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Sensors and Actuators A 167 (2011) 117–129

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

till motion process for improving the accuracy of latticed microstructures inrojection microstereolithography

n-Baek Park1, Young-Myoung Ha1, Seok-Hee Lee ∗

chool of Mechanical Engineering, Pusan National University, Busan 609-735, Republic of Korea

r t i c l e i n f o

rticle history:eceived 16 September 2010eceived in revised form9 November 2010ccepted 27 December 2010

a b s t r a c t

Projection microstereolithography (P�SL), which involves fabricating a microstructure using patternedlight with a dynamic mask such as a liquid crystal display (LCD) and a digital micromirror device (DMD), isone of the additive manufacturing technologies. A nonuniform light intensity distribution of the cross sec-tion to be illuminated affects the accuracy of the microstructure. In other words, under-cure or over-cureoccurs by light superposition in the corner of a latticed microstructure. This phenomenon will offset the

vailable online 31 December 2010

eywords:rojection microstereolithography (P�SL)igital micromirror device (DMD)till motion process

advantages of the P�SL (e.g., its simple process and fast fabrication time). Furthermore, accurate fabrica-tion 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 improvethe accuracy of latticed microstructure. We used continuous projection of a unit shape image, which isdetermined by the cross section in a layer. Some latticed microstructures have been more accurately

tion p

fabricated by the still mo

. Introduction

Microstereolithography (�SL), which is based on the stere-lithography (SL) process in additive manufacturing, is aicro-fabrication technology [1]. Recently, �SL technology has

een applied in various industrial fields due to advances in these of photocurable resin material. In biotechnology, scaffolds forissue generation have been studied using bio-adaptable materi-ls [2–4]. Micro-needles for transdermal delivery systems (TDDSs)ave been studied [5]. In the micro-fabrication area, a diffuser forhe dispersion of light in optical fibers, microlens arrays [6–8], and

icrofluidic devices [9,10] have been researched.�SL technology is divided into the scanning type (S�SL) and the

rojection type (P�SL). The scanning type builds a layer using thecanning of focused light. Galvano-mirror or X–Y stages are gener-lly used to guide the scanning path. The projection type completeslayer using one irradiation of patterned light that is controlled bydynamic pattern generator such as a liquid crystal display (LCD)

nd digital micromirror device (DMD) [11–17]. The S�SL can pre-isely fabricate a microstructure due to the size of the focused beamnd the precision of the stage, and the fabrication time is relativelyong. The P�SL has a relatively fast fabrication time, and the inten-

∗ Corresponding author. Tel.: +82 51 510 2327; fax: +82 51 582 0988.E-mail addresses: mechanic@pusan.ac.kr (I.B. Park), jinjjaguy@pusan.ac.kr

Y.M. Ha), sehlee@pusan.ac.kr (S.H. Lee).1 Tel.: +82 51 510 1476; fax: +82 51 510 0685.

924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2010.12.023

rocess compared to conventional P�SL processes.© 2011 Elsevier B.V. All rights reserved.

sity distribution of the patterned light has an effect on the accuracyof the microstructure [11].

To improve the accuracy of a layered microstructure, multiplefabrications of a sacrificial layer [1], a cross section segmenta-tion method considering the light intensity distribution [7], theapplication of a grayscale effect on a monochrome cross sec-tion [18–21], and control of the stacking direction [22] havebeen studied. However, these approaches have not shown a goodresult for microstructures with sharp edges (such as latticedmicrostructures) because the light intensity distribution is notcontrolled.

Several microfabrication technologies have been used to fabri-cate a latticed microstructure. A fused deposition modeling (FDM)process, which melts the material and squeezes it via a micro-nozzle, has been widely used to build latticed structures suchas a scaffold [23,24]. However, flections due to self-weight andan increase in fabrication time due to filling of the interior partdecrease the strength of the FDM [11]. The three-dimensional print-ing (3DP) process has a limitation on fabricating the microstructuredue to the difficulty of removing residual material in a structurewith a micro-hole [11].

In this research, the still motion process (SM-process), whichuses the continuous projection of a unit shape image based on the

P�SL, is introduced to improve the accuracy of a microstructure.The SM-process does not affect the system configuration and canbe applied to any dynamic pattern generator. In particular, it canbe effectively applied to fabricate latticed microstructures such asscaffolds with high porosity and sharp edges.

118 I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129

F

2

ila

Ps

Start

3D CAD model to STL format

Slicing STL with a certain layer thickness

Generating cross-sectional images

Initializing the fabrication system(PμSL)

Generating a new liquid layer

Transferring a cross-sectional image to

the DMD

Opening shutter for the exposure time

Last layer?

Pulling, rinsing, and post curing the part

N

Y

Pre

-pro

cess

Fab

rica

tion p

roce

ss

Post process

ig. 1. Schematic of our DMD-based projection microstereolithography system.

. Projection microstereolithography (P�SL)

Fig. 1 shows the configuration of our P�SL system. The systemncludes a DMD for dynamic pattern generation, optical devices foright delivery, an X–Y-stage for shifting the position of illumination,

Z-stage for stacking layers, a light source, and a controller [6–8].

Fig. 2 shows the conventional fabrication process used in the�SL system. A three-dimensional (3D) model in STL format isliced at a certain height. Each cross section is then translated into

End

Fig. 2. Flowchart of the conventional fabrication process in P�SL [13,19,20,26].

Ec

-w ww0

E

Emin Emin

Curing area

Ec

MicromirrorNo. 65432 7

Emin Emin

-w ww0

1

Curing area

E

(b) (a)

Fig. 3. Energy distribution of patterned beam by the DMD: (a) one micromirror and (b) several micromirrors [12].

3 1

2

Micromirror

1 2

42

2

3 1

Micromirror

Irradiation

Digital Mirror Device(DMD) 4

4 2

3 2

44

Cross-sectional image

Fig. 4. Under-cure and over-cure at the edge of a cross section due to Gaussian superposition.

I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129 119

F oss seca

aitrD

ig. 5. Intensities of a patterned beam at the position of the resin surface (a), (b) crt the corner.

monochrome image file using BMP format. After the system isnitialized, a sacrificial layer for leveling the first layer is built. Byhe movement of the Z-stage, a liquid resin covers the cured sac-ificial layer. The bitmap file for the first layer is imported into theMD, and patterned light via optical devices (lenses and a mir-

tions of bitmap format. (c) and (d) Intensity distribution of each cross section, and

ror) is illuminated on the resin surface. This procedure is continueduntil the last layer is built. After the fabrication is completed, theresidual liquid resin is removed by a solvent. The conventional fab-rication process in the P�SL is simple because one exposure fora layer is generally used. However, a non-uniform intensity dis-

120 I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129

itical

toe

3

3

lPpwgsotoc[

Fig. 6. Predicted curing shapes for various cr

ribution of a patterned beam causes building inaccuracy such asver-cure and under-cure. Hence, a solution that provides uniformxposure energy on the resin is required.

. Characteristics of patterned beam

.1. Energy distribution

In stereolithography, photopolymerization proceeds in the fol-owing order: photoinitiation, propagation, and termination [25].hotoinitiation is caused by free radicals produced from the decom-osition of initiators. The radical adds to a monomer and reactsith the double bond of another monomer. It keeps adding to a

rowing chain, one after another, which is called propagation. Thistep ends when the free radical site is inhibited by some impurities

r by a termination process [11]. During the photopolymerization,he critical exposure energy Ec, which is known as the thresh-ld exposure for phase-change, depends on photocurable resinomposed of a photoinitiator, a monomer, and a photoabsorber27].

exposures: (a) rectangle and (b) cross shape.

The distribution of a patterned beam by one micromirror of theDMD is assumed to be Gaussian. Several micromirrors correspond-ing to a cross section is considered to be a Gaussian superposition,as shown in Fig. 3. To build a layer by photopolymerization, energyover the critical exposure Ec is required. At that time, the curingwidth is −w to w as shown in Fig. 3(a). The intensity distributionof the patterned beam on the resin surface is affected by the mag-nification of an objective lens. Hence, a diffuser lens could be usedto reduce the uniformity of the intensity, but an aberration anddiffraction could also occur [19].

3.2. Effect of superposition

Superposition of a Gaussian distribution decreases the resolu-tion 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 con-sidered with respect to the wavelength. Gaussian superpositionenables the layer to cure evenly on the resin surface due to energycompensation between neighboring micromirrors. However, an

Actuators A 167 (2011) 117–129 121

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Objective lens

DMD

Unit-shape

Direction

Resin surface

Platform

Z-stage

1

1

5

5

Fig. 7. Schematic of the still motion method in P�SL.

Converting to an STL file

Slicing the STL file according to the layer

thickness

CAD model

Converting to a Bitmap image

(Cross-sectional images)

Segmenting the cross-sectional image by the

unit-shape

Pulling the fabricated part/ Rinsing and post

curing

Yes

Mo

del

ing

an

d P

re-p

roce

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abri

cati

on

Post-process

Transferring the unit shape to the DMD and

opening the shutter during a frame-time

Initializing the fabrication system

Generating a new liquid layer

Last Unit shape

Last Layer?

No

Yes

No

I.-B. Park et al. / Sensors and

naccurate shape such as over-cure and under-cure is normallyade at the edge of a cross section, as shown in Fig. 4. In Fig. 4,cross section of a bitmap format includes white and black pixels

o present the cross shape. The emitted light from the light sources patterned by the DMD displaying the cross section, and is trans-erred onto the resin surface. The degree of light intensity in therojected patterned beam is presented within a small rectangle,s shown in Fig. 4. As shown in the figure, micromirror numberplaced on a convex corner has less intensity than neighboringicromirrors (numbers 1 and 2). In contrast, micromirror numberplaced on a concave corner has higher intensities than the oth-

rs. This phenomenon causes under-cure and over-cure when thentensity is lower and higher, respectively, than the critical expo-ure.

To investigate the effect of light superposition, two cross sec-ions were used as shown in Fig. 5. The cross-sectional imageas transferred to the DMD, and the intensity distribution of theatterned beam was detected by a beam profiler (FX66TM, Ophirptronics Co., USA) that was placed on the platform. The maximum

ntensity of 16.4 mW/cm2 was used.Fig. 5(c) and (d) show the intensity distribution of each cross

ection. At the corner, the intensity is smaller than at other regions.he intensity at the center region is higher due to the superpositionf light. Therefore, the corner region is hard to cure or distort whenhighly viscous resin is used. Fig. 6 shows the predicted curing

hape implemented with pixels that have higher intensity than theritical exposures of 1–4 mJ/cm2. This means that the curing shapeainly depends on the critical exposure.When a patterned beam with an intensity of 16.4 mW/cm2 illu-

inates a resin at the critical exposure of 1 mJ/cm2 for 1 s, anver-cure shape is produced. The higher critical exposure the resinas, the more accurate the curing shape. However, the intensity ofmitted light from the lamp is limited, so resin with a high crit-cal exposure does not always produce an accurate curing shape.pecifically, under-cure can occur in the edge region, as shown inig. 4. To reduce inaccuracy due to superposition of the patternedeam, we introduce the still motion process.

. Still motion process in P�SL

.1. Method

Still motion is a method of displaying many images, one afternother, as frames in computer graphics [29]. Using this method,e applied a continuous projection of a unit shape for the comple-

ion of a layer in P�SL. This approach allowed us to improve theccuracy of the microstructure in P�SL.

Fig. 7 shows a schematic of the still motion method. The methodan be applied whenever a pattern generator such as LCD or DMDs used. Considering the curing characteristics of the resin, it isssumed that the cross section of a line with a certain width isomposed of five rectangular unit shapes. Each unit shape is num-ered in order according to the curing direction, and is imported

nto the DMD one-by-one for projection during the frame time. Therame time, which is the period required to change the unit shapen the DMD, has the same meaning as exposure time in conven-ional P�SL. The overall procedure for the still motion process ishown in Fig. 8.

.2. Unit shape

In P�SL, a microstructure is built by stacking cross sections. Topply the still motion method using variable cross sections, unithapes appropriate for each cross section must be generated. Thentensity of patterned light by a unit shape must be larger than

END

Fig. 8. Overall procedures for the still motion process in P�SL.

122 I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129

on dir

tut

tptgo

Fig. 9. Still motion method in P�SL. (a) Latticed cross section, (b) projecti

he critical exposure of the resin. Fig. 9 shows an example for thenit shape of a latticed cross section, and the projection direc-ion.

A rectangle was chosen for the unit shape of a latticed cross sec-

ion as shown in Fig. 9. For continuous curing on the resin surface,art of the unit shape was overlapped. To minimize the fabricationime, two groups of unit shape were used as shown in Fig. 9(b). Oneroup was continuously projected to the horizontal direction. Thether was then projected to the vertical direction. If a cross section

ection for the unit shape, and (c) continuous projection of the unit shape.

is changed, the unit shape must also be changed. In this case, thesame unit shape was used.

In scanning �SL (S�SL), a resin vat is generally moved to scanthe focused beam [21]. On the other hand, a series of unit shapes is

projected at intervals of shift-pixel during a frame time correspond-ing to an exposure time in the still motion method. Therefore, thismethod can prevent a fabrication error due to the movement ofthe system, and increase the accuracy of a microstructure due toimproved light intensity distribution.

I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129 123

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 frametime, (d) curing depth according to the beam intensity, and (e) curing depth according to the frame time.

124 I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129

1

2

3

4

Shift-pixel

Resin surface

Unit shape No.

Unit shape

31 1 41 2

Shape(Cd )

1 - 2 1 - 3 1 - 4

Resin surface

ding t

4

dath733tCcEb

C

4

4

catial

ttptc

a frame time within 40 ms provided enough accuracy in the unitshape of 100 pixels, as shown in Fig. 11(c). Therefore, the accuracyof the microstructure can be controlled by the frame time.

TP

1 2 3 4

(a)

Fig. 12. Curing shape accor

.3. Curing properties of photocurable resin

In this study, photocurable resin composed of 1,6-hexanedioliacrylate (HDDA, Miwon Chem. Tech. Co., Korea) as a monomer,nd 2,2-dimethoxy-2-phenylacetonphenone (DMPA, Fisher Scien-ific Co., USA) of 5 wt% as a photoinitiator, were used. HDDA hasigh light transmission and curing speed, and low viscosity (belowcps). DMPA has low yellowing and is proper to the wavelength of65 nm. These chemicals were mixed using a magnetic stirrer forh at room temperature. We called the mixture H2. Fig. 10 shows

he 3D model for the curing experiment, and a graph of curing depthd versus the exposure energy. For a specimen fabricated using theonventional P�SL process, the resin had a critical exposure energyc of 3.784 mJ/cm2, and a light penetration depth Dp of 440.728 �m,ased on the Beer–Lambert law as shown in Eq. (1).

d = Dp ln(

E

Ec

)(1)

.4. Control parameters

.4.1. Frame timeIn the still motion method, the parameters used to control the

uring shape are the intensity of the light source, the frame time,nd the size of the shift pixel. To investigate the curing characteris-ics according to these parameters, a specimen was used as shownn Fig. 11(a). The top layer of the specimen was fabricated with vari-ble conditions as shown in Table 1. Four pillars supporting the topayer were fabricated using the same conditions.

Generally, the curing width and depth is presented according to

he exposure energy. However, we used the beam intensity becausehe frame time is a major parameter, and the exposure energy of theatterned beam can be controlled by frame time corresponding tohe speed change of the unit shape in the still motion method. Theuring width and depth were proportional to the beam intensity

able 1rocess conditions for the specimen shown in Fig. 11.

Conditions Unit shape (pixel)

Width (W) Height (H)

Curing width (�m) (b) 50 100(c) 50 100–40

Curing depth (�m) (d) 50 100(e) 50 100–20

(b)

o the size of the shift pixel.

as shown in Fig. 11(b) and (d). The difference in curing width fora beam intensity over 6.56 mW/cm2 was within 40 �m, as shownin Fig. 11(b). The magnification of the optical system was about2.1 times; thus, the exact microstructure can be fabricated with anintensity of 6.56 mW/cm2.

In the same unit shape, the curing width and depth were propor-tional to the frame time as shown in Fig. 11(c) and (e). For example,

Fig. 13. Curing depth and width according to the shift pixel.

Shift (pixel) Beam intensity (mW/cm2) Frame time (ms)

10 3.28–16.4 10010 16.4 20–10010 3.28–16.4 10010 16.4 20–100

I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129 125

Fig. 14. Fabrication of cross shape microstructure using the conventional method in P�SL.

Fig. 15. Fabrication of cross shape microstructure using the still motion method in P�SL.

126 I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129

F l imagp

4

psbtscF

TF

ig. 16. Gridded microstructure using the still motion method: (a) cross sectionahotograph.

.4.2. Shift pixelIn this study, a shift pixel is defined as the distance between the

osition of a projected unit shape and that of the next projected unithape. The shift pixel plays a role in combining the cured regionsy unit shapes on the resin surface. A narrow shift pixel increaseshe overlapped area, and over-curing occurs due to the excessive

uperposition of light. On the other hand, a widened shift pixelauses uneven curing depth and incomplete building, as shown inig. 12.

able 2abrication conditions for the effect of the shift pixel.

Conditions

Unit shape (pixel) W: 50, H: 100Shift pixel (pixel) 2, 3, 4, 5Beam intensity (mW/cm2) 16.4Frame time (ms) 100

e, (b) unit shapes in each direction, and (c) scanning electron microscopy (SEM)

Table 2 shows the fabrication conditions for the experiment ofcuring 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 Tinu-vin, which is used to control the curing depth. Hence, the role ofthe shift pixel was limited to combining the cured regions of unitshape.

5. Fabrication examples

To compare the still motion method with the conventionalmethod in P�SL, a cross shape was fabricated as shown in Fig. 14.

The fabrication conditions are shown in Table 3. For the con-ventional method, the sharp corner was rounded due to thesuperposition of the patterned beam as shown in Fig. 14(b). Thisrounded shape has been founded in sharp corners, as shown inFig. 14(c). For the same cross section, the still motion method was

I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129 127

Fig. 17. Fabrication of lozenge pattern on the gridded microstructure using still motion process: (a) cross sectional image and (b) SEM photograph.

Table 3Fabrication conditions for the conventional method of Fig. 14.

One layer (b) Multiple layers (c)

Beam intensity (mW/cm2) 16.4 16.4

at

stw

asaTtt

Table 5Fabrication conditions for gridded microstructure shown in Fig. 16.

Conditions

Beam intensity (mW/cm2) 9.84Unit shape (pixel) W: 25, H: 25Layer thickness (�m) 10Total layer (EA) 5Total still motion image (EA/layer) 1804

TF

Layer thickness (�m) 10 10Total layer (EA) 1 20Total process time (min) – About 20

lso applied using the conditions shown in Table 4. Fig. 15 showshe results of the fabrication.

In Fig. 15(b), the over-cured shape was found because two unithapes in Fig. 15(a) were projected simultaneously in each direc-ion. When these unit shapes were fabricated separately, the resultas more accurate as shown in Fig. 15(c)–(f).

Fig. 16 shows a gridded microstructure, which is often calledscaffold in bio-engineering. The conditions for fabrication are

hown in Table 5. The unit shape of size of 25 × 25 pixels was usednd projected alternately in two directions, as shown in Fig. 16(b).he size of the holes in the completed microstructure was rela-ively accurate (about 50 �m × 50 �m). However, the fabricationime took slightly longer than the conventional method.

able 4abrication conditions for the still motion method of Fig. 15.

(b) (c)

Beam intensity (mW/cm2) 16.4 9.84Unit shape (pixel) W: 50, H: 100 W: 50, H: 10Layer thickness (�m)Total layer (EA)Total still motion image (EA/layer)Shift pixel (pixel)Frame time (ms) 100 100Total process time (min) About 10 About 10

Shift-pixel (pixel) 1Frame-time (ms) 100Total process time (min) About 20

To determine our ability to fabricate a scaffold pattern, a lozengepattern on gridded microstructure was fabricated under the con-ditions shown in Table 6. The two different patterns were wellbonded, and the shape of the pattern was uniform as shown inFig. 17(b).

In Fig. 18, an asterisk shape was fabricated to make a compar-ison of accuracy between the conventional process and the stillmotion method in P�SL. The fabrication conditions are shownin Table 7. While rounded corners were found due to light

(d) (e) (f)

9.84 16.4 16.40 W: 50, H: 40 W: 50, H: 100 W: 50, H: 40

203

10981

100 60 80About 10 About 7 About 9

128 I.-B. Park et al. / Sensors and Actuators A 167 (2011) 117–129

Fig. 18. Micro-asterisk using still motion method. (a) Cross-sectional image

Table 6Fabrication conditions for lozenge pattern microstructure shown in Fig. 17.

Conditions

Beam intensity (mW/cm2) 9.84Unit shape (pixel) H: 25, V: 25Layer thickness (�m) 25Total layer (EA) 20 (rectangle: 10 each, rhombus: 10 each)Total still motion image (EA/layer) (Rectangle: 1804, rhombus: 1913)Shift pixel (pixel) 1Frame time (ms) 100Total process time (min) About 22

Table 7Fabrication conditions for micro-asterisk in Fig. 18.

(b) (d)

Beam intensity (mW/cm2) 16.4 16.4Unit shape (pixel) – H: 25, V: 25Layer thickness (�m) 10 10Total layer (EA) 10 10Total still motion image (EA/layer) – 364Shift pixel (pixel) – 1Frame time (ms) – 100Total process time (min) About 10 About 16

, (b) SEM photograph, (c) unit shape image, and (d) SEM photograph.

superposition in the conventional process, sharp concave cor-ners were fabricated using the still motion method, as shown inFig. 18(b) and (c).

6. Conclusion

To improve the accuracy of a microstructure, we introduced thestill motion process for P�SL. In this process, a cross section wasdivided into a series of unit shapes, and they were, in turn, pro-jected onto the resin surface for a frame time. So, it seems to scanunit shape. In the still motion process, the intensity distribution fora cross section is almost uniform, hence the still motion processenables accurate fabrication of a latticed microstructure. The unitshape, beam intensity, and frame time were used as parameters tocontrol the accuracy. Considering the magnification of the imaging

system and the curing characteristics of the resin, a suitable valuefor each parameter was determined. This still motion process canbe applied to control the accuracy of a microstructure for applica-tions in various fields such as a scaffold for tissue regeneration, andmicro-devices for micro-actuators.

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I.-B. Park et al. / Sensors and

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