EFFECT OF IN-VIVO AND IN-VITRO DEGRADATION ON MOLECULAR AND MECHANICAL-PROPERTIES OF VARIOUS LOW-MOLECULAR-WEIGHT POLYLACTIDES

The in vivo and in vitro degradation of low-molecular-weight poly(L-la ctide), poly(L/D-lactide), and poly (L/DL-lactide) rods was investigat ed. The low-molecular-weight fast-degrading materials were used to acc elerate the degradation process and make the test conditions more crit ical. Ln the in vivo study the rods were implanted in the soft tissue of sheep and explanted at 1, 3, 6, and 12 months. In the in vitro expe riments the samples were subjected to aging at 37 degrees C in the pho sphate buffer using two different modes. In the so-called pseudodynami c mode the aging buffer was regularly replaced if the pH dropped more than 0.5. In the static mode the buffer was not changed over the whole testing period of 52 weeks. The mechanical, molecular, and crystallin e properties of the rods were measured and their appearance in the cou rse of aging was evaluated using scanning electron microscopy. It was found that the changes in the mechanical properties of poly(L-lactide) , poly(L/D-lactide), and poly(L/DL-lactide) samples subjected to in vi tro degradation tests in both the static and pseudodynamic modes are i n good approximation with data obtained from the in vivo study. The pH of the buffer solution had no evident effect on the mechanical proper ties or the rate of degradation as estimated from the drop in molecula r weight of the aged samples. The replacement of the aging buffer to m aintain a constant pH at 7.4 does not seem to be critical for the degr adation of the polylactides. In vitro degradation tests can be used as a relevant procedure for predicting the ill vivo functionality of imp lants from the polylactides used if the criteria for assessing such a functionality are the changes in mechanical properties and molecular w eight. (C) 1997 John Wiley & Sons, Inc.