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Abstract
Various factors affecting the extent of adhesion of hydrophilic particles to polymer surfaces have been evaluated. Specifically the kinetics of adhesion, and the influence of substrate surface tension, ionic strength, pH, and surface tension of the suspending liquid medium have been investigated. In addition the role of divalent cations has been assessed. The substrates examined exhibit a wide range of wettability and the hydrophilic particles used were both fresh and glutaraldehyde-fixed human erythrocytes.
The extent of particle adhesion to various polymer surfaces is also a function of solution pH and ionic strength. At a constant pH of 6 virtually no erythrocyte adhesion occurs at ionic strengths less than 0.05. Adhesion increases with increasing ionic strength and reaches a limiting plateau value at an ionic strength of approximately 0.1. The actual level of the plateau value is quite different for each polymer. Particle adhesion is also pH dependent. For conditions of constant ionic strength and variable pH the extent of erythrocyte adhesion at and below pH 6 is constant for each polymer substrate and extends over a substantial domain of high ionic strength and low pH. As the pH is increased erythrocyte adhesion decreases and reaches a second plateau at pH 8 and above.
The pH and ionic strength studies suggest that at low ionic strength double layer repulsion plays a critical role in preventing particle adhesion. With increasing ionic strength the double layer becomes more compressed allowing a closer approach of the particles to the substrate and hence an increased van der Waals attraction giving rise to increased erythrocyte adhesion.
For experiments in which the particles are suspended in solutions with an ionic strength of 0.15 or greater the kinetic studies reveal that the extent of adhesion increases rapidly initially and reaches a plateau value after approximately 30 mins. There is no evidence for a lag-time in the onset of particle adhesion, suggesting that electrostatic double layer forces are negligible under these experimental conditions. For a bulk particle concentration 1 × 106 cells/ml the plateau value of adhesion corresponds to a surface coverage of no more than 10% for the most densely covered substrate. For a given set of experimental conditions the level of saturation adhesion is determined by the wettability of the substrate material. The extent of particle adhesion to any given substrate material is influenced considerably by the respective surface tension of the adhering particles (γ PV ), the substrate material (γ SV ) and the suspending liquid media (γ LV ). For conditions where γ LV > γ PV the extent of particle adhesion decreases with increasing substrate surface tension, γ SV . For conditions where γ LV < γ PV the opposite pattern of behaviour is found.