  A plethora of wearable devices have been developed or commercialized for
continuous non-invasive monitoring of physiological signals that are crucial
for preventive care and management of chronic conditions. However, most of
these devices are either sensitive to skin conditions or its interface with the
skin due to the requirement that the external stimuli such as light or
electrical excitation must penetrate the skin to detect the pulse. This often
results in large motion artefacts and unsuitability for certain skin
conditions. Here, we demonstrate a simple fingertip-type device which can
detect clear pulse signals under all conditions, including fingers covered by
opaque substances such as a plaster or nail polish, or fingers immersed in
liquid. The device has a very simple structure, consisting of only a pair of
magnets and a magnetic sensor. We show through both experiments and simulations
that the detected pulsation signals correspond directly to the magnet
vibrations caused by blood circulation, and therefore, in addition to heartrate
detection, the proposed device can also be potentially used for blood pressure
measurement.
