The new method allows the team to automatically 3D print dynamic robots in a single step, with no assembly required, using a commercially-available 3D printer.
"Our approach, which we call 'printable hydraulics,' is a step towards the rapid fabrication of functional machines," said Daniela Rus, from Massachusetts Institute of Technology (MIT) in US.
"All you have to do is stick in a battery and motor, and you have a robot that can practically walk right out of the printer," said Rus.
Researchers 3D printed a tiny six-legged robot that can crawl via 12 hydraulic pumps embedded within its body.
Printing liquids is a messy process, which means that most approaches require an additional post-printing step such as melting it away or having a human manually scrape it clean.
That step makes it hard for liquid-based methods to be employed for factory-scale manufacturing.
With "printable hydraulics," an inkjet printer deposits individual droplets of material that are each 20 to 30 microns in diameter, or less than half the width of a human hair.
The printer proceeds layer-by-layer from the bottom up. For each layer, the printer deposits different materials in different parts, and then uses high-intensity UV light to solidify all of the materials except the liquids.
The printer uses multiple materials, though at a more basic level each layer consists of a "photopolymer," which is a solid, and "a non-curing material," which is a liquid.
"Inkjet printing lets us have eight different print-heads deposit different materials adjacent to one another, all at the same time," said Robert MacCurdy from MIT.
"It gives us very fine control of material placement, which is what allows us to print complex, pre-filled fluidic channels," he said.
Another challenge with 3D printing liquids is that they often interfere with the droplets that are supposed to solidify.
To handle that issue, the team printed dozens of test geometries with different orientations to determine the proper resolutions for printing solids and liquids together.
MacCurdy said that printing both liquids and solids is even more difficult with other 3D printing methods, such as fused-deposition modelling and laser-sintering.
The researchers 3D printed a small hexapod robot that weighs about 1.5 pounds and is less than 6 inches long.
To move, a single DC motor spins a crankshaft that pumps fluid to the robot's legs. Aside from its motor and power supply, every component is printed in a single step with no assembly required.
Among the robot's key parts are several set of "bellows" that are 3D printed directly into its body. To propel the robot, the bellows uses fluid pressure that is then translated into a mechanical force.