How do you design and build a robot that you can’t even see? And what would you use it for? In May, Horizon explores the developing field of nanorobotics and its potential applications. We speak to Prof. Brad Nelson at ETH Zurich in Switzerland whose team found that the nanobots they were working on destroyed the drugs they were meant to be delivering, so are now repurposing them to purify water. We speak to researchers who are using the origami-like properties of DNA to make tools such as nanorobotic boxes with lids that open, and others that have created a molecular robotic arm that can pick up, reposition and release molecules. And because tasks like going into a blood vessel to dissolve a dangerous clot would be ideal for a nanorobot, we find out how scientists are devising ways to enable nanorobots to travel through the bloodstream.