All posts by Max Planck Institute for Intelligent Systems

Scientists at the Max Planck Institute for Intelligent Systems in Stuttgart have developed a soft robotic tool that promises to one day transform minimally invasive endovascular surgery. The two-part magnetic tool can help to visualise in real time the fine morphological details of partial vascular blockages such as stenoses, even in the narrowest and most curved vessels. It can also find its way through severe blockages such as chronic total occlusions. This tool could one day take the perception of endovascular medical devices a step further.

Intravascular imaging techniques and microcatheter procedures are becoming ever more advanced, revolutionizing the diagnosis and treatment of many diseases. However, current methods often fail to accurately detect the fine features of vascular disease, such as those seen from within occluded vessels, due to limitations such as uneven contrast agent diffusion and difficulty in safely accessing occluded vessels. Such limitations can delay rapid intervention and treatment of a patient.

Scientists at the Max Planck Institute for Intelligent Systems in Stuttgart have looked at this problem. They have leveraged the concepts of soft robotics and microfabrication to develop a miniature soft magnetic tool that looks like a very slim eel. This tool may one day take the perception capabilities of endovascular devices one step further. In a paper and in a video, the team shows how the tool, which is propelled forward by the blood flow, travels through the narrowest artificial vessels – whether there is a sharp bend, curve, or obstacle.

When the tool reaches an occlusion like a partially blocked artery, it performs a wave-like deformation given the external magnetic field (more on that below). Then, the deformed soft body will be gently in contact with the surrounding occluded structures. Lastly, the real-time shapes of the device when we retract it will ‘visualize’ the morphological details inside the vessel, which facilitates the drug release at occlusion, as well as the sizing and placement of medical devices like stents and balloons for following treatment.

When there is a severe occlusion with only tiny microchannels for the blood to flow through, the tool can utilize the force from the blood to easily slide through these narrow channels. Which way was chosen indicates to the surgeon which access route to take for the following medical operation.

“The methods of diagnosing and treating endovascular narrow diseases such as vascular stenosis or chronic total occlusion are still very limited. It is difficult to accurately detect and cross these areas in the very complex network of vessels inside the body”, says Yingbo Yan, who is a guest researcher in the Physical Intelligence Department at MPI-IS. He is the first author of the paper “Magnetically-assisted soft milli-tools for occluded lumen morphology detection”, which was published in Science Advances on August 18, 2023. “We hope that our new soft robotic tool can one day help accurately detect and navigate through the many complex and narrow vessels inside a body, and perform treatments more effectively, reducing potential risks.”

This tiny and soft tool has a 20 mm long magnetic Active Deformation Segment (ADS) and a 5mm long Fluid Drag-driven Segment (FDS). The magnetization profile of ADS is pre-programmed with a vibrating-sample magnetometer, providing a uniform magnetic field. Under an external magnetic field, this part can deform into a sinusoidal shape, easily adapting to the surrounding environment and deforming into various shapes. Thus, continuous monitoring of the shape changes of ADS while retracting it can provide detailed morphological information of the partial occlusions inside a vessel.

The FDS was fabricated using a soft polymer. Small beams on its side are bent by the fluidic drag from the incoming flow. In this way, the entire tool is carried towards the area with the highest flow velocity. Therefore, learning the location of the FDS while advancing it can point to the location and the route of the microchannel inside the severe occlusions.

“Detection of vascular diseases in the distal and hard-to-reach vascular regions such as the brain can be more challenging clinically, and our tool could work with Stentbot in the untethered mode”, says Tianlu Wang, a postdoc in the Physical Intelligence Department at MPI-IS and another first author of the work. “Stentbot is a wireless robot used for locomotion and medical functions in the distal vasculature we recently developed in our research group. We believe this new soft robotic tool can add new capabilities to wireless robots and contribute new solutions in these challenging regions.”

“Our tool shows potential to greatly improve minimally invasive medicine. This technology can reach and detect areas that were previously difficult to access. We expect that our robot can help make the diagnosis and treatment of, for instance, stenosis or a CTO more precise and timelier”, says Metin Sitti, Director of the Physical Intelligence Department at MPI-IS, Professor at Koç University and ETH Zurich.

• PAPER – Magnetically assisted soft milli-tools for occluded lumen morphology detection. Yingbo Yan, Tianlu Wang, Rongjing Zhang, Yilun Liu, Wenqi Hu and Metin Sitti. Science Advances, 9(33), eadi3979.

Researchers at the Max Planck Institute for Intelligent Systems and the University of Colorado Boulder have developed a soft shape display, a robot that can rapidly and precisely change its surface geometry to interact with objects and liquids, react to human touch, and display letters and numbers – all at the same time. The display demonstrates high performance applications and could appear in the future on the factory floor, in medical laboratories, or in your own home.

Imagine an iPad that’s more than just an iPad—with a surface that can morph and deform, allowing you to draw 3D designs, create haiku that jump out from the screen and even hold your partner’s hand from an ocean away.

That’s the vision of a team of engineers from the University of Colorado Boulder (CU Boulder) and the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart, Germany. In a new study published in Nature Communications, they’ve created a one-of-a-kind shape-shifting display that fits on a card table. The device is made from a 10-by-10 grid of soft robotic “muscles” that can sense outside pressure and pop up to create patterns. It’s precise enough to generate scrolling text and fast enough to shake a chemistry beaker filled with fluid.

It may also deliver something even rarer: the sense of touch in a digital age.

“As technology has progressed, we started with sending text over long distances, then audio and later video,” said Brian Johnson, one of two lead authors of the new study who earned his doctorate in mechanical engineering at CU Boulder in 2022 and is now a postdoctoral researcher at the Max Planck Institute for Intelligent Systems. “But we’re still missing touch.”

The innovation builds off a class of soft robots pioneered by a team led by Christoph Keplinger, formerly an assistant professor of mechanical engineering at CU Boulder and now a director at MPI-IS. They’re called Hydraulically Amplified Self-Healing ELectrostatic (HASEL) actuators. The prototype display isn’t ready for the market yet. But the researchers envision that, one day, similar technologies could lead to sensory gloves for virtual gaming or a smart conveyer belt that can undulate to sort apples from bananas.

“You could imagine arranging these sensing and actuating cells into any number of different shapes and combinations,” said Mantas Naris, co-lead author of the paper and a doctoral student in the Paul M. Rady Department of Mechanical Engineering. “There’s really no limit to what these technologies could, ultimately, lead to.”

Playing the accordion

The project has its origins in the search for a different kind of technology: synthetic organs.

In 2017, researchers led by Mark Rentschler, professor of mechanical engineering and biomedical engineering, secured funding from the National Science Foundation to develop what they call sTISSUE—squishy organs that behave and feel like real human body parts but are made entirely out of plastic-like materials.

“You could use these artificial organs to help develop medical devices or surgical robotic tools for much less cost than using real animal tissue,” said Rentschler, a co-author of the new study.

In developing that technology, however, the team landed on the idea of a tabletop display.

The group’s design is about the size of a Scrabble game board and, like one of those boards, is composed of small squares arranged in a grid. In this case, each one of the 100 squares is an individual HASEL actuator. The actuators are made of plastic pouches shaped like tiny accordions. If you pass an electric current through them, fluid shifts around inside the pouches, causing the accordion to expand and jump up.

The actuators also include soft, magnetic sensors that can detect when you poke them. That allows for some fun activities, said Johnson.

“Because the sensors are magnet-based, we can use a magnetic wand to draw on the surface of the display,” he said.

Hear that?

Other research teams have developed similar smart tablets, but the CU Boulder display is softer, takes up a lot less room and is much faster. Each of its robotic muscles can move up to 3000 times per minute.

The researchers are focusing now on shrinking the actuators to increase the resolution of the display—almost like adding more pixels to a computer screen.

“Imagine if you could load an article onto your phone, and it renders as Braille on your screen,” Naris said.

The group is also working to flip the display inside out. That way, engineers could design a glove that pokes your fingertips, allowing you to “feel” objects in virtual reality.

And, Rentschler said, the display can bring something else: a little peace and quiet. “Our system is, essentially, silent. The actuators make almost no noise.”

Other CU Boulder co-authors of the new study include Nikolaus Correll, associate professor in the Department of Computer Science; Sean Humbert, professor of mechanical engineering; mechanical engineering graduate students Vani Sundaram, Angella Volchko and Khoi Ly; and alumni Shane Mitchell, Eric Acome and Nick Kellaris. Christoph Keplinger also served as a co-author in both of his roles at CU Boulder and MPI-IS.

• PAPER – A multifunctional soft robotic shape display with high-speed actuation, sensing, and control. Johnson, B.K., Naris, M., Sundaram, V., Volchko, A., Ly, K., Mitchell, S.K., Acome, E., Kellaris, N., Keplinger, C., Correll, N., Humbert, J.S., and Rentschler M.E. Nature Communications 14.1 (2023): 4516.

Scientists at the Max Planck Institute for Intelligent Systems in Stuttgart have developed a magnetically controlled soft medical robot with a unique, flexible structure inspired by the body of a pangolin. The robot is freely movable despite built-in hard metal components. Thus, depending on the magnetic field, it can adapt its shape to be able to move and can emit heat when needed, allowing for functionalities such as selective cargo transportation and release as well as mitigation of bleeding.

Pangolins are fascinating creatures. This animal looks like a walking pine cone, as it is the only mammal completely covered with hard scales. The scales are made of keratin, just like our hair and nails. The scales overlap and are directly connected to the underlying soft skin layer. This special arrangement allows the animals to curl up into a ball in case of danger.

While pangolins have many other unique features, researchers from the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems in Stuttgart, which is led by Prof. Dr. Metin Sitti, were particularly fascinated by how pangolins can curl up their scale-covered bodies in a flash. They took the animal as a model and developed a flexible robot made of soft and hard components that, just like the animal, become a sphere in the blink of an eye – with the additional feature that the robot can emit heat when needed.

In a research paper published in Nature Communications, first author Ren Hao Soon and his colleagues present a robot design that is no more than two centimeters long and consists of two layers: a soft layer made of a polymer studded with small magnetic particles and a hard component made of metal elements arranged in overlapping layers. Thus, although the robot is made of solid metal components, it is still soft and flexible for use inside the human body.

When the robot is exposed to a low-frequency magnetic field, the researchers can roll up the robot and move it back and forth as they wish. The metal elements stick out like the animal’s scales, without hurting any surrounding tissue. Once it is rolled up, the robot can transport particles such as medicines. The vision is that such a small machine will one day travel through our digestive system, for example.

Double useful: freely movable and hot

When the robot is exposed to a high-frequency magnetic field, it heats up to over 70oC thanks to the built-in metal. Thermal energy is used in several medical procedures, such as treating thrombosis, stopping bleeding and removing tumor tissue. Untethered robots that can move freely, even though they are made of hard elements such as metal and can also emit heat, are rare. The pangolin robot is therefore considered promising for modern medicine. It could one day reach even the narrowest and most sensitive regions in the body in a minimally invasive and gentle way and emit heat as needed. That is a vision of the future. Already today, in a video, the researchers are showing how they can flexibly steer the robot through animal tissue and artificial organs.

• PAPER – Pangolin-inspired untethered magnetic robot for on-demand biomedical heating applications. Ren Hao Soon, Zhen Yin, Metin Alp Dogan, Nihal Olcay Dogan, Mehmet Efe Tiryaki, Alp Can Karacakol, Asli Aydin, Pouria Esmaeili-Dokht, and Metin Sitti. Nature Communications, 14(1), 3320.

Most of the world is covered in oceans, which are unfortunately highly polluted. One of the strategies to combat the mounds of waste found in these very sensitive ecosystems – especially around coral reefs – is to employ robots to master the cleanup. However, existing underwater robots are mostly bulky with rigid bodies, unable to explore and sample in complex and unstructured environments, and are noisy due to electrical motors or hydraulic pumps. For a more suitable design, scientists at the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart looked to nature for inspiration. They configured a jellyfish-inspired, versatile, energy-efficient and nearly noise-free robot the size of a hand. Jellyfish-Bot is a collaboration between the Physical Intelligence and Robotic Materials departments at MPI-IS. “A Versatile Jellyfish-like Robotic Platform for Effective Underwater Propulsion and Manipulation” was published in Science Advances.

To build the robot, the team used electrohydraulic actuators through which electricity flows. The actuators serve as artificial muscles which power the robot. Surrounding these muscles are air cushions as well as soft and rigid components which stabilize the robot and make it waterproof. This way, the high voltage running through the actuators cannot contact the surrounding water. A power supply periodically provides electricity through thin wires, causing the muscles to contract and expand. This allows the robot to swim gracefully and to create swirls underneath its body.

“When a jellyfish swims upwards, it can trap objects along its path as it creates currents around its body. In this way, it can also collect nutrients. Our robot, too, circulates the water around it. This function is useful in collecting objects such as waste particles. It can then transport the litter to the surface, where it can later be recycled. It is also able to collect fragile biological samples such as fish eggs. Meanwhile, there is no negative impact on the surrounding environment. The interaction with aquatic species is gentle and nearly noise-free”, Tianlu Wang explains. He is a postdoc in the Physical Intelligence Department at MPI-IS and first author of the publication.

His co-author Hyeong-Joon Joo from the Robotic Materials Department continues: “70% of marine litter is estimated to sink to the seabed. Plastics make up more than 60% of this litter, taking hundreds of years to degrade. Therefore, we saw an urgent need to develop a robot to manipulate objects such as litter and transport it upwards. We hope that underwater robots could one day assist in cleaning up our oceans.”

Jellyfish-Bots are capable of moving and trapping objects without physical contact, operating either alone or with several in combination. Each robot works faster than other comparable inventions, reaching a speed of up to 6.1 cm/s. Moreover, Jellyfish-Bot only requires a low input power of around 100 mW. And it is safe for humans and fish should the polymer material insulating the robot one day be torn apart. Meanwhile, the noise from the robot cannot be distinguished from background levels. In this way Jellyfish-Bot interacts gently with its environment without disturbing it – much like its natural counterpart.

The robot consists of several layers: some stiffen the robot, others serve to keep it afloat or insulate it. A further polymer layer functions as a floating skin. Electrically powered artificial muscles known as HASELs are embedded into the middle of the different layers. HASELs are liquid dielectric-filled plastic pouches that are partially covered by electrodes. Applying a high voltage across an electrode charges it positively, while surrounding water is charged negatively. This generates a force between positively-charged electrode and negatively-charged water that pushes the oil inside the pouches back and forth, causing the pouches to contract and relax – resembling a real muscle. HASELs can sustain the high electrical stresses generated by the charged electrodes and are protected against water by an insulating layer. This is important, as HASEL muscles were never before used to build an underwater robot.

The first step was to develop Jellyfish-Bot with one electrode with six fingers or arms. In the second step, the team divided the single electrode into separated groups to independently actuate them.

“We achieved grasping objects by making four of the arms function as a propeller, and the other two as a gripper. Or we actuated only a subset of the arms, in order to steer the robot in different directions. We also looked into how we can operate a collective of several robots. For instance, we took two robots and let them pick up a mask, which is very difficult for a single robot alone. Two robots can also cooperate in carrying heavy loads. However, at this point, our Jellyfish-Bot needs a wire. This is a drawback if we really want to use it one day in the ocean”, Hyeong-Joon Joo says.

Perhaps wires powering robots will soon be a thing of the past. “We aim to develop wireless robots. Luckily, we have achieved the first step towards this goal. We have incorporated all the functional modules like the battery and wireless communication parts so as to enable future wireless manipulation”, Tianlu Wang continues. The team attached a buoyancy unit at the top of the robot and a battery and microcontroller to the bottom. They then took their invention for a swim in the pond of the Max Planck Stuttgart campus, and could successfully steer it along. So far, however, they could not direct the wireless robot to change course and swim the other way.

Knowing the team, it won’t take long to achieve this goal.

• PAPER – A versatile jellyfish-like robotic platform for effective underwater propulsion and manipulation. Wang, T., Joo, H. J., Song, S., Hu, W., Keplinger, C., and Sitti, M. Science Advances, 9(15), 2023, eadg0292.

Artificial muscles are a progressing technology that could one day enable robots to function like living organisms. Such muscles open up new possibilities for how robots can shape the world around us; from assistive wearable devices that can redefine our physical abilities at old age, to rescue robots that can navigate rubble in search of the missing. But just because artificial muscles can have a strong societal impact during use, doesn’t mean they have to leave a strong environmental impact after use.

The topic of sustainability in soft robotics has been brought into focus by an international team of researchers from the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart (Germany), the Johannes Kepler University (JKU) in Linz (Austria), and the University of Colorado (CU Boulder), Boulder (USA). The scientists collaborated to design a fully biodegradable, high performance artificial muscle – based on gelatin, oil, and bioplastics. They show the potential of this biodegradable technology by using it to animate a robotic gripper, which could be especially useful in single-use deployments such as for waste collection (watch the Youtube video). At the end of life, these artificial muscles can be disposed of in municipal compost bins; under monitored conditions, they fully biodegrade within six months.

We see an urgent need for sustainable materials in the accelerating field of soft robotics. Biodegradable parts could offer a sustainable solution especially for single-use applications, like for medical operations, search-and-rescue missions, and manipulation of hazardous substances. Instead of accumulating in landfills at the end of product life, the robots of the future could become compost for future plant growth,” says Ellen Rumley, a visiting scientist from CU Boulder working in the Robotic Materials Department at MPI-IS. Rumley is co-first author of the paper “Biodegradable electrohydraulic actuators for sustainable soft robots”, published in Science Advances.

Specifically, the team of researchers built an electrically driven artificial muscle called HASEL. In essence, HASELs are oil-filled plastic pouches that are partially covered by a pair of electrical conductors called electrodes. Applying a high voltage across the electrode pair causes opposing charges to build on them, generating a force between them that pushes oil to an electrode-free region of the pouch. This oil migration causes the pouch to contract, much like a real muscle. The key requirement for HASELs to deform is that the materials making up the plastic pouch and oil are electrical insulators, which can sustain the high electrical stresses generated by the charged electrodes.

One of the challenges for this project was to develop a conductive, soft, and fully biodegradable electrode. Researchers at Johannes Kepler University created a recipe based on a mixture of biopolymer gelatin and salts that can be directly cast onto HASEL actuators. “It was important for us to make electrodes suitable for these high-performance applications, but with readily available components and an accessible fabrication strategy. Since our presented formulation can be easily integrated in various types of electrically driven systems, it serves as a building block for future biodegradable applications,” states David Preninger, co-first author for this project and a scientist at the Soft Matter Physics Division at JKU.

The next step was finding suitable biodegradable plastics. Engineers for this type of materials are mainly concerned with properties like degradation rate or mechanical strength, not with electrical insulation; a requirement for HASELs that operate at a few thousand Volts. Nonetheless, some bioplastics showed good material compatibility with gelatin electrodes and sufficient electrical insulation. HASELs made from one specific material combination were even able to withstand 100,000 actuation cycles at several thousand Volts without signs of electrical failure or loss in performance. These biodegradable artificial muscles are electromechanically competitive with their non-biodegradable counterparts; an exciting result for promoting sustainability in artificial muscle technology.

“By showing the outstanding performance of this new materials system, we are giving an incentive for the robotics community to consider biodegradable materials as a viable material option for building robots”, Ellen Rumley continues. “The fact that we achieved such great results with bio-plastics hopefully also motivates other material scientists to create new materials with optimized electrical performance in mind.”

With green technology becoming ever more present, the team’s research project is an important step towards a paradigm shift in soft robotics. Using biodegradable materials for building artificial muscles is just one step towards paving a future for sustainable robotic technology.

• PAPER – Biodegradable electrohydraulic actuators for sustainable soft robots. Ellen H. Rumley, David Preninger, Alona Shagan Shomron, Philipp Rothemund, Florian Hartmann, Melanie Baumgartner, Nicholas Kellaris, Andreas Stojanovic, Zachary Yoder, Benjamin Karrer, Christoph Keplinger, and Martin Kaltenbrunner. Science Advances 9(12), 2023.

By Alexander Badri-Sprowitz, Alborz Aghamaleki Sarvestani, Metin Sitti and Linda Behringer

If a Tyrannosaurus Rex living 66 million years ago featured a similar leg structure as an ostrich running in the savanna today, then we can assume bird legs stood the test of time – a good example of evolutionary selection.

Graceful, elegant, powerful – flightless birds like the ostrich are a mechanical wonder. Ostriches, some of which weigh over 100kg, run through the savanna at up to 55km/h. The ostrich’s outstanding locomotor performance is thought to be enabled by the animal’s leg structure. Unlike humans, birds fold their feet back when pulling their legs up towards their bodies. Why do the animals do this? Why is this foot movement pattern energy-efficient for walking and running? And can the bird’s leg structure with all its bones, muscles, and tendons be transferred to walking robots?

Alexander Badri-Spröwitz has spent more than five years on these questions. At the Max Planck Institute for Intelligent Systems (MPI-IS), he leads the Dynamic Locomotion Group. His team works at the interface between biology and robotics in the field of biomechanics and neurocontrol. The dynamic locomotion of animals and robots is the group’s main focus.

Together with his doctoral student Alborz Aghamaleki Sarvestani, Badri-Spröwitz has constructed a robot leg that, like its natural model, is energy-efficient: BirdBot needs fewer motors than other machines and could, theoretically, scale to large size. On March 16th, Badri-Spröwitz, Aghamaleki Sarvestani, the roboticist Metin Sitti, a director at MPI-IS, and biology professor Monica A. Daley of the University of California, Irvine, published their research in the renowned journal Science Robotics.

Compliant spring-tendon network made of muscles and tendons

When walking, humans pull their feet up and bend their knees, but feet and toes point forward almost unchanged. It is known that Birds are different — in the swing phase, they fold their feet backward. But what is the function of this motion? Badri-Spröwitz and his team attribute this movement to a mechanical coupling. “It’s not the nervous system, it’s not electrical impulses, it’s not muscle activity,” Badri-Spröwitz explains. “We hypothesized a new function of the foot-leg coupling through a network of muscles and tendons that extends across multiple joints”. These multi-joint muscle-tendon coordinate foot folding in the swing phase. In our robot, we have implemented the coupled mechanics in the leg and foot, which enables energy-efficient and robust robot walking. Our results demonstrating this mechanism in a robot lead us to believe that similar efficiency benefits also hold true for birds,” he explains.

The coupling of the leg and foot joints and the forces and movements involved could be the reason why a large animal like an ostrich can not only run fast but also stand without tiring, the researchers speculate. A person weighing over 100kg can also stand well and for a long time, but only with the knees ‘locked’ in an extended position. If the person were to squat slightly, it becomes strenuous after a few minutes. The bird, however, does not seem to mind its bent leg structure; many birds even stand upright while sleeping. A robotic bird’s leg should be able to do the same: no motor power should be needed to keep the structure standing upright.

Robot walks on treadmill

To test their hypothesis, the researchers built a robotic leg modeled after the leg of a flightless bird. They constructed their artificial bird leg so that its foot features no motor, but instead a joint equipped with a spring and cable mechanism. The foot is mechanically coupled to the rest of the leg’s joints through cables and pulleys. Each leg contains only two motors— the hip joints motor, which swings the leg back and forth, and a small motor that flexes the knee joint to pull the leg up. After assembly, the researchers walked BirdBot on a treadmill to observe the robot’s foot folding and unfolding. “The foot and leg joints don’t need actuation in the stance phase,” says Aghamaleki Sarvestani. “Springs power these joints, and the multi-joint spring-tendon mechanism coordinates joint movements. When the leg is pulled into swing phase, the foot disengages the leg’s spring – or the muscle-tendon spring, as we believe it happens in animals,” Badri-Spröwitz adds. A video shows BirdBot walking in the research group’s laboratory.

Zero effort when standing, and when flexing the leg and knee

When standing, the leg expends zero energy. “Previously, our robots had to work against the spring or with a motor either when standing or when pulling the leg up, to prevent the leg from colliding with the ground during leg swing. This energy input is not necessary in BirdBot’s legs,” says Badri-Spröwitz and Aghamaleki Sarvestani adds: “Overall, the new robot requires a mere quarter of the energy of its predecessor.”

The treadmill is now switched back on, the robot starts running, and with each leg swing, the foot disengages the leg’s spring. To disengage, the large foot movement slacks the cable and the remaining leg joints swing loosely. This transition of states, between standing and leg swing, is provided in most robots by a motor at the joint. And a sensor sends a signal to a controller, which turns the robot’s motors on and off. “Previously, motors were switched depending on whether the leg was in the swing or stance phase. Now the foot takes over this function in the walking machine, mechanically switching between stance and swing. We only need one motor at the hip joint and one motor to bend the knee in the swing phase. We leave leg spring engagement and disengagement to the bird-inspired mechanics. This is robust, fast, and energy-efficient,” says Badri-Spröwitz.

Monica Daley observed in several of her earlier biology studies that the bird’s leg structure not only saves energy during walking and standing but is also adapted by nature so that the animal hardly stumbles and injures itself. In experiments with guineafowls running over hidden potholes, she quantified the birds’ remarkable locomotion robustness. A morphological intelligence is built into the system that allows the animal to act quickly – without having to think about it. Daley had shown that the animals control their legs during locomotion not only with the help of the nervous system. If an obstacle unexpectedly lies in the way, it is not always the animal’s sense of touch or sight that comes into play.

“The structure with its multi-jointed muscle-tendons and its unique foot movement can explain why even heavy, large birds run so quickly, robustly, and energy-efficient. If I assume that everything in the bird is based on sensing and action, and the animal steps onto an unexpected obstacle, the animal might not be able to react quickly enough. Perception and sensing, even the transmission of the stimuli, and the reaction cost time,” Daley says.

Yet Daley’s work on running birds over 20 years demonstrates that birds respond more rapidly than the nervous system allows, indicating mechanical contributions to control. Now that the team developed BirdBot, which is a physical model that directly demonstrates how these mechanisms work, it all makes more sense: the leg switches mechanically if there is a bump in the ground. The switch happens immediately and without time delay. Like birds, the robot features high locomotion robustness.

Whether it’s on the scale of a Tyrannosaurus Rex or a small quail, or a small or large robotic leg. Theoretically, meter-high legs can now be implemented to carry robots with the weight of several tons, that walk around with little power input.

The knowledge gained through BirdBot developed at the Dynamic Locomotion Group and the University of California, Irvine, leads to new insights about animals, which are adapted by evolution. Robots allow testing and sometimes confirming hypotheses from Biology, and advancing both fields.

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• PAPER – BirdBot achieves energy-efficient gait with minimal control using avian-inspired leg clutching. Alexander Badri-Spröwitz, Alborz Aghamaleki Sarvestani, Metin Sitti and Monica A. Daley. Science Robotics, Vol 7, Issue 64. DOI: 10.1126/scirobotics.abg4055