Protein adsorption on gradient surfaces on polyethylene prepared in a shielded gas plasma

High-Throughput and Combinatorial Approaches for the Development of Multifunctional Polymers

High-throughput (HT) development of new multifunctional polymers is accomplished by the combination of different HT tools established in polymer sciences in the last decade. Important advances are robotic/HT synthesis of polymer libraries, the HT characterization of polymers, and the application of spatially resolved polymer library formats, explicitly microarray and gradient libraries. HT polymer synthesis enables the generation of material libraries with combinatorial design motifs. Polymer composition, molecular weight, macromolecular architecture, etc. may be varied in a systematic, fine-graded manner to obtain libraries with high chemical diversity and sufficient compositional resolution as model systems for the screening of these materials for the functions aimed. HT characterization allows a fast assessment of complementary properties, which are employed to decipher quantitative structure-properties relationships. Moreover, these methods facilitate the HT determination of important surface parameters by spatially resolved characterization methods, including time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy. Here current methods for the high-throughput robotic synthesis of multifunctional polymers as well as their characterization are presented and advantages as well as present limitations are discussed.

Ordered Micro/Nanostructures with Geometric Gradient: From Integrated Wettability "Library" to Anisotropic Wetting Surface

Geometric gradients within ordered micro/nanostructures exhibit unique wetting properties. Well-defined and ordered microsphere arrays with geometric gradient (OMAGG) are successfully fabricated through combining colloidal lithography and inclined reactive ion etching (RIE). During the inclined RIE, the graded etching rates in vertical direction of etcher chamber are the key to generating a geometric gradient. The OMAGG can be used as an effective mask for the preparation of micro/nanostructure arrays with geometric gradient by selective RIE. Through this strategy, a well-defined wettability "library" with graded silicon cone arrays is fabricated, and the possibility of screening one desired "book" from the designated wettability "library" is demonstrated. Meanwhile, the silicon cone arrays with geometric gradient (SCAGG) can be applied to control the wetting behavior of water after being modified by hydrophilic or hydrophobic chemical groups. Based on this result, a temperature-responsive wetting substrate is fabricated by modifying poly n-isopropyl acrylamide (PNIPAM) on the SCAGG. These wettability gradients have great potential in tissue engineering, microfluidic devices, and integrated sensors.

Screening rat mesenchymal stem cell attachment and differentiation on surface chemistries using plasma polymer gradients

It is well known that the surface chemistry of biomaterials is important for both initial cell attachment and the downstream cell response. Surface chemistry gradients are a new format that allows the screening of the subtleties of cell-surface interactions in high throughput. In this study, two surface chemical gradients were fabricated using diffusion control during plasma polymerization via a tilted mask. Acrylic acid (AA) plasma polymer gradients were coated on a uniform 1,7-octadiene (OD) plasma polymer layer to generate OD-AA plasma polymer gradients, whilst diethylene glycol dimethyl ether (DG) plasma polymer gradients were coated on a uniform AA plasma polymer layer to generate AA-DG plasma polymer gradients. Gradient surfaces were characterized by X-ray photoelectron spectroscopy, infrared microscopy mapping, profilometry, water contact angle (WCA) goniometry and atomic force microscopy. Cell attachment density and differentiation into osteo- and adipo-lineages of rat-bone-marrow mesenchymal stem cells (rBMSCs) was studied on gradients. Cell adhesion after 24 h culture was sensitive to the chemical gradients, resulting in a cell density gradient along the substrate. The slope of the cell density gradient changed between 24 and 6 days due to cell migration and growth. Induction of rBMSCs into osteoblast- and adipocyte-like cells on the two plasma polymer gradients suggested that osteogenic differentiation was sensitive to local cell density, but adipogenic differentiation was not. Using mixed induction medium (50% osteogenic and 50% adipogenic medium), thick AA plasma polymer coating (>40 nm thickness with ∼11% COOH component and 35° WCA) robustly supported osteogenic differentiation as determined by colony formation and calcium deposition. This study establishes a simple but powerful approach to the formation of plasma polymer based gradients, and demonstrates that MSC behavior can be influenced by small changes in surface chemistry.

Gradient biomaterials and their influences on cell migration

Cell migration participates in a variety of physiological and pathological processes such as embryonic development, cancer metastasis, blood vessel formation and remoulding, tissue regeneration, immune surveillance and inflammation. The cells specifically migrate to destiny sites induced by the gradually varying concentration (gradient) of soluble signal factors and the ligands bound with the extracellular matrix in the body during a wound healing process. Therefore, regulation of the cell migration behaviours is of paramount importance in regenerative medicine. One important way is to create a microenvironment that mimics the in vivo cellular and tissue complexity by incorporating physical, chemical and biological signal gradients into engineered biomaterials. In this review, the gradients existing in vivo and their influences on cell migration are briefly described. Recent developments in the fabrication of gradient biomaterials for controlling cellular behaviours, especially the cell migration, are summarized, highlighting the importance of the intrinsic driving mechanism for tissue regeneration and the design principle of complicated and advanced tissue regenerative materials. The potential uses of the gradient biomaterials in regenerative medicine are introduced. The current and future trends in gradient biomaterials and programmed cell migration in terms of the long-term goals of tissue regeneration are prospected.

One-step method for generating PEG-like plasma polymer gradients: chemical characterization and analysis of protein interactions

In this work we report a one-step method for the fabrication of poly(ethylene glycol) PEG-like chemical gradients, which were deposited via continuous wave radio frequency glow discharge plasma polymerization of diethylene glycol dimethyl ether (DG). A knife edge top electrode was used to produce the gradient coatings at plasma load powers of 5 and 30 W. The chemistry across the gradients was analyzed using a number of complementary techniques including spatially resolved synchrotron source grazing incidence FTIR microspectroscopy, X-ray photoelectron spectroscopy (XPS) and synchrotron source near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Gradients deposited at lower load power retained a higher degree of monomer like functionality as did the central region directly underneath the knife edge electrode of each gradient film. Surface derivatization experiments were employed to investigate the concentration of residual ether units in the films. In addition, surface derivatization was used to investigate the reactivity of the gradient films toward primary amine groups in a graft copolymer of poly (L-lysine) and poly(ethylene glycol) (PLL-g-PEG copolymer) which was correlated to residual aldehyde, ketone and carboxylic acid functionalities within the films. The protein adsorption characteristics of the gradients were analyzed using three proteins of varying size and charge. Protein adsorption varied and was dependent on the chemistry and the physical properties (such as size and charge) of the proteins. A correlation between the concentration of ether functionality and the protein fouling characteristics along the gradient films was observed. The gradient coating technique developed in this work allows for the efficient and high-throughput study of biomaterial gradient coating interactions.

Spatial variation of the charge and sulfur oxidation state in a surface gradient affects plasma protein adsorption

A gradient of negative surface charge based on the 1D spatial variation from surface sulfhydryl to mixed sulfhydryl-sulfonate moieties was prepared by the controlled UV oxidation of a 3-mercaptopropylsilane monolayer on fused silica. The adsorption of three human plasma proteins--albumin (HSA), immunoglobulin G (IgG), and fibrinogen (Fgn)--onto such a surface gradient was studied using spatially resolved total internal reflection fluorescence (TIRF) and autoradiography. Adsorption was measured from dilute solutions equivalent to 1/100 (TIRF, autoradiography), 1/500, and 1/1000 (autoradiography) of protein physiological concentrations in plasma. All three proteins adsorbed more to the nonoxidized sulfhydryl region than to the oxidized, mixed sulfhydryl-sulfonate region of the gradient. In the case of HSA, the adsorption contrast along the gradient was largest when the adsorption took place from more dilute protein solutions. Increasing the concentration to 1/100 of the protein plasma concentration eliminated the effect of the gradient on HSA adsorption and, to the lesser extent, on IgG adsorption. In the case of Fgn, the greatest adsorption contrast was observed at the highest concentration used. On the basis of adsorption kinetics, the estimated binding affinity of HSA for the sulfhydryl region was twice the affinity for the mixed sulfhydryl-sulfonate region of the gradient. For IgG and Fgn, the initial adsorption was transport-limited and the initial adsorption rates approached the computed flux of the protein to the surface.

Patterning gradient properties from sub-micrometers to millimeters by magnetolithography

A new method is presented for patterning surfaces with gradient properties. The method is based on magnetolithography in which the surface patterning is performed by applying a gradient of a magnetic field on the substrate, using paramagnetic metal masks in the presence of a constant magnetic field. Superparamagnetic nanoparticles (NPs) are deposited on the substrate, and they assemble according to the field and its gradients induced by the mask. Once they pattern the substrate, they protect their sites on the substrate from interacting with any other species. The areas not protected by the NPs can be covered by molecules that chemically bind to the substrate. After these molecules are bound, the NPs are removed, and other molecules may be adsorbed on the newly exposed area. The new technique is based on a parallel process that can be carried out on a full wafer. It provides high resolution, it creates gradient continuously from sub-micrometers to millimeters, and it can be performed on surfaces that are not flat and that are even on the inside of a tube. The gradient that is formed is not limited to a specific property or type of substrate.

Molecular gradients of bioinertness reveal a mechanistic difference between mammalian cell adhesion and bacterial biofilm formation

Chemical gradients play an important role in guiding the activities of both eukaryotic and prokaryotic cells. Here, we used molecularly well-defined chemical gradients formed by self-assembled monolayers (SAMs) on gold films to reveal that mammalian cell adhesion and bacterial biofilm formation respond differently to a gradient of surface chemistry that resists cell attachment. Gradient self-assembled monolayers (SAMs) consisting of two mixed alkanethiols were fabricated by differential exposure of the gold film to one alkanethiol, followed by soaking in another alkanethiol solution. A gradient in bioinertness that resisted cell attachment was created on SAMs from a gradient in the surface density of HS(CH2)11(OCH2CH2)3OH, backfilled with either HS(CH2)11OH or HS(CH2)11CH3. Measurements of the amounts of mammalian cells and bacterial biofilms on these gradient surfaces reveal that, for mammalian cells, a critical density of adhesion ligands from absorbed proteins on surfaces exists for supporting maximum adhesion and proliferation, whereas for the bacterium Escherichia coli , the amount of biofilm formed on surfaces increased linearly with the surface density of adhesive groups (methyl or hydroxyl groups) in different media. These results are consistent with mammalian cell adhesion requiring an anchorage via specific molecular recognitions and suggest that biofilms can form by immobilization of bacteria via nonspecific interaction between bacteria and surfaces.

Responsive brush layers: from tailored gradients to reversibly assembled nanoparticles

We present a condensed overview of the recent developments of novel responsive thin polymer films from end-tethered chains (polymer brushes), which are different from conventional, uniform, and planar brush layers. For this discussion, we selected two types of recently introduced surface layers: binary brush layers with variable chemical composition forming a controllable gradient of composition and properties in a selected direction and brush layers either grafted directly to inorganic nanoparticles to form hybrid core-shell structures or combined with inorganic nanoparticles embedded into this layer. Unlike traditional brush layers, such a design brings a novel set of responsive surface properties allowing for capillary-driven microfluidic motion, combinatorial-like multiplexing response, reversible aggregation and dis-assembly of nanoparticles, fabrication of ultrahydrophobic coatings, and switchable mass transport across interfaces.

Surface-bound soft matter gradients

This feature article describes the progress realized over the past half century in the field of surface-bound gradient structures created on or from soft materials (oligomers and/or polymers), or those enabling the study of the behavior of soft materials. By highlighting our work in the field and accounting for the contribution of other groups, we emphasize the exceptional versatility of gradient assemblies in facilitating fast screening of physicochemical phenomena, acting as "recording media" for monitoring a process, and playing a key role in the design and fabrication of surface-bound molecular and macromolecular motors capable of directing a transport phenomenon.

Surface-chemical and -morphological gradients

Surface gradients of chemistry or morphology represent powerful tools for the high-throughput investigation of interfacial phenomena in the areas of physics, chemistry, materials science and biology. A wide variety of methods for the fabrication of such gradients has been developed in recent years, relying on principles ranging from diffusion to time-dependent irradiation in order to achieve a gradual change of a particular parameter across a surface. In this review we have endeavoured to cover the principal fabrication approaches for surface-chemical and surface-morphological gradients that have been described in the literature, and to provide examples of their applications in a variety of different fields.

Surface-enhanced nucleation of insulin amyloid fibrillation

Proteins can interact with biological surfaces such as cell membrane, chaperones, cornea, bone, arteries, veins, and heart cavities of the cardiovascular system and also with non-biological surfaces including dialysis membranes and tubing, catheters, invasive surgical instruments, needles, and artificial implants. Fibrillation of amyloid proteins is implicated in many human diseases, including Alzheimer's, Parkinson's, and type II diabetes. Here, we show that heterogeneous surfaces accelerate the human insulin nucleation process that is the rate-determining step during amyloid fibril formation. The observed shorter lag (nucleation) phase correlates both with surface wettability and surface roughness. Surfaces promote faster nucleation possibly by increasing the local concentration of protein molecules. A composite parameter combining both surface wettability and roughness suggests that the ideal surface for slower nucleation should be hydrophilic and smooth. These findings provide a basis for designing suitable biomaterials and biomedical devices, especially those to resist amyloidosis.

Synthesis of high-density grafted polymer layers with thickness and grafting density gradients

A novel approach was developed for the synthesis of tethered polymer layers with thickness and grafting density gradients. Poly(glycidyl methacrylate) (PGMA) was employed as a primary anchoring layer to attach the polymer chains to the surface of a silicon wafer. A linear temperature gradient heated stage was used for the generation of a gradual variation in the thickness of the anchoring PGMA film along the substrate. The obtained gradient was translated into the polymerization initiator gradient via the reaction between the epoxy groups of PGMA and the carboxyl functionality of 2-bromo-2-methylpropionic acid (BPA). The attachment of BPA to the surface modified with the monolayer of PGMA was confirmed by X-ray photoelectron spectroscopy experiments. To complete the experimental procedures, surface-initiated atom transfer radical polymerization was performed to synthesize the grafted polymer layers with thickness and surface densities that were varied along the substrate. The grafting density of the samples created in this three-step process ranged from 0.75 +/- 0.05 to 1.5 +/- 0.25 chains/nm(2). It was estimated, from a comparison of the surface densities of the initiator and the attached polymer, that the efficiency of the initiation from the surface was on the order of 5-10% and was dependent upon the surface concentration of the initiator and the time of polymerization.

Combinatorial approach to study enzyme/surface interactions

A fast combinatorial approach to access information about the immobilization behavior and kinetics of enzymes on a variation of surfaces is presented. As a test system, Candida Antarctica Lipase B was immobilized on a self-assembled monolayer bearing a gradient of surface energy. The respective immobilization behavior was monitored by Fourier transform infrared micro-spectroscopy. In addition, the activity of the immobilized enzyme was monitored over the entire film in real time with a specially developed fluorescence activity assay embedded into a siloxane gel. It was found that the highest amount of active protein was immobilized on the hydrophilic end of the gradient surface. This effect is associated with a higher surface roughness of this area resulting in hydrophobic micro-environments in which the enzyme gets immobilized.

On the role of CO2 laser treatment in the human serum albumin and human plasma fibronectin adsorption on zirconia (MGO-PSZ) bioceramic surface

The nature of the surface strongly influences the composition and recognizability of the adsorbed protein layer, which in turn affects the subsequent cellular interactions. Thus, to understand the biological response to a material, especially in vitro, one must fully understand the nature of the adsorbed protein film that forms on the material. This study investigates the fundamental interactions between the human serum albumin (no-cell adhesive) and human plasma fibronectin and bioinert ceramic following CO(2) laser treatment. The analysis of the albumin and fibronectin adsorption was conducted on the untreated and CO(2) laser-modified magnesia partially stabilized zirconia (MgO-PSZ) bioceramic using an ellipsometry. It was found that the adsorptions of albumin and fibronectin were influenced by the surface properties. The albumin adsorption was affected by the surface roughness and wettability characteristics of the MgO-PSZ and decreased with these properties, while the fibronectin adsorption was increased with wettability characteristics and predominantly governed by this property. Moreover, the considerable change in the polar component of surface energy, gamma(sv) (p), and its effect on protein adsorption implied that the albumin and fibronectin adsorption on the MgO-PSZ surfaces was probably due to the polar and chemical interactions. The value of this work is to provide a novel technique and useful information for manipulating protein adsorption and thereof cellular interactions.

A Simple, Reproducible Approach to the Preparation of Surface-Chemical Gradients

We demonstrate a very simple and reproducible preparative approach for the fabrication of surface-chemical gradients. A surface concentration gradient of adsorbed methyl- or hydroxyl-terminated thiolates was achieved upon gradually immersing a gold-coated substrate into a very dilute thiol solution (0.0033 mM) by means of a linear-motion drive. Subsequent immersion of the substrate into the complementary thiol solution provided a hydrophobicity gradient with a large range (50° of the water-contact angle) and over a significant distance (35 mm). The self-assembled monolayer gradient produced in this way also displayed a high packing density, as demonstrated by dynamic contact-angle and X-ray photoelectron spectroscopy measurements.

Influence of abciximab on the adhesion of platelets on a shielded plasma gradient prepared on polyethylene

INTRODUCTION

Thrombotic effects of biomaterial implants are mediated merely through activation of the platelet glycoprotein IIb-IIIa (GpIIb-IIIa) receptor. Consequently, platelet GpIIb-IIIa receptor inhibitors are successfully used during stent implantation procedures to prevent thrombosis. However, currently a new generation of stents contains surface coating, which changes the surface to more hydrophobic or hydrophilic. This change markedly affects the interaction of platelets and may influence the efficiency of GpIIb-IIIa inhibitors.

MATERIALS AND METHODS

To study the influence of the wettability of biomaterials on the effectiveness of abciximab, 5-cm polyethylene gradients with contact angles of 100 degrees to 40 degrees were made by means of glow discharge. Fresh whole blood with or without abciximab was recirculated over this gradient.

RESULTS

Inhibition of platelet adhesion by abciximab was maximal, but not complete, on the hydrophobic and moderate hydrophobic part of the gradient, with contact angles of 55 degrees to 90 degrees. Percentage inhibition by abciximab was maximal around 60 degrees.

CONCLUSIONS

Intermediate hydrophobicity of currently applied stent materials, such as stainless steel, seems optimal in combination with abciximab. However, on hydrophobic and particularly on hydrophilic materials, abciximab is less effective.

Platelet adhesion and activation on a shielded plasma gradient prepared on polyethylene

Contact of blood with foreign materials evokes thrombogenic effects to an extent determined partly by the wettability of the biomaterials surface. Tools to study blood response towards a variation in materials wettability with minimal variation in chemistry are "gradient surfaces". However, most gradients have been prepared by diffusion or density immersion techniques, which results in a limited gradient range. Through glow discharge with partial shielding, gradients on polymers were prepared over a length of 5 cm, which facilitated studies to platelet adhesion on separate gradient sections. On polyethylene, advancing water contact angles varied from 90 degrees to 40 degrees, with a hysteresis of 30 degrees. ESCA indicated an increasing incorporation of oxygen towards the hydrophilic end. To examine the role of materials wettability on the activation of adhering platelets, sections of shielded plasma gradients were incubated in anticoagulated whole human blood. Fewer platelets adhered to the hydrophobic end, but those platelets were more activated than those on the hydrophilic end, as judged from their morphology and exposure of GpIIb-IIIa complex. However, partly related to the increased binding of platelets, the clotting activation after platelet deposition was highest on the hydrophilic end. Concluding, this new technique results in a large gradient range, which facilitates studies of formed blood elements in relation to the wettability. Platelets are more activated on hydrophobic polyethylene, while on moderate hydrophilic polyethylene more platelet adhesion and activation of the clotting system occurs.