Mechanics and surface ultrastructure changes of poly(3-hydroxybutyrate) films during enzymatic degradation in pancreatic lipase solution

A study of in vitro biodegradation of poly(3-hydroxybutyrate) (PHB) in model conditions was performed. The porcine pancreatic lipase solutions at different concentrations in the two buffer systems (phosphate-buffered saline (PBS) and simulating body fluid (SBF)) were chosen as model biodegradation media. At first, optimal concentration of pancreatic lipase (0.25 mg/ml in PBS) was determined: in these media the decomposition of PHB films realized faster according to the data of gravimetric analysis. Differential scanning calorimetry showed an increase in the crystallinity of the samples (from 49% to 59%) after enzymatic degradation. These data are confirmed by method of nanoindentation, where the increase of the Young's modulus during the degradation (from 1.37 GPa to 4.4 GPa) was shown. This is due to the crystallization of the amorphous polymer component, and its decomposition and dissolving. During biodegradation three types of polymer ultrastructure changes were observed on the surfaces of the films: appearance of new lamellae, disappearance of lamellae and disintegration of lamellae into shorter fragments.