Tunable Morphology of Monodisperse Polymer Particles with Microfluidics

In this work, we propose a novel method to fabricate polymer particles with controlled morphology in microchannel by solidification of polymer solutions and study the transition in the shape of these particles from spheres, to cups, and to donuts. The non-spherical geometries are achieved through nonuniform diffusion of the solvent, leading to non-uniform solidification. By adjusting the flow rates of both continuous and dispersed phases, the spatial heterogeneity in the solvent diffusion rate and thus the morphology of the final polymer particles can be tuned. This approach offers a one-step continuous process for the fabrication of non-spherical polymer particles. The technique is applicable to different polymers under appropriate polymer-solvent combinations. The ability to tune the shapes of polymer particles easily will create new opportunities for applications that require anisotropic functional particles, such as in biomedical engineering. INTRODUCTION Functionalized polymer particles have found importance in many applications such as biosensors[1], tissue engineering[2], and diagnostic assay systems[3]. The availability of more sophisticated morphology of polymer particles like hollow microsphere enables significant advances in controlled release of drugs, pigments or chemical reagents[4-6]. At present, many techniques have been developed to fabricate polymer particles with a complex structure at the micrometer scale using either top-down[7, 8] or bottom-up strategies[9-15] . In particular, templating approach is widely used to produce core-shell structures, including hollow spheres of various materials such as silica[16], polyelectrolytes[17, 18] and polymers[19, 20]. Another approach to generate single-hole hollow polymer microspheres is to induce the concomitant phase separation and symmetrical volume shrinkage of shell materials by either solvent evaporation[21] or cross-linking reaction[22]. However, wide application of these polymer particles has been hampered in part by the lack of a continuous process for producing such particles. Microfluidic emulsification enables the continuous production of monodisperse droplets[23-26]. By applying monomer solutions as the dispersed phase, polymer particles have been generated by crosslinking of the monomer droplets[11, 27, 28]. An alternative approach for forming polymer particles is to solidify droplets of polymer solutions by solvent diffusion and evaporation. Interestingly, by solidifying these droplets in a straight microchannel, toroidal polymer particles are formed[29, 30]. Experimental Section Chemicals The continuous phase used in the experiments includes poly(dimethylsiloxane) oil (PDMS) (Sigma-Aldrich) with a viscosity of 1 cSt, containing 2 %wt Dow Corning 749 fluid (Dow Corning) as a surfactant; and the solvent used to dissolve polymers in the experiments was N,N-