A four-inch wafer of silicon has been turned into an army of one million microscopic, walking robots, thanks to some clever engineering employed by researchers at Cornell University in New York. In a paper, published Wednesday in the journal Nature, a team of roboticists detail the creation of their invisible army of robots, which are less than 0.1mm in size (about the width of a human hair) and cannot be seen with the naked eye. The robots ... take advantage of an innovative, new class of actuators, which are the legs of the microrobots. Controlling movement in these tiny machines requires the researchers to shine a laser on minuscule light-sensitive circuits on their backs, which propels their four legs forward. They've been designed to operate in all manner of environments such as extreme acidity and temperatures. One of their chief purposes, the researchers say, could be to investigate the human body from the inside. The team was able to build incredibly small legs, which are connected to two different patches on the back of the robot - one for the front pair of legs, one for the back. Alternating light between the patches propels the microrobot forward. The research team were able to show the microrobots devices could fit within the narrowest hypodermic needle and thus, could be "injected" into the body. The machines aren't intelligent enough to target a diseased cell or respond to stimuli, so there's no application for this invisible army. However, the researchers said that "their capabilities can rapidly evolve."