Stuttgart – Researchers at the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart developed a bullet-shaped, synthetic miniature robot with a diameter of 25 micrometers, which is acoustically propelled forward – a speeding bullet, in the truest sense of the word. Less than the diameter of a human hair in size, never before has such an actuated microrobot reached this speed. Its smart design is so efficient it even outperforms the swimming capabilities of natural microorganisms.
The scientists designed the 3D-printed polymer microrobot with a spherical cavity and a small tube-like nozzle towards the bottom (see figure 1). Surrounded by liquid such as water, the cavity traps a spherical air bubble. Once the robot is exposed to acoustic waves of around 330 kHz, the air bubble pulsates, pushing the liquid inside the tube towards the back end of the microrobot. The liquid’s movement then propels the bullet forward quite vigorously at up to 90 body lengths per second. That is a thrust force two to three orders of magnitude stronger than those of natural microorganisms such as algae or bacteria. Both are among the most efficient microswimmers in nature, optimized by evolution.
Figure 1. Microscopy image of the acoustically powered microrobot with a trapped spherical air bubble inside its body (left); Acoustic pressure field around the microrobot predicted by the numerical simulations (middle); Scanning electron microscopy image of the three-dimensionally printed microrobot with a sharp fin on the top (right).
Deploying ultrasound waves to actuate microrobots is not a new approach. However, previous robots with swimming capabilities have shown to be relatively slow, difficult to control near surfaces, and have had a relatively short operating time of only a few minutes. All of these factors are obstacles to their use in medical applications for targeted drug delivery, detoxification, or noninvasive surgeries. The scientists of the Physical Intelligence Department at the MPI-IS, Dr. Amirreza Aghakhani, Oncay Yasa, Paul Wrede, and Dr. Metin Sitti, who is the department’s Director, were able to fine-tune their robot’s steering capabilities while increasing the operating time to several hours. Their publication “Acoustically powered surface-slipping mobile microrobots“ was published in the Proceedings of the National Academy of Sciences PNAS in February 2020.
Figure 2. Schematics of a surface-slipping microrobot under ultrasound powering (top); Time-lapse image of the moving microbot on a substrate at 330 kHz acoustic excitation (bottom).
In addition to designing the robot with an inside cavity trapping a spherical air bubble with a small opening, they added a small fin to the microrobot’s cylindrical body surface (see figure 2), which biases the propulsion direction. They also coated the top of the polymer bullet with a soft magnetic nanofilm layer. With the help of surrounding external magnetic fields, they were then able to alter the direction of the bullet left or right, as well as up or down.
In several experiments, the researchers tested how capable their robot is of moving on different surfaces inside a microchannel similar to a blood vessel. They exposed the robot to acoustic waves and a magnetic field and succeeded in navigating it in this confined 3D space. They also showed that the trapping of cargo happens automatically while the microrobot moves. While the liquid is being pushed out of the robot’s cavity when exposed to ultrasound waves, a circular microstreaming pattern is generated towards the bottom (see figure 2). This current ensures that surrounding drug particles are guided towards the robot. There, the particles are amassed and are transported away with the movement of the robot. Thanks to this, the robot could one day be applied to collect cancer drug particles inside the bloodstream and specifically transport the drug towards a carcinoma, releasing the cargo at a close range for maximum impact.
Dr. Amirreza Aghakhani, a postdoctoral researcher in the Physical Intelligence Department and the co-lead author of the publication, summarizes the special features of the microrobot: “We can actuate our microrobots very efficiently, and they are also very fast. Ultrasound is harmless to the body and can penetrate into regions deep inside the body. We can move these robots on both flat and curved surfaces in a controlled manner and we can attach different cargo, such as drugs. This is impressive.”
Another benefit is ultrasound imaging. Inside the body, imaging is very challenging when the robot is only several micrometers in size. “However, the trapped air bubble can act as a contrast agent, making the robot and its location more visible,” Dr. Aghakhani adds.
“Our vision is to deploy such acoustically powered and magnetically steered microrobots inside the human body for various noninvasive medical applications in the near future,” Dr. Metin Sitti concludes.
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The Max-Planck-Institute for Intelligent Systems is located in two cities: Stuttgart and Tübingen. Research at the Stuttgart site of the Max Planck Institute for Intelligent Systems covers small-scale robotics, self-organization, haptic perception, bio-inspired systems, medical robotics, and physical intelligence. The Tübingen site of the institute concentrates on machine learning, computer vision, robotics, control, and the theory of intelligence.
Dr. Amirreza Aghakhani is a postdoctoral researcher in the Physical Intelligence Department of the Max Planck Institute for Intelligent Systems. He obtained his Ph.D. degree in Mechanical Engineering from Koc University in Istanbul, Turkey, in 2018. His research interests include acoustic actuation and imaging methods, micro-robotics, structural vibration, and energy harvesting. His current research focuses on the development of fast and efficient acoustic microswimmers for potential biomedical applications.
Oncay Yasa recently defended his doctoral dissertation in the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems in Stuttgart. He obtained his bachelor’s degree in 2012 from the Department of Molecular Biology and Genetics, Middle East Technical University (METU), Turkey. He then completed his master’s degree at Bilkent University and conducted research in the field of biomimetic self-assembled macromolecules for regenerative medicine at the National Nanotechnology Research Center (UNAM). Yasa’s research focusses on biohybrid microsystems and synthetic microswimmers. Yasa wants to combine his knowledge in molecular biology and materials science to create novel microswimmers that could be used in active drug delivery applications.
Paul Wrede joined the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart on February 1, 2019. Paul did his bachelor’s in Biomedical Engineering at the Technical University of Chemnitz. Besides his studies, he was a student assistant at the Leibniz Institute for Solid State and Materials Research in Dresden, where he worked on Biohybrid and Catalytic Microrobots. Paul is currently doing his master’s degree in Biomedical Technologies at the University of Tübingen. He focuses on designing and fabricating microrobotic approaches for future biomedical applications.
Professor Dr. Metin Sitti is the Director of the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems in Stuttgart.
Sitti received his BSc and MSc degrees in electrical and electronics engineering from Bogazici University in Istanbul in 1992 and 1994, respectively, and his PhD degree in electrical engineering from the University of Tokyo in 1999. He was a research scientist at University of California at Berkeley from 1999-2002. From 2002-2014, he was a Professor in the Department of Mechanical Engineering and Robotics Institute at Carnegie Mellon University in Pittsburgh, USA. Since 2014, he has been Director of the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems.
Sitti and his team aim to understand the principles of design, locomotion, perception, learning, and control of small-scale mobile robots made of smart and soft materials. Intelligence of such robots mainly come from their physical design, material, adaptation, and self-organization more than to their computational intelligence. Such physical intelligence methods are essential for small-scale milli- and micro-robots, especially due to their inherently limited on-board computation, actuation, powering, perception, and control capabilities. Sitti envisions that his novel small-scale robotic systems will be applied in healthcare, with the aim of having the greatest possible scientific and positive societal impact.