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The micropipette aspiration experiment remains a widely used micromanipulation technique for quantifying the mechanical properties of biological samples. Our study extends previous results by investigating the influence of sample size and adhesion area on the mechanical response of compressible thin biological samples. We thus defined a nonlinear relationship between aspirated length, Young's modulus, Poisson's ratio and sample thickness which allowed us to develop an original experimental protocol for simultaneous quantification of the Poisson's ratio and Young's modulus of adherent samples. We first validated our method by characterizing mechanical properties of polyacrylamide gels with tunable stiffness. We then considered application of these results to the quantification of cell elasticity, focusing on the influence of cell adhesion area onto the measured apparent cell stiffness.

To quantitatively elucidate myocyte-matrix adhesion, muscle cells were controllably peeled from narrow strips of collagen. Initial growth of myoblasts on collagen strips led to cell elongation and end-on fusion. Cell branching and lateral contact were minimized, enabling detailed study of myocyte-matrix adhesion. A micropipette was used to pull back one end of a cell; along the resulting detachment front, peeling velocity fluctuated as focal roughness, microns in scale, was encountered. Nonetheless, mean velocity generally increased with detachment force, consistent with forced disruption of adhesion bonds. Beta1-integrin distributions correlated with the focal roughness. In addition, the peeling forces and rates were found to be moderately well-described by a dynamical peeling model for receptor-based adhesion. Estimates were thereby obtained for the spontaneous, molecular off-rate and the receptor complex stiffness (0.01 to 0.001 mN/m) of adherent myocytes. Such a local stiffness is in the range of flexible proteins of the spectrin superfamily which includes the myocyte membrane protein dystrophin. Preliminary results with dystrophin-null myocytes, indicate that dystrophin's absence leads to a less adherent, faster peeling cell under a given load.

The red cell membrane's spectrin-actin skeleton is well-studied but continues to motivate deeper understanding. In the present set of studies, the skeleton is directly shown to be a thermally-soft surface structure capable of sustaining large, localized strains. Laser-photobleaching of a one micron line of fluorescent phalloidin-actin demonstrates not only that skeleton deformation is reversible, i.e. elastic, but also that the network can sustain in-plane extensions up to at least 200% and contractions down to 40%, referenced to an undistorted cell. Such kinematic quantities are, however, true thermodynamic-averages since thermal fluctuations of the F-actin nodes of the network are a significant fraction of the unstretched length of spectrin. The fluctuations, in fact, reveal the softness of the network and yield a simple estimate of the network's local shear modulus. Fluorescence polarization studies of phalloidin-actin indicate actin protofilaments lie tangent to the membrane, and, when the cell is pulled into a micropipette, filaments tend to align with extended spectrin. Central aspects of the microstretching, fluctuation, and rotation fields for the red cell network are captured in multi-scale, statistical mechanical simulations of polymer networks in deformation.