When it comes to 3D printing, most people think of a commonly used technology, namely Fused Deposition Modeling (FDM). This method involves extruding molten polymer through a nozzle and building the object in successive deposited layers as the polymer cools to a solid state. Another technique, called direct ink writing (DIW), also squeezes stuff out of a nozzle. In this case, though, the stuff is a gel-like polymer "ink" that turns into a solid through a chemical reaction. Compared to FDM, DIW is more cost-effective and energy-efficient, and objects can be made from more polymer.
However, a drawback of this technology is that toxic chemical catalysts and cross-linkers are often required to initiate and promote the liquid-to-solid transition. Not only can these chemicals be harmful to humans and the environment, but they are added in post-printing steps, adding to the duration and complexity of the production process.
That's where the new ink comes in. It was developed by Professor Jinhye Bae and colleagues at the University of California, San Diego, and uses a liquid polymer solution called poly(N-isopropylacrylamide), or PNIPAM for short. Functional materials, such as carbon nanotubes or graphene flakes, can be mixed into the liquid.
Because PNIPAM is initially viscous, it can be easily squeezed out of the needle with minimal force. When ink is squeezed into a calcium chloride brine solution, the salt ions immediately draw water molecules away from the ink, a phenomenon known as "salting out." The hydrophobic (water-repellent) polymer chains left in the ink clump together, causing the ink to instantly become solid. Any added functional materials will also be locked in.
Unlike traditional DIW printing, the PNIPAM printing method does not require the use of any post-print chemicals and can be performed at room temperature. Additionally, if desired, the printed solid objects can be subsequently converted into usable liquid PNIPAM.
The technology has been used to print circuit boards that power light bulbs.
A paper on the research, which was recently published in the journal Nature Communications, also involved scientists from Hanyang University in South Korea. You can see in the video below that the ink instantly forms a solid coil.