User Group: Researcher

It is not the first time that spiders have served as biological models in the research field of soft robotics. The hydraulic actuation mechanisms they apply to move their limbs when weaving their web or hunting for prey give them powers many roboticists and engineers have drawn inspiration from.

A team of researchers at the Max Planck Institute for Intelligent Systems and at the University of Colorado Boulder in the US has now found a new way to exploit the principles of spiders’ joints to drive articulated robots without any bulky components and connectors, which weigh down the robot and reduce portability and speed. Their slender and lightweight simple structures impress by enabling a robot to jump 10 times its height. At the end of May, the team’s work titled “Spider-inspired electrohydraulic actuators for fast, soft-actuated joints” was published in Advanced Science.

The high performance is enabled by Spider-inspired Electrohydraulic Soft-actuated joints—SES joints in short. The joints can be used in many different configurations—not just when creating an arachno-bot. In their paper, the scientists demonstrate a bidirectional joint, a multi-segmented artificial limb, and a three-fingered gripper, which can easily pick up delicate objects. All creations are lightweight, simple in their design, and exhibit high performance making them ideal for robotic systems that need to move rapidly and interact with many different environments.

The researchers developed their SES joints based on the HASEL technology which had previously been invented by the team to build artificial muscles. SES joints mimic a spider-inspired exoskeletal mechanism comprised of both rigid and softer elements, which function similarly to the animal’s leg extension through the use of hydraulic forces.

They built a flexible pouch made of thin plastic films (either polyester or polypropylene will do) which they filled with a liquid dielectric—a vegetable-based oil. They then placed electrodes on each side of the pouch. These liquid-filled pockets serve as actuators, in which the hydraulic power is generated through electrostatic forces. The pouch is attached to a rotary joint. When a high voltage is applied between the electrodes, the electrostatic forces cause the liquid dielectric to shift inside the pouch and the joint to flex. SES joints are capable of rotating up to 70 degrees, causing high torques, and can easily restore back to the starting position.

“The SES joints are very simple and light, as there are no peripheral components which weigh down the robot,” says Christoph Keplinger, Director of the Robotic Materials Department at the Max Planck Institute for Intelligence Systems. “Many applications for soft robots require versatile actuators. These spider-inspired joints allow for high functionality and consume only little power, they are easy and cheap to make—the plastics we are using are for food packaging—and their production is easily scalable. These are all qualities that are critical for the design of robots, which can move in many different ways and manipulate a variety of objects without breaking them.”

A three-fingered gripper was one application for which the team used SES joints to showcase their versatility. If the team had equipped the gripper with a muscle-like structure, it would have been in the way of the object that the gripper is grabbing. Using SES joints as the hinges of the gripper required much less space.  

“The research stands out because we can use a wide variety of materials, even the plastic used to make chips bags to create the pouches,” the first author of the publication Nicholas Kellaris says. “That way we can implement SES in a wide variety of geometries with specifically tuned actuation characteristics.”

“The ultimate goal of our research was not to make a spider robot,” Philipp Rothemund, the second author of the publication, adds. “We wanted to develop a state-of-the-art, active joint that you can put in any type of robot.”

Especially for small-scale robotic systems of only a few centimeters in size, where the limited space severely restricts the choice of actuator technologies, the SES-joints will come in very useful. For the soft robotics community, this invention is truly a leap forward.

As one would do with a Lego system, the scientists can randomly combine individual components. The building blocks or voxels – which could be described as 3D pixels – are made of different materials: from basic matrix materials that hold up the construction to magnetic components enabling the control of the soft machine. “You can put the individual soft parts together in any way you wish, with no limitations on what you can achieve. In this way, each robot has an individual magnetisation profile,” says Jiachen Zhang. Together with Ziyu Ren and Wenqi Hu he is first author of the paper entitled “Voxelated three-dimensional miniature magnetic soft machines via multimaterial heterogeneous assembly”. The paper was published in Science Robotics on April 28, 2021.

The project is the result of many previous projects conducted in the Physical Intelligence Department at MPI-IS. For many years, scientists there have been working on magnetically controlled robots for wireless medical device applications at the small scale, from millimeters down to micrometers size. While the state-of-the-art designs they have developed to date have attracted attention around the world, they were limited by the single material with which they were made, which constrained their functionality.

“When building soft-bodied miniature robots, we have many different and often complex designs. As a result of their small size, the available fabrication capability is very limited, and this poses a major challenge. For years, researchers have been trying to develop an innovative fabrication platform that provides scientists with completely new capabilities. Our team has now succeeded in demonstrating a new way to construct much more complex soft robots with different components rather than just one. By mixing and matching, we enable tailor-made functionalities and complex robot morphologies. Our new modular building platform will pave the way for many new functional wireless robots, some of which could potentially become the minimally-invasive medical devices of the future,” says Metin Sitti, who leads the Physical Intelligence Department and has pioneered many wireless medical and bio-inspired miniature robots.

“We have seen 3-D printing or mold casting of voxels with only one material. That limits the functionality – a single material can only do so many things,” says Ziyu Ren. “If you want more functionality like we did and a unique magnetisation profile, you have to introduce a whole set of different materials, for instance by mixing various non-magnetic and magnetic materials.

“Previously, each robot’s magnetisation profile was limited to certain patterns due to the strong coupling with the geometry of the robot. Now, we have created a platform that can achieve a flexible magnetisation profile. We can do so by freely integrating multiple magnetic parts together in one system,” Jiachen Zhang adds.

The new building platform enables many new designs and is an important milestone in the research field of soft robotics. The Physical Intelligence Department has already developed a wide variety of robots, from a crawling and rolling caterpillar-inspired robot, a spider-like construct that can jump high, to a robotic grasshopper leg and magnetically-controlled machines that swim as gracefully as jellyfish. The new platform will accelerate the momentum and open up a world of new possibilities to construct even more state-of-the-art miniature soft-bodied machines.

The scientists base each construction on two material categories. The base is mainly a polymer that holds up the matrix. But also other kinds of soft elastomers, including biocompatible materials like gelatin, are used. The second category comprises materials embedded with magnetic micro- or nanoparticles that make the robot controllable and responsive to a magnetic field.

Thousands of voxels are fabricated in one step. Like dough being distributed in a cookie tray, the scientists use tiny mold casts to create the individual blocks – each of which is no longer than around 100 micrometers. The composition then happens manually under a microscope, as automating the process of putting the particles together is still too complex. While the team integrated simulation before building a robot, they took a trial and error approach to the design until they achieved perfection. Ultimately, however, the team aims for automation; only then can they reap the economies of scale should they commercialize the robots in the future. “In our work, automated fabrication will become a high priority,” Jiachen Zhang says. “As for the robot designs we do today, we rely on our intuition based on extensive experience working with different materials and soft robots."