Abstract
Fibers of poly(4-hydroxybutyrate) (P4HB) have been submitted to both hydrolytic and enzymatic degradation media in order to generate samples with different types and degrees of chain breakage. Random chain hydrolysis is clearly enhanced by varying temperatures from 37 to 55 °C and is slightly dependent on the pH of the medium. Enzymatic attack is a surface erosion process with significant solubilization as a consequence of a preferent stepwise degradation. Small angle X-ray diffraction studies revealed a peculiar supramolecular structure with two different types of lamellar stacks. These were caused by the distinct shear stresses that the core and the shell of the fiber suffered during the severe annealing process. External lamellae were characterized by surfaces tilted 45° with respect to the stretching direction and a higher thickness, while the inner lamellae were more imperfect and had their surfaces perpendicularly oriented to the fiber axis. In all cases, WAXD data indicated that the chain molecular axis was aligned with the fiber axis and molecules were arranged according to a single orthorhombic structure. A gradual change of the microstructure was observed as a function of the progress of hydrolysis while changes were not evident under an enzymatic attack. Hydrolysis mainly affected the inner lamellar stacks as revealed by the direct SAXS patterns and the analysis of correlation functions. Both lamellar crystalline and amorphous thicknesses slightly increased as well as the electronic contrast between amorphous and crystalline regions. Thermal treatments of samples exposed to the hydrolytic media revealed microstructural changes caused by degradation, with the inner lamellae being those that melted faster.
Highlights
Synthetic bioabsorbable sutures have been commercialized since the early 1970s when braided polyglycolide sutures were developed [1]
Hydrolytic degradation of commercial P4HB sutures was evaluated through weight loss and molecular weight measurements using media of three different pH values and two temperatures (i.e., 37 and 55 ◦ C that are associated to physiological conditions and the higher available temperature before starting fusion, respectively)
Weight loss (Wl ) of the specimens was determined through Equation 1 where Wd is the sample weight after degradation and W0 is the initial sample weight: Wl = 100 × (W0 − Wd )/Wo
Summary
Synthetic bioabsorbable sutures have been commercialized since the early 1970s when braided polyglycolide sutures were developed [1]. Polymers 2020, 12, 2024 the 1980s when polydioxanone sutures were developed [2]. Advantages of this form involve the reduction of problems associated to tissue drag and the decrease of infection risk derived from a capillary effect. Traditional monofilament sutures have a fast or medium degradation rate, with a long term decomposition profile having been developed more recently. MonoMax® has been commercialized in 2009 for abdominal wall repair applications [3]. This suture is based on poly(4-hydroxybutyrate) (P4HB) and is currently probably the most pliable monofilament suture. The degradation process of the polymer P4HB in the human body is initiated by hydrolysis caused by the water diffused into the polymer bulk [5], but enzymes such as lipases are able to promote a surface attack [5,6]
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