Thermo-mechanical properties, microstructure and biocompatibility in poly-β-hydroxybutyrates (PHB) produced by OP and OPN strains of Azotobacter vinelandii

• Bacterial poly-β-hydroxybutyrates (PHBs) were obtained from cultured Azotobacter vinelandii strains.
• Tensile modulus and crystallinity are a growing function of molecular weight.
• Above 1000 kDa molecular entanglements suppress crystallization.
• PHB films promoted Human Embryonic Kidney 293 cells (HEK293) growth.

The thermo-mechanical properties, microstructure, and biocompatibility of a molecular weight series of poly-β-hydroxybutyrate (PHB) produced by Azotobacter vinelandii wild-type strain (OP) and mutant derivatives strains OPN, SOP1 and OPNA were determined. A broad molecular weight range was investigated, spanning from 160 kDa to 2030 kDa. Thermal properties were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results showed that the melting, crystallization and degradation temperatures of the PHBs initially increased with increasing the molecular weight. However, the highest molecular weight PHBs exhibited cold crystallization and reduction in melting temperature and melting enthalpy, suggesting difficulty to crystallize. Polarized optical microscopy (POM) showed a spherulitic morphology for all PHBs, with a ringed internal structure. Uniaxial tensile tests showed that the mechanical Young’s modulus is an increasing function of molecular weight, whereas the strain at fracture decreased, denoting brittleness as molecular weight increased. Interestingly, the physical properties of these bacterial PHBs were independent of the strain and aeration conditions used in its production. Finally, the PHBs obtained by this novel route did not exhibit any cytotoxic effect and allowed the growth of Human Embryonic Kidney 293 cells (HEK293) on as-cast films indicating the biocompatibility as a support for cell systems.

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