Surface characterization of the erosion of biodegradable polymer matrices using time-of-flight secondary ion mass spectrometry: Drug delivery and tissue engineering applications

The in vitro hydrolytic degradation of poly(α-hydroxy acid)-based biomedical devices has been investigated at the near surface phases, focusing on the initial rapid release of drugs coupled with the polymer degradation process at the initial stage of bulk erosion of biodegradable polyesters.; The precision of the molecular weight calculations of degradation products in quantitative time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been improved using a multiple ion summation method developed in Chapter 2. This method was used for the series of hydrolytic degradation studies of poly(glycolic acid) (PGA), poly(L-lactic acid) (PLLA), and poly(D,L-lactic acid-co-glycolic acid) (PLGA). This method of quantification leads to more precise results for the surface degradation kinetics of biodegradable polymers. In vitro studies of the hydrolytic degradation at the surface of PGA in Chapter 3 describe the role of the surface amorphous region during the induction of erosion of surface vs. bulk degradation processes. The kinetics of surface degradation are shown to be related to the enrichment of amorphous PGA in the surface phases. The surface chemistry of the induction period involves the initial rapid, pH dependent hydrolysis of surface segregated amorphous PGA, with little overall weight loss as water absorption approaches equilibrium in the reaction zone. In Chapter 4 results are presented for the simultaneous ToF-SIMS detection of both in vitro hydrolytic PLLA degradation kinetics and resultant triphenylamine (Ph₃N) release profile at/from Ph₃N/PLLA blend matrices. The environmental pH effects on the initial rapid release of drugs during the induction period are reported. The study of a model drug delivery system using the blend matrices leads to the conclusion that the initial rapid release of drugs from the surface is mostly attributed to the polymer degradation kinetics during the initial induction period. These results provide a better understanding of surface and interfacial reactions and can be related to the biocompatibility in many in vivo applications.