Tuning the formation and degradation of layer-by-layer assembled polymer hydrogel microcapsules

Engineered polymer capsules are finding widespread importance in the delivery of encapsulated toxic or fragile drugs. The effectiveness of polymer capsules as therapeutic delivery vehicles is often dependent on the degradation behavior of the capsules because it is often necessary to release the encapsulated drugs at specific times and in certain locations. Herein we investigate the parameters that govern the formation and degradation of a recently introduced new class of polymer hydrogel capsules based on disulfide cross-linked poly(methacrylic acid). We report a new and efficient method for the synthesis of thiol-functionalized poly(methacrylic acid) (PMA(SH)), the main component of the capsules. Polymeric capsules were synthesized by the layer-by-layer deposition of PMA(SH) and poly(vinylpyrrolidone) (PVPON) on silica particle templates, followed by cross-linking the PMA(SH) layers and removing PVPON and the template particles. The disulfide cross-links provided a redox-active trigger for degradation that was initiated by a cellular concentration of glutathione. We demonstrate that increasing the degree of PMA(SH) thiol modification affords direct control over the thickness of the polymer film and the degradation rate of the polymer capsules. Furthermore, the degradation rate of the PMA(SH) capsules was independent of film thickness, suggesting a bulk erosion process.