The above image is a shot of Georgia Tech’s latest robot posed next to a penny. The 3D-printed bot is roughly two millimeters in length — or about the size of the world’s smallest ants, per the school. The tiny devices are designed to move using vibration from a variety of sources, ranging from ultrasound to more traditional speakers.
https://techcrunch.com/2019/07/17/georgia-techs-ant-sized-micro-robots-move-through-vibration/