Engineers at the University of California, San Diego, have developed a miniature device that’s sensitive enough to feel the forces generated by swimming bacteria and hear the beating of heart muscle cells. The device is a nano-sized optical fiber that’s about 100 times thinner than a human hair. In cultures of beating heart muscle cells from mice, the nanofiber can detect sounds down to -30 decibels — a level that’s one thousand times below the limit of the human ear.