Crocker, John C
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Publication The Consensus Mechanics of Cultured Mammalian Cells(2006-07-05) Hoffman, Brenton D; Massiera, Gladys; Miranda, Kathleen A; Crocker, John CWhile understanding cells' responses to mechanical stimuli is seen as increasingly important for understanding cell biology, how to best measure, interpret and model cells' mechanical properties remains unclear. We determine the frequency-dependent shear modulus of cultured mammalian cells using four different methods, both novel and well established. This approach clarifies the effects of cytoskeletal heterogeneity, ATP-dependent processes and cell regional variations on the interpretation of such measurements. Our results clearly indicate two qualitatively similar but distinct mechanical responses, corresponding to the cortical and intracellular networks, each having an unusual, weak power-law form at low frequency. The two frequency dependent responses we observe are remarkably similar to those reported for a variety of cultured mammalian cells measured using different techniques, suggesting it is a useful consensus description. Finally, we discuss possible physical explanations for the observed mechanical response.Publication Role of configurational entropy in the thermodynamics of clusters of point defects in crystalline solids(2005-07-20) Kapur, Sumeet S; Crocker, John C; Prasad, Manish; Sinno, TalidThe internal configurational entropy of point defect clusters in crystalline silicon is studied in detail by analyzing their potential energy landscapes. Both on-lattice and off-lattice calculation approaches are employed to demonstrate the importance of off-lattice configurational states that arise due to a large number of inherent structures (local minima) in the energy landscape generated by the interatomic potential function. The resulting cluster configurational entropy of formation is shown to exhibit behavior that is qualitatively similar to that observed in supercooled liquids and amorphous solids and substantially alters the thermodynamic properties of point defect clusters in crystals at high temperature. This behavior is shown to be independent of interatomic potential and cluster type, and suggests that defects in crystals at high temperature should be generally described by a quasicontinuous collection of nondegenerate states rather than as a single ground state structure. The modified thermodynamic properties of vacancy clusters at high temperature are found to explain a longstanding discrepancy between simulation predictions and experimental measurements of vacancy aggregation dynamics in silicon.Publication Stress-Dependent Elasticity of Composite Actin Networks as a Model for Cell Behavior(2006-03-03) Gardel, M. L; Nakamura, F.; Hartwig, J.; Crocker, John C; Stossel, T. P; Weitz, D. ANetworks of filamentous actin cross-linked with the actin-binding protein filamin A exhibit remarkable strain stiffening leading to an increase in differential elastic modulus by several orders of magnitude over the linear value. The variation of the frequency dependence of the differential elastic and loss moduli as a function of prestress is consistent with that observed in living cells, suggesting that cell elasticity is always measured in the nonlinear regime, and that prestress is an essential control parameter.Publication Microrheology of polyethylene oxide using diffusing wave spectroscopy and single scattering(2002-05-20) Dasgupta, Bivash R.; Tee, Shang-You; Crocker, John C; Frisken, B. J.; Weitz, D. A.Experiments investigating the local viscoelastic properties of a simple uncross-linked flexible polymer are performed on polyethylene oxide solutions in the semidilute regime using polystyrene beads of varying sizes and surface chemistry as probes. We measure the thermal motions of the beads to obtain the elastic and viscous moduli of our sample. Two different dynamic light scattering techniques, diffusing wave spectroscopy and quasielastic light scattering (QELS), are used to determine the dynamics of the probe particles. Diffusing wave spectroscopy probes the short time dynamics of the scatterers while QELS or single scattering measures the dynamics at larger times. This results in a larger frequency overlap of the data obtained from the microrheological techniques with the data obtained from the conventional bulk measurements. The moduli are estimated using a modified algebraic form of the generalized Stokes-Einstein equation. Comparison of microrheology with bulk measurements shows excellent similarity confirming the applicability of this method for simple, uncross-linked polymeric systems.Publication Rheological Microscopy: Local Mechanical Properties from Microrheology(2003-03-14) Chen, D. T.; Crocker, John C; Weeks, E. R.; Islam, M. F.; Verma, R.; Gruber, J.; Lubensky, Thomas C.; Levine, A. J.; Yodh, A. G.We demonstrate how tracer microrheology methods can be extended to study submicron scale variations in the viscoelastic response of soft materials; in particular, a semidilute solution of lambda-DNA. The polymer concentration is depleted near the surfaces of the tracer particles, within a distance comparable to the polymer correlation length. The rheology of this microscopic layer alters the tracers’ motion and can be precisely quantified using one- and two-point microrheology. Interestingly, we found this mechanically distinct layer to be twice as thick as the layer of depleted concentration, likely due to solvent drainage through the locally perturbed polymer structure.Publication Multiple Particle Tracking and Two-Point Microrheology in Cells(2007-01-01) Crocker, John C; Hoffman, Brenton DMechanical stress and stiffness are increasingly recognized to play important roles in numerous cell biological processes, notably cell differentiation and tissue morphogenesis. Little definite is known, however, about how stress propagates through different cell structures or how it is converted to biochemical signals via mechanotransduction, due in large part to the difficulty of interpreting many cell mechanics experiments. A newly developed technique, two-point microrheology (TPM), can provide highly interpretable, quantitative measurements of cells’ frequency-dependent shear moduli and spectra of their fluctuating intracellular stresses. TPM is a non-invasive method based on measuring the Brownian motion of large numbers of intracellular particles using multiple particle tracking. While requiring only hardware available in many cell biology laboratories–a phase microscope and digital video camera, as a statistical technique, it also requires the automated analysis of many thousands of micrographs. Here we describe in detail the algorithms and software tools used for such large-scale multiple particle tracking, as well as common sources of error and the microscopy methods needed to minimize them. Moreover, we describe the physical principles behind TPM and other passive microrheology methods, their limitations, and typical results for cultured epithelial cells.Publication Fragility and Mechanosensing in a Thermalized Cytoskeleton Model with Forced Protein Unfolding(2007-11-01) Hoffman, Brenton D; Massiera, Gladys; Crocker, John CWe describe a model of cytoskeletal mechanics based on the force-induced conformational change of protein cross-links in a stressed polymer network. Slow deformation of simulated networks containing cross-links that undergo repeated, serial domain unfolding leads to an unusual state — with many cross-links accumulating near the critical force for further unfolding. This state is robust to thermalization and does not occur in similar protein unbinding based simulations. Moreover, we note that the unusual configuration of near-critical protein cross-links in the fragile state provides a physical mechanism for the chemical transduction of cell-level mechanical strain and extra-cellular matrix stiffness.Publication Universal Dripping and Jetting in a Transverse Shear Flow(2009-05-12) Meyer, Robert F; Crocker, John COne particularly efficient approach to making emulsions having monosized droplets is to push a fluid through an orifice into a transverse flow of a second immiscible fluid. We find that, at an intermediate particle Reynolds number, the final droplet size can be readily computed using a simple force balance. Remarkably like the well-known dripping faucet, this system displays both dripping and jetting behavior, controlled by the capillary, Weber and Ohnesorge numbers of the relevant fluids, and interesting nonlinear behavior such as period doubling near the transition between these two regimes.Publication Microrheology, Stress Fluctuations, and Active Behavior of Living Cells(2003-11-03) Lau, A. W. C.; Hoffman, Brenton D; Crocker, John C; Davies, A.; Lubensky, Thomas C.We report the first measurements of the intrinsic strain fluctuations of living cells using a recently developed tracer correlation technique along with a theoretical framework for interpreting such data in heterogeneous media with nonthermal driving. The fluctuations’ spatial and temporal correlations indicate that the cytoskeleton can be treated as a course-grained continuum with power-law rheology, driven by a spatially random stress tensor field. Combined with recent cell rheology results, our data imply that intracellular stress fluctuations have a nearly 1/ω2 power spectrum, as expected for a continuum with a slowly evolving internal prestress.Publication Microrheology of Entangled F-Actin Solutions(2003-10-07) Gardel, M. L.; Valentine, M. T.; Crocker, John C; Bausch, A. R.; Weitz, D. A.We measure the viscoelasticity of entangled F-actin over length scales between 1 and 100 µm using one- and two-particle microrheology, and directly identify two distinct microscopic contributions to the elasticity. Filament entanglements lead to a frequency-independent elastic modulus over an extended frequency range of 0:01–30 rad/ sec; this is probed with one-particle microrheology. Longitudinal fluctuations of the filaments increase the elastic modulus between 0.1 and 30 rad/ sec at length scales up to the filament persistence length; this is probed by two-particle microrheology.