Title: When Robots Become Materials

Author (None): Aram Bahmani,

Abstract:

Robotic collectives increasingly exploit physical interaction, dense packing, and mechanical coupling, producing systems whose behavior is difficult to classify as swarms, active matter, or materials. As a result, concepts such as coordination, embodiment, rigidity, and load bearing are often applied across different contexts, obscuring when a collective should be treated as a population of agents and when it admits a material description. This Review introduces a physics-based framework that organizes collective robotic systems according to how forces, constraints, and stress transmission shape collective function. We define three orthogonal dimensionless descriptorsphysical coupling, embodiment dominance, and stress persistenceand combine them with an activitycoupling phase space to distinguish informational coordination from embodied and material-level behavior. Applied across representative platforms, this framework delineates clear mechanical boundaries between swarms, active collectives, cohesive ensembles, and robotic matter, and highlights current mechanical limitations that constrain the development of programmable robotic materials.