Crawling in a line with a soft skin

We designed a snake robot by drawing insights from heavy-bodied snakes, which primarily use a rectilinear gait. In this gait, snakes move in a straight line by generating sequential waves through muscle contractions and relaxations, relying on their flexible bodies and the anisotropic friction properties of their ventral skin. We identified three critical elements in this locomotion: the flexible rib structure, the muscular system, and the anisotropic ventral skin. We employed flexible mechanical metamaterials to translate these biological principles into our robotic system: an origami backbone as a flexible skeleton and a kirigami skin with overlapping ventral scales. The robot's actuation uses an integrated tendon-driven system, and its modular design allows for connecting multiple segments. We characterized the robot's performance across various surfaces, confirming the anisotropic friction of the ventral region with macroscopic overlapping scales, which we enhanced using microstructures. Additionally, we identified the optimal frequency range for contraction-relaxation waves while navigating different surfaces, noting that surface roughness affects the robot's performance but influences body deformation concurrently. To address this, we developed an embedded proprioceptive sensor by transforming the actuating tendons into resistive sensors, offering insights into friction and surface roughness without compromising the robot's flexibility or interfering with environmental interactions. Lastly, we implemented a bioinspired central pattern generator (CPG) neural controller to produce stable, rhythmic gaits and smooth transitions across terrains. Our snake robot enhances our understanding of rectilinear locomotion and enables navigation through narrow spaces and diverse featureless terrains.