Residence-time dependent changes in fibrinogen adsorbed to polymeric biomaterials

Effect of surface wettability on the adhesion of proteins

Besides significantly broadening the scope of available data on adhesion of proteins on solid substrates, we demonstrate for the first time that all seven proteins (tested here) behave similarly with respect to adhesion exhibiting a step increase in adhesion as wettability of the solid substrate decreases. Also, quantitative measures of like-protein-protein and like-self-assembled-monolayer (SAM)-SAM adhesive energies are provided. New correlations, not previously reported, suggest that the helix and random content (as measures of secondary structure) normalized by the molecular weight of a protein are significant for predicting protein adhesion and are likely related to protein stability at interfaces. Atomic force microscopy (AFM) was used to directly measure the normalized adhesion or pull-off forces between a set of seven globular proteins and a series of eight well-defined model surfaces (SAMs), between like-SAM-immobilized surfaces and between like-protein-immobilized surfaces in phosphate buffer solution (pH 7.4). Normalized force-distance curves between SAMs (alkanethiolates deposited on gold terminated with functional uncharged groups -CH3, -OPh, -CF3, -CN, -OCH3, -OH, -CONH2, and -EG3OH) covalently attached to an AFM cantilever tip modified with a sphere and covalently immobilized proteins (ribonuclease A, lysozyme, bovine serum albumin, immunoglobulin, gamma-globulins, pyruvate kinase, and fibrinogen) clearly illustrate the differences in adhesion between these surfaces and proteins. The adhesion of proteins with uncharged SAMs showed a general "step" dependence on the wettability of the surface as determined by the water contact angle under cyclooctane (thetaco). Thus, for SAMs with thetaco < approximately 66 degrees, (-OH, -CONH2, and -EG3OH), weak adhesion was observed (>-4 +/- 1 mN/m), while for approximately 66 < thetaco < approximately 104 degrees, (-CH3, -OPh, -CF3, -CN, -OCH3), strong adhesion was observed (< or =8 +/- 3 mN/m) that increases (more negative) with the molecular weight of the protein. Large proteins (170-340 kDa), in contrast to small proteins (14 kDa), exhibit characteristic stepwise decompression curves extending to large separation distances (hundreds of nanometers). With respect to like-SAM surfaces, there exists a very strong adhesive (attractive) interaction between the apolar SAM surfaces and weak interactive energy between the polar SAM surfaces. Because the polar surfaces can form hydrogen bonds with water molecules and the apolar surfaces cannot, these measurements provide a quantitative measure of the so-called mean hydrophobic interaction (approximately -206 +/- 8 mN/m) in phosphate-buffered saline at 296 +/- 1 K. Regarding protein-protein interactions, small globular proteins (lysozyme and ribonuclease A) have the least self-adhesion force, indicating robust conformation of the proteins on the surface. Intermediate to large proteins (BSA and pyruvate kinase-tetramer) show measurable adhesion and suggest unfolding (mechanical denaturation) during retraction of the protein-covered substrate from the protein-covered AFM tip. Fibrinogen shows the greatest adhesion of 20.4 +/- 2 mN/m. Unexpectedly, immunoglobulin G (IgG) and gamma-globulins exhibited very little adhesion for intermediate size proteins. However, using a new composite index, n (the product of the percent helix plus random content times relative molecular weight as a fraction of the largest protein in the set, Fib), to correlate the normalized adhesion force, IgG and gamma-globulins do not behave abnormally as a result of their relatively low helix and random (or high sheet) content.

Mesoscopic analysis of conformational and entropic contributions to nonspecific adsorption of HP copolymer chains using dynamic Monte Carlo simulations

Dynamic Monte Carlo simulations of short linear HP-type copolymers exhibiting proteinlike characteristics are used to investigate both chain dynamics and changes in chain conformational entropy and their contributions to the energetics of adsorption onto a solid-liquid interface. The dMC results show that the conformations and energies of adsorbed chains are highly degenerate. The ensemble-averaged energy of the adsorbed state is dependent on temperature, chain sequence, native-state stability, and sorbent surface geometry and hydrophobicity. Mesoscopic thermodynamic analyses reveal that, although increased chain conformational entropy contributes to the driving force for adsorption in certain cases, many conditions exist where the change in conformational entropy is either negligible or unfavorable due to constraints imposed by the need to form a large and specific number of favorable intra- and intermolecular contacts and by the impenetrable nature of the sorbent surface. Step-number-averaged energy trajectories, based on sampling of a large number of energy trajectories and thus conformational states at each step number, suggest that the search for a global energy minimum is gradual, so that adsorption is first reversible but becomes apparently irreversible with longer exposure to the sorbent. These results appear to be connected to the conformational adaptability of the chain both on the surface and in solution, and an adsorption model taking chain conformational dynamics into account is proposed.

Blood compatible aspects of DNA-modified polysulfone membrane-protein adsorption and platelet adhesion

DNA was used as a biomaterial to modify the polysulfone (PSf) membrane by blending it with PSf. The blood compatibility of the membranes was then investigated. The water contact angle decreased, and the hydrophilicity increased when a single strand DNA was blended with PSf. Because of the hydrophilic surface, the DNA-blended PSf membranes had a lower protein adsorption than the PSf membrane, but it was not significantly decreased due to the interaction between the DNA and proteins. Circular dichroism (CD) spectroscopy was used to examine the changes in the secondary structure of the proteins after adsorption onto the polymer surface and desorption from the polymer surface into the SDS solution. The conformation of the proteins adsorbed onto the PSf membrane and desorbed from the PSf membrane significantly changed, but that of the proteins for the DNA-blended PSf membranes differed only slightly from the native one. The number of platelets that adhered on the surface of the DNA-blended PSf membranes was reduced compared to that on the PSf membrane. This suggested that DNA can be regarded as a biopolymer to modify PSf, and contributes to the hydrophilic and hemocompatible wipers on the surface of the hydrophobic PSf membranes.

Design and evaluation of novel blood incubation systems for in vitro hemocompatibility assessment of planar solid surfaces

Success in the development of hemocompatible biomaterials depends on adequate equipment and procedures for standardized analysis of blood-materials interactions in vitro. In view of the limited standard of knowledge on that important aspect, two novel incubation systems were designed, built, and evaluated for the in vitro assessment of the hemocompatibility of planar solid surfaces: A screening setup was introduced for the comparison of up to 12 different samples. A perfusion setup was developed to model the directed blood flow in the vascular system during incubation by a recirculation circuit, allowing the variation of the wall shear rate at the sample surface. The incubation procedures utilized freshly drawn, heparinized whole human blood. Hemocompatibility in terms of selected aspects of coagulation, thrombogenicity, and immune responses was quantified through plasma levels of characteristic molecules (immunoassays), cell counting, and analysis of adherent cells and fibrin formation (scanning electron microscopy), respectively. Prevention of blood-air contact and mechanical stress, constant temperature and blood pH during incubation, and the suitable choice of reference materials were found to be crucial for reliable testing. Considering those requirements, screening and perfusion system both provided sensitive discrimination between a given set of planar solid surfaces. In conclusion, the suggested methods for an in vitro hemocompatibility assessment permit versatile, sensitive, and efficient analysis of important blood-material interactions despite the unavoidable variability of blood characteristics in different experiments.

The effect of fluid shear and co-adsorbed proteins on the stability of immobilized fibrinogen and subsequent platelet interactions

The conformation adopted by the plasma protein fibrinogen upon its adsorption onto synthetic surfaces has been implicated to play an important role in determining the blood compatibility of biomaterials. It has recently been shown that adsorbed fibrinogen undergoes biologically significant conformational changes with increasing residence time on the surface of selected biomaterials. The purpose of this study was to examine the effects of co-adsorbed proteins and shear forces on such time-dependent functional changes in fibrinogen adsorbed onto polyethylene (PE), polytetrafluoroethylene (PTFE), and silicone rubber (SR). Fibrinogen was adsorbed onto these materials for 1 min and then allowed to 'reside' on these surfaces for up to 2 h prior to assessing its biological activity. Changes in fibrinogen reactivity were determined by measuring the adhesion of 51Cr-labeled platelets and the ability of blood plasma to displace previously adsorbed fibrinogen. The magnitude of platelet adhesion to substrates adsorbed with pure fibrinogen increased in the presence of shear, compared with static conditions; at the lowest shear rate of 200 s(-1), samples exhibited a 20-fold increase in adhered platelet levels. In contrast, at a higher shear rate of 1000 s(-1), the three polymers supported minimal levels of platelet attachment. Surfaces pre-adsorbed with 10% plasma did not promote a significant increase in the number of adherent platelets with increasing shear when compared with the pure fibrinogen-coated substrates. The presence of shear also significantly altered the materials' ability to retain fibrinogen. Under static conditions, the amount of fibrinogen retained following incubation in blood plasma increased on all materials with increasing fibrinogen residence time. However, the materials varied distinctly in their ability to retain adsorbed fibrinogen with increasing fibrinogen residence time, shear rate, and nature of the co-adsorbed proteins. Thus, the results from this study indicate that fluid shear, residence time of the adsorbed protein, nature of the co-adsorbed proteins, and surface chemistry of the material all play important roles in influencing platelet-surface interactions and that they act in a complex manner to influence the biocompatibility of a material.

Cell attachment and biocompatibility of polytetrafluoroethylene (PTFE) treated with glow-discharge plasma of mixed ammonia and oxygen

The plasma generated from a gas mixture of NH3 plus O2 (NH3 + O2) has been used to impart unique chemical and biological characteristics to polytetrafluoroethylene (PTFE). PTFE treated with NH3 + O2 plasma was physiochemically distinct from surfaces treated with plasma of either NH3 or O2 alone, as determined by electron spectroscopy for chemical analysis (ESCA). The contact angle analysis revealed that the PTFE surfaces became less hydrophobic after plasma treatments. ESCA results indicate the presence of oxygen-containing groups and nitrogen-containing groups at the plasma-treated surfaces. PTFE treated with NH3 + O2 plasma resisted the attachment of platelets and leukocytes in a manner similar to untreated PTFE; however, the attachment of bovine aorta endothelial cells was substantially increased. Once attached, these cells grew to confluency. The increased endothelial cell attachment was higher than that observed following plasma treatment with each gas used separately, which could be attributed to the considerable amount of CF(OR)2-CF2 formed on the NH3 + O2 plasma-treated PTFE surface. At 14 days after subcutaneous implantation in rats, the PTFE wafers treated with NH3 + O2 plasma demonstrated less encapsulation and lower levels of inflammatory cells compared to controls. Collectively, the results suggest that NH3 + O2 plasma treatment imparts a unique character to PTFE and could be useful in certain in vivo applications.

Hydrogels based on poly(ethylene oxide) and poly(tetramethylene oxide) or poly(dimethyl siloxane): synthesis, characterization, in vitro protein adsorption and platelet adhesion

In vitro protein adsorption, platelet adhesion and activation on new hydrogel surfaces, composed of poly(ethylene oxide) (PEO) and poly(tetramethylene oxide) (PTMO) or poly(dimethyl siloxane) (PDMS), were investigated. By varying PEO length (MW = 2000 or 3400), hydrophobic components (PTMO or PDMS) or polymer topology (block or graft copolymers), various physical hydrogels were produced. Their structures were verified by 1H NMR and ATR-IR and the molecular weights were determined by gel permeation chromatography. The hydrogels were soluble in a variety of organic solvents, while absorbed a significant amount of water with preserved three-dimensional structure by physical crosslinking. The dynamic contact angle measurement revealed that the surface hydrophilicity increased by incorporating longer PEO, PEO grafting, and adopting PDMS as a hydrophobic segment instead of PTMO. It was observed from in vitro protein adsorption study that the hydrogels exhibited significantly lower adsorption of human serum albumin (HSA), human fibrinogen (HFg), and IgG, when compared with Pellethane, a commercial polyurethane taken as a control. The hydrogels were attractive for HSA but not sensitive to HFg and IgG. And more than 65% of the proteins detected on the surfaces of the hydrogels were reversibly detached by being treated with an SDS solution. It was evident that the hydrogels synthesized in this study were much more resistant to platelet adhesion than the control, which might depend on the composition of proteins adsorbed on the surfaces and their degree of denaturation. Among the hydrogels tested, PEO3,4kPDMS exhibited albumin-rich and platelet-resistant surfaces, implying a potential candidate for biomaterial.

Poly2-methoxyethylacrylate (PMEA) coated oxygenator: an ex vivo study

PMEA is a hydrophilic polymer coating with a unique design that minimizes the adsorption and denaturation of proteins and blood cells. This study compares thrombus resistance, blood path resistance, thrombocyte profile, and blood trauma of the PMEA coated Capiox membrane oxygenator (Terumo, Japan) vs. an uncoated version.

METHOD

Six calves (mean bodyweight: 75.3 +/- 4.5kg) were placed on cardiopulmonary bypass for 6 hours and randomly assigned to the coated or uncoated oxygenator, with a low heparinisation protocol (ACT > 180s).

RESULTS

Macroscopically, red staining was observed in all uncoated oxygenators, and in none of the coated ones. Inlet pressure was significantly higher in the uncoated group (at 1 h: 279 +/- 25 vs. 175 +/- 11mmHg, p < 0.01 and at 6h: 217 +/- 10 vs. 171(8mmHg, p < 0.01). Thrombocyte count values (corrected for hematocrit and normalized by prebypass values) were significantly higher in the coated group (at 1 h: 76 +/- 6 vs. 53 +/- 13%, p < 0.01 and at 6 h: 70 +/- 6 vs. 44 +/- 26%, p < 0.01). Plasma hemoglobin was below 100mg/L in both groups throughout the experiments.

CONCLUSIONS

When compared with uncoated oxygenator, PMEA coated oxygenator exhibited increased thrombus resistance with lower inlet pressure and lower thrombocyte consumption. In both groups, trauma to red cells was minimal, emphasizing the efficient design of this type of oxygenator.

Hemocompatibility of heparin-coated surfaces and the role of selective plasma protein adsorption

Although several studies have shown that heparin-coated surfaces reduce the activation of both the complement system and the coagulation system, there is still inadequate understanding of the factors initiating and controlling blood activation at these surfaces. We investigated the adsorption profile of 12 common plasma proteins (and the platelet receptor CD41) to a heparin coating (Carmeda BioActive surface (CBAS)) compared to uncoated controls (PVC) by using an in vitro whole blood Chandler-Loop model. Surface bound proteins were studied kinetically by a direct ELISA technique. Western blots were performed on the SDS eluates in order to detect adsorbed cleavage products and denatured proteins. Changes in plasma levels of neutrophil activation markers, platelet activation, coagulation activation, complement activation and the inflammatory response were measured by conventional ELISAs. This study showed significant differences in adsorption patterns among the heparin-coated and the uncoated surfaces, notably for fibronectin, fibrinogen, C3 and high molecular weight kininogen (HMWK). The kinetic studies confirmed the results obtained from Western blots and indicated specific adsorption profiles of plasma proteins. We assume that at least some of the improved blood compatibility of the heparin-coated surfaces may be ascribed to the selective uptake and cleavage of plasma proteins.

Simultaneous measurement of biopolymer-mediated Mac-1 up-regulation and adherence of neutrophils: a novel flow cytometric approach for predicting initial inflammatory interaction with foreign materials

Implantation of any medical device normally causes an inflammatory cell interaction with the foreign material. In vitro cell activation of human neutrophils (Mac-1 upregulation) has been taken as one measure to assess the attributable risk of inflammation due to biopolymers before their clinical application. Mac-1 expression has generally been measured by flow cytometric assays, whereas quantification of neutrophil adhesion to the biopolymer surfaces has been performed by separate and time-consuming assays, e.g. microscopically by differential cell counting. However, due to an increasing number of surface-modified novel biopolymers entering clinical usage, effective testing of their inflammatory potential is now mandatory. To facilitate these analyses, we have developed a novel flow cytometric assay permitting simultaneous measurement of biopolymer-mediated neutrophil activation and adhesion. The biopolymers were used as beads (diameter 25+/-10 microm), and were demonstrated to be non-phagocytosable and non-fluorescent before being co-incubated with whole human blood (range of ratio granulocytes/beads from 5:1 to 1:1). Besides flow cytometric measurement of Mac-1 up-regulated neutrophils as fluorescing events, a fluorescence of the bead population indicates the adherence of activated neutrophils to the biopolymer surface.After establishing this assay, we evaluated it by comparing six different biopolymers. We observed high levels of Mac-1 expression (>70% of positive control) accompanied by increased adhesiveness (>60% of neutrophils) for polyurethane (PUR), polymethylmetacrylate (PMMA), and poly-DL-lactide (PDLLA) beads. Low Mac-1 expression levels (<10%) accompanied by a low percentage of adhering neutrophils (<10%) were observed for polyethylene (PE), polyisoprene (PI), and silicone (SI) beads.

Effects of fibrinogen residence time and shear rate on the morphology and procoagulant activity of human platelets adherent to polymeric biomaterials

Fibrinogen readily adsorbs to the surface of biomaterials and, because of its demonstrated ability to support platelet adhesion and aggregation, plays a role in thrombotic events associated with the implantation of synthetic materials in the human body. Thus, understanding the factors influencing the interactions of fibrinogen with biomaterials, and how platelet responses are affected, is crucial for the development of synthetic materials exhibiting improved blood compatibility. In this study, the effects of fibrinogen residence time and shear rate on the procoagulant activity of adherent platelets, along with their morphologic status, as deduced from scanning electron microscopy, were investigated. To examine whether adherent platelets promoted the generation of thrombin, polymeric materials (polytetrafluoroethylene, polyethylene, and silicone rubber) preadsorbed with fibrinogen were exposed to platelet suspensions at different wall shear rates and then incubated with clotting factors for 5 minutes under static conditions. The amount of thrombin generated per platelet was calculated from the optical density of the color developed by adding substrate S-2238. Scanning electron microscopy images of the platelets revealed that the platelets exhibited different morphologies, depending on the shear rate and residence time of the adsorbed fibrinogen. Platelets ranged from their normal discoid shape observed primarily under static conditions, to that of fully spread platelets. Results from this study show that platelets, in the presence of shear forces, undergo activation on exposure to surfaces on which adsorbed fibrinogen has resided for short residence times rather than long residence times. Interestingly, studies examining the procoagulant responses of such adherent platelets demonstrated that the platelets attached to the fibrinogen coated materials did not promote significant thrombin generation. Such low prothrombinase activity of adherent platelets suggests that adsorbed fibrinogen, while capable of supporting platelet adhesion and spreading on biomaterials, does not necessarily enhance the procoagulant activity of adherent platelets.

Aspects of the physical chemistry of polymers, biomaterials and mineralised tissues investigated with atomic force microscopy (AFM)

Beyond being merely a tool for measuring surface topography, atomic force microscopy (AFM) has made significant contributions to various scientific areas dealing with physical chemistry processes. This paper presents aspects of the physical chemistry at surfaces and interfaces of polymers, biomaterials and tissues investigated with AFM. Selected examples presented include surface induced self-assembly of polymer blends, copolymer interfacial reinforcement of immiscible homopolymers, protein adsorption on biomaterials and erosion of mineralised human tissues. In these areas, AFM is a useful and versatile tool to study structural or dynamic sample properties including thermodynamically driven surface evolution of polymer surfaces, lateral surface composition of interfaces, adsorption processes, and the metrology of demineralisation phenomena.

Blood compatible aspects of poly(2-methoxyethylacrylate) (PMEA)--relationship between protein adsorption and platelet adhesion on PMEA surface

Platelet adhesion and spreading is suppressed when a poly(2-methoxyethylacrylate) (PMEA) surface is used, compared with other polymer surfaces. To clarify the reason for this suppression, the relationship among the amount of the plasma protein adsorbed onto PMEA, its secondary structure and platelet adhesion was investigated. Poly(2-hydroxyethylmethacrylate) (PHEMA) and polyacrylate analogous were used as references. The amount of protein adsorbed onto PMEA was very low and similar to that absorbed onto PHEMA. Circular dichroism (CD) spectroscopy was applied to examine changes in the secondary structure of the proteins after adsorption onto the polymer surface. The conformation of the proteins adsorbed onto PHEMA changed considerably, but that of proteins adsorbed onto PMEA differed only a little from the native one. These results suggest that low platelet adhesion and spreading are closely related to the low degree of the denaturation of the protein adsorbed onto PMEA. PMEA could be developed as a promising material to produce a useful blood-contacting surface for medical devices.

Cytochrome c unfolding on an anionic surface

It is now well accepted that the adsorption of proteins to solid supports sometimes involves surface-mediated unfolding. A detailed understanding of the adsorption and surface-mediated unfolding process is lacking. We selected a well studied protein, horse heart cytochrome c, and a weakly ionic support to examine some of the characteristics of protein adsorption under near-physiological conditions. We used high-performance liquid chromatography (HPLC) to investigate the effect of temperature on surface-mediated unfolding. Samples of cytochrome c were introduced to an anionic support, and a NaCl gradient was used to desorb the protein at different times and temperatures. The profiles and retention times were monitored to examine the adhesive properties of cytochrome c to the anionic support. We found that protein retention increased with time at temperatures as low as 0 degrees C, and a significant loss of cytochrome c occurred between 55 degrees C and 70 degrees C. The loss of recovery of cytochrome c indicates irreversible surface-mediated unfolding. The changes in retention time may indicate more subtle transitions, including reversible surface-mediated unfolding of cytochrome c. These results suggest that perturbations in the structure as well as unfolding of cytochrome c can be detected at a lower temperature on an anionic surface than in solution thereby acting like a catalyst for protein unfolding.