Bio-hybrid soft robots with self-stimulating skeletons

Bioinspired hybrid soft robots combining living actuation and synthetic components are an emerging field in the development of advanced actuators and other robotic platforms (i.e. swimmers, crawlers, walkers). The integration of biological components offers unique properties (e.g. adaptability, response to external stimuli) that artificial materials cannot replicate with accuracy, being skeletal and cardiac muscle cells the preferred candidates for providing contractile actuation. Here, we present a skeletal-muscle-based swimming biobot with a 3D-printed serpentine spring skeleton that provides mechanical integrity and self-stimulation during the cell maturation process. The restoring force inherent to the spring system allows a dynamic skeleton compliance upon spontaneous muscle contraction, leading to a novel cyclic mechanical stimulation process that improves the muscle force output without external stimuli. Optimization of the 3D-printed skeletons is carried out by studying the geometrical stiffnesses of different designs via finite element analysis. Upon electrical actuation of the muscle tissue, two types of motion mechanisms are experimentally observed: i) directional swimming when the biobot is at the liquid-air interface and ii) coasting motion when it is near the bottom surface. The integrated compliant skeleton provides both the mechanical self-stimulation and the required asymmetry for directional motion, displaying its maximum velocity at 5 Hz (800 micrometer second-1, 3 body length second-1). This skeletal muscle-based bio-hybrid swimmer attains speeds comparable to cardiac-based bio-hybrid robots and outperforms other muscle-based swimmers. The integration of serpentine-like structures in hybrid robotic systems allows self-stimulation processes that could lead to higher force outputs in current and future biomimetic robotic platforms.