The needles start out straight, packed in a tidy array no wider than a fingertip. Press the patch onto warm skin and they begin to move. Within two minutes they have hooked themselves into a coil, like dozens of tiny fingers tightening their grip, pulling the edges of a wound together from the inside. Nobody is turning a screw or pulling a thread. The needles are simply responding to the heat of the body, doing what they were printed to do. That printed-in behaviour is the work of a team led by...