Antibiotic-free PHA-based antibacterial materials for bone regeneration

The current world problem of antibacterial resistance to antibiotics has driven the need to develop new therapeutics as alternatives to antibiotic to prevent and fight infections. In the biomedical field, bone regeneration is one of the applications with the highest risk of infection. Therefore, alongside biological and functional properties, materials used for this application should be able to provide antibacterial features able to inhibit bacterial adhesion and growth. In such a scenario, this work focused on the development of novel antibiotic-free materials based on polyhydroxyalkanoates (PHAs) for bone regeneration following two strategies.

For the first strategy, inherently antibacterial PHAs, thioester containing PHAs, were produced by bacterial fermentation. Such materials exhibited antibacterial activity by contact against S. aureus 6538P. Physical blending of thioester-PHA with P(3HHx-co-3HO-co-3HD) was investigated and a polymer ratio of 80:20 (P(3HHx-co-3HO-co-3HD): thioester-PHA) was identified as optimal to develop handleable and processable samples possessing antibacterial and in vitro biocompatibility properties.

For the second strategy, novel 2D antibacterial composite materials were produced by combining P(3HB) and P(3HO-co-3HD-co-3HDD) with a novel antibacterial co-substituted hydroxyapatite, containing two therapeutic ions, selenium and strontium, conferring osteoinductive and antibacterial properties to the ceramic. The materials developed showed to be able to induce an inhibitory effect against both Gram positive and Gram negative bacteria.

The possibility of developing 3D scaffolds using melt 3D printing of mcl-PHAs was investigated in this study for the first time. The process parameters were successfully optimized to produce well-defined and reproducible 3D P(3HO-co-3HD-co-3HDD) scaffolds. 3D scaffolds with dual porosity were also developed through the combination of 3D printing with salt leaching.

Finally, the obtained 2D antibacterial films were used as starting materials for the development of 3D scaffolds for antibacterial synthetic bone substitutes using the optimized melt 3D printing process. Novel 3D composite scaffolds were produced using the 2D composite materials based on P(3HO-co-3HD-co-3HDD) and the antibacterial hydroxyapatite. Based on the antibacterial and cell compatibility studies, the composite material with 10 wt% of Se-Sr-HA as a filler was identified as the optimal candidate for the development of antibacterial scaffolds for bone regeneration. Novel inherently antibacterial scaffolds were produced by melt 3D printing of 20:80 blend films of thioester-PHA with P(3HHx-co-3HO-co-3HD). The 3D blend scaffolds showed an inhibitory effect against S. aureus 6538P by direct contact testing and were able to support the growth of MC3T3-E1 cells, showing potential for bone regeneration.