Femtosecond laser writing of ant-inspired reconfigurable microbot collectives

Collective behaviors in animals enable complex tasks that exceed individual capabilities. Synthetic microbot collectives inspired by such systems have been created using magnetic, optical, electrical, and acoustic fields to achieve swarming and reconfigurable assemblies. Yet, current swarms often exhibit unstable connections, require continuous external stimuli to maintain assembled morphologies, and lack selective control over individual units. Biological systems such as ants achieve robust collectives via interlocking through mouthparts or limbs, providing both stability and controllable separation. Inspired by this second type of biological collective, this work aims to develop deformable, interlocking microbots that can be reversibly assembled and disassembled with strong mechanical stability, precise selectivity, and minimal reliance on continuous external inputs. The research question is whether combining magnetic actuation with light-triggered local deformation can enable robust, reconfigurable assemblies that enhance functionality such as gap traversal, navigation in constrained environments, cargo transport, and targeted drug delivery.

Prior studies demonstrated stimulus-responsive microactuators and microswimmers with programmable deformations using materials like liquid crystal elastomers and hydrogels, enabling 3D shape morphing and adaptive mobility (e.g., Stitt et al.; Huang et al.). Collective micro/nanorobot assemblies under magnetic, optical, electrical, and acoustic fields have shown rich morphology reconfiguration and swarm behaviors, but suffer from limited individual controllability and the necessity of continuous external stimuli to maintain assemblies. Biological collectives such as ants form stable structures by interlocking, inspiring mechanically locked robotic collectives. Laser direct writing and two-photon polymerization (TPP) facilitate multimaterial microfabrication with embedded functions; metal photoreduction enables integrating plasmonic/photothermal elements (e.g., Ag) onto microstructures for local, rapid actuation. The gap in the literature is the lack of stable, reversible, and selective mechanical connections among multiple deformable microbots to function as robust collectives.

Design and materials: Each ant-inspired microbot comprises a magnetic photoresist body, two thermo-responsive hydrogel joints forming mandibles, and a photoreduced silver nanoparticle (Ag NP) coating at the mandible tips to act as a photothermal transducer. The hydrogel formulation is based on N-isopropylacrylamide (NIPAM) with N,N'-methylenebis(acrylamide) (MBA) as crosslinker, Diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide as photoinitiator, and polyvinylpyrrolidone (PVP K30) to enhance mechanical strength while minimally affecting photothermal sensitivity. Hydrogel LCST is ~32–39 °C. The magnetic photoresist contains Fe3O4 nanoparticles (oleic-acid surface-modified) dispersed in a commercial photoresist (e.g., SZ2080/ZSB00-type). Fabrication (three steps via femtosecond TPP and photoreduction): 1) Magnetic body: 3D ant-like rigid body is written by TPP in the magnetic photoresist, followed by development (ethanol, DI water). 2) Hydrogel joints: A secondary TPP step fabricates asymmetrical bilayer hydrogel joints (approx. 5 μm length, 4 μm width) that connect the mandibles to the body. Asymmetry is programmed by varying scanning reset time (SRT) to produce different crosslink densities across layers, enabling bending upon thermal actuation. Processing parameters held constant include laser power and scanning speed (33 mW, 320 nm step), with SRT as the main variable; SRT = 1 s yields maximum shrinking ratio ε ≈ −0.39, SRT = 3 ms yields ε ≈ −0.10. 3) Ag NP deposition: A silver precursor is drop-cast, and site-selective photoreduction coats Ag NPs on the mandible surfaces to create efficient photothermal heaters. Characterization: SEM imaging (with gold sputter coating) visualizes structural details and Ag deposition; EDS confirms Fe and Ag presence. VSM shows near-zero remanence and coercivity, indicating superparamagnetism for magnetic responsiveness. Actuation principles: - Optical actuation: Focusing a laser on Ag-coated mandibles generates local heating, raising joint temperature above LCST to drive rapid, reversible joint bending and mandible opening; removal of light leads to cooling and closure. Joint dynamics under 20 mW show max opening angle ~86° in ~8 ms and recovery in ~12 ms. - Magnetic actuation: A permanent magnet or a Helmholtz coil (~45 mT) applies forces for translation and torque for orientation, aligning bodies with the field. Rotational response characterization shows response time ~13.05 s, standard deviation −0.71, overshoot 2.35 under step changes of field direction. Assembly strategy: Coordinated magnetic translation/orientation positions microbots, while local light triggers selective mandible opening to mechanically interlock with another unit’s tail, forming robust 90° or 180° assemblies. Assemblies are maintained without continuous external input and can be selectively disassembled via targeted illumination and magnetic repositioning. Multi-unit and multimorph reconfiguration: By sequencing 90° and 180° connections, two or more units are reconfigured into L, J, O, V, S, W, C, and I shapes. A spatial light modulator can generate multi-foci for parallel control of multiple mandibles/units. Variations: - Non-magnetic units can be incorporated using a magnetic unit to manipulate and interlock them. - Other geometries (rectangular or disk-shaped microbots with multiple claws/gripping points) are fabricated and assembled into diverse patterns. Robustness testing: A horizontal vibration generator shakes a slide where the first unit is anchored; subsequent units are assembled by the proposed method. Stability of 90° and 180° connections is assessed across vibration conditions, recording intact vs failed assemblies. Mobility and tasks: Magnetic-field-driven locomotion is quantified for single and assembled units. Gap traversal is tested with a defined gap (L = 120 μm, H = 20 μm) relative to microbot body length (b1 = 97 μm) and CG-mandible distance (D1 = 50 μm). Maze navigation and drug delivery: A TPP-fabricated photopolymer square maze (600 μm side) with micro-fences (150 μm side) houses HeLa-EGFP cells. A doxorubicin (DOX)-loaded hydrogel cubic carrier (20 μm) is prepared by soaking a hydrogel block in 5 mg mL⁻¹ DOX for 1 h. Three-unit collectives reconfigure between I- and C-shapes to enter/exit narrow entrances and to grip, carry, and release the carrier to a targeted micro-fence region. Cell culture and imaging follow standard protocols; fluorescence over 12 h quantifies delivery efficacy.

- Rapid, reversible local actuation: Ag NP-enabled photothermal heating opens mandibles to ~86° in ~8 ms under 20 mW laser and closes in ~12 ms; stable over at least 1000 on/off cycles, indicating robust fatigue resistance. - Magnetic controllability: Superparamagnetic bodies enable remote translation and alignment; rotational response characterization under a uniform ~45 mT field shows response time ~13.05 s, standard deviation −0.71, and overshoot 2.35 following step changes. - Reconfigurable mechanical assemblies: Two-unit assemblies can form stable 90° and 180° interlocks using coordinated magnetic positioning and light-triggered mandible opening; assemblies persist without continuous external stimuli and can be selectively disassembled. Multi-unit reconfiguration produces L, J, O, V, S, W, C, and I morphologies. The strategy extends to non-magnetic units with assistance from a magnetic unit and to different microbot geometries. - Robustness: Vibration tests demonstrate that both 90° and 180° assemblies maintain integrity under various horizontal vibration conditions (qualitative stability maps reported). - Enhanced functionality via collectives: Gap traversal experiments with a gap of L = 120 μm, H = 20 μm show that single (N=1) and two-unit (N=2) configurations fail, whereas three-unit (N=3) assemblies successfully traverse a gap approximately one body length. - Environmental adaptability and task execution: In a micro-maze (600 μm square), a three-unit collective reconfigures between I-shape (to pass narrow entrances/exits) and C-shape (to clamp and transport a 20 μm DOX-loaded hydrogel carrier), delivering cargo to targeted HeLa cell regions. Live-cell fluorescence over 12 h increases in the experimental micro-fence with delivered DOX relative to the control, with quantitative analysis (n = 3) showing higher fluorescence intensity in the treated region, confirming successful localized drug delivery.

The study addresses key limitations of swarming microbots by introducing mechanically interlocking, light-triggered connections that remain stable without continuous external fields. Combining magnetic global actuation with localized optical control enables selective engagement and disengagement of individual units, allowing precise reconfiguration among multiple morphologies suited to different tasks and constraints. This ant-inspired interlocking yields assembly robustness (vibration-insensitive) and broadens capabilities such as traversing gaps that single or two-unit systems cannot achieve, and adapting morphology for navigation through constrictions, cargo handling, and targeted delivery. The reconfigurable assembly of non-magnetic units via magnetic proxies further expands applicability to mixed-material collectives. The results demonstrate that a cooperative design that separates global transport (magnetic) from local locking/unlocking (optical) can significantly enhance functionality and control granularity in microbot collectives, relevant to microfluidics, on-chip manipulation, and biomedical interventions.

Inspired by ant interlocking, the authors developed femtosecond-laser-fabricated, multimaterial microbots whose Ag NP-enabled light-driven mandibles and magnetic bodies enable robust, reversible, and selective interlocking. The collectives form multiple stable morphologies (90°/180° and beyond), maintaining assemblies without continuous energy input, and perform advanced functions such as one-body-length gap traversal, maze navigation, cargo transport, and localized drug delivery to cells. This work showcases a generalizable strategy to construct reconfigurable, multimodal, multifunctional microbot collectives via coordinated magnetic and optical control. Future directions include extending from 2D planar assemblies to fully 3D collectives, integrating additional physical fields for more versatile assembly/disassembly, scaling control to larger numbers of units via advanced beam shaping and feedback, and optimizing materials and designs for in vivo compatibility and task-specific performance.

- Current demonstrations primarily manipulate and assemble units in a quasi-2D plane; extending to fully 3D environments and complex geometries is future work. - Shape reconfiguration relies on the presence of environmental gripping points to facilitate selective actuation during global magnetic control. - The thermo-responsive hydrogel actuation depends on local photothermal heating near LCST; environmental thermal conditions or biofluid heat dissipation may affect performance. - Parallel control of many units requires complex optical setups (e.g., spatial light modulators) and careful calibration; scalability to large collectives remains to be fully validated. - The robustness tests are qualitative and limited to horizontal vibrations; broader mechanical perturbations and long-term stability in biologically relevant fluids need evaluation.