Many drugs under development are delivered by nanoparticles in the bloodstream. to monitor changes in the nanoparticles, investigators can retrieve them from the blood by using a device with electrodes that apply oscillating electric fields. the nanoparticles, which are electrically conducting, are strongly attracted to the nearest electrode, while blood cells, which are poor conductors, experience only a weak force toward the nearest electrode, and suspended ions in the blood experience no net force