Shear-Induced Structuring for Multiple Parallel Gel Filaments Obtained from Casein–Alginate Hybrids

The addition of more than 1 wt % sodium alginate to a 7.5 wt % sodium caseinate solution induced aqueous two-phase separation (ATPS). Alginate and caseinate were effectively condensed in the upper and lower phases, respectively, thereby forming an alginate-casein assembly. The rigid structure of the alginate, which was formed by repeated regular hydrogen bonds between guluronic acid units, became supple when casein micelles penetrated and were adsorbed into the coiled alginate chains to weaken the diaxial linkages of the alginate hydrogen bonds. Protein-polysaccharide hybrid gel fibrils with a bundled structure were formed due to the deformation of the alginate-casein assembly in an aqueous solution by shear in a co-flow double tube, followed by cross-linking with Ca2+ supplied as the sheath fluid. The combination of ATPS and shear-induced elongation of the alginate-casein assemblies enabled the fabrication of hundreds of parallel gel filaments (φ = 1-10 μm) along the flow direction. These multiple parallel gel filaments can be applied to biomimetic chemistry for fibrous living tissues, as a biodegradable scaffold for cell engineering, and as a release carrier of physiologically active substances or drugs. Our proposed technique enables the formation of biomimetic protein-polysaccharide hybrid gel filaments with a bundled structure using Ca2+ chelation under laminar flow in a capillary, without the need for enzymatic cross-linking and multihole nozzles.