Polymer-blood interface

New experimental results on specific polymer material problems are presented in the last nine chapters. Several cases involve the study of polymers from commercial sources. The topics include (1) surface chemistry as induced by (a) outdoor weathering, (b) chemical reactions, and (c) plasma exposure (2) chemical bond formation at the polymer -metal interface and (3)biomaterials characterization and relationship to blood compatibility. 

In the case of the polymer-air interface, the ability to stick to the surface, resist wear or have a certain aesthetic characteristic can be dependent on the way the polymer molecules are organized at the interface. A number of physical phenomena are related to our understanding of the properties of an interface spreading of oil on a surface, adhesion of two bodies in contact (e.g. chewing gum sticking to a pavement), interaction of fluids with biological materials (e.g. blood in arteries), etc. 

Generally speaking, the surface free energy (Sre) reflects the chemical composition of the material and molecule orientation to its surface. In the case of PDLCs, orientation of the LC molecules inside the droplets and the interactions at the LC/ polymer matrix interface determine the capability of the material to be electrically controlled and used in the design of biosensors as blood or sperm testers (Lin et al. 2012). Moreover, values of SFE higher than 22 mN/m proved to be beneficial in maintaining multicellular structure (Hallab et al. 2001). 

Yoon el al. [112] reported an all-solid-state sensor for blood analysis. The sensor consists of a set of ion-selective membranes for the measurement of H+, K+, Na+, Ca2+, and Cl. The metal electrodes were patterned on a ceramic substrate and covered with a layer of solvent-processible polyurethane (PU) membrane. However, the pH measurement was reported to suffer severe unstable drift due to the permeation of water vapor and carbon dioxide through the membrane to the membrane-electrode interface. For conducting polymer-modified electrodes, the adhesion of conducting polymer to the membrane has been improved by introducing an adhesion layer. For example, polypyrrole (PPy) to membrane adhesion is improved by using an adhesion layer, such as Nafion [60] or a composite of PPy and Nafion [117],... 

The arrangement of molecular elements of a polymeric material at a blood-polymer interface generally is not known in detail x-ray photoelectron spectroscopy (XPS, also called ESCA) indicates that for block copolymers, polymers having large side groups of differing polarity and polyelectrolytes, the surface composition may be quite different from the bulk, stoichiometric composition (2). 

Therefore, this chapter presents preliminary evidence indicating the effect and interrelationship between primary and secondary molecular motions on thrombogenesis, independent of morphological order and/or crystallinity. The polymer selected for this study was an amorphous elastomeric hydrophobic polymer of poly[(trifluoroethoxy) (fluoroalkoxy)phosphazene] (PNF) I (5, 6). The salient aspects of this polymer are that (1) the onset of the secondary molecular motions occurs between -160° and - 120°C (2) the side chain motion can be altered by irradiation (ultraviolet, electron beam, or gamma) (3) no apparent ultrastructure morphology exists (4) the side chains can be derivatized (5) and (5) the polymer can be readily coated onto our extracorporeal test shafts (7) and irradiated accordingly. Additionally, contact angle measurements of the homopolymer (8) and the PNF (9), 19.7 and 15.0 dyn/cm2, respectively, indicated that the fluorinated side chains comprised the surface to be interfaced in the extracorporeal blood studies. 

The foreign body reaction occurring around soft tissue implants and thrombosis on surfaces in contact with blood are the major reactions encountered with implants. Both reactions involve the interaction of cells with the implant, especially in the later stages, and much previous study has therefore emphasized cellular events in the biocompatibility process. However, cells encounter foreign polymer implants under conditions that ensure the prior adsorption of a layer of protein to the polymer interface. The properties of the adsorbed layer are therefore important in mediating cellular response to the material. 

The first event that generally occurs after blood contacts a polymer surface is the formation of a protein layer at the blood-polymer interface (1). The formation of this protein layer is followed by the adherence of platelets, fibrin, and possibly leukocytes (2). Further deposition with entrapment of erythrocytes and other formed elements in a fibrin network constitutes thrombus formation. The growth of the thrombus eventually results in partial or total blockage of the lumen unless the thrombus is sheared off or otherwise released from the surface as an embolus (3). Emboli can travel downstream, lodge in vital organs, and cause infarction of tissues. The degree to which the polymer surface promotes thrombus formation and embolization, hemolysis, and protein denaturation determines its usefulness as a biomaterial (4). 

Probably no single causal mechanism functions in the calcification process of neointima-lined or smooth surface polyurethanes. Rather, surface calcification is most likely a result of the combination and interaction of mechanical and surface chemical effects at the blood-surface interface. Mechanical damage to or physical imperfections on the polymeric substrate in smooth surface devices or the neointima lining of textured bladders may be capable of inducing a deposition and mineralization process. Calcification of tissue valve leaflets has been proposed to result from the diffusion of blood elements into mechanically disrupted tissue (10), thus providing a site for mineralization to occur. Likewise, deposits of calcium-chelating proteins or lipids in defects in neointimal tissue or the polymer substrate may act as precursor binding sites for the observed mineralization. 

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