Researchers at the University of Cambridge have used supramolecular host-guest chemistry to fabricate cargo-carrying microcapsules from microfluidic droplets in a single step (Science, DOI: 10.1126/science.1215416). Oren A. Scherman, Chris Abell, and coworkers chose cucurbituril as the host molecule because it can form a ternary complex with two guests—in this case, methyl viologen attached to gold nanoparticles and a naphthol-containing copolymer. The researchers combined individual solutions of the three components in a microfluidic device to form a single aqueous phase. The oil phase shears droplets off the aqueous phase at a T junction. As the oil phase carries the droplets through a winding channel, the components combine to form hollow microcapsules consisting of a dispersion of gold nanoparticles in a polymer mesh held together by cucurbituril. The size of the microcapsules ranges from 10 to 24 µm and varies with the ratio of the oil and aqueous flow rates. The researchers can load a variety of cargo—such as drugs, biological molecules, and even cells—in the microcapsules by adding a fourth solution to the aqueous phase during fabrication.