Transition metal dichalcogenides (TMDCs) have emerged as promising materials for flexible and stretchable electronics due to their unique mechanical and electronic properties. Unlike conventional rigid semiconductors, TMDCs exhibit exceptional flexibility, making them suitable for applications requiring conformal integration with soft, deformable substrates. This article explores the critical aspects of TMDC integration into flexible systems, focusing on substrate compatibility, mechanical resilience, and performance under mechanical strain, with an emphasis on wearable sensor applications.

Substrate compatibility is a fundamental consideration when integrating TMDCs into flexible electronics. The choice of substrate directly influences the mechanical behavior and electronic performance of the resulting device. Common flexible substrates include polyimide (PI), polyethylene terephthalate (PET), and polydimethylsiloxane (PDMS). These materials offer high thermal stability, chemical resistance, and mechanical flexibility. TMDCs, such as MoS2 and WS2, can be transferred onto these substrates using techniques like wet transfer, dry transfer, or direct growth via chemical vapor deposition. The adhesion between TMDCs and the substrate is critical to prevent delamination under mechanical stress. Studies have shown that van der Waals interactions alone may not suffice for robust adhesion, prompting the use of interfacial layers such as hexagonal boron nitride (hBN) or thin polymer coatings to enhance bonding.

Mechanical resilience is a defining characteristic of TMDC-based flexible electronics. Monolayer and few-layer TMDCs exhibit high intrinsic mechanical strength, with Young’s moduli ranging from 200 to 300 GPa, comparable to steel. However, their performance under bending, stretching, and folding depends on the substrate and encapsulation strategies. For instance, when MoS2 is transferred onto a PDMS substrate, it can withstand tensile strains of up to 5% without significant degradation in electronic performance. Beyond this threshold, crack formation and propagation become evident, leading to increased resistance and eventual device failure. Encapsulation with elastomeric materials like polyurethane or silicone can mitigate these effects by distributing strain more evenly across the device.

The performance of TMDC-based devices under mechanical deformation is a key metric for wearable applications. Bending tests reveal that MoS2 field-effect transistors (FETs) on flexible substrates maintain stable operation at bending radii as small as 1 mm. Repeated bending cycles, however, introduce gradual degradation due to accumulated strain and interfacial slippage. For example, after 10,000 bending cycles at a 2 mm radius, the mobility of a MoS2 FET may decrease by approximately 15%. Folding introduces even more severe mechanical stress, often leading to irreversible damage if not properly engineered. Strategies such as strain-relief structures, serpentine interconnects, and neutral plane engineering have been employed to enhance foldability. Neutral plane engineering, in particular, positions the TMDC layer at the mechanical neutral axis of a multilayer stack, minimizing tensile or compressive strain during folding.

Wearable sensors represent one of the most promising applications for TMDC-based flexible electronics. Their high sensitivity to strain, pressure, and biochemical stimuli makes them ideal for health monitoring and human-machine interfaces. Strain sensors utilizing TMDCs exhibit gauge factors significantly higher than those of traditional metal or silicon-based sensors. For instance, a WS2 strain sensor can achieve a gauge factor of 300, compared to around 2 for metallic foil sensors. This high sensitivity enables detection of subtle physiological signals, such as pulse waves and joint movements. Pressure sensors based on TMDCs also demonstrate excellent performance, with detection limits as low as 1 Pa and response times in the millisecond range. These characteristics are critical for applications like prosthetic skin and tactile feedback systems.

Environmental stability is another critical factor for wearable TMDC devices. Exposure to moisture, oxygen, and temperature fluctuations can degrade performance over time. Encapsulation with barrier materials such as Al2O3 or SiNx deposited via atomic layer deposition (ALD) has proven effective in prolonging device lifetime. For example, encapsulated MoS2 sensors retain over 90% of their initial sensitivity after 30 days in ambient conditions, whereas unencapsulated devices degrade by more than 50% in the same period. Additionally, self-healing polymers are being explored to repair microcracks that may form during repeated deformation, further enhancing durability.

Integration with other flexible components, such as conductive polymers or carbon nanotubes, can improve the overall functionality of TMDC-based wearable systems. Hybrid structures combining MoS2 with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) have demonstrated enhanced mechanical flexibility and electrical conductivity. These composites can withstand strains of up to 20% while maintaining stable electrical properties, making them suitable for highly deformable applications like stretchable epidermal electronics.

Scalability and manufacturing present ongoing challenges for TMDC-based flexible electronics. While lab-scale demonstrations have shown promising results, large-area synthesis and transfer techniques need further development to enable commercial viability. Roll-to-roll processing and inkjet printing of TMDC inks are being investigated as potential pathways for mass production. Uniformity and defect control remain critical hurdles, as variations in layer thickness and crystallinity can significantly impact device performance.

In summary, TMDCs offer a compelling platform for flexible and stretchable electronics, particularly in wearable sensor applications. Their compatibility with various substrates, exceptional mechanical resilience, and high sensitivity under deformation make them stand out among emerging materials. Continued advancements in encapsulation, hybrid integration, and scalable manufacturing will be essential to unlock their full potential in real-world applications. The ability to monitor physiological signals with high precision and reliability positions TMDC-based wearables as a transformative technology in healthcare and human-computer interaction.