Multilayer Hydrogel Capsules of Interpenetrated Network for Encapsulation of Small Molecules

We report on a facile capsule-based platform for efficient encapsulation of a broad spectrum of hydrophilic compounds with molecular weight less than 1000 g mol^-1. The encapsulated compounds extend from low-molecular-weight anionic Alexa Fluor 532 dye and cationic anticancer drug doxorubicin (DOX) to fluorescein isothiocyanate-dextrans with M_w ranging from 4000 to 40 000 g mol^-1. The pH-sensitive hydrogel capsules with an interpenetrated network shell are synthesized by layer-by-layer assembly of poly(methacrylic acid) (PMAA, M_w = 150 000 g mol^-1) and poly( N-vinylpyrrolidone) (PVPON, M_w = 1 300 000 g mol^-1) on 5 μm silica microparticles followed by chemical cross-linking of the PMAA multilayers. Following core dissolution, the result is a hollow microcapsule with PVPON interpenetrated in the PMAA network. The capsules exhibit a reversible change in the diameter with a swelling ratio of 1.5 upon pH variation from 7.5 to 5.5. Capsules cross-linked for 4 h display high permeability toward molecules with molecular weight under 1000 g mol^-1 at pH = 7.5 but exclude dextran molecules with M_w ≥ 40 000 g mol^-1. Encapsulation of small molecules was achieved at pH = 7.5 followed by sealing the capsule wall with 40 000 g mol^-1 dextran at pH = 5.5. This approach results in negatively charged molecules such as Alexa Fluor being entrapped within the capsule cavity, whereas positively charged molecules such as DOX are encapsulated within the negatively charged capsule shell. Considering the simple postloading approach, the ability to entrap both anionic and cationic small molecules, and the pH-responsiveness of the interpenetrated network in the physiologically relevant range, these capsules offer a versatile method for controlled delivery of multiple hydrophilic compounds.