Have you ever wondered why robots are unable to walk and move their bodies as fluidly as we do? Some robots can run, jump, or dance with greater efficiency than humans, but their body movements also seem mechanical. The reason for this lies in the bones they lack.

Unlike humans and animals, robots do not have real bones or the flexible tissues that connect them; they have artificial links and joints made of materials like carbon fiber and metal tubes. According to Robert Katzschmann, a professor of robotics at ETH Zurich, these internal structures allow a robot to make movements, grab objects, and maintain different postures. However, since links and joints are made up of hard materials, robot bodies are not as flexible, agile, and soft as human bodies. This is what makes their body movements so stiff.

But they may not need to stay stiff for long. A team of researchers from the Swiss Federal Institute of Technology (ETH) Zurich and US-based startup Inkbit have figured out a way to 3D print the world’s first robotic hand with an internal structure composed of human-like bones, ligaments, and tendons. What makes the hand even more special is that it was printed using an entirely new 3D inkjet deposition method called vision-controlled jetting (VCJ).

3D printing vs. robots

Currently, robots that are 3D printed are typically made using fast-curing polyacrylates. These polymers are durable and solidify rapidly during deposition. However, to avoid any irregularities, “Each printed layer requires mechanical planarization [the process of smoothing an uneven surface by using mechanical force], which limits the levels of softness and the type of material chemistries that can be used,” the researchers note. This is why standard 3D-printed robots are not very elastic and are limited in their shapes and materials.

Due to the rapid solidification of the printed material, scientists don’t have the time to make modifications in different layers, and they must employ separate manufacturing steps and assembly to make the different components of a single robot. Once they are finished printing each part, they assemble these different pieces and thoroughly test them, making the process time-consuming and tedious.

This is where the proposed VCJ method can make a huge difference. This 3D printing process involved the use of soft, slow-curing thiolene polymers. “These have very good elastic properties and return to their original state much faster after bending than polyacrylates,” said Katzschmann, one of the authors on a new paper that describes the new method.

Rethinking 3D printing for robots

In a VCJ system, along with a 3D printer, there is a 3D laser scanner that visually inspects each layer for surface irregularities as it’s deposited. “This visual inspection makes the print process fully contactless, allowing for a wider range of possible polymers to be deposited. We, for example, printed with thiol-based polymers because it enabled us to create UV-light and humidity-resistant structures,” Katzschmann told Ars Technica.

After the scanning, there is no mechanical planarization of the deposited layer. Instead, the next layer is printed in such a way that it makes up for all the irregularities in the previous layer. “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 Wojciech Matusik, one of the study authors and a professor of computer science at MIT.

Moreover, the researchers claim that this closed-loop controlled system allows them to print the complete structure of a robot at once. “Our robotic hand can be printed in one go, no assembly is needed. This speeds up the engineering design process immensely—one can go directly from an idea to a functional and lasting prototype. You avoid expensive intermediate tooling and assembly,” Katzschmann added.

Using the VCJ technique, the researchers successfully printed a robotic hand that has internal structures similar to those of a human hand. Equipped with touch pads and pressure sensors, the robotic hand has 19 tendon-like structures (in humans, tendons are the fibrous connective tissues that connect bones and muscles) that allow it to move the wrist and fingers. The hand can sense touch, grab things, and stop fingers when they touch something. (The researchers used MRI data from a real human hand to model its construction.)

VCJ’s future

In addition to the hand, they also printed a robotic heart, a six-legged robot, and a metamaterial capable of absorbing vibrations in its surroundings. The researchers suggest that all these robots work like hybrid soft-rigid systems (robots that are made of both soft and hard materials) that can outperform hard robots in terms of flexibility and overcome the design- and scale-related issues faced by soft robots.

Since soft robots are made of flexible materials like fluids or elastomers, it is tricky for scientists to maintain their geometry and strength at larger scales, as the materials may struggle to retain their physical properties and structural integrity. Plus, it is much easier to control and power a centimeter or millimeter-scale soft robot; this is why they are made smaller. VCJ, on the other hand, has the potential to give rise to scalable hybrid soft-rigid robots.

“We foresee that VCJ will eventually replace all contact-based inkjet printing methods. With VCJ you can start producing functional parts for robotics, medical implants, and various other industries. The high resolution, suitable material properties, and their long lifetime make prints from the VCJ system very useful for both research and commercial applications,” Katzschmann told Ars Technica.

Nature, 2023. DOI: 10.1038/s41586-023-06684-3  (About DOIs)

Rupendra Brahambhatt is an experienced journalist and filmmaker. He covers science and culture news, and for the last five years, he has been actively working with some of the most innovative news agencies, magazines, and media brands operating in different parts of the globe.