the nanoparticles will talk to the wristband using light
When Google first announced the project they didn't discuss how the nanoparticles would relay their findings. But, in a video from The Atlantic, employees explain that they'll be using light signals to talk to the wristband through the superficial veins on the underside of the wrist. Of course, shining lights through the skin means factoring in a range of skin types and colors, and so Google's scientists have built fake arms with "the same autofluoresecence and biochemical components of real arms." Thus the fake skin.
The video itself is well worth a watch and offers a tantalizing glimpse into the goings-on at Google X. Andrew Conrad, the head of Google's Life Sciences department, also has a good response to those who might object that it's weird having nanoparticles floating through your body constantly tracking you. "It's way weirder," says Conrad, "to have cancer cells floating through your body that are constantly trying to kill you."