What is microbots

From artificial flagella to medical microbots - the beginning of a "fantastic journey"

Research report 2011 - Max Planck Institute for Intelligent Systems, Stuttgart location

Micro, nano and molecular systems
Scientific and popular science articles often speculate as to whether remotely controlled microbots can be sent through the human body. This would revolutionize (micro) medicine. We apply billions of micrometer-sized, chiral-colloidal screw propellers to a wafer. Then we magnetize them and let them drive through solutions with the help of computer-controlled magnetic fields. Similar to artificial flagella, so far they are the only microbots that can be controlled in liquids with micrometer precision.

Life is based on complex macromolecules that can assemble themselves into three-dimensional, functioning structures. The flagella motor of a bacterium is one such example where nature has managed to construct a rotary motor with a rotation axis, bearing and drive mechanism at 45 nm [1]. This motor rotates at a higher speed than that of a car engine and drives rotational flagella, which enable a bacterial cell to move through liquid media at a speed of 20 body lengths (approx. 40 µm) per second. This would correspond to a swimming speed of 120 km / h for a person. The construction of such a complex machine on these length scales with the help of chemical synthesis and nanotechnology is currently not yet conceivable. Although nanotechnology makes it possible to move individual molecules on defined surfaces in a vacuum, the production, movement and precise control of small structures in liquids represent a very special challenge.

Why are there no nano-submarines yet? After all, the sci-fi film, which was awarded an Oscar for special effects The fantastic voyage as early as 1966 presented a vehicle with which a minimally invasive journey into the brain can be undertaken. Other visions of the future speculate on similar vehicles that move in human blood vessels and are intended to assist the doctor of the future with diagnosis and therapy. A multitude of exciting questions arise: Are other physical forces important on the microscale and does this explain the particular technical challenge? What are suitable components for such small machines and how can they be manufactured? Why is it so far hardly possible to structure a wide variety of materials three-dimensionally on these small length scales? Where do the driving force and steering ability come from? How could such systems serve humanity? The laboratory for micro, nano and molecular systems at the MPI for Intelligent Systems in Stuttgart would like to face these tasks with an interdisciplinary team of physicists, chemists, and micro and bio engineers.

In his lecture on life on small scales (life at low Reynolds number) Harvard physicist Ed Purcell investigated this question: How does a microorganism swim? [2]. illustration 1 shows schematically which physical quantity is important. The Reynolds number is shown re for a swimming person, a fish and a bacterial cell. This dimensionless quantity of fluid dynamics describes how the inertial force relates to the drag force (viscosity). You can see how the Reynolds number decreases with the size of the object. The tenacity for a bacterial cell is thus around a hundred million times greater than that for a human. It is so dominant on small scales that a swimming motion that propels us forward would not be able to move a bacterium. For microorganisms, water is like a viscous syrup and requires an irreversible scourge of the sperm, and the bacterium literally "screwed" its way through the water. It is therefore also clear that a nano submarine needs a different propulsion propeller than a conventional ship's propeller for a fantastic journey.

 

If a flagella screw is to be recreated, three-dimensional nanostructuring is required. Although computer chips are already manufactured using 22 nanometer technology, these are always structures in a planar manner. The production of artificial flagella therefore requires special methods.

While still at the Rowland Institute of Harvard University, the author, together with his post-doc at the time, implemented a shadow vapor deposition method that allows a multitude of geometric shapes to be nanostructured in 3D. As in Figure 2 shown schematically, a material is vapor-deposited onto a strongly angled substrate. The growth is localized by casting a geometric shadow. A computer control allows the substrate to align and rotate during the growth process. This is how the glass screws that can be seen in Figure 2 were created. Compared to conventional two-photon or electron beam lithography, this process can be used to produce a large number of materials on large areas (> 50 cm2) to be edited. Silicon, oxides, magnetic materials, metals and alloys can be specifically nano-structured.

 

How do you control a nanopropeller? How is it possible to move such small objects through liquids? An elegant solution would be a chemical reaction in water with biological molecules as fuel. So far, however, this has only been possible in very strong acids through decomposing reactions. Even chemical swimmers need external fields to be controlled. It is therefore advisable to use a magnetic field for the drive and control at the same time. By magnetizing the glass screws in Figure 2, they can be turned and steered with a small magnetic field. In practice, this requires computer-controlled magnetic coils that allow the nanopropellers to move precisely in the smallest of spaces, as shown in Figure 3 will be shown.

 

Where is the journey going? In addition to fascinating fundamental questions that arise for any “life in low Reynolds numbers”, it is of great interest to develop further 3D manufacturing methods that should make it possible to realize complex objects on the smallest scales. Ideally, this should be possible with biologically compatible materials. This would e.g. B. allow colloidal molecules and complex components to be produced that cannot be chemically synthesized. Whether it can be used to build “intelligent” microsystems that autonomously influence reactions, move, explore their surroundings, communicate and be used in a minimally invasive manner is to be researched. The fantastic journey of this research area has only just begun.

The rotary motor of bacterial flagella
Annual Review of Biochemistry 72, 19-54 (2003)
Life at low Reynolds number
American Journal of Physics 45, 3-11 (1977)
Magnetically actuated propulsion at low Reynolds numbers: towards nanoscale control
Nanoscale 3, 557-563 (2010)
Controlled Propulsion of Artificial Magnetic Nanostructured Propellers
Nano Letters 9, 2243-2245 (2009)