Page 75 - Development of Functional Scaffolds for Bone Tissue Engineering Using 3D-Bioprinting of Cells and Biomaterials - Yasaman Zamani

the scaffold with larger void size, the scaffold enters the plastic deformation region at lower stress values which means lower tensile modulus. The tensile strength in the lower half of the 2-region gradient scaffold was higher compared with the 3-region gradient scaffold. The 2-region gradient scaffold had a void size of 0.6 mm in the lower half, while the 3-region gradient scaffold had a void size of 0.9 mm and 0.6 mm in the lower half resulting in reduced tensile strength.
We found that the compressive strength of 3D-printed PCL scaffolds was higher in the scaffold layer-by-layer building direction compared with the side direction. In addition, the stress- strain response of scaffolds to compressive force was different in the building direction compared with the side direction. When compressed in the building direction, scaffolds behaved elastically in the initial linear region and then reached a plateau stage of roughly constant or slightly increasing stress. However, when compressed in the side direction, scaffolds exhibited a distinctive yield point after the initial elastic region and then collapsed as evidenced by a sharp decrease in stress values. This can be explained by the orientation of struts in the scaffold in contrast to the direction of the force. When force was applied in scaffold building direction, struts of all layers were perpendicular to the loading direction. Therefore, struts were compressed against each other in their contact points and could recover when load was removed resulting in elastic deformation (Fig. 10a). Only at large compressive strains, the deformation was unrecoverable. On the other hand, when force was applied from the two sides, the 1 cm long struts of every other layer were aligned in the direction of the force. These struts had low resistance to plastic deformation due to strut buckling and formation of shear stress between layers that finally resulted in scaffold failure (Fig. 10a). Our results are in agreement with data by others who showed that when scaffold struts are aligned in the loading direction, scaffolds have low resistance against the imposed compressive load [47].
Unlike compressive strength, tensile strength of scaffolds was substantially low in the scaffold building direction. When tensile force was exerted along the scaffold building direction, the main mechanism of deformation was delamination of the layers (Fig. 10b). Layer delamination occurred through detachment of the fusion points between the struts. Stability of the fusion points depends on parameters such as polymer solidification kinetics, printing and environment temperatures, the layer printing time, deposition velocity, and scaffold height [48-50]. Since the bottom layers were bearing the weight of the top layers, the fusion points between the struts might have been stronger in the bottom layers. Thus, early fracture started from the top layers. On the other hand, when tensile force was exerted from the two sides, PCL struts of every other layer that were aligned in the direction of the force could elongate without early fracture because of PCL toughness. At higher strains, tensile stress on the struts aligned in the direction of the force
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