Quantitative analysis of factors determining the enzymatic degradation of poly(lactic acid).

The enzymatic degradation of poly(lactic acid) was catalyzed with Proteinase K and the effect of various factors on the rate of degradation was analyzed quantitatively with the help of appropriate kinetic models. The Michaelis-Menten model was modified for the purpose by considering the heterogeneous nature of the reaction and the denaturation of the enzyme. The results proved that Proteinase K degrades the polymer very efficiently. The rate of degradation increases considerably up to 0.1 mg/ml enzyme concentration, but remains constant at larger values. Temperature has an optimum at around 50 °C that is somewhat higher than the 37 °C extensively used in the literature as the most advantageous temperature. If degradation occurs in the same medium throughout the process, the formation of lactic acid results in the rapid decrease of pH and finally in the denaturation of the enzyme. The dropping of pH below 5 slows down and finally stops degradation completely. The daily change of the medium results in degradation with a constant rate and the entire amount of the polymer can be decomposed mainly into monomer or smaller oligomer fragments. Degradation rate decreases slightly with increasing molecular weight and increasing d-lactide content. The use of appropriate kinetic models allows quantitative analysis and the prediction of the rate of enzymatic degradation of PLA.

Poly-ε-Caprolactone/Halloysite Nanotube Composites for Resorbable Scaffolds: Effect of Processing Technology on Homogeneity and Electrospinning.

Polycaprolactone (PCL)/halloysite composites were prepared to compare the effect of homogenization technology on the structure and properties of the composites. Halloysite content changed from 0 to 10 vol% in six steps and homogeneity was characterized by various direct and indirect methods. The results showed that the extent of aggregation depends on technology and on halloysite content; the size and number of aggregates increase with increasing halloysite content. Melt mixing results in more homogeneous composites than the simple compression of the component powders or homogenization in solution and film casting. Homogeneity and the extent of aggregation determines all properties, including functionality. The mechanical properties of the polymer deteriorate with increasing aggregation; even stiffness depends on homogeneity. Strength and deformability decreases drastically as the number and size of aggregates increase. Not only dispersed structure, but also the physical state and crystalline structure of the polymer influence homogeneity and properties. The presence of the filler affects the preparation of electrospun fiber scaffolds as well. A part of the filler is excluded from the fibers while another part forms aggregates that complicates fiber spinning and deteriorates properties. The results indicate that spinning is easier and the quality of the fibers is better if a material homogenized previously by melt mixing is used for the production of the fibers.

Controlled degradation of poly-ε-caprolactone for resorbable scaffolds.

A lipase from Burkholderia cepacia was successfully adsorbed on the surface of halloysite nanotubes and the coated tubes were incorporated into poly-ε-caprolactone (PCL). The efficiency of the halloysite in the adsorption of the enzyme was characterized by the total protein content determined with the Bradford method. The activity of the adsorbed enzyme was estimated by the kinetic resolution of racemic 1-phenylethanol. The immobilized enzyme was mixed with the polymer and compression molded films were prepared at 70 °C. Activity measurements proved that the enzyme remains active even after adsorption; in fact, larger activities were measured for the immobilized enzyme than for the neat enzyme preparation. The supported enzyme degraded PCL efficiently, the rate of degradation depended on the amount of enzyme adsorbed. The kinetics of degradation was described quantitatively with an appropriate model accounting for two of the three steps of the process, i.e. degradation and the denaturation of the enzyme. The determination of time constants allows the adjustment of degradation rate. This is the first time that the enzyme, which catalyzes degradation, is incorporated into the polymer, and not into the degradation medium, thus allowing the preparation of resorbable scaffolds with controlled lifetime.