Flexible and wearable wireless passive surface acoustic wave (WP-SAW) strain sensors for on-body biomechanical monitoring: A review

Flexible and wearable surface acoustic wave (SAW) strain sensors, leveraging their inherent wireless and passive operation, have emerged as a transformative platform for on-body biomechanical monitoring. This capability is pivotal because key physiological stimuli such as pressure, force, and tissue deformation often manifest as measurable surface strain, making strain sensing a fundamental modality for non-invasive health assessment. This article systematically reviews recent advances in this field, with a specific focus on design principles, material systems, fabrication techniques, and system integration strategies that address the dual demands of high sensing performance and conformal wearability. It critically examines innovative strategies to address key challenges such as mechanical mismatch with soft tissues, temperature cross-sensitivity, motion artifacts, and signal attenuation in dynamic environments, including dual-channel differential designs, the adoption of flexible and stretchable materials, and intelligent signal processing algorithms. Furthermore, this review provides a critical assessment of barriers (e.g., long-term reliability, under deformation, biocompatibility, and system-level power efficiency) and outlines future research directions, including the development of intelligent systems, multi-modal sensing, and digital twins. By combining recent global research findings, this work serves as a comprehensive reference for developing next-generation, self-sustained wearable sensing systems for personalized biomechanical monitoring.