Speaker
Description
Magnetotactic bacteria are aquatic microorganisms able to swim along the Earth’s magnetic field lines thanks to an internal chain of magnetic nanoparticles that acts as a compass needle. These nanoparticles, called magnetosomes, are composed of a high purity crystalline magnetic core surrounded by a lipid bilayer membrane. For example, as a typical magnetotactic bacteria, the Magnetospirillum gryphiswaldense species can have up to 25 cuboctahedral magnetite (Fe$_{3}$O$_{4}$) particles, each measuring approximately 45 nm. This opens up the possibility of controlling their motion through external magnetic fields [1]. Being non-pathogenic makes them ideal for bio-applications such as detection and treatment of cancer through MRI, magnetic hyperthermia or targeted drug delivery [2]. To implement and improve these applications, their swimming mechanisms and response to external magnetic fields must be studied. While motility experiments often rely on the optical observation of the bacteria and on video recording for later analysis, we propose a different approach based on magnetic sensing, by detecting the stray field created by the magnetosome chain. The use of a device based on magnetic detection would enable a high level of integration as well as the potential development of a feedback loop for controlling bacteria trajectories.
In this work we describe the development of a microfluidic chip for the combined optical and magnetic detection of magnetotactic bacteria. The device consists of three series of magnetic microsensors integrated at the bottom of a fully transparent microfluidic channel. The fabrication process incorporates traditional microfabrication techniques such as photoresist lithography and sputtering deposition, as well as soft lithography using PDMS and SU-8. The microsensors are Permalloy-based anisotropic magnetoresistance (AMR) sensors. The size of the sensors, in the range of tens of microns, and their layout are determined by the compromise between the tiny size of the magnetotactic bacteria and the maximization of their response to the stray field of the magnetosome chain [3]. The microfluidic channel is patterned over the sensors ensuring proper alignment using SU-8 photoresist, and a final layer of flat PDMS is bonded on top to seal the device.
References
[1] S. Rismani Yazdi, R. Nosrati, C. A. Stevens, D. Vogel, P. L. Davies, and C. Escobedo, “Magnetotaxis Enables Magnetotactic Bacteria to Navigate in Flow,” Small, vol. 14, no. 5. Wiley, Dec. 04, 2017. doi: 10.1002/smll.201702982.
[2] M. L. Fdez-Gubieda, J. Alonso, A. García-Prieto, A. García-Arribas, L. Fernández Barquín, and A. Muela, “Magnetotactic bacteria for cancer therapy,” Journal of Applied Physics, vol. 128, no. 7. AIP Publishing, Aug. 17, 2020. doi: 10.1063/5.0018036.
[3] D. de Cos, N. Lete, M. L. Fdez-Gubieda, and A. García-Arribas, “Study of the influence of sensor permeability in the detection of a single magnetotactic bacterium,” Journal of Magnetism and Magnetic Materials, vol. 500. Elsevier BV, p. 166346, Apr. 2020. doi: 10.1016/j.jmmm.2019.166346.
