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Originally published as Biophys J. BioFAST on April 27, 2007.
doi:10.1529/biophysj.107.104091
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Biophysical Journal 93:363-372 (2007)
© 2007 The Biophysical Society

Force versus Axial Deflection of Pipette-Aspirated Closed Membranes

Volkmar Heinrich and Chawin Ounkomol

Department of Biomedical Engineering and Biomedical Engineering Graduate Group, University of California, Davis, California

Correspondence: Address reprint requests to Volkmar Heinrich, PhD, Dept. of Biomedical Engineering, UC Davis, 451 E. Health Sciences Dr., Davis, CA 95616. Tel.: 530-754-6644; Fax: 530-754-5739; E-mail: vheinrich{at}ucdavis.edu.

The axial deformation of a pipette-pressurized fluid membrane bag produces minuscule yet well-defined, reproducible forces. The stiffness of this ultrasensitive force transducer is tunable and largely independent of the constitutive membrane behavior. Based on a rigorous variational treatment, we present both numerical as well as approximate analytical solutions for the force-deflection relation of this unique biophysical force probe. Our numerical results predict a measurably nonlinear force-deflection behavior at moderate-to-large deformations, which we confirm experimentally using red blood cells. Furthermore, considering nearly spherical membrane shapes and enforcing proper boundary conditions, we derive an analytical solution valid at small deformations. In this linear regime the pressurized membrane bag behaves like a Hookean spring, with a spring constant that is significantly larger than previously published for the biomembrane force probe.




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Copyright © 2007 by the Biophysical Society.