Residence-time dependent changes in fibrinogen adsorbed to polymeric biomaterials

Enhancement of protein immobilization on polydimethylsiloxane using a synergistic combination of polydopamine and micropattern surface modification

Understanding protein interactions at biointerfaces is critical for the improved design of biomaterials and medical devices. Polydimethylsiloxane (PDMS) is used for numerous device applications, and surface modifications can enhance protein immobilization and the response to cells. A multifunctional approach combining topographical and biochemical modifications was applied to PDMS by fabricating 10-20 µm scale patterns onto PDMS surfaces and by coating with polydopamine (PDA). The modifications were confirmed by surface characterization and bovine serum albumin (BSA), fibrinogen (Fg), and fetuin-A (Fet-A) were radiolabeled with 125I. The amounts of protein attached to the surface before and after elution with sodium dodecyl sulfate (SDS) were quantified from single and complex multi-protein solutions to determine protein stability and competitive binding. The PDA coatings were the most stable and capable of immobilizing the highest levels of all proteins. Furthermore, combinations of PDA coatings with the smallest micropatterns provided an additional improvement, enhancing the amount immobilized and the stability. The adsorption of BSA and Fg from plasma demonstrated competitive binding and possible orientation changes, respectively. It was determined that Fet-A, a less studied protein, adsorbed from plasma at low levels, but the adsorption from fetal bovine serum (FBS) was significantly greater, providing important quantification data from radiolabeling that is relevant to many cell culture studies. Overall, combining topography and PDA modification has a synergistic effect on improving protein immobilization. These findings provide new insight on the quantities of proteins bound to PDMS and PDA coatings with implications for cell interactions in various biotechnology and medical applications.

The blood compatibility challenge. Part 3: Material associated activation of blood cascades and cells

Following protein adsorption/activation which is the first step after the contact of material surfaces and whole blood (part 2), fibrinogen is converted to fibrin and platelets become activated and assembled in the form of a thrombus. This thrombus formation is the key feature that needs to be minimized in the creation of materials with low thrombogenicity. Further aspects of blood compatibility that are important on their own are complement and leukocyte activation which are also important drivers of thrombus formation. Hence this review summarizes the state of knowledge on all of these cascades and cells and their interactions. For each cascade or cell type, the chapter distinguishes statements which are in widespread agreement from statements where there is less of a consensus. STATEMENT OF SIGNIFICANCE: This paper is part 3 of a series of 4 reviews discussing the problem of biomaterial associated thrombogenicity. The objective was to highlight features of broad agreement and provide commentary on those aspects of the problem that were subject to dispute. We hope that future investigators will update these reviews as new scholarship resolves the uncertainties of today.

Toward endothelialization via vascular endothelial growth factor immobilization on cell-repelling functional polyurethanes

We screened a family of nonspecific cell-repelling polyurethanes (PUs) whose backbones are attached with epoxy group-terminated polyethylene glycol (PEG) side chains. Water incubation of the PU films (with 9.2-31.1 wt % PEG) caused a surface enrichment of PEG chains where vascular endothelial growth factor (VEGF) was grafted by forming secondary amine linkages between VEGF molecules and the PEG spacer. These linkages are still ionizable similar to original primary amines in VEGF, thereby retaining the original charge distribution on VEGF macromolecules. This charge conservation together with PEG steric repulsion helped to preserve VEGF conformation and bioactivity. The PU substrates with suitable hard segments contents and VEGF surface densities can selectively induce endothelial cells (ECs) adhesion and proliferation toward endothelialization. Moreover, the PU substrates, even grafted with fibrinogen (Fg), cannot trigger platelet adhesion and deformation, suggesting an inactive conformation of the grafted Fg. Thus enough antithrombogenicity of the PU substrates could be expected before full endothelialization. These PU materials might be applied onto the lumens of vascular grafts, potentially stimulating luminal endothelialization in vivo. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 965-977, 2019.

Engineering Proteins at Interfaces: From Complementary Characterization to Material Surfaces with Designed Functions

Once materials come into contact with a biological fluid containing proteins, proteins are generally—whether desired or not—attracted by the material's surface and adsorb onto it. The aim of this Review is to give an overview of the most commonly used characterization methods employed to gain a better understanding of the adsorption processes on either planar or curved surfaces. We continue to illustrate the benefit of combining different methods to different surface geometries of the material. The thus obtained insight ideally paves the way for engineering functional materials that interact with proteins in a predetermined manner.

Understanding interactions between biomaterials and biological systems using proteomics

The role that biomaterials play in the clinical treatment of damaged organs and tissues is changing. While biomaterials used in permanent medical devices were required to passively take over the function of a damaged tissue in the long term, current biomaterials are expected to trigger and harness the self-regenerative potential of the body in situ and then to degrade, the foundation of regenerative medicine. To meet these different requirements, it is imperative to fully understand the interactions biomaterials have with biological systems, in space and in time. This knowledge will lead to a better understanding of the regenerative capabilities of biomaterials aiding their design with improved functionalities (e.g. biocompatibility, bioactivity). Proteins play a pivotal role in the interaction between biomaterials and cells or tissues. Protein adsorption on the material surface is the very first event of this interaction, which is determinant for the subsequent processes of cell growth, differentiation, and extracellular matrix formation. Against this background, the aim of the current review is to provide insight in the current knowledge of the role of proteins in cell-biomaterial and tissue-biomaterial interactions. In particular, the focus is on proteomics studies, mainly using mass spectrometry, and the knowledge they have generated on protein adsorption of biomaterials, protein production by cells cultured on materials, safety and efficacy of new materials based on nanoparticles and the analysis of extracellular matrices and extracellular matrix-derived products. In the outlook, the potential and limitations of this approach are discussed and mass spectrometry imaging is presented as a powerful technique that complements existing mass spectrometry techniques by providing spatial molecular information about the material-biological system interactions.

Anticardiolipin antibodies in various diseases in Taiwan: a retrospective analysis

The goals of this study are to determine the frequency of anticardiolipinantibodies (ACA) in patients with various diseases and to evaluate the clinical significance of ACA in Taiwan. We collected 690 patients from ACA laboratory records. They were divided into eight groups in order to compare ACA percentages. Positive rates of ACA in different disease groups were below 20%, except for 38.2% in autoimmune diseases with vascular thrombosis. Compared with old stroke, the ACA positivity in young stroke was not significantly different (P 0.482). The positive percentage of lupus anticoagulant (LA) (2.86%) was lower than that of ACA (15.66%) in young stroke (P 0.015). Among patients with pregnancy loss or prematurity, the ACA positivity in lupus patients (44.44%) was higher than without lupus (9.76%; P 0.01). The prevalence of ACA is higher in patients with vascular thrombosis complicated by autoimmune diseases than with thrombosis alone in Taiwan. Young and old stroke do not differ in ACA positivity. Moreover, ACA is more prevalent than LA for young stroke related coagulation.The ACA positivity for pregnancy loss or prematurity is very low in Taiwan. In summary, this is the first report on the frequency of ACA and other coagulation factors in various diseases in Taiwan.

pH Dependence of Adsorbed Fibrinogen Conformation and Its Effect on Platelet Adhesion

Quartz crystal microbalance with dissipation (QCM-D) and dual polarization interferometry (DPI) were used to investigate fibrinogen (Fib) adsorption behavior on different surfaces by changing the pH value. Moreover, integrin adhesion to the adsorbed Fibs was studied using DPI. Qualitative and quantitative studies of platelet adhesion to the adsorbed Fibs were performed using scanning electron microscopy (SEM), confocal laser scanning microscope (CLSM), and released lactate dehydrogenase (LDH) assay. Experimental results indicated that the conformation and orientation of the absorbed Fibs depended on surface property and pH cycling. For the hydrophilic surface, Fibs adsorbed at pH 7.4 and presented a αC-hidden orientation. As a result, no integrin adhesion was observed, and a small number of platelets were adhered because the αC-domains were hidden under the Fib molecule. By changing the rinsing solution pH from 7.4 to 3.2 and then back to 7.4, the adsorbed Fib orientation became αC-exposed via the transformation of Fib conformation during pH cycling. Therefore, integrin adhesion was more likely to occur, and more platelets were adhered and activated. For the hydrophobic surface, the adsorbed Fibs became more spread and stretched due to the strong interaction between the Fibs and surface. αC-exposed orientation remained unchanged when the rinsing solution pH changed from 7.4 to 3.2 and then back to 7.4. Therefore, a large number of integrins and platelets were adhered to the adsorbed Fibs, and almost all of the adhered platelets were activated.

Fabrication of Semiordered Nanopatterned Diamond-like Carbon and Titania Films for Blood Contacting Applications

Biomaterials with the ability to interface with, but not activate, blood components are essential for a multitude of medical devices. Diamond-like carbon (DLC) and titania (TiO2) have shown promise for these applications; however, both support platelet adhesion and activation. This study explored the fabrication of nanostructured DLC and TiO2 thin film coatings using a block copolymer deposition technique that produced semiordered nanopatterns with low surface roughness (5-8 nm Rrms). These surfaces supported fibrinogen and plasma protein adsorption that predominantly adsorbed between the nanofeatures and reduced the overall surface roughness. The conformation of the adsorbed fibrinogen was altered on the nanopatterned surfaces as compared with the planar surfaces to reveal higher levels of the platelet binding region. Planar DLC and TiO2 coatings supported less platelet adhesion than nanopatterned DLC and TiO2. However, platelets on the nanopatterned DLC coatings were less spread indicating a lower level of platelet activation on the nanostructured DLC coatings compared with the planar DLC coatings. These data indicated that nanostructured DLC coatings may find application in blood contacting medical devices in the future.

Multifaceted prospects of nanocomposites for cardiovascular grafts and stents

Cardiovascular disease is the leading cause of death across the globe. The use of synthetic materials is indispensable in the treatment of cardiovascular disease. Major drawbacks related to the use of biomaterials are their mechanical properties and biocompatibility, and these have to be circumvented before promoting the material to the market or clinical setting. Revolutionary advancements in nanotechnology have introduced a novel class of materials called nanocomposites which have superior properties for biomedical applications. Recently, there has been a widespread recognition of the nanocomposites utilizing polyhedral oligomeric silsesquioxane, bacterial cellulose, silk fibroin, iron oxide magnetic nanoparticles, and carbon nanotubes in cardiovascular grafts and stents. The unique characteristics of these nanocomposites have led to the development of a wide range of nanostructured copolymers with appreciably enhanced properties, such as improved mechanical, chemical, and physical characteristics suitable for cardiovascular implants. The incorporation of advanced nanocomposite materials in cardiovascular grafts and stents improves hemocompatibility, enhances antithrombogenicity, improves mechanical and surface properties, and decreases the microbial response to the cardiovascular implants. A thorough attempt is made to summarize the various applications of nanocomposites for cardiovascular graft and stent applications. This review will highlight the recent advances in nanocomposites and also address the need of future research in promoting nanocomposites as plausible candidates in a campaign against cardiovascular disease.

Proteins, platelets, and blood coagulation at biomaterial interfaces

Blood coagulation and platelet adhesion remain major impediments to the use of biomaterials in implantable medical devices. There is still significant controversy and question in the field regarding the role that surfaces play in this process. This manuscript addresses this topic area and reports on state of the art in the field. Particular emphasis is placed on the subject of surface engineering and surface measurements that allow for control and observation of surface-mediated biological responses in blood and test solutions. Appropriate use of surface texturing and chemical patterning methodologies allow for reduction of both blood coagulation and platelet adhesion, and new methods of surface interrogation at high resolution allow for measurement of the relevant biological factors.

Facets of protein assembly on nanostructured titanium oxide surfaces

One key for the successful integration of implants into the human body is the control of protein adsorption by adjusting the surface properties at different length scales. This is particularly important for titanium oxide, one of the most common biomedical interfaces. As for titania (TiO(2)) the interface is largely defined by its crystal surface structure, it is crucial to understand how the surface crystallinity affects the structure, properties and function of protein layers mediating subsequent biological reactions. For rutile TiO(2) we demonstrate that the conformation and relative amount of human plasma fibrinogen (HPF) and the structure of adsorbed HPF layers depend on the crystal surface nanostructure by employing thermally etched multi-faceted TiO(2) surfaces. Thermal etching of polycrystalline TiO(2) facilitates a nanoscale crystal faceting and, thus, the creation of different surface nanostructures on a single specimen surface. Atomic force microscopy shows that HPF arranges into networks and thin globular layers on flat and irregular crystal grain surfaces, respectively. On a third, faceted category we observed an alternating conformation of HPF on neighboring facets. The bulk grain orientation obtained from electron backscatter diffraction and thermodynamic mechanisms of surface reconstruction during thermal etching suggest that the grain and facet surface-specific arrangement and relative amount of adsorbed proteins depend on the associated free crystal surface energy. The implications for potentially favorable TiO(2) crystal facets regarding the inflammatory response and hemostasis are discussed with a view to the advanced surface design of future implants.

Time-dependent conformational changes in adsorbed albumin and its effect on platelet adhesion

Recent studies have shown that platelets can adhere to adsorbed albumin (Alb) through a receptor-mediated mechanism, but only if the Alb undergoes more than a critical degree of adsorption-induced unfolding. The objectives of this research were to investigate whether Alb that was initially adsorbed in a manner that induced unfolding that was less than this critical level would undergo further unfolding with time and, if so, whether this would induce the onset of platelet adhesion once this critical level was exceeded. To address these questions, CD spectropolarimetry was used to monitor the structure of Alb on OH- and CH(3)-functionalized alkanethiol self-assembled monolayer surfaces, with the Alb initially adsorbed under conditions resulting in degrees of unfolding that were below this critical level, and then the adsorbed Alb layers were aged over 6 months in sterile physiological saline at 37 °C. Platelet adhesion to Alb was quantified at selected time points via a lactate dehydrogenase (LDH) assay. The results indicate that an adsorbed Alb layer does undergo further structural changes with increasing residence time and supports platelet adhesion once it unfolds beyond the previously determined critical level. These results may be relevant to the clinically observed problem of the onset of late-thrombosis, which occurs on cardiovascular implants such as drug-eluting stents.

Exposure of the lysine in the gamma chain dodecapeptide of human fibrinogen is not enhanced by adsorption to poly(ethylene terephthalate) as measured by biotinylation and mass spectrometry

Conformational changes in adsorbed fibrinogen may enhance the exposure of platelet adhesive sites that are inaccessible in solution. To test this hypothesis, mass spectrometric methods were developed to quantify chemical modification of lysine residues following adsorption of fibrinogen to biomaterials. The quantitative method used an internal standard consisting of isotope-labeled fibrinogen secreted by human HepG2 cells in culture. Lysine residues in the internal standard were partially reacted with NHS-biotin. For the experimental samples, normal human fibrinogen was adsorbed to poly(ethylene terephthalate) (PET) particles. The adsorbed fibrinogen was reacted with NHS-biotin and then eluted from the particles. Constant amounts of internal standard were added to sample fibrinogen and analyzed by liquid chromatography/tandem mass spectrometry. Biotinylation of the lysine residue in the platelet-adhesive gamma chain dodecapeptide (GCDP) was quantified by comparison with the internal standard. Approximately 80% of the GCDP peptides were biotinylated when fibrinogen was reacted with NHS-biotin in solution or adsorbed onto PET. These results are generally consistent with previous antibody binding studies and suggest that other regions of fibrinogen may be crucial in promoting platelet adhesion to materials. The results do not directly address but are consistent with the hypothesis that only activated platelets adhere to adsorbed fibrinogen.

Surface characterization and protein interactions of segmented polyisobutylene-based thermoplastic polyurethanes

The surface properties and biocompatibility of a class of thermoplastic polyurethanes (TPUs) with applications in blood-contacting medical devices have been studied. Thin films of commercial TPUs and novel polyisobutylene (PIB)-poly(tetramethylene oxide) (PTMO) TPUs were characterized by contact angle measurements, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM) imaging. PIB-PTMO TPU surfaces have significantly higher C/N ratios and lower amounts of oxygen than the theoretical bulk composition, which is attributed to surface enrichment of PIB. Greater differences in the C/N ratios were observed with the softer compositions due to their higher relative amounts of PIB. The contact angles were higher on PIB-PTMO TPUs than on commercial polyether TPUs, indicating lower surface energy. AFM imaging showed phase separation and increasing domain sizes with increasing hard segment content. The biocompatibility was investigated by quantifying the adsorption of fouling and passivating proteins, fibrinogen (Fg) and human serum albumin (HSA) respectively, onto thin TPU films spin coated onto the electrode of a quartz crystal microbalance with dissipation monitoring (QCM-D). Competitive adsorption experiments were performed with a mixture of Fg and albumin in physiological ratio followed by binding of GPIIb-IIIa, the platelet receptor ligand that selectively binds to Fg. The QCM-D results indicate similar adsorbed amounts of both Fg and HSA on PIB-PTMO TPUs and commercial TPUs. The strength of the protein interactions with the various TPU surfaces measured with AFM (colloidal probe) was similar among the various TPUs. These results suggest excellent biocompatibility of these novel PIB-PTMO TPUs, similar to that of polyether TPUs.

Effects of protein solution composition on the time-dependent functional activity of fibrinogen on surfaces

Protein function affects subsequent biological processes such as cell adhesion and thrombus formation. We have developed tools to detect the biological activity of fibrinogen using AFM techniques. In this work, we measure the effects of solution concentration, residence time, and protein competition with BSA on the time-dependent functional changes in adsorbed fibrinogen on mica surface. AFM probes were functionalized with monoclonal antibodies recognizing fibrinogen gamma 392-411, which includes the platelet binding dodecapeptide region. Results show good correlation between changes in biological activity of adsorbed fibrinogen at the molecular scale measured by AFM and platelet adhesion measured at a macroscale. Furthermore, the results show that inclusion of BSA into the solution moves the peak biological activity of fibrinogen to earlier time points. These results illustrate a complex and dynamic biological interface and offer new opportunities for improved insights into the molecular basis for the biological response to biomaterials.

Assisted peptide folding by surface pattern recognition

Natively disordered proteins belong to a unique class of biomolecules whose function is related to their flexibility and their ability to adopt desired conformations upon binding to substrates. In some cases these proteins can bind multiple partners, which can lead to distinct structures and promiscuity in functions. In other words, the capacity to recognize molecular patterns on the substrate is often essential for the folding and function of intrinsically disordered proteins. Biomolecular pattern recognition is extremely relevant both in vivo (e.g., for oligomerization, immune response, induced folding, substrate binding, and molecular switches) and in vitro (e.g., for biosensing, catalysis, chromatography, and implantation). Here, we use a minimalist computational model system to investigate how polar/nonpolar patterns on a surface can induce the folding of an otherwise unstructured peptide. We show that a model peptide that exists in the bulk as a molten globular state consisting of many interconverting structures can fold into either a helix-coil-helix or an extended helix structure in the presence of a complementary designed patterned surface at low hydrophobicity (3.7%) or a uniform surface at high hydrophobicity (50%). However, we find that a carefully chosen surface pattern can bind to and catalyze the folding of a natively unfolded protein much more readily or effectively than a surface with a noncomplementary or uniform distribution of hydrophobic residues.

Conformational behavior of fibrinogen on topographically modified polymer surfaces

The influence of topographical surface features at the submicron scale on the structural changes in the surface-adsorbed fibrinogen was investigated on poly(lactic-co-glycolic-acid) (PLGA) films. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) was employed in this study for the induced conformational change of fibrinogen over various adsorption times, while the adsorption kinetics of fibrinogen was quantified by the enzyme linked immunosorbent assay (ELISA). When a PLGA surface is modified topographically, the adsorbed fibrinogen undergoes less conformational change when compared to adsorption on the pristine PLGA surface. The extent of conformational change is related to platelet adhesion. Reduced thrombogenicity was demonstrated by the higher ratios of alpha-helix to beta-turn and beta-sheet to beta-turn structures on the topographic PLGA film, which suggests that topographical manipulation of surfaces is a viable approach to influence the thrombogenicity of surfaces.

Sum Frequency Generation Studies on Bioadhesion: Elucidating the Molecular Structure of Proteins at Interfaces

The study of bioadhesion is significant to applications in a variety of scientific fields. Techniques that are surface sensitive need to be utilized to examine these kinds of systems because bioadhesion occurs at the interface between two surfaces. Recently, Sum Frequency Generation (SFG) has been applied to investigate different bioadhesive processes because of its intrinsic surface specificity, excellent sensitivity and its ability to perform experiments in situ. SFG studies on the bioadhesion of fibrinogen, factor XII and mefp-3 on various surfaces will be discussed in this review.

Early stage reactivity and in vitro behavior of silica-based bioactive glasses and glass-ceramics

The surface reactivity of different sets of glasses and glass-ceramics belonging to the SiO(2)-P(2)O(5)-CaO-MgO-K(2)O-Na(2)O system have been investigated. The attention was focused on the role of their composition on the bioactivity kinetics, in terms of pH modifications, silica-gel formation and its evolution toward hydroxycarbonatoapatite, after different times of soaking in simulated body fluid. Glasses and glass ceramics have been characterized by thermal analysis, SEM-EDS observations and phase analysis (XRD). XPS measurements have been carried out on the most representative set of sample in order to evaluate the evolution of the surface species during the growth of silica-gel and hydroxycarbonatoapatite. The response of murine fibroblast 3T3 to the material before and after a conditioning pre-treatment (immersion in SBF) has been investigated on the same set of samples in order to point out the role of the bioactivity mechanism on cell viability. The main differences among the various glasses have been related to the modifier oxides ratio and to the MgO content, which seems to have an influence on the glass stability, both in terms of thermal properties and surface reactivity. The surface characterization and in vitro tests revealed few variations in the reactivity of the different glasses and glass-ceramics in their pristine form. On the contrary, the different surface properties before and after the pre-treatment in SBF seem to play a role on the biocompatibility of both glass and glass-ceramics, due to the different ion release and hydrophilicity of the surfaces, affecting both cell viability and protein adsorption.

Immunological identification of fibrinogen in dual-component protein films by AFM imaging

The success of long-term blood-contacting implanted devices is largely dependent upon the interaction of the blood components with the device biomaterial surface. The ability to study these interactions has been hindered by a lack of methods to measure single-molecule interactions in complex multi-protein environments similar to the environment found in vivo. In this paper, we demonstrate the use of atomic force microscopy (AFM) in conjunction with gold nanolabels to detect the protein fibrinogen under aqueous conditions without the topographical clues usually necessary for high resolution visualization. BSA was patterned onto both muscovite mica and plasma-treated polydimethylsiloxane (PDMS) substrates and these test substrates were subsequently backfilled with fibrinogen to yield a featureless protein layer. The fibrinogen in this dual-protein layer was detected using high resolution AFM imaging following infusion of anti-fibrinogen conjugated with nanogold particles. This AFM immuno-detection technique will potentially be applicable to complex multi-component protein films adsorbed on clinically relevant polymers used in medical devices.

Measuring the time-dependent functional activity of adsorbed fibrinogen by atomic force microscopy

In this work, we measured time-dependent functional changes in adsorbed fibrinogen by measuring antigen-antibody debonding forces with atomic force microscopy (AFM). AFM probes were functionalized with monoclonal antibodies recognizing fibrinogen gamma 392-411, which includes the platelet binding dodecapeptide region. These probes were used to collect force measurements between the antibody and fibrinogen on mica substrates and the probability of antigen recognition was calculated. Statistical analysis showed that the probability of antibody-antigen recognition peaked at approximately 45 min postadsorption and decreased with increasing residence time. Macroscale platelet adhesion measurements on these mica substrates were determined to be greatest at fibrinogen residence times of approximately 45 min, which correlated well with the functional activity of adsorbed fibrinogen as measured by the modified AFM probes. These results demonstrate the utility of this approach for measuring protein function at or near the molecular scale and offers new opportunities for improved insights into the molecular basis for the biological response to biomaterials.

Modern biomaterials: a review - bulk properties and implications of surface modifications

This review concerns the importance of length and time on physicochemical interactions between living tissue and biomaterials that occur on implantation. The review provides information on material host interactions, materials for medical applications and cell surface interactions, and then details the extent of knowledge concerning the role(s) that surface chemistry and topography play during the first stage of implant integration, namely protein adsorption. The key points are illustrated by data from model in vitro studies. Host implant interactions begin nanoseconds after first contact and from then on are in a state of flux due to protein adsorption, cell adhesion and physical and chemical alteration of the implanted material. The many questions concerning the conformational form and control of bound proteins and how this may impact on cell adhesion in the first instance and later on cell signalling and implant integration can be answered by systematic investigations using model materials. Only then we will be in a more informed position to design new materials for use in the body.

Effect of biopassive and bioactive surface-coatings on the hemocompatibility of membrane oxygenators

Postoperative complications associated with cardiopulmonary bypass (CPB) surgery and extracorporeal circulation (ECC) procedures are still a major clinical issue. Improving the hemocompatibility of blood contacting devices used for ECC procedures may ameliorate various postpump syndromes. In a simulated CPB model using human blood, we investigated the hemocompatibility, fibrinogen adsorption, and platelet receptor (GPIIb-IIIa) binding capacity of surface-modified membrane oxygenators (Jostra Quadrox). Three groups were compared: (i) biopassive protein coatings (SafeLine), (ii) bioactive heparin coatings (BioLine), and (iii) noncoated controls. During the 2 h recirculation period, plasma concentrations of activation markers for platelets (beta-thromboglobulin), inflammation (elastase), complement (C5a), and coagulation (prothrombin fragment 1+2, thrombin-antithrombin III) were lower in the groups with biopassive and bioactive coatings compared to the noncoated group (p < 0.01). These parameters did not significantly differ between the two surface-coated groups, except for complement activation: C5a levels were higher in the biopassive group compared to the bioactive group (p < 0.01). Moreover, surface-coated oxygenators showed less fibrinogen adsorption, GPIIb-IIIa binding, and platelet/leukocyte adhesion (p < 0.01). We assume that fewer fibrinogen and platelet receptor molecules bound to the surface-coated oxygenator surfaces results in fewer platelet adhesion and activation, which will significantly contribute to the improved hemocompatibility of the biopassive and bioactive oxygenators. Our results suggest that the application of bioactive oxygenators (BioLine) during CPB surgery may reduce postoperative complications for the patient more effectively than biopassive oxygenators (SafeLine).

Computational modeling of factor Xa inhibition by immobilized tissue factor pathway inhibitor

Coating surfaces of implanted devices with anticoagulants can reduce thrombosis and studies using a recombinant form of endogenous tissue factor pathway inhibitor (rTFPI) are promising. The anticoagulant function of immobilized rTFPI is thought to occur primarily by its inhibition of plasma clotting factor Xa (FXa); however the kinetics of this reaction at a surface are as yet unknown. To better understand the surface inhibition reaction under flow conditions, a theoretical model was developed delineating the roles of mass transport and reaction kinetics for an in vitro parallel plate device used in prior experimental studies [Hall et al., J. Biomech. Eng. 120:484-490, 1998]. As a first approximation, the kinetics of inhibition of FXa by rTFPI reported for static, homogeneous systems was considered. The unsteady convection-diffusion equation was solved for different wall-shear rates and inlet concentrations of FXa using the computational fluid dynamics software CFD-ACE (ESI Software Group). The results show that the heterogeneous inhibition reaction is diffusion controlled prior to saturation of the rTFPI. The experimental results compare favorably with the model at the lower shear rates (100-400 s(-1)). At higher shear rates (>400 s(-1)) the theoretical results follow the same trend as the experimental results but show a greater inhibition of FXa, implying an effect of flow or shear on the inhibition reaction.

Cell adhesion on nanofibrous polytetrafluoroethylene (nPTFE)

Here, we described the in vitro biocompatibility of a novel nanostructured surface composed of PTFE as a potential polymer for the prevention of adverse host reactions to implanted devices. The foreign body response is characterized at the tissue-material interface by several layers of macrophages and large multinucleated cells known as foreign body giant cells (FBGC), and a fibrous capsule. The nanofibers of nanofibrous PTFE (nPTFE) range in size from 20 to 30 nm in width and 3-4 mm in length. Glass surfaces coated with nPTFE (produced by jet-blowing of PTFE 601A) were tested under in vitro conditions to characterize the amount of protein adsorption, cell adhesion, and cell viability. We have shown that nPTFE adsorbs 495 +/- 100 ng of bovine serum albumin (BSA) per cm2. This level was considerably higher than planar PTFE, most likely due to the increase in hydrophobicity and available surface area, both a result of the nanoarchitecture. Endothelial cells and macrophages were used to determine the degree of cell adsorption on the surface of the nanostructured polymer. Both cell types were significantly more round and occupied less area on nPTFE as compared to tissue culture polystyrene (TCPS). Furthermore, a larger majority of the cells on the nPTFE were dead compared to TCPS, at dead-to-live ratios of 778 +/- 271 to 1 and 23 +/- 5.6 to 1, respectively. Since there was a high amount of cell death (due to either apoptosis or necrosis), and the foreign body response is a form of chronic inflammation, an 18 cytokine Luminex panel was performed on the supernatant from macrophages adherent on nPTFE and TCPS. As a positive control for inflammation, lipopolysaccharide (LPS) was added to macrophages on TCPS to estimate the maximum inflammation response of the macrophages. From the data presented with respect to IL-1, TNF-alpha, IFN-gamma, and IL-5, we concluded that nPTFE is nonimmunogenic and should not yield a huge inflammatory response in vivo, and cell death observed on the surface of nPTFE was likely due to apoptosis resulting from the inability of cells to spread on these surface. On the basis of the production of IL-1, IL-6, IL-4, and GM-CSF, we concluded that FBGC formation on nPTFE may be decreased as compared to materials known to elicit FBGC formation in vivo.

The antithrombogenic potential of a polyhedral oligomeric silsesquioxane (POSS) nanocomposite

We have developed a nanocomposite using a silica nanocomposite polyhedral oligomeric silsesquioxane (POSS) and poly(carbonate-urea)urethane (PCU) for potential use in cardiovascular bypass grafts and the microvascular component of artificial capillary beds. In this study, we sought to compare its antithrombogenicity to that of conventional polymers used in vascular bypass grafts so as to improve upon current patency rates, particularly in the microvascular setting. Using atomic force microscopy (AFM) and transmission electron microscopy (TEM), surface topography and composition were studied, respectively. The ability of the nanocomposite surface to repel both proteins and platelets in vitro was assessed using thromboelastography (TEG), fibrinogen ELISA assays, antifactor Xa assays, scanning electron microscopy (SEM), and platelet adsorption tests. TEG analysis showed a significant decrease in clot strength (one-way ANOVA, p < 0.001) and increase in clot lysis (one-way ANOVA, p < 0.0001) on the nanocomposite when compared to both poly(tetrafluoroethylene) (PTFE) and PCU. ELISA assays indicate lower adsorption of fibrinogen to the nanocomposite compared to PTFE (one-way ANOVA, p < 0.01). Interestingly, increasing the concentration of POSS nanocages within these polymers was shown to proportionately inhibit factor X activity. Platelet adsorption at 120 min was also lower compared to PTFE and PCU (two-way ANOVA, p < 0.05). SEM images showed a "speckled" morphologic pattern with Cooper grades I platelet adsorption morphology on the nanocomposite compared to PTFE with grade IV morphology. On the basis of these results, we concluded that POSS nanocomposites possess greater thromboresistance than PTFE and PCU, making it an ideal material for the construction of both bypass grafts and microvessels.

Conformational Changes of Fibrinogen after Adsorption

The adsorption behavior of fibrinogen to two biomedical polyurethanes and a perfluorinated polymer has been investigated. Changes in the secondary structure of adsorbed fibrinogen were monitored using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and sum frequency generation vibrational spectroscopy (SFG). SFG measurements were performed in the amide I range as well as in the C-H/N-H stretching range. Amide I signals from SFG demonstrate that fibrinogen has post-adsorption conformational changes that are dependent upon the polymer surface properties. For example, strong attenuation of the amide I and N-H stretching signals with increasing residence time was observed for fibrinogen adsorbed to poly(ether urethane) but not for the other two polymers. This change is not readily observed by ATR-FTIR. Differences in the observed spectral changes for fibrinogen adsorbed to each polymer are explained by different initial binding mechanisms and post-adsorption conformational changes.

Formation of viscoelastic protein layers on polymeric surfaces relevant to platelet adhesion

The hemocompatibility of biomaterials is highly dependent on the adhesion and activation of platelets. Surface-adsorbed fibrinogen has a dominant role in promoting platelet adhesion to artificial surfaces by binding glycoprotein IIb-IIIa (GPIIb-IIIa), the major platelet membrane receptor. Using quartz crystal microbalance with dissipation monitoring (QCM-D), we have investigated the material-dependent binding kinetics of purified GPIIb-IIIa to polymer-adsorbed fibrinogen. The following ranking of polymer-adsorbed mass (fibrinogen and GPIIb-IIIa) to test polymers could be established: poly[desaminotyrosyl-tyrosine ethyl (DTE) carbonate]/poly(lactide-co-glycolide)>poly[DTE co-5% poly(ethylene glycol) carbonate]. The QCM-D fibrinogen adsorption data were confirmed using an immunofluorescence assay. A synthetic RGD-containing peptide, but not a control peptide, inhibited GPIIb-IIIa binding to polymer-adsorbed fibrinogen, demonstrating the specificity of binding. Importantly, the binding efficiency of purified GPIIb-IIIa to polymer-adsorbed fibrinogen correlated with increased platelet adhesion in an in vitro model. Theoretical simulations using a Voight-based model provided quantitative data on the thickness and viscoelastic properties of the polymer-adsorbed protein layers. The precision of the modeling technique was limited with respect to the shear moduli values, leading to large variations. However, the other modeling parameters showed reproducible results. The thickness of both protein layers was polymer-dependent and ranged from 5 to 35 nm and the viscosity from 0.001 to 0.005 kg/ms, whereas the protein layer densities showed little differences between the test polymers. These results suggest that material-dependent changes in the thickness and viscoelastic properties of adsorbed fibrinogen-GPIIb-IIIa layers are crucial factors in the binding behavior of platelets to biomaterials.

Effects of single-walled carbon nanotubes on the functions of plasma proteins and potentials in vascular prostheses

BACKGROUND

Nonwoven single-walled nanotubes (SWNTs) are novel materials with a nanotopography macroscopic surface and pure-carbon composition, which may serve as coatings on implants in blood-contact environments.

METHODS

The adsorption behavior of albumin, fibrinogen, and fresh plasma on SWNT membranes was studied using scanning electron microscopy (SEM), energy-dispersive x-ray, and direct enzyme-linked immunosorbent assay, respectively. Platelet adhesion and activation were investigated in vitro using SEM observation, and via flow cytometry analysis by PAC-1, which binds to GPIIb/IIIa expressed on activated platelets.

RESULTS

Nonwoven SWNTs clearly displayed greater adsorption preference for fibrinogen than for albumin. However, adhered platelets were not observed by SEM. Results obtained from flow cytometry analysis further proved very low expression of GPIIb/IIIa on platelets caused by nonwoven SWNTs.

CONCLUSION

The function of adsorbed fibrinogen to mediate platelet recognition, adhesion, activation, and aggregation was significantly suppressed, which induced extremely low levels of platelet adhesion and activation.

Surface Structures and Properties of Polystyrene/Poly(methyl methacrylate) Blends and Copolymers

Sum frequency generation (SFG) vibrational spectroscopy has been applied to study the molecular surface structures of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blends and the copolymer between PS and PMMA (PS-co-PMMA) in air, supplemented by atomic force microscopy (AFM) and contact angle goniometer. Both the blend and the copolymer have equal weight amounts of the two components. SFG results show that both components, PS and PMMA, can segregate to the surface of the blend and the copolymer before annealing, although PMMA has a slightly higher surface tension. Upon annealing both SFG results and contact angle measurements indicate that the PS segregates to the surface of the PS/PMMA blend more but no change occurs on the PS-co-PMMA surface. AFM images show that the copolymer surface is flat but the 1:1 PS/PMMA blend has a rougher surface with island like domains present. The annealing effect on the blend surface morphology has also been investigated. We collected amide SFG signals from interfacial fibrinogen molecules at the copolymer or blend/protein solution interfaces as a function of time. Different time-dependent SFG signal changes have been observed, showing that different surfaces of the blend and the copolymer mediate fibrinogen adsorption behavior differently.

Hemodialysis Membranes Based on Functionalized High-density Polyethylene

Grafting vinyl acetate (VAc) and maleic anhydride (MAn) comonomer onto high-density polyethylene (HDPE) was performed by means of gamma rays. Conditions for the minimum homopolymer formation, maximum grafting yield and alternate copolymer grafts were carried out. Further chemical treatment with sodium hydroxide, hydrochloric acid, ammonium hydroxide, sulfamic acid and amino pyridine were made on the grafted membranes to increase functionality. The swelling, mechanical, solute permeability and biocompatibility properties of these copolymers were evaluated for possible application as dialysis membranes. The introduction of functional groups on HDPE membranes enhanced their hydration and transport flux. The treated grafting membranes showed improved permeability towards urea, creatinine and uric acid over the ungrafted HDPE. The permeability rate of the solutes through the membranes depended on the molecular weight and the size of the solutes. The presence of hydrophilic groups on the membranes reduced protein adsorption and enhanced membrane transport. The swelling, solute dialysis permeability and protein low affinity properties of HDPE-g-(VAc-alt-MAn) treated with sulfamic acid or 2-aminopyridine indicate potential use as hemodialysis membranes.

Improving the blood compatibility of polyurethane using carbon nanotubes as fillers and its implications to cardiovascular surgery

Blood compatibility has been an occlusion for biomaterials used in the cardiovascular system. In this work, a multiwalled carbon nanotubes-polyurethane composite (MWNT-PU) was prepared through a controlled co-precipitation. The surface chemical composition of treated carbon nanotubes was analyzed with XPS and the thermal behaviors of composite were characterized by DSC. The platelet adhesion and activation caused by the composite were evaluated by using SEM and flow cytometric analysis, respectively, and the disruption of red blood cells was analyzed through measuring the absorbance of free hemoglobin. The experimental results demonstrated that: (1) Multiwalled carbon nanotubes (MWNTs) with oxygen-containing functional groups could be well dispersed in polyurethane matrix through a controlled coprecipitation; (2) the composite surface displayed a significantly improved anticoagulant function, which can be indicative of the promising potentials of carbon nanotube-based materials in the implants and medical devices applied in blood-contacting environments.

Time-dependent conformational changes in fibrinogen measured by atomic force microscopy

Tapping-mode atomic force microscopy was used to study the time-dependent changes in the structure of fibrinogen under aqueous conditions following adsorption on two model surfaces: hydrophobic graphite and hydrophilic mica. Fibrinogen was observed in the characteristic trinodular form, and the dimensions of the adsorbed molecules were consistent with previously reported values for these surfaces. On the basis of the differences in the relative heights of the D and the E domains, four orientation states were observed for fibrinogen adsorbed on both the surfaces. On graphite, the initial asymmetric orientation states disappeared with spreading over time. Some small lateral movements of the adsorbed proteins were observed on mica during repeated scanning, whereas no such movement was observed on graphite, indicating strong adhesion of fibrinogen to a hydrophobic surface. Spreading kinetics of fibrinogen on the two surfaces was determined by measuring the heights of the D and E domains over a time period of approximately 2 h. On graphite, the heights of both the D and E domains decreased with time to a lower plateau value of 1.0 nm. On mica, the heights of both the D and E domains showed an increase, rising to an upper plateau value of approximately 2.1 nm. The spreading of the D and E domains on graphite was analyzed using an 'exponential-decay-of-height' model. A spreading rate constant of approximately 4.7 x 10(-4) s(-1) was observed for the whole fibrinogen molecule adsorbed on graphite, corresponding to a free energy of unfolding of approximately 37 kT. Extrapolation of the exponential curve in the model to t = 0 yielded values of 2.3 and 2.2 nm for the heights of the D and the E domains at the time of contact with the hydrophobic graphite substrate, significantly less than their free solution diameters. A two-step spreading model is proposed to explain this observation.