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Biophysical Engineering Lab and Institute for Medicine and Engineering, School of Engineering and Applied Science, University of Pennsylvania, Philadelphia, Pennsylvania
Correspondence: Address reprint requests to Dennis E. Discher, Biophysical Engineering Lab, 112 Towne Bldg., University of Pennsylvania, Philadelphia, PA 19104-6315. Tel.: 215-898-4809; Fax: 215-573-6334; E-mail: discher@seas.upen.edu.
Membrane tension underlies a range of cell physiological processes. Strong adhesion of the simple red cell is used as a simple model of a spread cell with a finite membrane tension—a state which proves useful for studies of both membrane rupture kinetics and atomic force microscopy (AFM) probing of native structure. In agreement with theories of strong adhesion, the cell takes the form of a spherical cap on a substrate densely coated with poly-L-lysine. The spreading-induced tension, 
, in the membrane is 
1 mN/m, which leads to rupture over many minutes; and is estimated from comparable rupture times in separate micropipette aspiration experiments. Under the sharpened tip of an AFM probe, nano-Newton impingement forces (10–30 nN) are needed to penetrate the tensed erythrocyte membrane, and these forces increase exponentially with tip velocity (nm/ms). We use the results to clarify how tapping-mode AFM imaging works at high enough tip velocities to avoid rupturing the membrane while progressively compressing it to a 20-nm steric core of lipid and protein. We also demonstrate novel, reproducible AFM imaging of tension-supported membranes in physiological buffer, and we describe a stable, distended network consistent with the spectrin cytoskeleton. Additionally, slow retraction of the AFM tip from the tensed membrane yields tether-extended, multipeak sawtooth patterns of average force 200 pN. In sum we show how adhesive tensioning of the red cell can be used to gain novel insights into native membrane dynamics and structure.
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