"Microrobot" is a sometimes vague term used to refer to small robots. Sometimes it has been used to refer to robots smaller than a millimeter on a side, but has been used as a name for any unusually small robot. Microrobotics is a field in its infancy, but carries huge potential for the future. As a recent example of achievements in microrobotics, South Korean researchers have already demonstrated a six-legged microrobot less than a millimeter in size, small enough to travel through arteries. This robot can release drugs that unblock clogged arteries, which could help stop heart attacks. It is currently in the prototype phase.
The possibility of microrobotics was first analyzed in the 1970s by scientists doing classified research for the intelligence agencies of the US government. At the time, the necessary miniaturization technologies to build an actual microrobot did not exist, so the studies were merely theoretical for some time. In November 1987, roboticist Anita M. Flynn published the seminal paper, "Gnat Robots (and How They Will Change Robotics)," which ushered in the modern era of speculation and design of microrobots. Only in the 1990s did computer chips get miniaturized enough to make true microrobots possible. Still, the technical challenges are numerous. Few practical microrobots exist, and none of them are yet commercialized.
The technology required to create microbots is called MEMS — microelectromechanical systems. MEMS involves active mechanical and electronic systems at the micro-scale. For a robot to be less than a millimeter in size often means it will have design features measured in mere microns. Using the less stringent definition of "microrobot," as in a robot a few cm across, feature sizes are measured in the millimeters.
One of the most impressive microbotics projects have come out of Harvard's Microrobotics Lab. This team has created flying robots with a wingspan just larger than a penny. It makes you wonder — what will warfare or even daily life be like in a society with numerous microrobots? There are poisons so strong that only a few nanograms can kill — what would a war be like if one side had mosquito-like microrobots to inject these into enemy soldiers? Perhaps international treaties will be needed against the use of such weapons.