Undulatory Propulsion in Thin Rectangular Sheet Swimmers.

Undulatory body waves are a proven strategy for efficient motion, yet translating them into a flat, mechanically simple robotic swimmer has remained elusive. We introduce a magnetically driven thin rectangular sheet swimmer that can utilise a global magnetic field to produce two behavioural patterns: rapid travelling wave propulsion in low viscosity settings and large amplitude folding for grasping under high viscosity conditions. The swimmer is an ultrathin, micrometre-scale PDMS film with two edge-embedded micrometre-scale NdFeB cylinders. A finite element study guided this minimal dipole placement, balancing magnetic torque interference while preserving full flexibility. When the uniform magnetic field created by the 6-axis Helmholtz coil system oscillates at moderate frequencies, the sheet develops travelling waves that propel it horizontally and, by tilting the field axis, climb vertically, allowing the swimmer to traverse the entire workspace, squeeze through narrow gaps, and skirt obstacles with ease. Rotational magnetic field at lower frequencies triggers a reversible fold-unfold cycle that clutches and repositions passive micro-objects in highly viscous media, all without on-board electronics, pumps, or tethers. Endurance tests show neither magnet delamination nor film tearing, confirming the robustness of the embedded magnet approach. Because both locomotion and manipulation arise from the same global field, the platform is readily extendable to vision-guided closed-loop control. The synergy of field tunable undulatory propulsion, 3D workspace traversal, and viscosity-assisted grasping positions with this thin sheet swimmer is a promising tool for minimally invasive delivery and in-situ therapeutic procedures where a single soft robotic swimmer must navigate low-viscosity fluids yet interact with rigid particles or gel-like tissues.