Development of antibacterial Polyhydroxyalkanoates for their use in nerve tissue engineering

The fabrication of nerve conduits (NGCs) with the appropriate features for a successful nerve regeneration is not always possible/fulfilled. The currently available implants partially meet the requirements of such medical devices.
Despite the evolution of the polymer field and hydrogels, not much research has been conducted to exploit all possible bioplastics for their use in nerve tissue engineering. The drawbacks of current’s NGCs are promoting alternative
materials. In this perspective, this study utilized Polyhydroxyalkanoates (PHAs), linear bio-polyesters derived from bacterial fermentation process with biocompatible and biodegradable properties. PHAs are divided into two
categories, according on the number of carbon atoms in a monomer unit: short chain length (scl) and medium chain length (mcl). Depending on the type of PHA the physicochemical properties differ. The use of these bio-polyesters is gaining high interest in a vast range of biomedical applications. Additionally, previous studies have shown that PHAs are highly promising candidates for their use in nerve tissue engineering.

In this study, the main objective was to develop a NGC, based on PHAs enriched with inorganic and organic compounds (garlic powder, cerium doped phosphate fibres and graphene oxide) in order to impart antibacterial properties to the final design. Production of scl and mcl-PHAs, P(3HB) and P(3HO-co-3HD), achieved via bacterial fermentation. Blends of these two biopolymers in different ratios, 80/20, 75/25, 25/75 and 20/80, were fabricated and characterized for their
physicochemical and biocompatibility properties and future use as potential base materials for nerve tissue engineering. The addition of different amounts of organic and inorganic compounds resulted in the fabrication of novel composites.
These composites were fully characterized for their physicochemical and biological properties. The antibacterial activity of these composites was evaluated using a range of tests and bacterial strains whereas their cytocompatibility was investigated using L929 murine fibroblasts and NG108-15 neuronal cells. The promising composite films were then translated to nerve conduits via fabrication by dip moulding process. The fabricated tubes were used for further NGCs
development to insert guidance channels, formed by aligned electrospun PHA fibres. Alternatively, NGCs with aligned geometrical characteristic were developed by rolling films of PHA composites with phosphate fibres into tubular structures.

Two groups of NGCs prototypes were fabricated using the 75/25 blend of PHAs as the polymer matrix. Three different percentages of cerium doped phosphate fibres (37 wt%, 74 wt% and 90 wt%) and graphene oxide (0.5 wt%, 2 wt% and
5 wt%) were incorporated in the polymer blend. These composites have shown to possess sufficient antibacterial activity against both Gram-negative and Grampositive
bacteria without introducing cytotoxic effect to the L929 murine fibroblasts and the NG108-15 neuronal cells.

In summary, the mechanical, antibacterial and cytocompatibility properties of the developed composites from natural bio-polyesters; showed that they are excellent candidates for their use in nerve tissue engineering and the fabrication of novel bioresorbable and antibacterial NGCs.