Hélène Joisten

Comparison of dispersion and actuation properties of vortex and synthetic antiferromagnetic particles for biotechnological applications

Magnetic nanoparticles are receiving an increasing interest for various biotechnological applications due to the capability that they offer to exert actuation on biological species via external magnetic fields. In this study, two types of magnetic particles recently proposed for cancer cells treatment were compared. Both are prepared by top-down approaches and imitate the properties of superparamagnetic particles. One type is made of a single magnetic layer and has a magnetic vortex configuration. The second type has a multilayered structure called synthetic antiferromagnet. Once released in solution, the agglomeration/dispersion of these particles due to their magnetostatic interactions was compared as well as the mechanical torque that they can generate when submitted to an external magnetic field.

Self-polarization phenomenon and control of dispersion of synthetic antiferromagnetic nanoparticles for biological applications

Using a top-down approach, synthetic antiferromagnetic micro/nanoparticles usable for biological applications were prepared. These particles exhibit “superparamagneticlike” properties. Their magnetic susceptibility can be accurately controlled by the thickness of the constituting layers. When dispersed in solution, striking differences in their interactions are observed depending on their susceptibility. Above a susceptibility threshold, a phenomenon of self-polarization is observed in zero applied field, resulting in a gradual agglomeration of the particles. In contrast, below the susceptibility threshold, the particles get redispersed in zero field. This is interpreted by a self-consistent model taking into account dipolar interactions between particles and their magnetic susceptibility.

Tumbling motion yielding fast displacements of synthetic antiferromagnetic nanoparticles for biological applications

Synthetic antiferromagnetic micro/nanoparticles usable for biological applications were recently developed using a top-down approach, made of alternating NiFe layers and non magnetic Ru spacers. We describe here different types of motions of magnetic particles chains, controlled either by field gradients or alternating magnetic fields and combination of both. Of particular interest is a displacement named “tumbling motion” consisting in a combination of rotation and translation, with friction on the bottom surface of the container, as a bicycle wheel on a horizontal surface. This motion yields a translation speed 10–30 times faster than by using conventional gradient of magnetic field.

Fabrication of nanotweezers and their remote actuation by magnetic fields

A new kind of nanodevice that acts like tweezers through remote actuation by an external magnetic field is designed. Such device is meant to mechanically grab micrometric objects. The nanotweezers are built by using a top-down approach and are made of two parallelepipedic microelements, at least one of them being magnetic, bound by a flexible nanohinge. The presence of an external magnetic field induces a torque on the magnetic elements that competes with the elastic torque provided by the nanohinge. A model is established in order to evaluate the values of the balanced torques as a function of the tweezers opening angles. The results of the calculations are confronted to the expected values and validate the overall working principle of the magnetic nanotweezers.

Magneto-optical micromechanical systems for magnetic field mapping.

A new method for magnetic field mapping based on the optical response of organized dense arrays of flexible magnetic cantilevers is explored. When subjected to the stray field of a magnetized material, the mobile parts of the cantilevers deviate from their initial positions, which locally changes the light reflectivity on the magneto-optical surface, thus allowing to visualize the field lines. While the final goal is to be able to map and quantify non-uniform fields, calibrating and testing the device can be done with uniform fields. Under a uniform field, the device can be assimilated to a magnetic-field-sensitive diffraction grating, and therefore, can be analyzed by coherent light diffraction. A theoretical model for the diffraction patterns, which accounts for both magnetic and mechanical interactions within each cantilever, is proposed and confronted to the experimental data.

Magnetic superparamagnetic-like microparticles for cancer cells destruction

To improve cancer treatment with reduced side effects, several techniques are currently under investigation, such as targeted drug delivery or hyperthermia . This work is related to the study of a newly discovered technique: the triggering of cancer cell apoptosis by mechanical vibrations of magnetic microparticles attached to the cancer cells membrane . This method, which targets only diseased cells, was firstly demonstrated by Kim et al in 2010. The method consists in specifically attaching magnetic anisotropic particles to cancer cells membrane and then applying a weak alternative magnetic field . The induced particles vibrations generate a stress on the cells membrane. A chemical chain reaction is then triggered, which causes the reactivation of the apoptosis of cancer cells . The triggering of the apoptosis of human renal cancer cells was also demonstrated by SPINTEC. The magnetic particles used for this purpose have to respect a few specifications: they must have a sufficiently large volume, they must be anisotropic so that they can be actuated by a magnetic field rather than a gradient of magnetic field, and they have to be superparamagnetic-like in order to avoid any aggregation in solution . Two types of magnetic particles fulfilling these specifications were developed in this work: permalloy particles presenting a vortex structure and magnetite particles presenting a polycrystalline random anisotropy configuration .

Synthesis of mono disperse magnetic nanoparticles prepared on block copolymer templates for medical imaging techniques

The purpose of this study is to develop biocompatible magnetic nanoparticles serving as contrast agents in magnetic imaging with narrow size distribution. The nanoparticles are prepared by combined top-down and bottom-up approach which relies on the use of self-assembled PS-PMMA diblock copolymer templates. Materials being studied include nickel, permalloy, and magnetite nanoparticles. SEM and SQUID magnetometry are used for structural and magnetic characterization, respectively. Hysteresis loops show that the nanoparticles are superparamagnetic which is an important property for biomedical applications.