Droplet microfluidics-assisted fabrication of Fe-alginate microgels with complex morphology: effect of the composition of droplets

Although shape-controllable alginate-based microgels <i>via</i> droplet microfluidics have been widely investigated, studies have mostly focused on tuning the designs of microfluidic devices and parameters of the crosslinking media, and very little attention has been paid to studying the effect of droplets' compositions on the morphologies of the resulting microgels. In the present study, droplets of aqueous solutions of sodium alginate (SA) with different additives (<i>i.e.</i>, 10 wt% dextran, PEG, or glycerol) have been produced from microfluidics and collected in solutions with different concentrations of glycerol (0-70 wt%) and FeCl₃ (2-10 wt%). The effect of droplet compositions on the morphologies of resulting Fe-alginate microgels has been systematically investigated. The results showed that the morphologies of the microgels were remarkably influenced by the interactions between the droplets and the collecting solutions, leading to the formation of microgels with different shapes. Additionally, microgels prepared under specific conditions exhibited certain deformation patterns. For example, dimpled microgels with large cavities exhibited an increase in cavity size in collecting solutions with higher concentrations of glycerol and FeCl₃ (the value of <i>d</i>/<i>D</i> rose from 0.3 to 0.5, <i>D</i> and <i>d</i> represent the outer contour dimension and the cavity diameter of the microgel). Similarly, red blood cell-like microgels showed an increase in concavity depth as the concentrations of glycerol and FeCl₃ increased (the value of <i>D</i>/<i>L</i> rose from 1.7 to 3.7, <i>D</i> represents the horizontal length of the microgel, and <i>L</i> indicates the vertical depth of the concave). The dimple-like microgels were uniquely advantageous in terms of rapid and efficient payload release (up to 98% release in 5 hours, 16.68 μg of total release), significantly superior to the spherical microgels (82% release in 5 h, 12.54 μg of total release). These findings establish a framework for morphological engineering of microgels, offering promising opportunities for applications in payload delivery systems requiring rapid and controlled release profiles.