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Alginate Hydrogel Microspheres (2)

Preparation of alginate hydrogel microspheres with Microdroplets/Microsphere Generator

by ion-exchange cross-linking gelation

Experimental Purpose:In this application note, alginate hydrogel microspheres with high monodispersity are prepared with Microdroplet/Microsphere Generator. Drop-Surf Droplet Generation Oil is used as a continuous phase and 2 aqueous phases (chelated Ca-EDTA and sodium alginate, chelated Zn-EDDA and sodium alginate) are used as dispersed phases, and the microdroplets are cross-linking gelation by ion-exchange to obtain highly monodisperse alginate hydrogel microspheres (CV<5%).



Hydrogels are an important class of soft materials with far ranging applications in drug research, drug delivery, tissue engineering, food and pharmaceutical sciences. From a vast library of hydrogel forming (bio)polymers, alginate, as a natural polysaccharide, has attracted great attention in the biomedical field because of its low toxicity, mild ionic cross-linking conditions, good biocompatibility and biodegradability[1]. Currently, it is reported that micrometer-sized alginate particles act as bioscaffolds for 3D cell culture units to simulate the matrix environment of cell growth, which are widely used for the encapsulation, culture and monitoring of living cells in pharmaceutical research and tissue engineering[2]. Uyen N.T.T et al. found that alginate microgels particles can not only control the release rate of drugs, but also deliver drugs to specific therapeutic targets, and alginate can be directly degraded and excreted from the body through metabolism after drug release [3]. Typically, alginate microgels particles are prepared by ionic cross-linking of alginate solution with divalent cations such as Ca2+, Ba2+, Cu2+, Cd2+ and Sr2+ after the formation of droplets. For example, an aqueous sodium alginate is dropped into excess calcium chloride aqueous solution, and alginate microgels are prepared by stirring or ultrasound [4]. However, the microgels produced by this method have some shortcomings such as poor monodispersity of particle size and irregular shape.

Alginate microgels particles with precise control over size, shape and morphology can be produced by droplet-based microfluidics. Generally, an aqueous sodium alginate solution is emulsified and dispersed in oil phase in microfluidic device, and cross-linked gelation with Ca2+. This fast reaction of cross-linking occurs immediately and typically results in clogging and nonuniform drop formation. To overcome these problems, the generation of droplets and cross-linking gelation should be separated. Zhang et al. reported that CaCO3 nanoparticles are dispersed in the alginate solution, and can be dissolved under acidic conditions after drop formation[5]. However, the dissolution of solid CaCO3 particles causes a heterogeneous distribution of Ca2+ inside the droplets and diminishes the homogeneity of the resulting particles. In addition, the agglomeration of nanoparticles will also lead to the blockage of the small microfluidic channels. Therefore, Utech et al. introduced chelated Ca-EDTA as a cross-linking precursor that allows Ca2+ to homogenously distribute alginate aqueous solution before droplets formation. Subsequently, Ca2+ may be released by pH reduction by introduction of an acid, resulting in essentially instantaneous gelation [6]. However, the pH must be reduced below ~ pH 5 to release Ca2+ which may be prohibitive for cell or protein encapsulation.

To address the above problem, FluidicLab provides an effective strategy to control the release of Ca2+ into alginate aqueous solution to form hydrogel particles by ion-exchanging. The schematic diagram is shown in the follow picture: the dispersed phase 1 (containing Ca-EDTA chelate and sodium alginate), the dispersed phase 2 (containing Zn-EDDA chelate and sodium alginate) and the continuous phase (Drop-Surf Droplet Generation Oil) are pushed into a microfluidic chip by FluidicLab Microdroplet Generator, and droplets are generated after the dispersed phases mixed together and sheared by the continuous phase. Once two dispersed phases are mixed, the Zn2+ binds with EDTA due to its affinity with different ligands, resulting in release of Ca2+ which subsequently cross-links the alginate.




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