Wearable, compact photo- and speckle plethysmography device for continuous non-invasive vital signs monitoring

Continuous real-time monitoring of hemodynamic variables has become very relevant in modern medicine not only in clinical settings, but also for consumer wearables. Wearable technologies allow for vital sign monitoring (VSM) and help to promptly react in urgent situations. Photoplethysmography (PPG), an optical method based on absorption changes, is a well-known technique for non-invasive monitoring of cardiovascular biomarkers. However, PPG quality can be significantly compromised by skin melanin content, low perfusion, and ambient light changes. Unreliability under these conditions could potentially lead to severe consequences if threatening events remain undetected. Speckle Plethysmography (SPG) is based on the measurement of speckle variations and shows a robust signal quality independent of skin melanin content and ambient light, which indicates that SPG is a more reliable technology for continuous VSM. We present a compact, wearable device for simultaneous PPG and SPG VSM. The system consists of a laser diode illuminating the tissue, and a camera that captures the reflected light, forming a speckle pattern. Additionally, an acquisition platform (portable computer) is used to send the information wirelessly to a computer. The system allows for software-controlled tuning of parameters to optimize signal quality. SPG and PPG are calculated from the images, visualized in real-time and recorded to analyze cardiovascular biomarkers such as heart rate, heart rate variability and others. This compact wearable device is the first step towards full SPG/PPG sensor integration to enable robust, low-cost, wearable, non-invasive VSM.