Polymer depletion layers as measured by electrophoresis

Electroosmosis of Viscoelastic Fluids: Role of Wall Depletion Layer

We investigate electroosmotic flow of two immiscible viscoelastic fluids in a parallel plate microchannel. Contrary to traditional analysis, the effect of the depletion layer is incorporated near the walls, thereby capturing the complex coupling between rheology and electrokinetics. Toward ensuring realistic prediction, we show the dependence of electroosmotic flow rate on the solution pH and polymer concentration of the complex fluid. In order to assess our theoretical predictions, we have further performed experiments on electroosmosis of an aqueous solution of polyacrylamide (PAAm). Our analysis reveals that neglecting the existence of a depletion layer would result in grossly incorrect predictions of the electroosmotic transport of such fluids. These findings are likely to be of importance in understanding electroosmotically driven transport of complex fluids, including biological fluids, in confined microfluidic environments.

Effects of neutral polymers on the mechanics of red blood cell adhesion onto coated glass surfaces

BACKGROUND

Cell-cell and cell-surface adhesion modulated by water-soluble polymers continues to be of current interest, especially since prior reports have indicated a role for depletion-mediated attractive forces.

OBJECTIVE

To determine the effects of concentration and molecular mass of the neutral polymer dextran (40 kDa to 28 MDa) on the adhesion of human red blood cells (RBC) to coated glass coverslips.

METHODS

Confocal-reflection interference contrast microscopy (C-IRM), in conjunction with phase contrast imaging, was utilized to measure the adhesion dynamics and contact mechanics of RBC during the initial stages of cell contact with several types of substrates.

RESULTS

Adhesion is markedly increased in the presence of dextran with a molecular mass ⩾ 70 kDa. This increased adhesiveness is attributed to reduced surface concentration of the large polymers and hence increased attractive forces due to depletion interaction. The equilibrium deformation of adhering RBC was modeled as a truncated sphere and the calculated adhesion energies were in close agreement with theoretical results.

CONCLUSIONS

These results clearly demonstrate that polymer depletion can promote RBC adhesion to artificial surfaces and suggest that this phenomenon may play a role in other specific and non-specific cell-cell interactions, such as rouleau formation and RBC-endothelial cell adhesion.

Depletion-mediated red blood cell aggregation in polymer solutions

Polymer-induced red blood cell (RBC) aggregation is of current basic science and clinical interest, and a depletion-mediated model for this phenomenon has been suggested; to date, however, analytical approaches to this model are lacking. An approach is thus described for calculating the interaction energy between RBC in polymer solutions. The model combines electrostatic repulsion due to RBC surface charge with osmotic attractive forces due to polymer depletion near the RBC surface. The effects of polymer concentration and polymer physicochemical properties on depletion layer thickness and on polymer penetration into the RBC glycocalyx are considered for 40 to 500 kDa dextran and for 18 to 35 kDa poly (ethylene glycol). The calculated results are in excellent agreement with literature data for cell-cell affinities and with RBC aggregation-polymer concentration relations. These findings thus lend strong support to depletion interactions as the basis for polymer-induced RBC aggregation and suggest the usefulness of this approach for exploring interactions between macromolecules and the RBC glycocalyx.

Electrophoretic mobility of human erythrocytes in the presence of poly(styrene sulfonate)

The adsorption and depletion of the anionic polymer poly(styrene sulfonate) (PSS) on fresh human red blood cells (RBC) were investigated by measuring RBC electrophoretic mobility as a function of polymer molecular mass (48-2610 kDa), ionic strength (15 and 150 mM NaCl) and polymer concentration (<or= 1.5 g/dL). A subset of studies was carried out using fixed and PSS-coated cells. Our results indicate a marked increase of mobility with molecular mass and polymer concentration. Adsorption of PSS onto fresh RBC was weak, with normal mobility restored following washing cells in PSS-free buffer. Calculated zeta potentials based upon mobility and medium viscosity rose up to 618 mV for 2610 kDa PSS compared to 13 mV for control cells, thus suggesting significant polymer depletion at the cell surface; fixed and PSS-coated RBC were insensitive to medium viscosity, also validating this depletion layer hypothesis. Calculated values of increased RBC surface charge were used to estimate polymer adsorption per cell; these estimates indicated linear adsorption isotherms and binding levels consistent with studies employing neutral polymers. In overview, our results suggest the usefulness of microelectrophoresis methods for studies of RBC interactions with charged polymers or proteins, and the value of this approach for future studies using proteins known to affect RBC-RBC interactions.

Capillary electrophoresis of subcellular-sized particles

Utilization of capillary electrophoresis (CE) for characterization and analytical separation of submicron- and micron-sized organic and inorganic particles as well as biological vesicles is reviewed. CE has been applied to charged polystyrene size standards, inorganic and organic colloidal particles, lipoprotein particles, liposomes, microsomes and viruses. These particle separations generally occur in a size-dependent manner and provide values of electrophoretic mobility which are in good agreement with those obtained by other electrophoretic techniques.

Mechanistic insights derived from retardation and peak broadening of particles up to 200 nm in diameter in electrophoresis in semidilute polyacrylamide solutions

Rigid spherical particles in the size range of 5-200 nm diameter were subjected to capillary zone electrophoresis (CZE) in semidilute solutions of uncross-linked polyacrylamide of M(r) 5, 7 and 18 x 10(6) (PA-5, -7 and -18, respectively) of varying concentrations up to 1.6% and at field strengths varying from 68 to 270 V/cm. For all particles under study, the experimental Ferguson plots, log(mobility) vs. polymer concentration, permit a linear approximation. Their slope, the retardation coefficient KR = delta log (mobility)/delta (concentration), for particles smaller than 30 nm in diameter increased with particle size in PA-5 and -7 independently of electric field strength and polymer M(r). The KR of particles of 30 nm in diameter or more was found to be independent of particle size at the lowest field strength used but to decrease with it at the higher values of field strength. The decrease was parallel but shifted to higher values of retardation when the polymer M(r) increased from 5 to 7 x 10(6). With a decreasing ratio of average mesh size of the polymer network, zeta, to particle radius, R, the approach to "continuity" of the polymeric medium (zeta/R << 1) with both increasing particle size and polymer concentration does not result in the retardation behavior expected according to the macroscopic (bulk) viscosity of the solution. These experimental observations were hypothetically interpreted in terms of a transition to a retardation mechanism comprising the formation of a polymer depletion layer near the particle surface--polymer solution interface. Peak width exhibited an overall increase with PA-7 concentration for all particles studied. For particles of 30 nm in diameter or less, the increase was steepest when the radius of the particle was approximately commensurate with zeta at a given polymer concentration. For the largest particle, 205 nm in diameter, peak broadening with polymer concentration was found to correlate linearly with peak asymmetry. CZE of the particles in PA-18 solutions revealed abnormal behavior, with both mobility and peak width remaining near-constant up to a concentration of 0.08% and sharply declining at higher concentrations. The decline of relative mobility is the same-for the entire particle size range used, while peak width declines in direct relation to particle size.