Caveat - It has been more than decade since I worked in an optics lab, and I haven't looked behind the pay wall at the paper. --- -I'm pretty sure this works

Caveat - It has been more than decade since I worked in an optics lab, and I haven't looked behind the pay wall at the paper. --- -I'm pretty sure this works in a way analogous to "frustrated total internal reflection," which powers many fingerprint readers. (look up total internal reflection on Wikipedia) -By placing three mediums with different impedances in sequence, you can promote an "evanescent" (non-propagating) nearfield wave to partially couple into the the third medium. -The wave will propagate in the third medium if it has the correct impedance relationship to the other two, and if it supports a propagation mode of the incident wave. In this case it appears that the skin is being used as the third medium, based on the wording in the promo blurb from Stanford.