Tunable stiffness exhibited in entangled collectives of aquatic worms and biomimetic soft robots

The California blackworm (Lumbriculus variegatus) is an aquatic worm that forms aggregate structures called worm blobs by twisting their slender bodies around other worms of its kind. These blobs provide a variety of advantages to the worm in its natural environment, ranging from protection from the elements, nourishment and better mobility than what is achievable in isolation. These entangled collectives have been found to tune their rigidity under varied stimuli. Here, we modulate the dissolved oxygen (DO) concentration inside the water to tune the worm blob's rigidity/entanglement. We measure the force applied by the blob under different DO concentrations, when subjected to external untangling forces. It is found that the blob applies a significantly larger force when in a high DO environment compared to when in a low DO one. We also find that their structural integrity is higher when present in high oxygen environments, showing typical solid like effects such as toppling over in the presence of unbalanced forces. The insights obtained from these biological aggregates have applications in development of soft robotic systems which can act as a "programmable glue" between substrates.