Researchers from MIT, Pennsylvania State University, and Carnegie Mellon University have devised a new way to separate cells by exposing them to tilted sound waves as they flow through a tiny channel. Separating cells with sound offers a gentler alternative to existing cell-sorting technologies, which require tagging the cells with chemicals or exposing them to stronger mechanical forces that may damage them. The dime-sized device holds potential for for detecting extremely rare tumor cells that circulate in cancer patients’ blood, helping doctors predict whether a tumor is going to spread. To test the device’s usefulness for detecting circulating tumor cells, the researchers tried to separate breast cancer cells known as MCF-7 cells from white blood cells. These two cell types differ in size, density, and compressibility. The microfabricated device successfully recovered about 71 percent of the cancer cells.