Polymers surfaces, platelet retention

Several polymers were evaluated in the form of a surface coating on glass beads packed in columns to determine their ability to retain platelets when whole human blood passes over the surface. This ability was measured as the platelet retention index p, the fraction of platelets retained on the column. Lowest values of p were found for poly(ethylene oxide), polypropylene oxide), poly(tetramethylene oxide) (in the form of polyurethanes), and polydimethylsiloxane. Highest values (around 0.8) were found for cross-linked poly(vinyl alcohol) and the copolymers of ethylenediamine with diisocyanates. Intermediate values were found for polystyrene and its copolymers with methyl acrylate, for polyacrylate, and for poly(methyl methacrylate). The results are interpreted in terms of possible hydrophobic and hydrogen bonding interactions with plasma proteins. 

Figure 1. In vitro test for platelet retention, using plastic columns, packed with fine glass beads whose surfaces are thereafter coated with test polymer (ambient temperature = 37°C).

We tested the hypothesis that the platelet retention index should increase as polymer composition varied from 100 mol % methyl acrylate to 100 mol % styrene, the respective platelet retention indices p being about 0.25 and 0.55 for the homopolymers. The results are shown in Figure 3, wherein p increases to a maximum near 40% styrene. When the surfaces were incubated in platelet-poor plasma before contact with whole blood, the values of p were much reduced (from 0.25 to 0.05 for methyl acrylate) for copolymers containing up to 60 mol % styrene. Copolymers of higher styrene content were not rendered significantly less retentive by plasma pretreatment. 

Before long-term hemocompatibility can be expected for any material, the nature of polymer surface-protein interaction must be established in more detail the way in which the polymer surface alters itself (rotation of segments, side groups, chain refolding, etc.) in response to the protein species the way the protein is altered in conformation (if at all) upon adsorption by the surface and how this conformational change provokes platelet retention. Of course, longer-term ex vivo or in vivo studies also will be necessary. 

These polyurethanes (and their precursors or analogues) were cast from an appropriate solvent, usually DMF, onto the glass bead surfaces used in the in vitro test for platelet retention (4), or for the thrombin absorption test used previously (2). Crystals of KBr for Fourier transform infrared (FTIR) spectroscopy and glass microscopic slides for examination by XPS (ESCA) served as supports for polymers cast from the same solvents. Concentration of polymer (5 wt %), temperature of casting... 

By XPS, much less nitrogen was found in their surfaces than in most of the polymers reported in Table III, and < >, the fraction of ethereal carbon, ranged upward from 0.8. When PEO 3500 was used as the diol, the XPS scans of the segmented polyurethane were identical with the PEO calibration standard i.e., no nitrogen signal could be detected at any level of amplification, and the expanded C Is scan corresponded to only carbon bonded to ether i.e., cf> = 1. Platelet retention indices were 0.05 or less. 

Concentration of the polymer, temperature, and evaporation conditions under argon were maintained as closely as possible, specially with casting microscope slides and glass beads, so that the surface morphology in ESCA and platelet retention tests were about the same. 

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