Our understanding of the physical principles organizing the genome in the nucleus is limited by the lack of tools to directly exert and measure forces on interphase chromosomes in vivo and probe their material nature. Here, we introduce an approach to actively manipulate a genomic locus using controlled magnetic forces inside the nucleus of a living human cell. We observed viscoelastic displacements over micrometers within minutes in response to near-piconewton forces, which are consistent with a Rouse polymer model. Our results highlight the fluidity of chromatin, with a moderate contribution of the surrounding material, revealing minor roles for cross-links and topological effects and challenging the view that interphase chromatin is a gel-like material. Our technology opens avenues for future research in areas from chromosome mechanics to genome functions.

The mechanical manipulation of magnetic nanoparticles is a powerful approach to probing and actuating biological processes in living systems. Implementing this technique in high-throughput assays can be achieved using biocompatible micromagnet arrays. However, the magnetic properties of these arrays are usually indirectly inferred from simulations or Stokes drag measurements, leaving unresolved questions about the actual profile of the magnetic fields at the micrometer scale and the exact magnetic forces that are applied. Here, we exploit the magnetic field sensitivity of nitrogenvacancy color centers in diamond to map the 3D stray magnetic field produced by a single soft ferromagnetic microstructure. By combining this wide-field optical magnetometry technique with magneto-optic Kerr effect microscopy, we fully analyze the properties of the micromagnets, including their magnetization saturation and their size-dependent magnetic susceptibility. We further show that the high magnetic field gradients produced by the micromagnets, greater than 10 4 T•m −1 under an applied magnetic field of about 100 mT, enables the manipulation of magnetic nanoparticles smaller than 10 nm inside living cells. This work paves the way for quantitative and parallelized experiments in magnetogenetics and magnetomechanics in cell biology.

In many cell types, migration can be oriented towards a chemical stimulus. In mammals, for example, embryonic cells migrate to follow developmental cues, immune cells migrate toward sites of inflammation, and cancer cells migrate away from the primary tumour and toward blood vessels during metastasis. Understanding how cells migrate in 3D environments in response to chemical cues is thus crucial to understanding directed migration in normal and disease states. To date, chemotaxis in mammalian cells has been primarily studied using 2D migration models. However, it is becoming increasingly clear that the mechanisms by which cells migrate in 2D and 3D environments dramatically differ, and cells in their native environments are confronted with a complex chemical milieu. To address these issues, we developed a microfluidic device to monitor the behaviour of cells embedded in a 3D collagen matrix in the presence of complex concentration fields of chemoattractants. This tuneable microsystem enables the generation of (1) homogeneous, stationary gradients set by a purely diffusive mechanism, or (2) spatially evolving, stationary gradients, set by a convection-diffusion mechanism. The device allows for stable gradients over several days and is large enough to study the behaviour of large cell aggregates. We observe that primary mature dendritic cells respond uniformly to homogeneous diffusion gradients, while cell behaviour is highly position-dependent in spatially variable convection-diffusion gradients. In addition, we demonstrate a directed response of cancer cells migrating away from tumour-like aggregates in the presence of soluble chemokine gradients. Together, this microfluidic device is a powerful system to observe the response of different cells and aggregates to tuneable chemical gradients.

Current efforts in nanofluidics aimed at detecting scarce molecules or particles are focused mainly on the development of electrokinetic-based devices. However, these techniques require either integrated or external electrodes, and a potential drop applied across a carrier fluid. One challenge is to develop a new generation of electroless passive devices involving a simple technological process and packaging without embedded electrodes for micro- and nanoparticles enrichment with a view to applications in biology such as the detection of viral agents or cancers biomarkers. This paper presents an innovative technique for particles handling and enrichment based exclusively on a pressure-driven silicon bypass nanofluidic device. The device is fabricated by standard silicon micro-nanofabrication technology. The concentration operation was demonstrated and quantified according to two different actuation modes, which can also be combined to enhance the concentration factor further. The first, "symmetrical" mode involves a symmetric cross-flow effect that concentrates nanoparticles in a very small volume in a very local point of the device. The second mode, "asymmetrical" mode advantageously generates a streaming potential, giving rise to an Electroless Electropreconcentration (EL-EP). The concentration process can be maintained for several hours and concentration factors as high as ~200 have been obtained when both symmetrical and asymmetrical modes are coupled. Proof of concept for concentrating E. coli bacteria by the manual actuation of the EL-EP device is also demonstrated in this paper. Experiments demonstrate more than a 50-fold increase in the concentration of E. coli bacteria in only ~40 s.

The remote actuation of cellular processes such as migration or neuronal outgrowth is a challenge for future therapeutic applications in regenerative medicine. Among the different methods that have been proposed, the use of magnetic nanoparticles appears to be promising, since magnetic fields can act at a distance without interactions with the surrounding biological system. To control biological processes at a subcellular spatial resolution, magnetic nanoparticles can be used either to induce biochemical reactions locally or to apply forces on different elements of the cell. Here, we show that cell migration and neurite outgrowth can be directed by the forces produced by a switchable parallelized array of micro-magnetic pillars, following the passive uptake of nanoparticles. Using live cell imaging, we first demonstrate that adherent cell migration can be biased toward magnetic pillars and that cells can be reversibly trapped onto these pillars. Second, using differentiated neuronal cells we were able to induce events of neurite outgrowth in the direction of the pillars without impending cell viability. Our results show that the range of forces applied needs to be adapted precisely to the cellular process under consideration. We propose that cellular actuation is the result of the force on the plasma membrane caused by magnetically filled endo-compartments, which exert a pulling force on the cell periphery.

Our understanding of the physical principles organizing the genome in the nucleus is limited by the lack of tools to directly exert and measure forces on interphase chromosomes in vivo and probe their material nature. Here, we present a novel approach to actively manipulate a genomic locus using controlled magnetic forces inside the nucleus of a living human cell. We observe viscoelastic displacements over microns within minutes in response to near-picoNewton forces, which are well captured by a Rouse polymer model. Our results highlight the fluidity of chromatin, with a moderate contribution of the surrounding material, revealing the minor role of crosslinks and topological effects and challenging the view that interphase chromatin is a gel-like material. Our new technology opens avenues for future research, from chromosome mechanics to genome functions.

The remote actuation of cellular processes such as migration or neuronal outgrowth is a challenge for future therapeutic applications in regenerative medicine. Among the different methods that have been proposed, the use of magnetic nanoparticles appears to be promising since magnetic fields can act at a distance without interactions with the surrounding biological system. To control biological processes at a subcellular spatial resolution, magnetic nanoparticles can be used either to induce biochemical reactions locally or to apply forces on different elements of the cell. Here, we show that cell migration and neurite outgrowth can be directed by the forces produced by a switchable parallelized array of micro-magnetic pillars, following passive uptake of nanoparticles. Using live cell imaging, we first demonstrate that adherent cell migration can be biased toward magnetic pillars and that cells can be reversibly trapped onto these pillars. Second, using differentiated neuronal cells we were able to induce events of neurite outgrowth in the direction of the pillars without impending cell viability. Our results show that the range of forces applied needs to be adapted precisely to the cellular process under consideration. We propose that cellular actuation is the result of the force on the plasma membrane caused by magnetically filled endo-compartments, which exert a pulling force on the cell periphery.

A challenge in current stem cell therapies for Parkinson's disease (PD) is controlling neuronal outgrowth from the substantia nigra towards the targeted area where connectivity is required in the striatum. Here we present progress towards controlling directional neurite extensions through the application of iron-oxide magnetic nanoparticles (MNPs) labelled neuronal cells combined with a magnetic array generating large spatially variant field gradients (greater than 20 T m −1 ). We investigated the viability of this approach in both two-dimensional and organotypic brain slice models and validated the observed changes in neurite directionality using mathematical models. Results showed that MNP-labelled cells exhibited a shift in directional neurite outgrowth when cultured in a magnetic field gradient, which broadly agreed with mathematical modelling of the magnetic force gradients and predicted MNP force direction. We translated our approach to an ex vivo rat brain slice where we observed directional neurite outgrowth of transplanted MNP-labelled cells from the substantia nigra towards the striatum. The improved directionality highlights the viability of this approach as a remote-control methodology for the control and manipulation of cellular growth for regenerative medicine applications. This study presents a new tool to overcome challenges faced in the development of new therapies for PD.