Cancer cells often break free from their original locations and circulate through the bloodstream, allowing them to form new tumors elsewhere in the body. Detecting these cells could give doctors a new way to predict whether patients’ tumors will metastasize, or monitor how they are responding to treatment, but finding these rare cells has proven challenging. A team of engineers from MIT, Penn State University, and Carnegie Mellon University is developing a novel way to isolate these cells: using sound waves to separate them from blood cells. To sort cells using sound waves, which offer a gentler alternative, the researchers built microfluidic devices with two acoustic transducers, which produce sound waves, on either side of a microchannel. Their new cell-sorting device is 20 times faster than the original version that they first reported last year, approaching the speed that would be necessary to make it useful for testing patient blood samples.