3D Printing a Robotic Hand with Bones, Ligaments, and Tendons

Andrew Corselli

3D printing is advancing rapidly, and the range of materials that can be used has expanded considerably. While the technology was previously limited to fast-​curing plastics, it has now been made suitable for slow-​curing plastics as well. These have decisive advantages as they have enhanced elastic properties and are more durable and robust.

The use of such polymers is made possible by a new technology developed by researchers at ETH Zurich and a U.S. start-​up. As a result, researchers can now 3D print complex, more durable robots from a variety of high-​quality materials in one go. This new technology also makes it easy to combine soft, elastic, and rigid materials. The researchers can also use it to create delicate structures and parts with cavities as desired.

“We wouldn’t have been able to make this hand with the fast-​curing polyacrylates we’ve been using in 3D printing so far,” said First Author Thomas Buchner. “We’re now using slow-​curing thiolene polymers. These have very good elastic properties and return to their original state much faster after bending than polyacrylates.”

This makes thiolene polymers ideal for producing the elastic ligaments of the robotic hand. In addition, the stiffness of thiolenes can be fine-​tuned very well to meet the requirements of soft robots.

“Robots made of soft materials, such as the hand we developed, have advantages over conventional robots made of metal,” said Professor Robert Katzschmann. “Because they’re soft, there is less risk of injury when they work with humans, and they are better suited to handling fragile goods.”

3D printers typically produce objects layer by layer: nozzles deposit a given material in viscous form at each point; a UV lamp then cures each layer immediately. Previous methods involved a device that scraped off surface irregularities after each curing step. This works only with fast-​curing polyacrylates. Slow-​curing polymers such as thiolenes and epoxies would gum up the scraper.

To accommodate the use of slow-​curing polymers, the researchers developed 3D printing further by adding a 3D laser scanner that immediately checks each printed layer for any surface irregularities.

“A feedback mechanism compensates for these irregularities when printing the next layer by calculating any necessary adjustments to the amount of material to be printed in real time and with pinpoint accuracy,” said Co-Author Wojciech Matusik. This means that instead of smoothing out uneven layers, the new technology simply takes the unevenness into account when printing the next layer.

Inkbit, an MIT spin-​off, was responsible for developing the new printing technology. The ETH Zurich researchers developed several robotic applications and helped optimize the printing technology for use with slow-​curing polymers. The researchers from Switzerland and the U.S. have now jointly published the technology and their sample applications in the journal Nature.

At ETH Zurich, Katzschmann’s group will use the technology to explore further possibilities and to design even more sophisticated structures and develop additional applications. Inkbit is planning to use the new technology to offer a 3D printing service to its customers and to sell the new printers.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Katzschmann.

Tech Briefs: I’m certain there were way too many to count, but what was the biggest technical challenge you faced while developing this 3D-printing technique?

Katzschmann: The biggest challenges were to a) rapidly scan and analyze the scans for closed-loop control of each printed layer, and b) finding the correct print parameters that allowed us to create multimaterial robots with fine, functional features.

Tech Briefs: Can you explain in simple terms how it works?

Katzschmann: The print process uses 3D ink deposition. The printer deposits millions of droplets of ink which start to cure once they are exposed to UV light. After every printed layer, a 3D scanning system recovers the surface profile which is then used in the subsequent layers to adjust how large the next droplets are for the print. The method does not require any leveling after every printed layer. We can print multimaterial systems and robots using this method.

Tech Briefs: You’ve said the group will use the technology to explore further possibilities and to design even more sophisticated structures and develop additional applications. How is that coming along? Any updates you can share?

Katzschmann: We are exploring the use of the technology for tissue engineering, which is going well. We are also looking into the integration of conductive materials, which would expand the possibilities in terms of system design.

Tech Briefs: You’ve also said that Inkbit is planning to use the new technology to offer a 3D-printing service to its customers and to sell the new printers. How is that coming along? Any updates there?

Katzschmann: The printing service is already available; people can already contact Inkbit and ask to print on their machines for a service fee.

Tech Briefs: Going from that, what are your next steps?

Katzschmann: We are diving deeper into combining the printed materials with living cells. We are also exploring the use of artificial muscles in combination with the printed constructs.

Tech Briefs: Do you have any advice for engineers aiming to bring their ideas to fruition?

Katzschmann: Our advice to engineers is to iterate often and fail fast, something that our printer is letting us do now even better while using less resources. Much time can be spent on the drawing board, but actually putting the idea to reality will put it to test and will uncover many issues one has overlooked during the ideation phase.

Tech Briefs: Anything else you’d like to add?

Katzschmann: We hope many will get interested in soft robotics, an emerging field that moves away from rigid metals and replaces those with more bioinspired materials and softer materials that are similar to bone or muscle.

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