PLANAR MANIPULATION OF MAGNETO-TACTIC BACTERIA USING
UNIDIRECTIONAL MAGNETIC FIELDS
T.A.G. Hageman
1,2*, M.P. Pichel
1,2, M.O. Altmeyer
2, A. Manz
1, L. Abelmann
1,2 1KIST Europe, GERMANY
2
University of Twente, THE NETHERLANDS
ABSTRACT
We show for the first time that an alternating unidirectional magnetic field generated by a magnetic erase head allows planar manipulation of magneto-tactic bacteria (MTB), and is not restricted to parallel directions only. We used squared-shaped magnetic fields of approximately 4 mT while sweeping from 0.25 to 10 Hz, and found that at frequencies of over 3 Hz the mean orthogonal velocity becomes constant. The erase head offers a significant reduction in size and complexity over conventional manipulators.
KEYWORDS: Magnetotactic bacteria, erase head, magnetism, control INTRODUCTION
Modeling and control of MTB has been an interest by several groups [1,2]. Applications include transport of microscopic objects in microfluidics [3] and delivery of pay-loads such as drugs or micro-objects [4]. Electromagnets are the primary choice for generating these fields. The high fields required result in overheating of the magnets or bulky cooling systems [2]. Previously [5] we demonstrated our control of MTB using a motorized miniature permanent magnet.
Smaller and less complex magnetic manipulators will contribute to more portable solutions, and will open up possibilities in generating localized magnetic fields for local manipulation and control. We introduce the use of a commercial magnetic tape erase head. Although only generating a unidirectional field, an orthogonal velocity of MTB can be reached by exploiting the bacteria's radius of curvature under superimposed alternating fields (Figure 1). Compared to the motorized permanent magnet the erase head offers easier control over the field magnitude, higher switching frequencies and allows further miniaturization (for instance using commercial available parallel heads). The resulting field, however, is not uniform, harder to calibrate and requires close proximity to the sample.
Figure 1. An alternating magnetic field generated by a magnetic recording head allows orthogonal
manipulation of magnetotactic bacteria.
Figure 2. Sealed, thin coverglasses form a shallow channel for minimum distance to the recording head gap.
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978-0-9798064-8-3/µTAS 2015/$20©15CBMS-0001 19th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 25-29, 2015, Gyeongju, KOREA
THEORY
The magnetic torque τ exerted on magnetotactic bacteria is given by:
τ=m B sin(θ )
(1)With m the magnetic dipole moment, B the magnetic field strength and θ the angle between the field and diple moment. As a result of the self-propellant nature of MTB, as well as their rotational friction, under direct reversal of the external magnetic field the bacteria will make a U-shaped trajectory [5]. The torque is maximized when the bacteria is orthogonal to the magnetic field (θ = 90o). At increasing
reversal switching frequencies the bacteria will tend to stay in the region of maximum torque, and therefore will move orthogonal to the magnetic field.
EXPERIMENTAL
A sample was sealed between 200 µm thick coverglasses and placed on the surface of a magnetic erase head for minimum distance and thus maximum field strength (Figure 2). A square wave current profile created a magnetic field in the gap with a field strength of approximately 4 mT. During periodically sweeping the frequency from 0.25 to 10 Hz over a period of 10 s, the response of MTB was recorded by a camera at 100 fps. A custom-written Matlab script was used to track the bacteria over time.
RESULTS AND DISCUSSION
Figure 3 shows a typical trajectory of a MTB over time. A horizontal (orthogonal) displacement component can be observed with respect to the vertically directed magnetic field. The observed displacement is not completely orthogonal but would be tunable with the duty cycle of the magnetic field. A closer analysis shows that at increasing frequencies, the orthogonal displacement over time approximates a linear curve. Figure 4 shows that at frequencies of over 3 Hz, the orthogonal velocity fits well to a linear curve. The transition frequency is expected to change with the magnetic respons of the MTB, due to either a change in field strength or the bacteria's magnetic moment, which are both proportional to the magnetic torque.
Figure 3. Response of a magnetotactic bacterium to a magnetic field periodically sweeping from 0.25 to 10Hz, showing a significant orthogonal displacement.
Figure 4. The orthogonal displacement of a magnetotactic bacterium as a function of time, at increasing frequency of the magnetic field. At frequencies greater than 3Hz, the average displacement over time is linear, marking frequency-independent velocity.
CONCLUSION
We introduced the use of a magnetic erase head for manipulating MTB at a field strength of around 4mT. By applying periodically switching unidirectional fields between 0.25 and 10 Hz we showed it is possible to generate an orthogonal displacement by exploiting the radius of curvature of the bacteria. Fre-quencies of over 3 Hz result in saturation of the orthogonal velocity. The development towards smaller and less complex magnetic manipulators will contribute to more portable solutions, and will open up pos-sibilities in generating localized magnetic fields for local manipulation and control.
REFERENCES
[1] K. Erglis et al., “Dynamics of magnetotactic bacteria in a rotating magnetic field,” Biophysical
Journal, 93, 1402-1412, 2007.
[2] I. Khalil et al., “Experimental testbed for characterization and control of biological microrobots,”
International Symposium on Experimental Robotics, 88, 617-631, 2012.
[3] S. Martel, “Using a swarm of self-propelled natural microrobots in the form of flagellated bacte ria to perform micro-assembly tasks”, International Conference on Robotics and Automation, 500-505, 2010.
[4] S. Martel, “Microrobots in the vascular network: present status and next challenges,” Journal of
Micro-Bio Robot, 8, 41-52, 2013.
[5] M.P. Pichel et al., “Magnetic manipulation of bacteria in microfluidics,” International Conference
on Miniaturized Systems for Chemistry and Life Sciences, 18, 721-723, 2014.
CONTACT
* Tijmen Hageman; +49-681-9382-250; t.hageman@kist-europe.de
