Effect of hydrophilic and hydrophobic microdomains on mode of interaction between block polymer and blood platelets

Relationship between blood compatibility and water structure—Comparative study between 2-methoxyethylacrylate- and 2-methoxyethylmethacrylate-based random copolymers

We have proposed that the excellent blood compatibility of poly(2-methoxyethylacrylate (MEA)) is caused by freezing bound water contained in it on the basis of results on platelet activation (Tanaka and Mochizuki, J Biomed Mater Res A 2004; 68:684-695). To clarify the applicability of this mechanism to other indexes for blood compatibility, the relationship between complement activation and water structure was investigated by using two copolymers, poly(MEA-2-hydroxyethylmethacrylate (HEMA)) and poly(2-methoxyethylmethacrylate (MEMA)-HEMA), where HEMA content was varied from 25 to 90 mol %. ESCA analysis revealed that the surface compositions of these copolymers (dry state) agreed with the compositions determined by (1)H NMR. However, analysis by water contact angle (wet state) showed that their surfaces were quite different. The contact angle of poly(MEMA-HEMA) depended on the monomer composition, whereas the angle of poly(MEA-HEMA) was close to that of polyHEMA regardless of the monomer composition. The effect of HEMA content in the copolymers on complement activation (production of C3a) was investigated in an in vitro test. The activation by poly(MEMA-HEMA) was enhanced according to the HEMA content, while the activation by poly(MEA-HEMA) with 0-40 mol % of HEMA was weak and did not depend on the HEMA content. These properties are discussed from the viewpoints of the water structure observed by DSC and the surface structure.

Study on blood compatibility with poly(2-methoxyethylacrylate)—relationship between surface structure, water structure, and platelet compatibility in 2-methoxyethylacrylate/2-hydroxyethylmethacrylate diblock copolymer

Diblock copolymers composed of 2-methoxyethylacrylate (MEA) and 2-hydroxyethylmethacrylate (HEMA) were firstly prepared (the composition ratio = 90/10, 79/21, 66/34, and 48/52 mol/mol) by anion living polymerization. ESCA analysis of their surface structures (dry state) revealed that PMEA segment was segregated to the top surface in all of the polymers, whereas the results of contact angle of water (wet state) showed that the surfaces were covered with PHEMA segment. In vitro platelet adhesion test showed that these polymers had the excellent compatibility with platelet compared to PHEMA homopolymer. Water structure in the hydrated copolymers was investigated by DSC and freezing bound water was observed for all the polymers like PMEA homopolymer, whereas it was not found in PHEMA homopolymer. Further investigation of water structure based on the results of DSC and EWCMS (equilibrium water content by moisture sorption) suggested that freezing bound water existed in PHEMA segment in addition to PMEA segment. We have proposed that the water plays a key role in the appearance of good blood compatibility of the copolymer, according to our previous works (Tanaka et al. Biomacromolecules 2002;3:36-41, Tanaka et al. J Biomed Mater Res A 2004;68:684-695).

Nanostructured designs of biomedical materials: applications of cell sheet engineering to functional regenerative tissues and organs

Biomaterials surface design is critical for control of cell-materials interactions. Materials surface characteristics important to cell-materials interactions are the following: (a) nonfouling surfaces where cells cannot interact; (b) surfaces that interact with cells but do not alter cell morphology or metabolism (passive adhesion processes); and (c) surfaces that strongly interact with cells and cell-surface receptors to alter cell shape after metabolic interactions (active adhesion). In this paper, we briefly discuss the relationship between materials surface characteristics and cells for biomaterials designs in these categories. We have extensively investigated the thermoresponsive polymer, poly(N-isopropylacrylamide) (PIPAAm), as grafted surfaces allowing recovery of confluent cell monolayers as contiguous living cell sheets for tissue engineering applications. Cellular interactions with PIPAAm-grafted surfaces can be regulated vertically using the thickness of the PIPAAm-grafted layers in nanometer-scale levels, as well as laterally (spatially) using nano-patterned PIPAAm chemistry on various other surface chemistries. PIPAAm-grafted surfaces with 15-20-nm thick layers exhibit temperature-dependent cell adhesion/detachment control, while surfaces with PIPAAm layer thicknesses of more than 30 nm do not support cell adhesion. These changes in cell adhesion are explained by the limited mobility of the surface grafted polymer chains as a function of grafting, hydration, and temperature.

Effect of synthesis temperature of PEO-grafted PU/PS IPNs on surface morphology and in vitro blood compatibility

When hydrophilic/hydrophobic polymers have a microdomain structure, platelet adhesion and activation are effectively suppressed by prohibition of the excessive assembly of glycoproteins and adenosine triphosphate (ATP) consumption of the platelets on the surface. In this study, poly(ethylene oxide)-grafted hydrophilic polyurethane (PU)/hydrophobic polystyrene (PS) interpenetrating polymer networks (IPNs) were synthesized by varying the synthesis temperature to control the phase separation and the microdomain surface structure, and the effect of the degree of phase separation on the in vitro blood compatibility. The size of the dispersed PS-rich domains in the PU-rich matrix decreased, and the hydrophilicity also decreased as the synthesis temperature of the PS network during the IPN synthesis was decreased, as the phase separation was suppressed during the synthesis. The amount of the adsorbed bovine plasma fibrinogens (BPF) on the PEO-grafted PU/PS IPNs decreased as the synthesis temperature was decreased, and the in vitro adhesion of the platelets was also suppressed on the PEO-grafted PU/PS IPNs prepared at lower temperature. The microdomain structure on the surface affected the adhesion and the activation of the adhered platelets, and the suppression of the phase separation resulted in the decrease of the domain size, which also enhanced the blood compatibility of the PEO-grafted PU/PS IPNs.

Macrophage responses to vascular stent coatings

Diamond-like carbon (DLC) films have been proposed as potential coatings for blood-contacting devices. In this study, tetrahedral amorphous carbon (ta-C) films deposited by filtered cathodic vacuum arc system (FCVA) were compared with commercially deposited polyurethane coatings (PU) and uncoated stainless steel samples. X-ray reflectivity (XRR) measurements were performed to check density and thickness of the ta-C coatings, and contact angles measurements were used to assess surface wettability. J774 macrophages were used to assess the cell responses to the materials. Cell number, metabolic activity, and hydrogen peroxide production were measured by using biochemical assays, and the cell attachment and morphology were determined by using scanning electron microscopy and confocal laser scanning microscopy. Wettability measurements showed that of the materials, the stainless steel was the most hydrophilic, and the ta-C coatings were the most hydrophobic. Although the initial attachment and morphology did not appear to be dependent on the wettability, the cell numbers did increase with increasing wettability. Macrophages on the stainless steel samples were the most active in producing hydrogen peroxide. These data show that ta-C samples performed as well as commercial PU-coated samples in blocking cell reactions to the substrate and may prove to be effective coatings for blood-contacting materials.

Effect of water structure on blood compatibility? thermal analysis of water in poly(meth)acrylate

The purpose of this study is to clarify the main factor causing excellent blood compatibility of poly(2-methoxyethyl acrylate)(PMEA) by the comparison between PMEA and seven PMEA analogous polymers. The polymers have a typical functional group as ester side chains such as methoxyethyl, hydroxyethyl, phenoxyethyl, and alkyl groups. The properties of the polymers relating to water were investigated in terms of contact angle, equilibrium water content (EWC), and thermal analysis by differential scanning calorimetry. The water in PMEA could be classified into three types: nonfreezing water, freezing bound water, and free water while the water in the analogous polymers was classified into just two types: free and nonfreezing waters, regardless of the chemical structure of the side chain. The surface property represented by the contact angle of water corresponded to the content of the bound water (nonfreezing water + freezing bound water). The platelet compatibility in vitro did not depend on the contents of these waters, or on the contact angle. On the basis of the results of this work and the previous work on the platelet compatibility of poly(MEA-co-HEMA) (Tanaka et al. Biomacromolecules 2002;3;36-41), the main factor causing the excellent compatibility of PMEA is discussed.

Cardiopulmonary bypass technology transfer: musings of a cardiac surgeon

The development of cardiopulmonary bypass (CPB) has been one of the greatest technical advancements in cardiovascular medicine. With heparin anticoagulation, this device can safely replace the circulatory and gas-exchanging functions of the heart and lung, facilitating complex cardiac operations. Limitations still exist however, related to blood reactions at the biomaterial surface, such as cell activation, inflammation and low-grade thrombosis. In this brief review, the thought processes which paralleled the development of CPB biocompatible surfaces such as heparin-coating, will be explored, as well as current theories on the suspected mechanisms by which heparin-coated surfaces act as an anti-inflammatory device during CPB. Results with new surfaces for CPB designed to capitalize on superior protein adsorption properties, such as surface modifying additive (SMA) and poly (2-methoxyethylacrylate) (PMEA), will also be described. Finally, the significance of biomaterial-independent blood activation will be discussed, emphasizing the current need to develop strategies utilizing optimal biomaterials, modified surgical technique and pharmacologic therapy to minimize the systemic complications of CPB.

Serum protein adsorption and platelet adhesion on pluronic-adsorbed polysulfone membranes

We examined plasma protein adsorption and platelet adhesion to polysulfone (PSf) flat membranes coated with Pluronic with varying polyethylene oxide (PEO) block length. Adsorption of albumin, globulin and fibrinogen to Pluronic-coated PSf membranes was independent of plasma dilution when concentrations of human blood plasma above 20% were applied. Increasing coating concentrations of aqueous Pluronic solution resulted in decreased protein adsorption by the PSf membranes. Pluronic F68, which was more hydrophilic than Pluronic L62 or L64 and had 80% of PEO content, was the most effective at suppressing the adsorption of plasma proteins and platelet adhesion to PSf membranes. We developed a mixed protein solution containing human albumin, gamma-globulin and fibrinogen to attempt to mimic the competitive and cooperative binding effects found in plasma. Fibrinogen adsorption from plasma could be recapitulated by the mixed protein solution. The number of platelets adhering to the PSf membranes decreased as the coating concentration of Pluronic solution was increased, and platelet adhesion decreased in parallel with fibrinogen adsorption. These results suggest that the bioinert property of PEO segments in the Pluronic, which is ascribed to their high flexibility in aqueous media, suppresses the adsorption of plasma proteins and platelets to the Pluronic-coated PSf membranes.

Adhesion and growth of endothelial cell on amphiphilic PU/PS IPN surface: effect of amphiphilic balance and immobilized collagen

Since natural blood vessels are lined with an endothelial cell (EC) monolayer, it is proposed that synthetic biomaterial surfaces be covered or seeded with endothelial cells for ideal nonthrombogenicity under normal conditions. The effects of the surface energy of hydrophilic-hydrophobic IPN on EC adhesion and growth were investigated. The human umbilical vein endothelial cells (HUVECs) were cultured on polyurethane (PU)/polystyrene (PS) IPN prepared by changing only the amphiphilic balance and controlling the microphase separated surface structure. The collagens were immobilized on the IPN surfaces for enhanced adhesion of HUVECs, and the morphology of the collagens immobilized highly depended on the surface energy of the IPNs. The stranded rope structure of the collagen molecules in the solution state was maintained only on the surface of the IPN with intermediate hydrophilicity. The adhesion and the proliferation of ECs on the nontreated IPN surfaces increased by increasing the hydrophobicity of the IPNs, and they were optimized on the collagen-treated IPN surface having an intermediate hydrophilicity. Finally, platelet adhesion was significantly reduced on the EC-hybridized surface of the IPNs.

Surface Modified Cardiopulmonary Bypass Circuits: Modifying the Inflammatory Response

As a consequence of an aging population demographic, now more than ever, researchers in cardiac surgery must focus on means to improve the methods and technologies related to cardiopulmonary bypass. This review presents a classification of the currently available options for biomaterial modification for cardiopulmonary bypass circuits. Hypotheses are given relating the mechanism of action by which some of these surfaces afford improved biocompatibility. Finally, nonpharmacologic biomaterial-independent strategies for minimizing the effects of cardiopulmonary bypass, such as the use of hemofiltration and leukocyte filtration, and the minimization of the use of cardiotomy suction blood are outlined.

Study of blood compatibility with poly(2-methoxyethyl acrylate). Relationship between water structure and platelet compatibility in poly(2-methoxyethylacrylate-co-2-hydroxyethylmethacrylate)

Previously, we reported that poly(2-methoxyethylacrylate) (PMEA) showed excellent blood compatibility and implied that the water structure in PMEA contributed to the blood compatibility. In this study, the relationship between the water structure and the blood compatibility is clarified by studying the influence of the monomer composition of poly(MEA-co-HEMA) on the water structure and the blood compatibility of the copolymers. The water in the polymer was classified into three types: free water, freezing bound water, and nonfreezing water. The polymers with 0-30 mol % of HEMA content had a significant amount of freezing bound water, and the amount decreases greatly when the composition of HEMA exceeded 30 mol %. On the other hand, the amount of other water increased simply with an increase of HEMA content. The evaluation of the platelet compatibility of poly(MEA-co-HEMA) revealed that the adhesion number and the morphological change of platelet on the copolymer surface were least when the HEMA content was 0-20 mol %. These two results strongly suggest that the freezing bound water relates to the platelet compatibility of the polymers.

Hydrophilic hybrid IPNs of segmented polyurethanes and copolymers of vinylpyrrolidone for applications in medicine

The preparation and biocompatibility properties of thermoplastic apparent interpenetrating polymer networks (T-IPNs) of a segmented polyurethaneurea, Biospan (BS), and vinylpyrrolidone-dimethylacrylamide (VP-DMAm) copolymers, are described. The biological interaction between the obtained materials and blood was studied by in vitro methods. The addition of the VP-DMAm copolymers to form T-IPNs with BS substantially increased the equilibrium water uptake and water diffusion coefficients. Investigation of the proteins adsorption, platelet adhesion, thrombus formation and factor XII activation is presented. Investigations of the proteins adsorption of the BS/VP-DMAm T-IPNs surfaces show that the segmented polyurethane (BS) containing VP-DMAm copolymers with higher VP content adsorb more albumin than fibrinogen and gamma-globulin. The platelets adhesion, thrombus formation and factor XII activation are effectively suppressed with respect to the segmented polyurethane when VP-DMAm copolymers with high VP contents are incorporated into BS as T-IPNs.

The effect of fluorinated surface modifying macromolecules on the surface morphology of polyethersulfone membranes

Polyethersulfone (PES) has been recently adopted for membrane materials in applications such as ultrafiltration and haemodialysis. As a biomaterial, the factors which affect the blood compatibility of PES membranes include surface energetics, hydrophobicity, and surface morphology. Surface fluorination of materials has been found to create surfaces with improved blood compatibility and chemical stability. One novel approach to generating fluorinated polymer surfaces has included the use of fluorinated surface modifying macromolecules (SMMs). These macromolecules have been reported to establish fluorinated functional groups at surfaces of polymeric materials without significantly affecting the physical properties of the base polymer. However, to date there has been relatively little information published on the nature of the surface structure for PES materials containing these SMMs. In this study, synthesized SMMs with varying chemical compositions were characterized and blended with PES, and fabricated into flat sheet membranes. The bulk thermal transitions of PES materials were not significantly altered by the addition of 4 wt% SMMs. Contact angle data showed that the addition of SMMs in PES created more hydrophobic surfaces, accompanied by an increase in surface heterogeneity. X-ray photoelectron spectroscopy studies confirmed the presence of elemental fluorine at the surface. Through microscopy studies, it was shown that surface modification was achieved by the migration of SMM concentrated microdomains to the air-membrane interface. The generated microdomains (approximately 1-2 microm in diameter) are dispersed within the top 8 microm of the surface. The concentration of microdomains was gradually depleted from the surface to the bulk of the membrane. A schematic of the morphology for SMMs within the PES membrane surface was proposed.

New biocompatible polymer surface coating for stents results in a low neointimal response

Clinical studies indicate a more pronounced endothelial response after stent implantation than after balloon inflation. This might be related to the metal surface of the stent, and therefore it is speculated that coating of the stent might partially prevent hyperplasia. One coated and one noncoated Palmaz-Schatz stent were implanted in two separate coronary arteries of seven pigs. The coating was composed of methylmethacrylate (MMA) (hydrophobic, 70 mol %) and 2-hydroxyethyl methacrylate (HEMA) (hydrophilic 30 mol %). After sacrifice (3 weeks), cross sections were made of the stented areas. Vessel wall reaction was calculated both independently and dependently of local vessel wall injury due to the stent struts. Overall, vessel wall reaction of the coated stents was lower than that of the noncoated stents. The degree of hyperplasia was linearly related to the degree of stent-induced vessel wall injury. Analyses of all the struts showed that significantly less hyperplasia occurred in the coated versus noncoated stents. In this porcine coronary artery model, the MMA/HEMA stent coating resulted in significantly reduced vessel wall response. However, it remains to be determined whether this favorable outcome will also be present in humans.

Development of "heparin-like" polymers using swift heavy ion and gamma radiation. I. Preparation and characterization of the materials

The development of ideal antithrombogenic polymers, a major problem in biomaterials sciences, is a primary objective in the fields of cardiovascular prostheses, artificial hearts, and other devices. To decrease their thrombogenicity, which remains the major obstacle, we have developed polymeric materials endowed with a specific affinity for antithrombin III (ATIII) and thus able, like heparin, to catalyze the inhibition of thrombin by ATIII. Sulfonate and sulfonamide groups are introduced onto phenyl rings belonging to styrene residues, which are radiation grafted (using swift heavy ion and gamma radiation) onto poly(vinylidene difluoride) (PVDF) and also onto poly(vinylidene fluoride/hexafluoropropylene) [P(VDF-HFP)]. In contrast to gamma radiation, which leads to a homogeneous modification, the advantage of swift heavy ion grafting is that only small regions are modified; thus, the surface may present hydrophilic (corresponding to the modified areas) and hydrophobic microdomains (corresponding to the unmodified areas) of different sizes, depending on the absorbed dose and grafting yield. Surface topography and composition are characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Sulfur, sodium, fluorine, and carbon are determined by scanning electron microscopy combined with energy-dispersive X-ray analysis (SEM-EDXA). The amount of fluorine decreases as polystyrene (PS) is grafted, whatever the kind of radiation and polymer. When the polymers are functionalized, the amount of fluorine also decreases while sodium and sulfur appear. Functionalization seems to increase the roughness of the surface, and its area.

Two different types of nonthrombogenic surfaces: PEG suppresses platelet adhesion ATP-independently but HEMA-St block copolymer requires ATP consumption of platelets to prevent adhesion

Poly(ethylene glycol) (PEG) and a hydrophobic-hydrophilic microdomain structured block copolymer comprising poly(2-hydroxyethyl methacrylate) and polystyrene (HEMA-St) have been reported to show good blood compatibility owing to inhibition of platelet activation. By using a computer-assisted novel technique to analyze platelet behavior on the surfaces, we found two different mechanisms to prevent platelet adhesion. Platelets were prevented from adhesion and spreading on the microdomain surface and retained cell movement for a long time. The platelet movement velocity was not significantly different between PEG-grafted surfaces and HEMA-St block copolymer-cast surfaces. However, platelet motion was qualitatively different. Platelets on HEMA-St block copolymer-cast surfaces moved with rolling, spinning, and vibrating, whereas platelet movement was limited to oscillatory vibration on PEG-grafted surfaces. When platelets were treated with NaN(3), an adenosine triphosphate (ATP) synthesis inhibitor, before contacting the surfaces, platelets movement velocity was decreased only on HEMA-St block copolymer-cast surfaces. Such an inhibitory effect was hardly observed with platelets on PEG-grafted surfaces. We propose two different mechanisms to prevent platelet adhesion onto surfaces. One is ATP-independent as observed with PEG, and the other is ATP-dependent for HEMA-St block copolymer, where platelets consume ATP to prevent adhesion.

The in vitro blood compatibility of poly(ethylene oxide)-grafted polyurethane/polystyrene interpenetrating polymer networks

Poly(ethylene oxide) (PEO)-grafted polyurethane (PU)/polystyrene (PS) interpenetrating polymer networks (IPNs) were synthesized. The effects of the mobile pendant PEO chains with their microphase separated structure on blood-compatibility were investigated. The morphology of both the fracture surface as well as the top surface indicate that the size of the dispersed domains of the PS-rich phase decreased as the grafting with the PEO was increased. The swelling ratio also decreased as the grafting with the PEO was increased. However, the dynamic contact angle and the interfacial energy between IPN surface and water decreased, due to the structural reorganization of the pendant PEO chains. PU/PS IPNs have an excellent mechanical property as compared with PU homopolymers. The adsorption of bovine plasma fibrinogen (BPF) onto the PU/PS IPNs and PU homopolymers was effectively suppressed by the PEO-grafting. In the platelet adhesion test, the amount of platelets adsorbed, activated, and/or coagulated upon the PEO-grafted PU/PS IPNs were reduced when compared to the ungrafted PU homopolymers.

Experimental study of ectopic-free tissue transfer of rabbit epigastric flap using small-caliber vascular grafts

Ectopic intraperitoneal free-tissue grafting using small-caliber artificial vascular grafts is reported. The vascular grafts were polyurethane tubes (inner diameter 1.4 mm; outer diameter 1.9 mm; length 15 mm) with a coating of the antithrombotic agents argatroban and 2-hydroxyethylmethacrylate-styrene block copolymer (HS)on the inner surface. The patency of the artificial vessels was investigated in an ectopic-free rabbit epigastric flap in which grafts were used for anastomosis between the femoral and renal vessels. The mean patent duration of uncoated controls (n = 10) was 128 +/- 37 min. Flap viability after one week (n = 17) and the patency of the coated grafts were investigated histologically. The flap take rate was 65%, the arterial graft patency rate 41%, and the venous graft patency rate 24%. The HS-treated surface combined with the argatroban slow-release system exhibited improvement of flap survival in an intraperitoneal ectopic -free grafting model.

Effect of cross-link density and hydrophilicity of PU on blood compatibility of hydrophobic PS/hydrophilic PU IPNs

To investigate the effect of the hydrophilic and hydrophobic microdomain structure on blood compatibility, a series of interpenetrating polymer networks (IPNs) composed of hydrophilic polyurethane (PU) and hydrophobic polystyrene (PS) was prepared. One series was prepared with varying cross-link densities of each network, the other with varying hydrophilicity of the PU component. All PU/PS IPNs exhibited microphase-separated structures that had dispersed PS domains in the continuous PU matrix. The domain size decreased with decreasing the hydrophilicity of the PU component and increasing the cross-link density of each network. As the cross-link density and hydrophobicity of the PU component was increased, an inward shift of Tgs was observed, which was due to the decrease in phase separation between the hydrophobic PS component and hydrophilic PU component. In the in vitro platelet adhesion test, as the microdomain size of PU/PS IPN surface decreased, the number of adhered platelets on the PU/PS IPN surface was reduced and deformation of the adhered platelets decreased. It could be concluded that blood compatibility of PU/PS IPN was mainly affected by the degree of mixing between PU and PS component, which was reflected by the domain size of PS rich phase.

Active platelet movements on hydrophobic/hydrophilic microdomain-structured surfaces

The early motion and interaction of platelets on a microdomain-structured block copolymer surface composed of 2-hydroxyethyl methacrylate (HEMA)-styrene were analyzed and compared with those on a compositionally identical random copolymer, homopolymer poly (HEMA) (hydrophilic) and polystyrene (hydrophobic) surfaces. Contacting platelets were quantitatively more active, with motions including rolling, detachment, oscillatory vibration, and change of direction only on the HEMA-St block copolymer surface. Active platelet movements were observed for long time periods (>20 min) on HEMA-St block copolymer surfaces and were distinct from those for inert PSt latex particles on these same surfaces, demonstrating that platelet movements were not due to physical forces such as convection, hydrophobic interactions, or microbrownian movement. To study the cause and mechanism underlying the platelet movements, platelets treated with an adenosine triphosphate (ATP) synthesis inhibition, NaN3, or a membrane skeleton-disrupting chemical agent, dibucaine, were also studied on these surfaces. Both treatments reduced platelet movement and demonstrated that platelets in contact with the HEMA-St block copolymer surface require metabolic processes consuming ATP and involve dynamics of their membrane skeleton. These energy-consuming active movements might explain the previously observed lower platelet activation and low thrombogenicity of the HEMA-St block copolymers. Enhanced platelet movements on the HEMA-St block copolymer surface show that the microdomain surface interacts uniquely with platelets to hinder activation and preserve passive platelet function despite surface contact.

Sulfonated poly(ethylene oxide)-grafted polyurethane copolymer for biomedical applications

Sulfonated poly(ethylene oxide) (PEO-SO3)-grafted polyurethane (PU) copolymer (PU-PEO-SO3) was developed for utilization as a material for biomedical applications. Its bulk properties and biocompatibility were investigated and the results were compared to those of PU and PU-PEO (not sulfonated). PU-PEO-SO3 copolymer showed higher water uptake than PU and PU-PEO, indicative of increased hydrophilicity due to the grafting of PEO-SO3. Compared with PU, PU-PEO-SO3 displayed lower glass transition temperature (Tg) and melting endotherm from DSC analysis, suggesting the increase of microphase separation and the suppression of hard-segment packing. PU-PEO-SO3 had practically similar mechanical properties to PU, indicating that the chemical modification of PU was accomplished without reducing its bulk properties. From the results of biocompatibility tests, PU-PEO-SO3 copolymer showed significantly lower adhesion of platelets and S. epidermidis than PU control and PU-PEO. In addition, the platelet factor 4 (PF4) released from platelets was the lowest in PU-PEO-SO3 among the test materials. PU-PEO-SO3 copolymer, which has excellent mechanical properties and biocompatibility, is expected to be useful as coating, molding, and blending materials for artificial organs and medical devices.

Minimized platelet interaction with poly(2-hydroxyethyl methacrylate-block-4-bis(trimethylsilyl)methylstyrene) hydrogel showing anomalously high free water content

Novel hydrophilic-hydrophobic block copolymers - poly(2-hydroxyethyl methacrylate-block-4-bis(trimethylsilyl)methylstyrene) (poly(HEMA-block-BSMS) or BH polymer) - were prepared as materials potentially useful for blood-contacting devices. Among the BH(X) series including poly(HEMA) itself, platelet adhesion and activation are minimum on the surface of BH(10), the block copolymer composed of 10 mol% of the hydrophobic segment. The unique physicochemical characteristics of BH(10) such as the high free water content and the increased mobility of PHEMA segments seem to play an important role in the prevention of platelet adhesion and activation. The water content of BH(10) was twice that of poly(HEMA) itself, even in the presence of 10 mol% of the hydrophobic moiety [poly(BSMS)]. Further, an anomalous increase in free water content was observed for the BH(10) sample measured by DSC. The glass transition temperature of BH(10) in the dry state was determined to be c. 10 degrees C, which was much lower than that observed for each of the homopolymers (c.f.: Tg [poly(HEMA)] = 70 degrees C; Tg [poly(BSMS)] = 160 degrees C), suggesting an increased mobility of tethered PHEMA segments in the BH(10) film.

Reduction of surface-induced platelet activation on phospholipid polymer

omega-Methacryloyloxyalkyl phosphorylcholine (MA-PC) polymers which have been synthesized with attention to the surface structure of a biomembrane show excellent blood compatibility, i.e., resistance to protein adsorption and blood cell adhesion. To clarify the stability of platelets in contact with the MAPC polymer surfaces, cytoplasmic free calcium concentration ([Ca2+],) in the platelets was measured. A platelet suspension was passed through a column packed with various polymer beads after treatment with plasma, and the [Ca2+]i in the platelets eluted from the column was measured. The [Ca2+]i in contact with the MAPC polymers, i.e., poly[2-methacryloyloxyethyl phosphorylcholine-co-nbutyl methacrylate (BMA)] (PMEB) and poly(6-methacryloyloxyhexyl phosphorylcholine-co-BMA) (PMHB), was less than that in contact with poly(BMA). However, poly(10-methacryloyloxydecyl phosphorylcholine-co-BMA) (PMDB) was not effective in suppressing the increase in [Ca2+]i, and thus was at the same level as in the poly(BMA). This result indicated that platelets in contact with PMEB or PMHB were less activated compared with those in contact with PMDB and poly(BMA). Moreover, the state of the platelets adhered to these polymer surfaces, both morphologically and immunologically, was examined. Scanning electron microscopic observation of the polymer surface after contact with a platelet suspension revealed that many platelets adhered and changed their shape on the poly(BMA). The numbers of adhetent platelets were reduced on all MAPC polymer surface. The relative amount of alpha-granule membrane glycoprotein (GMP-140) which appears on the cell membrane by activation of platelets on the PMEB surfaces was less than that on poly(BMA) and poly(2-hydroxyethyl methacrylate). These results suggest that PMEB and PMHB suppressed not only platelet adhesion but also activation of the platelets in contact with these surface.

2-hydroxyethyl methacrylate-terminated polyurethane/polyurethane interpenetrating polymer networks

The interpenetrating polymer networks (INPs) of polyurethane (PU) and 2-hydroxyethyl methacrylate (MEHA)-terminated polyurethane (HPU) were prepared by solution polymerization. PU prepolymer was synthesized from 4,4-diphenyl methane diisocyanate (MDI) and poly(propylene oxide) glycol (PPG). HPU prepolymer was synthesized from MDI, poly(tetramethylene oxide) glycol and HEMA. Dynamic mechanical analysis showed that the resultant IPN membranes have good compatibility between their constituents. As the HPU content increased, the tensile strength of the IPNs first increased and then decreased. For the highest tensile strength, the optimum HPU content was about 25 wt %. The value of surface tension of IPNs varied from 44.4 to 50.5 dyne/cm, and polarity ranged from 0.59 to 0.91. The relative index of platelet adhesion (RIPA) of the IPN membranes was measured by the dynamic thrombosis test at constant shaking speed and temperature. By the criteria of this test, the IPN membranes with HPU content of about 25 wt% to the minimum platelet adhesion. When measured by the angular dependent ESCA technique on the surface of IPN samples, the variation in the RIPA correlated to the change in the surface soft segment to hard segment ratio. Higher HPU content resulted in more migration of soft segments toward the surface. The platelet adhesion was observed to be minimized when the surface O/N ratio was around 12.

Degradation of polyetherurethane by subcutaneous implantation into rats. II. Changes of contact angles, infrared spectra, and nuclear magnetic resonance spectra

In vivo degradation of polyetherurethanes (PEUs) was studied using two kinds of PEUs, U-3 and U-8, coated on a base film of ethylene/vinyl alcohol copolymer (EVAL). U-3 is a nonsegmented PEU prepared from 4,4'-diisocyanatodiphenylmethane (MDI) and poly(tetramethylene oxide) (PTMO 1000). U-8 is a segmented PEU prepared from MDI, PTMO 1000, and 1,4-butanediol. Previous studies of PEUs were conducted using gel permeation chromatography and scanning electron microscopy. In this study, the explanted materials were examined with contact angle measurement, ATR-FTIR, and nuclear magnetic resonance (NMR) spectroscopies. All data obtained by these methodologies indicate that the PTMO/MDI oligomers diffused to the material surface in the early stage of implantation. Then, the low-molecular-weight fraction of the oligomers leached out from the surface to the exudate. Degradation became dominant after 2-4 weeks. In the case of PU-8, the PTMO fraction decreased approximately 35-40% from the surface at 24 weeks postimplantation. In the case of PU-3, the loss of coating material (U-3) on the base film (EVAL) was observed after 10 weeks. The PTMO fraction of the surface U-3 remained on EVAL at 6 weeks postimplantation, however, it was 64% of the initial material. The molecular weight of the U-3 remaining on EVAL also decreased. Degradation of U-3 occurred more rapidly than that of U-8. The data obtained with our materials were insufficient in determining evidence of oxidative degradation with IR or NMR spectra.

Evaluation of the antithrombogenicity of a new microdomain structured copolymer

A new antithrombogenic material was studied, fluorine-acryl-styrene-urethane-silicone (FASUS) copolymer, the theoretical basis of which attributed to its hydrophilic and hydrophobic microdomain structure. In this series of experiments, the blood compatibility of this copolymer was evaluated in vitro and ex vivo. For the in vitro evaluation, a whole blood clotting test and the microsphere column test were performed. For the ex vivo evaluation, two series of shunt tests in rabbits were performed, one was the arterioartery (A-A) shunt model, and the other was the arteriovenous (A-V) shunt model. The antithrombogenicity was assessed by measuring the shunt obstructive time in the A-A shunt experiment. The A-V shunt experiment was assessed by measuring the circulating platelet counts, platelet aggregability, activated partial thromboplastin time (APTT), and prothrombin time (PT). In the whole blood clotting test, FASUS revealed the significantly longer blood clotting time than that of the control glass tubings (19.7 +/- 1.0 versus 6.5 +/- 0.7 min, p < 0.001). In the microsphere column test, the coated group had a 30% reduction of the platelet number in the eluted blood in contrast with a marked decrease of 70% in the control group (p < 0.05). In the ex vivo A-A shunt experiment, the occlusion time for the FASUS-coated group was significantly longer than that of the control (109.7 +/- 17.3 versus 3.0 +/- 0.4 min, p < 0.05). The A-V shunt experiment showed that the FASUS copolymer suppressed the decrease in platelet counts and tended to improve prolonged APTT compared with that of the control. Clinically, in 25 patients, we placed coated FASUS copolymer into the cannulas for use in percutaneous cardiopulmonary support (PCPS) procedures. There was no evidence of thrombus on the blood contacting surface and no thromboembolism in major organs clinically or upon postmortem examination. In summary, this new copolymer may be effective in preventing thrombus formation in vitro, ex vivo, and in clinical situations.

Antithrombogenicity of hydrophilic polyurethane-hydrophobic polystyrene IPNs. I. Synthesis and characterization

A series of interpenetrating polymer networks (IPNs) composed of hydrophilic polyurethane (PU) and hydrophobic polystyrene (PS) were prepared by the simultaneous polymerization method. The PU network was synthesized via the isocyanate-terminated PU prepolymer based on polyethylene glycol (PEG), a highly hydrophilic oligomer, and hexamethylene diisocyanate (HDI). The bulk and surface characteristics of these materials were analyzed by differential scanning calorimetry (DSC), tensile testing, scanning electron microscopy (SEM), attenuated total reflectance-Fourier transform infrared (ATR-FTIR), electron spectroscopy for chemical analysis (ESCA), and contact angle measurement. The PU/PS IPNs prepared in this study exhibited phase separated structures, which had dispersed PS domains in the continuous PU matrix, in both the bulk and surface showing two transition temperatures. The IPN containing 50 wt% of PS showed good mechanical properties. The enrichment of PU phase in the surface was revealed by SEM, ATR-FTIR, ESCA, and contact angle measurement.

Antithrombogenicity of hydrophilic polyurethane-hydrophobic polystyrene IPNs. II. In vitro and ex vivo studies

To investigate the effect of hydrophilic and hydrophobic surfaces with phase separated structure on their blood responses, interpenetrating polymer networks (IPNs) composed of hydrophilic polyurethane (PU) and hydrophobic polystyrene (PS) were prepared by simultaneous polymerization. In vitro protein adsorption, in vitro platelet adhesion, and ex vivo A-A shunt test were carried out to evaluate the blood compatibility of the PU/PS IPNs. The results of protein adsorption on the PU/PS IPN surfaces indicated that albumin preferentially adsorbed on the hydrophilic surface (PU), while fibrinogen preferentially adsorbed on the hydrophobic surface (PS). The PU/PS IPNs exhibited suppressive properties for both platelet adhesion and activation. The occlusion time of U50S50 IPN containing 50 wt% of PS was twice as long as that of the PU control (50 min), indicating enhanced blood compatibility, presumably due to the selective adsorption of plasma proteins and the suppression of the adhesion and activation of platelets.

New challenges in biomaterials

Significant opportunities and challenges exist in the creation and characterization of biomaterials. Materials have been designed for contact with blood, as replacements for soft and hard tissues, as adhesives, and as dental materials. Current methods of synthesis and characterization of these materials are outlined. Approaches for controlling the interface between tissue and biomaterials and ways in which the engineered materials may contribute to medicine are considered.

Prevention of changes in platelet cytoplasmic free calcium levels by interaction with 2-hydroxyethyl methacrylate/styrene block copolymer surfaces

Changes in platelet cytoplasmic free calcium levels were investigated in contact with cast film surfaces of a block copolymer containing 2-hydroxyethyl methacrylate (HEMA) and styrene (St) (0.5 mole fraction HEMA). These copolymer surfaces demonstrate microdomain alternating lamellae structures composed of hydrophilic HEMA domains (5 nm width) and hydrophobic St domains (20 nm width). The results were compared with those obtained from platelets contacting a random copolymer of HEMA-St (0.5 mole fraction HEMA) and from homopolymers of polystyrene (PSt) and poly(2-hydroxyethyl methacrylate) (PHEMA). Cytoplasmic free calcium levels in platelets contacting the microdomain structured surfaces of the HEMA-St block copolymer remained relatively constant in contrast to the significant increases observed for the radically prepared HEMA-St copolymer, PSt, and PHEMA surfaces. Adhering platelets were stimulated by exogenously introduced thrombin and calcium ionophore A23187 20 min after platelet adherence to the polymer surfaces. Only platelets on the block copolymer surfaces showed active metabolic responses. These results suggest that adhering platelets on the microdomain structured surfaces maintain high sensitivities to external stimulation due to an intrinsic strong inhibition of platelet functional changes induced by surface contact.