The versatile manipulation of micro and nano droplets is crucial across numerous scientific and engineering fields. An effective droplet manipulation technique, from a practical application standpoint, needs to be multifunctional, capable of transporting, merging, mixing, splitting, dispensing, and even heating droplets. This highly integrated approach simplifies the technological process and promotes broader adoption in standard laboratory settings. Furthermore, an ideal strategy must be applicable across scales. For instance, nanoliter (nL) scale droplet manipulation is highly demanded in high-precision analysis, while microliter (µL) scale manipulation is necessary in applications like microchemical plants. Currently, various external stimuli, including optical, electrical, acoustic, and magnetic fields, are employed for droplet manipulation. Magnetic manipulation stands out due to its advantages: it doesn't require complex circuitry and is independent of environmental transmittance and substrate surface charge. Existing magnetic actuation strategies for droplet manipulation fall into two categories: (I) incorporating magnetic additives into droplets and (II) integrating magnetically responsive materials into elastomeric substrates. The first category involves adding materials like hydrophobic magnetic powder, hydrophilic magnetic particles, steel beads, or ferrofluid to the droplet, enabling direct manipulation by a magnetic field for transport, merging, mixing, and dispensing. However, this leads to contamination, necessitating purification steps to obtain pure droplets. Some additives, like ferrofluid, might be incompatible with biological applications, limiting their use in biomedicine. In the second category, magnetic additives are embedded in elastomeric substrates to create magneto-responsive surfaces with microstructures. Droplet manipulation is achieved by the deformation of these structures, avoiding contamination. However, due to the fixed substrate and simple bending deformation, only basic manipulations like propulsion, mixing, capture, and release are possible; droplet splitting and dispensing remain unrealized. Both methods, while enabling basic manipulations like transport, merging, and mixing, have limitations: (a) droplet dispensing/splitting remains challenging, relying on structural morphology or surface energy traps; (b) on-demand droplet release after dispensing is not typically achieved, with dispensed droplets often entangled with additives or pinned to surfaces; (c) they mainly handle µL-scale droplets and struggle with nL-scale droplets. Therefore, a versatile manipulation strategy that integrates basic manipulations and extends to functionalities like daughter droplet dispensing, on-demand release, and cross-scale applicability (from nL to µL) is needed.

Numerous studies have explored magnetic droplet manipulation. Methods involving the addition of magnetic particles to droplets offer direct control but suffer from contamination issues. Conversely, incorporating magnetic materials into substrates avoids contamination but limits manipulation capabilities. Existing techniques often struggle with precise dispensing and on-demand release of daughter droplets, particularly at the nL scale. While some progress has been made in 3D manipulation and stirring, a comprehensive, cross-scale approach encompassing all these functionalities remains elusive. The literature highlights the need for a more versatile and less invasive method capable of handling droplets across a wide range of sizes, from nL to µL.

This research introduces a novel approach to droplet manipulation using a magnetically actuated Janus origami robot (JO-robot). The JO-robot is fabricated using polydimethylsiloxane (PDMS) doped with carbonyl iron particles, created through femtosecond laser writing and modification. This technique allows for the simultaneous cutting of the robot's contour, creation of creases, and surface modification in a single step, unlike other magnetic microrobot fabrication methods. The robot's rectangular sheet-like structure provides a simple design with various motion modes. Two creases are incorporated to enhance magnetic response and multi-mode droplet manipulation. During curing, the carbonyl iron particles form chains, improving magnetic response. The laser cutting direction ensures the chains are perpendicular to the long side of the robot, enabling stable tumbling motion. Selective laser modification creates a Janus structure: one side is superhydrophobic with low adhesion, and the other is hydrophobic with high adhesion. This Janus characteristic is crucial for the robot's interaction with droplets. The fabrication process involves creating a magnetic film using PDMS doped with carbonyl iron particles. A directional magnetic field aligns the particles into chains during spin coating and curing. Femtosecond laser writing is used to precisely cut and modify the magnetic film, forming the JO-robot's shape, creases, and Janus wetting properties. The laser parameters (power, speed, scanning pattern) are optimized to achieve the desired surface morphology and hydrophobicity. The superhydrophobic surface is created by laser modification, resulting in a contact angle of approximately 155.8°. The hydrophobic surface exhibits a contact angle of approximately 105.9°. The magnetic response of the JO-robot is characterized by observing its movement under a controlled magnetic field. The magnetic force and torque are calculated using equations (1) and (2) from the paper, relating the magnetization of the robot, the magnetic flux density, and the volume of the robot. The tumbling angle and moving distance are measured as a function of the magnets array's moving distance, quantifying the robot's mobility. The adhesion forces of the superhydrophobic and hydrophobic surfaces are measured using a surface force meter. Finally, the droplet morphology during dispensing and release is simulated using the Surface Evolver software, minimizing the total system energy to predict equilibrium shapes. This simulation considers the elastic energy of the robot, interfacial energies, gravitational energy, and magnetic energy. The simulation parameters, such as contact angles and surface tensions, are determined experimentally. For the nucleic acid extraction and purification application, the JO-robot is modified with chitosan. The laser-modified surface is activated using oxygen plasma, followed by soaking in a chitosan solution. This creates a chitosan coating on one side of the robot, enabling pH-responsive binding and release of nucleic acids. The extraction process involves sequential steps: (i) DNA binding with the chitosan-coated JO-robot in lysis buffer; (ii) transfer and washing steps using washing buffers; (iii) elution of the bound nucleic acids in elution buffer; (iv) precise dispensing of the eluted sample for PCR amplification. The entire process is carried out within a mineral oil environment to prevent droplet evaporation. A quantitative PCR instrument is used to analyze the amplified DNA and confirm the effectiveness of the extraction process.

The study demonstrates the successful fabrication and application of a magnetic Janus origami robot for versatile droplet manipulation. Key findings include: 1. **Omni-manipulation capabilities:** The JO-robot achieves three-dimensional transport, merging, splitting, dispensing, and release of daughter droplets, stirring, and remote heating of droplets across a wide size range, from 3.2 nL to 51.14 µL. The minimum volume of daughter droplet dispensed and released are 3.2 nL and 30.2 nL, respectively. This versatility surpasses the capabilities of existing magnetic actuation methods. 2. **Cross-scale droplet manipulation:** The JO-robot effectively handles droplets across multiple scales, from nL to µL. This cross-scale applicability is a significant advancement over previous methods that are limited to a narrower size range. 3. **On-demand droplet release:** Unlike previous methods, the JO-robot enables controlled release of dispensed daughter droplets, eliminating the need for surface traps or specialized structures. 4. **Efficient stirring and photothermal heating:** The robot's magnetically controlled rotation allows for efficient stirring, accelerating mixing. Moreover, its photothermal properties enable remote heating while stirring, enhancing mixing in viscous liquids. 5. **Successful nucleic acid extraction and purification:** The JO-robot, modified with chitosan, successfully extracts and purifies nucleic acids, demonstrating its potential for biological applications. The extraction and purification was validated by polymerase chain reaction (PCR) of human cytomegalovirus (HCMV), with successful amplification of HCMV DNA and a clear absence of signal in the no-template control (NTC). The Ct value obtained (32) confirms the successful detection of nucleic acids extracted using the developed JO-robot-based approach. 6. **Scalable Design:** The JO-robot design is inherently scalable, as demonstrated by the successful manipulation of both µL and nL scale droplets using JO-robots of varying sizes. Miniaturized JO-robots are shown to perform successfully even at the nL scale. 7. **Simulation Validation:** Numerical simulations using Surface Evolver effectively predict the droplet morphology during dispensing and release, supporting the experimental observations. The results obtained through simulations successfully match experimental outcomes, supporting the chosen methodology and its accuracy.

This research presents a significant advancement in droplet manipulation technology. The development of a versatile, cross-scale magnetic Janus origami robot provides a solution to the limitations of existing magnetic actuation strategies. The JO-robot's ability to perform multiple operations on droplets across a wide size range without contamination is a key contribution. The successful integration of functions such as dispensing, releasing, stirring, and photothermal heating makes it a powerful tool for various applications, including microfluidics, microchemistry, and biomedical diagnostics. The successful demonstration of nucleic acid extraction and purification highlights its potential for lab-on-a-chip devices and point-of-care diagnostics. The ability to manipulate nL-scale droplets extends the scope of magnetic droplet manipulation to high-sensitivity assays and biomedical analyses. The results suggest the JO-robot's design is readily adaptable to other experimental setups and conditions, promising wider applicability across various fields. Future research could focus on optimizing the robot's design for specific applications, exploring different materials for enhanced functionality and expanding the range of compatible liquids and biological samples.

This study demonstrates a novel magnetically actuated Janus origami robot for versatile and cross-scale droplet manipulation. The robot's unique design, combining origami deformation with Janus wetting properties, enables a wide range of functionalities, exceeding the capabilities of existing methods. The successful application in nucleic acid extraction and purification showcases its potential for biomedical applications. Future research can explore the robot's use in other microfluidic applications and investigate the use of different materials and designs for enhanced performance and broader applicability.

While the study demonstrates the effectiveness of the JO-robot, some limitations exist. The current fabrication method relies on femtosecond laser writing, which may not be readily accessible to all researchers. The performance of the robot might be affected by variations in the magnetic field strength and homogeneity. Further optimization of the robot's design and fabrication process could improve its efficiency and precision. The study focuses on specific liquids and biological samples; more research is needed to evaluate its compatibility with a wider range of materials. The current application in PCR is a proof-of-concept. More rigorous validation and comparison with existing methods is required for broader clinical applications.