Tunable and Selective Degradation of Amine-Reactive Multilayers in Acidic Media

We report the design of reactive and hydrolytically degradable multilayers by the covalent layer-by-layer assembly of an azlactone-containing polymer, poly(2-vinyl-4,4-dimethylazlactone), with an acid-degradable, acetal-containing, small-molecule diamine linker. This approach yields cross-linked multilayers that contain (i) residual azlactone reactivity that can be used for further functionalization after fabrication and (ii) acid-labile cross-links that can undergo pH-triggered degradation. Thin films and hollow capsules fabricated using this approach were relatively stable in slightly basic media (pH = 7.4) but eroded and degraded gradually in mildly acidic environments (pH = 5). The residual azlactones in these materials could be functionalized by reaction with hydrophilic or hydrophobic amines to tune physicochemical properties, including surface wetting and rates of degradation/erosion. Interestingly, our results reveal that rates of degradation could be tuned over a broad range (from ∼4 h to ∼10 days) simply by post-fabrication modification of the parent reactive material. We further demonstrate the potential of acetal-containing microcapsules to be used for the acid-triggered release of encapsulated cargo. The results of in vitro experiments reveal that microcapsules loaded with fluorescently labeled dextran can be internalized by mammalian cells and that cell uptake and intracellular degradation were also influenced by the types of functional groups installed post-fabrication. The introduction of acid degradability expands the range of stimuli that can be used to trigger the destruction of these reactive materials to include changes in pH relevant to chemical and biological processes. Our results also introduce an approach to tuning degradation profiles that differs from past strategies used to design degradable multilayers. We conclude that this approach provides a new, useful, and modular platform for the design of stimuli-responsive nano/biointerfaces with transient environmental stability.