Wet spinning and 3D printing of supramolecular hydrogels in acid-base and dynamic conditions

N-alkyl-D-galactonamides give biocompatible hydrogels that are very delicate and not injectable. To circumvent their mechanical fragility and the injectability issue, we have developed a method of injection based on solvent-water exchange. A solution of the gelator in a good solvent is injected vertically in a bath of water. The diffusion of water inside the solvent jet triggers the fast self-assembly of the N-alkyl-D-galactonamides into supramolecular fibers. It leads to the formation of well-defined gel filaments. We first used dimethylsulfoxide (DMSO) as the good solvent of N-alkyl-D-galactonamides. Then, we considered to get rid of this organic solvent by implementing an "all-aqueous method", paving the way to green chemistry methods. Despite the fact that these molecules do not have conventional acid-base functions in water, N-alkyl-D-galactonamides can be deprotonated and solubilized in highly concentrated NaOH aqueous solutions. As with DMSO, the basic solution is denser than the solution in the bath. Thus, a well-defined vertical jet falls down when the solution is injected in an acidic aqueous solution. The neutralization of the base at the acid-base interface triggers the gelation and leads to the formation of well-defined gel filaments as well. The acid-base reaction at the interface has been visualized by colored pH in-dicators. With phenol red (pKa = 7.9), a pink-to-yellow V-shaped "flame" transition is clearly observed. A similar transition from colorless-to-pink is observed with the pH indicator acid fuchsin, which pKa = 13 is close to the N-heptyl-D-galactonamide pKa. The distance from the nozzle of this transition zone is directly related to the injection rate, which is well-described by a model. We also applied this method to 3D printing, by a liquid-in liquid direct writing. It gives well-resolved patterns. The opportunity to get noodles or 3D printed patterns made of single small molecules by acid-base exchange opens new perspectives. Notably, the supramolecular fibers could be used to support and direct the formation of inorganic materials with original microstructures.