Flexible Strain Sensors with Ultra-High Sensitivity Using ZnO-Assisted Laser-Induced Graphene

The rapid progress in flexible and stretchable electronics is transforming fields such as wearable health monitoring, environmental sensing, and sports performance tracking. These applications demand real-time, high-precision sensing capabilities, making laser-induced graphene (LIG) a material of considerable interest due to its high electrical conductivity, mechanical flexibility, and scalability. LIG is produced by laser irradiation of polyimide (PI) substrates, which initiates a photothermal decomposition process, resulting in a porous graphene structure. This direct laser-writing method, requiring no masks, is highly adaptable for constructing complex, customizable sensors. Despite these advantages, conventional LIG-based strain sensors often suffer from limited sensitivity, which restricts their utility in precision monitoring scenarios.

This study introduces zinc oxide (ZnO) nanoparticles (NPs) as an enhancement to LIG, addressing these limitations by improving both photothermal efficiency and strain sensitivity. ZnO NPs, with their wide bandgap, UV responsiveness, and high thermal stability, are well-suited for augmenting the properties of LIG. By incorporating ZnO, the critical fluence (Φ_crit) required for effective LIG formation is reduced from 7.5 W to 3.75 W, enabling more efficient and selective graphenization. This integration has led to the development of two distinct sensors: a stretchable strain sensor optimized for elongational monitoring and a dual-functional sensor capable of both UV and bending strain detection.

The resulting stretchable strain sensor, fabricated by transferring LIG onto polydimethylsiloxane (PDMS), exhibited an ultrahigh gauge factor (GF) of 1214 at 10% strain, which is approximately 60 times greater than sensors without ZnO NPs. This enhancement in sensitivity is coupled with substantial durability, as the sensor maintains consistent performance over 2000 stretching cycles with minimal signal degradation, confirming its suitability for long-term wearable applications. The sensor also exhibits rapid response and recovery times, approximately 200 ms, which is advantageous for dynamic monitoring tasks such as motion tracking.

In addition to the stretchable strain sensor, a dual-functional ZnO-LIG sensor was developed. This sensor integrates UV sensing and bending strain detection on opposite sides, allowing it to simultaneously monitor UV exposure and angular displacement. The bending strain sensor aspect is optimized for applications that require precise angle measurements, achieving a GF of 155.5 at 0.32% strain and 101.4 at 1.03% strain, while maintaining sensitivity and stability over 2000 bending cycles. The UV-sensitive layer demonstrated an ON/OFF current ratio exceeding 1000, with response and decay times of 21.2 seconds and 18.9 seconds, respectively, under 365 nm UV light at an intensity of 1000 μW/cm². By implementing a correction equation to compensate for strain effects, the sensor accurately isolates UV signals from strain-induced resistance changes, ensuring reliable UV monitoring even during bending.

This dual-functional sensor was tested in a real-world setting, proving its efficacy in scenarios such as outdoor running, where simultaneous monitoring of UV exposure and joint movement is beneficial. The findings highlight the ZnO-LIG composite's versatility and robustness, underscoring its potential for advanced wearable electronics. Future research will explore additional enhancements, including the incorporation of nanomaterials like molybdenum disulfide (MoS₂) and carbon nanotubes, to further expand sensor capabilities. Moreover, integrating these sensors with wireless technologies and AI-based algorithms will facilitate predictive health monitoring and sophisticated environmental sensing, paving the way for the next generation of wearable devices.