Abstract
Poly(ortho esters) are a polymer system containing backbone linkages that are stable in base, hydrolyze at very slow rates at the physiological pH of 7.4, and become progressively more labile as the pH is lowered. A major rationale for developing this system was a need for a polymer capable of a wide variety of erosion rates and where erosion could be confined to the surface of a solid device. Then, any selected, constant release rate can be achieved for therapeutic agents physically incorporated into the matrix by maintenance of an appropriate device geometry. Surface erosion and variations in delivery rates can be achieved by either stabilizing the interior of the polymer with a base so that erosion can take place only in the surface layers where the basic excipient is neutralized by the external medium or by using acidic excipients (Heller, 1980) incorporated into the typically highly hydrophobic matrix. In this latter case, surface erosion takes place only in the surface layer where the excipient is exposed to water.
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