Microcapsules are of great interest in biomedical applications, the food industry, cosmetics and as catalyst supports. Multifunctional capsules that respond to more than one stimuli are needed in many of these applications to achieve controlled release and enable smart manipulation. With such capsules, targets can be addressed with one stimulus, while a second stimulus triggers release.

Here, we demonstrate a novel method to obtain microcapsules with polymer shells of controlled macroporosity and mechanical properties that can be tuned within a wide range. These capsules contain two liquid compartments: an aqueous core and an oil phase that permeates the macroporous polymer shell. Simultaneously loading of water-soluble chemicals in the core and oil-soluble chemicals in the liquid phase of the biphasic shell can potentially enable the creation of capsules that respond to multiple external stimuli.

Microcapsules are produced by microfluidics, using a co-flow flow-focusing glass capillary device to make water-oil-water (W/O/W) double emulsion templates. A mixture of acrylate monomers (glycidyl methacrylate and ethylene glycol dimethacrylate) and porogens (phthalate-based, alkane or alkanol) is used as oil phase. Heterogeneous polymerization of the acrylate monomers leads to a biphasic structure: a network of polymer particles permeated by the liquid porogen and covered with a thin, tight polymer skin. The diameter of the polymer particles and the pore size can be tuned by varying the amount of porogen or by mixing porogens with different solubility parameters. The permeability of the shell is determined by the polymer skin and the porous structure. Compression tests of single capsules show that the elastic modulus and the force at break depend on the microstructure of the porous network. Highly interconnected networks of many small polymer particles lead to strong and stiff capsules, while larger and fewer particles result in weaker microcapsules. The interstitial liquid porogen can also be loaded with an oil-soluble encapsulant or used as carrier for a chemical trigger. Incorporation of glycidyl methacrylate monomers results in polymer particles with epoxy-functionalized surfaces, which can be further reacted with amine-based functional compounds. In a proof of concept design, we exploited such surface amine groups to create magnetic capsules by covalently binding dopamine-coated iron oxide nano-particles to the outer shell surface.

The proposed multi-compartment structure provides a rich platform for the future design of microcapsule systems capable of responding to multiple external stimuli.

Eve Loiseau

Complex Materials, Department of Materials, ETH-Zurich

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