Biocompatibility evaluation of laser-induced AAm and HEMA grafted EPR. Part 1: In-vitro study

Samples based on ethylene-propylene rubber (EPR) have been surface grafted with acrylamide (AAm) and 2-hydroxyethyl methacrylate (HEMA) using CO2-pulsed laser as a stimulation source. Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXA) and attenuated total reflectance infrared (ATR-IR) spectra were performed on the modified samples. These techniques revealed the formation of grafted poly(AAm) and poly(HEMA) on the surface of EPR. The surface grafted poly(AAm) and poly(HEMA) were found to have a fractal type of morphology. EDXA showed insignificant grafted AAm and HEMA in regions where fractals were absent. Fractal patterned surfaces provide hydrophilic and hydrophobic sites, making EPR suitable as a biomaterial. In-vitro adhesion and spreading of alveolar macrophages (AMs) cultured on the surface of modified samples have been evaluated by hemocytometry and SEM, respectively, and compared with unmodified controls. Relationships between AM adhesion and their spreading, with surface morphology, graft level and water compatibility are also discussed. Generally, more AMs attach onto unmodified surfaces with a greater degree of spreading, than on the modified EPR. Samples grafted between 0.7 mg/cm2 and 1 mg/cm2 showed fairly low AM density compared with both unmodified EPR and lightly modified samples (less than 0.2 mg/cm2). AMs cultured on the unmodified EPR were larger and displayed pronounced ruffling of the plasma membrane, an increased capacity for adherence and spreading on the surface, and an increased number of extensive filopodia. Moreover, AMs attached onto the surface of modified samples appeared rounded, with minimal cytoplasmic spreading and ruffling.

Mechanism of cytoplasmic calcium changes in platelets in contact with polystyrene and poly(acrylamide-co-methacrylic acid) surfaces

Changes in cytoplasmic free calcium levels ([Ca2+]i) in platelets in contact with polystyrene (PSt) and poly(acrylamide-co-methacrylic acid) (PAAmMAc) particles were evaluated and results were compared with those from two representative biological calcium agonists; thrombin and calcium ionophore A23187. PSt particles stimulated a steep increase in cytoplasmic calcium levels in platelets as much as thrombin and A23187. Serratia protease-treated platelets showed a steep increase in [Ca2+]i by PSt particles, suggesting that PSt surfaces can initiate platelet activation independent of a glycoprotein Ib (GPIb)-mediated pathway. By contrast, dibucaine-treated platelets showed little increase in [Ca2+]i by PSt particles, indicating that microfilament assembly, including binding of GPIb with actin binding protein, should be required for platelet activation in contact with PSt surfaces. PAAmMAc particles induced little increase in cytoplasmic calcium levels in platelets. However, PAAmMAc particle-treated platelets demonstrated little response to thrombin in terms of an increase in [Ca2+]i and ATP release, suggesting the possibility that PAAmMAc surfaces may regulate [Ca2+]i by influencing platelet metabolism. Furthermore, sodium azide-treated platelets showed an increase in [Ca2+]i in platelets when contacting PAAmMAc particles, supporting the suggestion that PAAmMAc surfaces could regulate platelet functions. Fluorescence polarization measurements using 1,6-diphenyl-1,3,5-hexatriene-loaded platelets revealed that PAAmMAc particles increased membrane fluidity in platelets, which may be due to physicochemical interaction with PAAmMAc surfaces.

Surface properties and platelet reactivity of segmented poly(etherurethanes) and poly(etherurethaneureas)

Segmented poly(etherurethanes) (SPUs) and segmented poly(etherurethaneureas) (SPUUs) with various hard and soft segment components were prepared. The surface characterization of SPUs and SPUUs in the air-equilibrated state and in the hydrated-state was carried out by means of X-ray photoelectron spectroscopic (XPS) and dynamic contact angle measurements. XPS revealed that in the air-equilibrated state, lower surface free energy components were enriched at the air-solid interface, whereas in the hydrated-state higher surface free energy components were enriched at the water-solid interface. The change in environment from air to water induced the surface reorganization in order to minimize interfacial free energy. The large contact angle hysteresis was observed for SPUs and SPUUs. This hysteresis can be mainly ascribed to the surface reorganization process. Platelet adhesion tests on SPU and SPUU surfaces indicated that the existence of surface microphase separated structure played an important role in the attainment of blood compatibility.

Reduced thrombogenicity of polymers having phospholipid polar groups

The thrombogenicity of polymers having a phospholipid polar group, poly(2-methacryloyloxyethyl phosphorylcholine (MPC)-co-n-butyl methacrylate (BMA)), was evaluated by a microsphere-column method with attention to the activation and adhesion of platelets on the polymer surface. When citrated platelet-rich plasma (PRP) contacted with the polymers, a large number of platelets adhered and aggregated on poly(BMA). The number of adherent platelets decreased and deformation and aggregation were suppressed with increasing MPC composition. The same tendency was noted when Ca2(+)-re-added PRP came in contact with the polymers. In the case of poly(MPC-co-BMA) with 0.320 mole fraction of MPC, activation of platelets and formation of fibrin were completely suppressed. Therefore, MPC moieties in the polymer play an important role in the reduction of thrombogenicity of the polymer.

In vitro and ex vivo platelet interactions with hydrophilic-hydrophobic poly(ethylene oxide)-polystyrene multiblock copolymers

Hydrophilic-hydrophobic multiblock copolymers synthesized from telechelic oligomers of poly(ethylene oxide) (PEO) and polystyrene (PS) have been used to study the influence of hydrophilic and hydrophobic balance on interfacial interactions of these surfaces with blood components. In vitro coagulation assays show no inherent ability of these amphiphilic surfaces to affect contact activation or coagulation factors. In vitro platelet adhesion and release reactions from rabbit platelet-rich plasma were shown to be greatest on Biomer and PS homopolymer surfaces and least on cross-linked PEO surfaces, with the PEO-PS block copolymers demonstrating intermediate responses. These same substrates were tested in a new low-flow, low-shear arterio-artery shunt system in rabbits. Whole blood occlusion times were not a direct function of hydrophilic content as both PEO and PS homopolymers and Biomer showed short occlusion times, while PEO-PS block copolymers prolonged occlusion times considerably, depending on composition. Overall, results suggest that PEO-PS block copolymers promote unique whole blood responses in contrast to homopolymer and Biomer controls which are more complex than direct correlations to bulk hydrophilic and hydrophobic contents.

Bulk, surface, and blood-contacting properties of polyetherurethanes modified with polyethylene oxide

The bulk, surface, and blood-contacting properties of a series of polyether polyurethanes based on polyethylene oxide (PEO) (MW = 1450), polytetramethylene oxide (PTMO) (MW = 1000), and mixed PEO/PTMO soft segments were evaluated. The effect of varying the weight percentage of PEO, and thus the overall polarity of the mixed soft segment phase, was investigated. Two polymer blends prepared from a PTMO-based and a PEO-based polyurethane were also studied. Differential scanning calorimetry (DSC) and dynamic mechanical analysis indicated that the polyurethanes based on either the PEO or the PTMO soft segments are relatively phase mixed. The degree of phase mixing in the polymers increased with increasing weight fraction of PEO. As expected, water absorption and the hydrophilicity of the polymer increased with increasing PEO soft segment content. In vacuum, the PEO-rich polymers have a lower concentration of soft segment at the surface, possibly due to the migration of the polar PEO segments away from the polymer/vacuum interface. The blood-contacting results indicated that the higher PEO-containing polymers were more thrombogenic than the pure PTMO-based polyurethane. A threshold concentration of PEO in the polyurethane appeared to be required before the blood-contacting properties were significantly affected.

Surface molecular mobility and platelet reactivity of segmented poly(etherurethaneureas) with hydrophilic and hydrophobic soft segment components

Segmented poly(etherurethaneureas) (SPUUs) with hydrophilic and hydrophobic polyether components were prepared. The surface chemical composition of SPUUs in various environments was investigated by means of X-ray photoelectron spectroscopic and dynamic contact angle measurements. These measurements revealed that in a hydrated state, the higher surface free energy component is enriched on the surface of the SPUU in order to minimize the interfacial free energy between water and the solid surface. The surface molecular mobility showed a strong correlation with bulk molecular motion. Platelet adhesion tests and dynamic contact angle measurements after adsorption of bovine serum albumin revealed that the surface of SPUUs with hydrophilic soft segments has a non-adhesive nature. The platelet reactivity of hydrophobic SPUUs is influenced by microphase separation at the surface.

Synthesis and non-thrombogenicity of polyurethanes with poly(oxyethylene) side chains in soft segment regions

Epoxidized polybutadiene-urethanes were synthesized and grafted with poly(oxyethylene)s. The non-thrombogenicity of the graft polyurethanes was investigated in relation to the content of poly(oxyethylene). The grafting of poly(oxyethylene) to polyurethane suppressed adsorption and denaturation of plasma proteins and platelet adhesion. It was also found that there exists an optimum content of poly(oxyethylene) for the graft polyurethane to attain the highest non-thrombogenicity.

Introduction of surface functional groups onto biomaterials by glow discharges

An attempt was made to graft the monomer HEMA to the polymer surface by "Glow discharge" technique. Experiments were carried out for different surfaces varying the exposure times of samples to HEMA and also as a function of glow discharge time. It was found that as the percentage of grafting increases the hydrophilicity also increases. Contact angle measurements were performed on these substrates, which confirmed the hydrophilic nature of the grafted samples compared to the controls. The role of protein adsorption and their effects to modulate the blood polymer interaction is briefly discussed. When a foreign material comes in contact with blood, the initial event is the adsorption of plasma proteins in parallel with the adhesion of platelets to the material. Albuminated surfaces discourage platelet adhesion while fibrinogen enhances the platelet attachment and thrombosis. Hence a decreased ratio of fibrinogen to albumin on a substrate can be correlated as an indication to its improved blood compatibility. Fibrinogen to albumin ratios of the grafted samples showed a reduction, indicating that albumin adsorption is high; which may make the modified surfaces non-thrombogenic.

Hemocompatibility of hydrogel with polyethyleneoxide chains

We have synthesized a hydrogel with long polyethyleneoxide (PEO) chains and evaluated its hemocompatibility. The hydrogel significantly suppressed the adsorption of plasma proteins and adhesion of platelets. In addition, the marked retardation of thrombus formation on its surface was recognized. These findings seems to be caused by the volume restriction effects derived from the highly flexible and hydrophilic PEO chains.

Recent advances in thromboresistant materials

There are strong demands for innovative antithrombogenic materials, because of their necessity in fabricating artificial organs and progressive surgical prostheses. This paper reviews the recent advances in thromboresistant materials. Special emphasis is placed on the novel feature of the surface characterization of thromboresistant materials by X-ray photoelectron spectroscopy and its biomedical implications. Essential role of crystalline-amorphous microstructured surface in thromboresistant property of semicrystalline segmented copolymer is also discussed in detail.

Adsorption of proteins from solution at the solid-liquid interface

The purpose of this article is to present some general principles and rules for the adsorption of proteins from aqueous solution on solid surfaces, emphasizing conformational and reversibility aspects. Special attention is paid to the relation between structural properties of the protein molecule and its adsorption behavior and to the role of small ions in the overall adsorption process. Thermodynamic analysis reveals that, under many conditions, the adsorption is driven by an entropy increase that is (partly) related to changes in the structure of the protein molecules.

Ex vivo shunt testing of hydrogel-silicone rubber composite materials

Hydrogel polymers consisting of HEMA (2-hydroxyethyl methacrylate) alone or co-polymerized with NVP N-(vinyl pyrrolidone) were incorporated into silicone rubber tubing to produce hydrophilic composite materials. Polymerization methods of the interpenetrating network (IPN) and radiation-induced type were used to impregnate the silicone rubber substrates. These methods produced materials with varied wettability (contact angle with water ranged from 57 degrees to 95 degrees) but similar texture. The surface morphology as observed in the scanning electron microscope was the same for all the materials. Four types of composite materials and two sizes of silicone rubber control tubing in a total of 19 dogs were used as ex vivo femoral artery-to-vein (A-V) shunts and were sampled at 5, 15, 30, 45, and 60 min. Data obtained by using scanning electron microscopy (SEM) were used to determine the type and amount of adhering blood cells at the time intervals described. Large platelet aggregates and/or a variable initial response were found to be predictors of shunt failure (occlusion). Most samples showed only varied levels of platelet adhesion, but other cell types were seen in some samples. The adhesion was found to peak in all cases by 30 min and to reach lower levels by 60 min, suggesting that an initial phase of reaction was complete. The course of platelet adhesion for a 60-min period is presented.

Blood-materials interactions: the minimum interfacial free energy and the optimum polar/apolar ratio hypotheses

Numerous hypotheses exist to explain observed blood-materials interactions. It is the purpose of this article to test two popular hypotheses, namely, the minimum interfacial free energy hypothesis and the optimum polar/apolar ratio hypothesis. Methacrylate polymers and copolymers were characterized using the captive bubble underwater contact angle method; bulk water content was determined by gravimetric methods; streaming potential measurements were made; and surface roughness and possible particulate contamination were evaluated by reflected light microscopy. In vitro blood tests include whole blood clotting time measurements on polymer-coated tubes; centrifugal force platelet adhesion on polymer-coated coverslips; and a measure of the partial thromboplastin time, Russell's viper venom time (Stypven time), and the prothrombin time of native whole blood exposed to polymer-coated microscope slides. Results suggest that platelet adhesion correlates in the opposite direction of whole blood clotting time and partial thromboplastin time, emphasizing the need for a multiparameter approach to blood-materials testing. Based on these tests the minimum interfacial free energy hypothesis is not supported. In fact, the data suggest the opposite to be true. It is apparent that platelet adhesion can be a misleading indicator of blood compatibility. Neither hypothesis can explain the apparent conflict between the platelet adhesion data and the coagulation time data.

Blood compatibility of hydrophilic polymers

Blood compatibility has been studied for hydrophilic polymers such as poly(vinyl alcohol) (PVA), its derivative, and polyethylene grafted with water-soluble monomers. The surfaces in contact with electrolyte solutions have been characterized by measuring the zeta potentials. The study of plasma protein adsorption on these polymers has revealed that bovine serum albumin as well as bovine serum fibrinogen adsorbs to a lesser extent as the hydrophilicity of the polymers increases. Platelet deposition and fibrin formation, examined using platelet-rich plasma, have been found to take place less significantly on PVA as well as sodium acrylate- and acrylamide-grafted polyethylene than on nongrafted and acrylic acid-grafted polyethylene. Ex vivo experiments with canine whole blood have shown that formation of thrombus on PVA is less than on siliconized glass but increases upon heat treatment which reduces the hydrophilicity. When PVA tubes of about 1 mm diameter are anastomosed to the carotid artery of rat, the patency rate is found to depend strongly on the anastomotic technique. From the results on the zeta potential and the experiments in vitro and ex vivo it can be concluded that the material having a surface from which solvated, neutral chains are extended into the outer aqueous phase may exhibit excellent resistance to thrombus formation.

The interaction of plasma proteins with polymers. I. Relationship between polymer surface energy and protein adsorption/desorption

Adsorption of bovine albumin, gamma-globulin and fibrinogen from phosphate buffer solution (pH = 7.5) onto several polymer films was studied using the radioiodinated proteins (125I). The kinetics of desorption of the proteinated polymer films in bovine plasma was determined. Contact angle measurements on these same polymers allowed the calculation of dispersive (WA d) and polar (Ip) components of the polymer-protein solution system. Results from these measurements show that the nondispersive-dispersive force balance at the polymer-protein solution interface, expressed by the Ip/WA d ratio, is an important factor for binding of proteins on polymer surfaces. The purity of fibrinogen and the cleaning procedures for the polymer surfaces influence the absolute values of proteins adsorbed on polymer surfaces.