Heart rate monitoring with cellulose optical fiber

Cellulose, as a renewable, biodegradable, and biocompatible material, offers new possibilities for optical fiber (OF) sensors. Cellulose OFs are highly hygroscopic with fast wetting and drying properties, easily functionalized, and can be made both water-resistant and water-soluble. These fibers do not replace the glass or polymer OFs in communication or sensing applications, rather cellulose OF can open new applications. OFs have been used for heart rate and respiratory rate monitoring both with glass and polymer fibers. We have also shown carboxymethyl cellulose (CMC) and regenerated cellulose (RC) based OF breathing sensor.
Here we show CMC OF for heart rate monitoring. We prepared a 5 cm long CMC fiber loop using commercial ferrules and sleeves. We used single mode (SM) fiber with 9/125 µm core/cladding diameter and multimode (MM) fiber with 62.5/125 µm core/cladding diameter as connection fibers. SM fiber couples coherent 637 nm laser light to CMC MM fiber and modal interference generates speckle pattern type output intensity distribution. This light distribution is sensitive to CMC fiber movement, and the change is measured by coupling just a small part of intensity distribution to photodetector using smaller MM fiber as others have already shown before. Our sensor showed 55 beats per minute (bpm) from arm.