By Soham NandiReviewed by Frances BriggsJun 17 2025A recently published Nanotechnology and Precision Engineering article introduces a novel hydrophilic hard-magnetic soft robot (HMSR) for precise droplet manipulation.
Study: Hydrophilic hard-magnetic soft robots: A new approach for precise droplet manipulation. Image Credit: Gumpanat/Shutterstock.com
Made from NdFeB particles and Ecoflex elastomer, the porous HMSR enables high-speed droplet transport and splitting under weak magnetic fields, whilst its chemical stability allows handling of acidic/alkaline droplets and targeted microparticle removal, advancing applications in biochemical assays, material synthesis, and surgical robotics.
Background
Droplet manipulation via magnetic fields offers advantages like contactless control and biocompatibility, making it valuable for biochemical and medical applications.
Previous methods relied on either magnetic particles within droplets, which suffer from weak forces, contamination risks, and limited droplet volumes; or magnetically responsive soft materials, which are complicated to fabricate and droplet manipulation is less flexible.
Magnetic soft robots show promise, but their hydrophobic surfaces hinder droplet interactions.
This study introduced HMSR made of NdFeB particles and Ecoflex, treated with oxygen plasma for enhanced wettability. The HMSR achieved precise, high-speed droplet transport (200 mm/s for 50 µl) and splitting under low magnetic fields (20 mT).
Its chemical stability has improved acidic/alkaline droplet handling and microparticle removal, improving flexibility, force, and contamination, allowing for further applications in biomedicine and material synthesis.
Fabrication and testing of HSMRs
The HSMRs were developed in a multi-step process. HMSRs were prepared by mixing NdFeB particles (5 micrometers (μm)), sugar particles (800-1100 μm) as pore templates, and Ecoflex elastomer in a 0.3:0.2:1 mass ratio.
The mixture was cured at 85 ° C for two hours and then dissolved in water to create porous structures. It is then treated with oxygen plasma for hydrophilicity and magnetized under a 1200 Volts (V) pulsed field. Plasma treatment parameters were optimized using ink infiltration tests, with complete hydrophilicity throughout the porous structure achieved in seven minutes.
When driven by a moving magnet in a 20 mT field, the HMSRs showed excellent rolling capability across various surfaces such as polystyrene, paper, and sandpaper. The study found that the robots could transport droplets up to 900 μl at controlled speeds (10-200 mm/s), with three distinct manipulation modes based on speed and droplet volume: transport, splitting, and detachment.
The high hydrophilicity of the HMSR meant sodium hydroxide (NaOH) was easily captured by the HMSR, which then delivered it to a phenolphthalein solution, causing a chemical reaction. It then carried out a similar mechanism, transporting a HCl droplet to the NaOH and phenolphthalein reaction drople and carrying out another reaction. The HMSR used a magnet to roll inside the droplet and accelerate the reaction. It was also able to remove 500 μm sugar microparticles via water droplet adhesion and transport.
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Key Findings
The study demonstrated the robot’s robust magnetic responsiveness, rolling at 70 mm/s under a weak 20 mT field across diverse surfaces. Three distinct droplet manipulation modes were identified, which were transport (up to 900 µl at 10 mm/s), splitting (critical speed inversely proportional to droplet volume), and detachment (beyond 532 mm/s).
The HMSR achieved very high transport speeds (200 mm/s for 50 µl droplets), outperforming existing magnetic methods in both velocity and actuation efficiency. Mechanical analysis revealed that droplet behavior depended on the balance between magnetic driving forces and fluid adhesion.
The HMSR’s unique combination of magnetic responsiveness, wettability control, and biocompatibility addresses many of the limits in soft robotics. It has the potential to accelerate developments in advanced lab-on-a-chip systems and minimally invasive medical technologies. Future work could include improving performance through substrate engineering.
Journal Reference
Sun, X., Li, Z., Li, C., Zhang, H., Liu, W., Liu, M., Li, L., & Gui, L. (2025). Hydrophilic hard-magnetic soft robots: A new approach for precise droplet manipulation. Nanotechnology and Precision Engineering, 8(4). DOI:10.1063/5.0251223
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