Cell membrane experiments Dept. of Experimental Orthopaedics and Biomechanics Bioengineering Reza Abedian (M.Sc.)

Mechanical experiments: Best suited for spherical cells Yield the modulus of elasticity with respect to area change of the membrane The dominant stress resultants are circumferential and longitudinal tensions –Micropipette aspiration –Compression between two flat surfaces –Deflection of the surface by a rigid spherical particle

K is about 450 dyn/cm at 25ºC A pressure of the order of 10 5 dyn/cm 2 is needed N Φ membrane tension stress ∆P the pressure difference between the pipette interior and the outside medium L is the length of the aspirated projection of the membrane According to Laplace’s formula Micropipette aspiration:

Membrane shear experiments: Distortion of the membrane without change of area –Pure shear In x-direction by stretch ratio of λ Shrinkage in y-direction by stretch ratio of λ -1 –Simple shear General deformation of RBC includes both pure shear and area change RBC membrane can sustain a large shear but ruptures if the area changes by a few percent

Micropipette (continued) N m and N Φ membrane stress resultants Λ m is the stretch ration in the direction of N m Maximum shear stress resultant: μ is 6.6e-3 at 25ºC

Optical tweezers: The experimental set up used to impose large deformation on the human red blood cell using optical tweezers The key component of this set up is: –laser source connected to –inverted microscope The laser beam is used to trap a high-refractive-index silica bead which is attached to the cell surface The 1.5 W laser beam when used with silica microbeads of 4.12 μ in diameter can generate large forces that are nearly an order of magnitude higher than those used for deforming red blood cells in previous studies (HKenon et al., 1999; Sleep et al., 1999; Parker and Winlove, 1999) Two silica microbeads, which act as handles, are attached diametrically across the cell through non-specific binding As the present optical tweezers system is designed to consist only of a single optical trap, one of the microbeads is adhered to the glass surface while the other is free to be trapped using the laser beam By moving one of the beads with the laser beam, the cell is directly stretched The trapping force exerted on the microbead can be changed, up to a maximum value of about 600 pN, by varying the laser power setting Thereafter, changes in axial (in the direction of stretch) and transverse (normal to that of stretch) diameters are recorded using a CCD camera and video recorder. The red blood cell, placed in a phosphate buffered saline solution, was stretched by optical tweezers in the room temperature laboratory environment. Further details of sample preparation, force calibration, experimental set up and data collection can be found in Lim et al. (2004).

Optical tweezers:

Theoretical background and computational model Constitutive response the human red blood cell membrane comprises 1.the phospholipid bilayer 2.the underlying spectrin network 3.transmembrane proteins The (composite) cell membrane structure is commonly modelled as an incompressible effective continuum

Constitutive model: Evans (1973) suggested that the relationship between the membrane shear stress Ts (in units of force per unit length) and deformation is of the form:

References: Fung’s Cell Membrane experiments Mechanics of the human red blood cell deformed by optical tweezers, Journal of the Mechanics and Physics of Solids M. Daoa, C.T. Limb, S. Suresha; Department of Materials Science and Engineering, Massachusetts Institute of Technology, Division of Bioengineering and Department of Mechanical Engineering, Department of Mechanical Engineering, Massachusetts Institute of Technology THERMOELASTICITY OF RED BLOOD CELL MEMBRANE, R. WAUGH AND E. A. EVANS, Departments ofBiomedical Engineering and Physiology, Duke University, Durham, North Carolina