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Depending on the type of the initial degradation process (oxidative, thermal, photolytic), the end groups could be either methyl, hydroxymethyl, aldehyde or carboxyl groups Biodegradable Polymer 27 similar to those shown in the sequence above. The surface density or surface availability of such groups should vary inversely with sample molecular weight, unless there is a preference for such groups to either associate in the interior or migrate to the surface of the sample. However, an important unanswered question in this entire process is whether all of the reactions in the sequence above can occur at the cell surface, or must the hydrocarbon substrate be taken into the cell at some stage in order to carry out the final sequence of enzymatic reactions and fragmentation.

4) before and after it has been exposed to a very active hydrolase enzyme in an aqueous medium [60]. The surface of the film before degradation (on the left in Fig. 2) is essentially smooth with some undulations most likely resulting from the presence or absence of spherulitic crystalline regions just below a disordered surface layer. After partial degradation of the polymer by enzymatic hydrolysis (as shown on the right of Fig. 2), the surface has been deeply etched or eroded, and the exposed regions surrounded by pits and channels are most likely those spherulites referred to in the previous sentence.

Drug loading 10 wt% [1]. Reprinted with permission as the erosion process proceeds, kinetics of release will remain constant only for devices that maintain constant surface area. Kinetics of drug release from various geometrical shapes undergoing surface erosion can be calculated from the following equation [7]: M ^ = 1 - [1 - k 0 t / C 0 a ] " where k 0 is the erosion constant, t is the time, C 0 is the initial uniform concentration of the drug in the matrix, a is the radius of a sphere or cylinder or half-thickness of a slab and n = 3 for a sphere, n = 2 for a cylinder and n = 1 for a slab.

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