Magnetic tweezers optimized to exert high forces over extended distances from the magnet in multicellular systems

Abstract

Magnetic tweezers are mainly divided into two classes depending on the ability of applying torque or forces to the magnetic probe. We focused on the second category and designed a device composed by a single electromagnet equipped with a core having a special asymmetric profile to exert forces as large as 230 pN-2.8 μm Dynabeads at distances in excess of 100 μm from the magnetic tip. Compared to existing solutions our magnetic tweezers overcome important limitations, opening new experimental paths for the study of a wide range of materials in a variety of biophysical research settings. We discuss the benefits and drawbacks of different magnet core characteristics, which led us to design the current core profile. To demonstrate the usefulness of our magnetic tweezers, we determined the microrheological properties inside embryos of Drosophila melanogaster during the syncytial stage. Measurements in different locations along the dorsal-ventral axis of the embryos showed little variation, with a slight increase in cytoplasm viscosity at the periphery of the embryos. The mean cytoplasm viscosity we obtain by active force exertion inside the embryos is comparable to that determined passively using high-speed video microrheology.

Force measurements of Myosin II waves at the yolk surface during Drosophila dorsal closure

“…Further, we performed experiments to characterize the physical parameters of the yolk cell, in particular the elasticity of the yolk cortex as well as the viscosity of the cytoplasm inside the yolk cell. We found a mean viscosity value of h ¼ 18.4 51.9 Pa s. This is about 25 times larger than that of 0.75 50.13 Pa s we measured in the same cell before cellularization (17), which was close to the viscosity deterimined by Wessel et al at a similar stage. In contrast, Wessel et al have also measured the viscosity of the yolk cell cytoplasm slightly earlier than the dorsal closure stage, in particular before the formation of the cortical layer, and found a value of h ¼ 5 52.8 Pa s (34).…”

Environmentally controlled magnetic nano-tweezer for living cells and extracellular matrices

“…The environmental chamber ensures stable conditions during the measurements, such as controlled CO 2 supply and temperature. Other studies that employ magnetic tweezers for live cell measurements rely on a heated microscope stage for temperature control 29 , 39 , 47 , 48 which may induce temperature gradients and thus affect the mechanical properties of the cells. In fact, we analyzed time- and force-dependent viscoelastic properties of adherent cells under more controlled conditions at large forces, ranging from 1 to 5 nN.…”

Three-dimensional tissue stiffness mapping in the mouse embryo supports durotaxis during early limb bud morphogenesis

“…Magnetic devices have been employed primarily to measure cellular viscoelastic properties in vitro 17 , and in some instances to induce forces upon cells in vivo [18][19][20][21] . However, few magnetic devices have been employed to determine the spatial distribution of physical properties in vivo [22][23][24] . The application of conventional magnetic devices to quantify tissue mechanical properties in vivo is challenging due to low and non-uniform force generation, limited workspace permitted by conventional magnetic devices, and heat generation.…”