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.

We demonstrate a MEMS-compatible detector with a single channel that employs chemical vapor-deposited graphene (G _CVD ) in a chamberless, nondispersive infrared gas sensing configuration. The detector, designed to function as a thermistor bolometer, is intricately crafted by integrating serpentine-shaped G _CVD into flexible, epoxy-based SU-8 photopolymer, covering an area of 500 × 500 µ m ² . Experimental evaluations of the detector were carried out at a distance of 30 mm away from the light source within a chamberless setup to assess its performance metrics. The results unveil a peak responsivity of approximately 0.65 V/W for the G _CVD -based detector. This outcome underscores the simplicity of the fabrication process, eliminating the necessity for implementing intricate structures like p-n junctions, surface plasmons, or microcavities. Moreover, the detector demonstrates rapid resistance switching, with an impressive response time of 1.2 s and a recovery time of 1.7 s observed during the electrical chopping of the light source.

We present a flexible pH sensor which leverages the unique properties of reduced graphene oxide/polyaniline (rGO/PANI) composite films through an efficient and scalable hybrid microfabrication approach, wherein the rGO/PANI films are conformally coated on flexible polyethylene terephthalate (PET) substrates via dip-coating and thereafter lithographically patterned into precise arrays of interdigitated electrodes (IDEs), serving both as the pH-active medium and the electrical interface. Upon dip-coating, a thermal reduction process is performed to yield uniform rGO/PANI composite layers on PET substrates, where the PANI content is adjusted to 20% to optimize conductivity and protonation-driven response. Composition optimization is first performed using inkjet-printed silver (Ag) contacts and a conductometric readout mechanism is employed to explore pH-dependent behavior. Subsequently, IDE arrays are defined in the rGO/PANI using photolithography and oxygen-plasma etching, demonstrating clean pattern transfer and dimensional control on flexible substrates. Eliminating separate contact metals in the final design simplifies the stack and reduces cost. A set of IDE geometries is evaluated through I–V measurements in buffers of different pH values, revealing a consistent, monotonic change in electrical characteristics with pH and geometry-tunable response. The present study demonstrated that the most precise pH measurement was achieved with an 80:20 rGO/PANI composition within the pH 2–10 range. These results establish rGO/PANI IDEs as a scalable route to low-cost, miniaturized, and mechanically compliant pH sensors for field and in-line monitoring applications.

Drop-on-demand inkjet printing has shown great potential for wearable health monitoring applications because of its ability to directly pattern on flexible substrates that can conform to curved surfaces such as the skin. Surface biopotential measurements such as electrocardiography is one such example requiring conductive electrodes that can be attached to skin to record the electrical activity of the heart, otherwise known as an electrocardiogram (ECG). Typical pre-gelled, silver/silver chloride (Ag/AgCl) electrodes; also known as ”wet electrodes”, are known to cause skin irritations with performance degradation over time, and therefore remain largely non-ideal especially in long-term, mobile heath monitoring scenarios. This paper reports, for the first time, the development of a single, fully inkjet-printed graphene-on-plastic monolithic wearable armband, whose performance was benchmarked against commercial Ag/AgCl electrodes during a 1-hour-long ECG recording with five participants. The inkjet-printed graphene-on-plastic armband displayed excellent ECG reception with a signal-to-noise ratio (SNR) of up to 4.2 dB higher than that of commercial electrodes.