Three-dimensional deformation mechanism of a wire-actuated polytetrafluoroethylene fiber structure inspired by Euglena's pellicular strips (Experimental and modeling validation)

Conventional screw-type propellers are widely used but have pose environmental concerns, including sediment disturbance and potential harm to aquatic ecosystems. These concerns have prompted interest in biologically inspired propulsion mechanisms. Previous studies have explored biomimetic motion strategies, such as body-caudal fin (BCF) movements for high Reynolds number (Re) regimes and ciliary or undulatory locomotion for low Re environments. This study focuses on the latter and advancing a propulsion mechanism inspired by the pellicular strip gliding motion of Euglena, aiming for low environmental impact and cross-environmental adaptability. While our previous work demonstrated only two-dimensional deformation, the present study achieves three-dimensional motion by introducing a three-dimensional sliding deformation mechanism using circumferentially arranged porous polytetrafluoroethylene (PTFE) sheets actuated by tensioned wires. Unlike conventional sharp memory alloy (SMA) coil actuators, mechanical wire traction allows for rapid and spatially variable deformation. Time-series analyses of localized expansion and bending motion were conducted, and a geometric model was developed to describe surface profiles and fiber orientations along the longitudinal axis. Experimental results revealed asymmetric displacement distributions during both localized expansion and bending motions. The deformation patterns observed in localized expansion resembled the non-uniform peristaltic motion characteristic of Euglenoid movement. The surface function E(z1) and fiber orientation function D(z1), extracted via marker-based image tracking, effectively captured the deformation behavior. Although discrepancies in shrinkage ratios arose—likely due to structural limitations near constrained regions—the deformation cycle was completed within 1.2 seconds, corresponding to an actuation frequency of ~ 0.5 Hz. This frequency surpasses that of microorganisms like Euglena and is comparable to the lower end of swimming frequencies observed in fish. These findings demonstrate that the proposed mechanism can reproduce Euglena-like three-dimensional deformation and serve as a foundational platform for multimodal, bioinspired propulsion. With future improvements in constraint design and control coordination, this system shows strong potential for adaptive robotic applications across a wide range of Re conditions.