Fabrication of inflated LbL microcapsules with a ‘bead-in-a-capsule’ morphology

Abstract

The fabrication of inflated Layer-by-Layer (LbL) microcapsules with a unique ‘bead-in-a-capsule’ morphology is presented. Currently, the fabrication of LbL microcapsules using conventional aqueous LbL techniques usually results in microcapsules with a two-phase system (LbL capsular wall with an air, liquid, solid or hydrogel core). Here, we present the fabrication of inflated LbL microcapsules with a unique three-phase system (LbL capsular wall, hydrogel microbead in an aqueous core) by using the Reverse-Phase LbL (RP-LbL) technique. The RP-LbL technique is performed in an organic solvent and allows encapsulation of water-soluble templates and molecules with high efficiency. Firstly, the RP-LbL technique is used to coat polymer layers onto agarose microbeads containing TRIS buffer for the formation of LbL capsular walls onto the microbeads and to minimize out-diffusion of encapsulated TRIS. Next, the polymer-coated agarose microbeads are transferred from an organic to an aqueous solvent where the TRIS molecules induce an osmotic pressure in the microcapsules' interior. This pressure drives the inflation of the LbL microcapsules that causes the expansion of the LbL capsular walls. Fluorescence staining reveals that the inflated LbL microcapsules consist of an agarose microbead suspended within the aqueous interior of the capsule but still attached to the LbL capsular wall at one point; thereby displaying a ‘bead-in-a-capsule’ morphology. It was demonstrated that the degree of inflation depends on the concentration of pre-loaded TRIS and the number of coated polymer layers. Also, ADOGEN^® 464 (a cationic surfactant) is required for the fabrication of the inflated LbL microcapsules. The mass of dextran macromolecules (65–2000 kDa) diffusing through the LbL capsular wall had decreased by at least 49% after expansion of the capsular wall. Inflated microcapsules were shown to be capable of controlling the distribution of two different materials internally. Hence, it is possible that inflated microcapsules can permit localized control over chemical or enzymatic reactions for future uses in biomedical applications.