Droplet-based microfluidic preparation of shape-variable alginate hydrogel magnetic micromotors

This article introduces a facile droplet-based microfluidic method for the preparation of Fe₃ O₄-incorporated alginate hydrogel magnetic micromotors with variable shapes. By using droplet-based microfluidics and water diffusion, monodisperse (quasi-)spherical microparticles of sodium alginate and Fe₃ O₄ (Na-Alg/Fe₃ O₄ ) are obtained. The diameter varies from 31.9 to 102.7 µm with the initial concentration of Na-Alginate in dispersed fluid ranging from 0.09 to 9 mg/mL. Calcium chloride (CaCl₂ ) is used for gelation, immediately transforming Na-Alg/Fe₃ O₄ microparticles into Ca-Alginate hydrogel microparticles incorporating Fe₃ O₄ nanoparticles, i.e., Ca-Alg/Fe₃ O₄ micromotors. Spherical, droplet-like, and worm-like shapes are yielded depending on the concentration of CaCl₂, which is explained by crosslinking and anisotropic swelling during the gelation. The locomotion of Ca-Alg/Fe₃ O₄ micromotors is activated by applying external magnetic fields. Under the rotating magnetic field (5 mT, 1–15 Hz), spherical Ca-Alg/Fe₃ O₄ micromotors exhibit an average advancing velocity up to 158.2 ± 8.6 µm/s, whereas worm-like Ca-Alg/Fe₃ O₄ micromotors could be rotated for potential advancing. Under the magnetic field gradient (3 T/m), droplet-like Ca-Alg/Fe₃ O₄ micromotors are pulled forward with the average velocity of 70.7 ± 2.8 µm/s. This article provides an inspiring and timesaving approach for the preparation of shape-variable hydrogel micromotors without using complex patterns or sophisticated facilities, which holds potential for biomedical applications such as targeted drug delivery.