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Properties of Permalloy nanodiscs in magnetic vortex state and magneto-mechanical treatment of cancer cells.

Magnetic dots in vortex state have been intensively studied due to their attractive properties for emerging multidisciplinary applications such as magnetic information storage, spintronics and biomedicine. In the vortex ground state, the magnetic moments are curled in the dot plane and only at the center the core of the vortex points perpendicularly to the plane. This configuration gives rise to a characteristic magnetic behavior as a function of the in-plane applied magnetic field, displaying hysteresis loops with no remanence or coercive field and open lobes at high field [1]. Apart from the intrinsic interest that this peculiar magnetic structure offers from a fundamental point of view, some appealing applications for this type of particles have been proposed, such as the magneto-mechanical actuation for cancer cell destruction [2]. This therapy employs disc-shaped particles with vortex magnetic configuration that, under an low amplitude (about 10 mT) and low frequency (tens of Hertz) AC fields, are made to oscillate, hitting and damaging the integrity of cancer cells to which they were attached. Since this actuation does not imply heat generation, in principle, the magneto-mechanical actuation avoids the risk of damaging the surrounding healthy tissue as it can occur in magnetic hyperthermia. Additionally, it is thought that the magneto-mechanical actuation leads to the apoptosis of the cells instead of the necrotic pathway caused by heating, avoiding cell-leakage in the surrounding extracellular environment and inflammatory reactions caused by necrosis [3]. In this work, we present the results obtained in Permalloy circular dots fabricated by hole mask colloidal lithography (HCL) with diameters ranging from 60 to 140 nm, and different thicknesses from 20 to 60 nm. HCL is a bottom-up fabrication technique that basically uses a monolayer of self-assembled polystyrene nanospheres to create a template of holes in a polymer film deposited over a substrate. The holes are filled with sputtered Permalloy and the polymeric template removed to produce a dense pattern of dots on the substrate [4]. For their use in the in vitro experiments, the nanostructures are prepared on top of a sacrificial layer that is later removed to release the discs. In this case, the discs are prepared with a thin (4 nm) gold layer on both sides. The nanodiscs on the substrate are magnetically characterized by SQUID and MOKE magnetometries, and Magnetic Force Microscopy (MFM). Additional micromagnetic simulations and analytic calculations have been performed to clarify the magnetization configuration in the dots with different diameter to thickness ratios [5]. In the dots with a diameter of 140 nm, the MFM images reveal that the vortex core occupies about half the size of the dots. This vortex core diameter is approximately equal to the dot diameter for the smaller dots (60 nm). Micromagnetic calculations confirms that, in this case, the size of the core can be even greater that the dot size. Nevertheless, this does not prevent the existence of a clear magnetic vortex behavior as demonstrated by the measured hysteresis loops. The suitability of nanodiscs for cancer cell treatment using the magneto-mechanical actuation is evaluated using lung carcinoma cells for the in vitro experiments. For comparing with previous studies, we simultaneously perform the experiments using Permalloy discs with a diameter of 2 μm fabricated by photolithography. The internalization process in cancer cells, along with the cytotoxic effect, and the influence of a low magnetic field on the viability of the cells are studied. We observe that the discs do not disrupt the viability of the cells, but they seem to inhibit their proliferation. The application of an alternating magnetic field to produce the magneto-mechanical actuation seems to have a scarce impact on the cell viability when using large discs (2 um in diameter) but, with the use of nanodiscs, the destroyed cell rate is increased to 30 % among the cells that have internalized discs. Figure 2 show an example of cell death by the treatment. We must stress that these results are obtained in a small number of experiments, and that we have used manual counting of cells under the microscopy (no cytometers used). In any case, although the percentages may not be statistically sound, the results prove the ability of the nanodiscs to produce irreparable changes in cancer cell integrity. Acknowledgements This work was supported by the Spanish Government under Project MAT2014-55049-C2-R and by the Basque Government under the Micro4Fab Project (KK-2016/00030). K. G. acknowledges support by IKERBASQUE (Basque Foundation for Science) and by Spanish MINECO grant FIS2016-78591-C3-3-R.