Seeing inside the Body Using Wearable Sensing and Imaging Technologies.

The growing demand for personalized and real-time health assessment has driven rapid advancements in wearable health monitoring technologies. A critical shift is underway from reactive, treatment-centered care toward proactive, preventive healthcare models, motivating efforts to observe internal physiological states more accurately from outside the body. This review presents wearable monitoring technologies for evaluating internal organ function, focusing on two principal approaches: indirect sensing-based methods and direct imaging-based methods. Indirect methods utilize surrogate signals-including electrophysiological activity, physical responses, and biochemical markers-to infer physiological states. To address the limitations of single-modality sensing, recent developments in multimodal systems that integrate electrical, chemical, and physical signals are also highlighted. However, indirect approaches based on biomarkers are often insufficient for diagnostic applications that require direct visualization of organ structure and function. Therefore, this review further explores direct imaging technologies, such as wearable ultrasound and electrical impedance tomography (EIT), which offer more immediate insights into internal biological conditions. By combining indirect and direct strategies, wearable systems are evolving beyond surface-level monitoring to enable internal visualization and functional assessment of organs. This review outlines the key enabling technologies and discusses their potential to advance participatory, personalized, and precision healthcare through continuous and context-aware monitoring platforms.