A Pen-on-Paper Graphene Oxide-Based Nanocomposite for Multitype Strain Sensing

Flexible strain sensors that exploit nanotechnologies to achieve superior performances have become increasingly indispensable in a number of high-value-added applications, such as human body monitoring, structural health monitoring and human-machine interaction. For real-world implementations, it is utterly important to fabricate sensors that, on one hand, possess high sensitivities, and on other hand, are customizable and convenient to install.<br/><br/>Recently, the pen-on-paper (PoP) approach – a fundamental printing technique that makes use of ubiquitous and portable raw materials – demonstrates the potential to meet the installation requirements of flexible strain sensors in real-world applications. Moreover, many modified graphene-based nanofillers have been shown to exhibit high sensitivities to microscopic deformations. Owing to their long-term stable dispersibility in solutions, modified graphene-based nanofillers are considered high suitable ingredients for PoP inks. On this basis, a PoP ink was prepared from graphene oxide (GO) in this work, and the ink was used to fabricate a flexible GO/paper strain sensor that can capture multiple types of deformations (i.e., in-plane tension, in-plane compression and out-of-plane pressure).<br/><br/>To prepare a GO ink, graphene was first oxidized by the Hummers’ method to a certain degree of oxidation and then put into an ordinary ink pen, together with polymer dispersants including phenolic resin and amphoteric polyacrylamide. GO/paper sensor specimens were fabricated by using the ink pen to mark on printing paper. The dimensions of the sensor specimens are fully customizable. The ink demonstrates an excellent writability even when a piece of paper is in bent state. Furthermore, even if an ink is left unused for several months, sensor specimens that are fabricated from it would exhibit a similar sensing capacity to those that are fabricated from a freshly prepare ink.<br/><br/>The sensitivities of the GO/paper sensor to in-plane tensile strain and in-plane compressive strain were evaluated by subjecting sensor specimens to bending. Within 1% strain, the gauge factors were found to be 121 (tension) and 103 (compression). Thanks to the 3D structure of GO, the GO/paper sensor is also able to capture out-of-plane pressure, exhibiting a sensitivity of 0.34 kPa^-1within pressure range of 2-7 kPa. To simulate real-world implementations, sensor specimens were attached onto aluminum alloy substrates to monitor the tensile strain, compressive strain and pressure applied onto the substrates. The real-world practicality of the GO/paper sensor was further demonstrated through a number of applications, including gesture recognition, eye movement and intraocular pressure monitoring, and aerodynamic pressure measurement.<br/><br/>Through the abovementioned experiments, it was observed that the sensitivity of the GO/paper sensor to tensile and compressive strain is inversely proportional to the degree of oxidation of the GO used. From material characterization results, it was found that as the degree of oxidation of the GO in a sensor specimen increases, the energy band gap and the distances between the GO particles would increase, while the number of layers and the size of the GO particles would decrease. The result of these changes would be the damaging of the conductive network in the sensor specimen and the weakening of the tunneling effect across the GO particles, leading to a reduction in sensitivity.<br/><br/>To summarize, this work introduces a flexible GO/paper strain sensor fabricated by the PoP approach. The sensor demonstrates high sensitivities to various types of real-world deformations, including in-plane tension, in-plane compression and out-of-plane pressure. Moreover, the GO ink prepared possesses an outstanding long-term stability. Last but not least, the inverse relationship between sensitivity and the degree of oxidation of GO was discussed.