Microstructural Changes during Degradation of Biobased Poly(4-hydroxybutyrate) Sutures.

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.

Isothermal Crystallization Kinetics of Poly(4-hydroxybutyrate) Biopolymer.

Thermal properties and crystallization kinetics of poly(4-hydroxybutyrate) (P4HB) have been studied. The polymer shows the typical complex melting behavior associated to different lamellar populations. Annealing processes had great repercussions on properties and the morphology of constitutive lamellae as verified by X-ray scattering data. Kinetics of isothermal crystallization was evaluated by both polarizing optical microscopy (POM) and calorimetric (DSC) measurements, which indicated a single crystallization regime. P4HB rendered banded spherulites with a negative birefringence when crystallized from the melt. Infrared microspectroscopy was applied to determine differences on the molecular orientation inside a specific ring according to the spherulite sectorization or between different rings along a determined spherulitic radius. Primary nucleation was increased during crystallization and when temperature decreased. Similar crystallization parameters were deduced from DSC and POM analyses (e.g., secondary nucleation parameters of 1.69 × 105 K2 and 1.58 × 105 K2, respectively). The effect of a sporadic nucleation was therefore minimized in the experimental crystallization temperature range and a good proportionality between overall crystallization rate (k) and crystal growth rate (G) was inferred. Similar bell-shaped curves were postulated to express the temperature dependence of both k and G rates, corresponding to the maximum of these curves close to a crystallization temperature of 14-15 °C.

Non-Isothermal Crystallization Kinetics of Poly(4-Hydroxybutyrate) Biopolymer.

The non-isothermal crystallization of the biodegradable poly(4-hydroxybutyrate) (P4HB) has been studied by means of differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). In the first case, Avrami, Ozawa, Mo, Cazé, and Friedman methodologies were applied. The isoconversional approach developed by Vyazovkin allowed also the determination of a secondary nucleation parameter of 2.10 × 105 K2 and estimating a temperature close to 10 °C for the maximum crystal growth rate. Similar values (i.e., 2.22 × 105 K2 and 9 °C) were evaluated from non-isothermal Avrami parameters. All experimental data corresponded to a limited region where the polymer crystallized according to a single regime. Negative and ringed spherulites were always obtained from the non-isothermal crystallization of P4HB from the melt. The texture of spherulites was dependent on the crystallization temperature, and specifically, the interring spacing decreased with the decrease of the crystallization temperature (Tc). Synchrotron data indicated that the thickness of the constitutive lamellae varied with the cooling rate, being deduced as a lamellar insertion mechanism that became more relevant when the cooling rate increased. POM non-isothermal measurements were also consistent with a single crystallization regime and provided direct measurements of the crystallization growth rate (G). Analysis of the POM data gave a secondary nucleation constant and a bell-shaped G-Tc dependence that was in relative agreement with DSC analysis. All non-isothermal data were finally compared with information derived from previous isothermal analyses.

Nanostructured medical sutures with antibacterial properties.

Bacterial repellence in suture materials is a desirable property that can potentially improve the healing process by preventing infection. We describe a method for generating nanostructures at the surface of commercial sutures of different composition, and their potential for preventing biofilm formation. We show how bacteria attachment is altered in the presence of nanosized topographies and identify optimum designs for preventing it without compromising biocompatibility and applicability in terms of nanostructure robustness or tissue friction. These studies open new possibilities for flexible and cost-effective realization of topography-based antibacterial coatings for absorbable biomedical textiles.

The potential of photoacoustic microscopy as a tool to characterize the in vivo degradation of surgical sutures.

The ex vivo and in vivo imaging, and quantitative characterization of the degradation of surgical sutures (∼500 µm diameter) up to ∼1cm depth is demonstrated using a custom dark-field photo-acoustic microscope (PAM). A practical algorithm is developed to accurately measure the suture diameter during the degradation process. The results from tissue simulating phantoms and mice are compared to ex vivo measurements with an optical microscope demonstrating that PAM has a great deal of potential to characterize the degradation process of surgical sutures. The implications of this work for industrial applications are discussed.

A low memory cost model based reconstruction algorithm exploiting translational symmetry for photoacustic microscopy.

A model based reconstruction algorithm that exploits translational symmetries for photoacoustic microscopy to drastically reduce the memory cost is presented. The memory size needed to store the model matrix is independent of the number of acquisitions at different positions. This helps us to overcome one of the main limitations of previous algorithms. Furthermore, using the algebraic reconstruction technique and building the model matrix "on the fly", we have obtained fast reconstructions of simulated and experimental data on both two- and three-dimensional grids using a traditional dark field photoacoustic microscope and a standard personal computer.