Mechanical Principles of Caterpillar Locomotion: Implications for Soft Robot Design

In recent years, emulating natural organisms has become a primary strategy for developing innovative and intelligent devices for various medical applications. Consequently, there has been enhanced attention to the locomotion mechanisms of animals to design adaptable micro- or nano-robots capable of navigating through diverse environments. Among non-legged crawlers, caterpillars exhibit a distinctive ability to sequentially elongate and contract their body segments, enabling efficient movement thanks to the muscle contractions and substrate grip. Although several models describing caterpillar locomotion have already been widely discussed in the inherent literature, none of them, as far as the authors are aware, have examined the crucial competition between elasticity and adhesion on the substrate as the primary factor that allows and maintains stable their crawling gait. To address this gap, experimental observations were conducted on in vivo Pieris Brassicae larvae, and a simplified one-dimensional discrete model has been developed. Elastic inter-masses springs represent the soft body segments, while Winkler-like constraints –with a specific adhesion threshold– simulate the interaction of prolegs with the supporting substrate. Additionally, a purpose-built algorithm has been implemented to deal with the nonlinearity inoculated by the update of the mechanical configurations needed for tracing back the subsequent crawl-substeps during the larva motion. By assuming a quasi-static approach, parametric analyses have been carried out to evaluate the influence of the above-mentioned issues on the overall behaviour of the system. Finally, by also addressing dynamic conditions, elasto-static assumptions have been validated, thus confirming the reliability of the proposed mechanical model for describing caterpillar locomotion. Starting from the experimental and numerical findings, we believe that the proposed strategy not only allows to faithfully capture the mechanical behaviour of caterpillars but also offers a predictive tool for defining design criteria for customizable bio-inspired soft robots.