How physical force is sensed by cells and transduced into cellular signaling pathways is poorly understood. Previously, we showed that tyrosine phosphorylation of p130Cas (Cas) in a cytoskeletal complex is involved in force-dependent activation of the small GTPase Rap1. Here, we mechanically extended bacterially expressed Cas substrate domain protein (CasSD) in vitro and found a remarkable enhancement of phosphorylation by Src family kinases with no apparent change in kinase activity. Using an antibody that recognized extended CasSD in vitro, we observed Cas extension in intact cells in the peripheral regions of spreading cells, where higher traction forces are expected and where phosphorylated Cas was detected, suggesting that the in vitro extension and phosphorylation of CasSD are relevant to physiological force transduction. Thus, we propose that Cas acts as a primary force sensor, transducing force into mechanical extension and thereby priming phosphorylation and activation of downstream signaling.

This paper describes a procedure for making topologically complex three-dimensional microfluidic channel systems in poly(dimethylsiloxane) (PDMS). This procedure is called the "membrane sandwich" method to suggest the structure of the final system: a thin membrane having channel structures molded on each face (and with connections between the faces) sandwiched between two thicker, flat slabs that provide structural support. Two "masters" are fabricated by rapid prototyping using two-level photolithography and replica molding. They are aligned face to face, under pressure, with PDMS prepolymer between them. The PDMS is cured thermally. The masters have complementary alignment tracks, so registration is straightforward. The resulting, thin PDMS membrane can be transferred and sealed to another membrane or slab of PDMS by a sequence of steps in which the two masters are removed one at a time; these steps take place without distortion of the features. This method can fabricate a membrane containing a channel that crosses over and under itself, but does not intersect itself and, therefore, can be fabricated in the form of any knot. It follows that this method can generate topologically complex microfluidic systems; this capability is demonstrated by the fabrication of a "basketweave" structure. By filling the channels and removing the membrane, complex microstructures can be made. Stacking and sealing more than one membrane allows even more complicated geometries than are possible in one membrane. A square coiled channel that surrounds, but does not connect to, a straight channel illustrates this type of complexity.

A simple analytical model describing tip-surface interactions in an electrostatic force microscopy (EFM) experiment is proposed. Tip-surface capacitance is modeled as a sum of capacitances of cone, sphere, and plate with the substrate. Individual tips are calibrated according to this model by the choice of tip radius. Differences in EFM signal amplitude between probes are explained by differences in the sphere radii. Three tips with different sphere radii were used to detect EFM force gradients on an array of samples of dispersed Au nanoparticles with diameters ranging from 6 to 18 nm. The spatial distribution of the electric field created by an Au nanoparticle polarized by the inhomogeneous field of the tip is calculated analytically. The particle diameter and tip-surface separation dependence of the measured force gradient due to metal sphere polarization is compared to that predicted by the model. A statistically significant z-offset factor is introduced into the model to correct for the curvature mismatch between the model system and the actual tip. IntroductionSince the invention of scanning tunneling and atomic force microscopies, 1,2 various adaptations of the scanning probe technique have revolutionized the study of surfaces. Scanning probe methods allow the simultaneous mapping and correlation of surface topography and other physical properties. Electrostatic force microscopy (EFM), 3-9 measures the long-range electrostatic interactions between a sample and a conducting probe when a voltage is applied between them. This methodology, with slight variations, has been applied to electric field distributions in devices, 10-12 electrostatics of self-assembled monolayers on surfaces, 13 studies of surface potential variations in oxide bicrystals, 14,15 static and dynamic properties of ferroelectric materials, 16-21 charge measurements in single nanostructures, [22][23][24] as well as observation of charge storage and leakage in various materials. [25][26][27] Although some quantitative measurements of surface charges have been reported, 7,13,22,24-29 most applications of EFM have focused on the mapping of surface potential, which does not require a quantitative understanding of the tip-surface capacitance. However, surface potential does not uniquely determine the charge and polarizability distribution in the sample. To determine the distribution of static charges and polarizability, one must characterize the capacitive interactions between the surface and the probe. Because the EFM probe is in reality an irregular pyramid with a small rounded tip, there is no simple analytical solution. Hence, an approximate model must be used. A number of theoretical works exploring both analytical and numerical methods have addressed these issues, proposing different simplified geometries for the AFM probe, such as cone, sphere, parallel-plate, hyperboloid, as well as their various combinations. [30][31][32][33][34][35][36][37] Herein we develop the apparatus and modeling methodology to enable rigorous quantitative EFM int...