New 3D printing method uses polymer ink and salt water for sustainable material production

A new 3D printing process developed by a team at the University of California San Diego is so simple that it uses a polymer ink and a saltwater solution to create solid structures. The work, published in Nature Communications, has the potential to make material production more sustainable and environmentally friendly.

The process uses a liquid polymer solution known as poly(N-isopropylacrylamide) or PNIPAM. When this PNIPAM ink is extruded through a needle into a calcium chloride salt solution, it solidifies immediately upon contact with the salt water. The rapid solidification is driven by the so-called “salting-out” effect, in which the salt ions draw water out of the polymer solution. This removal of water causes the hydrophobic polymer chains in the PNIPAM ink to stick close together and form a solid shape.

“This is all done under ambient conditions, with no need for additional steps, specialized equipment, toxic chemicals, heat or pressure,” said study senior author Jinhye Bae, a professor in the Aiiso Yufeng Li Family Department of Chemical and Nano Engineering at the UC San Diego Jacobs School of Engineering.

Traditional methods for solidifying polymers usually require energy-intensive steps and aggressive substances. In contrast, this new process utilizes the simple interaction between PNIPAM and salt water at room temperature to achieve the same result, but without the environmental impact.

In addition, this process is reversible. The solid structures produced can be easily dissolved in fresh water and returned to their liquid form. This allows the PNIPAM ink to be reused for further printing processes. “This offers a simple and environmentally friendly approach to recycle polymer materials,” said Bae.

To demonstrate the versatility of their method, the researchers printed structures from PNIPAM inks containing other materials. For example, they printed a circuit using an ink made from PNIPAM and carbon nanotubes, which successfully lit up a light bulb. This printed circuit could also be dissolved in fresh water, demonstrating the potential to produce water-soluble and recyclable electronic components.

Bae and her team see this simple and reversible 3D printing technique as a contribution to the development of environmentally friendly polymer manufacturing technologies. This work was supported by the National Science Foundation and the National Research Foundation of Korea.