Non-linear interactions between muscle and scaffold in biohybrid robots

Bio hybrid robots use living muscles for actuation. These muscles are formed in situ by dispensing a liquid mixture of myoblasts and extra cellular matrix (ECM) on the robot scaffold. The cells compact the ECM after curing, apply a static force on the scaffold and differentiate to myotubes. This differentiation process depends on the force that the cells generate and the amount of self-stretch they experience. The evolution kinetics of the muscle results from a complex cross talk between the cells and the scaffold deformation history. If the scaffold is too compliant then the cells cannot generate the force and the differentiation is hampered. If the scaffold is too stiff, then excessive cell force may cause them slide with respect to each other limiting myotube formation. The detailed nature of this cross talk is yet to be resolved. Here, we propose a robot scaffold that serves as a non-linear spring – stiff during early phase of differentiation but soft as differentiation proceeds. This allows the cells to generate sufficient force and self-stretch during the formation of myotubes, but large deformation of the scaffold with small increase of muscle force during actuation of the robot. The principle is applied on a biohybrid swimmer robot as a proof of principle.