Recent progress on micro- and nano-robots: towards in vivo tracking and localization

Scientists have dreamed for long of miniature robots that can be controlled and navigated inside human body, to help the medical doctors to diagnose and treat the diseases. As shown by the old classic movie Fantastic Voyage released in 1966, a submarine with a group of crews was shrunk to the micro-scale so that it can navigate inside the human blood vessel for the removal of a blood clot. To explore the possibility to realize that wild idea, scientists have proposed and developed many types of miniature machines and robots, some of which are aimed to adapt various kinds of physiological environments for diagnosis and disease treatment. A variety of micro- and nano-scale robots have been developed among the past decade, which offer promise for diverse biomedical applications (1-14). The key factors for constructing the biomedical micro- and nano-scale robots are the controllability, visuality, functionality and biocompatibility. Apart from the macroscale robots, the micro- and nano-scale robots applied in the fluid environments may face to significant difficulties in motion due to the low Reynolds number. To actuate and steer the tiny robots in the low Reynolds number regime where inertia is negligible compared to the viscous force, special strategies for locomotion should be developed. Basically, the micro-/nanorobots can be classified into natural/biological, artificial and biohybrid types, and the artificial micro-/nanoagents are often divided into self-propelled, and external field propelled ones, according to the type of input energy (15). The self-propelled micro-/nano-robots usually generate continuous propulsion from the environment in the form of self-electrophoresis, self-thermophoresis, self-diffusiophoresis and tiny bubbles. In contrast, the external field propelled micro-/nanorobots can be actuated only if the external field, such as magnetic field, electric field, light, US waves and so on, is applied