Milli-scale cellular robots that can reconfigure morphologies and behaviors simultaneously

The study addresses the challenge of creating small-scale modular robots capable of reversible docking and detaching, conscious approaching, and diverse behavior reconfiguration. Conventional small-scale robots excel at locomotion but are typically monolithic and difficult to transform for varied tasks. Prior modular approaches at small scales face limitations due to global actuation, lack of selective docking, and limited behaviors, while swarms suffer from non-selective control and constraints to liquid environments. The authors propose milli-scale cellular robots (mCEBOT) using heterogeneous assembly of short and long, soft-magnetic frustum units to simultaneously reconfigure morphology and behavior, enabling adaptability in unstructured environments (narrow spaces, high barriers, wet surfaces, hanging targets).

The paper surveys small-scale robots with varied architectures and actuation, such as helical microrobots enabling 3D navigation in viscous fluids; soft, film-like millirobots with multimodal locomotion in harsh environments; and micro-granular robots for targeted drug delivery. It reviews modular robotics since the CEBOT concept (1988), highlighting progress in unit design, connectors, coordination algorithms, and extensibility, yet noting difficulties at small scales due to lack of reliable, selective assembly/detaching strategies. Swarm robots can form patterns under global magnetic actuation but lack monomer control and are typically limited to liquids. Assisted assembly methods can form structures but often lack reversible assembly or behavioral diversity. The work contrasts these with their heterogeneous assembled mCEBOT, which promises good behavior/morphology reconfiguration, reversible assembly, and independent unit control.

Design and materials: Units are frustum-shaped with large aspect ratios and two differentiated section radii to guide magnetic pole distribution and reduce repulsion in side-by-side assemblies. Soft magnetic iron particles are embedded in a biocompatible Eudragit L100-55 polymer matrix (30 wt% Eudragit solution in ethanol mixed with 30 wt% iron powder). Fabrication: A magnetic-field-assisted process forms arrays of frustum units between two non-magnetic plates separated by a set distance D. A vertical magnetic field drives unit growth from the bottom to the top substrate under magnetic force, surface tension, and gravity; units are cured and extracted. Characterization: SEM reveals iron particles aligned along the unit’s long axis (easy magnetization axis). VSM characterization (−2000 to 2000 mT) shows soft-magnetic behavior with low coercivity (~4.6 mT); in the practical actuation range (−200 to 200 mT), magnetization is near-linear (~0.56 emu g−1 mT−1). Fabrication parameter study: Without a top substrate limit, maximum growth height Hm increases with particle mass (10–40%) and field (40–120 mT), up to ~2750 µm at 40% and 120 mT. Bottom radius R primarily depends on particle mass (ranging ~0–100 µm at 10% to ~200–500 µm at 40%), with minimal dependence on field. Bottom angle α increases with field (40–120 mT) from ~68° to ~85°, with minimal dependence on particle mass (except abnormal at 10% under <80 mT). Standard fabrication conditions used: 30% magnetic particles and 100 mT, yielding bottom radius ~200 µm, bottom angle ~83°, and magnetization ~55 emu g−1 at 100 mT. Two unit heights are made by adjusting D: short ~850 µm and long ~1200 µm. Assembly and separation strategy: Units magnetize under applied fields, aligning poles at their ends. Depending on relative positions, they stably assemble end-by-end (x > 0 along long axis) or side-by-side (x < 0). Heterogeneous dimensions yield different step sizes/trajectories under identical fields, enabling selective approach and docking of targeted units via path planning. Separation uses field removal; an oscillating field decreasing from 30 to 0 mT aids detachment. Bonding force measurement: Two identical long units are assembled and manually separated under applied fields (50–100 mT) using a micromanipulator and analytical balance, pulling along the connection-normal: axial for end-by-end and radial for side-by-side. Behavioral configurations: Four typical configurations are defined with corresponding driving fields and gaits: (1) Independent monomer for slipping under an oscillating field (100 mT, 1 Hz, −20° to 20°). (2) Bastinade shape for rolling under a continuous rotating field (100 mT, 0.25 Hz). (3) Biped structure for walking under spatially varying field directions (100 mT), alternating foot contacts as fulcrums. (4) Hoe mode for climbing by anchoring an external pivot under a steering field (100 mT, up to 160°). Environmental tasks and compound demonstrations: The robots adapt to narrow slits by separating into monomers (slipping), overcome barriers by assembling into bastinade (rolling), traverse wet surfaces by forming a biped (walking), and reach hanging targets by hoe-mode climbing using an environmental pivot. A compound task integrates these modes across wet surface (~300 µm film), a narrow slit (~0.7 mm), a barrier (~4.0 mm), and a hanging target (~3.5 mm) with timed reconfigurations. Environment exploration and path marking: Using selective assembly/detaching and localized field actuation, units are stepwise unloaded as markers at forks or obstacles in a maze-like environment, enabling later path following and marker recycling by new units.

- Magnetic properties: Soft-magnetic behavior with coercivity ~4.6 mT; near-linear magnetization ~0.56 emu g−1 mT−1 in ±200 mT range. Under 100 mT, unit magnetization ~55 emu g−1. - Fabrication parameter effects: Maximum growth height Hm increases with particle mass and field, from ~0 µm (10% mass, 40 mT) to ~2750 µm (40% mass, 120 mT). Bottom radius R depends on particle mass (10%: ~0–100 µm; 40%: ~200–500 µm), largely independent of field. Bottom angle α increases with field from ~68° to ~85° (40–120 mT). - Standard units: Bottom radius ~200 µm, bottom angle ~83°; two heights produced: short ~850 µm, long ~1200 µm. - Heterogeneous assembly: Two stable docking modes achieved—end-by-end and side-by-side—determined by relative position. Selective approach is enabled by different step sizes of short vs long units under the same field. - Bonding forces (two long units): End-by-end ~4.0 µN at 50 mT rising to ~25.5 µN at 100 mT; side-by-side ~1.5 µN at 50 mT rising to ~12.5 µN at 100 mT (end-by-end ≈2× stronger). - Motion performance metrics: • Slipping (monomer, 100 mT, 1 Hz, ±20°): ~28% body length forward per gait; vertical height fluctuation ~34% body length. • Rolling (bastinade, 100 mT, 0.25 Hz): height fluctuation ~100% of body length; effective step size ~200% of body length per gait. • Walking (biped, 100 mT, spatially varying): unilateral step size ~75% of leg spacing. • Climbing (hoe mode, 100 mT, steering 160°): maximum height reach ~142% of body length; in demo with a pivot, reachable height increased to ~4000 µm (~125% of initial maximum height) enabling retrieval of a hanging target at ~3500 µm. - Environmental adaptability demonstrations: • Narrow channel traversal theoretically down to channel diameter ~unit diameter via slipping monomers. • Barrier overcoming: Independent monomers could not surmount a 2 mm barrier, but a bastinade of 3 units rolled over it; in compound task, a ~4.0 mm barrier was overcome via rolling at 100 mT, 0.1 Hz. • Wet surface (~300 µm water film): Biped walked ~30 mm in 70 s and exited water coverage by 80 s. • Compound task timing: Separation into monomers at 84 s; slit crossing completed by 410 s via slipping; bastinade assembly from 410–440 s; barrier overcome at 452 s by rolling; reassembly into hoe shape at 584 s; hanging target retrieved within ~50 s thereafter. - Environment exploration and path marking: In a maze-like terrain, mCEBOT passed 4 obstacle areas (2 barriers, 2 slits) within 450 s, leaving 5 markers (3 forks, 1 start, 1 destination). A new unit followed the marked path in 256 s and recycled markers, demonstrating guidance efficiency and resource reuse.

The work demonstrates that heterogeneous assembly of soft-magnetic frustum units enables simultaneous reconfiguration of morphology and behavior at the milli-scale, addressing the longstanding challenge of conscious, reversible docking/undocking and diversified locomotion under global magnetic actuation. By exploiting unit height heterogeneity, the authors achieve differentiated responses to a uniform field, allowing selective approach and docking (end-by-end or side-by-side) with controllable bonding strengths and on-demand separation. Mapping architectures to motion modes (slipping, rolling, walking, climbing) allows adaptation to diverse unstructured environments—narrow slits, high barriers, wet surfaces, and hanging targets—validated in a compound task. The selective detaching capability supports new functions such as environment exploration with path marking and subsequent path following with marker recycling, illustrating practical benefits over monolithic small robots or virtual swarms. These results suggest a generalizable strategy for small-scale modular robots to extend operational versatility and robustness.

This study introduces mCEBOT, a milli-scale modular robotic platform achieving reversible, selective heterogeneous assembly of short and long units, enabling multiple morphologies (end-by-end and side-by-side connections) and corresponding motion behaviors (slipping, rolling, walking, climbing). The robots exhibit strong environmental adaptability, completing compound tasks and demonstrating environment exploration with path marking and recycling. Quantitative characterization confirms controllable magnetization, tunable fabrication geometry, and sufficient micro-Newton bonding forces for stable operation under ≤100 mT fields. Future directions proposed include extending heterogeneity beyond geometry to materials responsive to pH, temperature, humidity, or light, and leveraging selective detaching for tasks such as multi-regional delivery and stepwise drug release, as well as integrating finer 3D-printed structures to broaden functionality.