# Additive Manufacturing of Bone Scaffolds Using PolyJet and Stereolithography Techniques
## 2 Materials and Methods  
### Design and Development of 3D-Printed Scaffold Structures  
S scaffolds are not structurally equivalent to natural human bone because of the complex and heterogeneous physiological structure of the native tissue; therefore, studies are needed focused on simple designs for scaffolds that are functionally equivalent to the tissue to be repaired in terms of the pore size, porosity, pore shape, and mechanical properties. Here, the pore sizes were chosen on the basis of cell responses. Three different porosity levels were chosen to reduce the mismatch between the stiffness of the scaffold and the native tissue in order to mitigate the stress shielding effect, while the pore shapes were chosen that could offer the best mechanical properties. A healthy human bone contains pore size ranges from 300 to 600 \(\upmu\)m and even ranging up to 3000 \(\upmu\)m in cases of osteoporosis, with porosity ranges of 30-90% ([PERSON], 2018). Keeping all of these points in mind, 3D CAD models of bone scaffold structures with cubic and hexagonal closed packed pore geometries were selected and designed with Creo Parametric 4.0 and SolidWorks. Cubic and hexagonal closed packed structures are the simplest porous unit cell designs, with all struts at angles of 90\({}^{\circ}\) and 60\({}^{\circ}\) to each other. Each CAD model was 15 mm in height and 30 mm in diameter, including the encased cylindrical structure (see Figures 1 and 2). The additively manufactured scaffolds had a maximum acceptable dimensional error of 0.54 mm, which is an acceptable dimensional error for the 3D printing of scaffolds ([PERSON], 2018). Previous studies have shown that these cubic and hexagonal closed packed structures can meet the mechanical loading requirements of bone scaffolds ([PERSON], 2018; [PERSON], 2018). Conventional manufacturing techniques, such as casting and injection molding, can be used to fabricate scaffolds with complex geometries; however, the use of computer-aided design (CAD) in additive manufacturing enables the fabrication of highly complex geometrical shapes for high-tech industries such as aerospace and biomedical engineering. In order to demonstrate the printing capabilities of the selected methods of additive manufacturing, 3D scaffold CAD models were designed for cubic and hexagonal closed packed structures, as presented in Table 1. Pore sizes were measured as the minimum diameter of the inscribed circle in hexagonal closed packed scaffold geometries and the inner length of the sides of the cube. Figure 1a,c and b,d show the isometric and front views of the 3D CAD models of hexagonal closed packed and cubic pore geometry scaffolds structures. Figure 1e,f represents the optical image of the 3D-printed scaffold structures taken using a Keyence Digital VHX-2000 microscope. Both cubic and hexagonal closed packed structures were encased into a cylindrical structure to clamp them into a rig designed for permeability testing. Before 3 DP, CAD models were saved in a simple, light, and easy-to-handle format, i.e., stl format, which is widely used in additive manufacturing. The PolyJet printer and stereolithography (\(\upmu\)SLA) printer were selected to print cubic and hexagonal closed packed scaffold structures. STRATASYS and ASIGA propose a large range of materials for 3D printing. In this study, commercially available VeroClear (STRATASYS) and PlasWhite (ASIGA) materials were chosen to print the scaffold structures. For PolyJet 3D printing (3 DP), the stl files of scaffold structures were sent to the PolyJet printer using integrated software. For the PolyJet printer, the time take to print the six scaffold samples was 90 min. All samples (28) were printed within 7 h. The printing time for the hexagonal pore geometry scaffold sample was slightly higher than the cubic pore geometry due to the complexity of the pore shape. After printing, the support material SUP706 was removed with a water jet at a pressure of 30 bars followed by compressed air cleaning at 4 bars for complete removal of residuals.