Effects of oligoethylene oxide monoalkyl(aryl) alcohol ether grafting on the surface properties and blood compatibility of a polyurethane

Efficient One-Step Passivation of Polyurethane Using Transurethanization

The uncontrolled accumulation of biological materials on the surface of medical devices through protein adsorption or cell adhesion causes adverse biological reactions in the living host system, leading to complications. In this study, poly(ethylene glycol) (PEG) is successfully grafted onto polyurethane (PU) surfaces by using a new strategy through a simple and efficient transurethanization reaction. The PEG hydroxyl group is deprotonated and then reacted with the PU surface to provide antiadhesive hydrophilic surfaces in a single step. Surface analysis techniques proved the grafting to be efficient and the formation of a hydrophilic polymeric layer at the surface of PU. Biological assays showed that the surface modification induced lower protein adsorption, cell, platelet, and bacterial adhesion than untreated surfaces, showing a potential for biomedical applications.

Poly(ether urethane)s incorporating long alkyl side-chains with terminal carboxyl groups as fatty acid mimics: synthesis, structural characterization and protein adsorption

The object of this work was to produce polyurethanes with greater affinity for albumin (Alb) and improved hemocompatibility by introduction of carboxyl-terminated alkyl side-chains that better mimic fatty acids, in contrast to methyl terminated alkyl side-chains used previously. Synthesis of poly(ether urethane)s (PEUs) with long alkyl side-chains via a multi-step solution addition polymerization is described. The synthesis is based upon the polymerization of a diisocyanate pre-polymer with various chain extenders and reaction with Br-terminated compound in the final stage. The side-chains had terminal methyl or carboxylic groups, and were attached either directly to the polymer backbone or to an oligo(ethylene glycol) spacer. The bulk structure of the PEUs was confirmed by 1H-NMR and the surface polymer structure was characterized by ToF-SIMS. The influence of the incorporated C16-alkyl, C16-carboxyalkyl and oxyethylene-C16-carboxyalkyl side-chains attached to the polymer backbone on fibrinogen (Fg) and Alb adsorption from blood plasma, and Fg adsorption from buffer solutions and binary mixtures with Alb was measured. Incorporation of C16-alkyl or C16-carboxyalkyl side-chains into PEUs caused relatively small changes in Fg and Alb adsorption. PEUs with oxyethylene-C16-carboxyalkyl side-chains exhibited the lowest Fg adsorption and the highest Alb adsorption among all the tested polymers.

Synthesis, permeability and biocompatibility of tricomponent membranes containing polyethylene glycol, polydimethylsiloxane and polypentamethylcyclopentasiloxane domains

The synthesis of "smart" tricomponent amphiphilic membranes containing poly(ethylene glycol) (PEG), polydimethylsiloxane (PDMS) and polypentamethylcyclopentasiloxane (PD(5)) domains is described. Contact angle hysteresis indicates that in air, the surfaces of such PEG/PD(5)/PDMS membranes are enriched by the hydrophobic components, PDMS and PD(5), while in water, the surfaces are rich in the hydrophilic PEG. The oxygen permeability of a series of membranes with varying M(c,hydrophilic) (M(n,PEG)=4600, 10,000 and 20,000 g/mol) and varying PEG/PD(5)/PDMS compositions was studied. Oxygen permeability increased with the amount of PDMS in the membrane. The molecular weight cut-off (MWCO) ranges and permeability coefficients of insulin through a series of PEG/PD(5)/PDMS(=29/14/57) membranes with varying M(c,hydrophilic) were determined. Insulin permeability is directly related to M(c,hydrophilic) of the membrane. MWCO studies show that the membranes are semipermeable to, i.e., allow the transport of smaller proteins such as insulin (M(n)=5733 g/mol, R(s)=1.34 nm) and cytochrome c (M(n)=12,400 g/mol, R(s)=1.63 nm), but are barriers to larger proteins such as albumin (M(n)=66,000 g/mol, R(s)=3.62 nm). Implantation of representative membranes in rats showed them to be biocompatible. According to these studies, PEG/PD(5)/PDMS membranes may be suitable for biological applications, e.g., immunoisolation of cells.

Improved blood compatibility and decreased VSMC proliferation of surface-modified metal grafted with sulfonated PEG or heparin

Although the technique of coronary stenting has remarkably improved long-term results in recent years, (sub)acute thrombosis and late restenosis still remain problems to be solved. Metallic surfaces were regarded as thrombogenic, due to their positive surface charges, and stenosis resulted from the activation and proliferation of vascular smooth muscle cells (VSMCs). In this study, a unique surface modification method for metallic surfaces was studied using a self-assembled monolayer (SAM) technique. The method included the deposition of thin gold layers, the chemisorption of disulfides containing functional groups, and the subsequent coupling of PEG derivatives or heparin utilizing the functional groups of the disulfides. All the reactions were confirmed by ATR-FTIR and XPS. The surface modified with sulfonated PEG (Au-S-PEG-SO3) or heparinized PEG (Au-S-PEG-Hep) exhibited decreased static contact angles and therefore increased hydrophilicity to a great extent, which resulted from the coupling of PEG and the ionic groups attached. In vitro fibrinogen adsorption and platelet adhesion onto the Au-S-PEG-SO3 or Au-S-PEG-Hep surfaces decreased to a great extent, indicating enhanced blood compatibility. This decreased interaction of the modified surfaces should be attributed to the non-adhesive property of PEG and the synergistic effect of sulfonated PEG. The effect of the surface modification on the adhesion and proliferation of VSMCs was also investigated. The modified Au-S-PEG-SO3 or Au-S-PEG-Hep surfaces also exhibited decreased adhesion of VSMCs, while the deposited gold layer itself was effective. The enhanced blood compatibility and the decreased adhesion of VSMCs on the modified metallic surfaces may help to decrease thrombus formation and suppress restenosis. It would therefore be very useful to apply these modified surfaces to stents for improved functions. A long-term in vivo study using animal models is currently under way.

Novel dendrimer based polyurethanes for PEO incorporation

A series of segmented polyurethanes based on methylene diisocyanate/poly (tetramethylene oxide) and chain extended with either ethylene diamine or butane diol in combination with a generation 2 polypropylenimine octaamine dendrimer were synthesized. For polymer synthesis, the dendrimers were protected with either t-boc or Fmoc groups and were incorporated into the polyurethane microstructure to permit further functionalization with biologically active groups. Following deprotection, the dendrimers were reacted with succinimidyl propionate polyethylene oxide (SPA-PEO) to improve the protein resistance of the polymers and to examine the potential of this technique for polymer functionalization. Different synthesis techniques were examined to optimize the incorporation of the PEO into the polymer microstructure. Incorporation of the dendrimers and the PEO were confirmed by NMR and FTIR. Gel permeation chromatography was used to examine the molecular weights of the various polyurethanes. The dendrimer incorporated polymers had significantly lower molecular weights than the ED or BDO chain extended controls, likely due to lower reactivity of the dendrimers as a result of steric factors. Following PEO reaction, the molecular weights of the resultant polymers were consistent with the levels of PEO incorporation noted by comparison of peak intensities in the NMR spectra. Due to the highly hydrophilic nature of the PEO, some migration to the polymer surface was expected. Water contact angles and XPS, used to characterize the surfaces, suggest that there was some PEO enrichment at the surface of the polymers. Adsorption of radiolabeled fibrinogen to the polymer surfaces was decreased by a factor of approximately 40% in some of the PEO incorporated polymers. There were also differences in the patterns of plasma protein adsorption on the various surfaces as evaluated by SDS PAGE and immunoblotting. Therefore, the use of dendrimers in biomaterials for incorporation of a large number of functional groups seems to be promising.

Surface modification using silanated poly(ethylene glycol)s

Surface-grafted poly(ethylene glycol) (PEG) molecules are known to prevent protein adsorption to the surface. The protein-repulsive property of PEG molecules are maximized by covalent grafting. We have synthesized silanated monomethoxy-PEG (m-PEG) for covalent grafting of PEG to surfaces with oxide layers. Two different trialkoxysilylated PEGs were synthesized and characterized. The first trialkoxysilylated PEG was prepared by direct coupling of m-PEG with 3-isocyanatopropyltriethoxysilane through a urethane bond (silanated PEG I). The other silanated PEG (silanated PEG II) containing a long hydrophobic domain between PEG and a silane domain was prepared by reacting m-PEG with 1,6-diisocyanatohexane and 10-undecen-1-ol in sequence before silylation with 3-mercaptopropyl trimethoxysilane. Silanated PEGs I and II were grafted onto glass, a model surface used in our study. The PEG-grafted glass surfaces were characterized by contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Although contact angle did not change much as the bulk concentration of silanated PEG used for grafting increased from 0.1 to 20 mg/ml for both PEGs I and II, the surface atomic concentrations from XPS measurements showed successful PEG grafting. Surface PEG grafting increased concentration of surface carbon but decreased silicone concentration. The high resolution C1s spectra showed higher ether carbon with lower hydrocarbon compositions for the PEG-grafted surfaces compared to the control surface. AFM images showed that more PEG molecules were grafted onto the surface as the bulk concentration used for grafting was increased. AFM images of the dried surfaces showed that the surfaces were not completely covered by PEG molecules. After hydration, however, the surface appears to be covered completely probably due to the hydration of the grafted PEG chains. Glass surfaces modified with silanated PEGs reduced fibrinogen adsorption by more than 95% as compared with the control surface. Silanated PEGs provides a simple method for PEG grafting to the surface containing oxide layers.

Complement activation by PEO-grafted glass surfaces

Activation of the complement system is one way in which the human body reacts to foreign materials that come in contact with blood. Poly(ethylene oxide) (PEO) has been used quite frequently to modify biomaterial surfaces to prevent protein adsorption and cell adhesion. Despite extensive use of PEO, however, PEO-induced complement activation has not been examined before. We examined the complement activation by PEO chains grafted to glass surfaces. PEO was grafted to trichlorovinylsilane-treated glass (TCVS-glass) by gamma-irradiation using PEO homopolymer, Pluronic F108 (PF108), and PEO-polybutadiene-PEO triblock copolymer (COP5000). Complement activation was assessed by measuring the plasma C3a level. Of the three polymers grafted (PEO, PF108, and COP5000), only PF108 showed significant increases in complement activation over controls. Complement C3a production on PF108-grafted glass was linearly dependent on surface concentration of grafted PF108. The C3a concentration increased from 46 ng/mL to 316 ng/mL as the surface PF108 concentration increased from 0-0.25 microg/cm(2). Kinetics of C3a generation by PF108-grafted surfaces show that 60% of the steady state C3a concentration was generated during the first hour of plasma exposure. When the same PF108-grafted glass surface was repeatedly exposed to fresh plasma, the amount of C3a generated decreased by 70% after the first exposure. This supports the "single-hit" mechanism in complement activation. PEO homopolymer did not activate complement in bulk solution, and, thus, it appears that C3a complement activation by PF108-grafted surfaces is due to the presence of poly(propylene oxide) units. Grafting of PEO using PEO-containing block copolymers requires examination of complement activating properties of the non-PEO segment.

Synthesis and characterization of segmented polyurethanes based on amphiphilic polyether diols

Segmented polyurethanes (SPUs) based on polyethylene glycol (PEG), polypropylene glycol (PPG) and a series of Pluronics with different ethylene oxide/propylene oxide ratios (EO/PO) and molecular weights were prepared. Different diisocyanates were used for making SPUs: 4,4-diphenylmethane diisocyanate (MDI), 4,4-dicyclohexylmethane diisocyanate (MDCI), hexamethylene diisocyanate (HMDI) and isophorone diisocyanate (IPDI). 1,4-Butane diol (BD) and ethylene diamine (ED) were used as chain extenders. The polymers obtained were characterized by infrared spectroscopy (IR), nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The microphase morphology (phase separation and phase mixing) is discussed in more detail.

Effect of forms of collagen linked to polyurethane on endothelial cell growth

Collagen has been widely coated or grafted onto polymer surfaces to improve the biocompatibility of materials. To better support the growth of endothelial cells on polyurethane (PU), collagen was grafted to the carboxyl group enriched PU through 1,2-bis(2,3-epoxypropoxy)ethane linking. Our results demonstrated that collagen in various conditions may result in different forms being grafted to the PU substrate, which subsequently affected the growth of endothelial cells. Collagen predialyzed against physiological phosphate buffered saline (PBS) could be reconstituted into native type fibrils with a bigger diameter at 37 degrees C than could collagen neutralized by titration with NaOH. At low temperature, titrated collagen formed floss-like fibrils packed in a ball with cobblestone-like morphology. The amount of collagen grafted was related to the condition of the collagen used, which in consequence affected the diameter of the collagen fibril formed and the growth of endothelial cells. In conclusion, reconstituted collagen fibrils formed from collagen in PBS at 37 degrees C grafted in the highest amounts to an epoxy-PU substrate and that optimally supported the growth of endothelial cells. Such prepared materials may be potentially good vascular bioprosthetic materials and may provide a wide range of biological applications.

Complement-mediated leukocyte adhesion on poly(etherurethane ureas) under shear stress in vitro

Blood-contacting biomaterials may activate the complement cascade, thus promoting leukocyte adhesion to the biomaterial surface. We hypothesize that the extent of complement activation is modulated by biomaterial formulation and the presence of fluid shear stress. To investigate this hypothesis, we tested base poly(etherurethane ureas) formulated with or without Santowhite antioxidant, a nucleophilic additive. We found that adherent leukocyte densities decreased with increasing shear stress. Moreover, leukocyte adhesion was decreased significantly further by Santowhite additive under shear stress but not under static conditions. Monocytes showed a higher propensity for adhesion than did neutrophils under shear and static conditions. Under static conditions, adherent cells on the Santowhite-containing polyurethane had a slightly more activated morphology than those on the base polyurethane. Cell adhesion under shear stress was significantly decreased when C3 or fibronectin was depleted from the suspension medium. Santowhite additive increased Factor B adsorption to the test surface while shear stress increased Factor H adsorption. The combination of Santowhite additive and shear stress increased the adsorption of both Factor B and Factor H and the serum protein S-terminal complement complex levels, but it did not further increase the state of activation of adherent cells. We conclude that leukocyte adhesion on poly(etherurethane urea) surfaces is sensitive to the levels of shear stress and that both C3 and fibronectin are required to maintain adhesion in the presence of shear stress. The low state of cellular activation and increased Factor H adsorption may explain the decreased adherent leukocyte density on the Santowhite-containing polyurethane.

Role of leucocytes in coagulation induced by artificial surfaces: investigation of expression of Mac-1, granulocyte elastase release and leucocyte adhesion on modified polyurethanes

Thrombus formation on artificial surfaces can be viewed as the sequential and concomitant involvement of protein adsorption, platelet reactions, activation of the coagulation system, participation of complement, fibrinolytic and kallikrein-kinin systems, and the interaction of cellular elements. This study examines the activation of leucocytes on a series of well-characterized polyurethanes with different ionic groups [sulphonate groups (negatively charged); quaterinary amine groups (positively charged)], in terms of adhesion, degranulation and cell surface integrin receptor expression. Leucocyte adhesion was monitored with radiolabelled neutrophils and scanning electron microscopy (SEM), degranulation by measurement of human neutrophil elastase using an indirect enzyme-linked immunosorbent assay and cell surface expression of the integrin receptor Mac-1, using fluorescent-activated cell sorting (FACS). Our results indicate a trend towards enhanced adhesion and degranulation with respect to the negatively charged polyurethane. Similar results were observed with respect to the integrin Mac-1 from recovered adherent cells. The findings of enhanced adhesion and spreading, Mac-1 up-regulation and granulocyte elastase release from the negatively charged sulphonated polyurethane indicate the potential of leucocytes to contribute towards thrombus formation on such surfaces.