High Molecular Weight PLLA Research Articles

Effects of chain end structures on pyrolysis of poly( l-lactic acid) containing tin atoms

Thermal degradation of high molecular weight PLLA containing residual tin atoms was investigated as a means of controlling the reaction for feedstock recycling to l, l-lactide. To clarify the pyrolysis mechanism of the PLLA, three samples with different chain end structures were prepared, namely, as-polymerized PLLA-ap, precipitated-with-methanol PLLA-pr, and purified PLLA-H. From pyrolyzate and kinetic analyses, typical degradation mechanisms of Sn-containing PLLA were clarified. In other words, it was assumed that the pyrolysis of PLLA-ap proceeds through a zero-order weight loss process with the apparent E a =80–90 kJ mol −1 , and with the occurrence of backbiting and transesterification reactions caused by Sn-alkoxide chain ends. The pyrolysis of PLLA-pr was also assumed to proceed via a zero-order weight loss process with apparent E a =120–130 kJ mol −1 , with the proposed mechanism being Sn-catalyzed selective lactide elimination caused by Sn-carboxylate chain ends. Both pyrolysis of PLLA-ap and PLLA-pr produced l, l-lactide selectively. These degradation mechanisms and products are in contrast to those of PLLA-H, in which a large amount of diastereoisomers and cyclic oligomers were formed by random degradation. From this study, the complicated PLLA pyrolysis behavior as reported previously could be explained properly.

Melt polycondensation ofL-lactic acid with Sn(II) catalysts activated by various proton acids: A direct manufacturing route to high molecular weight Poly(L-lactic acid)

Poly(L-lactic acid) (PLLA) was produced by the melt polycondensation of L-lactic acid. For the optimization of the reaction conditions, various catalyst systems were examined at different temperature and reaction times. It was discovered that Sn(II) catalysts activated by various proton acids can produce high molecular weight PLLA [weight-average molecular weight (Mw ) ≥ 100,000] in a relatively short reaction time (≤15 h) compared with simple Sn(II)-based catalysts (SnO, SnCl2 · 2H2O), which produce PLLA with an Mw of less than 30,000 after 20 h. The new catalyst system is also superior to the conventional systems in regard to racemization and discoloration of the resultant polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1673–1679, 2000

Sustained Local Delivery of Dexamethasone by a Novel Intravascular Eluting Stent to Prevent Restenosis in the Porcine Coronary Injury Model

Objectives. This study sought to assess the feasibility, safety and efficacy of sustained intracoronary delivery of dexamethasone by a novel polymer-coated eluting stent.Background. Development of techniques to provide sustained local drug delivery has focused on polymers as matrices for drug incorporation and elution.Methods. A tantalum wire stent was coated with dexamethasone (0.8 mg) suspended in a matrix of either low (∼80 kD) or high (∼321 kD) molecular weight poly-l-lactic acid (PLLA [0.4 mg]). Uncoated stents, stents coated with PLLA or stents coated with dexamethasone in PLLA were overexpanded by 30% to the normal vessel diameter in the coronary arteries of juvenile farm pigs. Animals were euthanized 28 days later, and neointimal thicknesses were measured. Additional pigs underwent placement of stents coated with high molecular weight PLLA–dexamethasone for assessment of arterial tissue and serum concentrations of dexamethasone at 1 h and 1, 2, 10 and 28 days after stent implantation.Results. In vitro dexamethasone release occurred over the first 6 days. Stents coated with low molecular weight PLLA produced an intense inflammatory neointimal response. Stents utilizing the high molecular weight PLLA were well tolerated within the coronary vessel during the 28-day experiment. However, dexamethasone did not decrease neointimal hyperplasia. Dexamethasone concentrations in the arterial tissue were ∼300,000-fold higher than those in the serum 24 h after stent implantation, remaining ∼3,000-fold higher at 28 days.Conclusions. The eluting stent utilizing high molecular weight PLLA appeared to be a well tolerated and effective means of providing sustained, site-specific drug delivery to the porcine coronary artery wall for at least 28 days.(J Am Coll Cardiol 1997;29:808–16)

Mechanical properties of dense polylactic acid structures fabricated by three dimensional printing

Polylactic acid (PLA) is a bioresorbable polymer that is used in a number of clinical situations. Complex shapes of PLA are commonly machined for bone fixation and reconstruction. Solid free form fabrication methods, such as 3D printing, can produce complex-shaped articles directly from a CAD model. This study reports on the mechanical properties of 3D-printed PLLA parts. 3D printing is a solid free-form fabrication process which produces components by ink-jet printing a binder into sequential powder layers. Test bars were fabricated from low and high molecular weight PLA powders with chloroform used as a binder. The binder printed per unit line length of the powder was varied to analyze the effects of printing conditions on mechanical and physical properties of the PLA bars. Furthermore, cold isostatic pressing was performed after printing to improve the mechanical properties of the printed bars. The maximum measured tensile strength for the low molecular weight PLLA (53 000) is 17.40 ± 0.71 MPa and for high molecular weight PLLA (312 000) is 15.94 ± 1.50 MPa.