Introduction
Graphene is a two-dimensional, honeycomb-structured carbon nanomaterial in which carbon atoms are arranged in hexagonal lattices. Carbon atoms are bonded through sp2 hybridization, producing an extremely stable structure and one of the hardest known materials. Graphene has a very large specific surface area, is relatively easy to produce, and exhibits excellent mechanical properties along with outstanding thermal and electrical conductivity. Because of its unique structure and properties, graphene has attracted broad interest from researchers worldwide.
The intact six-membered ring structure of graphene gives it chemical inertness and hydrophobicity, while strong van der Waals forces cause graphene sheets to aggregate. Many of graphene's superior properties manifest only at the single-layer level, so aggregation limits performance and reduces compatibility and uniform dispersion in polymer matrices. To overcome aggregation, researchers have developed functionalization and dispersion strategies to produce graphene-reinforced composites with improved properties. Functionalization not only enhances dispersion but also introduces new functionalities, expanding the application range of graphene-based sensors.
Why Flexible Sensors?
Sensors are broadly divided into flexible and rigid types. Rigid sensors, often based on metals and semiconductor materials, are widely used but face limitations when applied to irregular surfaces, which restricts their usability. Producing lightweight, flexible, and mechanically robust flexible sensors at low cost is therefore an important direction for sensor development.
Applications in Flexible Sensors
1. Flexible Strain Sensors
Flexible, stretchable, and wearable strain sensors have seen extensive attention for applications ranging from electronic skin to full-body health monitoring systems. Sensors with large stretchability, wide sensing range, and high sensitivity are ideal for comprehensive detection of human motion. The single-atom thickness of graphene allows graphene sheets to deform easily out of plane while providing good flexibility. Graphene-based flexible strain sensors have been widely explored and implemented.
For example, a multifunctional skin-like strain sensor was developed using graphene-coated spring-like mesh microstructures to monitor pressure, stretch, vibration, and bending. Reduced graphene oxide (rGO) sheets were attached to an elastic fabric to form a spring-shaped conductive mesh network. Two conductive fabric layers were packaged face to face, and copper foil at the fabric edge acted as electrodes to form a flexible strain sensor. The device demonstrated high sensitivity of 72 kPa^-1, a low detection limit of 1.38 Pa, a large sensing range, and could detect minimum tensile strain of 0.1% and vibration amplitude of 10 μm. It is capable of detecting signals from tiny physiological events to large-scale body movements.
Another example used rGO to produce a high-performance stretchable strain sensor with a fish-scale-like graphene sensing layer. The fish-scale structure provided a wide sensing range (82% strain), high sensitivity (gauge factor GF = 16.2), an ultralow detection limit (<0.1% strain), and excellent stability (>5000 cycles). The fabrication process is simple, low-cost, and energy-efficient, making it suitable for full-range human motion detection.
Because of their tactile sensitivity, flexible strain sensors are used in many scenarios. Graphene's intrinsic flexibility helps preserve electrical performance under mechanical stress, making it a promising material for highly stretchable and flexible sensors.
2. Flexible Gas Sensors
Graphene is a promising material for gas sensing due to its very large specific surface area: atoms in a single graphene sheet can adsorb gas molecules, creating a large active detection area. Many graphene-based gas sensors have been studied and implemented. One study produced a flexible NO2 sensor composed of rGO nanosheets combined with electrospun nylon-6 nanofibers. The composite fabric showed sensitive responses to NO2 at room temperature. Sensor response increased with NO2 concentration. The rGO-coated nanofiber network sensor (RGPNMF) was influenced by humidity: at 80% relative humidity, sensitivity to NO2 was twice that at 45% relative humidity, indicating that 80% RH provided optimal conditions for NO2 detection. Selectivity tests at room temperature showed RGPNMF had strong selectivity for NO2: the response to NO2 (13.6%) was more than ten times the response to other gases such as formaldehyde, acetone, benzene, and ammonia.
Graphene's large surface area makes it extremely sensitive to its environment, enabling detection down to single-molecule adsorption and desorption events. Flexible graphene-based gas sensors place an active sensing film on flexible electrodes and substrates, producing lightweight, bendable devices that can be fabricated over large areas. These characteristics meet requirements for portability and low power consumption in special environments.
3. Flexible Humidity Sensors
Humidity sensors are widely used for human comfort and environmental monitoring, and in industrial production, meteorology, and agriculture. Graphene is a promising low-dimensional hygroscopic conductive material with electrical sensitivity and a very large specific surface area. Graphene oxide (GO), which contains abundant oxygen-containing functional groups, is dispersible and hydrophilic. GO can interact with water molecules and serve as a humidity-sensitive film, aiding humidity sensor fabrication.
Humidity sensors are evolving from simple sensing elements to integrated, smart multiparameter detectors. Graphene-based flexible humidity sensors are attractive because of their low cost, low power consumption, and ease of fabrication.
4. Flexible Biosensors
Graphene composites are often used as modified materials to improve biosensor sensitivity. Functionalized graphene composites, combined with high-sensitivity graphene electrodes, enable low detection limits for biomolecule sensing. For instance, composite films formed from PEDOT:PSS and rGO can produce flexible, highly conductive films. Ethylene glycol treatment of PEDOT:PSS can increase conductivity by about 300 times; incorporating rGO into solvent-treated PEDOT:PSS improves electrochemical properties. Conductive paper biosensors based on such composites are low-cost, flexible, and environmentally friendly and have been applied to cancer biomarker detection.
A wide range of graphene-based flexible biosensors has been developed. Graphene has shown utility for detecting small biomolecules and for electrochemical enzymatic sensing. However, large-scale production and practical deployment still require further research.
Research Challenges and Trends
Flexible sensors present opportunities for innovative applications and disruptive technologies, with broad application prospects. Currently, many companies and research institutions in China are conducting research in this area. The main advantage of flexible materials is the ability to obtain accurate measurements while conforming to skin, enabling continuous monitoring of muscle activity and minimizing measurement errors caused by motion.
Flexible sensors are typically made by mixing conductive fillers with elastic polymers. The core component is a conductive film or a microstructured conductive film, but many such films are not breathable or moisture-permeable, causing heat buildup and discomfort during prolonged wear. Antimicrobial properties also affect long-term use. Achieving comfortable wearable experiences is therefore critical to expanding flexible sensor applications. Bridging the gap between laboratory flexible sensors and truly wearable devices remains an ongoing research challenge.
Graphene-based energy storage technologies are not yet mature: graphene's stability during charge-discharge cycles is often poor, and cycling performance can be inadequate. Most graphene-based flexible sensors currently require external power. Designing self-powered wearable electronics could provide persistent power and has significant scientific and practical importance; some research groups have begun experiments in this area. Self-sensing flexible sensors that operate autonomously without human intervention are a research hotspot because they can reduce operator error and safety risks. Self-healing flexible sensors are also under investigation; self-healing can be intrinsic or extrinsic, meaning repairs occur without external triggers or stimulation. Integrating multiple functions—such as self-powering and self-healing—into a single flexible sensor is a promising direction for multifunctional devices.
High-sensitivity flexible sensors also have potential in smart packaging, where they could detect small changes in packaged goods during transport and production, reflecting subtle environmental variations. Many emerging application areas remain to be explored. Future flexible electronics are expected to emphasize high flexibility, high sensitivity, portability, wearability, real-time response, and safety.
Conclusion
Graphene's exceptional electrical, thermal, optical, and mechanical properties have attracted global research interest. Although graphene production techniques have diversified, its large specific surface area predisposes it to aggregation, which affects adsorption capacity and the properties of graphene-enhanced composites. Appropriate functionalization can mitigate aggregation and broaden application possibilities. Functionalized graphene is a highly designable material with many emerging application areas awaiting further exploration.
Graphene and its derivatives possess unique physicochemical properties, and various graphene-based flexible sensors continue to emerge. Graphene-based flexible sensors can offer high sensitivity, low cost, and good stability, providing advantages in sensing applications. As manufacturing techniques mature and research methods advance, graphene applications across fields will likely receive increasing attention.
