[Mechanical strength and chemical stability of biodegradable block-polymerized and injection molded poly-L-lactide in...

Biodegradable poly-L-lactic acid rods made of block-polymerized material (BP; molecular weight 550,000) and injection-moulded rods with high (SGI; molecular weight 121,000) and low (SGA, molecular weight 118,000) molecular orientation were compared 2, 4 and 6 weeks after incubation in enzyme solutions with high hydrolytic activity (esterase, alpha chymotrypsin and peptidase) and in buffer solution (TRIS buffer). The molecular weight, modulus of elasticity, bending strength (three-point bending test), and cyclic bending load to failure applied to the rods in a newly developed testing machine (1 Hz, maximum 100,000 cycles) were compared. The molecular weight of BP material decreased to 36% after 2 weeks, in contrast to the injection-moulded materials, in which it decreased only to 66% even after 6 weeks. The bending strength of all specimen decreased significantly faster in alpha chymotrypsin than in the other media (MANOVA, P < 0.001). SGI had a significantly higher bending strength than SGA, and SGA a higher strength than BP. There was no difference after incubation in the other two enzymatic solutions. BP lost 80% of its initial bending strength (140 N/mm2) after 6 weeks, and SGI and SGA (120 N/mm2) only 20%. Under permanent cyclic loading BP initially resisted 100,000 cycles with an applied cyclic load of 12.5 N/mm2, decreasing after 6 weeks to only 9,500 cycles, in contrast to SGI and SGA, which resisted to 46,000 cycles. There was, however, no relevant difference in the mechanical characteristics of the two injection-moulded rods. These results confirm that BP is degraded significantly faster than SGA and SGI.(ABSTRACT TRUNCATED AT 250 WORDS)