Ethanol-induced gelation enables direct three-dimensional printing of sodium alginate hydrogel

Hydrogels have found wide applications in various fields, but further advances in hydrogel-based applications hinge upon overcoming the challenge of generating complex structures. Although three-dimensional (3D) printing enables customized designs, it remains challenging to print hydrogel structures directly without using supporting materials or incorporating polymeric or solid thickeners due to the low printability of hydrogel precursor solutions. Here, we demonstrate that direct 3D printing of sodium alginate (SA) hydrogel structures can be achieved by introducing ethanol into the SA solution. On the one hand, ethanol enables a sol-to-gel transition to occur by lowering the temperature. Our molecular dynamics (MD) simulations reveal that the gelation occurs due to disrupted hydrogen bonding interaction among SA chains and water. On the other hand, the presence of ethanol notably increases the viscosity, yield stress, and shear-thinning feature of SA solution, making it an ideal 3D printing ink. An isotropic shrinkage of the printed structures is observed after further gelation with Ca2+ ions and the removal of ethanol. Our study provides an effective approach for direct printing of SA hydrogels and an effective way to achieve enhanced printing resolution (>10 %), paving the way for more advanced hydrogel-based applications in various fields.