W. Zingg

Surface thermodynamics of leukocyte and platelet adhesion to polymer surfaces

Adhesion of leukocytes and platelets to solid substrates of different surface tensions and hence different wettability is studied from a thermodynamic point of view. A simple thermodynamic model predicts that cellular adhesion should increase with increasing surface tension of the solid substrate if the surface tension of the medium in which the cells are suspended is lower than the surface tension of the cells. If the surface tension of the suspending medium is higher than that of the cells, the opposite behavior is predicted. These predictions are borne out completely by neutrophil adhesion tests, where the surface tension of the aequeous suspending medium is varied by addition of dimethyl sulfoxide (DMSO). Platelet adhesion experiments also confirm these predictions, the only difference being that surface tensions of the suspending medium above that of the platelets cannot be realized, owing to exudation of surface active solutes from the platelets. Utilization of the thermodynamic prediction that cellular adhesion should become independent of the surface tension of the substrate when the surface tensions of the cells and that of the suspending medium are equal leads to a value of the surface tension of neutrophils of 69.0 erg/cm2,† in excellent agreement with the value obtained from contact angles measured on layers of cells.

Determination of the surface tension of proteins I. Surface tension of native serum proteins in aqueous media

The desorption patterns of serum proteins in hydrophobic chromatography suggest that serum proteins that remain immersed in an aqueous medium and do not become in a protein-air interface are very hydrophilic. Contact angle measurements on fairly thick layers of hydrated serum proteins, formed on ultrafiltration membranes, yield surface tensions that correlate well with the degree of hydrophilicity derived from desorption data obtained by hydrophobic chromatography. For further confirmation the absorptivity of four human serum proteins was measured with respect to surfaces of different polymers of various surface tensions, for solution in aqueous solvents of different surface tensions. The surface tension of the solvent from which a dissolved protein adsorbs to precisely the same extent onto all solid substrates (regardless of their surface tensions) is equal to the surface tension of that protein. The surface tensions found by the contact angle (first value given) and by the protein adsorption methods (second value given) were. in erg/cm2; alpha 2-macroglobulin, 71.0, 71.0; serum albumin, 70.5, 70.2; immunoglobulin M, 69.5, 69.4; immunoglobulin G, 67.4, 67.7.

Erythrocyte adhesion to polymer surfaces

Abstract Thermodynamic model considerations suggest that the adhesion of biological cells to polymeric surfaces depends on the relative magnitude of the surface tension γSV of the substrates, the surface tension γCV of the cells, and the surface tension γLV of the suspending liquid medium. Glutaraldehyde-fixed erythrocytes are identified as suitable model particles for such studies. Experimental results of the extent of erythrocyte adhesion from suspension of mixtures of buffer and varying amounts of dimethyl sulfoxide (DMSO) confirm qualitatively and in certain aspects quantitatively the thermodynamic predictions. Specifically, the prediction that the extent of cell adhesion should become independent of substrate surface tension when γLV = γCV is confirmed by experiment. On the other hand for this case the free energy of adhesion predicting zero cell adhesion is not experimentally confirmed completely. However, performing these experiments with distilled water rather than buffer reduces erythrocyte adhesion to a negligible level. It emerges that van der Waals interactions are, by a large margin, the most important forces involved in cell adhesion to polymer surfaces. Nevertheless, not only divalent cations (through bridging effects), but also monovalent cations play a certain, though limited, role.

Measurement of surface tensions of blood cells and proteins.

Department o f Mechanical Engineering Institute of Biomedical Engineering f Institute of Medical Sciences University of Toronto Toronto, Canada, M5S IA4 lJ Research Institute Hospital for Sick Children Toronto, Canada, M5G 1x8 Department o f Microbiology Department of Chemical Engineering State University of New York at Buffalo Buffalo, New York 14214 Separation Processes Branch NASAIGeorge C. Marshall Space Flight Center Huntsville, Alabama 35812

Determination of surface tensions of proteins II. Surface tension of serum albumin, altered at the protein-air interface

Serum albumin, which itself has a surface tension of congruent to 70.3 erg/cm2, when dissolved in water lowers the surface tension of water from 72.5 to congruent to 50 erg/cm2, as measured by a variety of means, including the pendant drop, the Wilhelmy plate and the platinum ring methods. Equally low and even lower surface tensions are found with the contact angle method, on a thin layer of albumin that had been adsorbed onto a low energy surface and subsequently exposed to air. Surface tensions of drops of albumin solutions varying in concentration from 0.01 to 5.5% (w/v) yielded, with a contact angle method, values that only varied between 67 and 61 erg/cm2. With the pendant drop, the Wilhelmy plate and the platinum ring methods, one essentially measures the surface tension at the air-liquid interface, at which proteins tend to adsorb, and where reversible or irreversible reorientation can be expected. The same holds for a thin layer of protein adsorbed onto a low energy surface, exposed to air. Thus, when through the very act of surface tension measurement, or after adsorbing protein onto a substrate, protein is exposed at the air-liquid interface, it apparently loses the pronounced hydrophilicity characteristic of its native hydrated state and manifests through reorientation a much more hydrophobic tertiary configuration.

Phagocytosis of bacteria by platelets: Surface thermodynamics

Abstract Engulfment of four species of bacteria by pig platelets was investigated under well defined in vitro conditions from a surface thermodynamics aspect. A simple thermodynamic model predicts that bacterial ingestion should increase with increasing bacterial surface tension if the surface tension of the liquid medium in which the platelets and bacteria are suspended is lower than the surface tension of the platelets themselves. The opposite behavior is predicted if the surface tension of the liquid medium is higher than that of the platelets. These predictions have been confirmed by phagocytosis experiments, where the platelets and bacteria opsonized and nonopsonized) have been suspended in aqueous media of different surface tensions achieved through the addition of varying amounts of dimethyl sulfoxide. Use of the thermodynamic prediction that bacterial engulfment should become independent of the surface tension of the ingested bacteria when the surface tensions of the platelets and that of the suspending medium are equal gives rise to an experimental value of the surface tension of platelets of 67.6 ergs/cm 2 , in excellent agreement with the value obtained via an equation—of-state approach from contact angles measured on layers of platelets. In addition the maximum value of the free energy of engulfment for any particular bacterium—platelet system occurs when the surface tension of the liquid equals that of the bacteria. The level of engulfment should be a minimum under such conditions thus permitting the surface tension determination of the bacteria. Experimental determinations of the minima for the four bacterial species employed gives rise to bacterial surface tensions which conform very closely to the values obtained from contact angle measurements on layers of bacteria. Thus platelet phagocytosis of bacteria provides an independent alternative method for determining the surface tension of both platelets and bacteria. The experimental data further suggest that platelet interaction with bacteria is nonspecific in the sense that the phagocytosis does not appear to be modulated by immunoglobulin F c receptors.

Contact angle kinetics of human albumin solutions at solid surfaces

Abstract Contact angle kinetics of sessile drops of albumin solution on hydrophilic acetal and hydrophobic FC 721 surfaces were measured using axisymmetric drop shape analysis. Youngs equation is used to calculate the solid/liquid interfacial tension from measured contact angles and surface tensions as a function of time. The change in solid/liquid interfacial tension is a result of protein adsorption. It indicates that at the hydrophilic acetal surface the albumin molecules, interact only weakly, whereas the interaction with the hydrophobic FC 721 surface is quite strong.

Determination of the surface tension of biological cells using the freezing front technique

The freezing front technique for solid surface tension measurements was used to obtain the surface tensions of glutaraldehyde-fixed human erythrocytes, and fresh human lymphocytes and grnulocytes in aqueous media. The results agree well with the values obtained by other methods and indicate that the freezing front technique is sufficiently sensitive to detect small differences (of the order of 0.1 ergs/cm2) in surface tension. This property, along with a number of applications for which it is uniquely suited makes the freezing front technique an important new approach to the measurement of the surface tensions of biological cells and of small particles in general.

Axisymmetric drop shape analysis (ADSA) applied to protein solutions

Abstract Axisymmetric drop shape analysis (ADSA) is applied to study time evolution of profiles of pendant and sessile insulin and protein solution drops. Human, bovine and porcine insulin preparations are studied with respect to surface tension kinetics (pendant drop) and contact angle kinetics on hydrophobic FC-721 (fluorocarbon) surfaces. ADSA permits high resolution between the different insulin preparations, which can be distinguished by slight but reproducible differences in surface tensions and contact angles. Application of Youngs equation, however, results in nearly identical solid-protein solution interfacial tensions. The time dependence of contact angles of albumin and fibrinogen solution drops on Acetal and FC-721 is presented. Scatter of data is more pronounced on the hydrophilic Acetal surface than on the hydrophobic FC-721 one. Further, an irregular motion of the three-phase contact line is observed. The applicability of Youngs equation to albumin and fibrinogen solutions is discussed in terms of the solid-protein solution interfacial tensions.

Platelet adhesion to smooth and rough hydrophobia and hydrophilic surfaces under conditions of static exposure and laminar flow

Abstract Platelet adhesion was measured on well-defined smooth surfaces and on the same surfaces with a known amount of roughness, under static conditions and in a new flow cell producing laminar flow. The results obtained with pig platelet suspensions show that under static conditions surface roughness does not influence platelet adhesion. With the flow cell an effect of roughness could be demonstrated, both with pig platelet suspension and with fresh canine blood: platelet adhesion decreased on a rough hydrophilic surface (glass) when compared with the same smooth surface, and increased on a rough hydrophobic surface (silane) when compared with the same smooth surface at the flow rates studied.