Page 74 - Development of Functional Scaffolds for Bone Tissue Engineering Using 3D-Bioprinting of Cells and Biomaterials - Yasaman Zamani

mechanical properties. One of the main parameters affecting the mechanical properties of 3D- printed scaffolds is the distance between the two struts or the void size [33-35]. Scaffold void size determines both the mechanical properties of the scaffold and ease of mass transfer into the scaffold [36, 37]. Void size should not be too large since it results in reduced mechanical stability. On the other hand, void size should not be too small to hamper nutrient transfer into the scaffold [38]. We 3D-printed scaffolds with 3 different void sizes, i.e. 0.3, 0.6, or 0.9 mm. As expected, scaffolds with larger void size had lower compressive strength. Larger void size means that there are less PCL struts in the whole structure to maintain the mechanical integrity of the scaffold, and therefore the scaffold has less resistance to the compressive force. Moreover, scaffolds with larger void size had lower compressive modulus which means that they enter the plastic deformation zone at lower stress values. This finding is in agreement with numerous other reports demonstrating decreased compressive strength with increasing scaffold pore size and porosity [39-41]. Next, using the results obtained from the homogeneous scaffolds, we designed and 3D- printed 2-region or 3-region gradient-structured scaffolds with increasing void size from top-to- bottom. By measuring the compressive strength of the gradient scaffolds in the upper half and in the lower half separately, we observed that both gradient scaffolds had higher compressive strength in the upper half compared with the lower half of the scaffold. This was relevant to the higher compressive forces induced to the upper part and lower compressive force induced to the lower part of the symphysis during jaw opening. Our results agree with published data showing that in a single scaffold, regions with smaller void size have higher mechanical properties compared with regions with larger void size [42]. Compressive strength of our scaffolds (4.1-12 MPa) was in the range of trabecular bone in the human mandible (0.22-10.44 MPa) [43].
Current studies on the mechanical properties of bone tissue engineering scaffolds are mostly based on compression testing, and data on the tensile strength of 3D-printed bone scaffolds are scarce [44-46]. However, tensile strength of bone scaffolds might be equally important depending on the location of the defect. Since our modeling results indicated that during opening and closing of the jaw, a tensile force was induced on the lower parts of the symphysis in the transverse direction, we tested the tensile strength of the scaffolds in the lower half. Similar to compressive strength, tensile strength of homogeneous scaffolds decreased with increasing scaffold void size. This can be explained by the extent of contact between the layers. When the scaffold has larger void size, there are less contact points between the layers, and therefore the scaffold has less resistance to failure against the imposed tensile force. Moreover, tensile modulus of homogeneous scaffolds decreased with increasing scaffold void size. Same explanation can be used for the tensile modulus. Due to less contact points between the struts in
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