Fibrinogen adsorption, platelet adhesion and activation on mixed hydroxyl-/methyl-terminated self-assembled monolayers

Bioengineered surfaces to improve the blood compatibility of biomaterials through direct thrombin inactivation

Thrombus formation, due to thrombin generation, is a major problem affecting blood-contacting medical devices. This work aimed to develop a new strategy to improve the hemocompatibility of such devices by the immobilization of a naturally occurring thrombin inhibitor into a nanostructured surface. Boophilin, a direct thrombin inhibitor from the cattle tick Rhipicephalus microplus, was produced as a recombinant protein in Pichia pastoris. Boophilin was biotinylated and immobilized on biotin-terminated self-assembled monolayers (SAM) via neutravidin. In order to maintain its proteinase inhibitory capacity after surface immobilization, boophilin was biotinylated after the formation of a boophilin-thrombin complex to minimize the biotinylation of the residues involved in thrombin-boophilin interaction. The extent of boophilin biotinylation was determined using matrix-assisted laser desorption/ionization-time of flight/time of flight mass spectrometry. Boophilin immobilization and thrombin adsorption were quantified using quartz crystal microbalance with dissipation. Thrombin competitive adsorption from human serum was assessed using ¹²⁵I-thrombin. Thrombin inhibition and plasma clotting time were determined using spectrophotometric techniques. Boophilin-coated SAM were able to promote thrombin adsorption in a selective way, inhibiting most of its activity and delaying plasma coagulation in comparison with boophilin-free surfaces, demonstrating boophilin's potential to improve the hemocompatibility of biomaterials used in the production of blood-contacting devices.

Platelet responses to dynamic biomaterial surfaces with different poly(ethylene glycol) and polyrotaxane molecular architectures constructed on gold substrates

Four different dynamic biomaterial surfaces with different molecular architectures were prepared using two hydrophilic polymers: poly(ethylene glycol) and polyrotaxanes containing α-cyclodextrin. Either one or both terminals of the poly(ethylene glycol) or polyrotaxanes were immobilized onto a gold substrate via Au-S bonds, resulting in poly(ethylene glycol)-graft, polyrotaxanes-graft, poly(ethylene glycol)-loop, and polyrotaxanes-loop structures. Human platelet adhesion was suppressed more effectively on the graft surfaces than on the loop surfaces for both poly(ethylene glycol) and polyrotaxanes due to the high mobility of graft polymer chains with a free terminal. Moreover, the platelets adhered to the polyrotaxane surfaces much less than the poly(ethylene glycol) surfaces, possibly because of the mobile nature of the α-cyclodextrin molecules that were threaded on the poly(ethylene glycol) chain. Actin filament assembly in adherent platelets was also greatly prevented on the poly(ethylene glycol)/polyrotaxanes-graft surfaces in comparison with the corresponding loop surfaces. A clear correlation between the numbers and areas of adherent platelets on these surfaces suggests that platelet adhesion and activation were dominated by the platelet GPIIb/IIIa-adsorbed fibrinogen interaction. These results indicate that both of the different modes of dynamic features, sliding/rotation of α-cyclodextrin and polymer chain mobility, effectively suppressed platelet adhesion in spite of the similar hydrophilicity. This research affords a novel chemical strategy for designing hemocompatibile biomaterial surfaces.

Thromboresistant and endothelialization effects of dopamine-mediated heparin coating on a stent material surface

Heparinization of surfaces has proven a successful strategy to prevent thrombus formation. Inspired by the composition of adhesive proteins in mussels, the authors used dopamine to immobilize heparin on a stent surface. This study aimed to assess the thromboresistant and endothelialization effects of dopamine-mediated heparin (HPM) coating on a stent material surface. The HPM was synthesized by bonding dopamine and heparin chemically. Cobalt-chromium (Co-Cr) alloy disks were first placed in the HPM solution and applied to surface stability then underwent thromboresistant tests and human umbilical vein endothelial cells (HUVEC) cytotoxicity assays. The results showed not only thromboresistant activity and a stable state of heparin on the surfaces after investigation with toluidine blue and thrombin activation assay but also proliferation of HUVEC in vitro. Studies on animals showed that the HPM-coated stent has no obvious inflammation response and increasing of restenosis rate compared to the bare metal stent (BMS) indicating good biocompatibility as well as safety in its in vivo application. Moreover, improving the endothelial cell (EC) proliferation resulted in a higher strut-covering rate (i.e., endothelialization) with shuttle-shaped EC in the HPM-coated stent group compared to that of the BMS group. These results suggest that this facile coating approach could significantly promote endothelialization and offer greater safety than the BMS for its much improved thromboresistant property. Moreover, it may offer a platform for conjugating secondary drugs such as anti-proliferative drugs.

Fibronectin binding to the Treponema pallidum adhesin protein fragment rTp0483 on functionalized self-assembled monolayers

Past work has shown that Treponema pallidum, the causative agent of syphilis, binds host fibronectin (FN). FN and other host proteins are believed to bind to rare outer membrane proteins (OMPs) of T. pallidum, and it is postulated that this interaction may facilitate cell attachment and mask antigenic targets on the surface. This research seeks to prepare a surface capable of mimicking the FN binding ability of T. pallidum in order to investigate the impact of FN binding with adsorbed Tp0483 on the host response to the surface. By understanding this interaction, it may be possible to develop more effective treatments for infection and possibly mimic the stealth properties of the bacteria. Functionalized self-assembled monolayers (SAMs) on gold were used to investigate rTp0483 and FN adsorption. Using a quartz crystal microbalance (QCM), rTp0483 adsorption and subsequent FN adsorption onto rTp0483 were determined to be higher on negatively charged carboxylate-terminated self-assembled monolayers (-COO(-) SAMs) compared to the other surfaces analyzed. Kinetic analysis of rTp0483 adsorption using surface plasmon resonance (SPR) supported this finding. Kinetic analysis of FN adsorption using SPR revealed a multistep event, where the concentration of immobilized rTp0483 plays a role in FN binding. An examination of relative QCM dissipation energy compared to the shift in frequency showed a correlation between the physical properties of adsorbed rTp0483 and SAM surface chemistry. In addition, AFM images of rTp0483 on selected SAMs illustrated a preference of rTp0483 to bind as aggregates. Adsorption on -COO(-) SAMs was more uniform across the surface, which may help further explain why FN bound more strongly. rTp0483 antibody studies suggested the involvement of amino acids 274-289 and 316-333 in binding between rTp0483 to FN, while a peptide blocking study only showed inhibition of binding with amino acids 316-333. Finally, surface adsorbed rTp0483 with FN bound significantly less anti-RGD and gelatin compared to FN adsorbed directly to -COO(-) SAMs, indicating that one or both binding regions may play a role in binding between rTp0483 and FN.

Platelet compatibility improvement by proper choice of acidic terminal functionality for mixed-charge self-assembled monolayers

In this study two different series of mixed-charge self-assembled monolayers (SAMs) prepared with -N(+)(CH(3))(3)-terminated alkanethiol and strong dissociated monovalent -SO(3)H acid-terminated or weaker dissociated divalent -PO(3)H(2) acid-terminated alkanethiol in pure ethanol were characterized. The influence of the acidity of the anionic functionality in the mixed-charge SAMs on the surface characteristics and platelet compatibility was investigated. X-ray photoelectron spectroscopy indicated that a nearly equivalent amount of countercharged terminal groups was noted on the surface of -SO(3)H/-N(+)(CH(3))(3) mixed SAMs, while "-N(+)(CH(3))(3) thiol poor" phenomena were found on -PO(3)H(2)/-N(+)(CH(3))(3) mixed SAMs instead. This was caused by the distinct differences in solvation capability between the acidic anionic functional groups and solvent molecules and/or the interactions among the terminal ends of the thiols. This acidity difference also affected other interfacial properties and the platelet compatibility. The mixed SAMs formed from the mixture of -SO(3)H- and -N(+)(CH(3))(3)-terminated thiols showed higher surface hydrophilicity and exhibited the least amount of platelets adhered, but these two mixed SAMs were all fairly negatively surface charged. The structure of the hydration layer near the surfaces was likely affected by the acidity of the anionic functionality, and this would cause such a distinct behavior in platelet compatibility. It was concluded that the hydrophilic surfaces with nearly equal amounts of surface positively and negatively charged components could exhibit better platelet compatibility. This work demonstrated that the nature of the acidic terminal ends of alkanethiol is also a key factor for preparing mixed-charge SAMs with good platelet compatibility.

Chemically well-defined self-assembled monolayers for cell culture: toward mimicking the natural ECM

The extracellular matrix (ECM) is a network of biological macromolecules that surrounds cells within tissues. In addition to serving as a physical support, the ECM actively influences cell behavior by providing sites for cell adhesion, establishing soluble factor gradients, and forming interfaces between different cell types within a tissue. Thus, elucidating the influence of ECM-derived biomolecules on cell behavior is an important aspect of cell biology. Self-assembled monolayers (SAMs) have emerged as promising tools to mimic the ECM as they provide chemically well-defined substrates that can be precisely tailored for specific cell culture applications, and their application in this regard is the focus of this review. In particular, this review will describe various approaches to prepare SAM-based culture substrates via non-specific adsorption, covalent immobilization, or non-covalent sequestering of ECM-derived biomolecules. Additionally, this review will highlight SAMs that present ECM-derived biomolecules to cells to probe the role of these molecules in cell-ECM interactions, including cell attachment, spreading and 'outside-in' signaling via focal adhesion complex formation. Finally, this review will introduce SAMs that can present or sequester soluble signaling molecules, such as growth factors, to study the influence of localized soluble factor activity on cell behavior. Together, these examples demonstrate that the chemical specificity and variability afforded by SAMs can provide robust, well-defined substrates for cell culture that can simplify experimental design and analysis by eliminating many of the confounding factors associated with traditional culture substrates.

Platelet and leukocyte adhesion to albumin binding self-assembled monolayers

This study reports the use of tetraethylene glycol-terminated self-assembled monolayers (EG(4) SAMs) as a background non-fouling surface to study the effect of an 18 carbon ligand (C18) on albumin selective and reversible adsorption and subsequent platelet and leukocyte adhesion. Surface characterization techniques revealed an efficient immobilization of different levels of C18 ligand on EG(4) SAMs and an increase of surface thickness and hydrophobicity with the increase of C18 ligands. Albumin adsorption increased as the percentage of C18 ligands on the surface increased, but only 2.5%C18 SAMs adsorbed albumin in a selective and reversible way. Adherent platelets also increased with the amount of immobilized C18. Pre-immersion of samples in albumin before contact with platelets demonstrated an 80% decrease in platelet adhesion. Pre-immersion in plasma was only relevant for 2.5%C18 SAMs since this was the only surface to have less platelet adhesion compared to buffer pre-immersion. EG(4) SAMs adhered negligible amounts of leukocytes, but surfaces with C18 ligands have some adherent leukocytes. Except for 10%C18 SAMs, which increased leukocyte adhesion after albumin pre-adhesion, protein pre-immersion did not influence leukocyte adhesion. It has been shown that a surface with a specific surface concentration of albumin-binding ligands (2.5%C18 SAMs) can recruit albumin selectively and reversibly and minimize the adhesion of platelets, despite still adhering some leukocytes.

Enhanced mesenchymal stromal cell recruitment via natural killer cells by incorporation of inflammatory signals in biomaterials

An exacerbated inflammatory response questions biomaterial biocompatibility, but on the other hand, inflammation has a central role in the regulation of tissue regeneration. Therefore, it may be argued that an 'ideal' inflammatory response is crucial to achieve efficient tissue repair/regeneration. Natural killer (NK) cells, being one of the first populations arriving at an injury site, can have an important role in regulating bone repair/regeneration, particularly through interactions with mesenchymal stem/stromal cells (MSCs). Here, we studied how biomaterials designed to incorporate inflammatory signals affected NK cell behaviour and NK cell-MSC interactions. Adsorption of the pro-inflammatory molecule fibrinogen (Fg) to chitosan films led to a 1.5-fold increase in adhesion of peripheral blood human NK cells, without an increase in cytokine secretion. Most importantly, it was found that NK cells are capable of stimulating a threefold increase in human bone marrow MSC invasion, a key event taking place in tissue repair, but did not affect the expression of the differentiation marker alkaline phosphatase (ALP). Of significant importance, this NK cell-mediated MSC recruitment was modulated by Fg adsorption. Designing novel biomaterials leading to rational modulation of the inflammatory response is proposed as an alternative to current bone regeneration strategies.

Immunoblot analysis of proteins associated with self-assembled monolayer surfaces of defined chemistries

Intact and fragmented proteins, eluted from self-assembled monolayer (SAM) surfaces of alkanethiols of different chemistries (-CH₃, -OH, -COOH, -NH₂), following exposure to human plasma (HP) or human serum (HS), were examined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting techniques. The SAM surfaces were incubated for 1 h with 10% (v/v) sterile-filtered, heat-inactivated (h.i.) HS or 1% (v/v) sterile-filtered h.i. HP preparations [both in phosphate buffered saline (PBS)]. Adsorbed proteins were eluted using 10% SDS/2.3% dithioerythritol for characterization of protein profiles. The type of incubating medium may be an important determinant of adsorbed protein profiles, since some variations were observed in eluates from filtered versus control unfiltered h.i. 10% HS or 1% HP. Albumin and apolipoprotein A1 were consistently detected in both filtered h.i 10% HS and 1% HP eluates from all SAM surfaces and from control tissue culture-treated polystyrene (TCPS). Interestingly, Factor H and Factor I, antithrombin, prothrombin, high molecular weight kininogen (HMWK), and IgG were present in eluates from OH, COOH, and NH₂ SAM surfaces and in eluates from TCPS but not in eluates from CH₃ SAM surfaces, following exposure to filtered h.i. 10% HS. These results suggest that CH₃ SAM surfaces were the least proinflammatory of all SAM surfaces. Overall, similar trends were observed in the profiles of proteins eluted from surfaces exposed to filtered 10% HS or 1% HP. However, the unique profiles of adsorbed proteins on different SAM surface chemistries may be related to their differential interactions with cells, including immune/inflammatory cells.

Immune responses to implants - a review of the implications for the design of immunomodulatory biomaterials

A key for long-term survival and function of biomaterials is that they do not elicit a detrimental immune response. As biomaterials can have profound impacts on the host immune response the concept emerged to design biomaterials that are able to trigger desired immunological outcomes and thus support the healing process. However, engineering such biomaterials requires an in-depth understanding of the host inflammatory and wound healing response to implanted materials. One focus of this review is to outline the up-to-date knowledge on immune responses to biomaterials. Understanding the complex interactions of host response and material implants reveals the need for and also the potential of "immunomodulating" biomaterials. Based on this knowledge, we discuss strategies of triggering appropriate immune responses by functional biomaterials and highlight recent approaches of biomaterials that mimic the physiological extracellular matrix and modify cellular immune responses.

Fibrinogen organization at the cell-material interface directs endothelial cell behavior

Fibrinogen (FG) adsorption on surfaces with controlled fraction of —OH groups was investigated with AFM and correlated to the initial interaction of primary endothelial cells (HUVEC). The —OH content was tailored making use of a family of copolymers consisting of ethyl acrylate (EA) and hydroxyl ethyl acrylate (HEA) in different ratios. The supramolecular distribution of FG changed from an organized network-like structure on the most hydrophobic surface (—OH 0) to dispersed molecular aggregate one as the fraction of —OH groups increases, indicating a different conformation by the adsorbed protein. The best cellular interaction was observed on the most hydrophobic (—OH 0) surface where FG assembled in a fibrin-like appearance in the absence of any thrombin. Likewise, focal adhesion formation and actin cytoskeleton development was poorer as the fraction of hydroxy groups on the surface was increased. The biological activity of the surface-induced FG network to provide 3D cues in a potential tissue engineered scaffold, making use of electrospun PEA fibers (—OH0), seeded with human umbilical vein endothelial cells was investigated. The FG assembled on the polymer fibers gave rise to a biologically active network able to direct cell orientation along the fibers (random or aligned), promote cytoskeleton organization and focal adhesion formation.

Influence of surface chemistry and protein concentration on the adsorption rate and S-layer crystal formation

Bacterial crystalline surface layers (S-layers) are the outermost envelope of prokaryotic organisms representing the simplest biological membranes developed during evolution. In this context, the bacterial protein SbpA has already shown its intrinsic ability to reassemble on different substrates forming protein crystals of square lattice symmetry. In this work, we present the interaction between the bacterial protein SbpA and five self-assembled monolayers carrying methyl (CH(3)), hydroxyl (OH), carboxylic acid (COOH) and mannose (C(6)H(12)O(6)) as functional groups. Protein adsorption and S-layer formation have been characterized by atomic force microscopy (AFM) while protein adsorption kinetics, mass uptake and the protein layer viscoelastic properties were investigated with quartz crystal microbalance with dissipation monitoring (QCM-D). The results indicate that the protein adsorption rate and crystalline domain area depend on surface chemistry and protein concentration. Furthermore, electrostatic interactions tune different protein rate adsorption and S-layer recrystallization pathways. Electrostatic interactions induce faster adsorption rate than hydrophobic or hydrophilic interactions. Finally, the shear modulus and the viscosity of the recrystallized S-layer on CH(3)C(6)S, CH(3)C(11)S and COOHC(11)S substrates were calculated from QCM-D measurements. Protein-protein interactions seem to play a main role in the mechanical stability of the formed protein (crystal) bilayer.

Role of surface chemistry in protein remodeling at the cell-material interface

Background

The cell-material interaction is a complex bi-directional and dynamic process that mimics to a certain extent the natural interactions of cells with the extracellular matrix. Cells tend to adhere and rearrange adsorbed extracellular matrix (ECM) proteins on the material surface in a fibril-like pattern. Afterwards, the ECM undergoes proteolytic degradation, which is a mechanism for the removal of the excess ECM usually approximated with remodeling. ECM remodeling is a dynamic process that consists of two opposite events: assembly and degradation.

Methodology/Principal Findings

This work investigates matrix protein dynamics on mixed self-assembled monolayers (SAMs) of –OH and –CH₃ terminated alkanethiols. SAMs assembled on gold are highly ordered organic surfaces able to provide different chemical functionalities and well-controlled surface properties. Fibronectin (FN) was adsorbed on the different surfaces and quantified in terms of the adsorbed surface density, distribution and conformation. Initial cell adhesion and signaling on FN-coated SAMs were characterized via the formation of focal adhesions, integrin expression and phosphorylation of FAKs. Afterwards, the reorganization and secretion of FN was assessed. Finally, matrix degradation was followed via the expression of matrix metalloproteinases MMP2 and MMP9 and correlated with Runx2 levels. We show that matrix degradation at the cell material interface depends on surface chemistry in MMP-dependent way.

Conclusions/Significance

This work provides a broad overview of matrix remodeling at the cell-material interface, establishing correlations between surface chemistry, FN adsorption, cell adhesion and signaling, matrix reorganization and degradation. The reported findings improve our understanding of the role of surface chemistry as a key parameter in the design of new biomaterials. It demonstrates the ability of surface chemistry to direct proteolytic routes at the cell-material interface, which gains a distinct bioengineering interest as a new tool to trigger matrix degradation in different biomedical applications.

Interactions of leukocytes and platelets with poly(lysine/leucine) immobilized on tetraethylene glycol-terminated self-assembled monolayers

Surfaces that bind heparin are important for biomaterials for blood deheparinization. In our recent work it was demonstrated that a polypeptide composed of L-lysine and L-leucine (pKL), after immobilization onto tetra(ethylene glycol) terminated self-assembled monolayers (EG4-SAMs), can bind heparin from blood plasma in a selective, concentration-dependent way. During this work the effect of this peptide on platelet adhesion and activation and leukocyte adhesion was studied. The surface charge of these nanostructured surfaces was evaluated in order to correlate the effect of positively charged amine groups and hydrophobic methyl groups on the behavior of platelets and leukocyte adhesion. The results demonstrated that the presence of pKL decreased leukocyte adhesion to EG4-SAMs at all concentrations used. This effect is even more pronounced when surfaces were pre-immersed in heparinized plasma. In contrast, there is an increase in platelet adhesion and activation with increased percentage immobilized pKL. This effect is enhanced when surfaces were pre-immersed in heparinized plasma. However, adsorbed pKL in very low amounts does not induce platelet adhesion and activation compared with EG4, even when pre-immersed in plasma. Since only low pKL amounts are necessary to induce heparin selectivity, these results are promising for the development of heparin-binding biomaterials for blood deheparinization.

Development of nanocomposite based on hydroxyethylmethacrylate and functionalized fumed silica: mechanical, chemico-physical and biological characterization

In this research work organic/inorganic nano composites were synthesized from poly-2-hydroxyethylmethacrylate and properly modified silica nanoparticles by in situ polymerization. In particular, fumed nanosilica was functionalized with methacryloylpropyltrimetoxy silane (MPTMS) in order to obtain a more homogeneous, reliable and mechanically performing nano composite. For comparison, nano composites with non functionalized silica were also prepared. Scanning electron microscopy was performed in order to visualize the effects of functionalization on the mode and state of dispersion. This analysis demonstrated that MPTMS grafted onto silica surface acts as an effective coupling agent and assures a good dispersion and distribution of nanoparticles as well as a strong nano particle/matrix interfacial adhesion. As a result of strong interactions occurring between phases, a pronounced increase of the glass transition temperature and mechanical parameters were recorded. Finally, these novel nano composites were seeded with murine fibroblast and human mesenchymal stem cells, and observed in time-lapse experiments proving an effective biological response.

Time sequence of blood activation by nanoporous alumina: Studies on platelets and complement system

In the present work, the time sequence of blood activation by alumina membranes with different porosities (20 and 200 nm in diameter) was studied. The membranes were incubated with whole blood from 2 min to 4 h. Platelet adhesion and activation in addition to complement activation was monitored at different time points. Evaluation of platelet adhesion and activation was done by determining the change in platelet number and the levels of thrombospondin-1 (TSP-1) in the fluid phase. Scanning electron microscopy studies were done to further evaluate platelet adhesion and morphology. Immunocytochemical staining was used to evaluate the presence of CD41 and CD62P antigens on the material surface. Complement activation was monitored by measuring C3a and sC5b-9 in plasma samples by means of enzyme immunoassays. Both alumina membranes displayed similar complement activation time profiles, with levels of C3a and sC5b-9 increasing with incubation time. A statistically significant difference between the membranes was found after 60 min of incubation. Platelet activation characteristics and time profile were different between the two membranes. Platelet adhesion increased over time for the 20 nm surface, while the clusters of microparticles on the 200 nm surface did not appreciably change during the course of the experiment. The release of TSP-1 increased with time for both membranes; however, much later for the 200 nm alumina (240 min) as compared to the 20 nm membrane (60 min). The surface topography of the alumina most probably influence protein transition rate, which in turn affects material platelet activation kinetics.

The stability of self-assembled monolayers with time and under biological conditions

The stability of self-assembled monolayers (SAMs) of different functionalities (CH(3)-, OH-, and EG4-) over time under appropriate storage conditions and when immersed in cell culture media was evaluated. X-ray photoelectron spectroscopy (XPS) was used to detect the oxidized sulfur species (S(2p) binding energy from 167 to 168 eV) resulting from oxidation of the surfaces. CH(3)-terminated SAMs stored for a period of 9 weeks in a nitrogen chamber were not altered. The same did not happen with OH- and EG4-SAMs, for which the XPS spectra evidenced oxidized peaks after 2 weeks. Regarding the stability of these surfaces under biological conditions, 30 min of immersion at 37 degrees C in serum-free or 10% fetal bovine serum (FBS) supplemented medium did not induce detectable oxidation. However, a small percentage of oxidized sulfur could have been washed out by the media, as confirmed in studies using SAMs immersed in water. Despite the possible rinsing out of oxidized thiols, high amounts of oxidation can still be detected by XPS. SAMs degradation during ethanol sterilization was not detectable by XPS, although a small increase on the wettability of OH-SAMs was observed. The data suggest that SAMs must be used freshly prepared, being recommended for short-term biological studies.

Relative influence of surface topography and surface chemistry on cell response to bone implant materials. Part 2: Biological aspects

A current medical challenge is the replacement of tissue which can be thought of in terms of bone tissue engineering approaches. The key problem in bone tissue engineering lies in associating bone stem cells with material supports or scaffolds that can be implanted in a patient. Beside bone tissue engineering approaches, these types of materials are used daily in orthopaedics and dental practice as permanent or transitory implants such as ceramic bone filling materials or metallic prostheses. Consequently, it is essential to better understand how bone cells interact with materials. For several years, the current authors and others have developed in vitro studies in order to elucidate the mechanisms underlying the response of human bone cells to implant surfaces. This paper reviews the current state of knowledge and proposes future directions for research in this domain.

Bioactivity of immobilized EGF on self-assembled monolayers: optimization of the immobilization process

Last trends in Biomaterials focus the mimic of cellular environments capable to control cellular responses. Epidermal growth factor (EGF) is a pleiotropic cytokine known to regulate cell proliferation, differentiation, and death. This study aims to optimize the immobilization of EGF on 11-mercapto-1-undecyl-tetra(ethylene)glycol (EG4)-self-assembled monolayers (SAMs) and to establish a new model surface to study EGF-mediated signaling. Gold substrates were modified with a monolayer of EG4 and N,N'-carbonyldiimidazole (CDI) was used to activate hydroxyl terminated groups of EG4-SAMs. EGF was then immobilized on activated EG4-SAMs at pH 7.4, 4 degrees C, and 100 rpm. Different immobilization reaction times were tested as well as different CDI concentrations to optimize the reaction conditions and obtain a range of immobilized EGF concentrations on the surfaces. Surface characterization of EGF-SAMs was performed using radiolabeling, water contact angle measurements, X-ray photoelectron spectroscopy, and ELISA. Phosphorylation of EGFR on BT-20 breast cancer cell line by EGF-SAMs was tested by immunostaining. EGF was successfully immobilized on EG4-SAMs, at 4 degrees C and pH 7.4 in a range of concentrations from 3.6 +/- 0.8 to 17.6 +/- 1.5 ng/cm(2). The concentration of EGF increases with immobilization time and with the CDI concentration reaching the maximum for surfaces activated with 30 mg/mL of CDI after 48 h. The bioactivity of EGF-SAMs was confirmed by immunostaining of phospho-EGFR of BT-20 cells. This study described EGF immobilization on EG4-SAMs at different concentrations, which could be important surface models to study specific protein interactions at the molecular level evolving EGF-family of proteins.

Supramolecular Surfaces Modulating Cellular Response

Polyrotaxane-immobilized surfaces were prepared as a platform of dynamic surfaces, which can prevent from non-specific interaction with plasma proteins and platelet, and then modulate cellular functions via specific interaction with receptor protein-ligand binding through movable polyrotaxane backbone. The immobilization of the polyrotaxane was carried out via two-step protocol, in which the polyrotaxane with tetraethyleneglycol dodecanethiol (TEGDT) anchoring group at both terminals was fixed onto Au substrate via Au-S bond, followed by the fixation of TEGDT molecule onto the Au substrate to complete the loop formation of polyrotaxane on the Au substrate with the help of self-assembled monolayer formation of TEGDT. Their surface properties were characterized by means dynamic contact angle measurements, and preliminary studies as biomaterials were performed in terms of plasma protein adsorption onto their surfaces.

Mixed-SAM surfaces monitoring CTX-protein part I: Using atomic force microscope measurements

Fast and efficient detection of Cobra cardiotoxin (CTX) protein molecules on biochip surfaces is an example of application in biotechnology. One potential application of mixed self assembled monolayers (SAMs) as chip surfaces yield different binding affinities of the CTX proteins, a series of studies on the interaction force between CTX proteins and the mixed SAMs surfaces formed from mixtures of two thiols with the same/different chain lengths and/or with the same/different terminal groups will be investigated. In these dual papers, the mixed SAMs of n-alkinethiol SAMs of different chain lengths are chosen as the first examples of this series due to the simple functions of the mixed SAMs surface structure. Thus, the adhesion force measurements of CTX protein molecules on mixed SAMs of n-alkinethiol SAMs of different chain lengths: 1-decanethiol (C9) and 1-hexanethiol (C5) with different mixing ratios are developed and conducted using atomic force microscope (AFM). There are two major tasks in Part I of the dual papers: the development of the AFM measurements providing reliable information, and selection of the surface with highest binding affinity among this mixed SAMs group. Results indicate that the adhesion forces for CTX protein molecules on mixed SAMs with mixing ratio (χ(C9)) of 0.25, 0.5, 0.75 and 1, are 1.26, 1.8, 1.38, and 1.25 folds respectively, compared with the adhesion force of CTX protein molecules on the C5 surface only. Therefore, the SAM surfaces of χ(C9) = 0.5 is the best choice as a biomaterial sensor of this group of mixed SAMs because the strongest binding force and highest efficiency. Effects of the loading force of the AFM operation, the radius of curvature of the AFM tip, and the AFM tip endurance as well as control experiments were examined to ensure the quantitative determination of adhesion force for AFM measurement. The physical mechanism of protein adsorption on SAM surfaces will be studied and analyzed by molecular dynamics (MD) simulations and will be reported in Part II of the dual papers to compensate the limited information on the interaction taking place at atomic level that experiments cannot provide.

Excess fibrinogen adsorption to monolayers of mixed lipids

Adsorption of fibrinogen to the monolayers of mixed lipids, dipalmitoyl phosphatidyl choline (DPPC) and eicosylamine (EA) was measured at a surface pressure of 20 mN/m by an in situ surface plasmon resonance technique. Pressure-area isotherms of DPPC+EA mixtures on water and buffer subphases indicated good lipid miscibility and some contraction of the monolayers at intermediate and higher surface pressures. Surface electric potential of the DPPC+EA monolayers showed excess values for intermediate DPPC:EA ratios. Fibrinogen adsorption and its adsorption rates from a dilute solution (0.03 mg/ml) were proportional to the fraction of EA in the monolayer indicating that protein binding was primarily driven by electrostatic interactions between positive EA charges in the monolayer and a net negative protein charge. At a higher protein concentration (0.06 mg/ml) both the fibrinogen adsorbed amount and its maximum adsorption rate showed excess values relative to the pure EA for 1:1, 2:1 and 3:1 DPPC+EA monolayers. This excess adsorption could be explained, in part, by the contraction of the monolayers with intermediate DPPC:EA ratios which resulted in an excess surface electric potential.

Conformational behavior of fibrinogen on topographically modified polymer surfaces

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

Profiles of carbohydrate ligands associated with adsorbed proteins on self-assembled monolayers of defined chemistries

Conserved protein-carbohydrate-lipid pathogen-associated molecular patterns (PAMPs) interact with cells of the innate immune system to mediate antigen recognition and internalization and activation of immune cells. We examined if analogous "biomaterial-associated molecular patterns" composed of proteins, specifically their carbohydrate modifications, existed on biomaterials, which can play a role in mediating the innate immune response to biomaterials. To probe for these carbohydrates in the adsorbed protein layer, as directed by the underlying biomaterial chemistry, self-assembled monolayers (SAMs) presenting -CH(3), -OH, -COOH, or -NH(2) were preincubated with serum/plasma, and the presence of carbohydrate ligands of C-type lectin receptors (CLRs) was investigated using lectin probes in an enzyme-linked lectin assay (ELLA). Presentation of CLR ligands was detected on control tissue culture polystyrene (TCPS). Absorbances of mannose or N-acetylglucosamine increased with decreasing incubating serum concentration, whereas absorbances of sialylated epitopes or fucose remained unchanged. Absorbances of alpha-galactose or N-acetylgalactosamine decreased with decreasing incubating serum concentration; beta-galactose was undetectable. Among SAM endgroups, preincubation with 10% serum resulted in differential presentation of CLR ligands: higher alpha-galactose on COOH SAMs than NH(2) or CH(3) SAMs, highest complex mannose on NH(2) SAMs, and higher complex mannose on OH SAMs than CH(3) SAMs. Least sialylated groups were detected on CH(3) SAMs. In summary, biomaterial chemistry may regulate protein adsorption and hence unique presentation of associated carbohydrates. The ultimate goal is to identify the effects of protein glycosylations associated with biomaterials in stimulating innate immune responses.

Synergistic effect of hydrophobic and anionic surface groups triggers blood coagulation in vitro

Biomaterial induced coagulation encompasses plasmatic and cellular processes. The functional loss of biomedical devices possibly resulting from these thrombotic reactions motivates the need for a better understanding of processes occurring at blood-biomaterial interfaces. Well defined model surfaces providing specific chemical-physical properties (self assembled monolayers (SAMs)) displaying hydrophobic or/and acidic terminal groups were used to uncover initial mechanisms of biomaterial induced coagulation. We investigated the influence of electrical charge and wettability on platelet- and contact activation, the two main actors of blood coagulation, which are often considered as separate mechanisms in biomaterials research. Our results show a dependence of contact activation on acidic surface groups and a correlation of platelet adhesion to surface hydrophobicity. Clot formation resulting from the interplay of blood platelets and contact activation was only found on surfaces combining both acidic and hydrophobic surface groups but not on monolayers displaying extreme hydrophobic/acidic properties.

The adherence of platelets to adsorbed albumin by receptor-mediated recognition of binding sites exposed by adsorption-induced unfolding

Although albumin (Alb) is the most abundant plasma protein, it is considered to be non-adhesive to platelets, as it lacks any known amino acid sequences for binding platelet receptors. Recent studies have suggested that platelets adhere to adsorbed Alb by mechanisms linked to its conformational state. To definitively address this issue we used circular dichroism (CD) spectropolarimetry to characterize the conformation of Alb adsorbed on a broad range of surface chemistries from a wide range of Alb solution concentrations, with platelet adhesion examined using a lactate dehydrogenase (LDH) assay and scanning electron microscopy (SEM). Our results prove that platelets bind to adsorbed Alb through receptor-mediated processes, with binding sites in Alb exposed and/or formed by adsorption-induced protein unfolding. Most importantly, beyond a critical degree of unfolding, the platelet adhesion levels correlated strongly with the adsorption-induced unfolding in Alb. The blockage of Arg-Gly-Asp (RGD) specific platelet receptors using an Arg-Gly-Asp-Ser (RGDS) peptide led to significant inhibition of platelet adhesion to adsorbed Alb, with the extent of inhibition and morphology of adherent platelets being similar for both Alb and Fg. Chemical neutralization of arginine (Arg) residues in the adsorbed Alb layer inhibited platelet-Alb interactions significantly, indicating that Arg residues play a prominent role in mediating platelet adhesion to Alb. These results provide deeper insight into the molecular mechanisms that mediate the interactions of platelets with adsorbed proteins, and how to control these interactions to improve the blood compatibility of biomaterials for cardiovascular applications.

Disruption and activation of blood platelets in contact with an antimicrobial composite coating consisting of a pyridinium polymer and AgBr nanoparticles

Composite materials made up from a pyridinium polymer matrix and silver bromide nanoparticles embedded therein feature excellent antimicrobial properties. Most probably, the antimicrobial activity is related to the membrane-disrupting effect of both the polymer matrix and Ag(+) ions; both may work synergistically. One of the most important applications of antimicrobial materials would be their use as surface coatings for percutaneous (skin-penetrating) catheters, such as central venous catheters (CVCs). These are commonly used in critical care, and serious complications due to bacterial infection occur frequently. This study aimed at examining the possible effects of a highly antimicrobial pyridinium polymer/AgBr composite on the blood coagulation system, i.e., (i) on the coagulation cascade, leading to the formation of thrombin and a fibrin cross-linked network, and (ii) on blood platelets. Evidently, pyridinium/AgBr composites could not qualify as coatings for CVCs if they trigger blood coagulation. Using a highly antimicrobial composite of poly(4-vinylpyridine)-co-poly(4-vinyl-N-hexylpyridinium bromide) (NPVP) and AgBr nanoparticles as a thin adherent surface coating on Tygon elastomer tubes, it was found that contacting blood platelets rapidly acquire a highly activated state, after which they become substantially disrupted. This implies that NPVP/AgBr is by no means blood-compatible. This disqualifies the material for use as a CVC coating. This information, combined with earlier findings on the hemolytic effects (i.e., disruption of contacting red blood cells) of similar materials, implies that this class of antimicrobial materials affects not only bacteria but also mammalian cells. This would render them more useful outside the biomedical field.

Blood interactions with noble metals: coagulation and immune complement activation

Noble metals are interesting biomaterials for a number of reasons, e.g., their chemical inertness and relative mechanical softness, silver's long known antimicrobial properties, and the low allergenic response shown by gold. Although important for the final outcome of biomaterials, little is reported about early events between pure noble metals and blood. In this article, we used whole blood in the "slide chamber model" to study the activation of the immune complement activation, generation of thrombin/antithrombin (TAT) complexes, and platelet depletion from blood upon contact with silver (Ag), palladium (Pd), gold (Au), titanium (Ti), and Bactiguard, a commercial nanostructured biomaterial coating comprised of Ag, Pd, and Au. The results show the highest TAT generation and platelet depletion on Ti and Au and lower on Pd, Ag, and the Bactiguard coating. The immune complement factor 3 fragment (C3a) was generated by the surfaces in the following order: Ag > Au > Pd > Bactiguard > Ti. Quartz crystal microbalance adsorption studies with human fibrinogen displayed the highest deposition to Ag and the lowest onto the Bactiguard coating. The adsorbed amounts of fibrinogen did not correlate with thrombogenicity in terms of TAT formation and platelet surface accumulation in blood. The combined results suggest, hence, that noble metal chemistry has a different impact on the protein adsorption properties and general blood compatibility. The low thrombogenic response by the Bactiguard coating cannot be explained by any of the single noble metal properties but is likely a successful combination of the nanostructure, nanogalvanic effects, or combinatory chemical and physical materials properties.

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.

Investigation of the effects of surface chemistry and solution concentration on the conformation of adsorbed proteins using an improved circular dichroism method

In this paper we present the development of methods using circular dichroism spectropolarimetry with a custom-designed cuvette to increase the signal-to-noise ratio for the measurement of the secondary structure of adsorbed proteins, thus providing enhanced sensitivity and reproducibility. These methods were then applied to investigate how surface chemistry and solution concentration influence both the amount of adsorbed proteins and their secondary structure. Human fibrinogen and albumin were adsorbed onto alkanethiol self-assembled monolayers (SAMs) on gold with CH3, OCH2-CF3, NH2, COOH, and OH terminal groups from both dilute (0.1 mg/mL) and moderately concentrated (1.0 mg/mL) solutions. An increase in surface hydrophobicity was found to cause an increase in both the amount of the protein adsorbed and the degree of structural change that was caused by the adsorption process, while an increase in solution concentration caused an increase in the amount of protein adsorbed but a decrease in the degree of conformational change, with these effects being more pronounced on the more hydrophobic surfaces. The combined use of these two parameters (i.e., surface chemistry and solution concentration) thus provides ameans of independently varying the degree of structural change following adsorption from the amount of adsorbed protein. Further studies are underway to examine which of these factors most strongly influences platelet response, with the overall goal of developing a better understanding of the fundamental factors governing the hemocompatibility of biomaterial surfaces.