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Abstract
In this study, ceramic lattice microstructures were fabricated via micro-stereolithography using optical fibers to irradiate a photocurable ceramic slurry containing β-tricalciumphosphate microparticles. Changing the optical fiber core diameter and incident laser power can produce microstructures of the desired linewidths and cured depths. Fabrication conditions, such as scanning distance, accumulation pitch, and number of lamination layers, were optimized using the proposed method. The fabricated lattice structures were degreased and sintered to produce ceramic lattice structures. Overall, a simple and compact fabrication system was proposed for producing 3D microparts fabricated from various kinds of ceramics for electronics, mechanics, optics, and medical applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical fibers have useful features such as fine diameters, flexibility, wide variety of core diameters, and the ability to guide ultraviolet (UV) and infrared light. Owing to these features, optical fibers have been used for a wide variety of microfabrication technologies such as interference lithography [1], nanoimprinting [2–4], micro lens fabrication [5], assembling of microparticles [6,7], self-written optical waveguides [8–10], scanning near-field optical microscope lithography [11], and three-dimensional (3D) printing [12–17]. For example, fine optical fibers make it possible to construct a simple, compact interference lithography system without complicated optics [1]. The usage of a bundle of fine optical fibers enables parallel replication of nanostructured patterns [3], mass production of micro lenses [5], and 3D microstructures [15]. Additionally, owing to the flexibility of optical fibers, the manipulation and assembly of microparticles in a liquid can be performed via insertion of the optical fiber from the side of a droplet [9,10].
Self-focusing of light irradiated from an optical fiber inside a photocurable resin has been utilized for fabricating a self-written waveguide that is automatically connected to the end of the optical fiber [8–10]. The self-written waveguide also enables connection of two optical fibers [9]. In addition, wavefront shaping of a laser beam introduced into a multimode optical fiber allows for the focusing and scanning of the laser at the end of the optical fiber, so that 3D microstructures can be created in a photocurable resin while moving the optical fiber along the vertical direction [12,13]. Unlike conventional direct laser writing based on single-photon and two-photon polymerization, these optical fiber-based techniques are novel types of microscale 3D printing without laser scanning [18,19]. The lensless fiber probe facilitates additive manufacturing of 3D microstructures inside a narrow, closed space. However, the size of the 3D microstructures is limited by the field of view of the multimode fiber.
More recently, we demonstrated large-scale optical fiber scanning lithography using the self-focusing phenomena inside a photocurable resin [14]. According to our method, the optical fiber is scanned inside the photocurable resin so that large-scale 3D structures can be fabricated beyond the diameter of the optical fiber. In addition, the lateral resolution can be adjusted over a wide range, from 1 µm to 1000 µm, by means of changing the core diameter of the optical fiber. Owing to the self-focusing phenomena in photocurable resins, the depth resolution can also be controlled by changing the distance between the tip of the fiber and the substrate. Therefore, the scanning of the optical fiber inside the photocurable resin enables multi-scale and multi-depth patterning that cannot be achieved by conventional lithography.
In this study, we employ a photocurable ceramic slurry for optical fiber scanning lithography. The ceramic slurry consists of an acrylate monomer, a photoinitiator, and ceramic microparticles. The laser beam irradiated from the optical fiber is highly scattered in the ceramic slurry because the slurry is opaque. As a result, the cured depth is limited to less than 50 µm, unlike that in the case of the photocurable resins used in our previous work [14]. The small cured depth enables this method to fabricate ceramic 3D microstructures, such as lattices and overhanging structures. Recently, additive manufacturing of ceramic 3D structures has attracted much attention for various applications such as bone regenerative medicine, dentistry, mechanical parts, and fluidic devices [20–23]. However, the range of fabrication resolutions of conventional 3D printing techniques for ceramics, such as powder-bed fusion and vat photopolymerization, is limited by the focus spot size of a laser beam; hence, various types of fabrication equipment are required to change the fabrication resolution to develop multi-scale 3D objects. In contrast, the optical fiber-based micro-stereolithography provides a wide range of linewidths between 1 µm and 1 mm using optical fibers with different core diameters in a single piece of fabrication equipment. In addition, using an optical fiber allows us to additively fabricate ceramic 3D microstructures in a narrow space that cannot be accessed by the optical systems used in conventional stereolithography systems. These unique features contribute to the rapid manufacturing of multi-scale 3D ceramic structures, such as scaffolds and filters.
In the following experiments, we examined the lateral and depth resolutions for micro-stereolithography using optical fibers with different core diameters with a photocurable ceramic slurry containing β-tricalciumphosphate (β-TCP) microparticles. In addition, we created lattice structures by optimizing fabrication conditions such as exposure conditions and laminating pitch. Finally, the fabricated lattice structures were sintered to produce ceramic lattice structures.
2. Experimental methods
2.1 Preparation of materials
In this study, a mixture of an acrylate-based monomer (SR499, Sartomer Inc.) and β-tricalciumphosphate (β-TCP) microparticles with high biocompatibility was used for the fabrication of 3D bioceramic structures. The average size of the β-TCP microparticles, which were provided by Tomita Pharmaceutical Co., Ltd, was 2.3 µm in diameter. For curing the ceramic slurry using UV light, 1 wt% photo-initiator (TPO: Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, Sigma-Aldrich Co.) and 2 wt% UV absorber (UVA: 2-(5-Chloro-2-benzotriazolyl)-6-tert-butyl-p-cresol, Tokyo Chemical Industry Co., Ltd.) were added to the acrylate-based monomer. In addition, for dispersing the ceramic particles in the monomer, 0.45 wt% dispersant (Malialim AFB-1521, NOF Corp.) was added to the β-TCP microparticles. A photocurable ceramic slurry having a volume ratio of β-TCP microparticles to the monomer of 4:6 was prepared. To prevent precipitation and aggregation of the β-TCP particles, the slurry was mixed using a mixer (ARE-250, Thinky Corp.) at 1200 rpm for 4 min before fabrication.
The surface of the glass substrate was modified using methacrylate groups via silane coupling treatment (3-methacryloxypropyltrimethoxysilane, Shinetsu Chemical Corp.) [24]. This surface treatment provides strong adhesion of the cured slurry structures to the substrate and prevents the structures from peeling during the fabrication and washing of the uncured slurry.
2.2 Multi-scale micro-stereolithography system using optical fibers
The multi-scale micro-stereolithography system employing optical fibers used in this study is shown in Fig. 1.
This method can enable a wide variety of linewidths to be achieved using different types of optical fibers with different core diameters and can also control the accumulation pitch via adjustment of the distance between the tip of the optical fiber and the substrate. A 377 nm UV diode laser (CUBE 375–16C, Coherent, Inc., wavelength: 377 nm) was used as the light source. To control the switching on and off of the laser irradiation, a mechanical shutter was used. An objective lens (4×, NA: 0.13, Olympus) was used to introduce the laser beam into the tip of the optical fiber. In this study, three types of optical fibers were used to change the linewidth. The laser powers were measured after passing through an optical fiber.
To fabricate 3D structures, the glass substrate was moved using a Y motorized stage, and the edge of the optical fiber was also moved using a XZ motorized stage (IMAGE MASTER 350PC Smart SMΩX Musashi Engineering, Inc.). This system also has observation optics consisting of an objective lens (M Plan Apo 10×/0.28 Mitutoyo Corp.), an imaging lens, and a complementary metal oxide semiconductor (CMOS) camera to determine the initial Z position between the tip of the optical fiber and the surface of the glass substrate. After the initial position of the optical fiber was determined, a droplet of the photocurable ceramic slurry was placed on the glass substrate. Subsequently, the tip of the optical fiber, which was fixed to the motorized XZ stage, was immersed into the droplet. A 2D structure was fabricated by moving the optical fiber line-by-line using the motorized Y and XZ stages, while irradiating the laser from the tip of the optical fiber. Further, a 3D structure was accumulated by moving the optical fiber layer-by-layer using the motorized XZ stage, while creating each cross-sectional shape of the 3D model. In addition, the temperature of the glass substrate was maintained at 45 °C using a stage heater equipped with the Y motorized stage during fabrication to decrease the viscosity of the slurry. After fabrication, the 3D structures were gently washed with ethanol and dried using a blower.
3. Investigation of the fabrication resolution
To investigate the fabrication resolution of the micro-stereolithography system using the photocurable ceramic slurry, the linewidth and the cured depth of single lines were measured by changing the core diameter of the optical fiber and the laser power emitted from the tip of the optical fiber. A schematic of the fabricated structures for evaluation of the resolution is shown in Fig. 2(a). The cantilever structure allows us to measure both the linewidth and cured depth simultaneously. Three types of optical fibers (core diameter: 10 µm, 25 µm, and 105 µm) were used in the fabrication of the cantilever structures. The scanning speed of the optical fibers during fabrication was set to 0.1 mm/s.
Fig. 2. (a) Schematic of fabricated cantilever structure. SEM images (top and side view) of the cantilever structures fabricated at different laser power and core diameter, i.e., (b) 50 µW, 25 µm; (c) 3 µW, 10 µm; (d) 3 µW, 105 µm, respectively.
Figure 2(b) shows a fabricated example of a cantilever structure obtained with an optical fiber with a core diameter of 25 µm and a laser power of 50 µW. The base of the cantilever was fabricated by stacking 7 layers under an accumulation pitch of 25 µm. The length of the cantilever on the base part was 550 µm and an overhang of approximately 236 µm was fabricated without any deformation or collapse. Figure 2(c) shows a cantilever model with the finest linewidth fabricated with a core diameter of 10 µm at a laser power of 3 µW. In this condition, the cantilever collapses due to insufficient laser power, and the cantilever is shortened. At a laser power of 1 µW, this collapse and shortening of the cantilever also occurred for all three core diameters (10 µm, 25 µm, and 105 µm). Figure 2(d) shows a cantilever model with the thinnest cured depth fabricated with a core diameter of 105 µm at a laser power of 3 µW. Although the cantilever shown in Fig. 2(d) was warped owing to the large aspect ratio after the washing process, the linewidth and cured depth could be measured.
Figure 3 shows the relationship between laser power, linewidth, and cured depth with each core diameter (CD). The cantilever structures were fabricated by a single scanning of the optical fiber while changing the laser power (1 µW, 3 µW, 6 µW, 12 µW, 25 µW, 50 µW) for each core diameter (10 µm, 25 µm, 105 µm). In the experiments, depending on the core diameter of the optical fiber and the laser power, the accumulation pitch (Z direction) varied from 10 µm–25 µm to fabricate the cantilever structures. The measured linewidth and cured depth were plotted as the average values of three points at the overhanging part of the cantilever, along with the standard deviations. As shown in Fig. 3(a), when the laser power range was 1 µW–50 µW, the linewidths were16 µm–64 µm, 27 µm–67 µm, and 80 µm–117 µm for the core diameters of 10 µm, 25 µm, and 105 µm, respectively. It was found that the linewidth can be adjusted within this range by changing the laser power. Moreover, most of the linewidths were larger than the corresponding core diameter. This result indicates that the laser light is scattered by the ceramic microparticles. From this result, it is possible to obtain the desired linewidth of 16 µm–117 µm by changing the core diameter and laser power.
Fig. 3. Relationship between the (a) linewidth as well as (b) cured depth and laser power with different core diameters of the optical fibers.
From Fig. 3(b), in the laser power range of 3 µW–50 µW, cured depths of approximately 25 µm–47 µm with the 10 µm core diameter, 25 µm–45 µm with the 25 µm core diameter, and 19 µm–42 µm with the 105 µm core diameter were obtained. At a laser power of 1 µW, the cured depth could not be measured because the cantilevers broke for all core diameters. The cause for this cantilever collapse is thought to be the insufficient strength of the cured part owing to low light intensity. Therefore, it was found that the laser power had to be adjusted to 3 µW or higher for the fabrication of a 3D structure with sufficient mechanical strength to be possible. In addition, because the cantilever collapses under its own weight, it is expected that the available laser power will be limited by the overhang length required for the fabrication of the lattice structures.
4. Fabrication of ceramic lattice structures
4.1 Fabrication of lattice structures using a photocurable ceramic slurry
To fabricate a 3D lattice structure, an optical fiber with a core diameter of 25 µm was used. The fabricated structures must be peeled off from the glass substrate before degreasing and sintering. Therefore, we prepared sacrificial layers (two layers) 1.4 mm${\times} $ 1.4 mm in size using the same photocurable ceramic slurry to make it easier to peel off the structures without collapse. To reduce the fabrication time of this layer, a laser power of 60 µW and a scanning speed of 0.2 mm/s were used. However, the laser power and scanning speed for fabricating the lattice structures were set to 6 µW and 0.1 mm/s, respectively.
Figure 4(a) shows the parameters that determine the pore size of the 3D lattice. The scanning distance (SD) is the distance between the center of the drawing lines. The accumulation pitch (AP) is the height of the optical fiber that is lifted to create a new layer. The AP is set to a value smaller than the cured depth to prevent the upper layer from peeling off from the lower layer. The number of laminations (NoL) is the number of stacks of the cross-sectional layer of the same shape. First, it was shown that the pore diameter in the lateral direction can be adjusted by changing the SD. Because the linewidth at a laser power of 6 µW was approximately 42 µm, SD was set to 80 µm, 90 µm, and 100 µm. The woodpile structures were fabricated with AP = 25 µm and NoL = 1 layer. Figures 4(b)–4(d) show the top and side views of the fabricated woodpile structures at each SD. The linewidths of all three structures were approximately 48 µm. These linewidths were approximately 10 µm larger than those obtained during the fabrication of the cantilever. It is speculated that partial curing of the β-TCP slurry was due to the repeated irradiation of laser light when the optical fiber was scanned. The average lateral pore sizes were 31 µm, 39 µm, and 47 µm for SD values of 80 µm, 90 µm, and 100 µm, respectively. From these results, it was confirmed that the pore size in the lateral direction can be adjusted by changing SD. The average pore diameters in the Z direction were 11 µm, 13 µm, and 15 µm for SD values of 80 µm, 90 µm, and 100 µm, respectively. A smaller SD results in a smaller pore size in the Z direction. Then, we showed that the pore size in the Z direction can be adjusted by changing the NoL. The AP and SD were set to 20 µm and 100 µm, respectively, and the NoLs were set to 2 layers, 3 layers, and 4 layers. The side view of each model is shown in Figs. 4(e)–4(g). The average pore diameters in the Z direction were 22 µm, 34 µm, and 56 µm when the NoLs were 2 layers, 3 layers, and 4 layers, respectively, and the pore diameters increased in proportion to NoL. Therefore, these results confirmed that the pore diameter can be controlled by changing the SD and NoL.
Fig. 4. (a) Schematic of the woodpile structure with explanation of fabrication parameters: scanning distance (SD), accumulation pitch (AP), and number of laminations (NoL). Top and side views of woodpile structures that were fabricated under different scanning distances: (b) 80 µm, (c) 90 µm, and (d) 100 µm with an AP of 25 µm and single-layer lamination with a laser power of 6 µW. Side view of woodpile structures that were fabricated under different NoL: (e) 2 layers, (f) 3 layers, (g) 4 layers with an AP of 20 µm and SD of 100 µm with a laser power of 6 µW.
4.2 Degreasing and sintering of the fabricated lattice structure
The fabricated structures are degreased by thermal decomposition to remove the acrylate-based monomer, the UV absorber, and the dispersant, and the remaining ceramic structures are sintered to improve both their mechanical properties and biocompatibility. As a representative example, the woodpile structure fabricated in the previous section with a SD of 100 µm and NoL of 3 layers was sintered and degreased. First, the structure was peeled off by gradually cutting the sacrificial layer using a knife. Figures 5(a) and 5(b) show the structures before and after peeling from the glass substrate. Owing to the sacrificial layer, it was possible to remove the fabricated object. Subsequently, the woodpile structure was placed on an alumina substrate and sintered in an electric furnace (FO810, Yamato Scientific Co., Ltd.). During sintering, the temperature was increased from 20 °C to 1000 °C at a heating rate of 12 °C/min, before increasing to 1150 °C at a heating rate of 5 °C/min. The temperature of 1150 °C was maintained for 3 h. Figure 5(c) shows the woodpile structure after degreasing and sintering. The shrinkage ratio calculated from the dimensions of the entire lattice model before and after heat treatment was 10% in the XY direction and 21% in the Z direction. It is considered that the difference in shrinkage ratio between the XY and Z directions is caused by the geometry of the structures and/or anisotropy of the structure in the stacking direction.
Fig. 5. Bird's eye view of the woodpile structure (a) after curing, (b) after peeling, and (c) after degreasing and sintering. The woodpile structure was fabricated with SD = 100 µm, AP = 20 µm, and NoL = 3 layers.
The approximate porosity of the woodpile structure was calculated from its linewidth, thickness in the Z direction, and pore diameter. As a result, the porosity was approximately 15%. Subsequently, the SD was changed between 80 µm–100 µm, and the stacking number was changed between 1–4 layers to fabricate 10 woodpile structures. They were degreased and sintered under the same heating profile as above. The shrinkage rate in the XY direction was approximately 10%–15%, the shrinkage rate in the Z direction was approximately 16%–21%, and the average shrinkage rate in each direction was 13% and 18%, regardless of SD and NoL.
4.3 Fabrication of a complex lattice structure
Finally, we demonstrated the fabrication of a more complex lattice structure that mimicked the shape of a tooth shown in Fig. 6(a). The size of the structure was 1 mm ${\times} $ 1 mm in the XY direction and 1.6 mm in the Z direction. Similar to the method described above, this method involved the preparation of a sacrificial layer with a laser power of 60 µW and fabrication of a tooth model on it with a laser power of 6 µW at a scanning speed of 0.1 mm/s. The SD, AP, and NoL were set to 100 µm, 20 µm, and 3 layers, respectively. As a result, we succeeded in fabricating a complex structure with a fine lattice. The production time was approximately 1.5 h. The tooth-like structure was peeled off from the glass substrate (Fig. 6(b)) and sintered as per the same profile mentioned above. Figure 6(c) shows the tooth-like structure after sintering. The results demonstrate that the complex shape of the tooth-like structure was maintained. A sintered body with a pore diameter of approximately 24 µm was obtained in both the XY and Z directions. The shrinkage rate of the sintered body was 15% in the XY direction and 18% in the Z direction. As the sintered body made from β-TCP slurry has low toxicity and high biocompatibility, its application to bone regenerative medicine can be facilitated if it is fabricated with an appropriate pore size and shape.
Fig. 6. CAD model (a) of the complex lattice structure (tooth shape), and the fabricated structure (b) before and (c) after degreasing and sintering.
5. Conclusion
We demonstrated the fabrication of ceramic lattice micro-structures with photocurable ceramic slurry by micro-stereolithography using optical fibers. First, we examined the lateral and depth resolutions of this fabrication method using optical fibers with different core diameters with a photocurable ceramic slurry containing β-TCP particles. The experimental results demonstrated multi-scale fabrication with linewidths of 16 µm−117 µm and cured depths of 25 µm−47 µm by changing the core diameter and laser power. Although the linewidth was widely varied by changing the core diameter and the laser power, the cured depth did not change significantly because of restriction by scattering and absorption of light.
In addition, 3D lattice structures were fabricated by adjusting the fabrication conditions. Here, we successfully fabricated a 3D lattice structure with a pore size of 30 µm or less. The pore diameters of the lattice structure increased in proportion to the number of laminations. From these results, it was confirmed that the pore diameter can be controlled by changing the fabrication conditions such as the scanning distance, accumulation pitch, and number of lamination layers. The fabricated lattice structures were degreased and sintered to produce ceramic lattice structures. The average shrinkage rates were 13% in the lateral direction and 18% in depth, regardless of the scanning distance and number of lamination layers. Under the optimal fabrication conditions, a tooth-like structure was fabricated without any harmful collapse occurring.
In the near future, fiber-based micro-stereolithography using photocurable ceramic slurry can be a useful technique for fabricating 3D bioceramic structures for application in dentistry [25] and bone regenerative medicine [26]. This method can also produce 3D ceramic parts with various types of ceramics applicable for electronics [27], mechanics [28], and optics [29].
Funding
Core Research for Evolutional Science and Technology (JPMJCR1905).
Acknowledgments
This work was supported by JST CREST (Grant Number JPMJCR1905, Japan).
Disclosures
The authors declare no conflicts of interest.
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