3D printing of certain items could soon become much faster, simpler, and more environmentally friendly. That's because scientists have developed a new 3D printing ink that's easily extruded as a liquid but then solidifies when it comes into contact with a saltwater solution.
When most people think of 3D printing, they think of a widely used technique known as fused deposition modeling (FDM). This method involves expelling a molten polymer through a nozzle, building objects in successively deposited layers as the polymer cools to a solid state.
Another technique, called direct ink writing (DIW), involves extruding something from a nozzle. But in this case, that something is a gelatinous polymer “ink” that chemically turns into a solid. Compared to FDM, DIW tends to be more cost-effective and energy-efficient, and also allows objects to be created from a wider range of polymers.
But one drawback of the technique is that toxic chemical catalysts and crosslinkers are often required to initiate and then enhance the transition from liquid to solid. Not only are these chemicals potentially harmful to humans and the environment, they are also added in a post-printing step that increases the time and complexity of the manufacturing process.
This is where the new ink comes into play.
Developed by Prof. Jinhye Bae and colleagues at the University of California San Diego, the product contains 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.
Dongwan Ji
Because PNIPAM is initially quite fluid, it can be easily extruded through a needle with very little force. When the ink is extruded into a calcium chloride saltwater solution, the salt ions immediately pull water molecules out of the ink – a phenomenon known as “salting out.” The remaining hydrophobic (water-repellent) polymer chains in the ink then clump together, causing the ink to solidify instantly. Any added functional materials are also locked in there.
Unlike traditional DIW printing, the PNIPAM method does not require the use of any post-printing chemicals and can be performed at room temperature. As an added bonus, printed solid objects can later be converted to reusable liquid PNIPAM if desired.
This technology has previously been used to print a circuit board used to power a light bulb.
An article on the research, which also involved scientists from Hanyang University in Korea, was recently published in the journal Nature CommunicationsIn the video below you can see the ink instantly turn into solid coils.
3D printing of solid polymers in salt water
Source: UC San Diego