Science 347, 543–548 (2015)

By combining a polyelectrolyte gel — a polymer that swells when it comes into contact with water — with a fluorescent label, scientists at the Massachusetts Institute of Technology in the USA have created a new type of high-resolution biological imaging that they call expansion microscopy. Adding the gel label and water to cultured human kidney cells or brain slices from a mouse caused the samples to expand in volume by about a factor of four, thus making it possible to perform super-resolution imaging using optical microscopy and resolve features that are ordinarily spaced closer than the diffraction limit of visible light. The team report a lateral resolution of 70 nm using the technique. Importantly, the gel's expansion is isotropic and does not introduce any distortion of the sample's anatomy. In addition, expanded specimens are largely transparent as they mostly consist of water. The team believe that by modifying the material properties of the polymer, in particular the density of its cross-links, imaging with even higher resolutions may be possible in the future.