Platelet-surface compatibility

Columns packed with small polymer-coated glass beads, because of their large surface area to void volume ratio, provide a severe test of platelet-surface compatibility. A 2.5 ml column packed with 0.35 - 0.50 mm diameter beads provides -200 cm of surface area which, at a point in time, is exposed to -0.6 ml of blood. This is equivalent to spreading a film of blood 30 urn thick on a flat surface and, assuming a mean capillary diameter of 7 urn, represents approximately one-twentieth the surface to volume ratio of a capillary bed. 

Ikada and coworkers also studied the blood compatibility and protein denaturation properties of heparin covalently and ionically bound onto polymer surfaces [513], Both types of bound heparin gave deactivation of the coagulation process. Clotting deactivation was attributed to a heparin/ antithrombin III complex by covalently bound heparin which gave adsorbed protein denaturation and platelet deformation as compared with lack of these features with ionically bound heparin. 

Problems of desorption and loss of activity encountered with natural heparin have led numerous workers to explore synthetic heparin-like polymers or heparinoids, as reviewed by Gebelein and Murphy [475, 514, 515]. The blood compatibility of 5% blended polyelectrolyte/polyfvinly alcohol) membranes was studied by Aleyamma and Sharma [516,517]. The membranes were modified with synthetic heparinoid polyelectrolytes, and surface properties (platelet adhesion, water contact angle, protein adsorption) and bulk properties such as permeability and mechanical characteristics were evaluated. The blended membrane had a lower tendency to adhere platelets than standard cellulose membranes and were useful as dialysis grade materials. 

Polyethylene glycols are well known as protein-compatible molecules when coated or grafted onto surfaces. Both protein and platelet adsorptions to polyethylene glycol (PEG)-modified surfaces were shown to be reduced by PEG chains when attached to surfaces at one end of a molecule. Adsorption and platelet attachment were shown to be inversely proportional to the length of the PEG chain 100 monomer units provided minimal adsorption and adherence.- ... 

Blood-compatible polymer materials are required to inhibit both platelet adhesion and coagulation just as the endothelial on the polymer surface. It is known that there are many investigations in the design and the synthesis of socalled antithrombogenic materials. The immobilization of biologically active substances such as heparin [74, 75], urokinase [76], and prostaglandins [77-81] is one of the practical approaches. 

Fig.7 Blood compatibility of PEU surfaces modified by MPEO-derived SMAs. A Simplified cascade model for material-induced blood coagulation highlighting three clotting pathways plasma fibrin formation, platelet aggregation, and hemolysis-inflammation, respectively characterized by B-C...

The in vitro study of the hemocompatibility of biomaterials requires the consideration of many parameters, static or dynamic contact, flow rate, wall shear rate, form of biomaterial to be tested, pathway to consider (platelet adhesion, platelet activation, complement activation, contact phase activation etc..) and duration of contact(39). It has previously been demonstrated t t hemodynamic circumstances play a significant role in determining localization, growth and fiagmentation of thrombi and platelet adhesion in vivo, and that flow rate controls platelet transport to a surface and their adhesion (40). This evidence is siqtpoited by observed differences in platelet activity predominance in venous and arterial flow (41). Qearly, defining the blood compatibility of a material is a conqrromise between a number of these factors. 

These studies indicate that heparin directly affixed to a surface does not provide optimal, solution-like, anticoagulant behavior. The immobilization of heparin directly to the polymer surface resulted in alterations of the surface properties relative to control surfaces, which greatly influenced the plasma protein adsorption characteristics, a controlling factor in platelet adhesion and overall blood compatibility (12). 

This approach has also been used in the treatment of acute Intoxication in pediatric patients [21], The amberlite resin hemoperfusion system has a high clearance for many drugs [22]. However, the problem of platelet depletion was noted with this system. More recently, our approach of using an albumin-coating to make the particle surface more blood compatible for hemoperfusion... 

In order to determine the blood compatibility of the plasma treated materials, platelet adhesion studies were conducted since platelet adhesion is one of the most important steps during blood coagulation on artificial surfaces. Blood samples were drawn from healthy canines (adult mongrel dog) which were not on medication. Platelet-rich plasma was obtained and the platelets were washed by albumin density gradient centrifugation in Hepes-Tyrodes buffer solution of pH 7.3 by a modification of Walsh and Griffin s... 

Liu JH, Jen HL, Chung YC (1999) Surface modification of polyethylene membrans using phos-phorylcholine derivatives and their platelet compatibility. J Appl Polym Sci 74 2947-2954 Loh SK, ChooYM, Cheng SF, Ma A (2006) Recovery and conversion of palm olein-derived used frying oil to methyl esters for biodiesel. J Oil Palm Res 18 247-252 Loo C-Y, Lee W-H, Tsuge T, Doi Y, Sudesh K (2005) Biosynthesis and characterization of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) from pahn oQ products in a Wautersia eutropha mutant. Biotechnol Lett 27 1405-1410... 

Surface modification with hydrophilic polymers, such as poly(ethylene oxide) (PEO), has been beneficial in improving the blo( compatibility of polymeric biomaterials. Surface-bound PEO is expected to prevent plasma protein adsoiption, platelet adhesion, and bacterial adhesion by the steric repulsion mechanism. PEO-rich surfaces have been prepared either by physical adsorption, or by covalent grafting to the surface. Physically adsorbed PEO homopolymers and copolymers are not very effective since they can be easily displaced from the surface by plasma proteins and cells. Covalent grafting, on the other hand, provides a permanent layer of PEO on the surface. Various methods of PEO grafting to the surface and their effect on plasma protein adsorption, platelet adhesion, and bacterial adhesion is discussed. 

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