Graphene-based inks for printed and flexible electronics: a review of materials, formulation strategies, printing techniques, and sensor applications

Graphene-based inks have emerged as promising functional materials for printed and flexible electronics because of their electrical, mechanical, and chemical properties, together with their compatibility with solution-based processing. Among the various graphene-based materials, graphene oxide (GO), reduced graphene oxide (rGO), graphene flakes, and graphene-based composites are particularly attractive for ink formulations due to their tunable dispersibility, electrical conductivity, and processability. This review discusses graphene-based inks from the perspective of material selection, ink formulation, and printability, with a focus on flexible sensor applications. First, the key graphene derivatives used in printable inks are outlined, followed by a discussion of their electrical, optical, thermal, and mechanical properties in the context of printed films. The major synthesis routes, including top-down and bottom-up approaches, are then compared with an emphasis on scalability and compatibility with ink-based processing. Particular attention is given to ink formulation strategies, including solvent selection, binder and additive incorporation, rheological control, dispersion stability, and post-print processing such as thermal annealing, chemical reduction, and photonic treatment. The role of these factors in determining print quality, interflake charge transport, and device performance is highlighted. Representative applications in flexible sensing, including biosensors, gas, humidity, strain, and pressure sensors, are also reviewed. Finally, current challenges related to dispersion stability, conductivity–printability trade-offs, reproducibility, and long-term device reliability are discussed, together with future directions involving hybrid inks, predictive rheological modelling, and sustainable processing routes. Overall, graphene-based inks offer a versatile platform for the development of printed and wearable sensors, with further advances in formulation, processing, and scale-up needed for broader technological adoption.