Although they provide valuable information about mechanical stress, conventional strain gauges can be bulky, costly to install and maintain, and often require complex signal processing schemes. Low-cost, flexible and easy-to-integrate strain gauges would therefore be much better, and sensors that can be printed onto a variety of substrates are a promising class of device that researchers are looking into.

Graphene-based strain sensors are especially attractive thanks to the carbon sheet  high conductivity, mechanical strength and flexibility. A research team led by Gianluca Fiori at the University of Pisa and Cinzia Casiraghi at the University of Manchester has now succeeded in making the first graphene strain gauge that is directly fabricated on a paper substrate via inkjet printing. The process developed by the team greatly simplifies how such devices can now be built.

Fiori, Casiraghi and colleagues, made their strain gauge by depositing conductive lines made from a network of graphene flakes (dispersed in water as the solvent) on a PEL P60 paper substrate using a simple Dimatix DMP-2850 inkjet printer. This printer can create and define patterns over an area of about 200 mm x 300 mm and handle substrates that are up to 25 mm thick. A waveform editor and a drop-watch camera system was used to manipulate electronic pulses to the jetting device for optimizing the drops  characteristics as they were ejected from the nozzle.

 We see a change in the conductivity of the lines when we apply different degrees of strain to the paper,  explains Fiori. This occurs because, at the microscopic level, the different degrees of strain lead to larger (compressive) or smaller (tensile) interactions between the graphene flakes, which at the macroscopic level, translate into a change in conductivity.

Gauge factor of 125

The researchers engineered their strain sensor by looking at the electrical behaviour of the graphene lines under different applied strains, ϵ, and for different printing parameters – for example, the number of printing passes and the drop spacing. As expected, the larger the number of printing passes (that is, printed layers), the smaller the resistance, R, of the graphene line. The sensitivity, S, of the device increases with the number of layers and as the substrate becomes more curved. Indeed, it reaches more than 100% for a curvature of 1 cm (that is, a strain of 1.25%).

The device also has a gauge factor (GF) of 125. This factor describes the change in resistance, ΔR, coming from mechanical deformation and can be expressed as GF= ΔR/R₀/ϵ = S/ϵ, where R₀ is the nominal resistance.

Towards heterostructure inkjet-printed strain gauges

Inkjet printing allows us to simply and quickly fabricate a sensor directly on the surface to be inspected, which opens up the possibly of introducing arrays of sensors over large areas or multi-sensing, by introducing different types of sensors in the array, says Fiori. The fact that the solvent in which the graphene flakes are dispersed is water is also a breakthrough in ink formulation in this context.

 We might be able to exploit combinations of different materials too, such as graphene and hexagonal boron nitride (hBN), which we have already starting looking into during this study.  Our preliminary results show that these heterostructure devices perform better than those based on graphene alone, so we are now studying these further and optimizing them, since this technology might allow for new types of efficient and multifunctional strain gauges, he tells nanotechweb.org.