In vivo assessment of dual-function submicron textured nitric oxide releasing catheters in a 7-day rabbit model

Catheter-induced thrombosis is a major contributor to infectious and mechanical complications of biomaterials that lead to device failure. Herein, a dualfunction submicron textured nitric oxide (NO)-releasing catheter was developed. The hemocompatibility and antithrombotic activity of vascular catheters were evaluated in both 20 h in vitro blood loop and 7 d in vivo rabbit model. Surface characterization assessments via atomic force microscopy show the durability of the submicron pattern after incorporation of NO donor S-nitroso-N-acetylpenicillamine (SNAP). The SNAP-doped catheters exhibited prolonged and controlled NO release mimicking the levels released by endothelium. Fabricated catheters showed cytocompatibility when evaluated against BJ human fibroblast cell lines. After 20h in vitro evaluation of catheters in a blood loop, textured-NO catheters exhibited a 13-times reduction in surface thrombus formation compared to the control catheters, which had 83% of the total area covered by clots. After the 7 d in vivo rabbit model, analysis on the catheter surface was examined via scanning electron microscopy, where significant reduction of platelet adhesion, fibrin mesh, and thrombi can be observed on the NO-releasing textured surfaces. Moreover, compared to relative controls, a 63% reduction in the degree of thrombus formation within the jugular vein was observed. Decreased levels of fibrotic tissue decomposition on the jugular vein and reduced platelet adhesion and thrombus formation on the texture of the NO-releasing catheter surface are indications of mitigated foreign body response. This study demonstrated a biocompatible and robust dual-functioning textured NO PU catheter in limiting fouling-induced complications for longer-term blood-contacting device applications. STATEMENT OF SIGNIFICANCE: Catheter-induced thrombosis is a major contributor to infectious and mechanical complications of biomaterials that lead to device failure. This study demonstrated a robust, biocompatible, dual-functioning textured nitric oxide (NO) polyurethane catheter in limiting fouling-induced complications for longer-term blood-contacting device applications. The fabricated catheters exhibited prolonged and controlled NO release that mimics endothelium levels. After the 7 d in vivo model, a significant reduction in platelet adhesion, fibrin mesh, and thrombi was observed on the NO-releasing textured catheters, along with decreased levels of fibrotic tissue decomposition on the jugular vein. Results illustrate that NO-textured catheter surface mitigates foreign body response.

In Vitro and In Vivo Assessment of the Infection Resistance and Biocompatibility of Small-Molecule-Modified Polyurethane Biomaterials

Bacterial intracellular nucleotide second messenger signaling is involved in biofilm formation and regulates biofilm development. Interference with the bacterial nucleotide second messenger signaling provides a novel approach to control biofilm formation and limit microbial infection in medical devices. In this study, we tethered small-molecule derivatives of 4-arylazo-3,5-diamino-1H-pyrazole on polyurethane biomaterial surfaces and measured the biofilm resistance and initial biocompatibility of modified biomaterials in in vitro and in vivo settings. Results showed that small-molecule-modified surfaces significantly reduced the Staphylococcal epidermidis biofilm formation compared to unmodified surfaces and decreased the nucleotide levels of c-di-AMP in biofilm cells, suggesting that the tethered small molecules interfere with intracellular nucleotide signaling and inhibit biofilm formation. The hemocompatibility assay showed that the modified polyurethane films did not induce platelet activation or red blood cell hemolysis but significantly reduced plasma coagulation and platelet adhesion. The cytocompatibility assay with fibroblast cells showed that small-molecule-modified surfaces were noncytotoxic and cells appeared to be proliferating and growing on modified surfaces. In a 7-day subcutaneous infection rat model, the polymer samples were implanted in Wistar rats and inoculated with bacteria or PBS. Results show that modified polyurethane significantly reduced bacteria by ∼2.5 log units over unmodified films, and the modified polymers did not lead to additional irritation/toxicity to the animal tissues. Taken together, the results demonstrated that small molecules tethered on polymer surfaces remain active, and the modified polymers are biocompatible and resistant to microbial infection in vitro and in vivo.

FXII contact activation products have an inhibitory effect on αFXIIa

It is accepted that the contact activation complex of the intrinsic pathway of blood coagulation cascade produces active enzymes that lead to plasma coagulation following biomaterial contact. In this study, FXII was activated through contact with hydrophilic glass beads and hydrophobic octadecyltrichlorosilane-modified glass beads from neat buffer solutions. These FXII contact activation products generated from material interaction were found to suppress the procoagulant activity of exogenous αFXIIa, and this inhibition was dependent on surface wettability and the concentration of exogenous αFXIIa. Higher relative inhibition rates were generally observed at low concentrations of αFXIIa (1-2 μg/mL) while both hydrophobic and hydrophilic materials showed similar inhibition levels (~39%) at high concentrations of αFXIIa (20 μg/mL). The presence of prekallikrein in the activation system increased the amount of FXIIa produced during FXII contact activation, and also suppressed the apparent levels of inhibitors on hydrophilic surfaces, while having no effect on apparent levels of inhibitors on hydrophobic surface. The combination of FXII contact activation products and activator surfaces was found to dramatically increase inhibition of αFXIIa activity compared to the activation products alone, regardless of activator surface wettability and the presence of prekallikrein. This finding of inhibitors in the suite of proteins generated by contact activation provides additional knowledge into the complex series of interactions that occur when plasma comes into contact with material surfaces.

In vitro evaluation of blood plasma coagulation responses to four medical-grade polyurethane polymers

Segmented polyurethane (PU) block copolymers are widely used in implantable cardiovascular medical devices due to their good biocompatibility and excellent mechanical properties. More specifically, PU Biospan MS/0.4 was used in ventricular assist devices over the past decades. However, this product is being discontinued and it has become necessary to find an alternative PU biomaterial for application in cardiovascular devices. One important criterion for assessing cardiac biomaterials is blood compatibility. In this study, we characterized the surface properties of four medical-grade PU biomaterials: Biospan MS/0.4, BioSpan S, BioSpan 2F, and CarboSil 20 80A, including surface chemistry, topography, microphase separation structure and wettability, and then measured the blood plasma coagulation responses using bovine and human blood plasma. Results showed that BioSpan 2F contains high amounts of fluorine and has the lowest surface free energy while the other materials have surfaces with silicone present. An in vitro coagulation assay shows that these materials demonstrated improved blood coagulation responses compared to the polystyrene control and there were no significant differences in coagulation time among all PU biomaterials. The chromogenic assay showed all PU materials led to low FXII contact activation, and there were no significant differences in FXII contact activation, consistent with plasma coagulation responses.

Surfaces modified with small molecules that interfere with nucleotide signaling reduce Staphylococcus epidermidis biofilm and increase the efficacy of ciprofloxacin

Staphylococcus epidermidis are common bacteria associated with biofilm related infections on implanted medical devices. Antibiotics are often used in combating such infections, but they may lose their efficacy in the presence of biofilms. Bacterial intracellular nucleotide second messenger signaling plays an important role in biofilm formation, and interference with the nucleotide signaling pathways provides a possible way to control biofilm formation and to increase biofilm susceptibility to antibiotic therapy. This study synthesized small molecule derivates of 4-arylazo-3,5-diamino-1 H-pyrazole (named as SP02 and SP03) and found these molecules inhibited S. epidermidis biofilm formation and induced biofilm dispersal. Analysis of bacterial nucleotide signaling molecules showed that both SP02 and SP03 significantly reduced cyclic dimeric adenosine monophosphate (c-di-AMP) levels in S. epidermidis at doses as low as 25 µM while having significant effects on multiple nucleotides signaling including cyclic dimeric guanosine monophosphate (c-di-GMP), c-di-AMP, and cyclic adenosine monophosphate (cAMP) at high doses (100 µM or greater). We then tethered these small molecules to polyurethane (PU) biomaterial surfaces and investigated biofilm formation on the modified surfaces. Results showed that the modified surfaces significantly inhibited biofilm formation during 24 h and 7-day incubations. The antibiotic ciprofloxacin was used to treat these biofilms and the efficacy of the antibiotic (2 µg/mL) was found to increase from 94.8% on unmodified PU surfaces to > 99.9% on both SP02 and SP03 modified surfaces (>3 log units). Results demonstrated the feasibility of tethering small molecules that interfere with nucleotide signaling onto polymeric biomaterial surfaces and in a way that interrupts biofilm formation and increases antibiotic efficacy for S. epidermidis infections.

Nucleotide Messenger Signaling of Staphylococci in Responding to Nitric Oxide - Releasing Biomaterials

Nitric oxide (NO) releasing biomaterials are a promising approach against medical device associated microbial infection. In contrast to the bacteria-killing effects of NO at high concentrations, NO at low concentrations serves as an important signaling molecule to inhibit biofilm formation or disperse mature biofilms by regulating the intracellular nucleotide second messenger signaling network such as cyclic dimeric guanosine monophosphate (c-di-GMP) for many Gram-negative bacterial strains. However, Gram-positive staphylococcal bacteria are the most commonly diagnosed microbial infections on indwelling devices, but much less is known about the nucleotide messengers and their response to NO as well as the mechanism by which NO inhibits biofilm formation. This study investigated the cyclic nucleotide second messengers c-di-GMP, cyclic dimeric adenosine monophosphate (c-di-AMP), and cyclic adenosine monophosphate (cAMP) in both Staphylococcus aureus (S. aureus) Newman D2C and Staphylococcus epidermidis (S. epidermidis) RP62A after incubating with S-nitroso-N-acetylpenicillamine (SNAP, NO donor) impregnated polyurethane (PU) films. Results demonstrated that NO release from the polymer films significantly reduced the c-di-GMP levels in S. aureus planktonic and sessile cells, and these bacteria showed inhibited biofilm formation. However, the effect of NO release on c-di-GMP in S. epidermidis was weak, but rather, S. epidermidis showed significant reduction in c-di-AMP levels in response to NO release and also showed reduced biofilm formation. Results strongly suggest that NO regulates the nucleotide second messenger signaling network in different ways for these two bacteria, but for both bacteria, these changes in signaling affect the formations of biofilms. These findings provide cues to understand the mechanism of Staphylococcus biofilm inhibition by NO and suggest novel targets for antibiofilm interventions.

Crosslinkable fluorophenoxy-substituted poly[bis(octafluoropentoxy) phosphazene] biomaterials with improved antimicrobial effect and hemocompatibility

Biomaterial-associated microbial infection is one of the most frequent and severe complications associated with the use of biomaterials in medical devices. In previous studies, we developed new fluorinated polyphosphazenes, poly[bis(octafluoropentoxy) phosphazene] (OFP) and crosslinkable OFP (X-OFP), and demonstrated the inhibition of bacterial adhesion and biofilm formation, thereby controlling microbial infection. In this study, two additional fluorinated polyphosphazenes (PPs, defined as LS02 and LS03) with fluorophenoxy-substituted side groups, 4-fluorophenoxy and 4-(trifluoromethyl)phenoxy, respectively, based on X-OFP general structure, were synthesized and applied as coatings on stainless steel. The linkage of fluorophenoxy groups to the P-N backbone of PPs was found to increase the surface stiffness and significantly reduced Staphylococcus bacterial adhesion and inhibited biofilm formation. It also significantly reduced microbial infection compared to OFP, our prior X-OFPs or poly[bis(trifluoroethoxy) phosphazene] (TFE). The biofilm experiments show that the newly synthesized PPs LS02 and LS03 are biofilm free up to 28 days. Plasma coagulation and platelet adhesion/activation experiments also demonstrated that new PPs containing fluorophenoxy side groups are hemocompatible. The development of new crosslinkable fluorinated PPs containing fluorophenoxy-substituted side groups provides a new generation of polyphosphazene materials for medical devices with improved resistance to microbial infections and thrombosis formation.

Surface Texturing and Combinatorial Approaches to Improve Biocompatibility of Implanted Biomaterials

Biomaterial associated microbial infection and blood thrombosis are two of the barriers that inhibit the successful use of implantable medical devices in modern healthcare. Modification of surface topography is a promising approach to combat microbial infection and thrombosis without altering bulk material properties necessary for device function and without contributing to bacterial antibiotic resistance. Similarly, the use of other antimicrobial techniques such as grafting poly(ethylene glycol) (PEG) and nitric oxide (NO) release also improve the biocompatibility of biomaterials. In this review, we discuss the development of surface texturing techniques utilizing ordered submicron-size pillars for controlling bacterial adhesion and biofilm formation, and we present combinatorial approaches utilizing surface texturing in combination with poly(ethylene glycol) (PEG) grafting and NO release to improve the biocompatibility of biomaterials. The manuscript also discusses efforts towards understanding the molecular mechanisms of bacterial adhesion responses to the surface texturing and NO releasing biomaterials, focusing on experimental aspects of the approach.

Submicron topography design for controlling staphylococcal bacterial adhesion and biofilm formation

Surface topography modification with nano- or micro-textured structures has been an efficient approach to inhibit microbial adhesion and biofilm formation and thereby to prevent biomaterial-associated infection without modification of surface chemistry/bulk properties of materials and without causing antibiotic resistance. This manuscript focuses on submicron-textured patterns with ordered arrays of pillars on polyurethane (PU) biomaterial surfaces in an effort to understand the effects of surface pillar features and surface properties on adhesion and colonization responses of two staphylococcal strains. Five submicron patterns with a variety of pillar dimensions were designed and fabricated on PU film surfaces and bacterial adhesion and biofilm formation of Staphylococcal strains (Staphylococcus epidermidis RP62A and Staphylococcus aureus Newman D2C) were characterized. Results show that all submicron textured surface significantly reduced bacterial adhesion and inhibited biofilm formation, and bacterial adhesion linearly decreased with the reduction in top surface area fraction. Surface wettability did not show a linear correlation with bacterial adhesion, suggesting that surface contact area dominates bacterial adhesion. From this, it appears that the design of textured patterns should minimize surface area fraction to reduce the bacterial interaction with surfaces but in a way that ensures the mechanical strength of pillars in order to avoid collapse. These findings may provide a rationale for design of polymer surfaces for antifouling medical devices.

Inhibition of bacterial adhesion and biofilm formation by a textured fluorinated alkoxyphosphazene surface

The utilization of biomaterials in implanted blood-contacting medical devices often induces a persistent problem of microbial infection, which results from bacterial adhesion and biofilm formation on the surface of biomaterials. In this research, we developed new fluorinated alkoxyphosphazene materials, specifically poly[bis(octafluoropentoxy) phosphazene] (OFP) and crosslinkable OFP (X-OFP), with improved mechanical properties, and further modified the surface topography with ordered pillars to improve the antibacterial properties. Three X-OFP materials, X-OFP3.3, X-OFP8.1, X-OFP13.6, with different crosslinking densities were synthesized, and textured films with patterns of 500/500/600 nm (diameter/spacing/height) were fabricated via a two stage soft lithography molding process. Experiments with 3 bacterial strains: Staphylococcal epidermidis, Staphylococcal aureus, and Pseudomonas aeruginosa showed that bacterial adhesion coefficients were significantly lower on OFP and X-OFP smooth surfaces than on the polyurethane biomaterial, and surface texturing further reduced bacterial adhesion due to the reduction in accessible surface contact area. Furthermore the anti-bacterial adhesion effect shows a positive relationship with the crosslinking degree. Biofilm formation on the substrates was examined using a CDC biofilm reactor for 7 days and no biofilm formation was observed on textured X-OFP biomaterials. The results suggested that the combination of fluorocarbon chemistry and submicron topography modification in textured X-OFP materials may provide a practical approach to improve the biocompatibility of current biomaterials with significant reduction in risk of pathogenic infection.

New cross-linkable poly[bis(octafluoropentoxy) phosphazene] biomaterials: Synthesis, surface characterization, bacterial adhesion, and plasma coagulation responses

Biomaterial-associated microbial infection and thrombosis represent major issues to the success of long-term use of implantable blood-contacting medical devices. The development of new poly[bis(octafluoropentoxy) phosphazene (OFP) biomaterials provides new routes for combatting microbial infection and thrombosis. However, the limited mechanical properties of OFP to date render them unsuitable for application in medical devices and inhibit any attempts at subsequent surface topography modification. In this study, we synthesized cross-linkable OFPs (X-OFPs) with the different degrees of cross-linking in an effort to improve the mechanical properties. The results showed that the surface chemistry and surface topography of X-OFPs do not change significantly, but the surface mechanical stiffness increased after cross-linking. Atomic force microscopic phase images showed that the polymer phase separation structures changed due to cross-linking. Experiments with three bacterial strains: Staphylococcal epidermidis, Staphylococcal aureus, and Pseudomonas aeruginosa showed that bacterial adhesion was significantly decreased on the OFP and X-OFPs for both the pre-cross-linked and cross-linked as compared to polyurethane biomaterials. Furthermore, bacterial adhesions were lower on X-OFP surfaces than on pre-cross-linked materials, suggesting that mechanical stiffness is an important parameter influencing bacterial adhesion. Blood plasma coagulation responses revealed longer coagulation times for OFP and X-OFP materials than on polyurethanes, indicating that the new cross-linked OFPs are resistant to plasma coagulation compared to currently used polyurethane biomaterials.

Antibacterial effects of low-temperature plasma generated by atmospheric-pressure plasma jet are mediated by reactive oxygen species

Emergence and spread of antibiotic resistance calls for development of non-chemical treatment options for bacterial infections. Plasma medicine applies low-temperature plasma (LTP) physics to address biomedical problems such as wound healing and tumor suppression. LTP has also been used for surface disinfection. However, there is still much to be learned regarding the effectiveness of LTP on bacteria in suspension in liquids, and especially on porous surfaces. We investigated the efficacy of LTP treatments against bacteria using an atmospheric-pressure plasma jet and show that LTP treatments have the ability to inhibit both gram-positive (S. aureus) and gram-negative (E. coli) bacteria on solid and porous surfaces. Additionally, both direct LTP treatment and plasma-activated media were effective against the bacteria suspended in liquid culture. Our data indicate that reactive oxygen species are the key mediators of the bactericidal effects of LTP and hydrogen peroxide is necessary but not sufficient for antibacterial effects. In addition, our data suggests that bacteria exposed to LTP do not develop resistance to further treatment with LTP. These findings suggest that this novel atmospheric-pressure plasma jet could be used as a potential alternative to antibiotic treatments in vivo.

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.

Increased circulating microparticles in streptozotocin-induced diabetes propagate inflammation contributing to microvascular dysfunction

KEY POINTS

Circulating microparticles (MPs) are elevated in many cardiovascular diseases and have been considered as biomarkers of disease prognosis; however, current knowledge of MP functions has been mainly derived from in vitro studies and their precise impact on vascular inflammation and disease progression remains obscure. Using a diabetic rat model, we identified a >130-fold increase in MPs in plasma of diabetic rats compared to normal rats, the majority of which circulated as aggregates, expressing multiple cell markers and largely externalized phosphatidylserine; vascular images illustrate MP biogenesis and their manifestations in microvessels of diabetic rats. Using combined single microvessel perfusion and systemic cross-transfusion approaches, we delineated how diabetic MPs propagate inflammation in the vasculature and transform normal microvessels into an inflammatory phenotype observed in the microvessels of diabetic rats. Our observations derived from animal studies resembling conditions in diabetic patients, providing a mechanistic insight into MP-mediated pathogenesis of diabetes-associated multi-organ microvascular dysfunction.

ABSTRACT

In various cardiovascular diseases, microparticles (MPs), the membrane-derived vesicles released during cell activation, are markedly increased in the circulation. These MPs have been recognized to play diverse roles in the regulation of cellular functions. However, current knowledge of MP function has been largely derived from in vitro studies. The precise impact of disease-induced MPs on vascular inflammation and disease progression remains obscure. In this study we investigated the biogenesis, profile and functional roles of circulating MPs using a streptozotocin-induced diabetic rat model with well-characterized microvascular functions. Our study revealed a >130-fold increase in MPs in the plasma of diabetic rats compared to normal rats. The majority of these MPs originate from platelets, leukocytes and endothelial cells (ECs), and circulate as aggregates. Diabetic MPs show greater externalized phosphatidylserine (PS) than normal MPs. When diabetic plasma or isolated diabetic MPs were perfused into normal microvessels or systemically transfused into normal rats, MPs immediately adhered to endothelium and subsequently mediated leukocyte adhesion. These microvessels then exhibited augmented permeability responses to inflammatory mediators, replicating the microvascular manifestations observed in diabetic rats. These effects were abrogated when MPs were removed from diabetic plasma or when diabetic MPs were pre-coated with a lipid-binding protein, annexin V, suggesting externalized PS to be key in mediating MP interactions with endothelium and leukocytes. Our study demonstrated that the elevated MPs in diabetic plasma are actively involved in the propagation of vascular inflammation through their adhesive surfaces, providing mechanistic insight into the pathogenesis of multi-organ vascular dysfunction that commonly occurs in diabetic patients.

Blood coagulation response and bacterial adhesion to biomimetic polyurethane biomaterials prepared with surface texturing and nitric oxide release

A dual functional polyurethane (PU) film that mimics aspects of blood vessel inner surfaces by combining surface texturing and nitric oxide (NO) release was fabricated through a soft lithography two-stage replication process. The fabrication of submicron textures on the polymer surface was followed by solvent impregnation with the NO donor, S-nitroso-N-acetylpenicillamine (SNAP). An in vitro plasma coagulation assay showed that the biomimetic surface significantly increased the plasma coagulation time and also exhibited reduced platelet adhesion and activation, thereby reducing the risk of blood coagulation and thrombosis. A contact activation assay for coagulation factor XII (FXII) demonstrated that both NO release and surface texturing also reduced FXII contact activation, which contributes to the inhibition of plasma coagulation. The biomimetic surface was also evaluated for bacterial adhesion in plasma and results demonstrate that this combined strategy enables a synergistic effect to reduce bacterial adhesion of Staphylococcus epidermidis, Staphylococcus aureus, and Pseudomonas aeruginosa microorganisms. The results strongly suggest that the biomimetic modification with surface texturing and NO release provides an effective approach to improve the biocompatibility of polymeric materials in combating thrombosis and microbial infection. STATEMENT OF SIGNIFICANCE: (1) Developed a dual functional polyurethane (PU) film that mimics blood vessel inner surface by combining surface texturing and nitric oxide (NO) release for combatting biomaterial associated thrombosis and microbial infection. (2) Studied the blood coagulation response and bacterial adhesion to such biomimetic PU surfaces, and demonstrated that the combination of surface texturing and NO release synergistically reduced the platelet adhesion and bacterial adhesion in plasma, providing an effective approach to improve the biocompatibility of biomaterials used in blood-contacting medical devices. (3) The NO releasing surface significantly inhibits the plasma coagulation via the reduction of contact activation of FXII, indicating the multifunctional roles of NO in improving the biocompatibility of biomaterials in blood-contacting medical devices.

Three Dimensional Structure of Human Fibrinogen under Aqueous Conditions Visualized by Atomic Force Microscopy

Fibrinogen plays a central role in surface-induced thrombosis. However, the interactions of fibrinogen with different substrata remain poorly understood because of the difficulties involved in imaging globular proteins under aqueous conditions. We present detailed three dimensional molecular scale images of fibrinogen molecules on a hydrophobic surface under aqueous conditions obtained by atomic force microscopy. Hydrated fibrinogen monomers are visualized as overlapping ellipsoids; dimers and trimers have linear conformations predominantly, and increased affinity for the hydrophobic surface compared with monomeric fibrinogen. The results demonstrate the importance of hydration on protein structure and properties that affect surface-dependent interactions.

Liquid Biopsy of Vitreous Reveals an Abundant Vesicle Population Consistent With the Size and Morphology of Exosomes

Purpose

To investigate the molecular components of the vitreous in order to better understand retinal physiology and disease.

Methods

Vitreous was acquired from patients undergoing vitrectomy for macular hole and/or epiretinal membrane, postmortem donors, and C57BL/6J mice. Unbiased proteomic analysis was performed via electrospray ionization tandem mass spectrometry (MS/MS). Gene ontology analysis was performed and results were confirmed with transmission electron microscopy, atomic force microscopy, and nanoparticle tracking analysis (NTA).

Results

Proteomic analysis of vitreous obtained prior to vitrectomy identified a total of 1121 unique proteins. Gene ontology analysis revealed that 62.6% of the vitreous proteins were associated with the gene ontology term “extracellular exosome.” Ultrastructural analyses, Western blot, and NTA confirmed the presence of an abundant population of vesicles consistent with the size and morphology of exosomes in human vitreous. The concentrations of vitreous vesicles in vitrectomy patients, postmortem donors, and mice were 1.3, 35, and 9 billion/mL, respectively.

Conclusions

Overall, these data strongly suggest that information-rich exosomes are a major constituent of the vitreous. The abundance of these vesicles and the presence of retinal proteins imply a dynamic interaction between the vitreous and retina. Future studies will be required to identify the cellular origin of vitreal exosomes as well as to assess the potential role of these vesicles in retinal disease and treatment.

Translational Relevance

The identification of vitreous exosomes lays the groundwork for a transformed understanding of pathophysiology and treatment mechanisms in retinal disease, and further validates the use of vitreous as a proximal biofluid of the retina.

A new textured polyphosphazene biomaterial with improved blood coagulation and microbial infection responses

A new poly[bis(octafluoropentoxy) phosphazene] (OFP) was synthesized for the purpose of blood contacting medical devices. OFP was further either developed into crosslinkable polyphosphazene (X-OFP) or blended with polyurethane (PU) as the mixture (OFP/PU) for improvement of mechanical property of polyphosphazene polymers. All the materials were fabricated as smooth films or further textured with submicron pillars for the assay of antimicrobial and antithrombotic properties. Results showed that crosslinkable OFP (X-OFP) and blends of OFP/PU successfully improved the mechanical strength of OFP and fewer defects of pillars were found on the textured polyphosphazene surfaces. The antithrombotic experiments showed that polyphosphazene OFP materials reduced human Factor XII activation and platelet adhesion, thereby being resistant to plasma coagulation and thrombosis. The bacterial adhesion and biofilm experiments demonstrated that OFP materials inhibited staphylococcal bacterial adhesion and biofilm formation. The surface texturing further reduced the platelet adhesion and bacterial adhesion, and inhibited biofilm formation up to 23 days. The data suggested that textured OFP materials may provide a practical approach to improve the biocompatibility of current biomaterials in the application of blood contacting medical devices with significant reduction in risk of pathogenic infection and thrombosis.

STATEMENT OF SIGNIFICANCE

The thromboembolic events and microbial infection have been the significant barriers for the long term use of biomaterials in blood-contacting medical devices. The development of new materials with multiple functions including anti-thrombosis and antibacterial surfaces is a high research priority. This study synthesized new biostable and biocompatible polyphosphazene polymers, poly[bis(octafluoropentoxy)phosphazene] (OFP) and crosslinkable OFP, and successfully improved the mechanical strength of polyphosphazenes. Polymers were fabricated into textured films with submicron pillars on the surfaces. The antimicrobial and antithrombotic assays demonstrated that new materials combined with surface physical modification have significant reduction in risk of pathogenic infection and thrombosis, and improve the biocompatibility of current biomaterials in the application of blood-contacting medical devices. It would be interest to biomaterials and bioengineering related communities.

Antimicrobial nitric oxide releasing surfaces based on S-nitroso-N-acetylpenicillamine impregnated polymers combined with submicron-textured surface topography

A novel dual functioning antimicrobial CarboSil 20 80A polymer material that combines physical topographical surface modification and nitric oxide (NO) release is prepared and evaluated for its efficacy in reducing bacterial adhesion in vitro. The new biomaterial is created via a soft lithography two-stage replication process to induce submicron textures on its surface, followed by solvent impregnation with the NO donor, S-nitroso-N-acetylpenicillamine (SNAP), to obtain long-term (up to 38 d) NO release. The NO releasing textured polymer surface is evaluated against four bacteria commonly known to cause infections in hospital settings and the results demonstrate that the combined strategy enables a synergistic effect on reducing the bacterial adhesion of Staphylococcus epidermidis and Pseudomonas aeruginosa bacteria.

Surface dependent contact activation of factor XII and blood plasma coagulation induced by mixed thiol surfaces

Studies of the activation of FXII in both platelet poor plasma and in neat buffer solutions were undertaken for a series of mixed thiol self-assembled monolayers spanning a broad range of water wettability. A wide spectrum of carboxyl/methyl-, hydroxyl/methyl-, and amine/methyl-thiol modified surfaces were prepared, characterized, and then utilized as the procoagulant materials in a series of FXII activation studies. X-ray photoelectron spectroscopy was utilized to verify the sample surface's thiol composition and contact angles measured to determine the sample surface's wettability. These samples were then used in in vitro coagulation assays using a 50% mixture of recalcified plasma in phosphate buffered saline. Alternatively, the samples were placed into purified FXII solutions for 30 min to assess FXII activation in neat buffer solution. Plasma coagulation studies supported a strong role for anionic surfaces in contact activation, in line with the traditional models of coagulation, while the activation results in neat buffer solution demonstrated that FXIIa production is related to surface wettability with minimum levels of enzyme activation observed at midrange wettabilities, and no statistically distinguishable differences in FXII activation seen between highly wettable and highly nonwettable surfaces. Results demonstrated that the composition of the solution and the surface properties of the material all contribute to the observation of contact activation, and the activation of FXII is not specific to anionic surfaces as has been long believed.

Reduction of Endothelial Nitric Oxide Increases the Adhesiveness of Constitutive Endothelial Membrane ICAM-1 through Src-Mediated Phosphorylation

Nitric oxide (NO) is a known anti-adhesive molecule that prevents platelet aggregation and leukocyte adhesion to endothelial cells (ECs). The mechanism has been attributed to its role in the regulation of adhesion molecules on leukocytes and the adhesive properties of platelets. Our previous study conducted in rat venules found that reduction of EC basal NO synthesis caused EC ICAM-1-mediated firm adhesion of leukocytes within 10-30 min. This quick response occurred in the absence of alterations of adhesion molecules on leukocytes and also opposes the classical pattern of ICAM-1-mediated leukocyte adhesion that requires protein synthesis and occurs hours after stimulation. The objective of this study is to investigate the underlying mechanisms of reduced basal NO-induced EC-mediated rapid leukocyte adhesion observed in intact microvessels. The relative levels of ICAM-1 at different cell regions and their activation status were determined with cellular fractionation and western blot using cultured human umbilical vein ECs. ICAM-1 adhesiveness was determined by immunoprecipitation in non-denatured proteins to assess the changes in ICAM-1 binding to its inhibitory antibody, mAb1A29, and antibody against total ICAM-1 with and without NO reduction. The adhesion strength of EC ICAM-1 was assessed by atomic force microscopy (AFM) on live cells. Results showed that reduction of EC basal NO caused by the application of caveolin-1 scaffolding domain (AP-CAV) or NOS inhibitor, L-NMMA, for 30 min significantly increased phosphorylated ICAM-1 and its binding to mAb1A29 in the absence of altered ICAM-1 expression and its distribution at subcellular regions. The Src inhibitor, PP1, inhibited NO reduction-induced increases in ICAM-1 phosphorylation and adhesive binding. AFM detected significant increases in the binding force between AP-CAV-treated ECs and mAb1A29-coated probes. These results demonstrated that reduced EC basal NO lead to a rapid increase in ICAM-1 adhesive binding via Src-mediated phosphorylation without de novo protein synthesis and translocation. This study suggests that a NO-dependent conformational change of constitutive EC membrane ICAM-1 might be the mechanism of rapid ICAM-1 dependent leukocyte adhesion observed in vivo. This new mechanistic insight provides a better understanding of EC/leukocyte interaction-mediated vascular inflammation under many disease conditions that encounter reduced basal NO in the circulation system.

Protein adsorption, platelet adhesion, and bacterial adhesion to polyethylene-glycol-textured polyurethane biomaterial surfaces

Traditional strategies for surface modification to enhance the biocompatibility of biomaterials often focus on a single route utilizing either chemical or physical approaches. This study combines the chemical and physical treatments as applied to poly(urethane urea) (PUU) biomaterials to enhance biocompatibility at the interface for inhibiting platelet-related thrombosis or bacterial adhesion-induced microbial infections. PUU films were first textured with submicron patterns by a soft lithography two-stage replication process, and then were grafted with polyethylene glycol (PEG). A series of biological response experiments including protein adsorption, platelet adhesion/activation, and bacterial adhesion/biofilm formation showed that PEG-grafted submicron textured biomaterial surfaces were resistant to protein adsorption, and greatly increased the efficiency in reducing both platelet adhesion/activation and bacterial adhesion/biofilm formation due to the additive effects of physical topography and grafted PEG. Results suggest that a combination of chemical modification and surface texturing will be more efficient in preventing biomaterial-associated thrombosis and infection of biomaterials. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 668-678, 2017.

The role of substrate topography on the cellular uptake of nanoparticles

Improving targeting efficacy has been a central focus of the studies on nanoparticle (NP)-based drug delivery nanocarriers over the past decades. As cells actively sense and respond to the local physical environments, not only the NP design (e.g., size, shape, ligand density, etc.) but also the cell mechanics (e.g., stiffness, spreading, expressed receptors, etc.) affect the cellular uptake efficiency. While much work has been done to elucidate the roles of NP design for cells seeded on a flat tissue culture surface, how the local physical environments of cells mediate uptake of NPs remains unexplored, despite the widely known effect of local physical environments on cellular responses in vitro and disease states in vivo. Here, we report the active responses of human osteosarcoma cells to fibrous substrate topographies and the subsequent changes in the cellular uptake of NPs. Our experiments demonstrate that surface topography modulates cellular uptake efficacy by mediating cell spreading and membrane mechanics. The findings provide a concrete example of the regulative role of the physical environments of cells on cellular uptake of NPs, therefore advancing the rational design of NPs for enhanced drug delivery in targeted cancer therapy.

An engineered micropattern to reduce bacterial colonization, platelet adhesion and fibrin sheath formation for improved biocompatibility of central venous catheters

Background

Catheter-related bloodstream infections (CRBSIs) and catheter-related thrombosis (CRT) are common complications of central venous catheters (CVC), which are used to monitor patient health and deliver medications. CVCs are subject to protein adsorption and platelet adhesion as well as colonization by the natural skin flora (i.e. Staphylococcus aureus and Staphylococcus epidermidis). Antimicrobial and antithrombotic drugs can prevent infections and thrombosis-related complications, but have associated resistance and safety risks. Surface topographies have shown promise in limiting platelet and bacterial adhesion, so it was hypothesized that an engineered Sharklet micropattern, inspired by shark-skin, may provide a combined approach as it has wide reaching anti-fouling capabilities. To assess the feasibility for this micropattern to improve CVC-related healthcare outcomes, bacterial colonization and platelet interactions were analyzed in vitro on a material common for vascular access devices.

Methods

To evaluate bacterial inhibition after simulated vascular exposure, micropatterned thermoplastic polyurethane surfaces were preconditioned with blood proteins in vitro then subjected to a bacterial challenge for 1 and 18 h. Platelet adhesion was assessed with fluorescent microscopy after incubation of the surfaces with platelet-rich plasma (PRP) supplemented with calcium. Platelet activation was further assessed by monitoring fibrin formation with fluorescent microscopy after exposure of the surfaces to platelet-rich plasma (PRP) supplemented with calcium in a flow-cell. Results are reported as percent reductions and significance is based on t-tests and ANOVA models of log reductions. All experiments were replicated at least three times.

Results

Blood and serum conditioned micropatterned surfaces reduced 18 h S. aureus and S. epidermidis colonization by 70% (p ≤ 0.05) and 71% (p < 0.01), respectively, when compared to preconditioned unpatterned controls. Additionally, platelet adhesion and fibrin sheath formation were reduced by 86% and 80% (p < 0.05), respectively, on the micropattern, when compared to controls.

Conclusions

The Sharklet micropattern, in a CVC-relevant thermoplastic polyurethane, significantly reduced bacterial colonization and relevant platelet interactions after simulated vascular exposure. These results suggest that the incorporation of the Sharklet micropattern on the surface of a CVC may inhibit the initial events that lead to CRBSI and CRT.

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.

Platelet adhesion to polyurethane urea under pulsatile flow conditions

Platelet adhesion to a polyurethane urea surface is a precursor to thrombus formation within blood-contacting cardiovascular devices, and platelets have been found to adhere strongly to polyurethane surfaces below a shear rate of approximately 500 s(-1). The aim of the current work is to determine the properties of platelet adhesion to the polyurethane urea surface as a function of time-varying shear exposure. A rotating disk system was used to study the influence of steady and pulsatile flow conditions (e.g., cardiac inflow and sawtooth waveforms) for platelet adhesion to the biomaterial surface. All experiments were conducted with the same root mean square angular rotation velocity (29.63 rad/s) and waveform period. The disk was rotated in platelet-rich bovine plasma for 2 h, with adhesion quantified by confocal microscopy measurements of immunofluorescently labeled bovine platelets. Platelet adhesion under pulsating flow was found to decay exponentially with increasing shear rate. Adhesion levels were found to depend upon peak platelet flux and shear rate, regardless of rotational waveform. In combination with flow measurements, these results may be useful for predicting regions susceptible to thrombus formation within ventricular assist devices.

Nanotopographic cell culture substrate: polymer-demixed nanotextured films under cell culture conditions

Modulating physical cell culture environments via nanoscale substrate topographic modification has recently been of significant interest in regenerative medicine. Many studies have utilized a polymer-demixing technique to produce nanotextured films and showed that cellular adhesion, proliferation, and differentiation could be regulated by the shape and scale of the polymer-demixed nanotopographies. However, little attention has been paid to the topographic fidelity of the polymer-demixed films when exposed to cell culture conditions. In this brief article, two polymer-demixing systems were employed to assess topographic changes in polymer-demixed films after fibronectin (FN) extracellular matrix protein adsorption and after incubation in phosphate-buffered saline at 37°C. We showed that FN adsorption induced very small variations (<2 nm) to the polystyrene/polybromostyrene (PS/PBrS)-demixed nanoisland textures, not substantially altering the nanotopographies given by the polymer demixing. In addition, poly(L-lactic acid)/PS (PLLA/PS)-demixed nanoisland topographies using PLLA with M w=50×10(3) did not show notable degradation up to day 24.

A comparison of blood factor XII autoactivation in buffer, protein cocktail, serum, and plasma solutions

Activation of blood plasma coagulation in vitro by contact with material surfaces is demonstrably dependent on plasma-volume-to-activator-surface-area ratio. The only plausible explanation consistent with current understanding of coagulation-cascade biochemistry is that procoagulant stimulus arising from the activation complex of the intrinsic pathway is dependent on activator surface area. And yet, it is herein shown that activation of the blood zymogen factor XII (Hageman factor, FXII) dissolved in buffer, protein cocktail, heat-denatured serum, and FXI deficient plasma does not exhibit activator surface-area dependence. Instead, a highly-variable burst of procoagulant-enzyme yield is measured that exhibits no measurable kinetics, sensitivity to mixing, or solution-temperature dependence. Thus, FXII activation in both buffer and protein-containing solutions does not exhibit characteristics of a biochemical reaction but rather appears to be a "mechanochemical" reaction induced by FXII molecule interactions with hydrophilic activator particles that do not formally adsorb blood proteins from solution. Results of this study strongly suggest that activator surface-area dependence observed in contact activation of plasma coagulation does not solely arise at the FXII activation step of the intrinsic pathway.

Atomic force microscopy visualization of poly(urethane urea) microphase rearrangements under aqueous environment

Polyurethane biomaterials are a critically important class of polymers used in a variety of medical devices. It has been suggested that the good blood compatibility of polyurethanes arises from nanoscale chemical heterogeneities at the surface as a consequence of the microphase separated morphology. In this study, we used tapping mode atomic force microscopy with phase imaging under aqueous conditions to visualize the distribution of the surface microphases for a series of poly(urethane urea) block co-polymers with varying hard segment content. The surfaces were prehydrated for 24 h under a flow of 1 mM phosphate buffer. Topographic images showed the formation of nanometer-sized raised features on the surface, having lateral dimensions of 50-70 nm and heights of 10-15 nm. Phase images, reflecting the local distribution of the mechanical properties under aqueous conditions, were quite different from those obtained in ambient conditions, consistent with water-induced structural reorientation. Images suggest that there is little soft phase material at the polymer surface in the presence of water, while images acquired after dehydration of the samples show that the surface layer remains rich in hard domains, indicating that the films do not return to their original states over the time period studied.

Characterization of surface microphase structures of poly(urethane urea) biomaterials by nanoscale indentation with AFM

Atomic force microscopy utilizing both tapping mode and force mode imaging is used to visualize the separated microphases in poly(urethane urea) films under ambient and aqueous conditions. The topography of the PUU surface changed upon hydration with the formation of nanometer-sized features on the surface. The surface becomes enriched in hard domains with hydration time and this enrichment is irreversible after dehydration. Force mode measurements were used to quantify mechanical properties as both indentation and modulus measurements. Analysis of the modulus during indentation reveals the three-dimensional nature of the structures, with the surface being covered by a 2-20-nm-thick soft segment overlayer under ambient conditions, while hydration leads to the loss of this overlayer. The force measurements also reveal the presence of regions having modulus values between those of the hard and soft phases and located spatially near the interface between the hard and soft domains. However, such regions with intermediate modulus were only rarely seen following hydration. Calculation of the Young's modulus from the compression data shows that hydration increases the modulus of the PUU surface by both enrichment of the amount of hard domain present and increasing the modulus of the individual hard and soft phases themselves. Direct visualization of the distribution of these different domains on the surface by nanoscale measurements provides an important path to characterizing the relationships between the surface properties of these materials and subsequent performance in biomedical applications.

Chronic in vivo testing of the Penn State infant ventricular assist device

The Penn State Infant Ventricular Assist Device (VAD) is a 12-14 ml stroke volume pneumatically actuated pump, with custom Björk-Shiley monostrut valves, developed under the National Heart, Lung, and Blood Institute Pediatric Circulatory Support program. In this report, we describe the seven most recent chronic animal studies of the Infant VAD in the juvenile ovine model, with a mean body weight of 23.5 ± 4.1 kg. The goal of 4-6 weeks survival was achieved in five of seven studies, with support duration ranging from 5 to 41 days; mean 26.1 days. Anticoagulation was accomplished using unfractionated heparin, and study animals were divided into two protocol groups: the first based on a target activated partial thromboplastin time of 1.5-2 times normal, and a second group using a target thromboelastography R-time of two times normal. The second group required significantly less heparin, which was verified by barely detectable heparin activity (anti-Xa). In both groups, there was no evidence of thromboembolism except in one animal with a chronic infection and fever. Device thrombi were minimal and were further reduced by introduction of the custom valve. These results are consistent with results of adult VAD testing in animals and are encouraging given the extremely low levels of anticoagulation in the second group.

Amidolytic, procoagulant, and activation-suppressing proteins produced by contact activation of blood factor XII in buffer solution

The relative proportions of enzymes with amidolytic or procoagulant activity produced by contact activation of the blood zymogen factor XII (FXII, Hageman factor) in buffer solution depends on activator surface chemistry/energy. As a consequence, chromogenic assay of amidolytic activity (cleavage of amino acid bonds in s-2302 chromogen) does not correlate with the traditional plasma coagulation time assay for procoagulant activity (protease activity inducing clotting of blood plasma). Amidolytic activity did not vary significantly with activator particle surface energy, herein measured as water adhesion tension τ(o)=γ(lv)(o)cosθ(a) ; where γ(lv)(o) is pure buffer interfacial tension and θ(a) is the advancing contact angle. By contrast, procoagulant activity varied as a parabolic-like function of τ(o), high at both hydrophobic and hydrophilic extremes of activator surface energy and falling through a broad minimum within a 20<τ(o)<40 mJ/m(2) (55°<θ(a) < 75°) range, corroborating and expanding previously-published work. It is inferred from these functional assays that an unknown number of protein fragments are produced by contact activation of FXII (a.k.a. autoactivation) rather than just αFXIIa and βFXIIa as popularly believed. Autoactivation products produced by activator particles within the 20<τ(o)<40 mJ/m(2) (55°<θ(a) < 75°) surface-energy range suppresses production of procoagulant enzymes by activators selected from the hydrophobic or hydrophilic surface-energy extremes through as-yet unknown biophysical chemistry. Suppression proteins may be responsible for the experimentally-observed autoinhibition of the autoactivation reaction.

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.

Effects of membrane cholesterol depletion and GPI-anchored protein reduction on osteoblastic mechanotransduction

We previously demonstrated that oscillatory fluid flow activates MC3T3-E1 osteoblastic cell calcium signaling pathways via a mechanism involving ATP releases and P2Y(2) puringeric receptors. However, the molecular mechanisms by which fluid flow initiates cellular responses are still unclear. Accumulating evidence suggests that lipid rafts, one of the important membrane structural components, may play an important role in transducing extracellular fluid shear stress to intracellular responses. Due to the limitations of current techniques, there is no direct approach to study the role of lipid rafts in transmitting fluid shear stress. In this study, we targeted two important membrane components associated with lipid rafts, cholesterol, and glycosylphosphatidylinositol-anchored proteins (GPI-anchored proteins), to disrupt the integrity of cell membrane structures. We first demonstrated that membrane cholesterol depletion with the treatment of methyl-β-cyclodextrin inhibits oscillatory fluid flow induced intracellular calcium mobilization and ERK1/2 phosphorylation in MC3T3-E1 osteoblastic cells. Secondly, we used a novel approach to decrease the levels of GPI-anchored proteins on cell membranes by overexpressing glycosylphosphatidylinositol-specific phospholipase D in MC3T3-E1 osteoblastic cells. This resulted in significant inhibition of intracellular calcium mobilization and ERK1/2 phosphorylation in response to oscillatory fluid flow. Finally, we demonstrated that cholesterol depletion inhibited oscillatory fluid flow induced ATP releases, which were responsible for the activation of calcium signaling pathways in MC3T3-E1 osteoblastic cells. Our findings suggest that cholesterol and GPI-anchored proteins, two membrane structural components related to lipid rafts, may play an important role in osteoblastic cell mechanotransduction.

Dynamics of hydrated polyurethane biomaterials: Surface microphase restructuring, protein activity and platelet adhesion

Microphase separation is a central feature of segmented polyurethane biomaterials and contributes to the biological response to these materials. In this study we utilized atomic force microscopy (AFM) to study the dynamic restructuring of three polyurethanes having soft segment chemistries of interest in biomedical applications. For each of the materials we followed the changes in near surface mechanical properties during hydration, as well as fibrinogen activity and platelet adhesion on these surfaces. Both AFM phase imaging and force mode analysis demonstrated that these polyurethane biomaterials underwent reorientation and rearrangement resulting in a net enrichment of hard domains at the surface. Fibrinogen activity and platelet adhesion on the polyurethane surfaces were found to decrease with increasing hydration time. The findings suggest that water-induced enrichment of hydrophilic hard domains at the surface changes the local surface physical and chemical properties in a way that influences the conformation of fibrinogen, changing the availability of the platelet-binding sites in the protein. This work demonstrates that the hydrated polyurethane biomaterial interface is a complex and dynamic environment where the surface chemistry is changing, altering the activity of fibrinogen and affecting blood platelet adhesion.

Surface-energy dependent contact activation of blood factor XII

Contact activation of blood factor XII (FXII, Hageman factor) in neat-buffer solution exhibits a parabolic profile when scaled as a function of silanized-glass-particle activator surface energy (measured as advancing water adhesion tension tau(a)(o)=gamma(lv)(o)cos theta in dyne/cm, where gamma(lv)(o) is water interfacial tension in dyne/cm and theta is the advancing contact angle). Nearly equal activation is observed at the extremes of activator water-wetting properties -36<tau(a)(o)<72 dyne/cm (0 degrees <or=theta<120 degrees), falling sharply through a broad minimum within the 20<tau(a)(o)<40 dyne/cm (55 degrees <theta<75 degrees) range over which activation yield (putatively FXIIa) rises just above detection limits. Activation is very rapid upon contact with all activators tested and did not significantly vary over 30 min of continuous FXII-procoagulant contact. Results suggest that materials falling within the 20<tau(a)(o)<40 dyne/cm surface-energy range should exhibit minimal activation of blood-plasma coagulation through the intrinsic pathway. Surface chemistries falling within this range are, however, a perplexingly difficult target for surface engineering because of the critical balance that must be struck between hydrophobicity and hydrophilicity. Results are interpreted within the context of blood plasma coagulation and the role of water and proteins at procoagulant surfaces.

Contributions of contact activation pathways of coagulation factor XII in plasma

Activation of human blood plasma coagulation by contact with hydrophilic or hydrophobic surfaces (procoagulants) is dominated by kallikrein (Kal)-mediated activation of the blood zymogen FXII (Hageman Factor). Mathematical modeling of prekallikrein (PK)-deficient platelet-poor plasma (d(PK)PPP) and PK-reconstituted d(PK)PPP (Rd(PK)PPP) coagulation shows that autoactivation of FXII (FXII-->[surface]FXII) produces no more than about 25% of the total FXIIa produced by the intrinsic pathway. Autoactivation and reciprocal-activation increase in the same proportion with procoagulant surface energy (water-wettability), whereas total amount of FXIIa produced per-unit-area procoagulant remains roughly constant for any particular procoagulant. These results suggest that procoagulant surfaces initiate the intrinsic cascade by producing a bolus of FXIIa in proportion to surface energy or surface area but play no additional role in subsequent molecular events in the cascade. Results further suggest that reciprocal-activation occurs in proportion to the amount of FXIIa produced by the initiating autoactivation step.

Moderation of prekallkrein-factor XII interactions in surface activation of coagulation by protein-adsorption competition

Traditional biochemistry of contact activation of blood coagulation suggesting that anionic hydrophilic surfaces are specific activators of the cascade is inconsistent with known trends in protein adsorption. To investigate contact activation reactions, a chromogenic assay was used to measure prekallkrein (PK) hydrolysis to kallikrein (Kal) by activated factor XII (FXIIa) at test hydrophilic (clean glass) and hydrophobic (silanized glass) surfaces in the presence of bovine serum albumin (BSA). Hydrolysis of PK by FXIIa is detected after contact of the zymogen FXII with a test hydrophobic surface only if putatively-adsorbed FXIIa is competitively displaced by BSA. By contrast, FXIIa activity is detected spontaneously following FXII activation by a hydrophilic surface and requires no adsorption displacement. These results (i) show that an anionic hydrophilic surface is not a necessary cofactor for FXIIa-mediated hydrolysis of PK, (ii) indicate that PK hydrolysis does not need to occur by an activation complex assembled directly on an anionic, activating surface, (iii) confirms that contact activation of FXII (autoactivation) is not specific to anionic hydrophilic surfaces, and (iv) demonstrates that protein-adsorption competition is an essential feature that must be included in any comprehensive mechanism of surface-induced blood coagulation.

Atomic force microscopy studies of the initial interactions between fibrinogen and surfaces

Atomic force microscopy (AFM) was used to analyze the interactions between fibrinogen and model surfaces having different levels of water wettability. In contrast to most AFM studies, proteins were coupled to the substrate while model surface colloids were attached to the end of the AFM probe, thereby ensuring that proteins undergo only a single compression/decompression cycle. Similar values of adhesion force were observed between fibrinogen and all of the highly wettable surfaces; in the same manner, fibrinogen showed similar adhesion forces against all poorly wettable surfaces, with a step-like transition observed between the two groups. Relationships between the adhesion forces and loading rates were used to analyze the energy profiles involved in protein/surface interactions. Multiple energy barriers were found in the interaction of proteins with poorly wettable surfaces; whereas a single energy barrier was found for protein interactions with highly wettable surfaces. Contact time-dependent adhesion data were fit to an exponential model and showed that the rate constants of the protein unfolding process on highly wettable surface were smaller at low loading rates, but increased rapidly to yield values similar to those on poorly wettable surfaces at high loading rates. The activation energies of protein unfolding derived from the data offer insight into the role of surface wettability in affecting adhesion, conformational changes, and ultimately, the activity of proteins at biomaterial surfaces.

Contact activation of blood-plasma coagulation

This opinion identifies inconsistencies in the generally-accepted surface biophysics involved in contact activation of blood-plasma coagulation, reviews recent experimental work aimed at resolving inconsistencies, and concludes that this standard paradigm requires substantial revision to accommodate new experimental observations. Foremost among these new findings is that surface-catalyzed conversion of the blood zymogen factor XII (FXII, Hageman factor) to the enzyme FXIIa (FXII [surface] --> FXIIa, a.k.a. autoactivation) is not specific for anionic surfaces, as proposed by the standard paradigm. Furthermore, it is found that surface activation is moderated by the protein composition of the fluid phase in which FXII autoactivation occurs by what appears to be a protein-adsorption-competition effect. Both of these findings argue against the standard view that contact activation of plasma coagulation is potentiated by the assembly of activation-complex proteins (FXII, FXI, prekallikrein, and high-molecular weight kininogen) directly onto activating surfaces (procoagulants) through specific protein/surface interactions. These new findings supplement the observation that adsorption behavior of FXII and FXIIa is not remarkably different from a wide variety of other blood proteins surveyed. Similarity in adsorption properties further undermines the idea that FXII and/or FXIIa are distinguished from other blood proteins by unusual adsorption properties resulting in chemically-specific interactions with activating anionic surfaces. IMPACT STATEMENT: This review shows that the consensus biochemical mechanism of contact activation of blood-plasma coagulation that has long served as a rationale for poor hemocompatibility is an inadequate basis for surface engineering of advanced cardiovascular biomaterials.

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.

Submicron poly(L-lactic acid) pillars affect fibroblast adhesion and proliferation

Controlling cell adhesion and proliferation on synthetic polymers is key to tissue engineering scaffold development. It is accepted that surface topography influences cell response but the mechanisms behind this remain unclear. In this work, cell response is assessed to topographies larger than focal complexes (FXs) but smaller than focal adhesions (FAs). Poly(L-lactic acid) was patterned with 400- and 700-nm pillars via replication molding. Human fibroblast adhesion and proliferation were assessed. The development of focal contacts and actin microfilaments were evaluated via immunofluorescence. Cell interactions with surface topography were observed via scanning electron microscopy. Initial fibroblast adhesion (<1 day) increased with texture as 400 nm > 700 nm > smooth, but proliferation (>1 day) decreased with texture. Increased FX formation was observed on textured surfaces. However, FAs were narrower on textured surfaces compared with smooth materials and confined to interpillar regions. SEM showed that fibroblasts deformed the 400-nm pillars. It is hypothesized that surface texture mediated FX formation and increased cell adhesion, possibly via increased material surface area. Texture geometry limited maturation of FXs to FAs, decreasing proliferation. We conclude that surface texture can alter cell adhesion and proliferation and propose geometric constraint as a mechanism for this process.