Plot of interfacial viscosity from single particle diffusion measurements as a function of <i>h/d</i>.

<p>Filled circles denote particles of diameter 0.1 <i>μ</i>m, open circles denote particles of diameter 0.18 <i>μ</i>m and diamonds denote particles of diameter 0.5 <i>μ</i>m. The horizontal light shaded region represents <i>η</i>_<i>int</i> = 1.42±0.74 nPa⋅s⋅m based on the mean and standard deviation of the data for <i>h</i>/<i>d</i> < 5. The vertical dark shaded region represents the crossover from physical behavior at small <i>h</i>/<i>d</i> to unphysical behavior at <i>h</i>/<i>d</i> > 5.2±0.9. The horizontal error bars are due to uncertainties of <i>h</i>, and vertical error bars are due to uncertainties of <i>h</i> and <i>η</i>_2<i>D</i> (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121981#pone.0121981.e004" target="_blank">Eq 4</a>).</p

Two particle correlations in a single soap film measurement as a function of particle separation <i>R</i>.

<p>Particles of diameter <i>d</i> = 0.18 <i>μm</i> were used, and soap film thickness was <i>h</i> = 0.46±0.04 <i>μm</i>. The solid lines are fits from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121981#pone.0121981.e006" target="_blank">Eq 6</a> with <i>A</i> = 1.09 <i>μ</i>m²/<i>s</i>, <i>B</i> = 0.12 <i>μ</i>m²/<i>s</i> and <i>L</i> = 81 <i>μ</i>m. The data are computed from all particle pairs and averaging over a wide range of lag times <i>τ</i>.</p

Cartoon depicting soap film of thickness <i>h</i>, with a single representative particle of diameter <i>d</i> < <i>h</i>.

<p>On both air-film interfaces, representative soap molecules are shown. As discussed in the text, we believe it is likely that particles with <i>d</i> < <i>h</i> sit in the interior of the film, but we cannot rule out the possibility that some particles are trapped at the air-film interface.</p

Fit parameters for all experiments as a function of <i>h</i>/<i>d</i>.

<p>Symbols denote particle diameters as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121981#pone.0121981.g003" target="_blank">Fig 3</a>. See Eqs <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121981#pone.0121981.e006" target="_blank">6</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121981#pone.0121981.e007" target="_blank">7</a> for the meaning of the fit parameters. The vertical error bars are from the standard deviations of each fit parameter calculated for the different <i>τ</i>’s.</p

Titin-Based Nanoparticle Tension Sensors Map High-Magnitude Integrin Forces within Focal Adhesions

Mechanical forces transmitted through integrin transmembrane receptors play important roles in a variety of cellular processes ranging from cell development to tumorigenesis. Despite the importance of mechanics in integrin function, the magnitude of integrin forces within adhesions remains unclear. Literature suggests a range from 1 to 50 pN, but the upper limit of integrin forces remains unknown. Herein we challenge integrins with the most mechanically stable molecular tension probe, which is comprised of the immunoglobulin 27th (I27) domain of cardiac titin flanked with a fluorophore and gold nanoparticle. Cell experiments show that integrin forces unfold the I27 domain, suggesting that integrin forces exceed ∼30–40 pN. The addition of a disulfide bridge within I27 “clamps” the probe and resists mechanical unfolding. Importantly, incubation with a reducing agent initiates SH exchange, thus unclamping I27 at a rate that is dependent on the applied force. By recording the rate of S–S reduction in clamped I27, we infer that integrins apply 110 ± 9 pN within focal adhesions of rat embryonic fibroblasts. The rates of S–S exchange are heterogeneous and integrin subtype-dependent. Nanoparticle titin tension sensors along with kinetic analysis of unfolding demonstrate that a subset of integrins apply tension many fold greater than previously reported