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

In-situ synchrotron quantitative analysis of competitive adsorption tendency of human serum proteins on polyether sulfone clinical hemodialysis membrane

Comprehensive understanding of protein adsorption phenomenon on membrane surface during hemodialysis (HD) is one of the key moments for development of hemocompatible HD membrane. Though many mechanisms and kinetics of protein adsorption on some surface have been studied, we are still far away from complete understanding and control of this process, which results in a series of biochemical reactions that causes severe complications with health and even the death among HD patients. The aim of this study is to conduct quantitative analysis of competitive adsorption tendency of human serum protein on polyether sulfone (PES) clinical dialysis membrane. In situ synchrotron radiation micro-computed tomography (SR-µCT) imaging available at the Canadian Light Source (CLS) was conducted to assess human serum proteinbinding and undertake the corresponding quantitative analysis.The competitive adsorption of Human protein albumin (HSA), fibrinogen (FB) and transferrin (TRF) were tested from single and multiple protein solution. Furthermore, in-vitro human serum protein adsorption on clinical dialyzers was investigated using UV-Visible to confirm the competitive adsorption tendency. Results showed that when proteins were adsorbed from their mixture, FB content (among proteins) in the adsorbed layer increased from 3.6% mass (content in the initial solution) to 18% mass and 12%, in case of in situ quantitative and invitro analysis, respectively. The increase in FB content was accompanied by the decrease in the HSA content, while TRF remained on approximately on the same level for both cases. Overall, the percentage of HSA adsorption ratio onto the HD membrane has dropped approximately 10 times when HSA was adsorbed in competition with other proteins, compared to the adsorption from single HSA solution. The substitution of HSA with FB was especially noticeable when HSA adsorption from its single solution was compared with the case of the protein mixture. Moreover, SR-µCT has revealed that FB when adsorbed from a protein mixture solution is located predominately in the middle of the membrane, whereas the peak of the distribution is shifted to membrane bottom layers when adsorption from FB single solution takes place. Results showed that HSA FB and TRF adsorption behavior observations are similar on both in-situ small scale and clinical dialyzer of the PES membrane.

Rutile facet-dependent fibrinogen conformation: Why crystallographic orientation matters

Previous studies implied that single crystalline rutile surfaces have the ability to guide the functionality of adsorbed blood plasma proteins. However, a clear relation between the rutile crystallographic orientation and conformation of adsorbed proteins is still missing. Here, we examine the adsorption characteristics of human plasma fibrinogen (HPF) on atomically flat single rutile crystals with (110), (100), (101) and (001) facets. By direct visualization of individual protein molecules through atomic force microscopy (AFM) imaging, the distinct conformations of HPF were determined depending on rutile surface crystallographic orientation. In particular, dominant trinodular and globular conformation was found on (110) and (001) facets, respectively. The observed variations of HPF conformation were reasoned from the surface water contact angle and surface energy point of view. By analyzing AFM-based force measurements, statistically significant changes in surface energies of rutile surfaces covered with HPF were determined and linked to HPF conformation. Furthermore, the facet-dependent structural rearrangement of HPF was indirectly confirmed through deconvolution of high-resolution X-ray photoelectron spectroscopy (XPS) carbon and nitrogen spectra. The globular, and thus native-like HPF conformation observed on (001) facet, was reflected in the lowest level of amino group formation. We propose that the mechanism behind the crystallographic orientation-induced HPF conformation is driven by the facet-specific surface hydrophilicity and energy. From the biomedical material perspective, our results demonstrate that the conformation of HPF can be guided by controlling the crystallographic orientation of the underlying material surface. This might be beneficial to the field of titanium-based biomaterials design and development.

Polysaccharide-Protein Multilayers Based on Chitosan-Fibrinogen Assemblies for Cardiac Cell Engineering

The cell and tissue culture substrates play a pivotal role in the regulation of cell-matrix and cell-cell interactions. The surface properties of the materials control a wide variety of cell functions. Amongst various methods, layer-by-layer (LbL) assembly is a versatile surface coating technique for creating controllable bio-coatings. Here, polysaccharide/protein multilayers are proposed, which are fabricated by immersive LbL assembly and based on the chitosan/fibrinogen pair for improving the adhesion and spreading of cardiomyocytes. Two approaches in LbL assembly are employed for clarifying the effect of the bilayers order and their concentration on cardiomyocytes viability and morphology. Fourier transform infrared spectroscopy (FTIR) measurements show that the adsorption of the biopolymers is enhanced during the LbL deposition in a synergistic manner. Contact angle measurements indicate that the multilayers are alternating from less to more hydrophilic behavior depending on the biopolymer that is added last. Confocal microscopy with immunostained fibrinogen reveals that the amount of the protein is higher when the concentration of the immersion solution is increased, however, for low solution concentration it is speculated that interdigitation between the separate biopolymer layers takes place. This work motivates the use of fibrinogen in polysaccharide/protein multilayers for enhanced cytocompatibility in cardiac tissue engineering.

The Role of pH and Ionic Strength in the Attraction-Repulsion Balance of Fibrinogen Interactions

Fibrinogen (Fg) self-assembly is sensitive to the physicochemical properties of an environment like pH and ionic strength. These parameters tune the direction and strength of noncovalent physical driving forces determining protein intermolecular interactions. The attraction-repulsion balance in intermolecular interactions of the multidomain protein Fg at pH values 3.5, 7.4, and 9.5 and varying ionic strengths of the water medium has been analyzed by the complex diffusive approach, proposed by us previously. The concentration dependence of protein collective diffusion was analyzed within the phenomenological approach, based on the frictional formalism of nonequilibrium thermodynamics proposed by H. Vink. The analysis of protein diffusion data has shown the fundamental difference in the physical nature and direction of interaction forces between protein molecules at different conditions. The paired interaction potential of protein molecules was characterized in terms of second virial coefficients and Hamaker constants within the Deryaguin-Landau-Verwey-Overbeek theory and the "porous" colloid particle model. Our results indicated the maximum Hamaker constant and dominance of the van der Waals attraction between Fg molecules at pH 7.4. The increase in pH up to 9.5 results in the zero values of the second virial coefficient and Hamaker constant, corresponding to the full reciprocal compensation for electrostatic repulsion and van der Waals attraction. In the acidic medium (pH 3.5), the strong electrostatic repulsion substantially exceeds the van der Waals attraction. A high ionic strength is characterized by a significant decrease of all intermolecular interactions, which is expressed in almost zero values of virial coefficients and the Hamaker constant. Thus, it is experimentally shown that the physiological conditions of the Fg environment (pH 7.4 and slight ionic strength) provide a high probability for peak physical attraction between fibrinogen molecules, which is used in nature to facilitate blood clotting.

The strategy of modulation blood responses by surface modification with different functional groups on polyester film

A main problem in the design of blood-contacting biomaterials has been the deficiency of a systematic understanding of blood-biomaterial interactions and the strategy to modulate blood responses. In this work, different functional groups including carboxyl (COOH), hydroxyl (OH) and zwitterionic sulfobetaine group (⊕N((CH₃ )₂ )(CH₂ )₃ SO_3-○- , SMDB) were grafted on the poly (butylene terephthalate) (PBT) film to study how the functional groups modulate blood responses and in terms of interaction with the coagulation system, the complement system, and platelets. The results showed protein absorption and platelet adhesion was stronger on the PBT bearing COOH group than PBT films bearing OH and zwitterionic sulfobetaine groups (total protein (μg/cm² ): 32.92 ± 5.89 vs. 22.02 ± 1.44 vs. 19.09 ± 1.59; platelet adhesion (/mm² ): 1,626.7 ± 120.1 vs. 1,395.6 ± 363.3 vs. 1,102.2 ± 373.7), which had a rougher and negatively charged surface, and the coagulation system was inhibited by binding fibrinogen (Fg) and coagulation factors. Meanwhile, PBT-PSMDB showed anticoagulant property and induced platelet activation. As a result, complement formation on these two films were less than PBT bearing OH groups by inhibiting the coagulation system (C3a (ng/ml): 3,745.4 ± 143.9 vs. 3,290.9 ± 249.7 vs. 4,887.9 ± 88.9; C5a (ng/ml): 22.1 ± 2.6 vs. 22.3 ± 1.8 vs. 27.9 ± 2.0). On the other hand, PBT bearing OH groups did not facilitate remarkable platelet adhesion and activation, and had no influence on platelet aggregation, hypotonic shock response, and coagulation system. The above results showed that the blood responses were highly interlinked, and could be modulated by grafting with different functional groups on the biomaterial surfaces. These findings may help identify a strategy to design materials with better hemocompatibility for blood contact, filtration, and purification applications.

Principles of Fibrinogen Fiber Assembly In Vitro

Fibrinogen nanofibers hold great potential for applications in wound healing and personalized regenerative medicine due to their ability to mimic the native blood clot architecture. Although versatile strategies exist to induce fibrillogenesis of fibrinogen in vitro, little is known about the underlying mechanisms and the associated length scales. Therefore, in this manuscript the current state of research on fibrinogen fibrillogenesis in vitro is reviewed. For the first time, the manifold factors leading to the assembly of fibrinogen molecules into fibers are categorized considering three main groups: substrate interactions, denaturing and non-denaturing buffer conditions. Based on the meta-analysis in the review it is concluded that the assembly of fibrinogen is driven by several mechanisms across different length scales. In these processes, certain buffer conditions, in particular the presence of salts, play a predominant role during fibrinogen self-assembly compared to the surface chemistry of the substrate material. Yet, to tailor fibrous fibrinogen scaffolds with defined structure-function-relationships for future tissue engineering applications, it still needs to be understood which particular role each of these factors plays during fiber assembly. Therefore, the future combination of experimental and simulation studies is proposed to understand the intermolecular interactions of fibrinogen, which induce the assembly of soluble fibrinogen into solid fibers.

How Nanotopography-Induced Conformational Changes of Fibrinogen Affect Platelet Adhesion and Activation

The conformational state of adsorbed human plasma fibrinogen (HPF) has been recognized as the determinant factor in platelet adhesion and thrombus formation on blood-contacting biomaterials. Studies have highlighted the ability to control the HPF conformation merely by tailoring surface nanotopographical features. However, a clear relationship between the conformational changes of adsorbed HPF and the degree of platelet adhesion and activation achieved with different surface nanotopographies is still unclear. Here, we examined HPF assembly characteristics on nanostructured polybutene-1 (PB-1) surfaces with nanosized lamellar crystals (LCs), needle-like crystals (NLCs), and a nanostructured high-density polyethylene (HDPE) surface with shish-kebab crystals (SKCs), at a biologically relevant HPF concentration. By exposing the nanostructured surfaces with preadsorbed HPF to human platelets, significant differences in platelet response on LCs/SKCs and NLCs were identified. The former presented a uniform monolayer in the advanced stage of activation, whereas the latter exhibited minimal adhesion and the early stage of activation. Distinct platelet response was related to the postadsorption conformational changes in HPF, which were confirmed by topography-dependent shifts of the amide I band in attenuated total reflection-Fourier transform infrared (ATR-FTIR) analysis. Supported by atomic force microscopy (AFM) characterization, we propose that the mechanism behind the nanotopography-induced HPF conformation is driven by the interplay between the aspect ratios of polymeric crystals and HPF. From the biomedical perspective, our work reveals that surface structuring in a nanoscale size regime can provide a fine-tuning mechanism to manipulate HPF conformation, which can be exploited for the design of thromboresistant biomaterials surfaces.

Adsorption of Fibrinogen and Fibronectin on Elastomeric Poly(butylene succinate) Copolyesters

Proteins adsorbed onto biomaterial surfaces facilitate cell-material interactions, including adhesion and migration. Of particular importance are provisional matrix components, fibrinogen (Fg) and fibronectin (Fn), which play an important role in the wound-healing process. Here, to assess the potential of a series of elastomeric poly(butylene succinate) (PBS) copolymers for soft tissue engineering and regenerative medicine applications, we examined the adsorption of Fg and Fn. We prepared spin-coated thin films of the poly(butylene succinate) homopolymer and a series of elastomeric poly(butylene succinate) copolymers with butylene succinate (PBS, hard segment) to succinate-dimer linoleic diol unit (dilinoleic succinate (DLS), soft segments) weight ratios of 70:30, 60:40, and 50:50. X-ray diffraction was used to assess crystallinity, whereas the obtained thin films were characterized using a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy. Protein adsorption was assessed using QCM-D, followed by data analysis using viscoelastic modeling. On all three copolymers, we observed robust adsorption of both key provisional matrix proteins. Importantly, for both proteins, viscoelastic modeling determined that the adlayers were 30-40 nm thick and had low shear modulus values (<25 kPa), thus indicating soft orientations (end-on for Fg) or conformations (open for Fn) of the hydrated proteins. Overall, our results are very encouraging, as they predict excellent cell adhesion and migration, key features enabling tissue integration of potential PBS-DLS scaffolds.

Effect of the Molecular Weight of Poly(2-methoxyethyl acrylate) on Interfacial Structure and Blood Compatibility

The blood-compatible polymer poly(2-methoxyethyl acrylate) (PMEA) is composed of nanometer-scale interfacial structures because of the phase separation of the polymer and water at the PMEA/phosphate-buffered saline (PBS) interface. We synthesized PMEA with four different molecular weights (19, 30, 44, and 183 kg/mol) to investigate the effect of the molecular weight on the interfacial structures and blood compatibility. The amounts of intermediate water and fibrinogen adsorption were not affected by the molecular weight of PMEA. In contrast, the degree of denaturation of adsorbed fibrinogen molecules and platelet adhesion increased as the molecular weight increased. Atomic force microscopy observation revealed that the domain size of the microphase separation structures observed at the PMEA/PBS interfaces drastically (nearly 3 times in the mean area of a domain) changed with the molecular weight. PMEA with a lower molecular weight showed a smaller polymer-rich domain size, as expected on the basis of the microphase separation of polymer-rich and water-rich domains. The small domain size suppressed the aggregation and denaturation of adsorbed fibrinogen molecules because only a few fibrinogen molecules were adsorbed on a domain. Increasing the domain size enhanced the denaturation of adsorbed fibrinogen molecules. Controlling the interfacial structures is crucial for ensuring the blood compatibility of polymer interfaces.

Analysis of Interaction Between Interfacial Structure and Fibrinogen at Blood-Compatible Polymer/Water Interface

The correlation between the interfacial structure and protein adsorption at a polymer/water interface was investigated. Poly(2-methoxyethyl acrylate)(PMEA), which is one of the best blood compatible polymers available, was employed. Nanometer-scale structures generated through the phase separation of polymer and water were observed at the PMEA/phosphate buffered saline interface. The interaction between the interfacial structures and fibrinogen (FNG) was measured using atomic force microscopy. Attraction was observed in the polymer-rich domains as well as in the non-blood compatible polymer. In contrast, no attractive interactions were observed, and only a repulsion occurred in the water-rich domains. The non-adsorption of FNG into the water rich domains was also clarified through topographic and phase image analyses. Furthermore, the FNG molecules adsorbed on the surface of PMEA were easily desorbed, even in the polymer-rich domains. Water molecules in the water-rich domains are anticipated to be the dominant factor in preventing FNG adsorption and thrombogenesis on a PMEA interface.

A study of polyethylene glycol backfilling for enhancing target recognition using QCM-D and DPI

Polyethylene glycol (PEG) is a promising candidate for protein resistance and preserving protein function in biomedical applications. In this study, a PEG-based bifunctional platform with antifouling for plasma proteins and high sensitivity for biomolecules was designed. Long PEG chains (PEG₂₄) were used to install functional biomolecules, and short PEG chains (PEG₄) served as a protective layer to backfill the surface and suppress nonspecific protein adsorption. Quartz crystal microbalance with dissipation (QCM-D) and dual polarization interferometry (DPI) were combined to investigate the dynamic process of PEG₄ backfilling and the recognition capacity of biomolecules with different ratios of PEG₄ and PEG₂₄ in real time. The amount of PEG₄ chain backfilling affected the flexibility of PEG₂₄ and exposed sites. The recognition capacity was improved by increasing the ratios of PEG₄ to PEG₂₄. Therefore, when the feeding ratio of PEG₄ to PEG₂₄ was 9 : 1, a highly efficient and sensitive platform was constructed for immobilization of antibodies and recognition of antigens either in pure PBS or in a complex biological environment.

Polymers and biopolymers at interfaces

This review updates recent progress in the understanding of the behaviour of polymers at surfaces and interfaces, highlighting examples in the areas of wetting, dewetting, crystallization, and 'smart' materials. Recent developments in analysis tools have yielded a large increase in the study of biological systems, and some of these will also be discussed, focussing on areas where surfaces are important. These areas include molecular binding events and protein adsorption as well as the mapping of the surfaces of cells. Important techniques commonly used for the analysis of surfaces and interfaces are discussed separately to aid the understanding of their application.

Effect of End-Grafted Polymer Conformation on Protein Resistance

Monte Carlo simulation combined with an experimental method was used to investigate the effect of the conformational structure of polymer brushes on their protein resistance. The end-grafted polymers with two conformational structures, i.e., linear and looped, were considered. Protein adsorption behaviors on the surfaces grafted with either linear or looped polymers were investigated. Different chain lengths and grafting numbers of end-grafted polymers were employed in this simulation. The simulation results indicated that for long polymer brushes the conformational change from linear to looped generally improved their protein-resistant property for all of the grafting numbers investigated here, and a remarkable improvement in protein resistance can be achieved at a certain grafting number. Moreover, the simulations revealed that the smoothness of the surface and the formation of a dense impenetrable layer are the two significant characteristics of the looped polymer brush in resisting protein adsorption. Meanwhile, experiment results also showed that for a given chain length and grafting number the protein-resistant property of the looped polymer brush was superior to that of the surface grafted with linear polymers, which is quite consistent with the simulation results. These results further elucidated the difference in the protein-resistant property between the linear and looped polymer brushes, which provided useful information for preparing excellent antifouling materials in future experiments.

Bio-inspired microcapsule for targeted antithrombotic drug delivery

Thrombosis or embolism is the leading cause of death and long-term adult disability worldwide. To reduce the risk of thrombosis and hemorrhaging in patients, a facile and versatile method was developed to fabricate microcapsules for targeted antithrombotic drug delivery. The microcapsules were prepared via oxidative polymerization of dopamine on polystyrene microspheres, followed by immobilization of fibrinogen onto the surface of poly(dopamine) layers. Subsequently, microcapsules were obtained by removing the cores with THF. Nattokinase was loaded into the microcapsules via diffusion. The loading amount was approximately 0.05 mg g⁻¹ at 37 °C, and the loading efficiency was nearly 75%, based on the initial concentration of nattokinase in PBS. The release of nattokinase was a gradual process at 37 °C, and the activity of the targeted activated platelets was highly efficient. The antithrombotic activity of the nattokinase microcapsules was evidenced by the sharp dissolution of fibrin clots and the blood clotting time indexes. A gradual release mechanism of platelet-inspired microcapsules used for targeted antithrombotic therapy was proposed. This strategy for targeted antithrombotic drug delivery, which lowers the demand dose and minimizes side effects while maximizing drug efficacy, provides a potential new way to treat life-threatening diseases caused by vascular disruption.

NK-loaded hollow microcapsules were fabricated and assessed as a potential antithrombosis therapy.

Transport-Limited Adsorption of Plasma Proteins on Bimodal Amphiphilic Polymer Co-Networks: Real-Time Studies by Spectroscopic Ellipsometry

Traditional hydrogels are commonly limited by poor mechanical properties and low oxygen permeability. Bimodal amphiphilic co-networks (β-APCNs) are a new class of materials that can overcome these limitations by combining hydrophilic and hydrophobic polymer chains within a network of co-continuous morphology. Applications that can benefit from these improved properties include therapeutic contact lenses, enzymatic catalysis supports, and immunoisolation membranes. The continuous hydrophobic phase could potentially increase the adsorption of plasma proteins in blood-contacting medical applications and compromise in vivo material performance, so it is critical to understand the surface characteristics of β-APCNs and adsorption of plasma proteins on β-APCNs. From real-time spectroscopic visible (Vis) ellipsometry measurements, plasma protein adsorption on β-APCNs is shown to be transport-limited. The adsorption of proteins on the β-APCNs is a multistep process with adsorption to the hydrophilic surface initially, followed by diffusion into the material to the internal hydrophilic/hydrophobic interfaces. Increasing the cross-linking of the PDMS phase reduced the protein intake by limiting the transport of large proteins. Moreover, the internalization of the proteins is confirmed by the difference between the surface-adsorbed protein layer determined from XPS and bulk thickness change from Vis ellipsometry, which can differ up to 20-fold. Desorption kinetics depend on the adsorption history with rapid desorption for slow adsorption rates (i.e., slow-diffusing proteins within the network), whereas proteins with fast adsorption kinetics do not readily desorb. This behavior can be directly related to the ability of the protein to spread or reorient, which affects the binding energy required to bind to the internal hydrophobic interfaces.