Non-destructive and localized assessment of acidic microenvironments inside biodegradable polyanhydrides by spectral spatial electron paramagnetic resonance imaging

Spectral spatial electron paramagnetic resonance imaging (e.p.r.i.) was applied to characterize the microenvironment inside 2mm thick poly(1,3-bis-p-carboxyphenoxypropane-co-sebacic anhydride) (1/4) polymer discs. Incorporated nitroxides are highly immobilized in the dry polymer. Exposure to 0.1 M phosphate buffer (pH 7.4) resulted in the formation of a front of degraded polymer from outside to inside. The localized measurement of pH values inside the polymer matrix was performed, using a pH sensitive nitroxide (pK_a 6.1). A pH gradient was found between the degrading polymer and the buffer, but also inside the polymer. The pH inside the delivery matrix rises with time for 4.7 to 7.4. The results demonstrate that difficulties in the characterization of the pH, one of the most important parameters for drug delivery and polymer degradation, can be overcome by spectral spatial e.p.r.i.