Force spectroscopy study of the adhesion of plasma proteins to the surface of a dialysis membrane: role of the nanoscale surface hydrophobicity and topography

An in vitro model that mimics the foreign body response in the peritoneum: Study of the bioadhesive properties of HA-based materials

A cascade of reactions known as the foreign body response (FBR) follows the implantation of biomaterials leading to the formation of a fibrotic capsule around the implant and subsequent health complications. The severity of the FBR is driven mostly by the physicochemical characteristics of implanted material, the method and place of implantation, and the degree of immune system activation. Here we present an in vitro model for assessing new materials with respect to their potential to induce a FBR in the peritoneum. The model is based on evaluating protein sorption and cell adhesion on the implanted material. We tested our model on the free-standing films prepared from hyaluronan derivatives with different hydrophobicity, swelling ratio, and rate of solubilization. The proteomic analysis of films incubated in the mouse peritoneum showed that the presence of fibrinogen was driving the cell adhesion. Neither the film surface hydrophobicity/hydrophilicity nor the quantity of adsorbed proteins were decisive for the induction of the long-term cell adhesion leading to the FBR, while the dissolution rate of the material proved to be a crucial factor. Our model thus helps determine the probability of a FBR to materials implanted in the peritoneum while limiting the need for in vivo animal testing.

Biocompatibility and osteoinductive ability of casein phosphopeptide modified polyetheretherketone

Polyetheretherketone (PEEK) is a potential implant material for dental application due to its excellent mechanical properties. However, its biological inertness and poor osteoinductive ability limited its clinical application. Based on a lay-by-layer self-assembly technique, here we incorporated casein phosphopeptide (CPP) onto PEEK surface by a simple two-step strategy to address the poor osteoinductive ability of PEEK implants. In this study, the PEEK specimens were positively charged by 3-ammoniumpropyltriethoxysilane (APTES) modification, then the CPP was adsorbed onto the positively charged PEEK surface electrostatically to obtain CPP-modified PEEK (PEEK-CPP) specimens. The surface characterization, layer degradation, biocompatibility and osteoinductive ability of the PEEK-CPP specimens were studied in vitro. After CPP modification, the PEEK-CPP specimens had a porous and hydrophilic surface and presented enhanced cell adhesion, proliferation, and osteogenic differentiation of MC3T3-E1 cells. These findings indicated that CPP modification could significantly improve the biocompatibility and osteoinductive ability of PEEK-CPP implants in vitro. In a word, CPP modification is a promising strategy for the PEEK implants to achieve osseointegration.

Dual-Functional Polyetheretherketone Surface with an Enhanced Osteogenic Capability and an Antibacterial Adhesion Property In Vitro by Chitosan Modification

A chitosan layer was covalently bonded to a polyetheretherketone (PEEK) surface using a simple facile self-assembly method to address inadequate biological activity and infection around the implant. The surface characterization, layer degradation, biological activity, and antibacterial adhesion properties of chitosan-modified PEEK (PEEK-CS) were studied. Through chitosan grafting, the surface morphology changed, the surface roughness increased, and the contact angle decreased significantly. PEEK-CS boosted cell adhesion, proliferation, increased alkaline phosphate activity, extracellular matrix mineralization, and expression of osteogenic genes. PEEK-CS demonstrated less adhesion to Porphyromonas gingivalis as well as less bacterial adhesion to P. gingivalis and Streptococcus mutans. According to our findings, chitosan modification significantly improved the osteogenic ability and antibacterial adhesion of PEEK in vitro.

Optimal conjugation of catechol group onto hyaluronic acid in coronary stent substrate coating for the prevention of restenosis

Although endovascular stenting has been used as an interventional therapy to treat cardio- and cerebro-vascular diseases, it is associated with recurrent vascular diseases following stent thrombosis and in-stent restenosis. In this study, a metallic stent was coated with dopamine-conjugated hyaluronic acid with different ratios of catechol group to improve hemocompatibility and re-endothelialization. Especially, we were interested in how much amount of catechol group is appropriate for the above-mentioned purposes. Therefore, a series of dopamine-conjugated hyaluronic acid conjugates with different ratios of catechol group were synthesized via a carbodiimide coupling reaction. Dopamine-conjugated hyaluronic acid conjugates were characterized with 1H-nuclear magnetic resonance and Fourier transform infrared spectroscopy, and the amount of catechol group in dopamine-conjugated hyaluronic acid was measured by ultraviolet spectrometer. Co-Cr substrates were polished and coated with various dopamine-conjugated hyaluronic acid conjugates under pH 8.5. Dopamine-conjugated hyaluronic acid amounts on the substrate were quantified by micro-bicinchoninic acid assay. Surface characteristics of dopamine-conjugated hyaluronic-acid-coated Co-Cr were evaluated by water contact angle, scanning electron microscopy, and atomic force microscopy. The hemocompatibility of the surface-modified substrates was assessed by protein adsorption and platelet adhesion tests. Adhesion and activation of platelets were confirmed with scanning electron microscopy and lactate dehydrogenase assay. Human umbilical vein endothelial cells were cultured on the substrates, and the viability, adhesion, and proliferation were investigated through cell counting kit-8 assay and fluorescent images. Obtained results demonstrated that optimal amounts of catechol group (100 µmol) in the dopamine-conjugated hyaluronic acid existed in terms of various properties such as hemocompatibility and cellular responses.

Quantifying fibronectin adhesion with nanoscale spatial resolution on glycosaminoglycan doped polypyrrole using Atomic Force Microscopy

BACKGROUND

The interaction of ECM proteins is critical in determining the performance of materials used in biomedical applications such as tissue regeneration, implantable bionics and biosensing.

METHODS

To improve our understanding of ECM protein-conducting polymer interactions, we have used Atomic Force Microscopy (AFM) to elucidate the interactions of fibronectin (FN) on polypyrrole (PPy) doped with different glycosaminoglycans.

RESULTS

We were able to classify four main types of FN interactions, including those related to 1) non-specific adhesion, 2) protein unfolding and subsequent unbinding from the surface, 3) desorption and 4) interactions with no adhesion. FN adhesion on PPy/hyaluronic acid showed a significantly lower density of surface adhesion with the adhesion restricted to nodule structures, as opposed to their peripheries, of the polymer morphology. In contrast, PPy/chondroitin sulfate showed a significantly higher density of surface adhesion to the point where the distribution of adhesion effectively masked the topography. Through conductive AFM imaging, we found that the conductive regions correlated with regions of FN adhesion.

CONCLUSIONS

Given that the conductivity requires doping of the polymer, these findings suggest that FN adhesion is mediated by interactions with chondroitin sulfate and hyaluronic acid at the polymer surface and may be indicative of specific interactions due to contributions from electrostatic attraction between the FN and sulfate/anionic groups of the dopants.

GENERAL SIGNIFICANCE

This study demonstrates the ability of AFM to resolve the protein-conducting polymer interactions at the molecular and nanoscale level, which will be important for interfacing these polymer materials with biological systems. This article is part of a Special Issue entitled Organic Bioelectronics - Novel Applications in Biomedicine.

Multiscale morphology of organic semiconductor thin films controls the adhesion and viability of human neural cells

We investigate how multiscale morphology of functional thin films affects the in vitro behavior of human neural astrocytoma 1321N1 cells. Pentacene thin film morphology is precisely controlled by means of the film thickness, Theta (here expressed in monolayers (ML)). Fluorescence and atomic force microscopy allow us to correlate the shape, adhesion, and proliferation of cells to the morphological properties of pentacene films controlled by saturated roughness, sigma, correlation length, xi, and fractal dimension, d(f). At early incubation times, cell adhesion exhibits a transition from higher to lower values at Theta approximately 10 ML. This is explained using a model of conformal adhesion of the cell membrane onto the growing pentacene islands. From the model fitting of the data, we show that the cell explores the surface with a deformation of the membrane whose minimum curvature radius is 90 (+/- 45) nm. The transition in the adhesion at approximately 10 ML arises from the saturation of xi accompanied by the monotonic increase of sigma, which leads to a progressive decrease of the pentacene local radius of curvature and hence to the surface area accessible to the cell. Cell proliferation is also enhanced for Theta < 10 ML, and the optimum morphology parameter ranges for cell deployment and growth are sigma <or= 6 nm, xi > 500 nm, and d(f) > 2.45. The characteristic time of cell proliferation is tau approximately 10 +/- 2 h.

Atomic force microscopy and surface plasmon resonance investigation of fibronectin interactions with group B streptococci

The interactions of fibronectin (Fn) with group B streptococci (GBS) were investigated using the atomic force microscope (AFM) and surface plasmon resonance (SPR) biosensing. Submonolayer amounts of Fn were immobilized onto the AFM tip by two different methods, using either a sulfosuccinimidyl-4-(N-maleimidomethyl) cycholhexane-1-carboxylate (SMCC) linker or a pyridyldithio poly(ethylene glycol) succinimidylpropionate (NHS-PEG-PDP) linker. Each step of both immobilization methods was characterized using x-ray photoelectron spectroscopy. Time-of-flight secondary ion mass spectrometry experiments indicated both methods produced Fn immobilized in a similar conformation. AFM force-distance curves from live GBS plated onto polystyrene exhibited several types of interactions between the Fn functionalized AFM tip and the surface of capsule-deficient GBS (no interactions, interactions with the cell wall, Fn unfolding, large specific unbinding events, and small specific unbinding events). From analysis of the force-distance curves that exhibited only a single specific unbinding event, the work of adhesion and rupture force for the SMCC immobilized Fn tips (11,131 pN nm and 213 pN) were larger than the corresponding values for the NHS-PEG-PDP immobilized Fn tips (8115 pN nm and 189 pN). The unbinding event occurred at distances approximately 100 nm further from the surface with the NHS-PEG-PDP immobilized Fn tip compared to SMCC immobilized Fn tip. The SPR experiments of soluble Fn with adsorbed serine protease C5a peptidase (Scp), the surface protein on GBS that binds Fn, showed that both low (millimolar) and high binding (nanomolar) affinity interactions were present. However, the low binding affinity interactions dominated the adsorption process and, with increasing Fn solution concentration, the amount of Scp bound to Fn via the high binding affinity interaction decreased. These data confirm that Scp binds only to adsorbed Fn at the Fn concentrations typically present in blood plasma.

Polymer adhesion at the solid-liquid interface probed by a single-molecule force sensor

A method based on atomic force microscopy is used to delineate the properties that determine single-molecule adhesion onto solid substrates in aqueous environment. Hydrophobicity as well as electrical properties of the substrate and the polymer are varied. In addition, the influence of the solvent composition, in particular the effect of ions, on the molecular adhesion at the solid-liquid interface is studied. Surprisingly, the polymer and surface-related properties account for only small changes in adhesion force, while dissolved ions show a much larger effect. These results point towards the energy of solvation as the most important contribution to adhesion for a wide variety of polymers and substrate materials.

AFM of biological complexes: what can we learn?

The term "biological complexes" broadly encompasses particles as diverse as multisubunit enzymes, viral capsids, transport cages, molecular nets, ribosomes, nucleosomes, biological membrane components and amyloids. The complexes represent a broad range of stability and composition. Atomic force microscopy offers a wealth of structural and functional data about such assemblies. For this review, we choose to comment on the significance of AFM to study various aspects of biology of selected nonmembrane protein assemblies. Such particles are large enough to reveal many structural details under the AFM probe. Importantly, the specific advantages of the method allow for gathering dynamic information about their formation, stability or allosteric structural changes critical for their function. Some of them have already found their way to nanomedical or nanotechnological applications. Here we present examples of studies where the AFM provided pioneering information about the biology of complexes, and examples of studies where the simplicity of the method is used toward the development of potential diagnostic applications.

Interactions of the streptococcal C5a peptidase with human fibronectin

Group B Streptococci (GBS) is a leading cause of sepsis and meningitis in neonates and immunocompromised adults in western countries. GBS do not bind to fibronectin (Fn) in solution, but will bind to Fn adsorbed onto a solid surface. The reason for the specificity of this binding is unknown. Single molecule force spectroscopy was used to test the hypothesis that GBS, through streptococcal C5a peptidase (ScpB) molecules present on the surface of the bacteria, binds to a motif created by the juxtaposition of multiple adjacent Fn molecules. Atomic force microscopy (AFM) topographical images of adsorbed Fn deposited from various Fn coating concentrations were used to determine the Fn surface concentration. ScpB was tethered to an AFM tip with all surface modifications characterized by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. At the lowest Fn coverages the probability of observing a ScpB-Fn binding event increased linearly with Fn surface coverage. As an Fn monolayer was reached the probability of a ScpB-Fn binding event occurring increased markedly ( approximately 50 fold), with a concomitant increase in the rupture force from 17 pN to 33 pN. These results are consistent with the hypothesis that ScpB binds to a motif created by the juxtaposition of multiple Fn molecules.

Structure and reactivity of adsorbed fibronectin films on mica

Understanding the interactions of adsorbed fibronectin (Fn) with other biomolecules is important for many biomedical applications. Fn is found in almost all body fluids, in the extracellular matrix, and plays a fundamental role in many biological processes. This study found that the structure (conformation, orientation) and reactivity of Fn adsorbed onto mica is dependent on the Fn surface concentration. Atomic force microscopy and x-ray photoelectron spectroscopy were used to determine the surface coverage of adsorbed Fn from isolated molecules at low surface coverage to full monolayers at high surface coverage. Both methods showed that the thickness of Fn film continued to increase after the mica surface was completely covered, consistent with Fn adsorbed in a more upright conformation at the highest surface-Fn concentrations. Time-of-flight secondary ion mass spectrometry showed that relative intensities of both sulfur-containing (cystine, methionine) and hydrophobic (glycine, leucine/isoleucine) amino acids varied with changing Fn surface coverage, indicating that the conformation of adsorbed Fn depended on surface coverage. Single-molecule force spectroscopy with collagen-related peptides immobilized onto the atomic force microscope tip showed that the specific interaction force between the peptide and Fn increases with increasing Fn surface coverage.

The effect of silica nanoparticulate coatings on serum protein adsorption and cellular response

Serum protein adsorption on colloidal silica surfaces was investigated using a quartz crystal microbalance with dissipation (QCM-D) monitoring. The amount of serum proteins adsorbed on colloidal silica-coated surfaces was not significantly different from the control silica surfaces, with the exception of 21nm colloidal silica which experienced significantly less (P<0.05) fibrinogen adsorption compared with control silica. The adhesion and proliferation of human endothelial cells (C11STH) on nano-scale colloidal silica surfaces were significantly reduced compared with control silica surfaces, suggesting that the conformation of adsorbed proteins on the colloidal silica surfaces plays a role in modulating the amount of cell binding. Fibronectin is one of the main extracellular matrix proteins involved in endothelial cell attachment to biomaterial surfaces. There was reduced binding of a monoclonal anti-fibronectin antibody, that reacted specifically with the cell-binding fragment, to fibronectin-coated colloidal silica surfaces compared with control silica surfaces. This suggests that the fibronectin adsorbed on the colloidal silica-coated surfaces was conformationally changed compared with control silica reducing the availability of the cell-binding domain of fibronectin.

Force microscopy studies of fibronectin adsorption and subsequent cellular adhesion to substrates with well-defined surface chemistries

Molecular force spectroscopy was used to study the mechanical behavior of plasma fibronectin (FN) on mica, gold, poly(ethylene glycol), and -CH(3), -OH, and -COOH terminated alkanethiol self-assembled monolayers. Proteins were examined at two concentrations, one resulting in a saturated surface with multiple intermolecular interactions referred to as the aggregate state and another resulting in a semiaggregate state where the proteins were neither completely isolated nor completely aggregated. Modeling of the force-extension data using two different theories resulted in similar trends for the fitted thermodynamic parameters from which insight into the protein's binding state could be obtained. Aggregated proteins adsorbed on hydrophobic surfaces adopted more rigid conformations apparently as a result of increased surface denaturation and tighter binding while looser conformations were observed on more hydrophilic surfaces. Studies of FN in a semiaggregate state showed heterogeneity in the model's thermodynamic parameters suggesting that, in the early stages of nonspecific adsorption, multiple protein conformations exist, each having bound irreversibly to the substrate. Proteins in this state all demonstrated a more rigid conformation than in the corresponding aggregate studies due to the greater number of substrate contacts available to the protein. Finally, the force spectroscopy experiments were examined for any biocompatibility correlation by seeding substrates with human umbilical vascular endothelial cells. As predicted from the models used in this work, surfaces with aggregated FN promoted cellular deposition while surfaces with FN in a semiaggregate state appeared to hinder cellular deposition and growth. The atomic force microscope's use as a means for projecting surface biocompatibility, although requiring additional testing, does look promising.

Force spectroscopy of the double-tethered concanavalin-A mannose bond

We present the measurement of the force required to rupture a single protein-sugar bond using a methodology that provides selective discrimination between specific and nonspecific binding events and helps verify the presence of a single functional molecule on the atomic force microscopy tip. In particular, the interaction force between a polymer-tethered concanavalin-A protein (ConA) and a similarly tethered mannose carbohydrate was measured as 47 +/- 9 pN at a bond loading rate of approximately 10 nN/s. Computer simulations of the polymer molecular configurations were used to determine the angles that the polymers could sweep out during binding and, in conjunction with mass spectrometry, used to separate the angular effects from the effects due to a distribution of tether lengths. We find that when using commercially available polymer tethers that vary in length from 19 to 29 nm, the angular effects are relatively small and the rupture distributions are dominated by the 10-nm width of the tether length distribution. In all, we show that tethering both a protein and its ligand allows for the determination of the single-molecule bond rupture force with high sensitivity and includes some validation for the presence of a single-tethered functional molecule on the atomic force microscopy tip.

High-throughput investigation of osteoblast response to polymer crystallinity: influence of nanometer-scale roughness on proliferation

A high-throughput method for analyzing cellular response to crystallinity in a polymer material is presented. Variations in crystallinity lead to changes in surface roughness on nanometer length scales, and it is shown that cells are exquisitely sensitive to these changes. Gradients of polymer crystallinity were fabricated on films of poly(L-lactic acid) using a gradient in annealing temperature. The resultant morphologies were characterized using an atomic force microscope. Root-mean-square (rms) roughness values ranging from 0.5 to 13 nm were created on a single sample. MC3T3-E1 osteoblastic cells were cultured for 1, 3 and 5 d, and the number of cells was measured using automated fluorescence microscopy. It is shown that the rate of proliferation on the smooth regions of the films is much greater than that on the rough regions, and a monotonic variation in rate is observed as a function of roughness. The critical rms roughness, above which a statistically significant reduction in rate of proliferation occurs, was approximately 1.1 nm. Fluorescence microscopy measurements on immunostained cells indicate there is no significant change in cell area, the number or type of adhesions formed, or the degree of actin polymerization. Results from enzyme-linked immunofluorescence assays indicated that there was no detectable change in adhesion protein accessibility, suggesting the cells directly respond to substrate topography. The use of the gradient library approach yielded the functional dependence of cell proliferation on nanometer-scale roughness and gave a sensitive estimate of the critical roughness for which a decrease in proliferation is observed.

Structural changes of fibronectin adsorbed to model surfaces probed by fluorescence resonance energy transfer

Structural changes of proteins during adsorption to biomaterials affect the presentation of molecular binding sites and, ultimately, biomaterial performance. We have applied fluorescence resonance energy transfer (FRET) spectroscopy to study structural changes of the cell adhesion protein, fibronectin (Fn), following adsorption to model hydrophilic and hydrophobic surfaces. Fn was labeled with donor and acceptor fluorophores using two labeling schemes and intramolecular energy transfer was calibrated against measured structural changes of Fn in denaturing solutions. FRET was then applied to measure Fn's structure on surfaces. Based on FRET, Fn underwent greater extension of its dimer arms on hydrophilic glass than on hydrophobic fluoroalklysilane-derivatized glass (fluorosilane), and this extension was insensitive to molecular packing over a range of adsorption concentrations. Fn's conformation on glass better promoted cell attachment than on fluorosilane; the roles of both global structural changes (movements of modules) and local structural changes (disruption of secondary structure) on Fn's cell integrin binding activity are discussed. Based on previous FRET work, we compare Fn's conformations on these surfaces with its conformations in fibroblast culture. FRET is unique in allowing direct comparison of protein structure between biomaterial surfaces and cell culture.

Polypropylene hollow fiber for cells isolation: methods for evaluation of diffusive transport and quality of cells encapsulation

Formulation of membrane properties is important prior the successful implantation of encapsulated cells producing therapeutically relevant compounds. The purpose of our study was to specify the methods allowing preliminary evaluation of hollow fibers (HF) chosen for immunoisolation. We have selected as estimates (1) diffusive permeability for small and large solutes, and HF cut off (in vitro), (2) histological evaluation of tissue overgrowth after sc. implantation into mice. It was found that diffusive coefficients were linearly dependent on the particle diameter except that of albumin (2-3 times higher than theoretically estimated). This discrepancy imply that for certain particles the interaction with membrane material may be significant. The histological evaluation showed that siliconized HF implanted for 105 days were accepted (there was thin fibrotic layer on the external surface of the HF, no surrounding haemopoietic cells were found). It is concluded that proposed methods for preliminary evaluation of hollow fibers chosen for immunoisolation seems to be reliable and suitable for testing diffusive permeability of each relevant cell product.

Interaction between fibroblast cells and fluorinated polyimide with nano-modified surface

In this study, we investigated the effect of surface nano-modification of aromatic fluorinated polyimide (6FDA-6FAP) derived from 2,2'-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 2,2'-bis (4-aminophenyl) hexafluoropropane (6FAP) on the interaction with proteins and cells. The surface of 6FDA-6FAP was modified by surface-rubbing showed nano-ordered stripes along the rubbing direction. The rat fibroblast FR cells formed multicellular spheroids with high cell density on the rubbed surface then expressed excellent collagen production similarly in vivo, while on the non-modified surface the cells formed two-dimensional monolayers and the collagen production was negligible. The modulation of cell function by the surface nano-modification along with surface micro-modification may be one of the most important considerations during the design and manufacture of novel biochips or tissue engineering materials.