Graphene ink for safe wearable devices

As the development of smaller electronics continues, the ability to manufacture various gadgets and gizmos into portable and even wearable designs has become the norm. Yet, a bottleneck has arisen from the limitations of the materials – they do not function as well on a micro-scale. However, researchers from the Eindhoven University of Technology have utilized a nanomaterial that capitalizes on its unique makeup and allows the conductive material to show its stuff, graphene ink.

The conductive components in printed electronics have so far been based on metals. Suitable metals, however, tend to be prone to electromigration, essentially electrons escaping through heat or wear; and the metals that have proven to be ideal conductors for printed electronics have their own drawbacks – gold and silver are expensive, and copper is highly susceptible to oxidization.

This led researchers to search in the direction of flexible conductive polymers, which could be integrated into wearable devices. However, these too, had their issues, as stability hindered their use in practical applications.

Filtering down the list of possible materials, researchers finally chose graphene, a carbon allotrope that is highly conductive, mechanically strong, and environmentally inert. This wonder material also happens to be readily available, as it is synthesized from graphite, which is fairly common on Earth.

Graphene is a one-atom-thick layer of carbon atoms, is transparent, conductive, bendable, and is one of the strongest materials ever discovered. However, despite its wonders, its abilities also make it difficult to specialize.

More specifically, in the case of its conductivity, graphene's values of sheet resistance – basically its measure of resistance throughout its uniform thickness – show its lowest values are vastly higher than the values of commercially available indium tin oxide.

However, if we change the idea that graphene functions similarly to other materials, it allows for some truly out-of-the-box innovations.

Graphene nanoplatelets, particles made up of stacks of graphene commonly known as GNPs, can be produced in a large-scale and cost-effective manner. These readily available nanoplatelets facilitate the easy production of GNP-based inks.

Previous experimentation using graphene-based sensors was manufactured via screen printing, and stretchable supercapacitors were printed using a conductive binder-based ink with a small amount of graphene.

Although printed stretchable sweat sensors were developed, it was found that there was too little flexibility in their form and function.

Thus, an ideal conductive direction for wearable electronic applications was found. To be practical, wearable electronics need to have high conductivity, high stretchability, high cyclic stability, and low gauge factor – meaning their electrical resistivity would not change very much.

With this direction in mind, the researchers at Eindhoven developed a GNP-based ink that met all these requirements for screen printing stretchable, durable, and skin-compatible graphene conductors.

The graphene ink now had a sheet resistance as low as 34 ohms per square on TPU (thermoplastic polyurethane) surfaces after drying at 120 degrees Celsius (248 degrees Fahrenheit), which was then further reduced to less than 10 ohms per square after another advanced process was applied, making it comparable to commercial indium-tin oxide that is commonly used for touchscreen devices.

The ink also demonstrated a low gauge factor and minimal fatigue with up to 1000 cycles at 20 to 50% strain (physical stress), ultimately comparing strongly to silver and gold conductors, and was even found to maintain its conductivity at 100% strain

Ultimately, the researchers were successful in developing a comparable conductive material in a near liquid state that can maintain its effectiveness if ever fashioned into a device that is worn on or near the skin.

As the possibilities of its use can be likened to anything ink has ever been used for and more, the future of conductive ink is bright indeed.