Like milk on a supermarket shelf, donated blood comes with a sell-by date. The U.S. mandates a 42-day shelf life for banked blood because of the numerous physical and biochemical changes that stored red blood cells undergo. For instance, they show evidence of altered metabolism and oxidative damage. They also have increased membrane stiffness, which is a problem because stiffer cells cannot squeeze through narrow capillaries to deliver oxygen. Established techniques for monitoring stiffness of red blood cells require specialized equipment and training. Now, Gabriel Popescu of the University of Illinois, Urbana-Champaign, and colleagues show that an optical technique that uses a modified white-light microscope can measure red-blood-cell stiffness noninvasively (Sci. Rep. 2014, DOI: 10.1038/srep06211). Popescu’s lab developed spatial light interference microscopy (SLIM), in 2011. The technique measures membrane fluctuations of up to 50 nm. Complementing prior results, the researchers report, membranes of red blood cells grow stiffer in storage regardless of whether they still look normal under a microscope. Popescu is cofounder and CEO of imaging start-up Phi Optics. His team hopes to develop SLIM into a simple assay for red-blood-cell functionality in banked blood.