Surface modification using silanated poly(ethylene glycol)s

Synthesis, permeability and biocompatibility of tricomponent membranes containing polyethylene glycol, polydimethylsiloxane and polypentamethylcyclopentasiloxane domains

The synthesis of "smart" tricomponent amphiphilic membranes containing poly(ethylene glycol) (PEG), polydimethylsiloxane (PDMS) and polypentamethylcyclopentasiloxane (PD(5)) domains is described. Contact angle hysteresis indicates that in air, the surfaces of such PEG/PD(5)/PDMS membranes are enriched by the hydrophobic components, PDMS and PD(5), while in water, the surfaces are rich in the hydrophilic PEG. The oxygen permeability of a series of membranes with varying M(c,hydrophilic) (M(n,PEG)=4600, 10,000 and 20,000 g/mol) and varying PEG/PD(5)/PDMS compositions was studied. Oxygen permeability increased with the amount of PDMS in the membrane. The molecular weight cut-off (MWCO) ranges and permeability coefficients of insulin through a series of PEG/PD(5)/PDMS(=29/14/57) membranes with varying M(c,hydrophilic) were determined. Insulin permeability is directly related to M(c,hydrophilic) of the membrane. MWCO studies show that the membranes are semipermeable to, i.e., allow the transport of smaller proteins such as insulin (M(n)=5733 g/mol, R(s)=1.34 nm) and cytochrome c (M(n)=12,400 g/mol, R(s)=1.63 nm), but are barriers to larger proteins such as albumin (M(n)=66,000 g/mol, R(s)=3.62 nm). Implantation of representative membranes in rats showed them to be biocompatible. According to these studies, PEG/PD(5)/PDMS membranes may be suitable for biological applications, e.g., immunoisolation of cells.

Solid supports for microarray immunoassays

Stimulated by the achievements of the first phase in genomics and the resulting need of assigning functions to the acquired sequence information, novel formats of immunoassays are being developed for high-throughput multi-analyte studies. In principle, they are similar in nature to the microarray assays already established at the level of nucleic acids. However, the biochemical diversity and the sheer number of proteins are such that an equivalent analysis is much more complex and thus difficult to accomplish. The wide range of protein concentration complicates matters further. Performing microarray immunoassays already represents a challenge at the level of preparing a working chip surface. Arrays have been produced on filter supports, in microtiter plate wells and on glass slides, the last two usually coated with one-, two- or three-dimensionally structured surface modifications. The usefulness and suitability of all these support media for the construction and application of antibody microarrays are reviewed in this manuscript in terms of the different kinds of immunoassay and the various detection procedures. Additionally, the employment of microarrays containing alternative sensor molecules is discussed in this context. The sensitivity of microspot immunoassays predicted by the current analyte theory is not yet a reality, indicating the extent of both the technology's potential and the size of the task still ahead.

Improved blood compatibility and decreased VSMC proliferation of surface-modified metal grafted with sulfonated PEG or heparin

Although the technique of coronary stenting has remarkably improved long-term results in recent years, (sub)acute thrombosis and late restenosis still remain problems to be solved. Metallic surfaces were regarded as thrombogenic, due to their positive surface charges, and stenosis resulted from the activation and proliferation of vascular smooth muscle cells (VSMCs). In this study, a unique surface modification method for metallic surfaces was studied using a self-assembled monolayer (SAM) technique. The method included the deposition of thin gold layers, the chemisorption of disulfides containing functional groups, and the subsequent coupling of PEG derivatives or heparin utilizing the functional groups of the disulfides. All the reactions were confirmed by ATR-FTIR and XPS. The surface modified with sulfonated PEG (Au-S-PEG-SO3) or heparinized PEG (Au-S-PEG-Hep) exhibited decreased static contact angles and therefore increased hydrophilicity to a great extent, which resulted from the coupling of PEG and the ionic groups attached. In vitro fibrinogen adsorption and platelet adhesion onto the Au-S-PEG-SO3 or Au-S-PEG-Hep surfaces decreased to a great extent, indicating enhanced blood compatibility. This decreased interaction of the modified surfaces should be attributed to the non-adhesive property of PEG and the synergistic effect of sulfonated PEG. The effect of the surface modification on the adhesion and proliferation of VSMCs was also investigated. The modified Au-S-PEG-SO3 or Au-S-PEG-Hep surfaces also exhibited decreased adhesion of VSMCs, while the deposited gold layer itself was effective. The enhanced blood compatibility and the decreased adhesion of VSMCs on the modified metallic surfaces may help to decrease thrombus formation and suppress restenosis. It would therefore be very useful to apply these modified surfaces to stents for improved functions. A long-term in vivo study using animal models is currently under way.

Anisotropic stretch-induced hypertrophy in neonatal ventricular myocytes micropatterned on deformable elastomers

Because cell shape and alignment, cell-matrix adhesion, and cell-cell contact can all affect growth, and because mechanical strains in vivo are multiaxial and anisotropic, we developed an in vitro system for engineering aligned, rod-shaped, neonatal cardiac myocyte cultures. Photolithographic and microfluidic techniques were used to micropattern extracellular matrices in parallel lines on deformable silicone elastomers. Confluent, elongated, aligned myocytes were produced by varying the micropattern line width and collagen density. An elliptical cell stretcher applied 2:1 anisotropic strain statically to the elastic substrate, with the axis of greatest stretch (10%) either parallel or transverse to the myofibrils. After 24 h, the principal strain parallel to myocytes did not significantly alter myofibril accumulation or expression of atrial natriuretic factor (ANF), connexin-43 (Cx-43), or N-cadherin (by indirect immunofluorescent antibody labeling and immunoblotting) compared with unstretched controls. In contrast, 10% transverse principal strain resulted in continuous staining of actin filaments (rhodamine phalloidin); increased immunofluorescent labeling of ANF, Cx-43, and N-cadherin; and upregulation of protein signal intensity by western blotting. By using microfabrication and microfluidics to control cell shape and alignment on an elastic substrate, we found greater effects for transverse than for longitudinal stretch in regulating sarcomere organization, hypertrophy, and cell-to-cell junctions.

PEGylated nanoparticles for biological and pharmaceutical applications

The utility of polymeric micelles formed through the multimolecular assembly of block copolymer was comprehensively described as novel core-shell typed colloidal carriers for drug and gene targeting. Particularly, novel approaches for the formation of functionalized poly(ethylene glycol) (PEG) layers as hydrophilic outer shell were focused to attain receptor-mediated drug and gene delivery through PEG-conjugated ligands with a minimal non-specific interaction with other proteins. Surface organization of block copolymer micelles with cross-linking core was also described from a standpoint of the preparation of a new functional surface-coating with a unique macromolecular architecture. The micelle-attached surface and the thin hydrogel layer made by layered micelles exhibited nonfouling properties and worked as the reservoir for hydrophobic reagents. Furthermore, the potential utility of multimolecular assembly derived from heterobifunctional PEGs and block copolymers were explored to systematically modify the properties of metal and semiconductor nanostructures by controlling their structure and their surface properties, making them extremely attractive for use in biological and biomedical applications.

Studies of a novel human thrombomodulin immobilized substrate: surface characterization and anticoagulation activity evaluation

Immobilization of the anticoagulative or antithrombogenic biomolecule has been considered as one of the important methods to improve the blood compatibility of artificial biomaterials. In this study, a novel immobilization reaction scheme was utilized to incorporate the human thrombomodulin, an endothelial cell associated glycoprotein, onto the cover glass surface with an aim to develop an anticoagulative substrate. Trichlorotriazine and amino-terminated silane were employed as the coupling agents, while the polyethylene glycol with a molecular weight of 1500 was used as the spacer in this reaction scheme. Protein C activation assay indicated the immobilized human thrombomodulin still has this coenzymatic activity but is lower, possibly due to the conformation variation by the coupling agents. In vitro platelet adhesion assay has demonstrated the surface with immobilized human thrombomodulin is much less platelet-activating than others. Therefore, the novel reaction scheme proposed here is very promising for future development of an anticoagulative silicon or cover glass substrate (e.g. implantable sensor or biochip) by the immobilization of antithrombogenic protein, such as the human thrombomodulin in this study.

Optimisation of glass surfaces for optical immunosensors

The surfaces of glass sensor chips were modified with dextran to generate a layer protecting the sensor surface from unspecific protein binding and also serving as a matrix for covalent protein immobilisation. Dextran was coupled to the glass surface in different concentrations either covalently on amino-functionalised glass chips or via biotin-avidin binding. Unspecific binding of BSA was monitored with the grating coupler system, and was increasingly suppressed with increasing dextran concentrations. Using a solution with 100 mg/ml carboxymethylated dextran decreased the signals to approximately 2% of those obtained at an untreated glass chip. Antibodies were successfully immobilised in the dextran and binding to the corresponding Cy5-labelled antigen was repeatedly monitored using a fluorescence sensor system (total internal reflection fluorescence (TIRF)).

Interactions between PEG and type I soluble tumor necrosis factor receptor: modulation by pH and by PEGylation at the N terminus

The effects of polyethylene glycol (PEG) on protein structure and the molecular details that regulate its association to polypeptides are largely unknown. These issues were addressed using type I soluble tumor necrosis factor receptor (sTNF-RI) as a model system. Changes in solution viscosity established that a truncated form of sTNF-RI bound free PEG in a pH-dependent manner. Above pH 5.3, the viscosity escalated as the pH increased, while no effect occurred below pH 5.0. Conjugation of 2 kD, 5 kD, or 20 kD PEG to the N terminus attenuated the viscosity at the higher pH values. Tryptophan phosphorescence spectroscopy correlated changes in the protein structure about Trp-107, at the C terminus, with the pH-dependent and PEGylation-dependent attenuation of the viscosity. The results indicate that specific interactions between PEG and the truncated form of sTNF-RI are elicited by an increased flexibility of the truncated protein combined perhaps with removal of steric or charge barriers. Covalently bound PEG at the N terminus reduced the protein affinity for the free polymer and induced a more rigid and polar configuration around Trp-107. Deprotonation of His-105, which is perpendicular to Trp-107, was integral to the binding mechanism producing a pH-dependent switching mechanism. These findings stress the importance of surface charge and structural plasticity in determining macromolecular binding affinities and demonstrate the ability of conjugated PEG to modify the localized surface structure in proteins away from the site of conjugation.

Biocompatibility evaluation of ePTFE membrane modified with PEG in atmospheric pressure glow discharge

ePTFE membranes were modified by poly(ethylene glycol) having a molecular weight of 600 (PEG-600) in atmospheric pressure glow discharge (APG) plasma treatment. ePTFE membranes were immersed in 1%, 3%, or 5% (w/v) PEG-600 in dehydrated ethanol. PEG-600-penetrated ePTFE membranes were dried in a vacuum to immediately remove ethanol, then treated with APG at 20 kHz and 60-70 W for 15 min and thoroughly washed with ethanol and water. PEG-600-modified ePTFE membranes were analyzed using contact angle measurement, Fourier transform infrared attenuated total reflectance (FTIR-ATR), and scanning electron microscopy (SEM). ePTFE membrane contact angles were reduced after PEG-600 plasma treatment. FTIR-ATR spectra showed an absorption band due to a PEG hydroxyl group (-OH). SEM showed that ePTFE fiber surfaces were uniformly immobilized with PEG-600 and retained their porous structure. A general biological evaluation of the PEG-modified ePTFE membranes showed no cytotoxicity on CHO-K1 cell lines and no hemolytic action. Albumin adsorption on the PEG-modified ePTFE membranes increased with increasing PEG-600 deposited on ePTFE membranes. Fibrinogen adsorption decreased with increasing PEG-600 deposited on ePTFE membranes. gamma-Globulin adsorption did not change before or after PEG plasma modification. 1% and 3% PEG-600 plasma-treated ePTFE only slightly increased platelet adhesion, but adhering platelets evidenced no pseudopod formation. 5% PEG-600-modified ePTFE showed relatively large numbers of platelet adhesion. We concluded that 3% PEG-600-modified ePTFE membrane had the best physical properties and biological compatibility, indicating 3% PEG-600-modified ePTFE membranes exhibit the potential for blood filter application.

Engineering protein and cell adhesivity using PEO-terminated triblock polymers

Previous studies on customizing cell culture environments have utilized a variety of microfabrication-based tools to control the spatial localization of adhesive proteins and subsequently mammalian cells. Others have used various methods to immobilize nonadhesive PEO-based polymers on surfaces to inhibit protein absorption and cell adhesion. In this study, we report the application of a well-characterized, commercially available, PEO-terminated triblock polymer (Pluronic F108) to create micropatterned nonadhesive domains on a variety of biomaterials that deter cell adhesion for up to 4 weeks in culture. The Pluronic can be applied using microfluidic tools or photolithographic techniques, and can be adsorbed to a variety of common surfaces including tissue culture polystyrene, methylated glass, silicone, and polylactic-co-glycolic acid. The effectiveness of the Pluronic in inhibiting cell adhesion in the presence of collagen I is also quantified. Finally, these patterning techniques are generalized to control tissue organization on a variety of common biomaterials. This simple method for micropatterning PEO and, therefore, proteins and cells should prove useful as a tool for biomolecular surface engineering.

Kinetics and mechanism of intercellular ice propagation in a micropatterned tissue construct

Understanding the effects of cell-cell interaction on intracellular ice formation (IIF) is required to design optimized protocols for cryopreservation of tissue. To determine the effects of cell-cell interactions during tissue freezing, without confounding effects from uncontrolled factors (such as time in culture, cell geometry, and cell-substrate interactions), HepG2 cells were cultured in pairs on glass coverslips micropatterned with polyethylene glycol disilane, such that each cell interacted with exactly one adjacent cell. Assuming the cell pair to be a finite state system, being either in an unfrozen state (no ice in either cell), a singlet state (IIF in one cell only), or a doublet state (IIF in both cells), the kinetics of state transitions were theoretically modeled and cryomicroscopically measured. The rate of intercellular ice propagation, estimated from the measured singlet state probability, increased in the first 24 h of culture and remained steady thereafter. In cell pairs cultured for 24 h and treated with the gap junction blocker 18beta-glycyrrhetinic acid before freezing, the intercellular ice propagation rate was lower than in untreated controls (p < 0.001), but significantly greater than zero (p < 0.0001). These results suggest that gap junctions mediate some, but not all, mechanisms of ice propagation in tissue.

Albumin adsorption onto pyrolytic carbon: a molecular mechanics approach

A number of implants of cardiac valve prosthesis, vascular prosthesis, and coronary stents present a pyrolytic carbon interface to blood. Plasma protein adsorption is essential for the hemocompatibility of the implanted devices. This work quantitatively evaluates the molecular interaction force between a biomaterial surface (pyrolytic carbon) and plasma protein (albumin) binding sites through a simplified molecular model of the interface consisting of (i) multioriented graphite microcrystallites; (ii) selected fragments of albumin; and (iii) a water environment. A number of simplifying assumptions were made in the calculation: the albumin molecule was divided into hydrophobic and hydrophilic subunits (helices); an idealized clean, nonoxidized polycrystalline graphite surface was assumed to approximate the surface of pyrolytic carbon. The interaction forces between albumin helices and pyrolytic carbon surfaces are evaluated from potential energy data. These forces are decomposed into a normal and a tangential component. The first one is calculated using a docking procedure (F( perpendicular tot MAX) = 4.16 x 10(-20) N). The second one (F( parallel)), calculated by mean of geometric models estimating the energy variation associated with the protein sliding on the material surface, varies within the range +/-9.62 x 10(-21) N. The molecular simulations were performed using the commercial software package Hyperchem 5.0 (Hyperchem, Hypercube, Canada).

Chain-length dependence of the protein and cell resistance of oligo(ethylene glycol)-terminated self-assembled monolayers on gold

Oligo(ethylene glycol) (O-EG(n))-terminated alkanethiol surface-assembled monolayers (SAMs) have been reported to demonstrate protein-resistant properties similar to those of poly(ethylene glycol) (PEG). In this study, we compared the relative protein resistance of short and long ethylene oxide chains, SAMs of PEG 5000, PEG 2000, O-EG(3) (molecular weight = 120), and O-EG(6) (molecular weight = 240), on gold surfaces. Surface plasmon resonance showed that these monolayers were all protein-resistant within the uncertainty of the measurement. However, they exhibited different adhesive properties toward 3T3 mouse fibroblast adhesion in supplemented Dulbecco's modified Eagles medium. The results show that the cell adhesion was sensitive to the concentration of proteins supplemented in the culture medium and to the length of PEG chains.