Design, Implementation and Application of High Throughput Magnetic Tweezers for Cell Mechanics Studies

Chen, La

Dissertation / PhD Thesis

FZJ-2023-01642

Design, Implementation and Application of High Throughput Magnetic Tweezers for Cell Mechanics Studies

Chen, L. (Corresponding author)FZJ*

2016
RWTH Aachen Aachen

Aachen : RWTH Aachen 113 p. (2016) = Dissertation, RWTH Aachen, 2016

Please use a persistent id in citations: http://hdl.handle.net/2128/34223

Abstract: Recently it has been shown that the mechanical properties of cells play a very important role in various biological processes. Most living cells are small, fragile and highly heterogeneous. It is frequently observed that there is a large inherent variation in mechanical properties from cell to cell. Therefore, high throughput microrheology methods are always favorable in cell mechanics studies. Furthermore, high forces are usually needed to study cells with high stiffness and to analyze nonlinear mechanical properties such as stiffening or fluidization phenomena in cells. However, most available microrheology tools are limited to small force, poor maneuverability, and low throughput. In this context, the current thesis presents the design and implementation of two types of magnetic probe based microrheometers: magnetic tweezers (MT) and magnetic twisting cytometry (MTC), in both of which high throughput, high force (stress), and good maneuverability were successfully achieved at the same time. With the help of these tools, the mechanics of rat cardiomyocytes and brain cells were characterized. The first part of this work focuses on the implementation of a high throughput, high force tri-pole electromagnetic tweezers which can achieve 2D actuation. For a given magnetic bead, the maximal force of tri-pole magnetic tweezers depends on the size of the workspace, the width of the magnetic tips, and on the saturation magnetization of the tip material. In order to conveniently calibrate and study the force behavior, an inverse force model based on a numerical solver and active video tracking based feedback control were implemented. Material with high permeability was adopted as the main yoke to reduce the coil current. The electronics and software were custom-made to achieve high performance. For example, with a workspace of 60×60 µm2, a force of up to 1 nN can be applied on a 2.8 µm superparamagnetic bead in any direction within the plane at a speed of up to 1 kHz. However, the practically achieved saturation forces are usually lower than predicted values, which can be ascribed to two factors: magnetic performance deterioration near the cutting edges of the tips and 3D geometrical effect. The high power laser used in cutting causes segregation and morphological roughness near the cutting edge. Moreover, the geometry of the magnetic tips plays an important role regarding the force behavior. In the second part, the corrosion of the magnetic tips in several cell culture media was characterized. Obvious accelerated corrosion was observed in cardiomyocyte and neuronal cell media, but not in HEK cell medium. Both the electrochemical deposition of polypyrrole and the pyrolytical deposition of parylene-C were examined for passivation. It was found that the quality of polypyrrole deposition is insufficient in the area near the edges of the tweezers tips where they had been laser-cut. However, the parylene coating exhibits excellent isolation properties. Both cardiomyocyte and primary neuronal cell can be cultured on parylene-coated magnetic parts for a long time. In addition, the coated parts can also withstand repeated high magnetic field application. In the third part, based on the magnetic tweezers setup, a novel optical 2D magnetic twisting cytometry was implemented, in which both the strength and direction of the twisting field can be controlled. In the MTC system, both polarization and twisting magnetic field were based on electromagnets. A separate high field electromagnet was utilized to magnetize the ferromagnetic particles bound on the surface of the cells. The existing hex-pole yoke electromagnet, but without tips, was used to apply the twisting magnetic field. When the twisting field is less than 100 G, good linearity and small phase error can be achieved. Using the heterodyning technology, the measurement frequencies were extended up to 1 kHz. In the last part of the thesis, these developed microrheology tools were used to study the mechanical properties of rat cardiomyocytes and brain cells. Both the creep and the frequency response of cardiomyocyte HL-1 cells were characterized with the instrument operated in magnetic tweezers mode and in magnetic twisting cytometry mode, respectively. In both modes, the stiffness of HL-1 cells exhibits approximately log-normal distributions. High heterogeneity of single cell stiffness was also noticed. When HL-1 cells were cultured on a stiff substrate, there was an obvious stiffening effect at low frequency, which depends on the prestress generated by myosin activity. In addition, the mechanical properties of rat neuronal and glial cells were studied with magnetic tweezers. It was found that with increasing maturity, the stiffness of both neuron and glia increases. The power-law exponent of neuronal cells decreases with increasing cell maturity, but the one of glia cells does not change. Especially in early stage, it was found that there is high tension in neurites. Furthermore, the neuronal somas become stiffer with the applied stress. Both the elastic modulus of neurons and glia were also sensitive to the rigidity of the substrate.

Note: Dissertation, RWTH Aachen, 2016

Contributing Institute(s):