Advanced methods of wearable ultrasound and photoacoustic imaging devices: A review

Ultrasound imaging (USI) and photoacoustic imaging (PAI) are being adapted from console-based examinations to body-conformal formats for continuous and point-of-care monitoring. USI offers real-time anatomical and motion information, whereas PAI adds optical absorption contrast related to hemoglobin, oxygenation, temperature, and molecular composition. The challenge is no longer a single component. Wearable USI and PAI require soft mechanics, stable acoustic coupling, compact light delivery, low-power acquisition, safe operation, reliable data handling, and reconstruction methods that tolerate motion and imperfect contact. This review examines recent wearable USI and PAI technologies from device level to system level. We cover micromachined transducers, flexible and stretchable ultrasound patches, laser-diode and VCSEL-based PA excitation, compact PA detectors, multimodal PA/US integration, wireless electronics, spatial encoding, and reconstruction methods. Representative systems are compared in terms of form factor, imaging target, hardware architecture, and remaining bottlenecks, with emphasis on the gap between wearable probes and autonomous imaging platforms.