Page 46 - Development of Functional Scaffolds for Bone Tissue Engineering Using 3D-Bioprinting of Cells and Biomaterials - Yasaman Zamani

Introduction of functional hydroxyl and carboxyl groups to a material surface results in increased hydrophilicity [39]. Treatment of the PCL surface with NaOH results in the scission of PCL ester bonds and therefore, more hydroxyl and carboxyl groups are created on the surface [40]. Moreover, since the degradation of PCL occurs by ester bond hydrolysis, treatment of the PCL surface with NaOH accelerates PCL degradation [41]. It has been reported that PCL with Mw 66000-80000 fully degrades in about 3-4 years in vivo [42]. When PCL surface is treated with NaOH, depending on the treatment time and NaOH concentration used, the polymer chains at the surface are cleaved while the bulk of the polymer remains intact. Therefore, accelerated surface degradation occurs in vivo until the surface layer that was treated with NaOH fully degrades and the bulk of the polymer is exposed [40]. From this point on, degradation of the scaffold occurs with the same rate as an unmodified PCL scaffold. We found an increase in the intensity of C-O-H (hydroxyl and carboxyl) compared to C-O-C (ester) bonds in NaOH-treated scaffolds. Moreover, a higher density of hydrophilic functional groups (hydroxyl and carboxyl) were introduced to the surface of 72 h NaOH-treated scaffolds compared to 24 h NaOH-treated scaffolds. It is well known that super hydrophilic and super hydrophobic surfaces inhibit cell attachment and moderately hydrophilic surfaces (water contact angel measurements averaging 40°) optimally promote cell attachment [39,43,44]. On the other hand, immobilization of RGD on the surface of PCL facilitates cell attachment and subsequent proliferation. Therefore, both 24 h NaOH-treated scaffolds, having moderate hydrophilicity, and RGD-immobilized scaffolds, having cell recognition sites, might be favorable for pre-osteoblast attachment and proliferation.
Surface topography of scaffolds strongly affects cell behavior [45,46]. In the current study, RGD immobilization on the surface of 3D-printed PCL scaffolds did not change the smooth surface topography of PCL, while NaOH-treated scaffolds displayed a honeycomb-like surface pattern consisting of oval pores. An increase in pore dimensions with increasing treatment time from 24 to 72 h was expected since it has been reported that the longer PCL strands are subjected to NaOH, the more surface degradation occurs [40,47]. Our results agree with data by others who observed a rougher topography on PCL surface after NaOH treatment, with larger pores for longer immersion times [46,48,48]. It has been shown that surfaces with relatively high roughness (high Ra values) and smooth surfaces with no roughness (low Ra values) are neither suitable for cell activities [50-52]. If the surface has increased micrometer-scale roughness, it is possible that cells cannot establish effective cell-cell and/or cell-matrix interactions because it is difficult to bridge the irregularities of the surface. Very deep valleys in the surface with high roughness might also interfere with osteoblast migration [53]. On the other hand, surface roughness and stiffness play an important role in directing osteogenic differentiation [54]. Therefore, it can be speculated that
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