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Abstract
A multi-material microstereolithography system in which multiple photocurable resins are stored on a single glass palette was developed to produce multicolor three-dimensional (3D) models. Multiple photocurable resins with different colors are replaced by moving a linear translational X-stage that supports the glass palette. A Z-stage moves radially to remove the air bubbles that adhere around the 3D model when replacing the resins. The uncurable resin was washed out by sequentially immersing the 3D structure in two tanks containing a cleaning solvent. This makes it possible to produce multicolor 3D models without contaminating the resins and air bubbles.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, multi-material 3D printing, which enables the production of highly functional 3D structures by integrating multiple materials, has attracted attention and a variety of fabrication methods have been developed [1,2]. For example, full-color 3D printing that uses a material injection method has been demonstrated [3]. Multi-color 3D polymer structures and metal structures have been fabricated by using the powder bed fusion method [4,5]. Recently, multi-nozzle extrusion 3D printing has been developed, which is used to create multi-color 3D models and functional devices such as actuators and soft robots [6,7].
In addition, stereolithography, which is a high-resolution and precise 3D printing technique, has extended the materials that can be used at the same time and it enables the creation of functional devices that use multiple materials [8–12]. For example, Maruo et al. developed a multi-material fabrication method based on the free-surface method that uses a cylinder [8]. For this method, a photocurable resin is replaced layer-by-layer by adding different materials to the upper part of the cylinder tank while lowering the base substrate in the cylinder without washing processes. Optical waveguides have been fabricated using two types of resins with different refractive indices [8]. In this method, the apparatus is simple, and it is easy to switch the materials. However, because a different resin is added to the upper part of the cylinder, contamination of the resins occurs, and the resins cannot be reused. Another essential problem is that multiple resins cannot be used in the same plane without the washing processes of the residual resin because of the layer-by-layer process.
Hana et al. proposed another approach for multi-material stereolithography in which the material in the area is extruded and replaced by the next different material using channels with valves for dynamic fluid control [9]. This approach can be employed to create multi-material 3D structures with multiple resins in the same plane. However, since the resin is replaced by extruding the previous resin with the next resin inside the channel, contamination of resins may occur due to direct contact of resins. Therefore, a large amount of resin must be supplied to replace the resins completely inside the chamber to eliminate contamination, and the amount of waste resin increases.
In addition, Mayer et al. introduced a material replacement system in two-photon lithography that uses a similar microchannel system and suppresses the contamination by injecting a cleaning solvent; further, the method can be used to fabricate complicated 3D microstructures using five types of resins [10]. Lamont et al. also demonstrated multi-material two-photon lithography using a disposable polydimethylsiloxane (PDMS) microchannel to supply and replace resins [11]. However, they still encountered a problem in that the resins mixed with each other due to exchange in the channel; thus, the resins cannot be reused. Therefore, there is a large amount of waste material compared with the volume of the 3D models.
On the other hand, multi-material stereolithography techniques that are combined with a cleaning process to prevent contamination and large amounts of waste have been developed by some research groups [12,13]. For example, Wang et al. developed a fabrication system that uses a rotating wheel for delivering polymer solutions and ethanol for cleaning, and they fabricated mechanical metamaterials with a negative thermal expansion [12]. Zhou et al. also developed a similar system that includes a cleaning tank with an ultrasonic cleaner on a rotary stage [13]. However, in these methods, a rotary stage is used to replace multiple materials and cleaning solvents; thus, only two types of materials are used owing to the limitation of space for storing materials.
Kowsari et al. developed another method capable of storing multiple materials; they used a translating linear stage in which multiple droplets of different resins are placed to provide several types of resins for making a multi-material 3D model [14]. This method has the advantage that the material can be replaced easily without supplying materials by fluidic control. However, in the current apparatus, the uncured resin that adheres to the surroundings of the 3D model is simply blown off using an air blower without a cleaning solution. Therefore, the process of removing the uncured resin may not be effective depending on the shape and complexity of the 3D model. As described above, some problems remain for multi-material stereolithography.
In this study, we developed a multi-material microstereolithography system in which a glass palette that stores multiple droplets of resins and two tanks that store a cleaning solvent for a two-step cleaning process were mounted on a linear translational stage. In addition, an air blowing unit was embedded in the second tank to completely remove the cleaning solvent. This fabrication system can be employed to produce multi-material 3D microstructures without contaminating the resins and generating large amounts of resin waste.
In the experiment, we investigated whether the two-step cleaning process suppressed contamination of the resins. To remove most of the air bubbles that are included in the 3D model when the resins are replaced, we devised a method to remove the air bubbles that adhere to the 3D model by moving the translational XY stage that supports a 3D model at a certain distance between the palette and the 3D model. As a result, we fabricated a cube model consisting of multiple resins with five different colors. Furthermore, we measured the absorption spectra of the multi-colored resins and determined the optimal layer structures to express black color by accumulating multiple layers of each color resin. Finally, we formed a cross-shaped model using the multicolor resins.
2. Multi-material microstereolithography system with a linear translation stage for the exchange of materials and cleaning solvent
We have developed several types of microstereolithography systems that use a blue laser [15–17]. In this study, we installed a translational stage to support a glass palette for arranging multiple droplets of different photocurable resins and two cleaning tanks in our lab-made fabrication system that we had previously developed [17].
Figure 1 shows a schematic diagram of the fabrication system. The outer dimensions of the multi-material microstereolithography system are 85 cm wide, 85 cm deep, and 40 cm high. In this set-up, the laser light is emitted from a semiconductor laser with a wavelength of 405 nm (Cobolt 06-MLD, Cobolt AB, Solna, Sweden), which passes through a variable neutral density (ND) filter to adjust the laser power and automatic opening/closing of a shutter to turn the laser on and off. Then, the laser diameter is expanded by the beam expander (magnification: ×10). Next, after passing through the beam splitter cube to the observation optical system, the laser light is incident on the galvano mirror (GM-1015, Canon Inc, Japan), and is focused on the boundary surface of the photocurable resin and the glass palette using an objective lens with a numerical aperture of 0.1. The galvano mirror is scanned in the horizontal plane based on the slice data of the 3D model to harden the cross-sectional shape. The input laser output was 2 mW when passing through the variable ND filter in our setup. The scanning speed of the laser beam was set to 1 mm/s. The hatching distance was 3 µm. In addition, the laser light reflected by the glass palette is reflected by the beam splitter cube and then it focuses on the charge coupled device (CCD) camera. At the start of fabrication, the laser beam is used to adjust the fabrication start position. In addition, the translational XZ-stage (OSMS(CS)33-300(X), OSMS60-10ZF, SIGMAKOKI Co. Ltd., Japan) supporting the glass palette is moved to change the resins to create multi-material 3D models. The 3D model is formed on a glass plate attached to the XYZ-stage (OSMS20-85(XYZ), SIGMAKOKI Co. Ltd., Japan), and the Z-stage is pulled up to accumulate each layer. These stages are automatically controlled using a lab-made software to create a 3D model according to 3D-CAD model data; the procedures include laser writing, accumulation, replacement of resins, removing process of bubbles, and washing process.
Fig. 1. Multi-material stereolithography system with a palette storing multiple droplets of different photocurable resins.
To prevent the 3D model from adhering to the palette, we attached a fluorine-coated film to the palette. In contrast, the glass plate on the Z-stage side should be modified to improve adhesion of the resin. Some methods have been proposed to enhance adhesion of polymer 3D microstructures to a glass substrate [18,19]. In our experiments, the glass plate was treated with (3-methacryloxypropyl) trimethoxysilane based on the method proposed by Baldacchini et al. [18]. Performing this surface treatment ensures that the adhesive force of the photocurable resin to the glass plate exceeds that of the fluorine-coated film, making it possible to pull up the 3D model in a stable manner during layer-by-layer accumulation.
Figure 2 illustrates the material replacement and cleaning procedure. In Step 1, the blue laser beam is focused on the upper surface of the glass palette, which is scanned and fabricated in the droplet of resin A. In Step 2, the 3D model is immersed in ethanol stored in the primary cleaning tank for 10 s, and the uncured resin adheres to the periphery of the 3D model and the glass plate is dissolved and washed out. In Step 3, the 3D model is washed for 10 s in ethanol stored in the secondary cleaning tank using the same approach. In Step 4, the 3D model is pulled up from the cleaning tank, and the ethanol that adhered to the 3D model is blown off and dried for 10 s using an air jet. Air jetting is useful both for drying the 3D model and for removing uncurable resin around the 3D model. Finally, in Step 5, the 3D model is dipped in resin B, and the cross-sectional shape is cured using the same procedure as described in Step 1 to create a multi-material 3D model. Therefore, by performing Steps 1 to 4 as one cycle and repeating these steps for each photocurable resin, it is possible to automatically fabricate a multi-material 3D model without the harmful contamination of multiple resins. This process generates air bubbles, which are included in the resultant 3D model when the resin is replaced, as described in detail in Section 5. To solve this problem, the glass plate where the 3D model is formed is radially moved in the horizontal plane when inserting the 3D model into the next droplet of a resin to remove the air bubbles.
3. Preparation of multicolor photocurable resins
For high-precision 3D fabrication using single-photon polymerization with a blue laser, it is desirable to use a photocurable resin that can suppress excessive curing and selectively cure the resin near the focus. We developed an acrylate-based photocurable resin suitable for a blue laser with a wavelength of 405 nm by mixing a photoinitiator, a curing inhibitor, and a blue light absorber [17]. In this investigation, an acrylate resin (SR499, Sartomer Inc., Exton, PA, USA, 95.9 wt%), photopolymerization initiator (TPO, Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, Sigma-Aldrich, St. Louis, MO, USA, 1.0 wt%), polymerization inhibitor (2-tert-Butyl-4-methylphenol, Sigma-Aldrich, St. Louis, MO, USA, 3.0 wt%), and a light absorber (FDB-009, Yamada Chemical Co. Ltd., Kyoto, Japan, 0.1 wt%) were added. The absorbance of the blue laser of the resin prevents excess curing by limiting the blue light absorber; therefore, the curing depth of each layer can be reduced.
To prepare multi-color photocurable resins, we used this resin as a base material and dyed it red, blue, green, or yellow by mixing Sudan IV (0.05 wt%), Quinizarin Blue (0.1 wt %), Quinizarin Green SS (0.1 wt %), and Quinizarin (0.1 wt %), respectively. After mixing each dye, they were stirred and degassed with a mixer (ARE-250, Thinky Corp., Japan). The dye was filtered with a 0.22 µm filter using a syringe to prevent the absorption and scattering of laser light by the agglomerated dye.
4. Contamination-free multi-material microstereolithography using two-step cleaning with air blowing
To evaluate the effectiveness of the two-step cleaning process in which the 3D model is immersed in ethanol with two cleaning tanks sequentially, we first fabricated a cubic model with a side of 900 µm, as shown in Fig. 3(a), as the evaluation model. In the experiments, we used two types of resins; one is green, and the other is transparent without a dye. In this model, accumulation is performed in the z direction so that each layer is cured by replacing two types of resins. Figure 3(b) shows an example of a 3D model fabricated only by primary cleaning (Step 2 in Fig. 2). This prototype model has a total of 30 layers; hence, the total number of material changes is 60. Therefore, looking at the left side of Fig. 3(b), the left part that should have been transparent is slightly dyed green because of contamination of the green resin. It is considered that this is due to insufficient cleaning because the secondary cleaning process shown in Step 3 and Step 4 in Fig. 2 was not performed. Therefore, we fabricated the same cubic model by performing the primary and secondary cleaning processes as shown in Fig. 2. The fabricated cube model is shown in Fig. 3(c). In this case, contamination of the green resin is avoided; thus, the left side of the cube model is almost transparent. This indicates that secondary cleaning effectively suppressed the contamination of resins.
Fig. 3. Contamination evaluation experiment by the difference in the cleaning process. The cube model consists of two types of resins used for contamination comparison. (a) Cube model using transparent and green resin, (b) Side view of the cube model with only primary cleaning, (c) Side view of the cube model with both primary and secondary cleaning.
Although the contamination of two resins could be successfully eliminated, it was observed that the cube model (Fig. 3(b)) has a stacking deviation of approximately 20 μm. This is due to positioning error in accumulation. As the first step of the fabrication procedure to fabricate cube models with two types of resins, the laser focus is set to the surface of the glass palette at each XY-position of multiple resins by manual operation. In the experiments, the initial position of the laser focus was determined using the reflection beam profile on the CCD camera shown in Fig. 1. The numerical aperture of the objective lens is 0.1; thus, the depth of focus is approximately 60 µm in our setup. Therefore, the manual operation to define each focal position for two resins may cause Z-position error in the range from few µm to several 10 µm. However, in the case where the focal position was defined correctly, the stacking error was small as shown in Fig. 3(c). Therefore, if we introduce an autofocus system in this setup, the stacking error can be reduced up to approximately 3 µm, which is the repeated position accuracy of the Z-stage. We have recently demonstrated a method based on image processing of cured voxel images as a promising autofocus system [20].
5. Removal of air bubbles in multi-material 3D models
In addition to the issue of contamination of resins, when making a multi-material 3D model while swapping multiple photocurable resins, another problem that may occur is that air bubbles are generated at the time of swapping resins, which are included in the 3D model [13]. In the 3D model shown in Fig. 3(c), contamination can be suppressed by secondary cleaning; however, it can be confirmed that air bubbles are included in the 3D model. It is hypothesized that air bubbles are mixed in the resin when the 3D model is inserted into the droplet of the resin, and the bubbles are attached to a palette in a state of being sandwiched between the palette and the 3D model; therefore, the area where bubbles exist results in a void in the 3D model. It is difficult to suppress the generation of air bubbles only by reducing the speed at which the 3D model penetrates into the droplet of the resin.
Therefore, we considered a method of removing the bubbles that are mixed in the droplet. In the proposed method, we moved the 3D model in the horizontal plane while holding it above the palette at a small gap. This was set to 30 µm in our experiments; the states of the 3D model touching the droplet are shown in Fig. 4. Using this method, the air bubbles sandwiched between the palette and the 3D model can be separated from the 3D model and pushed out of the fabricating area (Fig. 5). Furthermore, by moving the 3D model radially around the fabricating area, most of the air bubbles in the fabrication area can be pushed out almost outside the fabrication area, as shown in Fig. 4. As a result, a 3D model can be fabricated without voids due to air bubbles. Figure 6 shows a cube model fabricated by removing bubbles using this method. A comparison of this result and Fig. 3(c) shows that although some tiny bubbles with a diameter of less than 20 µm still exist, it can be observed that the inclusion of air bubbles can be significantly reduced.
Fig. 6. Cube model consisting of two types of resin fabricated by the removal process of air bubbles. (a) Bird’s-eye view. (b) Front view. (c) Top view.
6. Fabrication of 3D multicolor models
Next, we fabricated a 3D model as shown in Fig. 7(a) to verify whether it is possible to integrally fabricate many colors of resins. The model is a cube with 1.5 mm on each side, the stacking pitch is 30 µm, the stacking number is 50 layers, and it was fabricated by exchanging five colors of resins 250 times. As a result, we confirmed that it is possible to integrally fabricate different color resins without contamination and voids, as shown in Figs. 7(b)–(d). In addition, by stacking four colors of resins (red, blue, yellow, and green), the light transmission spectrum of the model changes; therefore, it can be demonstrated that the color of the cube changes depending on the viewing direction.
Fig. 7. Cube model fabricated with multicolor resins. (a) Multicolor 3D model. (b) Bird’s-eye view. (c) Side view. (d) Top view.
The total fabrication time of the multi-color cube model was 360 min. In contrast, when 3D fabrication is performed using a single resin without replacing the resins, the fabrication time was only 14 min. Therefore, to shorten the fabrication time, it is necessary to replace the resin and move the stage to remove air bubbles as fast as possible.
7. Color tuning of multicolor 3D object by adjusting the number of layers
The light transmission spectrum of the 3D model can be adjusted by performing layer fabrication with five different colors of photocurable resins. First, we measured the transmittance of each color resin with an ultraviolet-visible spectrophotometer (UV-1700, Shimadzu, Japan). In the measurement, a thin-film resin was cured to a thickness of 300 µm, which is located between two glass plates, and was used as the sample. Figure 8 shows the average value of the transmittance at wavelengths of 380 nm to 780 nm, which were measured three times. The almost zero transmittance in the region near the wavelength of 400 nm is due to the absorption of the blue light absorber, which contains the resins.
To express black color (which is achieved by combining these four colored resins), we examined the combinations in which visible light is absorbed uniformly by adjusting the number of lamination layers of each color resin and controlling the magnitude of each absorption. By calculating the absorption of each color resin, it was confirmed that a combination of nine layers of green resin (thickness of a single layer: 30 µm) and six layers of other color resins can provide the magnitude of absorbance with a visible wavelength of one or more. This means that 90% or more light in the visible range can be absorbed; thus, black is expressed by accumulating these layer structures with each color.
Figure 9 shows the absorption spectra of the laminated structures (green: nine layers of green resin; red, yellow, and blue: six layers of each color resin; black: combination of nine layers of green resin and six layers of red, yellow, and blue resins). As a result, by adjusting the number of layers for each color resin according to their absorption spectrum, it is demonstrated that black can be expressed by accumulating each color resin with the proper thickness. From the calculation results of these absorption spectra, we fabricated a cross shape as an example of a 3D model that includes red, green, blue, yellow, and black. The total fabrication time of this model was 155 min. Figure 10 shows the 3D model and its top view. The ring that looks black from the top is a cylinder in which the number of layers of each color required to reproduce black is stacked. Therefore, the ring made of four-color resins absorbs light in the visible range uniformly; thus, the top view of the stacked ring is black in color. Therefore, it is possible to use it as a color filter for a variety of colors by adjusting the number of accumulations for each color resin.
Fig. 9. Calculated absorption spectra of the accumulated layer structures (red, blue, and yellow line: six layers of each resin; green line: nine layers of green resin; black line: combination of six layers of red, blue, and yellow resins and nine layers of green resin).
Fig. 10. Fabrication of multicolor cross shape. (a) Bird's-eye view of the 3D model. (b) Top view of the 3D model. (c) Bird's-eye view. (d) Top view.
8. Conclusions
We have developed a multi-material stereolithography system that uses a glass palette with multicolor photocurable resins. In this system, it is possible to significantly reduce the contamination that occurs when the resins are replaced by introducing a two-step cleaning process. Furthermore, we devised a method of scanning a 3D model fabricated on a glass plate in the horizontal plane inside a droplet of resin to remove the air bubbles that are included in the 3D model. Using this bubbles removal process, most of the air bubbles were pushed out of the fabrication area of each layer when the resins are replaced. As a result, we succeeded in producing a multi-material 3D model with almost no bubbles. In addition to the transparent resin, four color resins were prepared and placed on the same palette. Subsequently, accumulation was performed while swapping the resins according to the 3D multicolor object such as a cube model and the cross-shaped model. In the near future, the variety of color photocurable resins will be expanded to absorption types as well as fluorescent types [10,21,22]. Using these multi-color photocurable resins, multi-material 3D printing based on single-photon and two-photon polymerization will provide functional devices such as 3D optical memories, optical filters, light emitting devices, optical sensors, and light-driven actuators [10,23–25].
Funding
Core Research for Evolutional Science and Technology (JPMJCR1905).
Acknowledgments
Part of this work was supported by the JST CREST (Grant Number JPMJCR1905), Japan.
Disclosures
The authors declare no conflicts of interest.
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