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Still motion process for improving the accuracy of latticed microstructures in projection microstereolithography
Sensors and Actuators A 167 (2011) 117–129
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Still motion process for improving the accuracy of latticed microstructures in projection microstereolithography In-Baek Park 1 , Young-Myoung Ha 1 , Seok-Hee Lee ∗ School of Mechanical Engineering, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 September 2010 Received in revised form 19 November 2010 Accepted 27 December 2010 Available online 31 December 2010 Keywords: Projection microstereolithography (PSL) Digital micromirror device (DMD) Still motion process
a b s t r a c t Projection microstereolithography (PSL), which involves fabricating a microstructure using patterned light with a dynamic mask such as a liquid crystal display (LCD) and a digital micromirror device (DMD), is one of the additive manufacturing technologies. A nonuniform light intensity distribution of the cross section to be illuminated affects the accuracy of the microstructure. In other words, under-cure or over-cure occurs by light superposition in the corner of a latticed microstructure. This phenomenon will offset the advantages of the PSL (e.g., its simple process and fast fabrication time). Furthermore, accurate fabrication of a microstructure is indispensable for various applications such as a scaffold for tissue regeneration, and micro-devices for micro-actuators. In this study, the still motion method is introduced to improve the accuracy of latticed microstructure. We used continuous projection of a unit shape image, which is determined by the cross section in a layer. Some latticed microstructures have been more accurately fabricated by the still motion process compared to conventional PSL processes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Microstereolithography (SL), which is based on the stereolithography (SL) process in additive manufacturing, is a micro-fabrication technology [1]. Recently, SL technology has been applied in various industrial fields due to advances in the use of photocurable resin material. In biotechnology, scaffolds for tissue generation have been studied using bio-adaptable materials [2–4]. Micro-needles for transdermal delivery systems (TDDSs) have been studied [5]. In the micro-fabrication area, a diffuser for the dispersion of light in optical fibers, microlens arrays [6–8], and microfluidic devices [9,10] have been researched. SL technology is divided into the scanning type (SSL) and the projection type (PSL). The scanning type builds a layer using the scanning of focused light. Galvano-mirror or X–Y stages are generally used to guide the scanning path. The projection type completes a layer using one irradiation of patterned light that is controlled by a dynamic pattern generator such as a liquid crystal display (LCD) and digital micromirror device (DMD) [11–17]. The SSL can precisely fabricate a microstructure due to the size of the focused beam and the precision of the stage, and the fabrication time is relatively long. The PSL has a relatively fast fabrication time, and the inten-
∗ Corresponding author. Tel.: +82 51 510 2327; fax: +82 51 582 0988. E-mail addresses: [email protected] (I.B. Park), [email protected] (Y.M. Ha), [email protected] (S.H. Lee). 1 Tel.: +82 51 510 1476; fax: +82 51 510 0685. 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.12.023
sity distribution of the patterned light has an effect on the accuracy of the microstructure [11]. To improve the accuracy of a layered microstructure, multiple fabrications of a sacrificial layer [1], a cross section segmentation method considering the light intensity distribution [7], the application of a grayscale effect on a monochrome cross section [18–21], and control of the stacking direction [22] have been studied. However, these approaches have not shown a good result for microstructures with sharp edges (such as latticed microstructures) because the light intensity distribution is not controlled. Several microfabrication technologies have been used to fabricate a latticed microstructure. A fused deposition modeling (FDM) process, which melts the material and squeezes it via a micronozzle, has been widely used to build latticed structures such as a scaffold [23,24]. However, flections due to self-weight and an increase in fabrication time due to filling of the interior part decrease the strength of the FDM [11]. The three-dimensional printing (3DP) process has a limitation on fabricating the microstructure due to the difficulty of removing residual material in a structure with a micro-hole [11]. In this research, the still motion process (SM-process), which uses the continuous projection of a unit shape image based on the PSL, is introduced to improve the accuracy of a microstructure. The SM-process does not affect the system configuration and can be applied to any dynamic pattern generator. In particular, it can be effectively applied to fabricate latticed microstructures such as scaffolds with high porosity and sharp edges.
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Start
Pre-process
3D CAD model to STL format Slicing STL with a certain layer thickness Generating cross-sectional images Initializing the fabrication system(PμSL) Fabrication process
Generating a new liquid layer
Fig. 1. Schematic of our DMD-based projection microstereolithography system.
Transferring a cross-sectional image to the DMD Opening shutter for the exposure time
2. Projection microstereolithography (PSL)
N
Last layer?
Fig. 1 shows the configuration of our PSL system. The system includes a DMD for dynamic pattern generation, optical devices for light delivery, an X–Y-stage for shifting the position of illumination, a Z-stage for stacking layers, a light source, and a controller [6–8]. Fig. 2 shows the conventional fabrication process used in the PSL system. A three-dimensional (3D) model in STL format is sliced at a certain height. Each cross section is then translated into
Post process
Y
Pulling, rinsing, and post curing the part End Fig. 2. Flowchart of the conventional fabrication process in PSL [13,19,20,26].
E
E
Curing area Curing area
Emin
Emin
Emin
w0
-w
w
Emin
Ec
Ec
MicromirrorNo. 1 -w
(a)
2
3
4 w0
5
6
7 w
(b)
Fig. 3. Energy distribution of patterned beam by the DMD: (a) one micromirror and (b) several micromirrors [12].
Cross-sectional image
Irradiation Micromirror 4
4
1
3
Digital Mirror Device(DMD)
3
4
2
2 4
2
2 2
4
3 1
1 2
Micromirror
Fig. 4. Under-cure and over-cure at the edge of a cross section due to Gaussian superposition.
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Fig. 5. Intensities of a patterned beam at the position of the resin surface (a), (b) cross sections of bitmap format. (c) and (d) Intensity distribution of each cross section, and at the corner.
a monochrome image file using BMP format. After the system is initialized, a sacrificial layer for leveling the first layer is built. By the movement of the Z-stage, a liquid resin covers the cured sacrificial layer. The bitmap file for the first layer is imported into the DMD, and patterned light via optical devices (lenses and a mir-
ror) is illuminated on the resin surface. This procedure is continued until the last layer is built. After the fabrication is completed, the residual liquid resin is removed by a solvent. The conventional fabrication process in the PSL is simple because one exposure for a layer is generally used. However, a non-uniform intensity dis-
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Fig. 6. Predicted curing shapes for various critical exposures: (a) rectangle and (b) cross shape.
tribution of a patterned beam causes building inaccuracy such as over-cure and under-cure. Hence, a solution that provides uniform exposure energy on the resin is required. 3. Characteristics of patterned beam 3.1. Energy distribution In stereolithography, photopolymerization proceeds in the following order: photoinitiation, propagation, and termination [25]. Photoinitiation is caused by free radicals produced from the decomposition of initiators. The radical adds to a monomer and reacts with the double bond of another monomer. It keeps adding to a growing chain, one after another, which is called propagation. This step ends when the free radical site is inhibited by some impurities or by a termination process [11]. During the photopolymerization, the critical exposure energy Ec , which is known as the threshold exposure for phase-change, depends on photocurable resin composed of a photoinitiator, a monomer, and a photoabsorber [27].
The distribution of a patterned beam by one micromirror of the DMD is assumed to be Gaussian. Several micromirrors corresponding to a cross section is considered to be a Gaussian superposition, as shown in Fig. 3. To build a layer by photopolymerization, energy over the critical exposure Ec is required. At that time, the curing width is −w to w as shown in Fig. 3(a). The intensity distribution of the patterned beam on the resin surface is affected by the magnification of an objective lens. Hence, a diffuser lens could be used to reduce the uniformity of the intensity, but an aberration and diffraction could also occur [19]. 3.2. Effect of superposition Superposition of a Gaussian distribution decreases the resolution of a patterned beam due to the Rayleigh criterion [28]. To improve the resolution, light with a short wavelength can be used, but the curing characteristics of a photocurable resin must be considered with respect to the wavelength. Gaussian superposition enables the layer to cure evenly on the resin surface due to energy compensation between neighboring micromirrors. However, an
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4.1. Method Still motion is a method of displaying many images, one after another, as frames in computer graphics [29]. Using this method, we applied a continuous projection of a unit shape for the completion of a layer in PSL. This approach allowed us to improve the accuracy of the microstructure in PSL. Fig. 7 shows a schematic of the still motion method. The method can be applied whenever a pattern generator such as LCD or DMD is used. Considering the curing characteristics of the resin, it is assumed that the cross section of a line with a certain width is composed of five rectangular unit shapes. Each unit shape is numbered in order according to the curing direction, and is imported into the DMD one-by-one for projection during the frame time. The frame time, which is the period required to change the unit shape on the DMD, has the same meaning as exposure time in conventional PSL. The overall procedure for the still motion process is shown in Fig. 8.
Unit-shape DMD
5
1
Objective lens
Resin surface 5
1
Platform Z-stage Fig. 7. Schematic of the still motion method in PSL.
CAD model
Converting to an STL file Modeling and Pre-process
4. Still motion process in PSL
Direction
Slicing the STL file according to the layer thickness Converting to a Bitmap image (Cross-sectional images) Segmenting the cross-sectional image by the unit-shape Initializing the fabrication system
Generating a new liquid layer Transferring the unit shape to the DMD and opening the shutter during a frame-time Fabrication
inaccurate shape such as over-cure and under-cure is normally made at the edge of a cross section, as shown in Fig. 4. In Fig. 4, a cross section of a bitmap format includes white and black pixels to present the cross shape. The emitted light from the light source is patterned by the DMD displaying the cross section, and is transferred onto the resin surface. The degree of light intensity in the projected patterned beam is presented within a small rectangle, as shown in Fig. 4. As shown in the figure, micromirror number 3 placed on a convex corner has less intensity than neighboring micromirrors (numbers 1 and 2). In contrast, micromirror number 3 placed on a concave corner has higher intensities than the others. This phenomenon causes under-cure and over-cure when the intensity is lower and higher, respectively, than the critical exposure. To investigate the effect of light superposition, two cross sections were used as shown in Fig. 5. The cross-sectional image was transferred to the DMD, and the intensity distribution of the patterned beam was detected by a beam profiler (FX66TM , Ophir Optronics Co., USA) that was placed on the platform. The maximum intensity of 16.4 mW/cm2 was used. Fig. 5(c) and (d) show the intensity distribution of each cross section. At the corner, the intensity is smaller than at other regions. The intensity at the center region is higher due to the superposition of light. Therefore, the corner region is hard to cure or distort when a highly viscous resin is used. Fig. 6 shows the predicted curing shape implemented with pixels that have higher intensity than the critical exposures of 1–4 mJ/cm2 . This means that the curing shape mainly depends on the critical exposure. When a patterned beam with an intensity of 16.4 mW/cm2 illuminates a resin at the critical exposure of 1 mJ/cm2 for 1 s, an over-cure shape is produced. The higher critical exposure the resin has, the more accurate the curing shape. However, the intensity of emitted light from the lamp is limited, so resin with a high critical exposure does not always produce an accurate curing shape. Specifically, under-cure can occur in the edge region, as shown in Fig. 4. To reduce inaccuracy due to superposition of the patterned beam, we introduce the still motion process.
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Last Unit shape
No
Yes Last Layer?
No
Yes Post-process
Pulling the fabricated part/ Rinsing and post curing
4.2. Unit shape
END In PSL, a microstructure is built by stacking cross sections. To apply the still motion method using variable cross sections, unit shapes appropriate for each cross section must be generated. The intensity of patterned light by a unit shape must be larger than
Fig. 8. Overall procedures for the still motion process in PSL.
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Fig. 9. Still motion method in PSL. (a) Latticed cross section, (b) projection direction for the unit shape, and (c) continuous projection of the unit shape.
the critical exposure of the resin. Fig. 9 shows an example for the unit shape of a latticed cross section, and the projection direction. A rectangle was chosen for the unit shape of a latticed cross section as shown in Fig. 9. For continuous curing on the resin surface, part of the unit shape was overlapped. To minimize the fabrication time, two groups of unit shape were used as shown in Fig. 9(b). One group was continuously projected to the horizontal direction. The other was then projected to the vertical direction. If a cross section
is changed, the unit shape must also be changed. In this case, the same unit shape was used. In scanning SL (SSL), a resin vat is generally moved to scan the focused beam [21]. On the other hand, a series of unit shapes is projected at intervals of shift-pixel during a frame time corresponding to an exposure time in the still motion method. Therefore, this method can prevent a fabrication error due to the movement of the system, and increase the accuracy of a microstructure due to improved light intensity distribution.
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Fig. 10. Curing experiment of H2 using conventional fabrication process: (a) 3D specimen model and (b) curing depth according to exposure energy.
Fig. 11. Curing experiment H2 using still motion method. (a) 3D specimen model, (b) curing width according to the beam intensity, (c) curing width according to the frame time, (d) curing depth according to the beam intensity, and (e) curing depth according to the frame time.
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Shift-pixel 4 3 2
Resin surface
1
1-3
1-2
1-4
Unit shape
Shape(Cd )
Resin surface
1
1 2
Unit shape No. 1 2
3
3
1
4
4
(a)
(b) Fig. 12. Curing shape according to the size of the shift pixel.
4.3. Curing properties of photocurable resin In this study, photocurable resin composed of 1,6-hexanediol diacrylate (HDDA, Miwon Chem. Tech. Co., Korea) as a monomer, and 2,2-dimethoxy-2-phenylacetonphenone (DMPA, Fisher Scientific Co., USA) of 5 wt% as a photoinitiator, were used. HDDA has high light transmission and curing speed, and low viscosity (below 7 cps). DMPA has low yellowing and is proper to the wavelength of 365 nm. These chemicals were mixed using a magnetic stirrer for 3 h at room temperature. We called the mixture H2. Fig. 10 shows the 3D model for the curing experiment, and a graph of curing depth Cd versus the exposure energy. For a specimen fabricated using the conventional PSL process, the resin had a critical exposure energy Ec of 3.784 mJ/cm2 , and a light penetration depth Dp of 440.728 m, based on the Beer–Lambert law as shown in Eq. (1). Cd = Dp ln
E
as shown in Fig. 11(b) and (d). The difference in curing width for a beam intensity over 6.56 mW/cm2 was within 40 m, as shown in Fig. 11(b). The magnification of the optical system was about 2.1 times; thus, the exact microstructure can be fabricated with an intensity of 6.56 mW/cm2 . In the same unit shape, the curing width and depth were proportional to the frame time as shown in Fig. 11(c) and (e). For example, a frame time within 40 ms provided enough accuracy in the unit shape of 100 pixels, as shown in Fig. 11(c). Therefore, the accuracy of the microstructure can be controlled by the frame time.
(1)
Ec
4.4. Control parameters 4.4.1. Frame time In the still motion method, the parameters used to control the curing shape are the intensity of the light source, the frame time, and the size of the shift pixel. To investigate the curing characteristics according to these parameters, a specimen was used as shown in Fig. 11(a). The top layer of the specimen was fabricated with variable conditions as shown in Table 1. Four pillars supporting the top layer were fabricated using the same conditions. Generally, the curing width and depth is presented according to the exposure energy. However, we used the beam intensity because the frame time is a major parameter, and the exposure energy of the patterned beam can be controlled by frame time corresponding to the speed change of the unit shape in the still motion method. The curing width and depth were proportional to the beam intensity
Fig. 13. Curing depth and width according to the shift pixel.
Table 1 Process conditions for the specimen shown in Fig. 11. Conditions
Curing width (m) Curing depth (m)
Unit shape (pixel)
(b) (c) (d) (e)
Width (W)
Height (H)
50 50 50 50
100 100–40 100 100–20
Shift (pixel)
Beam intensity (mW/cm2 )
Frame time (ms)
10 10 10 10
3.28–16.4 16.4 3.28–16.4 16.4
100 20–100 100 20–100
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Fig. 14. Fabrication of cross shape microstructure using the conventional method in PSL.
Fig. 15. Fabrication of cross shape microstructure using the still motion method in PSL.
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Fig. 16. Gridded microstructure using the still motion method: (a) cross sectional image, (b) unit shapes in each direction, and (c) scanning electron microscopy (SEM) photograph.
4.4.2. Shift pixel In this study, a shift pixel is defined as the distance between the position of a projected unit shape and that of the next projected unit shape. The shift pixel plays a role in combining the cured regions by unit shapes on the resin surface. A narrow shift pixel increases the overlapped area, and over-curing occurs due to the excessive superposition of light. On the other hand, a widened shift pixel causes uneven curing depth and incomplete building, as shown in Fig. 12.
Table 2 shows the fabrication conditions for the experiment of curing depth and width according to the change of the shift pixel. The curing depth decreased when the shift pixel was widened, as shown in Fig. 13. The curing depth also decreased slightly. This phenomenon is similar to a photo-absorber such as Tinuvin, which is used to control the curing depth. Hence, the role of the shift pixel was limited to combining the cured regions of unit shape.
5. Fabrication examples Table 2 Fabrication conditions for the effect of the shift pixel. Conditions Unit shape (pixel) Shift pixel (pixel) Beam intensity (mW/cm2 ) Frame time (ms)
W: 50, H: 100 2, 3, 4, 5 16.4 100
To compare the still motion method with the conventional method in PSL, a cross shape was fabricated as shown in Fig. 14. The fabrication conditions are shown in Table 3. For the conventional method, the sharp corner was rounded due to the superposition of the patterned beam as shown in Fig. 14(b). This rounded shape has been founded in sharp corners, as shown in Fig. 14(c). For the same cross section, the still motion method was
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Fig. 17. Fabrication of lozenge pattern on the gridded microstructure using still motion process: (a) cross sectional image and (b) SEM photograph.
Table 5 Fabrication conditions for gridded microstructure shown in Fig. 16.
Table 3 Fabrication conditions for the conventional method of Fig. 14. One layer (b) Beam intensity (mW/cm2 ) Layer thickness (m) Total layer (EA) Total process time (min)
16.4 10 1 –
Multiple layers (c)
Conditions
16.4 10 20 About 20
Beam intensity (mW/cm2 ) Unit shape (pixel) Layer thickness (m) Total layer (EA) Total still motion image (EA/layer) Shift-pixel (pixel) Frame-time (ms) Total process time (min)
also applied using the conditions shown in Table 4. Fig. 15 shows the results of the fabrication. In Fig. 15(b), the over-cured shape was found because two unit shapes in Fig. 15(a) were projected simultaneously in each direction. When these unit shapes were fabricated separately, the result was more accurate as shown in Fig. 15(c)–(f). Fig. 16 shows a gridded microstructure, which is often called a scaffold in bio-engineering. The conditions for fabrication are shown in Table 5. The unit shape of size of 25 × 25 pixels was used and projected alternately in two directions, as shown in Fig. 16(b). The size of the holes in the completed microstructure was relatively accurate (about 50 m × 50 m). However, the fabrication time took slightly longer than the conventional method.
9.84 W: 25, H: 25 10 5 1804 1 100 About 20
To determine our ability to fabricate a scaffold pattern, a lozenge pattern on gridded microstructure was fabricated under the conditions shown in Table 6. The two different patterns were well bonded, and the shape of the pattern was uniform as shown in Fig. 17(b). In Fig. 18, an asterisk shape was fabricated to make a comparison of accuracy between the conventional process and the still motion method in PSL. The fabrication conditions are shown in Table 7. While rounded corners were found due to light
Table 4 Fabrication conditions for the still motion method of Fig. 15.
2
Beam intensity (mW/cm ) Unit shape (pixel) Layer thickness (m) Total layer (EA) Total still motion image (EA/layer) Shift pixel (pixel) Frame time (ms) Total process time (min)
(b)
(c)
(d)
(e)
(f)
16.4 W: 50, H: 100
9.84 W: 50, H: 100
16.4 W: 50, H: 100
16.4 W: 50, H: 40
100 About 10
100 About 10
9.84 W: 50, H: 40 20 3 1098 1 100 About 10
60 About 7
80 About 9
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Fig. 18. Micro-asterisk using still motion method. (a) Cross-sectional image, (b) SEM photograph, (c) unit shape image, and (d) SEM photograph.
Table 6 Fabrication conditions for lozenge pattern microstructure shown in Fig. 17. Conditions Beam intensity (mW/cm2 ) Unit shape (pixel) Layer thickness (m) Total layer (EA) Total still motion image (EA/layer) Shift pixel (pixel) Frame time (ms) Total process time (min)
9.84 H: 25, V: 25 25 20 (rectangle: 10 each, rhombus: 10 each) (Rectangle: 1804, rhombus: 1913) 1 100 About 22
Table 7 Fabrication conditions for micro-asterisk in Fig. 18.
Beam intensity (mW/cm2 ) Unit shape (pixel) Layer thickness (m) Total layer (EA) Total still motion image (EA/layer) Shift pixel (pixel) Frame time (ms) Total process time (min)
(b)
(d)
16.4 – 10 10 – – – About 10
16.4 H: 25, V: 25 10 10 364 1 100 About 16
superposition in the conventional process, sharp concave corners were fabricated using the still motion method, as shown in Fig. 18(b) and (c).
6. Conclusion To improve the accuracy of a microstructure, we introduced the still motion process for PSL. In this process, a cross section was divided into a series of unit shapes, and they were, in turn, projected onto the resin surface for a frame time. So, it seems to scan unit shape. In the still motion process, the intensity distribution for a cross section is almost uniform, hence the still motion process enables accurate fabrication of a latticed microstructure. The unit shape, beam intensity, and frame time were used as parameters to control the accuracy. Considering the magnification of the imaging system and the curing characteristics of the resin, a suitable value for each parameter was determined. This still motion process can be applied to control the accuracy of a microstructure for applications in various fields such as a scaffold for tissue regeneration, and micro-devices for micro-actuators.
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Still motion process for improving the accuracy of latticed microstructures in projection microstereolithography In-Baek Park 1 , Young-Myoung Ha 1 , Seok-Hee Lee ∗ School of Mechanical Engineering, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 September 2010 Received in revised form 19 November 2010 Accepted 27 December 2010 Available online 31 December 2010 Keywords: Projection microstereolithography (PSL) Digital micromirror device (DMD) Still motion process
a b s t r a c t Projection microstereolithography (PSL), which involves fabricating a microstructure using patterned light with a dynamic mask such as a liquid crystal display (LCD) and a digital micromirror device (DMD), is one of the additive manufacturing technologies. A nonuniform light intensity distribution of the cross section to be illuminated affects the accuracy of the microstructure. In other words, under-cure or over-cure occurs by light superposition in the corner of a latticed microstructure. This phenomenon will offset the advantages of the PSL (e.g., its simple process and fast fabrication time). Furthermore, accurate fabrication of a microstructure is indispensable for various applications such as a scaffold for tissue regeneration, and micro-devices for micro-actuators. In this study, the still motion method is introduced to improve the accuracy of latticed microstructure. We used continuous projection of a unit shape image, which is determined by the cross section in a layer. Some latticed microstructures have been more accurately fabricated by the still motion process compared to conventional PSL processes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Microstereolithography (SL), which is based on the stereolithography (SL) process in additive manufacturing, is a micro-fabrication technology [1]. Recently, SL technology has been applied in various industrial fields due to advances in the use of photocurable resin material. In biotechnology, scaffolds for tissue generation have been studied using bio-adaptable materials [2–4]. Micro-needles for transdermal delivery systems (TDDSs) have been studied [5]. In the micro-fabrication area, a diffuser for the dispersion of light in optical fibers, microlens arrays [6–8], and microfluidic devices [9,10] have been researched. SL technology is divided into the scanning type (SSL) and the projection type (PSL). The scanning type builds a layer using the scanning of focused light. Galvano-mirror or X–Y stages are generally used to guide the scanning path. The projection type completes a layer using one irradiation of patterned light that is controlled by a dynamic pattern generator such as a liquid crystal display (LCD) and digital micromirror device (DMD) [11–17]. The SSL can precisely fabricate a microstructure due to the size of the focused beam and the precision of the stage, and the fabrication time is relatively long. The PSL has a relatively fast fabrication time, and the inten-
∗ Corresponding author. Tel.: +82 51 510 2327; fax: +82 51 582 0988. E-mail addresses: [email protected] (I.B. Park), [email protected] (Y.M. Ha), [email protected] (S.H. Lee). 1 Tel.: +82 51 510 1476; fax: +82 51 510 0685. 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.12.023
sity distribution of the patterned light has an effect on the accuracy of the microstructure [11]. To improve the accuracy of a layered microstructure, multiple fabrications of a sacrificial layer [1], a cross section segmentation method considering the light intensity distribution [7], the application of a grayscale effect on a monochrome cross section [18–21], and control of the stacking direction [22] have been studied. However, these approaches have not shown a good result for microstructures with sharp edges (such as latticed microstructures) because the light intensity distribution is not controlled. Several microfabrication technologies have been used to fabricate a latticed microstructure. A fused deposition modeling (FDM) process, which melts the material and squeezes it via a micronozzle, has been widely used to build latticed structures such as a scaffold [23,24]. However, flections due to self-weight and an increase in fabrication time due to filling of the interior part decrease the strength of the FDM [11]. The three-dimensional printing (3DP) process has a limitation on fabricating the microstructure due to the difficulty of removing residual material in a structure with a micro-hole [11]. In this research, the still motion process (SM-process), which uses the continuous projection of a unit shape image based on the PSL, is introduced to improve the accuracy of a microstructure. The SM-process does not affect the system configuration and can be applied to any dynamic pattern generator. In particular, it can be effectively applied to fabricate latticed microstructures such as scaffolds with high porosity and sharp edges.
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Start
Pre-process
3D CAD model to STL format Slicing STL with a certain layer thickness Generating cross-sectional images Initializing the fabrication system(PμSL) Fabrication process
Generating a new liquid layer
Fig. 1. Schematic of our DMD-based projection microstereolithography system.
Transferring a cross-sectional image to the DMD Opening shutter for the exposure time
2. Projection microstereolithography (PSL)
N
Last layer?
Fig. 1 shows the configuration of our PSL system. The system includes a DMD for dynamic pattern generation, optical devices for light delivery, an X–Y-stage for shifting the position of illumination, a Z-stage for stacking layers, a light source, and a controller [6–8]. Fig. 2 shows the conventional fabrication process used in the PSL system. A three-dimensional (3D) model in STL format is sliced at a certain height. Each cross section is then translated into
Post process
Y
Pulling, rinsing, and post curing the part End Fig. 2. Flowchart of the conventional fabrication process in PSL [13,19,20,26].
E
E
Curing area Curing area
Emin
Emin
Emin
w0
-w
w
Emin
Ec
Ec
MicromirrorNo. 1 -w
(a)
2
3
4 w0
5
6
7 w
(b)
Fig. 3. Energy distribution of patterned beam by the DMD: (a) one micromirror and (b) several micromirrors [12].
Cross-sectional image
Irradiation Micromirror 4
4
1
3
Digital Mirror Device(DMD)
3
4
2
2 4
2
2 2
4
3 1
1 2
Micromirror
Fig. 4. Under-cure and over-cure at the edge of a cross section due to Gaussian superposition.
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Fig. 5. Intensities of a patterned beam at the position of the resin surface (a), (b) cross sections of bitmap format. (c) and (d) Intensity distribution of each cross section, and at the corner.
a monochrome image file using BMP format. After the system is initialized, a sacrificial layer for leveling the first layer is built. By the movement of the Z-stage, a liquid resin covers the cured sacrificial layer. The bitmap file for the first layer is imported into the DMD, and patterned light via optical devices (lenses and a mir-
ror) is illuminated on the resin surface. This procedure is continued until the last layer is built. After the fabrication is completed, the residual liquid resin is removed by a solvent. The conventional fabrication process in the PSL is simple because one exposure for a layer is generally used. However, a non-uniform intensity dis-
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Fig. 6. Predicted curing shapes for various critical exposures: (a) rectangle and (b) cross shape.
tribution of a patterned beam causes building inaccuracy such as over-cure and under-cure. Hence, a solution that provides uniform exposure energy on the resin is required. 3. Characteristics of patterned beam 3.1. Energy distribution In stereolithography, photopolymerization proceeds in the following order: photoinitiation, propagation, and termination [25]. Photoinitiation is caused by free radicals produced from the decomposition of initiators. The radical adds to a monomer and reacts with the double bond of another monomer. It keeps adding to a growing chain, one after another, which is called propagation. This step ends when the free radical site is inhibited by some impurities or by a termination process [11]. During the photopolymerization, the critical exposure energy Ec , which is known as the threshold exposure for phase-change, depends on photocurable resin composed of a photoinitiator, a monomer, and a photoabsorber [27].
The distribution of a patterned beam by one micromirror of the DMD is assumed to be Gaussian. Several micromirrors corresponding to a cross section is considered to be a Gaussian superposition, as shown in Fig. 3. To build a layer by photopolymerization, energy over the critical exposure Ec is required. At that time, the curing width is −w to w as shown in Fig. 3(a). The intensity distribution of the patterned beam on the resin surface is affected by the magnification of an objective lens. Hence, a diffuser lens could be used to reduce the uniformity of the intensity, but an aberration and diffraction could also occur [19]. 3.2. Effect of superposition Superposition of a Gaussian distribution decreases the resolution of a patterned beam due to the Rayleigh criterion [28]. To improve the resolution, light with a short wavelength can be used, but the curing characteristics of a photocurable resin must be considered with respect to the wavelength. Gaussian superposition enables the layer to cure evenly on the resin surface due to energy compensation between neighboring micromirrors. However, an
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4.1. Method Still motion is a method of displaying many images, one after another, as frames in computer graphics [29]. Using this method, we applied a continuous projection of a unit shape for the completion of a layer in PSL. This approach allowed us to improve the accuracy of the microstructure in PSL. Fig. 7 shows a schematic of the still motion method. The method can be applied whenever a pattern generator such as LCD or DMD is used. Considering the curing characteristics of the resin, it is assumed that the cross section of a line with a certain width is composed of five rectangular unit shapes. Each unit shape is numbered in order according to the curing direction, and is imported into the DMD one-by-one for projection during the frame time. The frame time, which is the period required to change the unit shape on the DMD, has the same meaning as exposure time in conventional PSL. The overall procedure for the still motion process is shown in Fig. 8.
Unit-shape DMD
5
1
Objective lens
Resin surface 5
1
Platform Z-stage Fig. 7. Schematic of the still motion method in PSL.
CAD model
Converting to an STL file Modeling and Pre-process
4. Still motion process in PSL
Direction
Slicing the STL file according to the layer thickness Converting to a Bitmap image (Cross-sectional images) Segmenting the cross-sectional image by the unit-shape Initializing the fabrication system
Generating a new liquid layer Transferring the unit shape to the DMD and opening the shutter during a frame-time Fabrication
inaccurate shape such as over-cure and under-cure is normally made at the edge of a cross section, as shown in Fig. 4. In Fig. 4, a cross section of a bitmap format includes white and black pixels to present the cross shape. The emitted light from the light source is patterned by the DMD displaying the cross section, and is transferred onto the resin surface. The degree of light intensity in the projected patterned beam is presented within a small rectangle, as shown in Fig. 4. As shown in the figure, micromirror number 3 placed on a convex corner has less intensity than neighboring micromirrors (numbers 1 and 2). In contrast, micromirror number 3 placed on a concave corner has higher intensities than the others. This phenomenon causes under-cure and over-cure when the intensity is lower and higher, respectively, than the critical exposure. To investigate the effect of light superposition, two cross sections were used as shown in Fig. 5. The cross-sectional image was transferred to the DMD, and the intensity distribution of the patterned beam was detected by a beam profiler (FX66TM , Ophir Optronics Co., USA) that was placed on the platform. The maximum intensity of 16.4 mW/cm2 was used. Fig. 5(c) and (d) show the intensity distribution of each cross section. At the corner, the intensity is smaller than at other regions. The intensity at the center region is higher due to the superposition of light. Therefore, the corner region is hard to cure or distort when a highly viscous resin is used. Fig. 6 shows the predicted curing shape implemented with pixels that have higher intensity than the critical exposures of 1–4 mJ/cm2 . This means that the curing shape mainly depends on the critical exposure. When a patterned beam with an intensity of 16.4 mW/cm2 illuminates a resin at the critical exposure of 1 mJ/cm2 for 1 s, an over-cure shape is produced. The higher critical exposure the resin has, the more accurate the curing shape. However, the intensity of emitted light from the lamp is limited, so resin with a high critical exposure does not always produce an accurate curing shape. Specifically, under-cure can occur in the edge region, as shown in Fig. 4. To reduce inaccuracy due to superposition of the patterned beam, we introduce the still motion process.
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Last Unit shape
No
Yes Last Layer?
No
Yes Post-process
Pulling the fabricated part/ Rinsing and post curing
4.2. Unit shape
END In PSL, a microstructure is built by stacking cross sections. To apply the still motion method using variable cross sections, unit shapes appropriate for each cross section must be generated. The intensity of patterned light by a unit shape must be larger than
Fig. 8. Overall procedures for the still motion process in PSL.
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Fig. 9. Still motion method in PSL. (a) Latticed cross section, (b) projection direction for the unit shape, and (c) continuous projection of the unit shape.
the critical exposure of the resin. Fig. 9 shows an example for the unit shape of a latticed cross section, and the projection direction. A rectangle was chosen for the unit shape of a latticed cross section as shown in Fig. 9. For continuous curing on the resin surface, part of the unit shape was overlapped. To minimize the fabrication time, two groups of unit shape were used as shown in Fig. 9(b). One group was continuously projected to the horizontal direction. The other was then projected to the vertical direction. If a cross section
is changed, the unit shape must also be changed. In this case, the same unit shape was used. In scanning SL (SSL), a resin vat is generally moved to scan the focused beam [21]. On the other hand, a series of unit shapes is projected at intervals of shift-pixel during a frame time corresponding to an exposure time in the still motion method. Therefore, this method can prevent a fabrication error due to the movement of the system, and increase the accuracy of a microstructure due to improved light intensity distribution.
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Fig. 10. Curing experiment of H2 using conventional fabrication process: (a) 3D specimen model and (b) curing depth according to exposure energy.
Fig. 11. Curing experiment H2 using still motion method. (a) 3D specimen model, (b) curing width according to the beam intensity, (c) curing width according to the frame time, (d) curing depth according to the beam intensity, and (e) curing depth according to the frame time.
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Shift-pixel 4 3 2
Resin surface
1
1-3
1-2
1-4
Unit shape
Shape(Cd )
Resin surface
1
1 2
Unit shape No. 1 2
3
3
1
4
4
(a)
(b) Fig. 12. Curing shape according to the size of the shift pixel.
4.3. Curing properties of photocurable resin In this study, photocurable resin composed of 1,6-hexanediol diacrylate (HDDA, Miwon Chem. Tech. Co., Korea) as a monomer, and 2,2-dimethoxy-2-phenylacetonphenone (DMPA, Fisher Scientific Co., USA) of 5 wt% as a photoinitiator, were used. HDDA has high light transmission and curing speed, and low viscosity (below 7 cps). DMPA has low yellowing and is proper to the wavelength of 365 nm. These chemicals were mixed using a magnetic stirrer for 3 h at room temperature. We called the mixture H2. Fig. 10 shows the 3D model for the curing experiment, and a graph of curing depth Cd versus the exposure energy. For a specimen fabricated using the conventional PSL process, the resin had a critical exposure energy Ec of 3.784 mJ/cm2 , and a light penetration depth Dp of 440.728 m, based on the Beer–Lambert law as shown in Eq. (1). Cd = Dp ln
E
as shown in Fig. 11(b) and (d). The difference in curing width for a beam intensity over 6.56 mW/cm2 was within 40 m, as shown in Fig. 11(b). The magnification of the optical system was about 2.1 times; thus, the exact microstructure can be fabricated with an intensity of 6.56 mW/cm2 . In the same unit shape, the curing width and depth were proportional to the frame time as shown in Fig. 11(c) and (e). For example, a frame time within 40 ms provided enough accuracy in the unit shape of 100 pixels, as shown in Fig. 11(c). Therefore, the accuracy of the microstructure can be controlled by the frame time.
(1)
Ec
4.4. Control parameters 4.4.1. Frame time In the still motion method, the parameters used to control the curing shape are the intensity of the light source, the frame time, and the size of the shift pixel. To investigate the curing characteristics according to these parameters, a specimen was used as shown in Fig. 11(a). The top layer of the specimen was fabricated with variable conditions as shown in Table 1. Four pillars supporting the top layer were fabricated using the same conditions. Generally, the curing width and depth is presented according to the exposure energy. However, we used the beam intensity because the frame time is a major parameter, and the exposure energy of the patterned beam can be controlled by frame time corresponding to the speed change of the unit shape in the still motion method. The curing width and depth were proportional to the beam intensity
Fig. 13. Curing depth and width according to the shift pixel.
Table 1 Process conditions for the specimen shown in Fig. 11. Conditions
Curing width (m) Curing depth (m)
Unit shape (pixel)
(b) (c) (d) (e)
Width (W)
Height (H)
50 50 50 50
100 100–40 100 100–20
Shift (pixel)
Beam intensity (mW/cm2 )
Frame time (ms)
10 10 10 10
3.28–16.4 16.4 3.28–16.4 16.4
100 20–100 100 20–100
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Fig. 14. Fabrication of cross shape microstructure using the conventional method in PSL.
Fig. 15. Fabrication of cross shape microstructure using the still motion method in PSL.
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Fig. 16. Gridded microstructure using the still motion method: (a) cross sectional image, (b) unit shapes in each direction, and (c) scanning electron microscopy (SEM) photograph.
4.4.2. Shift pixel In this study, a shift pixel is defined as the distance between the position of a projected unit shape and that of the next projected unit shape. The shift pixel plays a role in combining the cured regions by unit shapes on the resin surface. A narrow shift pixel increases the overlapped area, and over-curing occurs due to the excessive superposition of light. On the other hand, a widened shift pixel causes uneven curing depth and incomplete building, as shown in Fig. 12.
Table 2 shows the fabrication conditions for the experiment of curing depth and width according to the change of the shift pixel. The curing depth decreased when the shift pixel was widened, as shown in Fig. 13. The curing depth also decreased slightly. This phenomenon is similar to a photo-absorber such as Tinuvin, which is used to control the curing depth. Hence, the role of the shift pixel was limited to combining the cured regions of unit shape.
5. Fabrication examples Table 2 Fabrication conditions for the effect of the shift pixel. Conditions Unit shape (pixel) Shift pixel (pixel) Beam intensity (mW/cm2 ) Frame time (ms)
W: 50, H: 100 2, 3, 4, 5 16.4 100
To compare the still motion method with the conventional method in PSL, a cross shape was fabricated as shown in Fig. 14. The fabrication conditions are shown in Table 3. For the conventional method, the sharp corner was rounded due to the superposition of the patterned beam as shown in Fig. 14(b). This rounded shape has been founded in sharp corners, as shown in Fig. 14(c). For the same cross section, the still motion method was
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Fig. 17. Fabrication of lozenge pattern on the gridded microstructure using still motion process: (a) cross sectional image and (b) SEM photograph.
Table 5 Fabrication conditions for gridded microstructure shown in Fig. 16.
Table 3 Fabrication conditions for the conventional method of Fig. 14. One layer (b) Beam intensity (mW/cm2 ) Layer thickness (m) Total layer (EA) Total process time (min)
16.4 10 1 –
Multiple layers (c)
Conditions
16.4 10 20 About 20
Beam intensity (mW/cm2 ) Unit shape (pixel) Layer thickness (m) Total layer (EA) Total still motion image (EA/layer) Shift-pixel (pixel) Frame-time (ms) Total process time (min)
also applied using the conditions shown in Table 4. Fig. 15 shows the results of the fabrication. In Fig. 15(b), the over-cured shape was found because two unit shapes in Fig. 15(a) were projected simultaneously in each direction. When these unit shapes were fabricated separately, the result was more accurate as shown in Fig. 15(c)–(f). Fig. 16 shows a gridded microstructure, which is often called a scaffold in bio-engineering. The conditions for fabrication are shown in Table 5. The unit shape of size of 25 × 25 pixels was used and projected alternately in two directions, as shown in Fig. 16(b). The size of the holes in the completed microstructure was relatively accurate (about 50 m × 50 m). However, the fabrication time took slightly longer than the conventional method.
9.84 W: 25, H: 25 10 5 1804 1 100 About 20
To determine our ability to fabricate a scaffold pattern, a lozenge pattern on gridded microstructure was fabricated under the conditions shown in Table 6. The two different patterns were well bonded, and the shape of the pattern was uniform as shown in Fig. 17(b). In Fig. 18, an asterisk shape was fabricated to make a comparison of accuracy between the conventional process and the still motion method in PSL. The fabrication conditions are shown in Table 7. While rounded corners were found due to light
Table 4 Fabrication conditions for the still motion method of Fig. 15.
2
Beam intensity (mW/cm ) Unit shape (pixel) Layer thickness (m) Total layer (EA) Total still motion image (EA/layer) Shift pixel (pixel) Frame time (ms) Total process time (min)
(b)
(c)
(d)
(e)
(f)
16.4 W: 50, H: 100
9.84 W: 50, H: 100
16.4 W: 50, H: 100
16.4 W: 50, H: 40
100 About 10
100 About 10
9.84 W: 50, H: 40 20 3 1098 1 100 About 10
60 About 7
80 About 9
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Fig. 18. Micro-asterisk using still motion method. (a) Cross-sectional image, (b) SEM photograph, (c) unit shape image, and (d) SEM photograph.
Table 6 Fabrication conditions for lozenge pattern microstructure shown in Fig. 17. Conditions Beam intensity (mW/cm2 ) Unit shape (pixel) Layer thickness (m) Total layer (EA) Total still motion image (EA/layer) Shift pixel (pixel) Frame time (ms) Total process time (min)
9.84 H: 25, V: 25 25 20 (rectangle: 10 each, rhombus: 10 each) (Rectangle: 1804, rhombus: 1913) 1 100 About 22
Table 7 Fabrication conditions for micro-asterisk in Fig. 18.
Beam intensity (mW/cm2 ) Unit shape (pixel) Layer thickness (m) Total layer (EA) Total still motion image (EA/layer) Shift pixel (pixel) Frame time (ms) Total process time (min)
(b)
(d)
16.4 – 10 10 – – – About 10
16.4 H: 25, V: 25 10 10 364 1 100 About 16
superposition in the conventional process, sharp concave corners were fabricated using the still motion method, as shown in Fig. 18(b) and (c).
6. Conclusion To improve the accuracy of a microstructure, we introduced the still motion process for PSL. In this process, a cross section was divided into a series of unit shapes, and they were, in turn, projected onto the resin surface for a frame time. So, it seems to scan unit shape. In the still motion process, the intensity distribution for a cross section is almost uniform, hence the still motion process enables accurate fabrication of a latticed microstructure. The unit shape, beam intensity, and frame time were used as parameters to control the accuracy. Considering the magnification of the imaging system and the curing characteristics of the resin, a suitable value for each parameter was determined. This still motion process can be applied to control the accuracy of a microstructure for applications in various fields such as a scaffold for tissue regeneration, and micro-devices for micro-actuators.
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