Researchers from Harvard University in Massachusetts and the University of California San Diego have developed what they describe as the first robot with a 3-D-printed body with a rigid core and a soft exterior. The robot is capable of more than 30 untethered jumps at a time and is powered through a mix of butane and oxygen. In bringing together soft and rigid materials, the researchers hope to create a “new generation of fast, agile robots that are more robust and adaptable than their predecessors and can safely work side by side with humans”. The robot consists of two nestled hemispheres. The top hemisphere is 3-D-printed in once piece with nine different layers of stiffness. The result is a structure that goes from rubber-like flexibility on the exterior to full rigidity near the core. The researchers experimented with several design and concluded that a fully rigid top would make for higher jumps, while a more flexible top was more likely to survive impacts on landing. The bottom hemisphere of the robot is flexible and includes a small chamber where oxygen and butane are injected before it jumps. After the gases are ignited, this half propels the robot into a jump. When the chemical charge is exhausted, the bottom hemisphere goes back to its original shape. The two hemispheres surround a rigid core module that houses a custom circuit board, high-voltage power source, battery, miniature air compressor, butane fuel cell, and other components. In a series of tests, the robot jumped two and a half feet in height and half a foot laterally. In experiments, the robot jumped more than 100 times and survived an additional 35 falls from a height of almost four feet.