Alginate Hydrogel Formation by Microdroplet Reactions on the Open-Channel Microfluidics

The alginate hydrogel is a natural polysaccharide that has high biocompatibility and is used for drug delivery, dressing for wound healing, scaffold for 3D bioprinting, etc. It can be readily prepared by mixing sodium alginate with divalent cations such as Ca2+ in solution to crosslink the alginate polymer chains. The most common method to facilitate the crosslinking of sodium alginate is to add a small amount of solution of sodium alginate into a bulk solution of CaCl2 slowly or in a dropwise manner. CaCl2 permeates into a blob of the alginate solution to immediately crosslink the alginate and form a hydrogel. Morphologies of the alginate solution being added (droplet or a stream of liquid, droplet size, etc.) affect the morphologies and properties of the resulting hydrogel, which are important for different applications. Since CaCl2 solution is normally of a bulk volume, if this reaction was conducted in a setting where both reactants are in microdroplets, it may lead to unique characteristics. However, it is difficult to facilitate efficient reactions between microdroplets by conventional techniques.<br/><br/>In this study, crosslinking reactions between sodium alginate and CaCl₂ were conducted by spraying them on the substrate surface with wettability patterning that collects and mixes microdroplets on the surface. We fabricated highly-branched “open microfluidic channels” [1,2,3] by patterning different wetting properties such as hydrophilic and hydrophobic ones. When aqueous microdroplets are sprayed on the open microfluidic channels, they are captured and transported along a hydrophilic channel surrounded by a hydrophobic surface, driven by capillary action. Superhydrophobic coating of TiO₂ and Capstone (R) ST-100 turns to be superhydrophilic upon irradiation with UV light, which makes complex patterning possible by simple photolithography. This photo-patternable surface was used to fabricate a highly branched tree-like microfluidic channels to collect microdroplets and facilitate mixing of solution at a large number of branching points that exists throughout the surface.<br/><br/>On the fabricated microfluidic surface, aqueous solution of 20 g/L sodium alginate (500-600 cP at 10g/L, 20 °C) and that of 20 g/L CaCl₂ were prepared. Droplets of different aqueous solutions were sprayed onto the unused channels using a spray nozzle with a controlled rate: (1) sodium alginate solution, (2) CaCl₂ solution, (3) both using two spray bottles containing each of them. The positions of liquid blobs after spraying were recorded by a digital camera. When droplets of CaCl₂ solution, which has low viscosity, were sprayed on the channels, they were transported through the channels to form a large liquid blob at the focal point (the root of the tree structure) within 1 s. When droplets of sodium alginate solution were sprayed on the unused channels, they were also transported to the focal point despite their high viscosity (estimated viscosity is 1 × 10³ cP). On the other hand, when the two kinds of droplets were simultaneously sprayed on the channels, the transport of droplets to the focal point was apparently hindered. This hindered transport can be attributed to crosslinking of alginate by Ca²⁺ upon merging of fine droplets of sodium alginate and those of CaCl₂ at the branching points, which turned the fluid droplet to a non-fluid hydrogel.<br/><br/>In conclusion, we demonstrated mixing and reactions of microdroplets of sodium alginate and CaCl₂ to form hydrogel on the microfluidic surface. More detailed investigation of the reaction dynamics between droplets as well as properties of the resulting alginate hydrogel is in progress.<br/><br/>[1] H. Kai, R. Toyosato, M. Nishizawa, <i>RSC Adv.</i>, 8, 15985-15990, 2018.<br/>[2] H. Kai, <i>µTAS 2020</i>.<br/>[3] H. Kai, <i>2021 MRS Fall Meeting & Exhibit</i>.