Sustainable circuits printed from graphite-loaded shellac ink

For many years, researchers have been developing means of producing circuits using additive manufacturing techniques. Although this has been quite successful – allowing for cheap, precise, quick-to-produce electronics – the metal particles used to make these “inks” conductive are exacerbating the problem of electronic waste. This is particularly concerning given the likelihood that printed electronics will be largely used in cheap, disposable IOT sensors.

“There is an urgent need for materials that balance electronic performance, cost, and sustainability,” said Professor Gustav Nyström, head of Empa’s Cellulose and Wood Materials lab. 

In order to develop an environmentally friendly ink, Nyström and his colleagues set the goal of making it metal-free, non-toxic, and biodegradable. Having practical applications in mind, they also aimed to ensure it was easily formable and stable to moisture and moderate heat.

They chose as their conductive material inexpensive carbon; specifically, they used elongated graphite platelets mixed with tiny soot particles to establish electrical contact between the platelets. This was contained within a matrix of shellac: a resin produced by certain scale insects and used widely as a nail varnish. Shellac does not only fit the desired material profile; it is also soluble in alcohol, which evaporates after the ink is applied, causing it to dry.

Despite these useful ingredients, the task proved challenging. This was due to the need for the ink to exhibit shear thinning behaviour.  At “rest”, 3D-printing ink is viscous. However, at the moment it is printed, it is subjected to a lateral shear force and becomes more fluid, like a non-drip paint that acquires a soft consistency after being applied to a wall with a roller. It is particularly important to refine this characteristic in additive manufacturing; an ink that is too viscous would be too tough, but if it becomes too liquid during printing, the solid components could separate and clog the narrow nozzle.

To meet these requirements, the researchers tinkered intensively with their ink formulation. They tested two sizes of graphite platelets: 40μm, and 7-10μm in length. Many variations were also needed in the mixing ratio of graphite and carbon black, because too much carbon black makes the material brittle, risking cracking as the ink dries. Eventually, by optimising the formulation and the relative composition of the components, the team was able to develop several variants of the ink that can be used in different 2D and 3D printing processes.

Xavier Aeby, a doctoral student involved in the Scientific Reports study, explained: “The biggest challenge was to achieve high electrical conductivity and at the same time form a gel-like network of carbon, graphite, and shellac.”

The researchers investigated the behaviour of the material to ensure it was appropriate for additive manufacturing processes such as robocasting, confirming that the ink performed well as an electrical conductor, as well as having good tensile strength and stability underwater. The material has now been patented.

Nystöm said: “We hope that this ink system can be used for applications in sustainable printed electronics. For example, for conductive tracks and sensor elements in smart packaging and biomedical devices or in the field of food and environmental sensing.”