Thomas M. Fischer

Shape Memory of Human Red Blood Cells

The human red cell can be deformed by external forces but returns to the biconcave resting shape after removal of the forces. If after such shape excursions the rim is always formed by the same part of the membrane, the cell is said to have a memory of its biconcave shape. If the rim can form anywhere on the membrane, the cell would have no shape memory. The shape memory was probed by an experiment called go-and-stop. Locations on the membrane were marked by spontaneously adhering latex spheres. Shape excursions were induced by shear flow. In virtually all red cells, a shape memory was found. After stop of flow and during the return of the latex spheres to the original location, the red cell shape was biconcave. The return occurred by a tank-tread motion of the membrane. The memory could not be eliminated by deforming the red cells in shear flow up to 4 h at room temperature as well as at 37 degrees C. It is suggested that 1). the characteristic time of stress relaxation is >80 min and 2). red cells in vivo also have a shape memory.

Singular perturbations for the exterior three-dimensional slow viscous flow problem☆

Abstract In this paper the rigorous justification of the formal asymptotic expansions constructed by the method of matched inner and outer expansions is established for the three-dimensional steady flow of a viscous, incompressible fluid past an arbitrary obstacle. The justification is based on the series representation of the solution to the Navier-Stokes equations due to Finn, and it involves the reductions of various exterior boundary value problems for the Stokes and Oseen equations to boundary integral equations of the first kind from which existence as well as asymptotic error estimates for the solutions are deduced. In particular, it is shown that the force exerted on the obstacle by the fluid admits the asymptotic representation F = A 0 + A 1 Re + O ( Re 2 ln Re −1 ) as the Reynolds number Re → 0 + , where the vectors A 0 and A 1 can be obtained from the method of matched inner and outer expansions.

Is the surface area of the red cell membrane skeleton locally conserved

The incompressibility of the lipid bilayer keeps the total surface area of the red cell membrane constant. Local conservation of membrane surface area requires that each surface element of the membrane skeleton keeps its area when its aspect ratio is changed. A change in area would require a flow of lipids past the intrinsic proteins to which the skeleton is anchored. in fast red cell deformations, there is no time for such a flow. Consequently, the bilayer provides for local area conservation. In quasistatic deformations, the extent of local change in surface area is the smaller the larger the isotropic modulus of the skeleton in relation to the shear modulus. Estimates indicate: (a) the velocity of relative flow between lipid and intrinsic proteins is proportional to the gradient in normal tension within the skeleton and inversely proportional to the viscosity of the bilayer; (b) lateral diffusion of lipids is much slower than this flow; (c) membrane tanktreading at frequencies prevailing in vivo as well as the release of a membrane tongue from a micropipette are fast deformations; and (d) the slow phase in micropipette aspiration may be dominated by a local change in skeleton surface.

Bending stiffness of lipid bilayers. I. Bilayer couple or single-layer bending?

To describe the resistance of a bilayer to changes in curvature two mechanisms are distinguished which are termed bilayer couple bending and single-layer bending. In bilayer couple bending, the resistance arises from the 2-D isotropic elasticity of the two layers and their fixed distance. Single-layer bending covers the intrinsic bending stiffness of each monolayer. The two mechanisms are not independent. Even so, the distinction is useful since bilayer couple bending can relax by a slip between the layers from the local to the global fashion. Therefore, the bending stiffness of a bilayer depends on the time scale and on the extent of the deformation imposed on the membrane. Based on experimental data, it is shown by order of magnitude estimates that (a) the bending stiffness determined from thermally induced shape fluctuations of almost spherical vesicles is dominated by single-layer bending; (b) in the tether experiment on lipid vesicles and on red cells, a contribution of local bilayer couple bending can not be excluded; and (c) at the sharp corners at the leading and the trailing edge of tanktreading red cells, local bilayer couple bending appears to be important.

Cross binding and stiffening of the red cell membrane

Abstract Cross binding and stiffening of the human red cell membrane was studied using treatments with SH, amino, and carboxyl reagents, oxidizing and denaturing treatments and acidification. Membrane cross binding was initiated when, after red cell treatment, opposite areas of the cytoplasmic face of the red cell membrane were brought into contact by cell shrinking. Membrane cross binding was detected by light microscopy when this contact persisted upon swelling the cells in a hypotonic medium. Membrane stiffening was recorded as a decrease in elongation of red cells in the shear field of a viscous dextran solution. No correlation was found between membrane cross binding and membrane stiffening. The results are explained by the existence of two modifications of spectrin, type I causing solely membrane stiffening, type II causing membrane cross bonding as well as membrane stiffening. The amino and carboxyl reagents caused only type I modification. The other treatments caused both types of modification although with varying proportions. The results support the previously suggested mechanism of membrane cross bonding which involves a rearrangement of spectrin similar to denaturation by heat or urea, a decrease in associations within the membrane skeletal network, and a lateral aggregation of membrane proteins. These changes are proposed to occur by the type II modification. The data further substantiate the membrane stiffening effect of inter- and intra-molecular cross linking of spectrin which is identified with the type I modification. Finally, hypotheses are presented concerning the mechanism of membrane stiffening due to type II modifications of spectrin.

Smart Home Precipitator for Biomass Furnaces: Design Considerations on a Small-Scale Electrostatic Precipitator

With the use of wood pellets becoming more popular for heating in private households, an increase in particulate matter problems across urban areas has been detected. As this contributes to significant health risks, effective reduction of dust emissions will become mandatory even for comparably small heating systems. The smart home precipitator was designed for this scope of application. This paper demonstrates the development steps aiming at a cost-effective precipitator that can be used for central heating processes up to 30 kW. It starts with research on the mechanical design and on the collection efficiency for the development of a space-saving structure with comparable large collection area. In a next step, two different circuit topologies for the high-voltage power supply have been analyzed. The latest and final work step in the design phase was the development of a smart and efficient control algorithm using fuzzy logic for dynamic optimization.

On two-dimensional slow viscous flows past obstacles in a half-plane

We consider a cylinder with arbitrary cross section moving in a viscous incompressible fluid parallel to a plane wall. Formal asymptotic expansions of the solution for small Reynolds numbers are constructed by using boundary integral equations of the first kind. In contrast to the problem without a wall, we show that there exists a unique solution to the zero th order problem. However, the problem considered here is still singular in the sense that we find the Stokes paradox in the next higher order problem. A justification of the formal asymptotic expansion for the first two terms is established rigorously.

Angle of Inclination of Tank-Treading Red Cells: Dependence on Shear Rate and Suspending Medium

Red cells suspended in solutions much more viscous than blood plasma assume an almost steady-state orientation when sheared above a threshold value of shear rate. This orientation is a consequence of the motion of the membrane around the red cell called tank-treading. Observed along the undisturbed vorticity of the shear flow, tank-treading red cells appear as slender bodies. Their orientation can be quantified as an angle of inclination (θ) of the major axis with respect to the undisturbed flow direction. We measured θ using solution viscosities (η0) and shear rates (γ˙) covering one and three orders of magnitude, respectively. At the lower values of η0, θ was almost independent of γ˙. At the higher values of η0, θ displayed a maximum at intermediate shear rates. The respective maximal values of θ increased by ∼10° from 10.7 to 104 mPas. After accounting for the absent membrane viscosity in models by using an increased cytoplasmic viscosity, their predictions of θ agree qualitatively with our data. Comparison of the observed variation of θ at constant γ˙ with model results suggests a change in the reference configuration of the shear stiffness of the membrane.

High voltage power supply and control technologies for electrostatic precipitators in biomass applications

This paper will show the specific requirements of high voltage power supplies and their control technology related to small size electrostatic precipitators as they will be used in applications of biomass fired furnaces in the near future. Unlike large ESPs, which are installed in heavy industrial plants (e.g. coal fired power stations or steelworks), the small electrostatic precipitators will mainly be used on private properties or small businesses and hence, be operated by people even without any technical background. Therefore, additional requirements need to be considered in the design of the power supply and the control of such a system, that full automatic operation with continuous optimization can be achieved without operator interaction or maintenance. Available power electronic topologies e.g. soft switched, resonant, or pulsed mode will be evaluated and classified by a strength and weakness analysis. Additionally, the collection efficiency is strongly depending on the control performance of the system. Critical operating situations like heat up phases with fairly unstable combustion processes need exact adaption of voltage and current dynamics to achieve the required precipitator performance. Unlike in large electrostatic precipitator plants, flashovers must be merely avoided due to the noise and EMI, which is not acceptable in domestic environments. Fuzzy Logic was tested for the control system to process current and voltage setpoints for the power electronic internal control system. Finally, an example ESP system setup in laboratory state and measured data will show some results to be discussed.

Smart Home Precipitator for biomass-furnaces: Design considerations on a small scale electrostatic precipitator

With the use of wood pellets becoming more popular for heating in private households, an increase in particulate matter problems across urban areas has been detected. As this contributes to significant health risks, effective reduction of dust emissions will become mandatory even for comparably small heating systems. The smart home precipitator was designed for this scope of application. This paper demonstrates the development steps aiming at a cost-effective precipitator that can be used for central heating processes up to 30 kW. It starts with research on the mechanical design and on the collection efficiency for the development of a space-saving structure with comparable large collection area. In a next step, two different circuit topologies for the high-voltage power supply have been analyzed. The latest and final work step in the design phase was the development of a smart and efficient control algorithm using fuzzy logic for dynamic optimization.