Multinational team devises method for directly printing flexible, washable integrated circuits
Researchers from the Cambridge Graphene Centre have led a project that has successfully demonstrated printing of fully-integrated electronic circuits directly onto fabric for the first time. The circuits, which contain complex layered semiconductor devices, are printed using inks containing graphene and a similar two-dimensional conductive material, hexagonal boron nitride (h-BN).
Previous attempts at printing electronics on fabric are have used inks containing metal oxides or conductive polymers, but these have run into problems because of the limited semiconductor properties of the materials, making it impossible to construct components such as transistors. Although graphene’s potential for printed electronics has been recognised for some time, it has not yet been realised because previous attempts to make such inks have required toxic solvents.
The Cambridge team, working with other researchers in Milan and Wuxi in China, have devised a method to make a graphene: h-BN ink using non-toxic, low-boiling point solvents. This, they claim in a paper in Nature Communications, produces fully-working circuits containing field-effect transistors using conventional inkjet printing techniques. Moreover, the fabric remains breathable and the circuits are stretchable and survive up to 20 machine-wash cycles, they add.
The key to the innovation is the way the ink is formulated, the team explains. Graphene flakes were dispersed in a high boiling-point solvents ands treated ultrasonically to separate out the graphene layers, then the grapheme was extracted from the first solvent into ethanol, forming one ink. The h-BN was dispersed in water with a stabiliser and subjected to a high-shear process to separate its layers, forming another ink with a water solvent. Both inks were suitable for printing from a drop-on-demand inkjet printer of the type commonly used to print coloured pigments onto fabric.
The team printed onto polyester, forming transistors by printing layers of the two inks, whose differing semiconductor properties form the same sort of junction as a conventional solid-state doped-silicon component. They used a silver-containing ink for the electrical connections within the component. Another key discovery was that roughening the surface of the fabric improved the performance of the printed devices.
“Our inks are both cheap, safe and environmentally-friendly, and can be combined to create electronic circuits by simply printing different two-dimensional materials on the fabric,” said Dr Felice Torrisi of the Cambridge Graphene Centre, the paper’s senior author. “Turning textile fibres into functional electronic components can open to an entirely new set of applications from healthcare and wellbeing to the Internet of Things. Thanks to nanotechnology, in the future our clothes could incorporate these textile-based electronics, such as displays or sensors and become interactive.”
Co-author Prof Roman Sordan of the Polytechnic University of Milan added: “Digital textile printing has been around for decades to print simple colorants on textiles, but our result demonstrates for the first time that such technology can also be used to print the entire electronic integrated circuits on textiles. Although we demonstrated very simple integrated circuits, our process is scalable and there are no fundamental obstacles to the technological development of wearable electronic devices both in terms of their complexity and performance.” The other team members are affiliated to the School of Textiles and Clothing at Jiangnan University in Wuxi.