Physiological Monitoring Applications of Wearable Multimodal Fusion Systems Based on ECG and PPG: A Comprehensive Review.

Wearable technology has become popular today not only for clinical uses but also among the general public for everyday activities such as fitness tracking and activity recognition. It's no secret that wearable devices need to be more comfortable and that there is increasing attention to their reliability and accuracy. Therefore, researchers are making various attempts to improve these specifications. Recent research has employed multimodal fusion, rather than relying on results from a single source, such as electrocardiography (ECG) or photoplethysmography (PPG), because it enables a more comprehensive understanding of physical conditions. There are very few extensive reviews of ECG and PPG wearable multimodal fusion application systems, and they are limited to a single practical use. However, this review fills that gap and provides a comprehensive overview of recent wearable physiological monitoring technologies based on ECG and PPG, along with their modern multimodal fusion applications in the medical field. These include cuffless blood pressure estimation, stress monitoring, heart rate and heart rate variability monitoring. This presents the theoretical background, including the characteristics of ECG and PPG signals, recently developed wearable monitoring techniques for ECG and PPG, and the advantages and disadvantages of multimodal fusion. It also provides a comprehensive and comparative analysis of recent studies employing modern multimodal fusion approaches in the aforementioned medical field, as well as a discussion of the limitations and challenges of ECG-PPG wearable multimodal fusion systems reported in the literature. Therefore, this review will enable researchers to gain a complete and comprehensive understanding of the development of wearable multimodal fusion applications based on ECG and PPG.