Hydrophilic hard-magnetic soft robots: A new approach for precise droplet manipulation

Abstract

Precise droplet manipulation is critical in material synthesis, biochemical detection, and tissue engineering. However, the droplet velocity and volume manipulated by magnetic techniques are restricted owing to the low magnetic force exerted on magnetic particles and beads. Furthermore, magnetic particles are prone to contaminate droplets owing to residues and corrosion. To address these issues, this paper proposes a hydrophilic hard-magnetic soft robot (HMSR) with strong magnetic controllability and chemical stability for precise droplet manipulation. A porous HMSR was synthesized by incorporating NdFeB particles and sacrificial sugar particles into an Ecoflex elastomer. Oxygen plasma treatment was applied to make the HMSR become hydrophilic and thus enhance the driving force exerted on droplets. Three forms of droplet manipulation were demonstrated: droplet transport, droplet splitting, and robot–magnet detachment. Theoretical analysis and experimental results revealed that the critical HMSR speed requisite for droplet transport and splitting was inversely proportional to the droplet volume. Notably, a 50 μl droplet was transported in a 20 mT magnetic field at a maximum velocity of 200 mm/s. The maximum droplet volume that the HMSR could transport reached 900 μl. Benefiting from its chemical stability, HMSR successfully manipulated chemical reactions of acidic and alkaline droplets. Additionally, the HMSR achieved targeted removal of microparticles through droplet adhesion to them. This HMSR with precise droplet manipulation capability holds broad prospects for applications in biochemical detection, material synthesis, and surgical robotics.