It really looks like a plain bandage but if you detach it you’ll discover a patch, not bigger that a stamp that is able to sense variations of tension on your skin with a very high precision.
You can see it in the photo on the side (credit: L.A. Cicero/Stanford University): it has been designed to monitor not just your heart but also the cardio-vascular system as a whole.
When our heart beats it sends a pressure wave that is detected by the sensor under the bandage. Its strength and periodicity can provide important information on your heart workings. That first pressure wave is followed by a much tinier set of waves generated by the tissue response to the first wave (like a spring that is compressed by the first wave and then bounces back once the pressure wave is gone). These further waves can tell a lot about our vascular system: a sclerotic vein, or artery, generates a different response to the pressure wave than a normal vessel. By being able to capture and measure the tiny variations of these waves the sensor can provide most useful information on the status of our cardio vascular system. Obviously, the sensor is just picking up the variations of skin tension but these variations are transmitted to a computer, the one in your smart phone would be perfect, for analyses and comparison with previous sets of measurements so that even more meaning can be derived.
Notice that the cell phone might act as an integrator (this is my speculation, not presented by the researchers at Stanford), picking up information about your movement, as an example: if you are jogging (and this can be inferred by the cell phone accelerator, or if you are walking in a city rather than in a forest, by the seaside or on a mountain (and this is known through the positioning system in your cell phone) the data coming from the sensor lead to a different sort of information and all together, taken in different situations, can provide an amazingly accurate picture of the health of your cardio vascular system.
To create such a precise, flexible and unobtrusive sensor researchers at Stanford have overlaid on a thin rubber sheet two electrodes. The whole is thinner than a dollar bill. The rubber band is composed of tiny pyramids. Variation in the tension of the skin produces a variation in these pyramids, just a few microns each, and in turns changes the distance between the two electrodes leading to a variation in the electromagnetic field. Et voila! these is the data provided by the sensor to the computer for analyses.
The system is both sophisticated (a very precise distribution of the pyramids) and simple and so it is easy to manufacture at a very low cost. We can expect to find these kinds of monitoring devices on our body in the near future with our cell phone acting as the local interpreter of what is going on and as a relay point to more sophisticated analyses.