3D printed robots will be made of both solids and liquids

To demonstrate the concept, researchers 3D printed a tiny six-legged robot that can crawl via 12 hydraulic pumps embedded within its body. They also 3D printed robotic parts that can be used on existing platforms, such as a soft rubber hand for the Baxter research robot.

For all of the progress in 3D printing, liquids continue to be a big problem. 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 (minus, of course, the liquids). The printer uses multiple materials, though at a more basic level each layer consist of a "photopolymer," which is a solid, and "a non-curing material," which is a liquid.

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.

To demonstrate their method, 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. (As an alternative to the bellows, the team also demonstrated they could 3D print a gear pump that can produce continuous fluid flow.)

Lastly, the team 3D printed a silicone-rubber robotic hand with fluid-actuated fingers. This "soft gripper" was developed for Baxter, a robot that was designed by former CSAIL director Rodney Brooks as part of his spinoff company Rethink Robotics.