Platelet interactions with plasma-polymerized ethylene oxide and N-vinyl-2-pyrrolidone films and linear poly(ethylene oxide) layer

The influence of different rewetting procedures on the thrombogenicity of nanoporous poly(ether imide) microparticles

Nanoporous microparticles prepared from poly(ether imide) (PEI) are discussed as candidate adsorber materials for the removal of uremic toxins during apheresis. Polymers exhibiting such porosity can induce the formation of micro-gas/air pockets when exposed to fluids. Such air presenting material surfaces are reported to induce platelet activation and thrombus formation. Physical or chemical treatments prior to implantation are discussed to reduce the formation of such gas nuclei. Here, we report about the influence of different rewetting procedures - as chemical treatments with solvents - on the thrombogenicity of hydrophobic PEI microparticles and PEI microparticles hydrophilized by covalent attachment of poly(vinyl pyrrolidone) (PVP) of two different chain lengths.Autoclaved dry PEI particles of all types with a diameter range of 200 - 250 μm and a porosity of about 84% ±2% were either rewetted directly with phosphate buffered saline (24 h) or after immersion in an ethanol-series. Thrombogenicity of the particles was studied in vitro upon contact with human sodium citrated whole blood for 60 min at 5 rpm vertical rotation. Numbers of non-adherent platelets were quantified, and adhesion of blood cells was qualitatively analyzed by bright field microscopy. Platelet activation (percentage of CD62P positive platelets and amounts of soluble P-Selectin) and platelet function (PFA100 closure times) were analysed.Retention of blood platelets on the particles was similar for all particle types and both rewetting procedures. Non-adherent platelets were less activated after contact with ethanol-treated particles of all types compared to those rewetted with phosphate buffered saline as assessed by a reduced number of CD62P-positive platelets and reduced amounts of secreted P-Selectin (P < 0.05 each). Interestingly, the hydrophilic surfaces significantly increased the number of activated platelets compared to hydrophobic PEI regardless of the rewetting agent. This suggests that, apart from wettability, other material properties might be more important to regulate platelet activation. PFA100 closure times were reduced and within the reference ranges in the ethanol group, however, significantly increased in the saline group. No substantial difference was detected between the tested surface modifications. In summary, rewetting with ethanol resulted in a reduced thrombogenicity of all studied microparticles regardless of their wettability, most likely resulting from the evacuation of air from the nanoporous particles.

Poly(ethylene glycol) interfaces: an approach for enhanced performance of microfluidic systems

Microfluidic systems are extensively used platform for analytical and therapeutic applications. One of the major problems encountered in these systems is the loss of material due to non-specific surface interactions. When biological solutions are flowed through microchannels, they tend to adsorb on the surface due to the negative charge of the surface. This results in a reduced efficiency of the system which can be critical in sensitive biological analysis. Poly(ethylene glycol) (PEG) is known to form non-fouling interfaces on silicon and glass which are common materials used in microfluidic systems. The most common approach for modifying silicon/glass with PEG involves a solution phase protocol. Since the micro/nanofluidic systems have channel sizes ranging from hundreds of microns to submicron with variety of complicated network, this surface modification approach is not sufficient in forming uniform, conformal, and ultrathin films on the surface. Due to the enclosed features in these systems, the properties of liquids such as viscosity and surface tension play an important role in the clogging and eventually biofouling of these microchannels. Hence, we have developed a solvent-free vapor deposition protocol for modifying silicon/glass surfaces with PEG. Various concentrations of protein solutions were flowed through unmodified and PEG-modified glass microcapillaries of different lengths at different flow rates. PEG surfaces formed on silicon have shown 80% reduction in protein adsorption in static conditions.

Surface modification of nanoporous alumina surfaces with poly(ethylene glycol)

Nanoporous alumina surfaces have a variety of applications in biosensors, biofiltration, and targeted drug delivery. However, the fabrication route to create these nanopores in alumina results in surface defects in the crystal lattice. This results in inherent charge on the porous surface causing biofouling, that is, nonspecific adsorption of biomolecules. Poly(ethylene glycol) (PEG) is known to form biocompatible nonfouling films on silicon surfaces. However, its application to alumina surfaces is very limited and has not been well investigated. In this study, we have covalently attached PEG to nanoporous alumina surfaces to improve their nonfouling properties. A PEG-silane coupling technique was used to modify the surface. Different concentrations of PEG for different immobilization times were used to form PEG films of various grafting densities. X-ray photoelectron spectroscopy (XPS) was used to verify the presence of PEG moieties on the alumina surface. High-resolution C1s spectra show that with an increase in concentration and immobilization time, the grafting density of PEG also increases. Further, a standard overlayer model was used to calculate the thickness of PEG films formed using the XPS intensities of the Al2p peaks. The films formed by this technique are less than 2.5 nm thick, suggesting that such films will not clog the pores which are in the range of 70-80 nm.

XPS and AFM analysis of antifouling PEG interfaces for microfabricated silicon biosensors

In the past two decades, the biological and medical fields have seen great advances in the development of biosensors capable of quantifying biomolecules. Many of these biosensors have micro- and nano-scale features, are fabricated using biochip technology, and use silicon as a base material. The creation of antifouling sensor interfaces is critical to avoid serious consequences that arise due to their contact with biological fluids. To this end, we have created thin PEG interfaces of various grafting densities on silicon using a single-step PEG-silane coupling reaction scheme. Initial PEG concentration (5-50 mM) and coupling time (0.5-24 h) were varied to attain different grafting densities, and different PEG interfaces so created were analyzed using XPS and AFM. Furthermore, all the PEG interfaces were evaluated using XPS and AFM for their antifouling abilities using fibrinogen as the model protein. Results indicated that PEG interfaces created in this investigation are appropriate for biosensors with micro- and nano-scale features, and are efficient in controlling protein fouling.

Evaluation of the stability of nonfouling ultrathin poly(ethylene glycol) films for silicon-based microdevices

The creation of nonfouling surfaces is one of the major prerequisites for microdevices for biomedical and analytical applications. Poly(ethylene glycol) (PEG), a water soluble, nontoxic, and nonimmunogenic polymer has the unique ability of reducing nonspecific protein adsorption and cell adhesion and, therefore, is generally coupled with a wide variety of surfaces to improve their biocompatibility. The performance of these modified surfaces for long-term biomedical applications largely depends on the stability of these PEG films. To this end, we have investigated the stability of covalently coupled ultrathin PEG films on silicon in aqueous in vivo like conditions for a period of 4 weeks. The PEG-modified silicon substrates were incubated in PBS (37 degrees C, pH 7.4, 5% CO2) for different periods of time and then characterized using the techniques of ellipsometry, contact angle measurement, X-ray photoelectron spectroscopy, and atomic force microscopy. The ability of the PEG-modified surfaces to control protein fouling was examined by protein adsorption studies using fluorescein isothiocyanate labeled bovine serum albumin and ellipsometry. Furthermore, the ability of these films to control fibroblast adhesion was examined. Studies suggest that the PEG-modified surfaces retain their protein and cell repulsive nature even though the PEG film thickness decreases for the period of investigation.

Shear flow-induced detachment kinetics of Dictyostelium discoideum cells from solid substrate

Using Dictyostelium discoideum as a model organism of specific and nonspecific adhesion, we studied the kinetics of shear flow-induced cell detachment. For a given cell, detachment occurs for values of the applied hydrodynamic stress above a threshold. Cells are removed from the substrate with an apparent first-order rate constant that strongly depends on the applied stress. The threshold stress depends on cell size and physicochemical properties of the substrate, but is not affected by depolymerization of the actin and tubulin cytoskeleton. In contrast, the kinetics of cell detachment is almost independent of cell size, but is strongly affected by a modification of the substrate and the presence of an intact actin cytoskeleton. These results are interpreted in the framework of a peeling model. The threshold stress and the cell-detachment rate measure the local equilibrium energy and the dissociation rate constant of the adhesion bridges, respectively.

Shear-induced platelet activation and adhesion on human pulmonary artery endothelial cells seeded onto hydrophilic polymers

We evaluated platelet activation and adhesion on two plasma polymerized surfaces, N-vinyl pyrrolidone (NVP) and gamma-butyro lactone (GBL), which have been shown previously to promote endothelial cell growth and adhesion as well as fibronectin-coated glass (1 microg/cm(2)) coverslips. Human pulmonary artery endothelial cells were seeded onto coverslips at a low density ( approximately 20,000 cells/cm(2)) and grown to confluence (3-5 days). The materials, both with and without ECs, were then exposed to a shear rate of 400 s(-1) in a closed loop recirculating flow system containing human platelet-rich plasma. Plasma samples were taken at 0, 5, 15, 30, and 60 min and analyzed for platelet and coagulation activation. The coverslips were examined for EC coverage and platelet adherence. EC retention over a 1-h period was approximately 75% for all three materials. All three materials without ECs were highly platelet activating having similar P-selectin expression, platelet factor 4 (PF4) release, mepacrine uptake, and microparticle production. Both microparticle production and platelet adhesion were significantly lower in EC-seeded materials. Dense granule and PF4 release were both slightly diminished in all three materials seeded with ECs. P-selectin expression was reduced slightly for GBL, but remained the same for the other two materials. The EC-seeded materials displayed favorable characteristics with respect to platelet activation and adhesion; however, they still demonstrated some thrombogenic tendencies due to EC loss and exposure of the underlying substrate. Therefore, both EC coverage and EC hemostatic function are important factors in determining the thromboresistance of an EC-seeded surface.

In vitro and in vivo studies of PEO-grafted blood-contacting cardiovascular prostheses

The initial step of thrombus formation on blood-contacting biomaterials is known to be adsorption of blood proteins followed by platelet adhesion. Poly(ethylene oxide) (PEO) has been frequently used to modify biomaterial surfaces to minimize or prevent protein adsorption and cell adhesion. PEO was grafted onto a number of biomaterials in our laboratory. Nitinol stents and glass tubes were grafted with PEO by priming the metal surface with trichlorovinylsilane (TCVS) followed by adsorption of Pluronic and y-irradiation. Nitinol stents were also coated with Carbothane for PEO grafting. Chemically inert polymeric biomaterials, such as Carbothane, polyethylene, silicone rubber, and expanded polytetrafluoroethylene (e-PTFE), were first adsorbed with PEO-polybutadiene-PEO (PEO-PB-PEO) triblock copolymers and then exposed to gamma-irradiation for covalent grafting. For PEO grafting to Dacron (polyethylene terephthalate), the surface was sequentially treated with PEO-PB-PEO and Pluronics followed by gamma-irradiation. In vitro studies showed substantial reduction in fibrinogen adsorption and platelet adhesion to the PEO-grafted surfaces compared with control surfaces. Fibrinogen adsorption was reduced by 70-95% by PEO grafting on all surfaces, except for e-PTFE. The platelet adhesion corresponded to the fibrinogen adsorption. When the PEO-grafted surfaces were tested ex vivo/in vivo, however, the expected beneficial effect of PEO grafting was inconsistent. The beneficial effect of the PEO grafting was most pronounced on the PEO-grafted nitinol stents. Thrombus formation was reduced by more than 85% by PEO grafting on metallic stents. Only moderate improvement (i.e. 35% decrease in platelet deposition) was observed with PEO-grafted tubes of polyethylene, silicone rubber, and glass. For PEO-grafted heart valves made of Dacron, however, no effect of PEO grafting was observed at all. It appears that the extent of thrombus formation on PEO-grafted biomaterials was directly related to the extent of tissue damage during implantation surgery. Platelets can be activated and form aggregates in the bulk blood, and the formed platelet aggregates may be able to deposit on the PEO monolayer overcoming its repulsive property. Our studies indicate that the testing of in vitro platelet adhesion should include adhesion of large platelet aggregates, in addition to adhesion of individual platelets. Furthermore, the surface modification methods should be improved over the current monolayer grafting concept so that the repulsive force by the grafted PEO layers is large enough to prevent adhesion of platelet aggregates formed in the bulk blood before arriving at the biomaterial surface.

Effects of surface-coupled polyethylene oxide on human macrophage adhesion and foreign body giant cell formation in vitro

Surface immobilized polyethylene oxide (PEO) has been shown to efficiently reduce protein adsorption and cellular adhesion, resulting in a biologically passive surface. To explore the in vitro effects of surface immobilized PEO on the human inflammatory cells, macrophages, and foreign body giant cells (FBGCs), we developed a diisocyanate-based method for coupling PEO to amine-modified glass, a surface previously shown to enhance macrophage adhesion and FBGC formation. Contact angle analysis and X-ray photoelectron spectroscopy confirmed the presence of PEO molecules bound to the surface and revealed that PEO molecular weight significantly influenced the efficiency of PEO coupling. We used a 10-day human monocyte culture protocol to demonstrate that the presence of surface coupled PEO molecules does not significantly decrease initial monocyte density or monocyte-derived macrophage density after 3 days. However, PEO-coupled surfaces significantly reduced long-term monocyte-derived macrophage density and virtually eliminated interleukin-4-induced FBGC formation observed at day 10. The cellular response to these PEO-coupled surfaces was related to the molecular weight of the PEO chains, which was varied between 200 Da and 18.5 kDa. These results suggest that an optimized PEO surface treatment may be effective in reducing inflammatory cell adhesion and possible degradation during the inflammatory response to an implanted biomedical device.

A novel method for surface modification to promote cell attachment to hydrophobic substrates

The ability to study and regulate cell behavior at a biomaterial interface requires strict control over material surface chemistry. Perhaps the greatest challenge to researchers working in this area is preventing the fouling of a given surface due to uncontrolled protein adsorption. This work describes a method for coupling peptides to hydrophobic materials for the purpose of simultaneously preventing nonspecific protein adsorption and controlling cell adhesion. A hexapeptide containing the ubiquitous RGD cell-adhesion motif was coupled to polystyrene (PS) via a polyethylene oxide (PEO) tether in the form of a modified PEO/PPO/PEO triblock copolymer. Triblocks were adsorbed onto PS at a density of 3.3 +/- (5.14 x 10(-4)) mg/m2 (1.4 x 10(5) +/- 2.12 x 10(1) molecules/microm2), which was determined by isotope 125I labeling. The peptide, GRGDSY, was activated at the N terminus with N-Succinimidyl 3-(2-pyridyldithio) propionate and coupled to immobilized triblocks where the terminal hydroxyls had been converted to sulfhydryl groups. Surface peptide density was measured by amino acid analysis and found to be 1.4 x 10(4) +/- 0.47 x 10(4) molecules/microm2. PS modified with PEO/PPO/PEO copolymers alone was found to be inert to cell adhesion both in the presence of serum proteins and when exposed to activated RGD peptide. In contrast, PS conjugated with RGD via endgroup-activated PEO/PPO/PEO copolymers supported cell adhesion and spreading. The surface coupling scheme reported here should prove valuable for studying cell-ligand interactions under simplified and highly controlled conditions.