By creating hydrogel microparticles that mimic the flexibility of red blood cells, scientists at the University of North Carolina, Chapel Hill, have been able to extend the blood circulation lifetime of the particles, which can be used in drug delivery applications (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1010013108). Red blood cells can contort themselves to wriggle through narrow channels, and this flexibility helps them remain in circulation. Joseph M. DeSimone and coworkers synthesized red blood cell mimics from 2-hydroxyethyl acrylate that they cross-linked with poly(ethylene glycol) diacrylate. The researchers tuned the hydrogel flexibility by varying the amount of cross-linker. The researchers injected the hydrogel particles into mice and imaged them flowing through the animals’ blood vessels. They found that as the flexibility of the particles increased, the circulation time likewise increased. The most flexible microparticles, which contained only 1% cross-linker, had a circulation half-life of more than three days, compared with less than three hours for those with 10% cross-linker. In addition, the more flexible microparticles bypassed organs with complex vasculature, such as the lungs, which trapped more rigid particles.