Photoplethysmography: New trends and future directions

The chapters of this book have covered a wide range of topics in Photoplethysmography (PPG), from basic theory and technology to signal analysis, including a wide range of PPG applications for clinical and consumer use, etc. Owing to its practicality and ease of use, the interest in PPG-based technologies such as wearables continuous to grow year-on-year. The use of PPG for the monitoring of our health and wellbeing has been the corner stone and the main motivator for many of its applications. However, over the years researchers have gone beyond such applications and have explored the utilization of PPG in new directions such as biometrics. This is a subject of major interest as it impacts greatly our daily lives and the security of our private information, and hence it was deemed necessary to contribute parts of this last chapter and inform the readers of all the current efforts made in using PPG as a biometric indicator. Also, as it can be distilled from all the chapters in this book, there is still room for more basic understanding of the origin of the PPG and its relation to various pathophysiological phenomena. Contributors of this book have described and hailed the successes and the potential of the PPG, but at the same time have highlighted the need for more research and better understanding of the signal. Hence, another trend of the recent years, by many PPG research communities, is the return to basic PPG research. Such research can be accomplished by building the right tools and processes which will enable the further understanding of the PPG. Clearly the more we understand the PPG the better we can use it in specific applications. For example, throughout the book there were references for the need of better and more detailed understanding of the light–tissue interactions in PPG in multiple wavelengths (not just red, near-infrared, and green), in both transmission and reflection sensor topologies. Also, there were references for the need of standardization in PPG processes, such as data acquisition, waveform analysis, etc. Hence, it was deemed necessary to share with the readers some of the latest efforts in research in creating custom made phantoms and simulators for PPG, which could aid in the better understanding and utilization of the signal. This being the last chapter, it is also most fitting to offer some final thoughts in the potential future directions of this most exciting vascular optics technology.