Check out this Pulse Sensor which monitors Heart Beats and easily interfaces to an Arduino. This could be used for lots of creative applications. Think of cool clothing that blinks to your heart beat. It would be neat for fitness instructors to be able to look at the ear lobe of their students to determine their approximate heart rate and if the exercise plan should be altered to optimize heart rate. The Pulse Sensor team just finished a successful Kickstarter and are not moving full steam ahead with production.
“When blood is pumped through your body, it gets squeezed into the capillary tissues, and the volume of those tissues increases very slightly. Then, between heart beats, the volume decreases. The change in volume effects the amount of light that will transmit through. This fluctuation is very small, but we can sense it with electronics.”