ABSTRACT
Experiments aimed at the determination of mechanical properties of biological membranes were begun in the 1930s using sea urchin eggs and subsequently, nucleated red blood cells. Mechanical properties characterize the structure of the membrane as a continuum. The estimation of an elastic modulus for the red blood cell membrane were based on osmotic swelling experiments in which the transmembrane pressure had to be derived from the equations of chemical equilibrium. The apparent variation of the elastic modulus of the red blood cell membrane by several orders of magnitude in different tests was a result of oversimplification of the analyses. The resolution of the widely different estimates of the red blood cell membrane elastic moduli was accomplished when the restrictions imposed on membrane behavior by its ultrastructure were recognized. Surface isotropy means that the material properties do not depend on the orientation of surface coordinates chosen at a specific location; however, properties may be nonuniform, i.e., depend on surface location.
