# Additive Manufacturing of Bone Scaffolds Using PolyJet and Stereolithography Techniques
## 4 Discussions  
Various 3D-printed scaffold structures have been widely used in tissue engineering, especially in orthopedics for the replacement of damaged and diseased bone [39, 40]. The mechanical properties of 3D-printed bone scaffold structures have to be such that they can withstand applied loads during daily activities. Apart from the mechanical properties, the transportation of nutrients, metabolic waste removal, and gaseous exchange through porous scaffolds are crucial. In 3D-printed scaffold structures, the porosity and pore size play pivotal roles, as these two parameters enable nutrient transportation through the structure for bone regeneration, in addition to providing adequate load-bearing capability to the structure. Therefore, the 3D printed scaffold structures must imitate the natural bone structure. The traditional manufacturing techniques do not provide design freedom to produce complex geometry structures. The use of additive manufacturing techniques such as Polylet and \(\upmu\)SLA enable the fabrication of porous scaffold structures with complex features and varied porosity. In this study, scaffold structures were printed using Polylet and \(\upmu\)SLA techniques. The application of these two techniques for 3D printing of trabecular bone scaffolds was evaluated based on the permeability and mechanical properties. Based on the results, it was observed that samples with a cubic pore shape produced via microstereolithography showed 0.7 times higher permeability compared to Polylet-printed samples. For hexagonal closed packed samples, the microstereolithography results showed 1.5 times higher permeability comparing to Polylet-printed samples; however, the \(\upmu\)SLA technique has some limitations regarding the printing time and build area as compared to the Polylet techniques. The experimentally measured and numerically calculated permeability values for 3D-printed scaffold structures manufactured using both techniques were found to be within the permeability range of bone in healthy individuals. The effects of the specific surface area and surface energy on the permeability of 3D-printed scaffold structures were analyzed. The obtained results demonstrated that an increased specific surface area results in decreased permeability; thus, the specific surface area of the scaffold plays a very important role, because porous structures with different geometries can have different specific surface areas with the same porosity. Several previous studies have been performed on the permeability levels of synthetic and natural bone porous structures, as summarized in Table 8. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Mechanical Properties (MPa)**} & \multicolumn{3}{c}{**Rectangular**} & \multicolumn{3}{c}{**Cylindrical**} \\ \cline{2-7}  & \(\mathbf{x}\)**-Direction** & \(\mathbf{y}\)**-Direction** & \(\mathbf{z}\)**-Direction** & \(\mathbf{x}\)**-Direction** & \(\mathbf{y}\)**-Direction** & \(\mathbf{z}\)**-Direction** \\ \hline Elastic Modulus & 1400 & 1300 & 1600 & 1300 & 1200 & 1600 \\ Yield Strength & 82 & 80 & 96 & 78 & 74 & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Mechanical properties of the 3D-printed standard samples. The permeability levels and mechanical properties of 3D-printed scaffolds structures must match the mechanical properties of natural bone to avoid the stress shielding effect. The mechanical properties of the 3D-printed scaffolds manufactured using both techniques were in the range of healthy bone; however, the microstereolithography results indicated 1.3 times higher mechanical properties for these structures as compared to the Polyjet structures. For trabecular bone, the range of the elastic modulus is 0.2-2 GPa [51]. The yield strength is another important factor that must be taken into account for suitable 3D-printed scaffold structures. The Yield strength values for natural bone have been reported in numerous studies to be between 20 and 193 MPa [51].