Bioactive glass scaffolds reinforced with tailor-made polyurethane coatings for Bone Tissue Engineering

In the last decades, bioactive glasses have gained great importance in bone tissue engineering (BTE) applications due to their chemical similarity to the inorganic phase of bone and their excellent bioactivity. In fact, this class of biomaterials not only provides a biocompatible surface, but can also lead and enhance the deposition of new bone tissue. However, their relatively low mechanical properties, low resistance to fracture under loads and high brittleness limit their application in BTE approaches, which require robust mechanical structure and high resistance to applied stress. The aim of this work was the design and characterization of 45S5 Bioglass®-based scaffolds coated with tailor-made polymers. The coating is expected to improve the mechanical resistance and stiffness of the scaffolds, with no significant changes in biocompatibility and bioactivity. In detail, the scaffolds were fabricated by the foam replica technique and coated by a dipping procedure with a custom-made poly(urethane-urea)s (PUR) or commercial poly(ε-caprolactone) (PCL) (80000 Da) which was used as control. Two polyurethanes were used as coatings, differing in their soft segment: poly(ε-caprolactone) (2000 Da) for KHC2000 polyurethane and a mixture of poly(ε-caprolactone) (2000 Da) and polyethylene glycol (2000 Da) in 70/30 weight ratio for KHC2000E2000. For what concerns PUR hard segment, both the materials were based on 1,6-hexamethylene diisocyanate and L-lysine ethyl ester. The foam replica scaffolding technology allowed the fabrication of highly porous scaffolds (>90%) with interconnected porosity that are expected to be easily colonized by cells. But, this highly porous structure is one of the contributors to the poor mechanical properties of this kind of matrices. In previous works scaffold dipping procedure has been optimized in terms of concentration of the polymeric solution prepared in chloroform and immersion time: 1% w/v concentration and 2.5 minutes for poly(𝜀-caprolactone), 0.5%w/v concentration and 1 minute for KHC2000 and 1%w/v and 1 minute for KHC2000E2000. In this work, in order to increase the amount of polymer, and subsequently enhance the overall scaffold mechanical properties, the dip coating procedure was repeated three times on each scaffold. Coated and uncoated scaffolds were characterized in terms of morphology by scanning electron microscopy (SEM) that revealed a high porous structure even after three coatings. Scaffold bioactivity was investigated after immersion in Simulated Body Fluid (SBF) by SEM, Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD) to assess the successful deposition of hydroxyapatite. The results showed that the presence of polymers did not inhibit the well-known bioactivity of 45S5 Bioglass®. Concerning mechanical properties, compressive tests were carried out in dry and wet conditions, to better simulate the in-vivo conditions and after immersion in SBF to investigate the effect of hydroxyapatite deposition on the mechanical behaviour. Scaffold compressive strength was considerably increased by coating them with both polyurethanes and PCL. In fact, the compressive stress of uncoated of 45S5 Bioglass® scaffolds was determined to be 0.14 ± 0.04 MPa, while it reached 0.31 ± 0.09 MPa, 0.33 ± 0.08 MPa and 0.27 ± 0.07 MPa for the scaffolds coated with KHC2000 and KHC200E2000 and PCL, respectively. KHC2000-coated scaffolds showed a similar improvement in mechanical properties compared