Magnetic Janus origami robot for cross-scale droplet omni-manipulation

Versatile manipulation of micro- and nanoliter droplets is needed for practical applications where an integrated technique should support transport, merging, mixing, splitting, dispensing, and heating. Cross-scale applicability is also important, spanning nL for high-precision analysis to µL for microchemical applications. Existing external-field strategies include optical, electrical, acoustic, and magnetic approaches. Magnetic manipulation offers advantages such as no complex circuitry and independence from optical transmittance or surface charge. Prior magnetic strategies fall into two categories: (I) adding magnetic additives into droplets and (II) embedding magnetically responsive materials into elastomeric substrates. Additive-based approaches enable transport, merging, mixing, and dispensing but risk contamination, require purification, and can be incompatible with biological samples. Substrate-based methods avoid contamination but are limited by fixed geometries and simple bending, typically enabling propulsion, mixing, capture, and release; splitting and dispensing remain challenging. Moreover, many magnetic strategies primarily handle µL-scale droplets and struggle with nL-scale manipulation due to method constraints and fabrication limits. Therefore, a versatile, integrated, and cross-scale droplet manipulation strategy that adds functions like daughter-droplet dispensing and on-demand release is urgently needed. Here, a facile, versatile magnetic Janus origami robot (JO-robot) is presented, combining origami deformation and Janus wetting to enable omni-manipulation (3D transport, merging, splitting, dispensing and releasing pure daughter droplets, stirring, photothermal stirring) across droplets from several nL to tens of µL. The JO-robot tumbles under a magnetic field to wrap droplets for directional transport and acts as a detachable smart tool to execute multiple tasks without contaminating droplets.

Magnetic droplet manipulation methods include: (1) Magnetic additives within or on droplets (e.g., hydrophobic/hydrophilic magnetic particles, steel beads, printed frames, ferrofluids), which allow direct manipulation but contaminate samples, require subsequent purification, and can be biologically incompatible. (2) Magneto-responsive elastomeric substrates (microcolumns, microdimples, cilia, plates, tubes) that deform under fields to move droplets without contamination. However, fixed substrates and limited deformation modes restrict capabilities to propulsion, mixing, capture, and release, with droplet splitting/dispensing and on-demand release not achieved. Both approaches largely handle µL-scale droplets; nL-scale manipulation remains difficult. The need remains for a contamination-free, integrated, cross-scale method enabling dispensing, splitting, and on-demand release without reliance on surface traps or additives.

Design and materials: The JO-robot is a rectangular PDMS sheet doped with carbonyl iron particles, fabricated via femtosecond laser cutting and selective surface modification to form a Janus wetting structure. Typical size is 2 mm × 7 mm (varied for scaling studies). Two shallow creases are laser-written on the top side to guide folding. During PDMS curing in a directional magnetic field, iron particles align into chains perpendicular to the long edge to enable stable tumbling around the long side under a periodic magnetic field. Surface properties: One laser-modified side is superhydrophobic (contact angle 155.8° ± 1.4°, low adhesion; lateral adhesion force ~5.78 µN), while the unmodified side is hydrophobic (105.9° ± 2.1°, higher adhesion; lateral adhesion force ~67.54 µN). Crease dimensions are ~138.1 ± 7.5 µm wide and ~56.2 ± 3.9 µm deep, optimized to balance folding and unfolding without residue. Magnetic actuation: A moving magnets array generates a periodic magnetic field producing force F = Vm (M·∇)B and torque T = Vm M × B to drive tumbling, approaching, and manipulation. The JO-robot tumbles 0–360° synchronously with magnet motion; with wrapped droplets it maintains controlled tumbling and directional transport. Rotational stirring uses a rotating permanent magnet (25 mm cube) generating a rotating field; torque drives robot rotation on droplet surfaces. Droplet manipulation workflows: - Wrapping and transport: Capillary forces during contact fold the robot along creases to wrap part/all of a droplet; the robot then tumbles to transport on superhydrophobic substrates, including 3D transport on vertical and inverted surfaces. - Daughter droplet dispensing: The hydrophobic side first contacts the droplet; sequential folding forms a triangular cross-section around a portion of liquid. Continued tumbling creates and thins a neck at the joint to the parent droplet; breakage under Laplace pressure dispenses a daughter droplet. Folding time is controlled by magnet array speed (~68.75 ms to ~14.48 s) and is not affected by liquid surface tension. - On-demand daughter droplet release: By tilting, increasing field strength (magnetic flux density >265.0 ± 24.7 mT in x, ~0.5 ± 0.2 mT in y, ~0.6 ± 0.2 mT in z), and applying squeeze then push-off via tumbling, the robot extrudes and releases a spherical daughter droplet without adhesion. - Splitting: The robot divides a large droplet into two similar droplets without auxiliary structures. - Stirring and photothermal stirring: Under a rotating magnetic field, the robot spins on the droplet to induce rapid mixing. Near-infrared (808 nm) irradiation exploits carbonyl iron photothermal absorption for simultaneous heating and stirring, beneficial for viscous fluids (e.g., glycerol). Simulations: Surface Evolver minimizes total energy E = Er + Ex + Ey + Eg + Em to predict equilibrium droplet shapes during dispensing and release (quasi-static: Ca ~10^-3, We ~10^-6). COMSOL is used for magnetic field and internal flow simulations; flow simulations show velocity fields during stirring (edge regions away from the rotation axis move fastest). PCR application (nucleic acid extraction): The robot surface is plasma-activated and one side coated with chitosan (0.2 g in 1% acetic acid; 18 h soak) to create a pH-responsive capture surface. In mineral oil to prevent evaporation, steps include: (i) DNA binding at pH 5 (rotation 120 rpm, 5 min) at the water–oil interface with the chitosan side facing aqueous phase; (ii) sliding the unfolded, DNA-bound robot out of the droplet along the interface with a single magnet and transferring to washing buffer 1; (iii) washing (120 rpm, 30 s); (iv) transfer; (v) washing buffer 2; (vi) transfer; (vii) elution at pH 9 (120 rpm, 5 min); (viii) precise dispensing of ~2 µL eluted nucleic acid using a size-optimized robot (1.52 mm × 5.24 mm); (ix–xi) transfer, mix with PCR reagents, and perform PCR. A microdroplet chip comprises a silicone frame on a superhydrophobic glass with local hydrophilic pads for droplets and is overlaid with mineral oil. Fabrication details: Magnetic film prepared by mixing PDMS:iron powder:cross-linker (10:4:1 by mass), degas, spin-coat (1200 rpm, 15 s), align particles in field between NdFeB magnets (100 × 50 × 10 mm), cure at 100 °C for 30 min to yield ~95.9 ± 3.5 µm thick film. Femtosecond laser (800 nm, <100 fs, 1 kHz) performs, in one step, contour cutting (300 mW, 10 mm/s, 14 passes), crease writing (300 mW, 50 mm/s, 14 passes), and surface modification (300 mW, 50 mm/s, 3 passes, 60 µm grid) to realize Janus wetting. Setups and characterization: Superhydrophobic substrates prepared by commercial spray; imaging by CCD; contact angles by goniometer; lateral adhesion via custom force meter; fluorescence mixing monitored with LED excitation (450–460 nm) and green filter; NIR laser 808 nm, 430 mW (spot ~7 × 5 mm, 107 mm distance) for heating.

- Omni-manipulation functions demonstrated: 3D transport (including vertical and inverted surfaces), merging, splitting, daughter droplet dispensing and on-demand release, stirring, and photothermal stirring. - Cross-scale operation: Droplets from ~3.2 nL to ~51.14 µL handled. Minimum dispensable daughter droplet ~3.2 nL using a mini robot (0.23 mm × 0.94 mm). Maximum dispensable ~51.14 µL (robot 4.03 mm × 14.44 mm). Minimum releasable daughter droplet ~30.2 nL; releasable range 42.21 nL ± 9.5 nL to 16.46 µL ± 0.39 µL. - Transport performance: Directional transport speed up to 68.47 ± 2.1 mm/s for a 6.5 µL droplet; long-distance/high-speed ejection achievable over ~33.04 mm at ~75.09 mm/s with simple surface modification. Liquid metals with high surface tension can be transported. - Magnetic actuation ranges: Typical droplet manipulation (tumbling, dispensing, transport) requires magnetic flux density between 81.5 ± 5.7 mT and 155.4 ± 10.4 mT; on-demand release requires stronger fields (>265.0 ± 24.7 mT in x, ~0.5 ± 0.2 mT y, ~0.6 ± 0.2 mT z). - Surface properties: Superhydrophobic side contact angle 155.8° ± 1.4° with lateral adhesion ~5.78 µN; hydrophobic side 105.9° ± 2.1° with adhesion ~67.54 µN. Crease width ~138.1 ± 7.5 µm, depth ~56.2 ± 3.9 µm optimized for folding and release. - Dispensing control: Folding time tunable from ~68.75 ms to ~14.48 s via magnet array speed. Microdroplet arrays generated with good uniformity: three sequential daughter droplets of ~5.3, ~5.9, and ~5.8 µL. - Stirring and mixing: Internal flow simulations show peak liquid speed ~52.86 mm/s at robot angular speed 8π rad/s. In water, active stirring achieves near-homogeneous mixing within ~5 s versus ~100 s by diffusion (~20× faster). In glycerol, photothermal stirring heats from 24.3 °C to ~81–82 °C in 180 s and achieves mixing within ~240 s versus ~30 min without heating. - Integrated operations: Demonstrated micro-sampling from one droplet, transport, merging with another, and rapid mixing. Chemical reaction example (NaOH + HCl) followed by on-demand sampling and indicator test (Litmus turning blue) for instant readout. - nL-scale integration: Mini JO-robot (0.23 mm × 0.94 mm) dispensed ~7.8 nL from a ~177 nL droplet, transported and merged with ~132 nL droplet, and rapidly mixed. - PCR-related application: Chitosan-coated JO-robot enabled on-chip nucleic acid extraction and purification in mineral oil with precise ~2 µL template dispensing; successful qPCR detection of HCMV with Ct = 32 and no amplification in NTC.

The JO-robot addresses key limitations of existing magnetic droplet manipulation by eliminating magnetic additives within droplets, thereby avoiding contamination and purification steps, and by enabling functions beyond fixed substrate deformations. Its origami mechanics combined with Janus wetting enable reliable capillary folding for wrapping, controlled neck formation for precise dispensing, and low-adhesion release of pure daughter droplets on demand without surface energy traps. The strategy operates robustly across a wide droplet volume range (nL to tens of µL), meeting cross-scale application needs. The integration of rotational stirring and photothermal heating substantially accelerates mixing, especially in viscous media, facilitating rapid reactions. The demonstrated biochemical workflow shows that the JO-robot can carry out sequential, on-chip operations (binding, washing, elution, precise aliquoting, reagent mixing) culminating in successful PCR detection, highlighting relevance to diagnostics and microassays. Overall, the findings demonstrate a multifunctional, untethered, and scalable platform for precise droplet manipulation, offering versatility and practicality across chemistry, biology, and microfluidics.

A magnetic Janus origami robot is presented that enables omni-manipulation of droplets, including 3D transport, merging, splitting, precise dispensing, on-demand release, stirring, and remote photothermal heating. Leveraging capillary-driven folding, magnetic actuation, and Janus wetting, the system handles droplets from ~3.2 nL to ~51.14 µL, outperforming prior magnetic strategies that either contaminate droplets or lack advanced functions like dispensing and release. The platform integrates multiple operations to support continuous workflows and has been validated for on-chip nucleic acid extraction and purification with successful PCR amplification. With its contamination-free operation, cross-scale capability, and functional integration, the JO-robot shows strong potential for fine chemical processing, medical diagnostics, and microfluidics requiring precise reagent handling and rapid reactions. Future work can further expand application-specific designs, optimize magnetics and photothermal control for diverse reagents, and integrate with lab-on-chip systems for automation.