topmost surface region during the initial hydrolysis period the f polyester having a short fluorocarbon group f7c1 showed a progressive decrease of surface fluorocarbon concentration as a function of hydrolysis time this result may be explained by a decrease in water permeation into the sample bulk of f10c2 l pla that arises from the greater segregation of fluorocarbon groups at the topmost polymer surface another example of copolymers are those that are prepared incorporating pdms segments at very low ca 0 7 3 by weight compositions the esca data for these polyester b pdms b polyester triblock copolymers not shown demonstrated that the film surface is enriched in pdms greater than 80 of the surface area when compared to the bulk polymer composition like siloxanes and fluorocarbon groups the segregated pmds component can serve to inhibit water penetration of the barrier layer the barrier layer is designed to block the deleterious species agents for a desired amount of time which can be selected and to then resorb biodegrade biodegradation and membrane breakdown are enhanced by the ability of the barrier layer to absorb water thus for example the addition of siloxane groups to polymers to create siloxane polymers as described above generally results in a polymer which forms a layer with increased water resistance and break down time in general some parameters which can be varied in order to produce a layer with the desired breakdown time are layer thickness density and porosity as well as the ph of the local microenvironment thickness is dictated by preparation parameters increased density can generally be achieved by the use of polymers with a longer chain length the porosity of the polymer can be.