Basics of Bacterial motion (The trivial):
One of the most fascinating areas of research is biomimetics, where researchers attempt to replicate nature's creative accomplishments in the lab. For pure inspiration, look no further than to bacteria. Bacteria seem to go everywhere and do pretty much everything. The natural tendency of those of us with a inclination towards geekiness is to think of micro-robots. There are a number of natural roles that micro-robots could play, such as cleaning plaques from artery walls or defouling pipelines.
One of the problems with micro-robots has been making them small enough. One millimeter might seem small to us, but not to a bacteria. One of the primary constraints on shrinking these devices is the method of locomotion, which, because it requires moving parts, tends to be on the bulky side. This is about to change as some recent results published in Applied Physics Letters show researchers building their very own artificial bacterial flagella. In the publication, controlled motion and the simple task of separating cell-sized polystyrene beads were both demonstrated.
The bacteria flagellum is an interesting little piece of nature. Apart from providing endless amusement and artificial controversy, they also self-assemble—put the right proteins and some fuel in a test tube of water and the result is a bunch of little flagella propelling themselves about at random. Physicists are interested in them for two reasons: they are one of the few mechanisms for transporting micro-robots that might be replicable using inorganic materials, and, given the size of the micro-robot and the viscosity of relevant mediums (close to water in the human body), flagella are very close to an optimal propulsion system.
Researchers from ETH Zurich have recently published the results from their successful attempts to replicate flagella at the same size scale as natural flagella. To make a flagellum, they deposited layers of aluminium gallium arsenide, indium gallium arsenide, gallium arsenide, and chromium on top of a gallium arsenide crystal. They then use an ion beam to mill away most of the layers, leaving a long, thin rectangle of material.
At one end of this, they deposited a small square of chromium-nickel-gold to provide a magnetic head. The rectangle of material is separated from the gallium arsenide substrate by dissolving the aluminium gallium arsenide. The remaining strip of material is stressed because of the layering, so it curls up into a helix. This process is so well-controlled that the researchers can choose any helix they desire simply by changing the materials and thickness of the different layers.
This tiny scroll was then dropped into a swimming pool sitting under a microscope, where the researchers could observe and control it. The magnetic head of the scroll responds to any magnetic field—changing the field causes the orientation of the scroll to change and induces swimming motions. As a result, the flagella swims in controlled curves, allowing researchers to direct it to any location with any orientation. This was demonstrated by swimming the flagella to a polystyrene sphere, after which they were directed to rotate and push the sphere. The researchers also showed that two spheres could be separated by drilling a flagella between them. Videos of the swimmers in action can be found here.
This is, of course, a very cool piece of research, but it probably won't be used for any sort of remote activity. Although external fields are used to power and direct the swimmer, we still require sensors to see where it is. This probably eliminates any chance of putting these deep inside the human body to scrape plaque from artery walls or doing the equivalent inside mechanical systems. So there is certainly more research required in that direction. On the other hand, the rotary motion can certainly be used as a power source for micro-robots, and that doesn't require being able to sense where the micro-robot is.
A number of robotic swimming methods have been proposed at relatively small scales. Because many of these methods rely on reciprocating motions, they do not scale downwards. Yet over three billion years ago bacteria evolved a swimming strategy at micrometer dimensions that nature has had difficulty improving upon. Just over thirty-five years ago their swimming technique using rotating flagella was first described by H.C. Berg and R.A. Anderson (Nature vol. 245, pp. 380-382, 1973). Inspired by the flagellar motion of bacteria such as Escherichia Coli (see video 4), we have recently developed artificial bacterial flagella (ABF). Our ABF represent the first demonstration of wireless swimming microrobots similar in size and geometry to natural bacterial flagella, and are many orders of magnitude smaller than existing artificial helical swimmers.
Helical swimming robot consists of two parts: a helical tail and a soft-magnetic metal head. The tails are 27 to 42 nm thick, less than 2um wide, and coil into diameters smaller than three microns. The fabrication of ABF is based on a self-scrolling technique. The helical tail is patterned in 2D as an InGaAs/GaAs bilayer nanoribbon or an InGaAs/GaAs/Cr trilayer nanoribbon. The metal head is fabricated from a Cr/Ni/Au thin film using a lift-off process. The 2D films detach from the GaAs wafer and self-organize to form tethered helical robots. To untether the helical swimming microrobots from the substrate, micromanipulation is performed to cut, pick, and release them in water. After that, the helical swimming microrobots are propelled and steered precisely in water by a low-strength (1-2 mT), rotating magnetic field. Details of the experimental process can be found in (APL, 94, 064107, 2009).
By adjusting the rotating speed and direction of the magnetic field, the velocity and direction of motion of the helical swimmer can be tuned in a controlled fashion. The figure below shows an example where a 74µm long helical swimmer is driven to reach a target. The average velocity is approximately 5µm/s at 470 rpm. By inverting the rotating magnetic field, the swimmer turns in the opposite direction, and the linear motion is reversed.Ref : here.
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