With the growing demand for vital signs monitoring using wearables, there is a gap between what is offered and the desire for clinical accuracy. Advancements in electronic miniaturization, materials and signal processing have increased possibilities for wearable technology. Optical sensing using photoplethysmography (PPG) is prominent within the field of wearable healthcare due to the non-invasive and versatile nature of the modality. Different aspects related to cardiovascular health can be screened and monitored with PPG, such as oxygen saturation (SPO2), heart rate (HR), heart rate variability (HRV) and possibly blood pressure. Although the principles and possibilities for vital sign monitoring with PPG are present, there is only sparse evidence of clinically relevant signals from commercial optical sensors. Furthermore, there is still a big challenge regarding motion artifact within a wearable device. Currently three wavelengths are commonly used within commercial wearables; green (550nm), red (660nm) and infrared (850nm). There is increased research on including shorter wavelengths (blue) potentially providing increased robustness to motion artifacts during wear.
Within this study, the design challenges for a universal wearable optic patch for improved signal accuracy were investigated. A dual-photodiode and multi-wavelength flexible wearable optic patch was fabricated using hybrid printed electronics. The design was evaluated as a function of patch-skin contact pressure during motion. Our preliminary results show a more robust PPG signal with increased patch-skin contact pressure. In addition, this study demonstrates the capability of our wearable and flexible optical patch at measuring simultaneous multi-wavelength PPG.
Photoplethysmography (PPG) is an optical technique to study light absorption variations by blood pulsation. We have developed a flexible thin-film photodiode array combined with flexible LED light sources, both integrated into a wearable form factor, to measure PPG signals at various wavelengths and at any location on the human body in reflection. The array contains 256 photodiodes over an area of approximately 4 cm2, which can be read out at high frequencies up to 5.4 kHz. The large number of photodiodes operated at high speeds enables spatiotemporal PPG information with a high signal quality. Furthermore, applying independent component analysis to the data array allows for signal quality mapping and enhancement of the PPG signals over the full array. These wearable sensor arrays allow for new types of cardiovascular health related analysis, such as local biomarker mapping and in-situ probing of blood pulsation at different depths underneath the skin.
The next generation medical imaging will benefit significantly from artificial intelligence – therefore, not only advances in computing power are required, but also further materials and technology improvements will lead to better image quality. The strong X-ray absorption, high charge carrier mobility and lifetime recommend perovskites like methylammonium lead triiodide (MAPbI3) as novel direct X-ray converting materials. A major obstacle to the commercialization is the limited stability and lifetime, as reported until now. Here, we show for the first time that degradation is limited in our X-ray detector after 1.5 years of storage in ambient conditions.
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