Researchers at Zhejiang University in Hangzhou, China, figured out how to precisely control a knot's geometry and friction so that they could 'program' it to open when tugged on with a given force. This allows a surgeon - or a robot - stitching up a wound to pull a suture closed with just the right amount of force, simply by tugging the free end of the knotted thread and stopping when the knot unfurls.