Synthesis of phospholipid polymers having a urethane bond in the side chain as coating material on segmented polyurethane and their platelet adhesion-resistant properties

Protein, cell and bacterial fouling resistance of polypeptoid-modified surfaces: effect of side-chain chemistry

Peptidomimetic polymers consisting of poly-N-substituted glycine oligomers (polypeptoids) conjugated to biomimetic adhesive polypeptides were investigated as antifouling surface coatings. The polymers were immobilized onto TiO(2) surfaces via an anchoring peptide consisting of alternating residues of 3,4-dihydroxyphenylalanine (DOPA) and lysine. Three polypeptoid side-chain compositions were investigated for antifouling performance and stability toward enzymatic degradation. Ellipsometry and XPS analysis confirmed that purified polymers adsorbed strongly to TiO(2) surfaces, and the immobilized polymers were resistant to enzymatic degradation as demonstrated by mass spectrometry. All polypeptoid-modified surfaces exhibited significant reductions in adsorption of lysozyme, fibrinogen and serum proteins, and were resistant to 3T3 fibroblast cell attachment for up to seven days. Long-term in vitro cell attachment studies conducted for six weeks revealed the importance of polypeptoid side-chain composition, with a methoxyethyl side chain providing superior long-term fouling resistance compared to hydroxyethyl and hydroxypropyl side chains. Finally, attachment of both gram-positive and gram-negative bacteria for up to four days under continuous-flow conditions was significantly reduced on the polypeptoid-modified surfaces compared to unmodified TiO(2) surfaces. The results reveal the influence of polypeptoid side-chain chemistry on short-term and long-term protein, cell and bacterial fouling resistance.

Covalent grafting of fibronectin onto plasma-treated PTFE: influence of the conjugation strategy on fibronectin biological activity

Surface coating of synthetic materials is often considered to improve biomedical devices biocompatibility. In this study, we covalently bound fibronectin (FN) onto ammonia plasma-treated PTFE via two crosslinkers, namely glutaric anhydride (GA) and sulfosuccinimidyl-4-(p-maleimidophenyl)butyrate (sulfo-SMPB). With respect to clean PTFE, cell adhesion increased markedly on both FN grafted surfaces, although it was twice higher on PTFE-GA-FN than on PTFE-SMPB-FN. ELISA experiments performed with a polyclonal antibody revealed that the amount of FN is identical on both surfaces while monoclonal antibody specific to the RGD binding site clearly demonstrated a greater availability when FN is surface grafted through GA. These results provide evidence of a variation in protein conformation correlated with the surface conjugation strategy.

Synthesis, platelet adhesion and cytotoxicity studies of new glycerophosphoryl-containing polyurethanes

In this work we synthesized new MDI -based poly(ether)urethanes (PEUs) with phospholipid-like residue as chain extender. Polymers were prepared by a conventional two-step solution polymerization procedure using 4,4' diphenylmethanediisocyanate (MDI) and poly(1,4- butanediol) with 1000 as molecular weight to form prepolymers which were successively polymerized with 1 glycerophosphorylcholine (1-GPC), 2-glycerophosphorylcholine (2-GPC) or glycerophosphorylserine (GPS) as chain extenders. Two reference polymers bearing 1,4-butandiol (BD) have been also synthesized. The polymers obtained were characterized by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and modulated scanning calorimetry (MDSC). The biocompatibility of synthesized segmented polyurethanes was then investigated by platelet-rich plasma contact studies and related scanning electron microscopy (SEM) photographs for blood compatibility and cytotoxicity assay (MTT test) on material elution to assess the effect of any toxic leachables on cellular viability. Three polymers among all have given very satisfactory results suggesting to investigate more deeply their possible use in biomedical devices.

Biomembrane mimetic polymer poly (2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate) at the interface of polyurethane surfaces

Polyurethane (PU) is widely used to make artificial heart and blood vessel wells; however, its thrombogenicity in vivo is still in question. The biomembrane-mimetic and water-soluble polymers, poly (2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate) (PMB), were used to modify polyurethane (PU) surfaces for improving their hemocompatibility. The morphologies of the PMB modified PU surfaces were examined by using atomic force microscopy and the parameters of the PMB absorption kinetics were extracted from dynamic water contact angle measurements. Two-phase domains, the hard and soft segment phases, were observed on the PU surfaces under the aqueous conditions. The absorbed PMB molecules formed the isolated layers first on the hydrophobic hard segments, and subsequently networked as the PMB concentration increased. The increments of PMB concentration led to the decrement of the effective molar activation (wetting) free energy DeltaG.

Blood compatibility of surface-engineered poly(ethylene terephthalate) via o-carboxymethylchitosan

Poly(ethylene terephthalate) (PET) films were treated by argon plasma following by graft copolymerization with acrylic acid (AAc). The obtained PET-surface grafted PAA (PET-g-PAA) was coupled with chitosan (CS) and o-carboxymethylchitosan (OCMCS) molecules, respectively. Their surface physicochemical properties were characterized by X-ray photoelectron spectroscopy (XPS), water contact angle and streaming potential measurements. The PET-g-PAA surface containing carboxylic acid, CS immobilized PET surface containing amino and OCMCS immobilized PET surface containing both carboxylic acid and amino groups, make the PET surface exhibited a hydrophilic character. The blood compatibility was evaluated by platelet contacting experiments and protein adsorption experiments in vitro. The results demonstrate that the PET surface coupling OCMCS shows much less platelet adhesive and fibrinogen adsorption compared to the other surface modified PET films. The anticoagulation of PET-OCMCS is ascribed to the suitable balance of hydrophobicity/hydrophilicity, surface zeta potential and the low adsorption of protein.

Review paper: Principles and Applications of Surface Analytical Techniques at the Vascular Interface

Surface properties have been found to be one of the key parameters which cause degradation and of thrombogenicity in all polymers used in biomedical devices, thus signifying the importance and the necessity for quantitative and accurate characterization of the polymer surface itself as used in the construction of the device. The characterization techniques employed generally involve thermal and spectroscopic measurements, in which class the electrochemical investigations and scanning probe microscopies can also be included. Current hypotheses on the correlations that exist between surface parameters and hemocompatibility and degradation of polymers are examined herein, but concentrating on the field of clinically utilized polymeric materials as used within medical devices themselves. Furthermore, this review provides a brief but complete synopsis of these techniques and other emerging ones, which have proven useful in the analysis of the surface properties of polymeric materials as used in the construction of cardiovascular devices. Statements and examples are given as to how specific information can be acquired from these differing methodologies and how it aids in the design and development of new polymers for usage in biomedical device construction.

Stress response of adherent cells on a polymer blend surface composed of a segmented polyurethane and MPC copolymers

To better understand the effect of 2-methacryloyloxyethyl phosphorylcholine (MPC) copolymer in improving the biocompatibility of segmented polyurethane (SPU), the expression of heat shock protein (HSP) mRNA in HeLa S3 cells adhered on SPU blended with MPC copolymers was measured. Conventionally, MPC copolymers (PMEH) were synthesized by changing the feed ratios of MPC and 2-ethylhexyl methacrylate. X-ray photoelectron spectroscopic analysis of the SPU/PMEH film indicated that the surface concentration of MPC units on the SPU/PMEH film increased with an increase in PMEH composition. HeLa S3 cells were cultured on SPU/PMEH films. The number of adherent cells on the SPU/PMEH films decreased with an increase in the concentration of PMEH. When the PMEH composition was greater than 0.5 wt %, cell adhesion and proliferation decreased markedly. Expressions of HSP27 and HSP47 mRNA were detected using the reverse transcription-polymerase chain reaction (RT-PCR). After incubation for 24 h, both the HSP mRNA expressions in the HeLa S3 cells showed no significant differences among all samples. In HeLa S3 cells that adhered to the SPU film for 48 h, the expressions of HSP27 and HSP47 mRNA increased significantly when compared with those incubated for 24 h. In contrast, the two kinds of mRNA expressions decreased in the HeLa S3 cells that adhered to the SPU/PMEH films for 48 h. From these results, we concluded that PMEH was quite important in suppressing the stress response of adherent HeLa S3 cells. Therefore, SPU/PMEH blend polymers are useful as implantable biomedical materials.

Asymmetrically functional surface properties on biocompatible phospholipid polymer membrane for bioartificial kidney

To obtain a bioartificial kidney composed of a porous polymer membrane and renal cells, a polysulfone (PSf) membrane (PSM) blended with 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer was prepared. The PSM flat membrane with a porous structure could be prepared from the polymer blend containing 1 wt % of the MPC polymer in PSf by the phase inversion technique in a dry-wet process. Asymmetrical surface properties were observed on both sides of the membrane surfaces. That is, the sponge layer formed at the substrate-contacting surface of the membrane had 10-20 microm pores, but the pores in the micrometer range could not be observed for a skin layer formed at the air-contacting surface of the membrane. At the sponge layer surface, the MPC unit composition was 7 times larger than that at the skin layer surface. The amount of proteins adsorbed on the surface corresponded to the MPC unit composition. On the skin layer, a small amount of adsorbed proteins and platelet adhesion could be suppressed compared with those on the sponge layer. However, the skin layer had a moderate protein adsorption, so it showed a sufficient cytocompatibility to enable renal tubule epithelial cells to adhere and proliferate in the membrane. Thus, it functioned well as a renal tubule. Therefore, because of both its hemocompatibility and cytocompatibility, we could conclude that the PSM membrane is useful for as a renal tubule device for a bioartificial kidney.

Segmented polyurethane modified by photopolymerization and cross-linking with 2-methacryloyloxyethyl phosphorylcholine polymer for blood-contacting surfaces of ventricular assist devices

To improve the biocompatibility of pulsatile ventricular assist devices (VADs), the blood-contacting surface of the segmented polyurethane (SPU) diaphragm employed in an electromechanical VAD was modified by introducing 2-methacryloyloxyethyl phosphorylcholine (MPC) units into its surface and forming an interpenetrating polymer network (IPN) structure, which contained independently cross-linked MPC polymer and SPU. The SPU diaphragm modified with an IPN structure was then assembled into a target test pump and underwent continuous pump operation at 37 degrees C for 2 weeks in a simulated systemic circulation using a mock circulatory loop. The surface characteristics of the pump diaphragm after 2 weeks of pump operation were then analyzed with an X-ray photoelectron spectroscope (XPS) and gold-colloid-labeled immunoassay. The XPS surface analysis of the IPN-modified SPU indicated the firm anchoring of MPC units even after 2 weeks of pump operation (the phosphor : carbon ratio was reduced by only 0.09%). The IPN-modified diaphragm prevented protein adsorption as well as cell adhesion in comparison to the unmodified SPU surface. This result thus validated that (1) the IPN structure could firmly secure MPC units to the SPU surface even in a high-mechanical-stress and high-shear environment, (2) the antithrombogenic power of MPC units remained unchanged after 2 weeks of continuous exposure to a high-shear environment, and (3) the IPN modified SPU cross-linked with MPC could be a powerful antithrombogenic surface for blood pumps used for chronic circulatory support of cardiac patients.

In vitro biological performances of phosphorylcholine-grafted ePTFE prostheses through RFGD plasma techniques

Arterial prostheses made of microporous Teflon (ePTFE) are currently used in vascular surgery as bypasses for small and medium vessels. However, several clinical complications, such as thrombosis, frequently occur in these prostheses when implanted in humans. In this work, an original strategy was developed to improve the hemocompatibility of ePTFE prostheses, based on glow-discharge surface modification followed by chemical grafting of phosphorylcholine, known for its hemocompatible properties. This procedure leads to a covalent attachment of the molecules, therefore preventing their removal by shear stress induced by blood flow at the implant wall. The improvement of the blood compatibility properties of the modified ePTFE arterial prostheses have been investigated by in vitro tests such as thromboelastography, neutrophil adsorption, platelet aggregation, and cell cultures. These in vitro tests put in evidence that thrombogenicity index, platelet aggregation, and neutrophil adhesion were decreased by the molecule grafted on the prostheses. Moreover, the cell growth on the surface of the PRC-grafted prostheses was greatly enhanced in comparison to the virgin prosthesis. Based on these results, it could be concluded that PRC grafting on ePTFE prostheses permit to improve in vitro hemocompatibility and biocompatibility in comparison with their virgin counterpart.

Effects of the chemical structure and the surface properties of polymeric biomaterials on their biocompatibility

Polymeric biomaterials have extensively been used in medicinal applications. However, factors that determine their biocompatibility are still not very clear. This article reviews various effects of the chemical structure and the surface properties of polymeric biomaterials on their biocompatibility, including protein adsorption, cell adhesion, cytotoxicity, blood compatibility, and tissue compatibility. Understanding these aspects of biocompatibility is important to the improvement of the biocompatibility of existing polymers and the design of new biocompatible polymers.

Nano-scale surface modification of a segmented polyurethane with a phospholipid polymer

Nano-scale modification of a segmented polyurethane (SPU) with cross-linked 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer was performed to obtain a biocompatible elastomer. To control the domain size and the depth of the modified layer, various compositions of monomers, including MPC, 2-ethylhexyl methacrylate (EHMA), and glycerol 1,3-diglycerolate diacrylate, were examined. SPU film was immersed in the monomer solution and visible light irradiation was applied to initiate polymerization to the SPU film that was held by mica to condense MPC units at the surface. The surfaces of the obtained film were analyzed by X-ray photoelectron spectroscopy and water contact angle measurement. The surface density of MPC units changed with the monomer concentration, and the density was the highest when the ratio between MPC and EHMA was 7:3. In modified SPU films, 6- to 25-nm MPC unit-enriched domains were observed and the density of these domains gradually decreased with depth. The sizes of the domains depended on the MPC composition in the monomer solution. The mechanical properties of the modified films as evaluated by tensile strength measurement under wet conditions were not significantly different from those of SPU. With increase in the existence of MPC unit-enriched domains on the MEG film surface, platelet adhesion and activation were remarkably reduced compared to the SPU film. This nano-scale surface modification may be a useful technique for applying elastic polymer biomaterials.

Biocompatible glucose sensor prepared by modifying protein and vinylferrocene monomer composite membrane

This paper proposes a very simple procedure for preparing a biocompatible sensor based on a protein (bovine serum albumin, BSA), enzyme and vinylferrocene (VF) composite membrane modified electrode. The membrane was prepared simply by first casting vinylferrocene and then coating it with BSA and glucose oxidase immobilised with glutaraldehyde. The sensor response was independent of dissolved oxygen concentration from 3 to 10 ppm and showed good stability for serum sample measurement, unlike the commonly used BSA/enzyme modified electrode. The sensor response was almost unchanged over the measurement time (>10 h) whereas the responses of a BSA and glucose oxidase modified platinum electrode and an osmium-polyvinylpyridine wired horseradish peroxidase modified electrode (Ohara et al., 1993) fell to 68% of their initial value in a serum sample containing 10mM glucose.

Surface modification using photocrosslinkable chitosan for improving hemocompatibility

Immobilization of the anticoagulative or antithrombogenic biomolecule has been considered as one of the important methods to improve the blood compatibility of artificial biomaterials. In this study, a novel immobilization reaction scheme was utilized to incorporate O-butyrylchitosan (OBCS) onto the activated glass surface with an aim to develop an anticoagulative substrate. Activation of the glass surface was carried out by silanization and then OBCS was grafted to the silanized surface via a radiation grafting technique. The OBCS-grafted glass surfaces were characterized by electron spectroscopy for chemical analysis (ESCA) and atomic force microscopy (AFM). The blood compatibility of the OBCS-grafted glass was evaluated by platelet rich plasma (PRP) contacting experiments and protein adsorption experiments in vitro. These results have demonstrated that the surface with immobilized OBCS shows much less platelet adhesive and fibrinogen adsorption compared to the control surface. Therefore, the novel reaction scheme proposed here is very promising for future development of an anticoagulative glass substrate.

Polyethylene/phospholipid polymer alloy as an alternative to poly(vinylchloride)-based materials

To develop new biomaterials for making medical devices, polymer alloys composed of a phospholipid polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and polyethylene (PE) were prepared. The PE/PMPC alloy membrane could be obtained by a combination of solution mixing and solvent evaporation methods using xylene and n-butanol mixture as a solvent. Moreover, thermal treatment was applied to improve the mechanical properties of the PE/PMPC alloy membrane. In the PE/PMPC alloy membrane, the PMPC domains were located not only inside the membrane but also at the surface. Surface analysis of the PE/PMPC alloy membrane with X-ray photoelectron spectroscopy, wettability evaluation, and dynamic contact angle measurements revealed that the phospholipid polar groups in the PMPC covered the surface even after thermal treatment. Blood compatibility tests with attention to platelet adhesion and change in morphology of adhered platelets showed that the PE/PMPC alloy membrane had excellent platelet adhesion resistance. We finally concluded that the PE/PMPC alloy could be used as biomaterials instead of poly(vinyl chloride)-based materials.

Protein adsorption-resistant hollow fibers for blood purification

Nonfouling polysulfone (PSf) hollow fiber membranes resistant to protein adsorption and deposition were newly developed by the addition of 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer. To improve hydrophilicity, permeability, and nonfouling characteristics of the PSf hollow fiber in a hemodialyzer, we synthesized a MPC polymer which can be blended with PSf for preparing the polymer alloy (PSf/MPC polymer). The composition of the MPC polymer blended in the PSf was in the range between 7.0 and 15 wt%. From the PSf/MPC polymer solution, flat membranes and hollow fibers could be prepared. These membranes took an asymmetric structure, and its mechanical strength was good. The surface characterization of the PSf/MPC polymer hollow fiber membrane by X-ray photoelectron spectroscopy revealed that the MPC units were concentrated at the surface. The permeability for solutes through the PSf/MPC polymer membrane was higher, and the amount of protein adsorbed on the PSf/MPC polymer membrane was lower than those of the PSf membrane. Moreover, platelet adhesion was also effectively inhibited on the PSf/MPC polymer membrane.

Physical properties and blood compatibility of surface-modified segmented polyurethane by semi-interpenetrating polymer networks with a phospholipid polymer

Segmented polyurethanes, (SPU)s, are widely used in the biomedical fields because of their excellent mechanical property. However, when blood is in contact with the SPU, non-specific biofouling on the SPU occurs which reduces its mechanical property. To obtain novel blood compatible elastomers, the surface of the SPU was modified with 2-methacryloyloxyethyl phosphorylcholine (MPC) by forming a semi-interpenetrating polymer network (semi-IPN). The SPU film modified by MPC polymer with the semi-IPN (MS-IPN film) was prepared by visible light irradiation of the SPU film in which the monomers were diffused. X-ray photoelectron spectroscopy confirmed that the MPC units were exposed on the MS-IPN film surface. The mechanical properties of the MS-IPN film characterized by tensile testing were similar to those of the SPU film. Platelet adhesion on MS-IPN films was also investigated before and after stress loading to determine the effects of the surface modification on the blood compatibility. Many platelets did adhere on the SPU film before and after stress loading. On the other hand, the MS-IPN film prevented platelet adhesion even after repeated stress loading.

The vascular prosthesis without pseudointima prepared by antithrombogenic phospholipid polymer

On the luminal surface of the common synthetic vascular prostheses, blood coagulation can occur and a thrombus membrane is formed when blood flow passes through it. The thrombus membrane should be organized according to the wound healing process and it becomes a pseudointima which could serve as a blood conduit. However, the small-diameter vascular prosthesis may be quickly occluded by the initial thrombus. Therefore, no clinically applicable small-diameter prostheses have been developed to date. 2-Methacrylovloxyethyl phosphoryleholine (MPC) polymers resemble the structure of an outer cell membrane similar to the fluid mosaic model and demonstrate excellent antithrombogenicity. The purpose of this study is to develop a clinically applicable small-diameter prosthesis based on the new concept of the MPC polymer. We prepared vascular prostheses (2mm ID) from polymer blend composed of segmented polyurethane and the MPC polymer. The prostheses were placed in rabbit carotid arteries. The luminal surface retrieved at eight weeks after implantation was clear without thrombus and pseudointima. We now realize that the vascular prosthesis having the MPC polymer can be applied as a small-diameter prosthesis because it functions without thrombus and pseudointima formation.

Antithrombogenic polymer alloy composed of 2-methacryloyloxyethyl phosphorylcholine polymer and segmented polyurethane

To evaluate the antithrombogenicity of a new polymeric biomaterial in vivo, a polymer alloy tube composed of poly[2-methacryloyloxyethyl phosphorylcholine(MPC)-co-2-ethylhexyl methacrylate](PMEH) polymer and a segmented polyurethane (SPU) was prepared by a solvent evaporation method on a Teflon rod from a homogeneous solution containing both the PMHE and SPU. The composition of the PMEH vs the SPU was 10 wt%. The inner and outer surfaces of the polymer alloy tubing were characterized by X-ray electron spectroscopic (XPS) measurements. The MPC units were located on the inner surface of the polymer alloy tubing rather than the outer surface. After immersion in aqueous media, a higher concentration of the MPC units was observed on both surfaces. Selective staining of the MPC units with osmium tetraoxide was carried out to observe the morphology of the PMEH domain on the surface of the polymer alloy. There were large-sized PMEH domains on the inner surface of the tubing but small-sized domains were found on the outer surface. This result was in good agreement with the XPS results. Blood compatibility of the polymer alloy was evaluated by observation of fibrinogen adsorption and platelet adhesion from human plasma. A lot of fibrinogen was adsorbed and many platelets adhered to the inner surface of the original SPU tubing. On the other hand, the PHEH/SPU polymer alloy tubing suppressed these adsorptions and adhesions. When the PMEH/SPU polymer alloy tubing was implanted into a rabbit's artery, thrombus could not be observed even after a 7-day implantation but the original SPU tubing was almost totally occluded only after a 90-min implantation due to serious thrombus deposition on the surface. These results clearly indicated that the PMEH in the SPU matrix acted as an antithrombus reagent by suppression of protein adsorption and platelet adhesion and activation. Particularly, the MPC units played a significant role in this function.

Preparation and performance of protein-adsorption-resistant asymmetric porous membrane composed of polysulfone/phospholipid polymer blend

To obtain protein-adsorption-resistant membrane for hemodialysis, we prepared a polymer blend composed of polysulfone and 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer (PSf/MPC polymer). The content of the MPC polymer in the PSf was 7 and 15 wt%. The asymmetric porous membrane was obtained by the dry/wet membrane processing method. The surface characterization of the PSf/MPC polymer membrane by X-ray photoelectron spectroscopy revealed that the MPC polymer located at the surface. The mechanical strength of the PSf/MPC polymer membrane did not change compared with that of the PSf membrane. On the other hand, the permeability of solute below a molecular weight (Mw) of 2.0 x 10(4) through the PSf membrane increased with the addition of the MPC polymer, which is considered to be an effect of the hydrophilic character of the MPC polymer. The amount of protein adsorbed on the PSf membrane from plasma was reduced by the addition of the MPC polymer. The permeability of low-molecular-weight protein (Mw = 1.2 x 10(4)) did not change even after the PSf/MPC polymer membrane was contacted with plasma protein solution for 4 h, whereas it decreased dramatically in the case of the PSf membrane. Platelet adhesion was also effectively suppressed on the PSf/MPC polymer membrane. Based on these results, the MPC polymer could serve as a doubly functional polymeric additive, that is, to generate a protein-adsorption-resistant characteristic and to render the membrane hydrophilic.

Photoinduced graft polymerization of 2-methacryloyloxyethyl phosphorylcholine on polyethylene membrane surface for obtaining blood cell adhesion resistance

Phospholipid polymer, poly[2-methacryloyloxyethyl phosphorylcholine (MPC)], was grafted with polyethylene (PE) membrane using photoinduced polymerization technique to make the membrane resistant to cell adhesion. The water contact angle on the PE membrane grafted with poly(MPC) decreased with an increase in the photopolymerization time. This decrease corresponded to the increase in the amount of poly(MPC) grafted on the PE surface. The same graft polymerization procedure was applied using other hydrophilic monomers, such as acrylamide (AAm), N-vinylpyrrolidone (VPy) and methacryloyl poly(ethylene glycol) (MPEG). These monomers were also polymerized to form grafted chains on the PE membrane, and the grafting was confirmed with X-ray photoelectron spectroscopy. Analysis of amount and distribution of plasma proteins at the plasma-contacting surface of the original and the modified PE membranes were analyzed using immunogold assay. The grafting of poly(MPC) and poly(VPy) on PE membrane reduced the plasma protein adsorption significantly compared with that on the original PE membrane. However, the PE membranes grafted with poly(AAm) or poly(MPEG) did not show any effects on protein adsorption. Platelet adhesion on the original and modified PE membranes from platelet-rich plasma was also examined. A large number of platelets adhered and activated on the original PE membrane. Grafting with poly(AAm) did not suppress platelet adhesion, but grafting with poly(MPC) or poly(VPy) on the PE membrane was effective in preventing platelet adhesion. It is concluded that the introduction of the phosphorylcholine group on the surface could decrease the cell adhesion to substrate polymer.

Phosphorylcholine-based polymers and their use in the prevention of biofouling

This article provides an overview of work carried out on the synthesis and non-fouling properties of phosphorylcholine (PC)-containing polymers. The concept of biomimicry is outlined and the major classes of synthetic PC-based materials described. Studies on the interaction of these materials with various proteins are collated and the mechanism for their protein-resistant nature is discussed. Similarly, cellular interactions are also reviewed, with ex-vivo and in-vivo clinical data provided to demonstrate the usefulness of these materials for improving the properties of medical devices.

Short-term in vivo evaluation of small-diameter vascular prosthesis composed of segmented poly(etherurethane)/2-methacryloyloxyethyl phosphorylcholine polymer blend

A small-diameter vascular prosthesis with potential for clinical use was prepared from a Dacron prosthesis coated with nonthrombogenic polymeric materials. As a coating material, segmented poly(etherurethane) (SPU; Tecoflex 60) was blended with a phospholipid polymer, 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer, which has excellent blood compatibility. The Dacron prosthesis, 2 mm in diameter, was immersed in a solution of the SPU/MPC polymer blend and dried to evaporate the solvent. The SPU/MPC polymer prosthesis was nonwater permeable and could be sewn to a natural vessel by a microsurgical technique. The SPU solution was used instead of the SPU/MPC polymer blend solution to prepare a control prosthesis (SPU prosthesis). The SPU/MPC polymer prosthesis and the SPU prosthesis were placed as interposition grafts in rabbit carotid arteries. A massive red thrombus became attached to the surface of the SPU prosthesis as early as 90 min after implantation. In the SPU/MPC polymer prosthesis case, the surface was maintained clear even after 5-day implantation. These observations indicated that the MPC polymer in the SPU could improve the nonthrombogenicity of SPU, and the SPU/MPC polymer blend had potential for preparation of small-diameter vascular prostheses.

Growth of L929 cells on polymeric films prepared by Langmuir-Blodgett and casting methods

The growth and spreading of fibroblast, L929 cells, on various polymeric films prepared by the Langmuir-Blodgett (LB) and casting methods were investigated. L929 cells, which were cultivated on collagen and synthetic polymeric films prepared by the LB method, adhered and spread much more than those on synthetic films prepared by the casting method. This is explained by the fact that cell growth and cell spreading are suitable for L929 cells on the films having serum proteins that contain a high alpha-helix content, because LB films adsorbed those serum proteins estimated from the circular dichroism measurements of the films immersed in cell culture medium. An exponential relationship was observed from the plot of the cell density vs root mean square of roughness of the films, which is estimated by atomic force microscopy, whereas a linear relationship was observed from the plot of the spreading ratio vs the root mean square of roughness. It is suggested that the correlation between the cell growth or spreading ratio and surface roughness of the films where L929 cells were cultivated is considered to be more important than the correlation between the cell growth or spreading ratio and the contact angle of the films.

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.

Chemical modification of silk fibroin with 2-methacryloyloxyethyl phosphorylcholine. II. Graft-polymerization onto fabric through 2-methacryloyloxyethyl isocyanate and interaction between fabric and platelets

2-Methacryloyloxyethyl phosphorylcholine (MPC) was grafted onto silk fabric in a two-step heterogeneous system through the vinyl bonds of 2-methacryloyloxyethyl isocyanate (MOI) modified on the fabric. First, habutae silk fabric was modified with the MOI monomer in anhydrous dimethyl sulfoxide using di-n-butyltin (IV) dilaurate and hydroquinone at 35 degrees C. The saturated weight gain of modified MOI monomer on the fabric was 7.3 wt% versus the original silk. Second, graft polymerization with MPC onto the MOI modified silk was conducted using 2,2'-azo bis[2-(2-imidazolin-2-yl)propane dihydrochloride] (VA-044) as an azo polymerization initiator. The weight of the grafted MPC eventually gained was about 26.0 wt%. The MOI-modified and MPC-grafted silk fabrics were analyzed by Fourier transform infrared (FT-IR) spectroscopy. To confirm the improved biocompatibility of the silk fabric, platelet adhesion was preliminarily tested measuring lactate dehydrogenase. The number of platelets adhering to polyMPC-grafted silk fabric decreased by about one tenth compared to original and MOI-modified silk after 60 min of contact with human platelet-rich plasma (1.0 x 10(6) platelets cm(-2)).

Small diameter vascular prosthesis with a nonthrombogenic phospholipid polymer surface: preliminary study of a new concept for functioning in the absence of pseudo- or neointima formation

The purpose of this study was to prepare a small diameter vascular prosthesis functioning without pseudointima formation. A nonthrombogenic phospholipid polymer, the 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer, has a cell membrane-like structure and has demonstrated strong nonthrombogenicity. We have recently prepared 2 kinds of vascular prostheses, 2 mm in diameter, composed of the MPC polymer and segmented polyurethane (SPU). One includes 7.5 wt% MPC polymer (SPU/MPC[7.5] prosthesis), and the other includes 10.0 wt% (SPU/MPC[10] prosthesis). These prostheses were placed in rabbit carotid arteries and were retrieved at 1 and 4 weeks after implantation. A pseudointima was observed at 4 weeks on the SPU/MPC(7.5). For the SPU/MPC(10), the surface was macroscopically clear without a pseudointima even after a 4 week implantation. It appears that the SPU/MPC(10) prosthesis, functioning without a pseudointima, possesses a stronger nonthrombogenicity and would be more applicable for clinical use.

Platelet adhesion on the gradient surfaces grafted with phospholipid polymer

We have synthesized omega-methacryloyloxyalkyl phosphorylcholine (MAPC) polymers as new blood-compatible materials, with attention to the surface structure of the biomembrane and investigated their blood compatibility. The blood compatibility observed on the MAPC polymers is due to their strong affinity to phospholipids. When the blood comes in contact with the MAPC polymer, phospholipids in the plasma preferentially adsorb on the surface, compared with the plasma proteins or cells. The adsorbed phospholipids construct a biomembrane-like structure on the MAPC polymer surface. The MAPC polymers then have an excellent blood compatibility. In this study, we prepared a gradient poly(MAPC)-grafted polyethylene (PE) surface using a corona discharge treatment method to clarify the effect of the chemical structure of the MAPC unit on the blood compatibility of the MAPC polymers. The surface composition of MAPC and the hydrophilicity on the poly(MAPC)-grafted PE surface were determined by X-ray photoelectron spectroscopic (XPS) analysis and contact angle measurement with water, respectively. The phosphorus/carbon (P/C) ratio determined by the XPS analysis increased, but the water contact angle decreased with increasing corona irradiation energy. These results indicated that the surface density of the MAPC unit was increased. More than 2.5 cm from the starting point of the corona irradiation, the P/C ratio and water contact angle of the surface achieved a constant level. Thus, the surface was completely covered with the grafted poly(MAPC) chain. The effect of the methylene chain length of the MAPC unit on surface properties was also observed. The phospholipid polar group of the MAPC unit was effectively exposed on the surface as the chain length became longer. Moreover, the hydrophobicity of the surface was increased with the increase in the methylene chain length of the MAPC unit. The number of platelets adhering to the poly(MAPC)-grafted PE surface was reduced from the same point where the P/C ratio became constant.

Elastomers for biomedical applications

Current topics in elastomers for biomedical applications are reviewed. Elastomeric biomaterials, such as silicones, thermoplastic elastomers, polyolefin and polydiene elastomers, poly(vinyl chloride), natural rubber, heparinized polymers, hydrogels, polypeptides elastomers and others are described. In addition biomedical applications, such as cardiovascular devices, prosthetic devices, general medical care products, transdermal therapeutic systems, orthodontics, and ophthalmology are reviewed as well. Elastomers will find increasing use in medical products, offering biocompatibility, durability, design flexibility, and favorable performance/cost ratios. Elastomers will play a key role in medical technology of the future.

Why do phospholipid polymers reduce protein adsorption?

The amount of plasma protein adsorbed on a phospholipid polymer having a 2-methacryloyloxyethyl phosphorylcholine (MPC) moiety was reduced compared to the amount of protein adsorbed onto poly[2-hydroxyethyl methacrylate (HEMA)], poly[n-butyl methacrylate (BMA)], and BMA copolymers with acrylamide (AAm) or N-vinyl pyrrolidone (VPy) moieties having a hydrophilic fraction. To clarify the reason for the reduced protein adsorption on the MPC polymer, the water structure in the hydrated polymer was examined with attention to the free water fraction. Hydration of the polymers occurred when they were immersed in water. The differential scanning calorimetric analysis of these hydrated polymers revealed that the free water fractions in the poly(MPC-co-BMA) and poly(MPC-co-n-dodecyl methacrylate) with a 0.30 MPC mole fraction were above 0.70. On the other hand, the free water fractions in the poly(HEMA), poly(AAm-co-BMA), and poly(VPy-co-BMA) were below 0.42. The conformational change in proteins adsorbed on the MPC polymers and poly(HEMA) were determined using ultraviolet and circular dichroism spectroscopic measurements. Proteins adsorbed on poly(HEMA) changed considerably, but those adsorbed on poly(MPC-co-BMA) with a 0.30 MPC mole fraction differed little from the native state. We concluded from these results that fewer proteins are adsorbed and their original conformation is not changed on polymer surfaces that possess a high free water fraction.