Magnetic steering at the nanoscale

The ability to manipulate individual objects in microscopical scenarios is a long sought part in the tool chest of engineers. Manipulation presupposes steering, which is obvious at the human scale, but cannot be taken as a given at scales that differ in several orders of magnitude from ours. As a rule of thumb, the smaller the scale of an object, the more challenging it is to steer it. In response to the challenge of steering microscopic objects immersed in fluids, researchers have developed different concepts. These include microfluidic devices [1], microrobots governed by magnetic forces [2], and devices in which polarized objects move directed by electrical fields [3], to name a few. Of them, the ones based on magnetic fields have the appeal that they do not interfere with the internal activity of biological tissues, a property that makes them attractive for prospective biomedical applications. Until now, the efforts to attain magnetic steering have been successful at scales down to tens of microns [4]. The goal of this thesis was to investigate the challenges of (or limitations to) magnetic steering at the nanoscale. We did so by considering the magnetic forces acting on a magnetic nanoparticle that can move in the proximity of a flat surface. We started the study with theoretical considerations, essentially comparing the magnitude of magnetic steering forces with the magnitude of those forces that can undermine the guidance efforts (e.g. friction forces). These calculations encouraged an experimental realization, which we designed around a well-controlled model-system: microtubules propelled by surface-bound kinesins [5, 6]. In this system, we replaced the substrate by a magnetically patterned film, and the microtubules were decorated with magnetic nanoparticles, which mediate the steering. We describe the design and construction of the experiment, with particular attention to the fabrication and characterization of the magnetic films, their adaptation for use in protein assays, and the methodology to characterize the motion of the microtubules through their trajectories and speed. The discrepancy between numerous experiments and our expectations raised more fundamental questions, such as the role played by Brownian motion in nanoscale systems. We present theoretical arguments to characterize the dynamics of magnetic steering acting on a particle which is also subject to Brownian motion. Because the ability of Brownian motion to iii DISS ETH. No. 20284 P. Stickar Magnetic steering at the nanoscale diffuse a system over the saddle point of a potential barrier depends primarily on the depth of the potential well, we studied the changes in the energy landscape that can be obtained with different selections of the magnetic components (geometry and material). The theoretical arguments allow us not only to understand the experimental results, but also to outline the limitations to magnetic steering at the nanoscale. Furthermore, the analytical tools developed in this study enable us to outline design guidelines for future experiments involving magnetic steering at sub-micron scale. The design guidelines are presented as a recipe that can guide the selection of magnetic materials and geometric dimensions for both the magnetic film and the magnetic particles. [1] H. A. Stone, A. D. Stroock, and A. Ajdari. Engineering flows in small devices: microfluidics toward a lab-on-a-chip. Annual Reviews of Fluid Mechanics, 36:381–411, 2004. [2] Kemal Berk Yesin, Karl Vollmers, and Bradley J. Nelson. Analysis and design of wireless magnetically guided microrobots in body fluids. In Proceedings of the IEEE International Conference on Robotics and Automation, pages 1333–1338, New Oleans, USA, 2004. [3] Martin G. L. van den Heuvel, Martijn P. de Graaff, and Cees Dekker. Molecular sorting by electrical steering of microtubules in kinesin-coated channels. Science, 312:910–914, 2006. [4] David W. Inglis, R. Riehn, R. H. Austin, and J. C. Sturm. Continuous microfluidic immunomagnetic cell separation. Applied Physics Letters, 85:5093–5095, 2004. [5] Jonathon Howard. Mechanics of motor proteins and the cytoskeleton. Sinauer Associates, Inc., 2001. [6] Anita Goel and Viola Vogel. Harnessing biological motors to engineer systems for nanoscale transport and assembly. Nature Nanotechnology, 3:465–475, 2008. DISS ETH. No. 20284 iv Magnetic steering at the nanoscale P. Stickar