Polymers have previously been made in the presence of living cells, for such purposes as encapsulating the cells or engineering their surfaces. But people haven’t known whether synthetic polymers could be made inside cells, given the presence of free-radical scavengers. Mark Bradley and coworkers at the University of Edinburgh now show that radical polymerization can indeed happen inside living cells (Nat. Chem. 2019, DOI: 10.1038/s41557-019-0240-y). The researchers gave cells a biocompatible photoinitiator and a variety of monomers, such as N-(2-hydroxypropyl) methacrylamide and sodium 4-styrenesulfonate, and observed how polymerization affected cells. “Polymerized cells” experienced delays in their cell cycle relative to unpolymerized cells, but they didn’t have significant DNA damage or reduced viability. Polymerized cells exhibited changes in their ability to migrate, possibly due to interactions of the polymers with the cells’ actin scaffolding. Intracellular formation of fluorescent polymers resulted in enhanced fluorescence that continued even when cells were used in multiple generations of cell cultures. The team also formed polymer nanoparticles inside cells. The success of the radical polymerization inside cells raises questions about free-radical-scavenging mechanisms, the researchers note.