Unobtrusive Photoplethysmographic Monitoring Under the Foot Sole while in a Standing Posture

Abstract

Photoplethysmography (PPG) of the foot sole could provide additional health-related information compared with traditional PPG of the finger or wrist. Previously, foot PPG required the procedural binding of a light-emitting diode (LED)-photodetector (PD) pair. We achieved PPG of the foot sole without binding any sensors to the foot while the participant stood in a natural standing position on the testing device. Foot PPG was performed using multiple LED-PD pairs to overcome motion artefacts caused by stabilization. We identified regions of the sole suitable for reliable sensor positioning with optimal LED-PD pairs on the basis of the estimated heart rate (HR) and signal quality index derived by dynamic time warping (wSQI). The first experiment included four participants with direct skin-to-sensor contact, and the results showed a mean HR estimation error of 0.01 beats/min and a wSQI of 0.909. The extended experiment with 53 participants, which involved including a gap between the skin and sensors to consider real-life applications, yielded a mean HR estimation error of 0.638 beats/min and a wSQI of 0.751. Based on the selection ratio of optimal LED-PD pairs, the best region of the sole for PPG was the midfoot, except the medial longitudinal arch. In conclusion, we confirmed that foot PPG using multiple LED-PD pairs is appropriate for HR evaluation and further applications.

Insole-Based Systems for Health Monitoring: Current Solutions and Research Challenges

“…(1) Image processing (IP)-based [49][50][51][52][53][54]; (2) floor sensors (FS)-based [55][56][57][58]; and (3) wearable sensors (WS)-based [12][13][14][15][16][17][18][19][20][21] which are worn and carried by the user [59]. The IP-based system captures image data of the subject's gait through one or multiple optical sensors and extracts gait information through digital image processing [49][50][51][52][53][54].…”

“…In order to overcome these limitations, researchers have been focusing on wearable sensors (WS)-based gait and activity monitoring systems [12][13][14][15][16][17][18][19][20][21]. Modern miniature and lightweight wearable sensors with wireless connectivity have made it possible to develop user-friendly gait analysis systems and to obtain real-time gait data without hindering the user's regular day-to-day activities.…”

“…In [17], heart rate was determined from the sole of the foot by obtaining signals from the lateral plantar artery at the plantar aspect of the foot. A major challenge with obtaining heart rate measurements from the sole of the foot is that fat pads exist for the protection of the foot and for distributing an individual's weight.…”

“…Therefore, wearable pressure sensors and inertial measurement units (IMUs), coupled with the high-speed communication and computing technologies of current smart devices can be used for real-time monitoring of plantar pressure, activity and gait pattern, and for performing a quantitative assessment of foot health in the day-to-day activities of the individuals. Furthermore, insole-based and related systems can potentially be used to realize a comprehensive health monitoring tool by integrating additional sensors for monitoring heart rate [17,18], temperature [19], and muscle activity [20]. The nonobtrusive nature of such a system also results in gait being minimally impacted, allowing for the measurements obtained to be more reflective of our natural gait.…”

Illumination Adaptation in a Multi-Wavelength Opto-Electronic Patch Sensor

“…Wu, W et al [7] used a broadband light source and color filters to determine pulse rate and arterial blood oxygen saturation. Hong, S et al [8] selected the sole of the foot as the sampling site rather than the finger or the wrist and optimized the sensor position using LED-PD pairs. Hernando, A et al [9] showed that a forehead Pulse Amplitude Variability (PAV) signal has a non-respiratory component that was unable to estimate respiratory rate (RR).…”