A rigid assembly of alginates is formed in aqueous media primarily via hydrogen bonding between guluronic units. A flow of aqueous alginate solution in a co-flow capillary can form alginate gel fibers by contact with Ca2+ ions in sheath flow. Mixing with polyols [e.g., polyethylene glycol (PEG)] facilitates the shaping of the alginate assembly because PEG disrupts the assembly of the extended alginate chains to instead form alginate–PEG complexes that exhibit shear-thinning behavior. The shear-induced fibrous domains of the globular alginate–PEG complexes can be partitioned by a PEG-rich phase, resulting in multiple parallel alginate gel filaments when the strong ionic-field-induced PEG-rich phase is adjusted and an alginate–PEG complex phase is used as the aqueous two-phase separation system.