Unsegmented marine annelids as biomechanical models for soft robotics

This work investigates marine worms as a source of bioinspiration for soft robotics, focusing on Phascolosoma stephensoni (Annelida), an unsegmented sipunculan species with a fully eversible introvert capable of remarkable elongations. High-resolution micro-computed tomography was used to resolve the internal musculoskeletal architecture across functional configurations. Morphometric analyses of live specimens revealed strong differentiation between body regions: trunk length remains nearly constant during motion (7.26 ± 3.40 mm retracted vs. 7.70 ± 3.47 mm extended), whereas total body length more than doubles (from 8.87 ± 4.30 mm to 18.75 ± 7.35 mm), driven by introvert eversion at the tip. Tensile tests further highlighted distinct mechanical properties, with the trunk sustaining substantially higher strains before failure (≈ 90–110%) compared to the introvert (≈ 60–65%). Peristaltic locomotion was investigated using a mathematical model reproducing wave-like propulsion in unsegmented bodies at characteristic speeds of 0.5–5 mm s⁻¹ in confined media and showing close agreement with experimental observations. As an exemplary translation of these mechanisms, a soft robotic architecture based on magneto-responsive silicone was developed enabling stimulus-driven protrusions up to 2.5 times the initial length. Overall, this study provides a biologically grounded framework for innovative soft robotic systems inspired by unsegmented worms.