Life on Magnets: Stem Cell Networking on Micro-Magnet Arrays

et al. (2013) Life on Magnets: Stem Cell Networking on Micro-Magnet Arrays. PLoS ONE 8(8): e70416. doi:10.1371/journal.pone.0070416 Life on Magnets: Stem Cell Networking on Micro-Magnet Arrays Vitalii Zablotskii 0 Alexandr Dejneka 0 Sa rka Kubinova 0 Damien Le-Roy 0 Fre de ric Dumas-Bouchiat 0 Dominique Givord 0 Nora M. Dempsey 0 Eva Sykova 0 Pranela Rameshwar, Rutgers - New Jersey Medical School, United States of America 0 1 Institute of Physics AS CR, v.v.i. , Prague , Czech Republic , 2 Institute of Experimental Medicine AS CR, v.v.i. , Prague , Czech Republic , 3 Institut Ne el, CNRS/UJF , Grenoble , France Interactions between a micro-magnet array and living cells may guide the establishment of cell networks due to the cellular response to a magnetic field. To manipulate mesenchymal stem cells free of magnetic nanoparticles by a high magnetic field gradient, we used high quality micro-patterned NdFeB films around which the stray field's value and direction drastically change across the cell body. Such micro-magnet arrays coated with parylene produce high magnetic field gradients that affect the cells in two main ways: i) causing cell migration and adherence to a covered magnetic surface and ii) elongating the cells in the directions parallel to the edges of the micro-magnet. To explain these effects, three putative mechanisms that incorporate both physical and biological factors influencing the cells are suggested. It is shown that the static high magnetic field gradient generated by the micro-magnet arrays are capable of assisting cell migration to those areas with the strongest magnetic field gradient, thereby allowing the build up of tunable interconnected stem cell networks, which is an elegant route for tissue engineering and regenerative medicine. - Funding: GACR: P304/11/0653, P304/12/1370; Academy of Sciences of the Czech Republic: grant M100101219 and the French Agence Nationale de la Recherche (ANR, grant ANR-11-BSV5-014 02). Substrate Patterning was carried out at the Plateforme Technologique Amont (PTA-Grenoble). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Our planet produces a small magnetic field, about 50 mT, which varies on a length scale much larger that the size of humans, animals and cells. Nevertheless, even a small and quite homogenous magnetic field is crucial for many aspects of the lives of both humans and microorganisms, e.g. left-right inversion in the human brain [1]; magnetoreception observed in magnetotactic bacteria and believed to occur in certain animals, such as birds. But what happens when a living cell interacts with a strong magnet of similar size to itself? The stray field produced by such a micromagnet will dramatically change in value and direction across the cell body and the question is: how will the cell respond and adapt itself to a high magnetic field gradient? In spite of tremendous recent progress in cell biology and the ever growing use of magnetic materials in bio-medical applications, little is known of the long-term influence of a magnetic field at the cellular level. In studies of the effects of a magnetic field on living cells, mesenchymal stem cells are the subject of particular interest because of their ability to differentiate into adipocytes, chondrocytes and osteoblasts as well as other cell types [2], thus allowing tissue regeneration and providing therapeutic effects on diseases for which there is no other effective therapy. For tissue growth, the spatial organization of a stem cell colony and its geometrical and mechanical constrictions play an important role [35]. Thus, manipulating the fate of stem cells, their spatial organization and the creation of an interconnected cell network with externally applied magnetic fields is of great potential interest for tissue engineering applications. Here, we describe experiments with micro-magnets and living cells that reveal the dramatic impact of a high magnetic field gradient on the spatial organization and growth of stem cells. The observed magnetic control of the stem cells is discussed from the points of view of both physics and biology. Let us start with a brief description of the relevant effects of a magnetic field on biological objects. The influence of a magnetic field on materials is a familiar process not expected to show surprises an externally applied magnetic field can either pull or push an object depending on the sign of the objects magnetic susceptibility (paramagnetic, ferromagnetic, ferrimagnetic and superparamagnetic objects being attracted, diamagnetic objects being repelled). In this sense, living objects organisms, cells and biomolecules are not different; nevertheless, due to their inherent complexity it is difficult to distinguish between the different types of magnetism inside a living cell. The forces and effects induced by magnetic fields may offer unique control of cell motion, proliferation and machinery as well as a new opportunity for promising applications ranging from micro/nano-scale control, such as cell sorting, drug and gene delivery [6], to controlling the behavior of animals [7] and even humans [1]. Depending on cell type, exposure to a low or moderate static magnetic field may either increase or decrease Ca2+ influx; for a review, see [8]. The possibility of monitoring and remotely controlling cellular endocytosis and/or exocytosis rates of superparamagnetic iron oxide (SPIO) nanoparticles using a magnetic field was recently demonstrated [9,10]. A study of the direct influence of a magnetic field on a cell and the possibilities of magnetically controlling cellular motion, trapping and patterning, without the use of SPIO nanoparticles inserted in, or attached to the cells, is especially important because this approach avoids problems related to nanoparticle toxicity and removal. Such a direct influence of magnetic fields on living cells may exhibit itself in high magnetic field gradients, when the external magnetic field varies at the same scale as the cell size, i.e. in the close environment of micron-sized magnetic flux sources [11,12]. Arrays of micro-magnets, which produce magnetic field gradients up to 106 T/m [11], have indeed been used to diamagnetically trap arrays of Jurkat cells, in the presence of a paramagnetic contrast agent [13]. Such micromagnets can also be used to attract and trap cells functionalized with SPIO nanoparticles [9,14,15]. In this work we studied the behavior of SPIO nanoparticle-free mesenchymal stem cells in standard medium without any added paramagnetic contrast agent, in the presence of high magnetic field gradients generated by patterned micro-magnets. Materials and Methods Micro-magnet Arrays Si substrates were patterned, using lithography and deep Reactive Ion Etching (RIE), to produce arrays of Si pillars of lat (...truncated)