Polyanhydride Microcapsules Exhibiting a Sharp pH Transition at Physiological Conditions for Instantaneous Triggered Release

Stimulus-responsive microcapsules pose an opportunity to achieve controlled release of the entire load instantaneously upon exposure to an external stimulus. Core–shell microcapsules based on the polyanhydride poly(bis(2-carboxyphenyl)adipate) as a shell were formulated in this work to encapsulate the model active substance pyrene and enable a pH-controlled triggered release. A remarkably narrow triggering pH interval was found where a change in pH from 6.4 to 6.9 allowed for release of the entire core content within seconds. The degradation kinetics of the shell were measured by both spectrophotometric detection of degradation products and mass changes by quartz crystal microbalance with dissipation monitoring and were found to correlate excellently with diffusion coefficients fitted to release measurements at varying pH values. The microcapsules presented in this work allow for an almost instantaneous triggered release even under mild conditions, thanks to the designed core–shell morphology.