Living cells can aggregate and work collectively to perform tasks such as wound healing. Modular robotic systems have been designed to mimic these kinds of processes, but they usually require some degree of centralized control over the component parts or involve a complex design that limits scalability. In this week’s issue, Hod Lipson and his colleagues present ‘particle robots’, which consist of a number of individual units and could offer a different approach to biomimicking. The disc-shaped particles can expand and contract like a camera aperture, but cannot move independently. By loosely coupling the particles together using magnets, the researchers were able to show that an external stimulus, in this case light, can induce the particles to change their behaviour from individual oscillation to collective motion towards the stimulus. The team demonstrated the effect in a particle robot composed of 24 units and simulated it in systems of up to 100,000 components, indicating that the collective behaviour should be readily scalable. The simulation also revealed that the collective should still function even if one-fifth of the component particles malfunctioned.