First-ever 3-D Printed Robots Made of Both Solids and Liquids

One reason we don’t yet have robot personal assistants buzzing around doing our chores is because making them is hard. Assembling robots by hand is time-consuming, while automation — robots building other robots — is not yet fine-tuned enough to make robots that can do complex tasks.

But if humans and robots can’t do the trick, what about 3-D printers?

In a new paper, researchers at MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL) present the first-ever technique for 3-D printing robots that involves printing solid and liquid materials at the same time.

The new method allows the team to automatically 3-D print dynamic robots in a single step, with no assembly required, using a commercially-available 3-D printer.

“Our approach, which we call ‘printable hydraulics,’ is a step towards the rapid fabrication of functional machines,” says CSAIL Director Daniela Rus, who oversaw the project and co-wrote the paper. “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.”

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

The paper, which was recently accepted to this summer’s IEEE International Conference on Robotics and Automation (ICRA), was co-written by MIT postdoc Robert MacCurdy and PhD candidate Robert Katzschmann, as well as Harvard University undergraduate Youbin Kim.

The printing process

For all of the progress in 3-D printing, liquids continue to be a big hurdle. 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.

“Inkjet printing lets us have eight different print-heads deposit different materials adjacent to one another, all at the same time,” MacCurdy says. “It gives us very fine control of material placement, which is what allows us to print complex, pre-filled fluidic channels.”

Another challenge with 3-D 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.

While it’s a painstaking process, MacCurdy says that printing both liquids and solids is even more difficult with other 3-D printing methods, such as fused-deposition modeling and laser-sintering.

“As far as I’m concerned,” he says, “inkjet-printing is currently the best way to print multiple materials.”

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