Biomedical polymers erosion

The past two decades have produced a revival of interest in the synthesis of polyanhydrides for biomedical applications. These materials offer a unique combination of properties that includes hydrolytically labile backbone, hydrophobic bulk, and very flexible chemistry that can be combined with other functional groups to develop polymers with novel physical and chemical properties. This combination of properties leads to erosion kinetics that is primarily surface eroding and offers the potential to stabilize macromolecular drugs and extend release profiles from days to years. The microstructural characteristics and inhomogeneities of multi-component systems offer an additional dimension of drug release kinetics that can be exploited to tailor drug release profiles. 

Amino acid ester side groups are not the only units that sensitize the system to hydrolysis. The imidazolyl group has an even greater effect.197-198-219 For example, polymer 3.86, prepared by the reaction of poly(dichlorophosphazene) with imidazole, is so unstable hydrolytically that it decomposes in moist air to imidazole, phosphate, and ammonia. This is too high a sensitivity for most biomedical applications. Hence, an emphasis has been placed on the study of polymers such as 3.87 in which a hydrophobic cosubstituent group, such as aryloxy, is present to reduce the rate of erosion. 

Poly(L-lactide) is an ideal FDA approved polymer for biomedical applications because of slow-degrading characteristics and good tensile strength as compared to polyglycolide. The rate of degradation of poly(L-lactide) is very low and depends on the polymer crystallinity and the porosity of the matrix. Bulk erosion of the ester backbone via hydrolytic degradation generates lactic acid, which is broken down via the citric acid cycle into water and carbon dioxide [23]. 

What affects the biocompatibility of a polymeric material are some inherent properties, such as material chemistry, molecular weight, solubility, hydro-philicity/hydrophobicity, absorption, degradation and erosion mechanism, etc. Consequently, given the complexity and the variety of biomedical applications for which biodegradable polymers are currently used, it underlines the need for developing a wide range of biodegradable materials available for requirements of each medical application. 

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