Nima Khatibzadeh

University of California, Riverside, Riverside, California, United States

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Publications (5)11.31 Total impact

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    ABSTRACT: In this study, we investigated the effects of size and surrounding media viscosity on trapping of microspheres. A continuous wave ytterbium fiber laser with a 1064 nm wavelength was used to create an optical tweezers system for optical manipulation experiments. Briefly, the system consisted of an inverted microscope, and a 100X 1.4 NA oil immersion objective through which the laser beam converged to form the optical trap. The laser beam was collimated, steered, and coupled to the microscope through the epifluorescence microscope port. The laser power at the trap focal spot was determined by measuring the input power at the back aperture of the objective multiplied by the objective transmission factor at 1064 nm measured by a modified dual objective method. Polystyrene microspheres varying in diameter from 5 to 15 microns were suspended in liquid media in glass bottom petri dishes prior to trapping experiments. The microspheres were trapped at different trapping powers, and fluidic viscous drag forces where applied to the optically trapped microspheres by driving a computer controlled 2D motorized microscope stage at known velocities. The drag forces were calculated at the point that the microspheres fell out of the trap, based on the Stokes equation for flow around spheres. The data show a linear relationship between trapping force and trap power within the range of the microsphere diameters and media viscosity values used. The work includes calculation of the dimensionless trap efficiency coefficient (Q) at 1064 nm wavelength and the corresponding effects of media viscosity and microsphere size on (Q).
    02/2014;
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    ABSTRACT: In this study, we investigated nanomechanical properties of cell membranes in response to elongation at different rates. Membrane nanotubes (tethers) were pulled at different pulling rates by an optically-trapped fluorescent microsphere as recorded and analyzed for low (1 μm/s) and high (100 μm/s) pulling rates. The force relaxation response of membrane nanotubes exhibited a bi-phasic behavior including fast and slow relaxation processes at low and high pulling rates. The fast and slow force relaxation time constants were 0.388+/-0.21 s and 11.74+/-3.35 s, in response to pulling rate of 1 μm/s, respectively and significantly decreased at higher pulling rates. These reductions in the time constants are suggestive of reduced viscous effects and weakened adhesions between the membrane and the cytoskeleton during rapid pulling.
    01/2014;
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    ABSTRACT: Protrusions are deformations that form at the surface of living cells during biological activities such as cell migration. Using combined optical tweezers and fluorescent microscopy, we quantified the mechanical properties of protrusions in adherent human embryonic kidney cells in response to application of an external force at the cell surface. The mechanical properties of protrusions were analyzed by obtaining the associated force-length plots during protrusion formation, and force relaxation at constant length. Protrusion mechanics were interpretable by a standard linear solid (Kelvin) model, consisting of two stiffness parameters, and (with > ), and a viscous coefficient. While both stiffness parameters contribute to the time-dependant mechanical behavior of the protrusions, and in particular dominated the early and late stages of the protrusion formation and elongation process, respectively. Lowering the membrane cholesterol content by 25% increased the stiffness by 74%, and shortened the protrusion length by almost half. Enhancement of membrane cholesterol content by nearly two-fold increased the protrusion length by 30%, and decreased the stiffness by nearly two-and-half-fold as compared with control cells. Cytoskeleton integrity was found to make a major contribution to protrusion mechanics as evidenced by the effects of F-actin disruption on the resulting mechanical parameters. Viscoelastic behavior of protrusions was further characterized by hysteresis and force relaxation after formation. The results of this study elucidate the coordination of plasma membrane composition and cytoskeleton during protrusion formation.
    PLoS ONE 01/2013; 8(2):e57147. · 3.73 Impact Factor
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    ABSTRACT: In this study, we investigated the effects of membrane cholesterol content on the mechanical properties of cell membranes by using optical tweezers. We pulled membrane tethers from human embryonic kidney cells using single and multi-speed protocols, and obtained time-resolved tether forces. We quantified various mechanical characteristics including the tether equilibrium force, bending modulus, effective membrane viscosity, and plasma membrane-cytoskeleton adhesion energy, and correlated them to the membrane cholesterol level. Decreases in cholesterol concentration were associated with increases in the tether equilibrium force, tether stiffness, and adhesion energy. Tether diameter and effective viscosity increased with increasing cholesterol levels. Disruption of cytoskeletal F-actin significantly changed the tether diameters in both non-cholesterol and cholesterol-manipulated cells, while the effective membrane viscosity was unaffected by F-actin disruption. The findings are relevant to inner ear function where cochlear amplification is altered by changes in membrane cholesterol content.
    Soft Matter 01/2012; 8(32):8350-8360. · 3.91 Impact Factor
  • Biophysical Journal 01/2010; 98(3). · 3.67 Impact Factor