Research
We investigate
the swimming response of microorganisms to intense magnetic fields and magnetic forces.
We have developed a
method based on magneto-Archimedes principle that employs
magnetic forces to simulate variable gravity environments for the study of gravi-sensitivity of cells. 
The May 2006 issue of Scientific American has a short paragraph about our
research (PDF).
My experiments are performed on single cell protozoan Paramecium caudatum. A rather large (200 micrometer long) and commonly used ciliate, which possesses gravi-sensing abilities. We carried out most of our experiments at the National High Magnetic Field Laboratory.
Introduction
All our experiments are performed in solenoid magnet systems that produce non-homogenous magnetic fields. The field is at its maximum at the center of the magnet whereas the magnetic force, since it is proportional to the field-field gradient, has two maxima off the center as shown with the blue curve. When a diamagnetic object is placed in non-homogenous magnetic field, the induced magnetic moments are such that the object is repelled from the maximum field region (away from the center). Therefor the objects placed above the center of the magnet feel a decreased gravitation pull and vice versa. The total force profile is shown in red, the arrows show the relative magnitude of the force. Since biological matters are very weakly diamagnetic, we need strong forces of order of few thousand T2m-1 to manipulate them.
On the lefthand side of the above figure, (a), a schematic of a levitated object (movie) is illustrated. The righthand side, (b), shows the magnetic force (blue) and the total force (red) after taking into consideration the effect of gravity. The arrows show the direction and the magnitude of the total force.
Experiments
- Guiding paramecia with magnetic fields
- Simulating gravity for the study of gravi-responses of cells
- Measuring the diamagnetic susceptibility of Paramecium
The picture on the left shows the trajectories of swimming Paramecium caudatum in the absence of magnetic field. The trajectories have their helical shape (shows as wiggles in 2D) and there is no preferred swimming direction. As soon as we put them in strong magnetic fields, their trajectories align parallel to the field direction with no polarity (movie).
This phenomenon is also observed in immobilized Paramecium (movie). We have shown that this magneto-orientation effect in Paramecium is a passive response to the magnetic torque applied to it.
Swimming Paramecium fights sedimentation, a phenomenon known as negative gravikinesis. In simple words, a Paramecium swims harder against the force than with the force. This response is magnified when the amplitude of the gravity is increased by centrifugation, for example, and it vanishes in microgravity. The exact mechanism by which a swimming Paramecium detects "up" from "down" is not yet fully understood.
In our experiments, by exploiting the interaction of magnetic fields with the intrinsically magnetic components of the cells we are able to exert forces, simulated gravities, on swimming paramecia. This advance has the potential to be used as a new method for investigating the force sensitivity of cells. More information about our findings on this subject can be found elsewhere PDF (846K).
The swimming trajectories of Paramecium in simulated gravities. In increased simulated gravity, the downward paramecia swim faster whereas the response is inverted in inverted simulated gravity. The picture on the left corresponds to simulated gravity 4.5 times Earth's gravity and the one on the right is inverted -2.5 times. The lengths of the tracks are the distances traveled in two seconds.
By changing the effective buoyancy of an immobilized Paramecium the cell moves up or down depending on the
direction of the force (movie). Since the hydrodynamics are in low Reynolds number regime,
the moving velocity is proportional to the total force. From the balance of
forces we were able to
measure the diamagnetic susceptibility of the whole cell.