A sustainable 3D printing technique is developed using the reversible salting-out effect of aqueous salt solutions. This method enables the spontaneous and instant formation of physical crosslinks in poly(N-isopropylacrylamide) (PNIPAM) solutions at room temperature, leading to rapid solidification upon contact with salt solutions. The PNIPAM solutions are extrudable through needles and solidify immediately without the need for rheological modifiers, chemical crosslinkers, or additional post-processing steps. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, offering potential for water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators. The salting-out effect, influenced by salt ions, lowers the phase transition temperature of PNIPAM solutions to below 10 °C, enabling the spontaneous formation of physical crosslinks (hydrophobic interactions) between PNIPAM chains and consequential solidification of aqueous PNIPAM solutions. The PNIPAM solutions are extrudable through syringe needles without high-extruding force due to their low viscosity and shear-thinning behavior. The addition of salt ions, such as CaCl₂, significantly lowers the lower critical solution temperature (LCST) of PNIPAM, enabling the extruded PNIPAM solution inks to immediately undergo phase transition and solidification upon contact with a salt solution at room temperature. The formation of physical crosslinks and solidification, resulting from the salting-out effect, is consistently observed even when the PNIPAM solution contains functional materials or other polymers. The salting-out effect was reversible, allowing the PNIPAM solution to solidify and then return to a coil state upon cooling to subzero temperatures. This phenomenon implies that the LCST of PNIPAM is decreased by the salt ions causing the salting-out effect. The solidification caused changes in the internal structures and intermolecular interactions among PNIPAM chains. Cross-sectional scanning electron microscopy (SEM) images of lyophilized samples of the PNIPAM solution exhibited highly porous structures due to the high water content, whereas the lyophilized sample of the PNIPAM solidified within the CaCl₂ solution showed a non-porous dense structure. Fourier-transform infrared (FTIR) analysis confirmed that the salt ions significantly increased interactions between PNIPAM chains in the globule state. The PNIPAM solution exhibited strong peaks corresponding to O-H stretching and bending vibrations due to numerous hydrogen bonds of water molecules. Heating resulted in PNIPAM dehydration and the appearance of peaks corresponding to N-H stretching, C=O stretching, and N-H bending vibrations. The salt ions contributed to more effective dehydration than temperature elevation and more formation of hydrophobic interactions among the PNIPAM chains. TheA sustainable 3D printing technique is developed using the reversible salting-out effect of aqueous salt solutions. This method enables the spontaneous and instant formation of physical crosslinks in poly(N-isopropylacrylamide) (PNIPAM) solutions at room temperature, leading to rapid solidification upon contact with salt solutions. The PNIPAM solutions are extrudable through needles and solidify immediately without the need for rheological modifiers, chemical crosslinkers, or additional post-processing steps. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, offering potential for water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators. The salting-out effect, influenced by salt ions, lowers the phase transition temperature of PNIPAM solutions to below 10 °C, enabling the spontaneous formation of physical crosslinks (hydrophobic interactions) between PNIPAM chains and consequential solidification of aqueous PNIPAM solutions. The PNIPAM solutions are extrudable through syringe needles without high-extruding force due to their low viscosity and shear-thinning behavior. The addition of salt ions, such as CaCl₂, significantly lowers the lower critical solution temperature (LCST) of PNIPAM, enabling the extruded PNIPAM solution inks to immediately undergo phase transition and solidification upon contact with a salt solution at room temperature. The formation of physical crosslinks and solidification, resulting from the salting-out effect, is consistently observed even when the PNIPAM solution contains functional materials or other polymers. The salting-out effect was reversible, allowing the PNIPAM solution to solidify and then return to a coil state upon cooling to subzero temperatures. This phenomenon implies that the LCST of PNIPAM is decreased by the salt ions causing the salting-out effect. The solidification caused changes in the internal structures and intermolecular interactions among PNIPAM chains. Cross-sectional scanning electron microscopy (SEM) images of lyophilized samples of the PNIPAM solution exhibited highly porous structures due to the high water content, whereas the lyophilized sample of the PNIPAM solidified within the CaCl₂ solution showed a non-porous dense structure. Fourier-transform infrared (FTIR) analysis confirmed that the salt ions significantly increased interactions between PNIPAM chains in the globule state. The PNIPAM solution exhibited strong peaks corresponding to O-H stretching and bending vibrations due to numerous hydrogen bonds of water molecules. Heating resulted in PNIPAM dehydration and the appearance of peaks corresponding to N-H stretching, C=O stretching, and N-H bending vibrations. The salt ions contributed to more effective dehydration than temperature elevation and more formation of hydrophobic interactions among the PNIPAM chains. The