As the size of a robot decreases, the ratio of its surface area to its volume increases. Because the mass of a robot is proportional to its volume, the increase in this ratio means that surface forces (electrostatic attraction, for example) become large compared to inertial forces. So, as robots (and machines in general) become smaller, friction in their moving parts can become a major source of energy loss, wear, and unpredictable behavior. In the Biomimetic Millisystems lab, we have developed a process called "Smart Composite Microstructures" (SCM) that enables us to build small, strong, lightweight, robots and structures whose ability to move comes from bending of compliant polymer hinges that connect rigid links made from carbon fiber and other composites. These structures are made as single flat pieces and are folded up to form more complicated shapes and linkages. They can also be integrated with smart actuators like piezoelectrics and shape memory alloy to provide motion.
DASH 16 gram Hexapedal Robo
Using compliant fiber board as structural material, and a single main driver motor, the DASH robot is capable of 15 body lengths per second on flat surfaces. The structure is resilient and survives ground impact at terminal velocity of 10 meters per second. DASH: A Dynamic 15g Hexapedal Robot.
The RoACH Robot
In the Biomimetic Millisystems Lab we have combined our expertise in building millirobots with an interest in legged systems to build what we believe is the smallest untethered, legged robot to date - a 2.5 gram legged robot called the Robotic Autonomous Crawling Hexapod (RoACH). This robot makes use of the Smart Composite Microstructures fabrication process and integrated shape memory alloy (SMA) wire actuators. All power, control, and communication electronics are carried onboard and the entire robot is powered with a 20maHr Lithium-polymer battery from the Full River corporation. 
