New 3D-printer ink creates recyclable electric circuit

A team of US and Korean researchers has developed a 3D-printing ink that makes easy-to-recycle structures without the need for any heat or light.

The ink, made from a polymer, solidifies on contact with salt and dissolves back into re-usable ink on contact with fresh water.

The researchers say their ink could be useful for disposable electronics, robotic components, and prototyping.

They’ve published their findings in Nature Communications.

Polymer inks are useful tools for 3D-printing complex, small-scale devices. But they typically need high amounts of energy or extra solvents to print properly.

The researchers’ method uses a polymer called poly(N-isopropylacrylamide), or PNIPAM. This is a non-toxic substance used by the pharmaceutical industry for drug delivery systems.

PNIPAM dissolves in water to make a liquid, but it solidifies when it comes into contact with a salty calcium chloride solution.

Three different PNIPAM polymer inks being extruded into a salt water solution: plain PNIPAM (white), PNIPAM mixed with food coloring (red), and PNIPAM mixed with carbon nanotubes (black). Credit: Donghwan Ji

The researchers used a commercial healthcare-grade 3D printer to pump PNIPAM into mixtures of calcium chloride and water. It solidified into neat structures immediately.

“This is all done under ambient conditions, with no need for additional steps, specialised equipment, toxic chemicals, heat or pressure,” says senior author Professor Jinhye Bae, a researcher at the University of California – San Diego, USA.

The team made an electric circuit mixed with carbon nanotubes using their method, which they used to power a small light bulb.

They could also dissolve the structures they made in fresh water. After evaporating the water in an oven at 70°C, the researchers had dry, re-usable PNIPAM which could be re-dissolved in water to make fresh ink.

“This offers a simple and environmentally friendly approach to recycle polymer materials,” says Bae.