# Additive Manufacturing of Bone Scaffolds Using PolyJet and Stereolithography Techniques
## 1 Introduction
Permeability has prime importance in synthetic bone scaffolds, and must be considered when designing a scaffold because it can affect cell migration, cell metabolism, and mass transport of oxygen and nutrients; furthermore, it must be kept within the range found in human bones [15]. [PERSON] et al. showed that experimentally measured and numerically calculated permeability increased with increases in pore size and porosity [18]. [PERSON] et al. displayed the effects of synthetic scaffold permeability fabricated from salt leaching and stereolithography on nutrient transportation by comparing the results from cell culturing and cell seeding experiments, showing better cell interconnectivity and enhanced cell proliferation in the center of the scaffold's interior [19]. [PERSON] et al. used experimental and computational approaches to examine the effects of permeability on bone production for PCL (poly 3-caprolactone) scaffolds printed from a Solidscape 3D printer. The results showed that cartilaginous tissue was well proliferated in the less permeable environment, while for bone tissue regeneration and cell differentiation, higher permeability was more promising [20]. [PERSON] et al. investigated the relationships among the processing conditions, tensile strength, porosity, and permeability of synthetic scaffolds fabricated through SLS and showed that the permeability increases with increases in porosity [21]. With the help of experimental and numerical analyses of cancellous bone, [PERSON] et al. proposed that there is a direct relationship between permeability, porosity, and surface area, and also found that structures with similar porosity can have different levels of permeability because of their different surface areas [22]. [PERSON] et al. utilized [PERSON]'s law to find out the permeability of foam replica porous synthetic scaffolds, the permeability values of which were measured using deionized water and were similar to the permeability of human trabecular bone [23]. With the use of a micro-CT-based FE method and the experimentally verified Kozeny-Carman formulation, [PERSON] et al. showed a correlation between permeability and the trabecular bone microstructure with the use of a constant head permeameter [24]. [PERSON] et al. developed a simple test rig consisting of a constant head to provide a constant hydraulic pressure on the top surface of a test sample, in which they determined the experimental permeability and the determined permeability as 1.1 \(\times\) 10\({}^{-10}\) m\({}^{2}\)[25]. Different test fluids have been used for this type of analysis, such as water and gasses. [PERSON] et al. used gas as a test fluid in permeability analysis in his study to avoid pore blockage and hydrolysis [26]. [PERSON] et al. determined the permeability values for a human bone using raw linseed oil as a test fluid, which ranged between 0.40 \(\times\) 10\({}^{-9}\) m\({}^{2}\) and 11 \(\times\) 10\({}^{-9}\) m\({}^{2}\)[27]. Various experimental studies have been carried out to measure the intrinsic permeability of natural bone using water as a test fluid in different directions, with values lying between 10\({}^{-10}\) m\({}^{2}\) and 10\({}^{-9}\) m\({}^{2}\)[27; 28; 29]. The permeability values measured by authors in previous studies for human natural bone varied between 10\({}^{-11}\) m\({}^{2}\) and 10\({}^{-8}\) m\({}^{2}\)[29]. Synthetic bone scaffolds must have mechanical properties, such as the compressive strength and elastic modulus, similar to those of natural human bone to avoid the effects of stress shielding and osteoporosis [30]. Compression tests can be performed to investigate the mechanical properties of synthetic scaffolds. [PERSON] et al. numerically investigated the mechanical properties of poly(D, L-lactide-co-e-caprolactone)-poly (D, L-lactide)-resin-based synthetic scaffolds in terms of the architectural parameters, with the results showing that the mechanical properties of the synthetic scaffolds were within the same range as those of human bone [31].