  Wearable devices are becoming increasingly important, addressing needs in
both the fitness and the medical markets. In this paper, we describe a novel
sensing platform based on a hollow-core polyurethane optical fiber, operating
through capillary guidance, that acts as a conformal sensor of pressure or
deformation. The novelty is achieved by combining a simple structure (hollow
capillary) and a simple detection technique (intensity-based measurement) with
unconventional material properties (extreme deformability and high optical
absorbance). Used on the wrist and ankle, the sensor allows detailed features
of the cardiac pulse wave to be identified with high fidelity, while on the
chest it allows the simultaneous measurement of breathing rate and walking
cadence. Used together, an array of such sensors (with others) could be
incorporated into clothing and provide physiologically rich real-time data for
health monitoring.
