Development of eco-friendly poly(hydroxyalkanoate)-based biomaterials for tissue engineering applications

The need to reduce the environmental impact of the production pipeline of many synthetic biomaterials combined with the requirements of biomedical applications (e.g., cytocompatibility and bioactivity) have directed the latest biomedical research towards the engineering of novel optimised biomaterials. Poly(hydroxy alkanoate)s, PHAs, represent in this regard a green alternative to traditional polymeric materials, because they are produced by bacteria and only require few chemical compounds for their production. Poly(ester urethane)s, PURs, are also interesting candidates due to the possibility to ad-hoc select their building blocks to satisfy specific requirements for tissue engineering (TE) applications. In this scenario, this thesis project focused on the synthesis and chemical modification of a medium chain length PHA (mcl-PHA), which synthesis was carried out at The University of Sheffield (UK) (Prof. I. Roy’s group). This material was used (i) to develop new polymeric blends with three different thermoplastic PURs based on poly(ε-caprolactone), PCL, and (ii) as building block in the synthesis of a novel PUR containing both PHA and PCL. For blend production, the mcl-PHA was subjected to a hydrolysis process to reduce the existing discrepancy in average molecular weights between mcl-PHA (Mw∽100 kDa) and PCL-based PURs (Mw∼30-40 kDa). The optimised hydrolysis protocol gave a Mw comparable to the PURs after 6h of reaction. Hydrolysed mcl-PHA was then combined with PCL-based PURs, at different weight ratios (25:75, 50:50, 75:25 w/w) and blends were produced by solvent casting. Blend compatibility was assessed by Differential Scanning Calorimetry (DSC) and infrared (IR) spectroscopy, and the best results were found for 25:75 and 75:25 PHA:PUR weight ratios. Compared to pristine PUR, blends showed a considerable improvement in their toughness and elasticity due to the influence of the PHA counterpart. Although further mechanical investigations would be necessary to better assess the tuning of mechanical properties, these preliminary results highlighted that the combination of the two poly(ester)s could widen the TE applications of these materials (e.g., cartilage replacement). On the other hand, the mcl-PHA was also directly included in the PUR structure. A transesterification process was carried out to obtain a macrodiol to be used during PUR synthesis and the protocol was optimised to obtain a suitably low molecular weight diol (Mn∽6 kDa); reaction success was confirmed by Size Exclusion Chromatography (SEC), infrared and 1H Nuclear Magnetic Resonance (1H NMR) spectroscopy. The PUR synthesis was then carried out using a combination of PCL diol (Mn 2 kDa) and PHA diol (25:75 w/w), 1,6-diisocyanatohexane (HDI) and 1,8-octanediol as chain extender. Synthesis success was proved by IR spectroscopy and DSC confirmed the presence of both PCL and PHA in the PUR structure. However, 1H NMR evidenced that the ratio of the two components was different from the expected one (PHA:PCL 25:75 w/w) and the final PUR Mw reached around 12 kDa. This was probably due to a limited reactivity of the PHA diol compared to PCL diol. Hence, a further optimisation of the transesterification protocol will be needed, although the preliminary results in terms of thermal and mechanical properties were promising. In conclusion, the combination of PUR chemical versatility and PHA superior properties resulted in the production of biomaterials with promising features to target several biomedical applications.