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@PHDTHESIS{Chen:1005797,
      author       = {Chen, La},
      title        = {{D}esign, {I}mplementation and {A}pplication of {H}igh
                      {T}hroughput {M}agnetic {T}weezers for {C}ell {M}echanics
                      {S}tudies},
      school       = {RWTH Aachen},
      type         = {Dissertation},
      address      = {Aachen},
      publisher    = {RWTH Aachen},
      reportid     = {FZJ-2023-01642},
      pages        = {113 p.},
      year         = {2016},
      note         = {Dissertation, RWTH Aachen, 2016},
      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.},
      cin          = {IBI-3},
      cid          = {I:(DE-Juel1)IBI-3-20200312},
      pnm          = {5241 - Molecular Information Processing in Cellular Systems
                      (POF4-524)},
      pid          = {G:(DE-HGF)POF4-5241},
      typ          = {PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/1005797},
}