Tungsten disulfide (WS2) is a transition metal dichalcogenide (TMDC) that has gained significant attention for its exceptional mechanical and electronic properties, making it a promising candidate for strain sensing applications in flexible electronics. Unlike conventional metal or silicon-based strain sensors, WS2 offers advantages such as high strain sensitivity, flexibility, and compatibility with various substrates, enabling integration into wearable and stretchable devices. The piezoresistive effect in WS2, where mechanical deformation alters its electrical resistance, is central to its functionality as a strain sensor. This article explores the piezoresistive properties of WS2, substrate compatibility, strain sensitivity, durability, and recent innovations in heterostructure designs.

The piezoresistive behavior of WS2 arises from changes in its band structure and carrier mobility under mechanical strain. When subjected to tensile or compressive strain, the atomic lattice of WS2 deforms, leading to modifications in its electronic bandgap and charge transport properties. Monolayer WS2, with its direct bandgap of approximately 2.1 eV, exhibits a strong piezoresistive response due to the strain-induced shift in the conduction and valence bands. Multilayer WS2, while having an indirect bandgap, also demonstrates measurable piezoresistivity due to interlayer interactions and altered carrier scattering mechanisms. Studies have shown that the gauge factor, a measure of strain sensitivity, for WS2 can exceed 200, significantly higher than that of traditional metal foil strain gauges, which typically have gauge factors around 2–5.

Substrate compatibility is a critical factor in the performance of WS2 strain sensors. Flexible substrates such as polyimide, polydimethylsiloxane (PDMS), and polyethylene terephthalate (PET) are commonly used due to their mechanical flexibility and thermal stability. The transfer process of WS2 onto these substrates must ensure minimal defects and strong adhesion to prevent delamination under repeated strain. Chemical vapor deposition (CVD)-grown WS2 films can be directly synthesized on certain substrates, while exfoliated WS2 flakes may require transfer techniques involving polymer supports. The choice of substrate also influences the sensor’s performance; for example, PDMS provides excellent stretchability but may introduce hysteresis due to its viscoelastic nature. Optimizing the substrate-WS2 interface is essential for achieving reliable and reproducible strain sensing.

Strain sensitivity in WS2 sensors is influenced by factors such as layer thickness, crystallographic orientation, and defect density. Monolayer WS2 typically exhibits higher sensitivity compared to thicker layers due to its pronounced bandgap modulation under strain. However, multilayer WS2 can offer improved durability and lower noise levels, making it suitable for applications requiring long-term stability. The crystallographic orientation of WS2 relative to the applied strain direction also affects the piezoresistive response; strain applied along certain lattice directions can induce more significant changes in resistance. Defects, such as sulfur vacancies, can alter the strain sensitivity by introducing additional scattering centers or localized states that modify charge transport.

Durability under mechanical stress is a key consideration for practical applications of WS2 strain sensors. Repeated stretching and bending cycles can lead to crack formation, interfacial slippage, or degradation of the WS2 film. Encapsulation layers, such as thin films of Al2O3 or parylene, can protect the WS2 from environmental factors and mechanical damage. Studies have demonstrated that encapsulated WS2 sensors can withstand thousands of strain cycles with minimal performance degradation. The intrinsic mechanical strength of WS2, with a Young’s modulus of approximately 270 GPa, contributes to its resilience, but the overall durability depends on the integration method and the surrounding materials.

Innovations in heterostructure designs have further enhanced the capabilities of WS2 strain sensors. Combining WS2 with other 2D materials, such as graphene or hexagonal boron nitride (hBN), can improve sensitivity, reduce hysteresis, and enable multifunctional sensing. For example, WS2-graphene heterostructures leverage the high carrier mobility of graphene and the strong piezoresistivity of WS2 to achieve superior performance. Vertical heterostructures, where WS2 is stacked with other TMDCs, can also create strain-dependent junction properties that amplify the sensing response. Additionally, hybrid systems incorporating WS2 with organic materials or nanoparticles have been explored to tailor the mechanical and electronic properties for specific applications.

The applications of WS2 strain sensors span various fields, including wearable health monitoring, human-machine interfaces, and structural health monitoring. In wearable devices, WS2 sensors can detect subtle physiological signals such as pulse, respiration, and joint movements due to their high sensitivity and flexibility. For human-machine interfaces, they enable precise motion tracking and force feedback in prosthetics or virtual reality systems. In structural health monitoring, WS2 sensors integrated into composites or coatings can detect microcracks or deformations in real time, providing early warning of potential failures.

Challenges remain in scaling up the production of WS2 strain sensors and ensuring uniformity across large areas. Advances in growth techniques, such as roll-to-roll CVD or area-selective deposition, are being investigated to address these issues. Furthermore, the development of standardized testing protocols will be crucial for comparing the performance of different WS2-based sensors and facilitating their adoption in industrial applications.

In summary, tungsten disulfide strain sensors represent a cutting-edge technology in flexible electronics, offering high sensitivity, substrate versatility, and robust performance under mechanical stress. Innovations in heterostructure design and encapsulation techniques continue to push the boundaries of their capabilities, enabling new applications in wearable and industrial systems. As research progresses, WS2-based strain sensors are poised to play a pivotal role in the next generation of smart and responsive electronic devices.