Toughening robocast chitosan/biphasic calcium phosphate composite scaffolds with silk fibroin: Tuning printable inks and scaffold structure for bone regeneration

The present work aims the production of composite bioceramic scaffolds by robocasting suppressing sintering as post printing process. To achieve this purpose, extrudable ink compositions containing a high concentration of bioceramic powders (hydroxyapatite and β-tricalcium phosphate) embedded in aqueous polymeric solutions of chitosan and silk fibroin were fine-tuned. Polymeric solutions of chitosan/silk fibroin with different ratios were tested, maintaining the total amount of bioceramic solids at 30 vol%. The inks were characterized by rheological studies in viscometry and oscillatory modes, being the printable ones selected to produce scaffolds with different macropore sizes (300 μm and 500 μm). The scaffolds were characterized by mechanical properties (dry and wet conditions) and morphological features, as well as its degradability. In vitro studies were also evaluated in the scaffolds that presented the best structural performance. The addition of 2 wt% silk fibroin to a 5 wt% chitosan matrix allows to significantly improve the mechanical performance of the printed composite scaffolds, reflected in high values for Young's modulus and maximum compressive strength. This trend was continued in wet scaffolds with a concomitant reduction of mechanical properties. Regarding degradability, the scaffolds in general presented a weight loss in the range of 14–18% after 28 days incubation in HEPES solution at two different pH values at 37 °C, with an associated release of calcium and phosphorus ions. The scaffold with 300 μm porosity comprising the both polymers in its composition presented the less rate degradation when compared to the scaffolds with similar porosity and containing only chitosan as base matrix. Moreover, the combined natural polymers gave rise to a significant increase in the metabolic activity of human osteoblasts grown on the scaffolds with both macropore' size, being in line with the full cellular filling of their surfaces, demonstrated by SEM and confocal imaging. The advances presented in this work are a promising path in the ink's development for extrusion-based additive manufacturing techniques and subsequent biomaterials, encompassing suitable physical and chemical characteristics with high potential to be used as bone substitutes.