A hydrophilic plasma polymerized film composite with potential application as an interface for biomaterials

Vapor-based grafting of crosslinked poly(N-vinyl pyrrolidone) coatings with tuned hydrophilicity and anti-biofouling properties

Vapor-based one-step synthesis and grafting of poly(N-vinyl pyrrolidone) enable potent and durable anti-biofouling coatings with tailored structures.

Thermally cross-linked PNVP films as antifouling coatings for biomedical applications

Protein repellent coatings are widely applied to biomedical devices in order to reduce the nonspecific adhesion of plasma proteins, which can lead to failure of the device. Poly(N-vinylpyrrolidone) (PNVP) is a neutral, hydrophilic polymer with outstanding antifouling properties often used in these applications. In this paper, we characterize for the first time a cross-linking mechanism that spontaneously occurs in PNVP films upon thermal annealing. The degree of cross-linking of PNVP films and their solubility in water can be tailored by controlling the annealing, with no need for additional chemical treatment or irradiation. The physicochemical properties of the cross-linked films were investigated by X-ray photoelectron spectroscopy, infrared spectroscopy, neutron and X-ray reflectometry, ellipsometry, and atomic force microscopy, and a mechanism for the thermally induced cross-linking based on radical formation was proposed. The treated films are insoluble in water and robust upon immersion in harsh acid environment, and maintain the excellent protein-repellent properties of unmodified PNVP, as demonstrated by testing fibrinogen and immunoglobulin G adsorption with a quartz crystal microbalance. Thermal cross-linking of PNVP films could be exploited in a wide range of biotechnological applications to give antifouling properties to objects of any size, essentially making this an alternative to high-tech surface modification techniques.

Chemical vapor deposition of conformal, functional, and responsive polymer films

Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric films directly on the surface of a substrate. CVD polymers are desirable as conformal surface modification layers exhibiting strong retention of organic functional groups, and, in some cases, are responsive to external stimuli. Traditional wet-chemical chain- and step-growth mechanisms guide the development of new heterogeneous CVD polymerization techniques. Commonality with inorganic CVD methods facilitates the fabrication of hybrid devices. CVD polymers bridge microfabrication technology with chemical, biological, and nanoparticle systems and assembly. Robust interfaces can be achieved through covalent grafting enabling high-resolution (60 nm) patterning, even on flexible substrates. Utilizing only low-energy input to drive selective chemistry, modest vacuum, and room-temperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductive polymers, and controllably crosslinked networks and for the potential to reduce environmental, health, and safety impacts associated with solvents. Quantitative models aid the development of large-area and roll-to-roll CVD polymer reactors. Relevant background, fundamental principles, and selected applications are reviewed.

Macrophage Serum-Based Adhesion to Plasma-Processed Surface Chemistry is Distinct from That Exhibited by Fibroblasts

Plasma-polymerized films deposited from AlAm, HxAm, NVP, NVFA, AA and FC were compared to TCPS and PS surfaces in supporting cellular attachment, viability, and proliferation in serum-based culture in vitro for extended periods of time (>7 d). Surface patterns were created using multi-step depositions with physical masks. Cell adhesion in the presence of serum was compared for (monocyte-) macrophage and fibroblast cell lines. Cellular response was tracked over time, reporting adhesive behavior, proliferative rates, and morphological changes as a function of surface chemistry. Micropatterned surfaces containing different surface chemistries and functional groups (e.g. -NH(2), -COOH, -CF(3)) produced differential cell adhesive patterns for NIH 3T3 fibroblasts compared to J774A.1, RAW 264.7 or IC-21 (monocyte-) macrophage cell types. Significantly, macrophage adhesion is substantial on surfaces where fibroblasts do not adhere under identical culture conditions.

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.

Organic surface chemistry on titanium surfaces via thin film deposition

In order to develop a synthetic strategy for the fine tuning of the interfacial properties of titanium-based implants and implant parts, a thin polymeric film was deposited from ethylene plasma on the surfaces of Ti foils. The intended aim was to further modify the adherent, delamination-resistant organic coating using the techniques of surface modification of polymers to direct interfacial interactions at the metal foil-biological phase interface. In particular, air-plasma treatment and Ce(IV)-induced hydroxyethylmethacrylate grafting, two typical reactions of biomedical polymers surface chemistry, were used to improve cell adhesion or to impart cell resistance to the plasma-coated Ti. Results indicate that a plasma-deposited thin polymeric film effectively can act as a viable substrate for further surface chemical modifications and allow the application of a huge background of surface-modification polymers to metallic devices.

Human endothelial cell growth and coagulant function varies with respect to interfacial properties of polymeric substrates

The in vitro coagulant function of human aortic endothelial cells (HAECs) was investigated when grown on a series of polymer surfaces that ranged from hydrophobic to hydrophilic. The polymer interface materials were prepared by radiofrequency plasma polymerization from hexamethyl-disilazane, gamma-butyrolactone, and N-vinyl-2-pyrrolidone and deposited onto tissue culture Permanox. The three plasma polymers were noncytotoxic. When precoated with fibronectin (FN), HAECs on all four polymer surfaces were similar with respect to cell proliferation and coagulant function. Without FN precoating, cell proliferation and spreading increased with increasing surface hydrophilicity. Normalized production of tissue-type plasminogen activator increased with increasing hydrophilicity of the polymers during early incubation times, as did tissue plasminogen activator/plasminogen activator inhibitor-1 ratios. In comparison, normalized von Willebrand factor release decreased on the more hydrophilic surfaces. Thus, both endothelial cell growth and some coagulant/fibrinolytic functions are improved with increasing substrate hydrophilicity.

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

Dimethyldichlorosilane (DDS)-treated glass (DDS-glass) was modified with either poly(ethylene oxide) (PEO) films or poly(N-vinyl-2-pyrrolidone) (PNVP) films by plasma polymerization. The thickness of the plasma polymerized films was varied between 40 and 700 nm. The results showed that the hydrophilic plasma polymerized PEO and PNVP films on DDS-glass did not prevent platelet adhesion and activation. The film thickness had only marginal influence on the prevention of platelet activation. In contrast, platelet adhesion was prevented on DDS-glass absorbed with a PEO-containing block copolymer (Pluronic F-108 surfactant) even at a calculated thickness of the PEO layer of less than 40 nm. This study shows that surface hydrophilization is not sufficient for prevention of platelet adhesion and activation. The contrasting results in platelet adhesion between cross-linked plasma polymers and linear PEO-containing block copolymers may be explained qualitatively by a steric repulsion mechanism that is achieved by the conformational freedom of the linear PEO chains interacting with water.

Recruitment of tissue resident cells to hydrogel composites: in vivo response to implant materials

A model of local cellular recruitment was established using hydrogel matrices composed of alginate implanted subcutaneously into mice. Cells which trafficked to the matrix blocks were recovered and characterized for surface phenotype using fluorescently labelled antibodies and flow cytometry (fluorescence activated cell sorting). Temporal information of the differential recruitment of cells was determined. The basic pattern of recruitment in response to the hydrogels was established and mimicked that seen in a local inflammatory response. Neutrophils (PMN) were rapidly recruited (1 d) followed by macrophages and lymphocytes (1-3 d). Cell surface phenotype studies included the determination of CD3+, CD4+ and CD8+ cells, Mac-1+ cells, and immunoglobulin bearing cells. Microscopic analysis revealed numerous activated PMNs and monocyte derived foamy macrophages. Fluorescence immunocytochemistry of frozen sections of the block revealed that macrophages, CD3+ and natural killer cells were all recruited to the interior of the block. Ultrastructural analysis (transmission electron microscopy) showed highly activated macrophages, with abundant rough endoplasmic reticulum and secretory vesicles. Cells which remained on the surface of the matrix block were CD44 positive migratory cells. Electron microscopic evidence showed foamy macrophages with a varying degree of involvement with the hydrogel material. Surface scanning electron microscopy revealed numerous fibroblast-like cells coating the surface of the block. We suggest that these methods may be used to address the inflammatory response elicited with a a variety of implanted materials such as hydrogels, silicones, ceramics and metals. Furthermore, this model has been useful in determining cellular responses to cytokines and growth factors under similar conditions.

Activity of free and immobilized glucose oxidase: an electrochemical study

The activity of free and immobilized glucose oxidase was determined using a sandwich type thin-layer electrochemical cell. The thin-layer cell consisted of a gold electrode deposited on a glass microscope slide, 165 microns thick Teflon TFE spacers, and a glass cover. Enzyme activity was determined by using cyclic voltammetry to measure the amount of hydrogen peroxide produced in the glucose oxidase catalyzed redox reaction between glucose and oxygen in the thin-layer cell. The specific activity of 13.4 nM glucose oxidase in 0.2 M aqueous sodium phosphate, pH 5.2 at room temperature, was calculated to be 4.34 U/mg GOx. Under the same experimental conditions, qualitative detection of the activity of glucose oxidase covalently immobilized to a thin radiofrequency plasma modified poly(etherurethaneurea) film was demonstrated.

Immobilization of high-affinity heparin oligosaccharides to radiofrequency plasma-modified polyethylene

Oligosaccharides of heparin with high affinity for antithrombin III (ATIII) have been immobilized onto surface-modified NHLBI Primary Reference low density polyethylene (PE). PE was modified by radiofrequency plasma polymerized (< 150 nm thick) films derived from N-vinyl-2-pyrrolidone (PPNVP) or allyl alcohol (PPAA), and coupled by chemical derivatization to either 3-aminopropyltriethoxysilane or amino-terminated poly(ethylene oxide). High affinity heparin oligosaccharides (HA-heparin, anti-factor Xa activity of 592 +/- 120 IU/mg) prepared by partial deaminative cleavage of commercial crude heparin and fractionated by agarose-ATIII affinity chromatography, were immobilized to surface-modified PE by reductive amination. The anticoagulant activity, as determined by a chromogenic assay for the inhibition of factor Xa, was estimated to be 30-70 mIU/cm2, with binding estimated to be 56-119 ng/cm2. The highest activity was obtained for the HA-heparin immobilized to PE modified by PPNVP with a PEO spacer. Visual confirmation of ATIII binding to immobilized HA-heparin was demonstrated by a gold-labeled double antibody method with imaging by SEM.

Biocompatibility studies on plasma polymerized interface materials encompassing both hydrophobic and hydrophilic surfaces

The biocompatibility of radiofrequency plasma polymerized films (less than 100 nm thick) deposited on biomedical polymer supports has been characterized by in vitro and in vivo methods. The polymer interface materials covered a wide range of elemental composition and surface properties, and were prepared from N-vinyl-2-pyrrolidone, gamma-butyrolactone, n-hexane, and hexamethyldisilazane (PPHMDSZ). The biocompatibility studies showed that the interface materials were noncytotoxic to mouse and human fibroblasts, as shown by morphologic evaluation, and by determination of extracellular LDH; and they did not stimulate interleukin-1-like production from human monocytes, as indicated by a thymocyte proliferation assay. The human fibroblast proliferation assay showed that three of the polymers supported cell growth at levels comparable to, or greater than, polymer controls, while the hydrophobic PPHMDSZ inhibited both cell attachment and proliferation. The response to subcutaneous implantation for all test materials was indicative of biocompatibility, with rapid resolution of the acute phase response and normal wound healing. The wide range of composition and surface properties for the plasma polymerized films evaluated in this study suggest that this general class of materials is likely to exhibit excellent biocompatibility.

Glow discharge plasma deposition of tetraethylene glycol dimethyl ether for fouling-resistant biomaterial surfaces

The glow discharge plasma deposition (GDPD) of tetraethylene glycol dimethyl ether is introduced as a novel method for obtaining surfaces that are resistant to protein adsorption and cellular attachment. Analysis of films by x-ray photoelectron spectroscopy and several biological assays indicate the formation of a fouling-resistant, PEO-like surface on several substrata (e.g., glass, polytetrafluoroethylene, polyethylene). Adsorption of 125I-radiolabelled proteins (fibrinogen, albumin and IgG) from buffer and plasma was very low (typically less than 20 ng/cm2) when compared to the untreated substrata, which exhibited much higher levels of protein adsorption. Not all coated substrata adsorbed equal amounts of protein (e.g., coated glass samples typically adsorbed more protein than coated polyethylene or coated polytetrafluoroethylene samples), suggesting that the substratum used may affect the amount of protein adsorbed. Measurement of dynamic platelet adhesion, using epifluorescent video microscopy, and endothelial cell attachment further demonstrates the short-term nonadhesiveness of these surfaces.