Schuerle and Danino in The Scientist:
In the 1966 movie Fantastic Voyage, a team of scientists is shrunk to fit into a tiny submarine so that they can navigate their colleague’s vasculature and rid him of a deadly blood clot in his brain. This classic film is one of many such imaginative biological journeys that have made it to the big screen over the past several decades. At the same time, scientists have been working to make a similar vision a reality: tiny robots roaming the human body to detect and treat disease.
Although systems with nanomotors and onboard computation for autonomous navigation remain fodder for fiction, researchers have designed and built a multitude of micro- and nanoscale systems for diagnostic and therapeutic applications, especially in the context of cancer, that could be considered early prototypes of nanorobots. Since 1995, more than 50 nanopharmaceuticals, basically some sort of nanoscale device incorporating a drug, have been approved by the US Food and Drug Administration. If a drug of this class possesses one or more robotic characteristics, such as sensing, onboard computation, navigation, or a way to power itself, scientists may call it a nanorobot. It could be a nanovehicle that carries a drug, navigates to or preferentially aggregates at a tumor site, and opens up to release a drug only upon a certain trigger. The first approved nanopharmaceutical was DOXIL, a liposomal nanoshell carrying the chemotherapeutic drug doxorubicin, which nonselectively kills cells and is commonly used to treat a range of cancers. The intravenously administered nanoshells preferentially accumulate in tumors, thanks to a leaky vasculature and inadequate drainage by the lymphatic system. There, the nanoparticles slowly release the drug over time. In that sense, basic forms of nanorobots are already in clinical use.
Scientists can manipulate the shape, size, and composition of nanoparticles to improve tumor targeting, and newer systems employ strategies that specifically recognize cancer cells. Still, precise navigation to tumor sites remains a holy grail of nanorobot research and development.