the rate of polymerization is much greater when the ratio of bis-GMA exceeds 50 wt%^[16]. As such, we fixed the weight ratio at 50 wt%/50 wt% because of its lower viscosity, easier handleability, and reasonably fast rate of polymerization. To test 3D printability, we created CAD data of cuboid specimens with dimensions of 33 mm × 5 mm × 1 mm, and the 3D printability of the formulated resin (BT) was confirmed before proceeding to VP-3DP of the composite resins.

Next, a series of photoresins were formulated upon mixing BT and mica flakes with different sizes, bulk densities, and aspect ratios (Supplementary Table 1). The sol–gel boundaries at room temperature were evaluated for mixtures of these mica flakes and BT by varying the weight fraction of mica flakes by 5 wt% and the maximum loading while maintaining flowability for A-11, A-21S, AB-25S, J-31 M, Y-1800, Y-2300, and Y-3000 was determined to be 30, 30, 30, 40, 30, 35, and 35 wt%, respectively (Fig. 2a). As mixtures containing the mica flakes at these boundaries were highly viscous, we used the loading of 25 wt% for further 3DP investigations. Interestingly, at a loading of 25 wt%, which is equivalent to that mixed with typical cosmetic eyeshadows, the formulated resins reflect and scatter light like such cosmetic products due to their angle-dependent optical effects (Fig. 2b)^[17]. This visual feature enhances the design and functionality of the final products. The formulated composite resins containing a series of mica flakes (A-11, A-21S, AB-25S, J-31M, Y-1800, Y-2300, and Y-3000) at a loading of 25 wt% were then subjected to VP-3DP with the CAD model with three different layer addition (printing) directions (Fig. 2c). Importantly, simple 3DP operation without any special treatment readily afforded the desired CPMCs with highly precise geometries (Fig. 2d), demonstrating the feasibility of the fabricated mixtures for ready-to-use photoresins for 3D printers.

Analysis of anisotropy based on SEM observation

Having developed composite resins and confirmed their excellent printability, we next analyzed the anisotropy of the printed CPMCs containing a series of mica flakes (CPMC_A−11, CPMC_A−21S, CPMC_AB−25S, CPMC_J−31M, CPMC_Y−1800, CPMC_Y−2300, and CPMC_Y−3000) by scanning electron microscopy (SEM). SEM provides valuable insights into the filler distribution and orientation within composites. As the thickness (t) of each layer during VP-3DP was set to 25 µm (Fig. 1b), the rotational barrier and alignment of the mica flakes in the presence of compressional force in the confined space would be different depending on their sizes. Thus, mica flakes with a size smaller than 25 µm would be more isotropically distributed and behave more like isotropic materials. On the other hand, if there is a rotational barrier to the micas in the confined space (Fig. 1b), the micas will be aligned, and the resulting printed objects become anisotropic materials. To reveal this, SEM images were taken from the direction perpendicular to the printing direction (xy-plane) and from the direction in which the stacked layers were observed from the side (yz-plane) (Fig. 3a). Indeed, the distributions of A-11, where the average size (3 µm) is sufficiently smaller than 25 µm, in the xy- (Fig. 3b) and yz- (Fig. 3c) planes observed for CPMC_A−11 were quite similar. Although slightly more faces of the flakes can be observed in the xy-plane (Fig. 3b), no significant differences were observed that could clearly indicate the stacking of A-11 along the layer addition direction. In sharp contrast, the in-plane aligned morphology of J-31M, where the average size (35 µm) is sufficiently larger than t, was observed in the SEM image of the xy-plane (Fig. 3d) with CPMC_J−31M. Interestingly, the SEM image of the yz-plane sample showed a longitudinal streaked morphology (Fig. 3e). At first glance, the pattern appears to be a side view of the stacked layers during VP-3DP, but the spacing between the stacked mica flakes in the z-direction is apparently narrower than that of each layer (25 µm). Thus, the compressional stress during VP-3DP (Fig. 1b) could be sufficient to stack several layers of mica flakes in the confined space within t, and it is also worth noting that as mica flakes are stacked, it becomes more difficult for them to rotate within the layer (Fig. 1b). If this mechanism is true, it should in principle be possible to print anisotropic materials even with fillers reasonably smaller than 25 µm as long as there is a sufficient aspect ratio. As all mica flakes except A-11 used in this study commonly have high aspect ratios (Table S1), we continued SEM observations of the CPMCs to determine the limits of how small the size of mica flakes can be for printing anisotropic materials. Within the tested range, AB-25S, A-21S, and Y-3000 have average sizes of 24, 23, and 23 µm, respectively (Table S1), which are comparable to t. From the comparison of SEM images of CPMC_AB−25S (Figure S1a, b), CPMC_A−21S (Figure S1c, d), and CPMC_Y−3000 (Figure S1e, f) taken to observe the distribution of mica particles in the xy-plane and yz-plane, it becomes evident that all of these mica flakes are aligned so that their plate faces are parallel to the xy-plane. Moreover, despite the average sizes of Y-2300X and Y-1800 mica being 19 µm and 10 µm, respectively (Table S1), which are apparently smaller than 25 µm, they aligned in similar manners (Figure S2). Therefore, with a t of 25 µm, there should be a threshold between 5–10 µm, which is the barrier for restricting the rotation of mica flakes in the present confined space. This led us to further consider the narrowing effect of t down to 10 µm, which is the minimum layer thickness that can be set with the 3D printer used in this study. However, the distributions of A-11 in the xy- (Figure S3a) and yz- (Figure S3b) planes were quite similar, indicating that mica flakes with a size of approximately 1/3 of t can rotate in a confined space within a layer to be cured. Considering that Y-1800 was well aligned despite being less than half the t, a size of approximately above 1/3 of t would be a rough indication for inducing anisotropy.

Evaluation of the mechanical anisotropy based on flexural tests

Achieving the simple, affordable, and easily manipulable fabrication of highly precise and intricate anisotropic materials upon AVP-3DP without requiring external field-generating modules could be a catalyst for the dissemination of anisotropic materials. With the anisotropy observed for our CPMCs, realizing this intent may now be possible. Flexural stress–strain curves of test specimens derived from CPMCs were measured for three different directions, i.e., the layer (L), transverse (T), and perpendicular (P) directions, to evaluate direction-dependent flexural strengths (σs: σ_L, σ_T, and σ_P) and flexural moduli (Es: E_L, E_T, and E_P) (Fig. 4a). These directions were determined based on the difference in the relationship between the layer addition (printing) and indentation directions. For the control experiment, first, the σs and Es of BT were evaluated based on its flexural stress–strain curve (Supplementary Fig. 2), and the average σs (σ_L = 150 MPa, σ_T = 141 MPa, and σ_P = 130 MPa) (Fig. 4b, BT) and Es (E_L = 2.90 GPa, E_T = 3.22 GPa, and E_P = 4.59 GPa) (Fig. 4c, BT) were comparable to those reported in the previous literature^[16]. Perhaps as a consequence of the layer addition operation, which inevitably has a directional effect, slight anisotropy in the mechanical properties could also occur for samples without mica flakes. This effect corresponds to those reported in the previous literature^[18]. However, in the present case, the observed differences are not significant, as suggested by the range of error bars (Fig. 4b, c). With mica flakes, the hard and brittle properties derived from inorganic materials are imparted to BT, and it is evident that the σs (Fig. 4b) are reduced and the Es (Fig. 4c) are increased for all CPMCs in comparison with those of BT. For CPMC_A−11, essentially small differences between the three directions were observed for σs (Fig. 4b, A-11) and Es (Fig. 4c, A-11). This could be reasonably explained by the fact that the distributions of A-11 in the xy- and yz-planes were quite similar, and little anisotropy was confirmed in the comparison of the SEM images (Fig. 3a, b). In sharp contrast, except for those of CPMC_A−11, σ_L and σ_T are significantly greater than σ_P and, moreover, in the case of CPMC_Y−3000, the σ_L/σ_P ratio reaches 4.23. In addition, the E_P tends to be smallest within Es (Fig. 4c). Interestingly, CPMC_Y−3000 again showed outstanding directional dependence among all the tested CPMCs. The average Es values were 11.3 GPa (E_T), 5.24 GPa (E_L), and 5.64 GPa (E_P), respectively, and both the E_T/E_P and E_T/E_P ratios exceeded 2. Given that the tensile modulus and Vickers hardness of composites reinforced with platelet Al₂O₃, fabricated based on magnetic-field-assisted VP-3DP in the previous literature, in the strong-axis direction were approximately 900 MPa and 7.5, respectively, and those in the weak-axis direction were approximately 700 MPa and 6, respectively^[11a], the present differentiation effect on the mechanical properties would be prominent.

Moreover, the exceptionally high average E of 13.6 GPa, which is comparable to that of metal lead (E = 13–15 GPa), achieved in the T direction with CPMC_Y−2300X signifies the effectiveness of our approach in fabricating highly precise and intricate 3D-printed objects with excellent mechanical properties. It is also worth mentioning that the observed differentiation or decreasing effect in the mechanical properties are primarily concentrated in the P direction, as intended for the AVP-3DP design (Fig. 1b).

This study presented a simple, affordable, and easily accessible approach for fabricating precise and intricate anisotropic composite materials based on external-module-free AVP-3DP. We successfully addressed the challenge of fabricating anisotropic composites with consumer-use 3DP devices from photoresins containing a series of mica flakes while maintaining excellent printability and resolution. The key for fabricating anisotropic materials is the effective utilization of the preprogrammed force field generated during the repetitive descending and ascending motion of the platform in the typical VP-3DP process. This motion generates compressional stress in a confined space within a layer to be cured when descending the platform, resulting in the restricted rotational motion of mica flakes within the confined space and in the stacking of mica flakes perpendicular to the printing direction. Eventually, this alignment of mica flakes resulted in significant differentiation in the mechanical properties, particularly in the direction parallel to the printing platform.

This paves the way for diverse applications, from high-strength structural components to applications that are worth breaking as desired. For example, the demonstrated anisotropy with our CPMCs can be used to preprogram a fracture mechanism with printed models that allow for easy renewal of specific surfaces, such as a safe cutter knife with snap-off blades. Another interesting possible application is an easily breakable prop on a movie. For such applications, the method of molding sugar or cornstarch after heating and melting has been employed. However, there are limits to the shapes that can be made by the melt molding process. If 3DP can be used for effective and inexpensive modeling of materials that can be easily broken in the desired direction on the spot, the range of expression in film works will be expanded. No two film works are the same, and all small prop on a movie to be used in different scenes would be different, and this would be exactly the situation where modeling with 3DP is advantageous.

The concept of exploiting a preprogrammed force field generated in a confined space during a typical 3DP process represents a significant advancement in the field of 3D-printed functional materials and could open the door for the fabrication of functional printed materials with tailored mechanical properties based on the developed external-module-free AVP-3DP.

Materials. All reagents and solvents were purchased from commercial sources (Wako and TCI) and used as received.

Resin Formulation. The base resin BT (bis-GMA/TEGDMA, 50 wt%/50 wt%) containing TPO (1 wt%) was weighed and transferred to a mixing container. The desired amount of mica flakes was added to the resin, and the mixture was subjected to vacuum-type planetary centrifugation on a Kurabo MAZERUSTAR mixer for a predetermined time (e.g., 30 minutes) until complete removal of bubbles in the mixture. The resulting composite resin was directly subjected to AVP-3DP.

AVP-3DP. The formulated composite resin was poured into a vat for an ELEGOO MARS4 3D printer equipped with an LED with an emission wavelength of 405 nm and a high-resolution liquid crystal display (LCD) (9K) with the size of an LCD pixel of 18 µm × 18 µm (i.e., xy-resolutions: 18 µm × 18 µm). A digital model for a flexural test specimen (33 mm × 5 mm × 1 mm) was prepared using an Autodesk Fusion 360 CAD software. The thickness of each layer was set to 25 µm, and the exposure time for each layer was 2.5 s. To fix the printed objects on the platform surface applied with double-sided tape, 5 bottom layers were formed with an exposure time of 30 s. The printed flexural test specimens were removed from the platform, thoroughly rinsed with isopropyl alcohol, air-dried, and irradiated with light (λ = 405 nm) in an ANYCUBIC Cure and Wash Plus secondary curing machine for 30 min.

Scanning Electron Microscope Observation. The observation of surface morphology was performed on a Hitachi TM3030 microscope.

Three-point flexural tests. Three-point flexural tests (ASTM D790) were performed on a Shimadzu AGS-X mechanical testing machine with a 5 kN load cell operating at a crosshead speed of 0.5 mm/min until fracture. The mechanical properties of the printed samples, i.e., the flexural strength (σ) and flexural modulus (E), were evaluated based on the obtained stress–strain curves.

Competing interest

A patent related to this work is planned by the University of Tokyo with S.H. as an inventor.

Author contributions

Satoshi Honda: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing, Supervision.

Acknowledgments

We thank Mr. Takuto Sunaga (clarinetist, The University of Tokyo) and Mr. Seishiro Sasaki (saxophonist, The University of Tokyo) for their kind experimental support. This work was supported by JSPS KAKENHI (Grant Numbers 21H01632 and 23K17337 S.H.), the Fuji Seal Foundation (S.H.), and the UTEC-UTokyo FSI Research Grant Program (S.H.). We are also grateful to the Tokyo Metropolitan Government and our industrial collaborators for their financial support.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information. All the data may be obtained from the corresponding author upon reasonable request. Source data are provided with this paper.

Acknowledgments

Additional information

Supplementary information The online version contains supplementary material available at
https://doi.org/10.1038/xxxxxx.

Correspondence and requests for materials should be addressed to Satoshi Honda.

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Yes there is potential Competing Interest. A patent related to this work is planned by the University of Tokyo with S.H. as an inventor.

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A preprogrammed force field generated in a confined space enables external-module-free anisotropic vat-photopolymerization 3D printing

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Version 1

Abstract

Figures

Introduction

Results

Composite Resin Formulation and VP-3DP

Analysis of anisotropy based on SEM observation

Evaluation of the mechanical anisotropy based on flexural tests

Discussion

Methods

Declarations

Competing interest

Author contributions

Acknowledgments

Data availability

References

Additional Declarations

Supplementary Files

Status:

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