While research and clinical use of nanoparticles for medical purposes is on the rise, scientists are raising new concerns over the ability of these tiny metal particles to gain access to privileged sites in the body. Gevdeep Bhabra, from Southmead Hospital (United Kingdom), and colleagues studied the effects of particles made from cobalt and chromium, either 30 billionths of a metre or four millionths of a meter wide, and typically used in implants such as artificial hips or knees. The researchers grew a thin, artificial membrane from human cells and placed the particles on the membrane; underneath, they placed fibroblast cells, which promote the formation of connective tissue. Although the team showed that the particles had not crossed the membrane, the fibroblast cells beneath were shown to have about 10 times as many damage sites in their DNA than the case in which no particles were used. The researchers conclude that: “The outcome, which includes DNA damage without significant cell death, is different from that observed in cells subjected to direct exposure to nanoparticles. Our results suggest the importance of indirect effects when evaluating the safety of nanoparticles. The potential damage to tissues located behind cellular barriers needs to be considered when using nanoparticles for targeting diseased states.”