Getting exposed to too much ultraviolet light from the sun can lead to premature aging of the skin, cell damage, and even cancer. Sunscreens containing organic UV-filtering compounds reduce the amount of damage. But these sunscreens have problems of their own. In particular, they can penetrate the skin and generate harmful reactive oxygen species. A team of researchers at Yale University, led by biomedical engineer W. Mark Saltzman and dermatologist Michael Girardi, has made a new type of sunblock with bioadhesive nanoparticles that stay put on the skin’s surface (Nat. Mater. 2015, DOI: 10.1038/nmat4422). The nanoparticles are made of a polylactic acid-hyperbranched polyglycerol block copolymer that encapsulates the UV filter padimate O. The researchers made the nanoparticles stick to skin’s top layer by converting hydroxyls in the polyglycerol to aldehydes, which react with primary amines in proteins. Because of the bonds formed between the nanoparticles and skin proteins, the new sunblock is water resistant and must be removed by towel drying. In tests on mouse skin, the nanoparticle sunblock provided better UV protection than conventional sunscreens. Saltzman and Girardi’s team plans to test it next on humans.