Living cells are a non-equilibrium mechanical system, largely because intracellular molecular motors consume chemical energy to generate forces that reorganize and maintain cytoskeletal functions. Persistently under tension, the network of cytoskeletal proteins exhibits a nonlinear mechanical behavior where the network stiffness increases with intracellular tension. We examined the nonlinear mechanical properties of living cells by characterizing the differential stiffness of the cytoskeletal network for HeLa cells under different intracellular tensions. Combining optical tweezer-based active and passive microrheology methods, we measured non-thermal fluctuating forces and found them to be much larger than the thermal fluctuating force. From the variations of differential stiffness caused by the fluctuating non-thermal force for cells under different tension, we obtained a master curve describing the differential stiffness as a function of the intracellular tension. Varying the intracellular tension by treating cells with drugs that alter motor protein activities we found the differential stiffness follows the same master curve that describes intracellular stiffness as a function of intracellular tension. This observation suggests that cells can regulate their mechanical properties by adjusting intracellular tension.
Cell division plays an important role in regulating cell proliferation and differentiation. It is managed by a complex sequence of cytoskeleton alteration that induces dividing cells to change their morphology to facilitate their division. The change in cytoskeleton structure is expected to affect the intracellular viscoelasticity, which may also contribute to cellular dynamic deformation during cell division. However, the intracellular viscoelasticity during cell division is not yet well understood. In this study, we injected 100-nm (diameter) carboxylated polystyrene beads into the cytoplasm of HeLa cells and applied video particle tracking microrheology to measure their intracellular viscoelasticity at different phases during cell division. The Brownian motion of the intracellular nanoprobes was analyzed to compute the viscoelasticity of HeLa cells in terms of the elastic modulus and viscous modulus as a function of frequency. Our experimental results indicate that during the course of cell division, both intracellular elasticity and viscosity increase in the transition from the metaphase to the anaphase, plausibly due to the remodeling of cytoskeleton and redistributions of molecular motors, but remain approximately the same from the anaphase to the telophase.
The transverse force profile of a particle in an optical trap is important for the designs of optical trapping-based force
transducers. We mapped these force profiles for micron-size polystyrene beads using a pair of overlapping optical traps
produced by two highly focused Gaussian beams with unequal intensity; the stronger trap serves as a force transducer to
measure the force of the weaker trap in both linear and nonlinear regimes. For particles with size smaller or comparable
to the laser wavelength, the force profiles follow closely the gradient of the Gaussian profile, but as the particle size
increases, the force profiles deviate from the shape of the gradient of Gaussian for the distance beyond the position of the
maximum force. The distance from the center of the trap to the position of the maximum trapping force was found to
increase linearly with the particle size. The experimental results are in good agreements with our theoretical model,
based on a combination of the Mie theory, vector Debye integral, and Maxwell stress tensor; except that the
experimental particle-size dependence of the maximum trapping forces was found to be weaker than that predicted by
the theory.
In this paper, we report the viscoelastic properties of sodium polystyrene sulfonate (NaPSS) solution with
different concentrations (in the range of 10-4 M to 10-3 M) and with different molecular weights (70 kDa vs. 200 kDa)
investigated via Diffusing Wave Spectroscopy (DWS). The viscoelastic properties of the sample solutions are
characterized in terms of the elastic modulus G' and the viscous modulus G" as a function of frequency (f), and also
in terms of the polymer disentanglement time (τ); the effect of polymer concentration and molecular weight on these
parameters are presented. Our experimental results indicate that (1) both the viscous modulus G" and the
disentanglement time (τ) increase with molecular concentration, whereas the elastic modulus G' is relatively
insensitive to molecular concentration, and (2) for the same concentration, all the 3 parameters (i.e., the elastic
modulus G', the viscous modulus G", and disentanglement time τ) increase as the molecular weight increases.
Intracellular stresses generated by molecular motors can actively modify cytoskeletal network, which causes
changes in intracellular mechanical properties. We study the out-of-equilibrium microrheology in living cells. This paper
reports measurements of the intracellular mechanical properties using passive and optical tweezers-based active
microrheology approaches and endogenous organelle particles as probes. Using the fluctuation-dissipation theorem, we
compared the two approaches measurements and distinguished thermal and non-thermal fluctuations of mechanical
properties in living cells.
A dual-trap optical tweezers is used for deforming the red blood cell (RBC) in suspension and studying its elasticity. The
radiation force is applied directly to the cell without physical contact. The 3D radiation stress distribution was computed
by ray tracing, the generalized Lorentz-Mie theory with the T-matrix and the FDTD via the Maxwell stress tensor. The
3D deformation of the cells was computed with the elastic membrane theory. The calculated deformation can fit to
experimental data resulting in cell's elasticity coefficient. The static approach is valid only for small deformation (5-
10%). For a large deformation such as that of the RBC, we consider re-distribution of the radiation stress on the
morphologically deformed cell. This stress re-distribution in turn induces subsequent deformation of the deformed cell
and new stress re-distribution. The recursive process continues until a final equilibrium state is achieved. This iterative
computation was implemented with the finite element method using the COMSOLTM multi-physics models. The
deformation results can fit to the experimental data for cell's deformation up to 20%.
It has been known that the shape, the locomotion, and the growth of cells and bacteria are often affected by their
interactions with extra cellular matrix (ECM). However, it is difficult to quantify such interactions with conventional
biochemical methods. In this paper we report the application of oscillatory optical tweezers to trap and oscillate
three types of E. coli, in 0.2% LB agar substrate to quantify the E. coli - substrate interactions in terms of the
elasticity modulus G'. The three types of E. coli are BW25113 (wild-type, normal with flagellum), BW25113
(normal with flagellum, but subjected to UV light exposure for 1 hr to deactivate the flagellum), and JW1923 (a
null-flagellum mutant of BW25113). Our results indicate that the value of G' for the later two is significantly higher than
that for the normal wild-type (WT). We speculated that the interaction with the surrounding is perturbed, and hence
reduced, mainly by the motion of the flagellum in the case of the WT.
Optical binding has been proposed to be responsible for the cluster formation of micron size dielectric
spheres in coherent light fields. However, a direct measurement of the forces involved in binding is
missing. We report an experimental study of optical binding forces between two optically trapped
dielectric spheres. Results for optical forces are presented as a function of three parameters: inter
particle separation, particle size, and respective polarizations. A comprehensive calculation based on
the generalized Mie scattering theory for the experiment has also been conducted. This paper will
present a comparison between experimental and theoretical results.
In this paper, we begin with a brief overview of optical trapping of micro- and nano- particles and of various techniques for the measurement of optical force constants in the linear spring model. We then move on to introduce two complimentary approaches to implement optical forced oscillation of the trapped particle, one by an oscillatory optical tweezers, and the other by chopping (i.e., switching on-and-off) one of the beams in a twin set of optical tweezers. In each implementation, we have measured the steady state amplitude and phase of the oscillating particle as a function of frequency (from ~ 10Hz to 600Hz) with the aid of a quadrant photo-diode in conjunction with a lock-in amplifier. For the case of optical forced oscillation of a "free" particle involving only the optical force and the viscous drag, the experimental data fit fairly well the theoretical curve obtained from the simple linear spring model; both the optical force constant and the viscosity of the surrounding fluid can be deduced with fairly high precision as the fitting parameters from the best fit of the experimental data to the theoretical curves.
When one or more external forces, in addition to the optical force and the drag force, were applied to the oscillating particle via mechanisms such as protein-protein interaction or DNA stretching, the oscillating amplitude and phase varied in response to the external forces. Preliminary data showing the change in oscillating amplitude and phase as a function of time in response to external forces will be presented, and potential biomedical applications of this approach will be discussed.
Lipopolysaccharide (LPS) is one of the cell wall components of Gram-positive bacteria recognized by
and interacted with receptor proteins such as CD14 on macrophage cells. Such a process plays an
important role in our innate immune system. In this paper, we report the application of optical tweezers
(λ = 1064nm Gaussian beam focused by a water-immersed objective lens with N.A. = 1.0) to the study
of the dynamics of the binding of a LPS-coated polystyrene particle (diameter = 1.5μm) onto the
plasma membrane of a macrophage cell. We demonstrated that the binding rate increased significantly
when the macrophage cell was pre-treated with the extract of Reishi polysaccharides (EORP) which
has been shown to enhance the cell surface expression of CD14 (receptor of LPS) on macrophage cells.
We report the first direct experimental mapping of three-dimensional optical force field on a silica micro-particle trapped in a counter-propagating dual-beam trap. We tracked the three-dimensional Brownian motion of the trapped particle (by optical position sensing) and analyzed the particle position distribution to obtain the force constant of the optical force field on the particle along each direction. The trapping beams scattered by the trapped particle along two directions (mutually orthogonal to each other and also to the trapping beams) were projected on a pair of quadrant photo-detectors (QPDs) to facilitate high-speed (20 KHz) three-dimensional position tracking. Position tracking over two mutually orthogonal planes intrinsically provides one set of redundant data for a self-consistency check. At optical wavelength λ = 532nm, the force constants of the three-dimensional optical force field on a silica micro-particle (diameter = 2.58μm) were determined to be kx = 1.61×10-1 pN/μm, ky = 1.49×10-1 pN/μm, and kz = 4.43×10-2 pN/μm when the total trapping power was about 21mW and the distance between the two fiber end-faces was 125μm. The set of force constants (kx, ky, and kz) completely defines the optical force field E(x, y, z) = [kxx2 + kyy2 + kzz2]/2 (in the parabolic potential approximation) on the trapped particle.
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