Stuttgart – In nature, chemical motors are everywhere. They can be as tiny as a molecule to only a few millimeters in size, like for instance the locomotion of semi-aquatic insects. Speaking of which: Some semi-aquatic bugs can move on the water surface by secreting chemicals that reduce the local surface tension behind them. These chemicals generate a force known as “Marangoni propulsion”. In an effort to replicate these natural motors, different self-propelled chemical motors based on Marangoni propulsive forces have been developed by researchers over the years, utilizing a wide range of materials and fuels.
However, most self-propelled motor systems have significant limitations that constrain their overall performance: low efficiency (high volume of fuel is required for locomotion), short mobility lifetime, lack of control, difficult miniaturization, and toxicity. Such restrictions pose quite a few challenges: regarding the design, performance or control. These challenges must be addressed to replicate such motor systems and apply them to real world scenarios.
Figure 1: Fabrication of protein-based self-propelled motors: from cephalopod-derived proteins to microrobots
A team of researchers at the Max Planck Institute for Intelligent Systems in Stuttgart, all members of the Physical Intelligence Department lead by Metin Sitti, took on the task of overcoming these limitations by developing new self-propelled motors from biologically derived materials. In their recent publication in Nature Communications, Abdon Pena-Francesch, Joshua Giltinan and Metin Sitti developed multifunctional and biodegradable self-propelled motors from cephalopod-derived proteins and an anesthetic metabolite. Due to the tunable nanostructure of the protein materials and the physical/chemical properties of the fuel, their protein-based motors have a higher performance output, efficiency, and mobility lifetime than previously reported Marangoni motors. “We optimized the motor design, miniaturized it down to a hundred microns – with speeds up to 400 mm/s –, and controlled the locomotion via motor design and magnetic steering, achieving programmable complex locomotion,” says Pena-Francesch.
Figure 2: Programmable complex locomotion of protein motors: a) diverse locomotion modes from protein design, b) programmable trajectories via active magnetic steering
After understanding the propulsion mechanism and optimizing the performance across length scales (speed, efficiency, mobility lifetime, control, etc.), the scientists asked themselves how they could push their protein motors past current limitations of self-propelled systems. After all, regardless of the performance and efficiency, all motors need fuel to work. What happens when they eventually run out of fuel?
“An intrinsic limitation of self-propelled motors is the availability of chemical fuel, since motors have no means of doing work when the fuel is completely exhausted,” Giltinan explains. “However, we can induce locomotion to the protein motors after the fuel is exhausted via non-contact photothermal propulsion, giving the motors a second operational lifetime.”
Figure 3: Locomotion after fuel exhaustion: after the chemical fuel is exhausted, UV wide-field illumination induces thermal Marangoni forces that propel the motor forward
One might ask, whether the motors are toxic to the environment, or whether they are biocompatible or even biodegradable. “Most synthetic chemical motors present biocompatibility challenges because: i) the motor itself is not biocompatible, ii) the fuel is highly cytotoxic, and iii) the propulsion mechanism requires hazardous chemical reactions in the swimming media. However, our protein motors present a fully biocompatible alternative,” Sitti says and gives several reasons: “Firstly the protein that constitutes the motor is not only biocompatible but also biodegradable via disruption of the hydrogen bonding network and enzymatic degradation; second the fuel is a metabolite of sevoflurane – a FDA-approved and widely used inhalation general anesthetic –, which can be excreted in urine; and thirdly hazardous chemicals are not required in the liquid media, thus the protein motors can operate in water and other physiological fluids.”
Figure 4: Protein motor on-demand self-destruction for cargo delivery: free-swimming motor reacts to a pH stimulus by moving towards low pH and reducing its lifetime. Once stopped, the motor swells and degrades over 30 minutes, releasing its encapsulated cargo (scale bar: 10 mm)
Ok, so the motors perform great. So what? Most previously reported self-propelled motors are non-functional passive elements, solely used for fuel storage, and with little control over the locomotion. To demonstrate their use in diverse applications, the scientists explored multiple functionalities of their protein motors by taking advantage of the programmable protein nanostructure and properties. For example, they demonstrated stimuli-responsive drug release via the self-destruction of the motors for targeted cargo delivery, removal of heavy metal water contaminants for environmental remediation, and modular powering of inanimate objects (such as microrobots and small-scale devices).
Figure 5: Multifunctional modular motors: protein coatings are applied to inanimate objects as integrated power sources for locomotion: a) shark fin swimmers, b) autonomous gear trains (reducer and multiplier), c) spinning rotors, and d) magnetic microrobots with programmable complex locomotion (arrows show the direction of the magnetic field).
“These protein motors can be integrated onto virtually any material as a modular propulsion source, and can be functionalized with diverse nanoparticles and biomolecules, opening up the design space for control, sensing, and actuation schemes for small-scale robots, machines, and devices,” says Pena-Francesch. “To our knowledge, this is the first biodegradable self-propelled surface motor that is capable of such multifunctionality, high performance, and precise control at the air-liquid interface. Future work will explore protein motors as modular biodegradable propulsion sources in microrobotics for minimally invasive medical operations in physiological environments with natural or engineered air-liquid interfaces for sensing and therapeutic applications.”
For more details, you can find the full paper here: https://www.nature.com/articles/s41467-019-11141-9
Dr. Abdon Pena-Francesch is from Tarragona in Spain. He is a Humboldt Postdoctoral Fellow in the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems. Abdon received his BSc and MSc degrees in Mechanical and Chemical Engineering from Institut Químic de Sarrià in Barcelona in 2011 and 2013, respectively, and his Ph.D. degree in Engineering Science and Mechanics from The Pennsylvania State University (USA) in 2017. He then joined the Physical Intelligence Department at the MPI-IS in 2018. He works on developing new multifunctional materials for soft and small-scale robotics, with focus on new biodegradable, self-healing, and self-assembled materials (proteins, biomimetic polymers, etc.).
Dr. Joshua Giltinan is a post-doctoral researcher in the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems. Joshua received the B.S. from Towson University (Towson, MD, USA) in 2011 and his Ph.D. from Carnegie Mellon University (Pittsburgh, PA, USA) in 2019. He is interested in the fabrication control of magnetic microrobots for manufacturing tasks.
Dr. Metin Sitti is the Director of the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems, based in Stuttgart. Sitti received his BSc and MSc degrees in electrical and electronics engineering from Bogaziçi 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 during 1999-2002. During 2002-2016, he was a Professor in the Department of Mechanical Engineering and Robotics Institute at Carnegie Mellon University in Pittsburgh, USA. Since 2014, he is a Director 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 his novel small-scale robotic systems to be applied in healthcare, bioengineering, manufacturing, or environmental monitoring to name a few.