INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 3, pp. 445-449 MARCH 2012 / 445 DOI: 10.1007/s12541-012-0057-8

1. Introduction

Microstereolithography (MSTL) technology can be classified into line-scan-based MSTL and projection-based MSTL (pMSTL) according to the method used for generating 2-D patterns. In the line-scan-based MSTL, a 2-D pattern on a layer is generated by scanning a focused laser spot line by line. In contrast, in the pMSTL, one 2-D image generated by a digital micromirror device (DMD) or a liquid crystal display (LCD) is illuminated at a time, allowing increased fabrication speed of a 3-D structure by stacking 2-D patterns layer by layer.

This technology has recently been applied to the fabrication of porous 3-D scaffolds for artificial tissue regeneration. Lu et al. fabricated a porous 3-D scaffold by pMSTL using photo-cross-linkable poly (ethylene glycol) diacrylates. Cho et al. developed an indirect method for fabricating 3-D scaffolds, combining pMSTL technology with a sacrificial molding process. Many of the studies related to scaffold fabrication have focused on the biomaterials. However, fabrication of a structurally suitable scaffold for a target tissue is also important when applying these technologies to clinical

medicine. To fabricate a complex structure having the mimetic shape of a target tissue, we have developed a new algorithm based on projection image generation. The procedure uses the STL file format as the raw data for a 3-D model. This file format is widely employed in various fields related to manufacturing technology. An STL file can be obtained as a model designed by a commercial CAD program, or as a bio-image file from computerized tomography (CT) or magnetic resonance imaging (MRI). Hence an algorithm employing the STL file format can be utilized to fabricate a structurally complex scaffold from the bio-imaging data of a target tissue, as well as from a 3-D model designed with a commercial CAD program.

As stated before, the format of fabrication data used in the pMSTL is different from that of the line-scan-based MSTL. The pMSTL requires fabrication data based on bitmap information of a 2-D image, whereas the line-scan-based MSTL uses coordinates. In our study, we used a slicing process, which is widely used for line-scan-based MSTL, in the algorithm for converting STL file format into 2-D closed loops, as shown in Figs. 1-4. This algorithm also includes a new process for converting the sliced loops into bitmap

Projection Image-generation Algorithm for Fabrication of a Complex Structure using Projection-based Microstereolithography

Jin Woo Jung1, Hyun-Wook Kang1, Tae-Yun Kang1, Jeong Hun Park1, Jaesung Park1 and Dong-Woo Cho1,2,# 1 Department of Mechanical Engineering, Pohang University of Science and Technology, 77 Cheongam-ro, Nam-gu, Pohang, Gyeongbuk, Korea, 790-784

2 Division of Integrative Biosciences and Biotechnology, Pohang University of Science and Technology, 77 Cheongam-ro, Nam-gu, Pohang, Gyeongbuk, Korea, 790-784 # Corresponding Author / E-mail: dwcho@postech.ac.kr, TEL: +82-54-279-2171, FAX: +82-54-279-5899

KEYWORDS: Microstereolithography, Slicing algorithm, STL, Tissue engineering, Vascular network

Microstereolithography (MSTL) is a SFF technology that has been used to fabricate 3-D scaffolds in tissue engineering. Projection-based microstereolithography (pMSTL) offers the advantage of increased fabrication speed compared with a line-scan-based MSTL by creating 2-D patterns with single-section image exposure and then stacking them. To fabricate a complex 3-D structure for a target tissue (liver, blood vessel, etc.) using the pMSTL system, we introduce a new algorithm that automatically generates projection image information. The procedure uses the STL file format as the raw data for a 3-D model. First, the STL file data are converted into slicing data composed of closed loops, including layer thicknesses. Projection image data are then generated from the closed loops calculated during the slicing process. Finally, the projection image data are converted into pixel information. The proposed technique is evaluated by fabricating a complex 3-D vascular network structure, and is shown to be quite practical for automated fabrication of complex 3-D structures in tissue engineering.

Manuscript received: March 28, 2011 / Accepted: September 28, 2011

© KSPE and Springer 2012

446 / MARCH 2012 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 3 data composed of pixels, as described in Figs. 5-7.

Fig. 1 The new projection image-generation algorithm

Fig. 2 Process of slicing for loop construction on each layer

Fig. 3 Valid or invalid intersection cases between a polygon and a plan

Fig. 4 Calculation of an intersection line between a polygon and a plane

To evaluate the new algorithm, we fabricated the structure of a vascular network using projection image data converted from a 3-D STL model.

2. Projection Image-generation Algorithm

2.1 Slicing process for STL file data pMSTL is a technology for making 3-D structures via a layer-

by-layer process of stacking 2-D patterns in the z-direction. Fabrication data for the 2-D pattern of each layer are essential for this type of procedure. The slicing process uses the STL file data to generate closed loops for each layer, and these loops are the basis of the 2-D patterns.

The STL file format incorporates polygon data consisting of one unit normal vector and three vertices in rectangular coordinates. The surface of the model is approximated by polygons. In the slicing calculations, planes parallel to the x–y plane are generated at regular intervals. Valid intersections between the polygons and the

Fig. 5 Procedure for projection image generation

Fig. 6 Projection image generation using a narrower lines interval

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 3 MARCH 2012 / 447

planes are then determined according to the rule illustrated in Fig. 3. The lines of intersection between the polygons and the planes are calculated and used to construct the closed loops for each layer. Fig. 2 and Fig. 4 illustrate the calculation of the lines of intersection and the slicing process.

2.2 Projection image generation

Generally speaking, a 2-D image is composed of pixels in a graphical editing program. A pixel can be regarded as a single point in the shape of a square. Images in the closed loops must also be composed of pixels. Accordingly, we have devised an effective and stable method for obtaining images composed of pixels.

In the first step, parallel lines of infinite length are generated on the layers at small, uniform intervals. Next, lines lying outside the closed loops are eliminated by calculating the points of intersection between the lines and the loops. A line width equal to the interval between the lines is then added to the line information. In this process, a higher resolution for the projection image data can be obtained by using a narrower interval between the lines, but this will result in a longer execution time for the algorithm. Fig. 5 and Fig. 6 illustrate the method of projection image generation.

In the second step, projection image data have to be changed into bitmap data composed of pixels. The color (white or black) of each pixel is determined by checking whether the pixel occupies projection image data. If a pixel is placed on projection image data,

the pixel will be white. If not, the pixel will be black. Fig. 7 shows how the ends of some lines are converted into bitmap data.

3. Design and Fabrication of a Solid Model

3.1 Design of a vascular network Several attempts have been made to construct a vascular

network within a scaffold for the delivery of oxygen and various nutrients and the removal of wastes. A 3-D biomimetic vascular network requires a number of rules of geometric organization, and it is necessary to utilize a commercial CAD program for constructing such a complex design. Therefore, we have used our algorithm to construct a 3-D vascular network from a CAD model. The vascular network was designed selectively based on Murray’s law, and the branching plane of each daughter branch was chosen to be perpendicular to that of the parent. Fig. 8 shows the 3-D vascular network and its dimensions, designed via SolidWorks, a commercial CAD program.

3.2 Conversion of 3-D CAD model into projection image data

Fig. 10 describes the procedure for generating the projection images from the STL file format. First, the vascular network design was exported to STL format via SolidWorks. As Fig. 10(a) shows, the surface of the model was represented by triangular facets in STL format. Second, the STL data were converted to slicing data and then to projection image data via our algorithm. Slicing data consist solely of loop shapes for each layer, whereas projection image data also include the lines filling up the interiors of the loops. In the process, the layer thickness and line pitch/width were set at 50 μm and 10 μm, respectively.

3.3 Fabrication using the pMSTL system

Fig. 9 shows the pMSTL system used in this research. It is composed of a 500 W mercury ultra-violet (UV) lamp (used as the light source), a digital micromirror device (DMD), and a 3-axis

Fig. 7 Generation of bitmap data

(a) Vascular network (b) Diameter of each of tube and height

Fig. 8 Design of 3-D vascular network using a commercial CAD program

448 / MARCH 2012 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 3 stage system with a 0.1 μm resolution/100 mm stroke. When the DMD (1,024 x 768 pixels) projects a 2-D image onto the surface of a liquid photopolymer with sub-micron resolution, the image is solidified by photopolymerization, and a 3-D structure can then be constructed by stacking the 2-D patterns.

The generated projection image data were converted into CNC code which is compatible with pMSTL system. And FA1260T (SK Cytec Inc., Korea), UV-curable polymer, was used to fabricate the structure. The fabrication condition is shows in Table 1. 4. Conclusions

The final goal of tissue engineering is the regeneration of

artificial tissue with perfect biological and mechanical functions. Given that tissue generally has complex and random 3-D shapes, a technology for fabricating such a structure is a significant factor in realizing this goal. In this research, a new automated image-generation algorithm was proposed for constructing complex biomimetic 3-D shapes with pMSTL. A 3-D vascular network was successfully fabricated using this algorithm. In the future, we will attempt to apply this technology to fabricate a new scaffold, including a vascular network for nutrient and oxygen supply.

Fig. 9 Schematic drawing and photograph of pMSTL system

Table 1 The fabrication conditions in pMSTL system

Power 500 W Wavelength 375 nm Resolution 14 μm

Illumination time of a layer 10 sec

(a) STL file format

(b) Slicing data

(c) Projection image data

Fig. 10 Procedure for generating projection images from the STL file format of the 3-D vascular network model

Fig. 11 Vascular network fabricated by the pMSTL system

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 13, No. 3 MARCH 2012 / 449

ACKNOWLEDGEMENT This research was supported by the National Research

Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0018294) and the World Class University program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-2008-000-10105-0). REFERENCES

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