In-Air Microfluidic Strategy for the Production of Sodium Alginate Fibers with Regular Inclusions at Very High Throughput

Scalable technologies for the production of biocompatible complex microfibers of controllable size and composition at competitively high throughput are urgently needed in order to meet the growing demand for such microstructures in pharmaceutical and biomedical applications. Here, we introduce an in-air microfluidic strategy with throughput greater than $1400$ ml $\mathrm{h}{}^{\ensuremath{-}}1$ (corresponding to more than $17\phantom{\rule{0.1em}{0ex}}000$ m of fiber per hour). The microfibers of uniform diameter have regular inclusions, which can potentially be used for encapsulating cells into a protecting and nutrient environment, or for finely tuning the release of various actives at individualized doses. With the help of a recently developed prototype, we test seven different liquid combinations and obtain seven types of fibers, whose average ``dry'' diameter ranges between $94\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$ and $170\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$. The principle of our approach is to solidify the complex liquid structures generated by the controlled collisions of a drop stream with a continuous liquid jet, in air, via ionic cross-linking. After the stream of water-based droplets, which constitute the inclusions, collides in-air with the alginate-based jet (jet 1), the generated drops-in-jet compound is brought into contact with a second jet (jet 2) containing divalent cations (${\mathrm{Sr}}^{2+}$ or ${\mathrm{Ca}}^{2+}$) to initiate the solidification. Finally, the fibers are collected ``on the fly'' via a horizontal spinning plate, allow to dry (i.e., to fully equilibrate under controlled conditions), and characterized by their elongation at break and Young's modulus.