Picoinjection of Microfluidic Chip Enables Controllable Concentration of Magnetic Particle inside Droplet Microrobots

Abstract Although magnetically controlled microrobots hold great promise for application in next-generation interventional medical procedures, the insufficient magnetic particle concentration and limited drug-loading capacity of these robots remain significant challenges to their practical implementation. Herein, a droplet microfluidic platform integrated with picoinjection technology was developed to enable controlled adjustment of the magnetic particle concentration and drug load in droplet microrobots. The flow-focusing microfluidic chip, integrated with a liquid metal electrode, was fabricated to inject nanoliter-scale volumes of the alginate (ALG)-Fe3O4 precursor solution into preformed calcium chloride (CaCl2) droplets. The effects of gas pressure on droplet size and generation frequency were investigated, and the influence of injection rate on droplet diameter and injected volume was characterized. Furthermore, the effects of voltage, frequency, and duty cycle on injection success rates were analyzed through experiments and numerical simulations. By adjusting the injection rate, three types of microrobots with different magnetic particle concentrations were successfully fabricated. Characterization results indicate that these microrobots exhibit superparamagnetic behavior and distinct magnetic responsiveness. The motion performance of the microrobots was also verified. Finally, dual picoinjection was employed to incorporate DOX into the microrobots, demonstrating the feasibility of the platform for therapeutic-agent loading. Overall, this work provides a microfluidic strategy for quantitatively tuning the magnetic particle concentration and magnetic responsiveness of droplet microrobots, while demonstrating its potential for drug-delivery applications.