Bone tumour removal and traumas with large defects are the main catastrophic events impeding complete bone healing. Autologous and allogenic bone grafting, and biologically inert metallic devices have limitations such as non-availability of autogenous bone, risk of infectious disease transmission, surgical removal, and bacterial infections. In the last decades, bone tissue engineering (BTE) emerged as a promising field to overcome those limitations by using a variety of biomaterials such as natural and synthetic polymers, ceramic, metals, carbon-based materials, and composite materials, to develop artificial implants with suitable biocompatibility, mechanical properties, and bioactivity to act as a support structure and enhance bone regeneration. However, further improvements are needed in order to provide physicians with implants displaying higher levels of biocompatibility, mechanical properties closer to the bone tissue’s ones, and suitable rate of biodegradation in vivo allowing enough time for tissue regeneration to occur before losing mechanical stability due to degradation. In this study, the development of innovative composite materials for BTE applications has been explored, using a natural polymer from the family of polyhydroxyalkanoates (PHAs), called poly(3-hydroxybutyrate) [P(3HB)], and a set of different organic and inorganic fillers as reinforcing and bioactive agents. PHAs are a family of natural and sustainable polymers produced through a bacteria-driven fermentation process using renewable carbon sources. They have been widely proved to be highly biocompatible and able to biodegrade in vivo without any toxic by-products. Among them, P(3HB) has been selected as suitable candidate for bone tissue engineering due to its stiffness. However, PHAs do not possess the required osteoinduction and
osteconduction capacity to enhance bone formation. Therefore, a borosilicate-based bioactive glasses doped with zinc oxide (BS-Zn) and a set of carbon-based materials have been selected as fillers to provide bioactivity to the composite materials. Firstly, a large-scale production of P(3HB) has been optimised and performed, and the polymer extensively characterised to confirm its suitability for the study. Subsequently, solvent casting technique was exploited to develop P(3HB)/BS-Zn scaffolds with different high concentrations
(% v/v) of filler, resulting in improved mechanical properties, high level of biocompatibility, and enhanced mineralisation and osteogenic activity of human primary osteoblast cells compared to the neat polymer and positive control. Moreover, the antibacterial efficacy due to the presence of ZnO in the bioglass network has been demonstrated against E. coli and S. aureus pathogen bacteria strains, revealing the potential of the composites to prevent bacterial infections post-implantation of the scaffolds. The same processing approach was used to create P(3HB)/carbon-based material (CBMs) composite scaffolds, and their mechanical and thermal properties as well as the interaction and distribution of fillers and polymeric matrix was evaluated. Starbon, Activated Carbon, and
Inkjet were selected as innovative carbon materials obtained from different natural and synthetic sources. The study proved the enhanced osteoconductive capacity of the composite materials in vitro. In particular, P(3HB)/Inkjet and P(3HB)/Starbon demonstrated the highest level of biocompatibility and mineralisation as well as the most homogeneous distribution of the filler within the polymer network. Therefore, the two composites were selected to proceed with further evaluation of their potential by developing 3D constructs exploiting a melt extrusion-based 3D printing process. After a preliminary investigation, P(3HB)/Inkjet showed the best performances in terms of interconnected porosity and filler dispersion in the 3D structure. Therefore, the bone regeneration capacity of the composite was evaluated using human primary osteoblast cells, resulting in improved cell adhesion and proliferation, as well as osteogenic activity and calcium deposit as sign of mineralisation compared to neat P(3HB) and positive control. In conclusion, this study achieved the successful development of innovative promising composite scaffolds suitable to act as bone substitutes and strengthen the potential of BTE to help bone healing in the clinic.
