A microrobot able to swim freely in the human bloodstream has been named the Proteus by its creators, after the 1966 film Fantastic Voyage. Take a look at this microrobot video detailing the capabilities of the real-life Proteus, as wide as two human hairs.
The device uses a tiny tail modeled after bacteria for movement; it is controlled from outside the body. It's piezoelectric motor can actually power the device through human blood. Technovelgy readers are already familiar with this research; see the 2006 article Microrobot To Swim Like Bacteria With Flagellar Propeller for more details and pictures of Dr. Friend and his microrobots.
As Professor Friend explains, "Opportunities for micro-motors abound in fields as diverse as biomedicine, electronics, aeronautics and the automotive industry. Responses to this need have been just as diverse, with designs developed using electromagnetic, electrostatic, thermal and osmotic driving forces. Piezoelectric designs however have favourable scaling characteristics and, in general, are simple designs, which have provided an excellent platform for the development of micro-motors."
Although the new Proteus will not have miniature human passengers, the designers of the device, led by Dr. James Friend, from the Nanophysics Laboratory at Monash University, Australia, hope that it can perform a variety of life-saving functions within the human body.