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Surface and bulk erosion
Surface and bulk erosion are two different forms of erosion that describe how a degrading polymer erodes. In surface erosion, the polymer degrades from the exterior surface. The inside of the material does not degrade until all the surrounding material around it has been degraded.〔Ulery, B.D., Nair, L.S., and Laurencin, C.T. (2011) Biomedical Applications of Biodegradable Polymers. ''Journal of polymer science'', 49, 832-864.〕 In bulk erosion, degradation occurs throughout the whole material equally. Both the surface and the inside of the material degrade.〔 Surface erosion and bulk erosion are not exclusive, many materials undergo a combination of surface and bulk erosion.〔Uhrich, K.E., Cannizzaro, S.M., Langer, R.S., and Shakesheff, K.M. (1999) Polymeric Systems for Controlled Drug Release. ''Chem Rev'', 99, 3181−3198.〕 Therefore, surface and bulk erosion can be thought of as a spectrum instead of two separate categories.
==Erosion Kinetics==
In surface erosion, the erosion rate is directly proportional to the surface area of the material. For very thin materials, the surface area remains relatively constant when the material degrades, which allows surface erosion to be characterized as zero order release since the rate of degradation is constant.〔〔 In bulk erosion, the erosion rate depends on the volume of the material.〔 Due to degradation, the volume of the material decreases during bulk erosion causing the erosion rate to decrease over time. Therefore, bulk erosion rates are difficult to control since it is not zero order.〔 To determine whether a polymer will undergo surface or bulk erosion, the degradation rate of the polymer in water (how fast the polymer reacts to water) and the rate of diffusion of water penetrating through the material must be considered. If the degradation process is faster than the diffusion process, surface erosion will occur since the material’s surface will quickly degrade before water has time to diffuse and penetrate through the material. If the diffusion process is faster than the degradation process bulk erosion will occur because water penetrates through the material before significant erosion occurs on the surface.〔Burkersroda, F., Schedl, L., and Göpferich, A. (2002) Why degradable polymers undergo surface erosion or bulk erosion. ''Biomaterials'', 23, 4221-4231.〕 A kinetics of the erosion of a polymer can be modified by changing the diffusion process or the degradation process. For example, blending a polymer with another polymer that is very reactive to water will speed up the degradation process and cause surface erosion. On the other hand, decreasing the dimensions of a material will allow water to travel to the center of the material more quickly, which speeds up the diffusion process and causes bulk erosion.〔〔
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