Working in organic electronics, one of the many pitches for the use of these materials is the possibility of having flexible/stretchable electronics (i.e., skin patch equivalent of the current fitbit or yoga shirts that track your vitals and postures). All these wearables are already very close to commercialization. With these amazing futuristic sensors being made possible by organic semiconductors, let's begin this science blogging series by understanding the current everyday technology: How does the fitbit sense my heart rate? What’s that green light that keeps flickering in the back? How accurate is the heart rate sensor? and, of course, where are they headed?
Health trackers have become a rage in the current generation with every fortune 500 company coming out with a new fitness tracker: Fitbit, Apple watch, Fossil (now acquired by Google), Samsung gear, Garmin, AmazFit (Amazon) and the list goes on. So, while all these companies have proprietary sensors, the basic concept remains the same right now: pulse oximetry a 50-year-old concept. The basic idea is that the absorbance of oxygenated hemoglobin (Hb) is different from de-oxygenated hemoglobin (Hb) in the blood. So the higher the heart rate, the more oxygenated Hb is in your blood, estimated by the absorbance of the reflected light off your skin. This concept has been used widely in bulky medical devices that are connected to your finger. On your fitness tracker: a tiny LED light source (the green flickering light on the fitbit charge HR) hits the blood flowing under the wrist and a part of it is reflected back to the optical sensor which converts it to a heart rate reading (more details available on https://www.microcontrollertips.com/inside-fitbit-charge/). The accuracy of these sensors have been a part of some controversies, but the overall verdict has been that they are accurate to about ~3 bpm and this varies from LED and sensors in use. For example the red light LED source is more accurate than the green light source mostly because the skin does not absorb any red and hence a larger amount of light samples the blood.
While the technology is impressive, it’s still an old school way to think about wearables with every sensor and transistor on the device being rigid and not fitting the entire human form (https://www.wareable.com/wearable-tech/future-of-stretchable-sensors-8764) The organic semiconductor industry is trying to push these wearables to a new limit and to be able to keep pushing limits, we need to understand their materials properties better. In the coming series of blogs I write, I will dive into more organic semiconductor literature and try to keep it related to the future technologies that are almost at the cusp of commercialization.
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