FreeBOTs bring together aspects of swarm bots and modular bots. Each one is an iron sphere with motorized wheels and a permanent magnet. They can do a lot by themselves, but when a few of these spheres get together, their functionality expands greatly. More from IEEE Spectrum:
Since each robot has a ferromagnetic shell plus an internal permanent magnet, attaching one robot to another robot is relatively simple. Two robots can touch each other without connecting, since the iron shells are not permanent magnets. To make the attachment, the permanent magnet on the bottom of the little internal vehicle has to get close to the point at which the two spheres are touching, and when it does, the permanent magnet excites a magnetic field in the shells of both robots, causing them to stick together. The exact alignment is very forgiving, and the connection can happen absolutely anywhere on each robot, which is far more versatile than just about any other modular robotic system. Disconnecting simply involves moving the internal vehicle away from the connection point, which removes the magnetic field. Combining multiple FreeBOTs is where things get interesting, since it’s possible to create blobs of robots or chains of robots or use a small pile of robots to help one module overcome obstacles. Ferromagnetic surfaces can be leveraged even more by a swarm than by a single module.