On a lab bench in Philadelphia sits a tiny box lined with nearly invisible nanotubes and gold. A clear plastic pipe runs through it, and a thicket of pins, each sprouting a red or blue wire, protrudes from its end. As air from the pipe wafts over the nanotubes, electrical signals surge out of the box along the wire threads. The whole apparatus is situated near a vial of blood, “sniffing” the air above it through the pipe.