Vast swarms of miniature robots are coming -- and they might be the answer to scrubbing our waters clean of lead. "Microbots" smaller than the width of a human hair could be highly effective and cost-efficient tools for removing lead and other contaminants from industrial wastewater, according to a new study published in the journal Nano Letters last month. In the space of a single hour, the study showed, self-propelled microbots could remove up to 95 percent of lead from water. Lead is commonly found in wastewater from mines or factories that make batteries and electronic devices, and can pose a serious risk to public health, as the water crisis in Flint, Michigan demonstrates. Heavy metal pollution can cost big cities billions of dollars a year, said Samuel Sánchez, co-author of the study and a research group leader at the Max Planck Institute for Intelligent Systems in Germany.