Development of a microfluidic method for preparing monodispersed microparticles with controllable sizes and mechanical properties

In this article, we present a microfluidic method for the production of monodispersed hydrogel microparticles of Na-/Ca-alginate with controllable sizes (micrometer range) and mechanical properties without using surfactant. Our ultimate aim is to produce microparticles that mimic red blood cells (RBCs). Ex situ gelation is carried out to realize the transformation from Na-alginate to Ca-alginate microparticles. Gelation proves essential to obtaining desirable microparticle properties, such as insolubility in water and RBC-like mechanical properties. For both Na-alginate and Ca-alginate microparticles, a smooth surface and a porous inner structure are observed under a scanning electron microscope. A platform of microgrippers is successfully developed to manipulate the microparticles. The Young's modulus, measured using an atomic force microscope on the surface of Ca-alginate microparticles, is of the same order as that of RBCs. Combined with size control to micrometer range, this confirms the feasibility of using Ca-alginate microparticles to mimic RBCs. Introduction Microfluidics has emerged as a powerful technique to form discrete and uniform hydrogel structures at the microscale (<100µm). 1 This is because droplet-based microfluidic systems permit generation of discrete and uniform microdroplets which are dispersed in a continuous phase. 2 Each microdroplet can be the site of a gelation reaction forming microparticles. 1, 3-5 In this paper, we explore the generation of monodispersed hydrogel microparticles for potential applications in medical and biological fields. 6 Alginate is a well-known biopolymer obtained from brown algae. It has been widely applied in drug delivery and tissue engineering due to its biocompatibility, low toxicity, nonimmunogenicity, relatively low cost, and simple gelation with divalent cations such as Ca 2+ (covalent cross-linking). Alginate is a linear polysaccharide containing varying amounts of 1,4-linked -D-Mannuronic acid and -L-Guluronic acid residues arranged in a block-wise manner along the backbone. The composition and residue sequence vary depending on the source. 7 Seeking to control the mechanical and/or swelling properties of alginate gels, researchers have investigated covalent cross-linking with different types of molecules and cross-linking densities. 8 The purpose of our study was to produce microparticles with size (8-10µm) and mechanical properties (E=26±7kPa 9) similar to red blood cells (RBCs). Calcium alginate (Ca-alginate) was chosen for its mechanical properties. 8 Alginate forms hydrogel by simple gelation with divalent cations, such as calcium cation (Ca 2+). However, direct mixing of a calcium solution with an alginate solution in a microfluidic chip gives rise to instant gelation, likely resulting in clogging and non-uniform microdroplet formation 1. To