Biodegradable polymer surface-erosion

The polymers are useful for the fabrication or coating of implantable medical devices, in particular stents. They not only have the requisite mechanical properties but they also biodegrade by surface erosion rather than by bulk erosion. They thus would be expected to drastically reduce the formation and the release of large particles during biodegradation (74). 

Because enzymes are too big to diffuse into the bulk of a polymer, biodegradation is an erosion process that takes place at the surface of the plastic article [5]. Therefore, the thickness of a plastic article is a decisive parameter in determining the time needed for complete degradation. 

A much more desirable erosion mechanism is surface erosion, where hydrolysis is confined to a narrow zone at the periphery of the device. Then, if the drug is weU-immobihzed in the matrix so that drug release due to diffusion is minimal, the release rate is completely controlled by polymer erosion, and an ability to control erosion rate would translate into an ability to control dmg delivery rate. For a polymer matrix that is very hydrophobic so that water penetration is limited to the surface (thus Hmiting bulk erosion), and at the same time, allowing polymer hydrolysis to proceed rapidly, it should be possible to achieve a drug release rate that is controlled by the rate of surface erosion. Two classes of biodegradable polymers successfully developed based on this rationale are the polyanhydrides [31] and poly (ortho esters) [32], the latter of which is the subject of this chapter. 

Aliphatic polycarbonates are a class of surface erosion biodegradable polymers attracting great interests due to their good biocompatibility, favorable mechanical properties and low toxicity (18,19), The polymerization of aliphatic cyclic carbonates such as trimethylene carbonate (TMC) have been extensively studied (20,21). 

Figure 2 (a) Bulk erosion of biodegradable controlled-release polymer implants leads to unpredictable release profiles, (b) Polymers exhibiting surface erosion release drug at nearly constant rate (zero-order kinetics) as they dissolve in water. (From Ref. 61.)... 

Table 8.5 Critical thickness (Lcritiail) of biodegradable polyesters where hydrolytic degradation mechanism changes from bulk erosion to surface erosion [200], (Reprinted from Biomaterials, vol. 23, von Burkersroda et al., Why degradation polymers undergo surface erosion or bulk erosion, pp. 4221-4231. Copyright Elsevier, 2002.)...

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