Abstract
High accuracy measurement of mechanical strain is critical and broadly practiced in several application areas including structural health monitoring, industrial process control, manufacturing, avionics and the automotive industry, to name a few. Strain sensors, otherwise known as strain gauges, are fueled by various nanomaterials, among which graphene has attracted great interest in recent years, due to its unique electro-mechanical characteristics. Graphene shows not only exceptional physical properties but also has remarkable mechanical properties, such as piezoresistivity, which makes it a perfect candidate for strain sensing applications. In the present review, we provide an in-depth overview of the latest studies focusing on graphene and its strain sensing mechanism along with various applications. We start by providing a description of the fundamental properties, synthesis techniques and characterization methods of graphene, and then build forward to the discussion of numerous types of graphene-based strain sensors with side-by-side tabular comparison in terms of figures-of-merit, including strain range and sensitivity, otherwise referred to as the gauge factor. We demonstrate the material synthesis, device fabrication and integration challenges for researchers to achieve both wide strain range and high sensitivity in graphene-based strain sensors. Last of all, several applications of graphene-based strain sensors for different purposes are described. All in all, the evolutionary process of graphene-based strain sensors in recent years, as well as the upcoming challenges and future directions for emerging studies are highlighted.
Highlights
Pattern and transfer graphene have resulted in different types of graphene with diverse electrical and mechanical properties that can be helpful towards development of ultra-sensitive and stretchable strain sensors, provided that the said properties can be accurately controlled with excellent run-to-run repeatability
It is due to the shortcomings of accurate process control in graphene synthesis methgraphene and a dynamic range from 0 mbar to 700 mbar for the pressure sensor was obtained (Figure 9b)
Strain sensors have rapidly developed in the modern era due to the wide range of applications that they find use in, and graphene is a compelling candidate in this field
Summary
With the advent of the internet-of-things (IoT), smart, ubiquitous, pervasive sensing is rapidly gaining importance for providing reliable information at unprecedented sensitivity to enable new applications in consumer electronics [1,2,3], healthcare [4,5,6], manufacturing and structural monitoring [7,8], transportation [9,10], defense and surveillance [11,12,13]; as well as to fuel research in fundamental, applied and translational science [14]. There have been multiple studies which have reported graphene-based strain sensors built on a variety of substrates; the fabricated strain sensors display a wide range of gauge factors [31,32,33,34,35,36], making it inconclusive to verify the eventual advantages of graphene over standard metal-foil gauges suffering from low gauge factors We argue that such variation in performance of graphene-based strain sensors is largely due to the problems in obtaining high-quality graphene, in a repeatable, uniform, scalable fashion and one that allows semiconductor process integration. We provide examples of real-life applications of graphene-based strain gauges along with recommendations and future outlook on the development of graphene-based strain sensors as well as the challenges that lie ahead
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