Session L19 - Biocompatible Surfaces and Adsorbed Polymers.
ORAL session, Tuesday afternoon, March 13
Room 310, Washington State Convention Center

[L19.001] Fibrinogen Relaxation Kinetics on Hydrophobic Surfaces

Upon adsorption onto a hydrophobic interface, fibrinogen molecules denature and unfold, exposing internal, hydrophobic domains to the surface. Here, we seek to characterize the unfolding kinetics during the spreading process, using C16 self-assembled monolayers as model hydrophobic surfaces. For single-species fibrinogen runs, the initial spreading rate is constant, however, lateral interactions between neighboring molecules stunt spreading before an ultimate protein footprint can be realized. Therefore, to measure the spreading rate during the entire relaxation process, we report kinetic runs on unsaturated fibrinogen layers using fibrinogen and lysozyme as probes to measure the consumption of free surface space as the preadsorbed fibrinogen layers unfold. Both probes reveal a constant initial spreading rate for fibrinogen that begins to level off after 30 minutes and reaches a maximum footprint after 1 hour. The lysozyme probe gives a larger ultimate fibrinogen footprint than the fibrinogen probe, which may be due to lysozyme's structural stability and lack of post-adsorption denaturing.

[L19.002] Osteoblast Cell Adhesion and Spreading on Modified Surfaces

The biocompatibility of biomaterials is strongly related to cell adhesion to their surface. Surface characteristics of materials, particular functionality, play essential roles on osteoblast adhesion to biomaterials. Here we will show how surfaces modified by aminofunctional organosilane and the tripeptide, RGD, influence cell attachment, spreading and function.

[L19.003] Smart Polymer Surfaces

The aim of this study is to develop the methodology for creating polymer surfaces, which can respond to their environment. We demonstrate the methodology for surfaces that can alter their wetting characteristics when exposed to water vapor. For this, we take advantage of the surface partitioning of block copolymers at the polymer/air interface and utilize a hydrophilic group at the end of the surface-active block. The surface-active block of the copolymer (anchor) drives the surface segregation while the other block forms a dangling tail mixed with the matrix homopolymer. The hydrophilic end-group is hidden below the surface when it is in contact with air. However, when the surface is exposed to water vapor, the hydrophilic end-group re-surfaces thus significantly reducing the water contact angle. It is shown that this function can be reversed and repeated again by successive exposures of the polymer surface to dry or wet environment, respectively.

[L19.004] Molecular Surface Structure and Biocompatibility

An important component of biocompatibility is protein resistance. Our studies of ordered and disordered self-assembled monolayers of methyl 1-thiahexa(ethylene oxide) on gold reveal that disordered monolayers exhibit better inhibition of protein adsorption. We are developing internal reflection infrared spectroscopy methods to improve our understanding of the molecular structures and processes responsible for protein resistance and biocompatibility. To illustrate, we have characterized octadecanethiol and methyl 1-thiahexa(ethylene oxide) monolayers through thin gold layers. The internal reflection geometry permits in situ measurements and the observation of both the s- and p-polarized spectra. From this information, we are able to deduce the surface concentration and the orientations of the functional groups of the monolayers and proteins. The development of these measurements and their results will be discussed.

[L19.005] Methyl 1-Thiahexa(ethylene oxide) Monolayers for the Inhibition of Protein Adsorption

Self-assembled monolayers (SAMs) of methyl 1-thiahexa(ethylene oxide) [HS(CH_2CH_2O)_6CH_3] on gold were evaluated for their inhibition of protein adsorption. Reflection-adsorption infrared spectroscopy (RAIRS) data for HS(CH_2CH_2O)_6CH_3 SAMs assembled from either 95% or 100% ethanol showed that the 1-thiahexa(ethylene oxide) segment adopts the well ordered 7/2 helical conformation of the folded-chain crystal polymorph of poly(ethylene oxide). Less ordered SAMs of HS(CH_2CH_2O)_6CH_3 were obtained upon assembly from tetrahydrofuran (THF), which RAIRS data indicated are mixtures of the 7/2 helical and non-helical conformations. Exposure of these surfaces to bovine serum albumin and lysozyme solutions showed that the less ordered HS(CH_2CH_2O)_6CH_3 SAMs assembled from THF exhibit better inhibition of protein adsorption. Thus the inhibition of protein adsorption cannot be attributed to highly ordered, helical conformations.

[L19.006] INTERACTIONS BETWEEN PEG BRUSHES IN AQUEOUS MEDIUM: SOFT SURFACE MODIFICATION FOR IMPLANTS AND BIOMATERIALS

Poly(ethylene glycol) (PEG) is a water soluble polymer compatible with the immune system of the human body. Thus it may be a good candidate to modify surfaces of artificial implants, or to serve as a soft spacer for modifying surfaces with different biomolecules. To examine this, direct measurements of the normal and shear forces between two atomically smooth mica surfaces bearing functionalised PEG immersed in pure (conductivity) water have been carried out as a function of surface separation. PEG (Mw=3.4k) that has been functionalised on both ends, was introduced into water and adsorbed only at one end onto the mica surface. A monotonically-increasing force-distance law was indicated, beginning at surface separations of ~100nm. This repulsion is shorter ranged than in the conductivity water, due to the shorter Debye length in the presence of higher ion concentration. High repulsion forces starting at ~5nm are due to the extensive compression of the grafted polymer layers. The range of the high repulsion forces (3-5nm) and the AFM images indicate that the modified PEG molecules form a uniform monolayer on the surface. Shear motion was then applied between the surfaces, at separations from some tens of nanometers down to closest approach distance, and the lubricating properties of the layers were investigated. Results, which suggests that such PEG molecules can be readily used for surface modification of implants as well as precurser for active biomolecules, will be presented at the meeting.

[L19.007] In Situ Evaluation of Thickness of Swollen Adsorbed Polymer Layers by Ellipsometry

It has often been communicated that the application of ellipsometry to the study of highly solvated, thin films is limited to the extraction of a surface concentration. This limitation is not an inherent flaw in the technique itself, but lies in the performance of the instrument. Most ellipsometers have a maximum sensitivity of 0.01 degrees in both Psi and Delta. However, by increasing the sensitivity another order of magnitude, not only can the surface concentration be resolved, but also the first moment of the concentration profile, which can then be used to provide an unambiguous measure of layer thickness, independent of a model.

We have employed a phase-modulated ellipsometer, which has a resolution of 1*10^-5 in both the real and imaginary parts of the ellipsometric ratio, to measure the adsorption of a series of diblock copolymers comprised of polystyrene and poly(4-vinylpyridine) from toluene to silicon dioxide surfaces. We have been able to track the height evolution of an adsorbing layer with thicknesses between 60 and 350 Angstroms. These thicknesses have been compared to the thicknesses determined with the surface forces apparatus (SFA), which provides a measure of the overall height of the adsorbed layer. The ellipsometrically determined thickness is approximately 3/8 the overall height, which is indicative of a diffuse profile, as has been seen for polymer brushes in good solvent conditions.

[L19.008] Differences between tethered polyelectrolyte chains adsorbed onto bare mica and hydrophobically modified mica, comparison with theory.

End-grafted polymers generated from the adsorption of asymmetric diblock copolymers on solid surface play an important role in many areas of science and technology. While the small insoluble block acts as an anchor, the charged soluble block confers useful properties to the surface. This study looks at tethered layers of poly(styrene sulfonate)/poly(t-butyl styrene) (PtBS-PSS) adsorbed on both mica (hydrophilic) and octadecyltriethoxysilane (OTE) modified mica (hydrophobic). Normal compressing forces at two different constant grafting densities (bare and modified mica) were measured with the surface force apparatus and compared with theoretical prediction. The effect of salt concentration (Cs) upon the thickness of the self-assembled layers (Lo) was measured in each case. For adsorption of diblock copolymers onto OTE the resulting scaling relationship is much closer to the brush theory, Lo~Cs^-1/3. This result suggests that the adsorbed amount on mica is not high enough to form a brush.

[L19.009] Conformation of PNIPAM grafted chains at the silicon/water interface as a function of temperature by neutron reflectivity

Poly(N-isopropyl acrylamide) (PNIPAM) is perhaps the most well known member of the class of responsive polymers. Free chains of PNIPAM in water exhibit a lower critical solution temperature at about 31 ^\circC. This very sharp transition (\sim 5 ^\circ) is attributed to the disruption of the hydrogen bonding of water molecules around the amide group of the side chain. We have investigated the conformation of grafted PNIPAM chains at the silicon/water interface using neutron reflection. Grafted PNIPAM layers were prepared by reacting COOH-terminated PNIPAM to OH-terminated self-assembled monolayers on silicon. The grafting density was varied by controlling the composition of the SAM. Reflectivity data from the protonated layers in deuterated water were obtained over a range of temperature from 10 ^\circ to 70 ^\circ using a liquid cell. To date, a range of PNIPAM molecular weights from 3K to 33K have been examined. Surprisingly, whereas the samples all have a cloud point at about 31 ^\circ in aqueous solution, we find little or no contraction of the grafted PNIPAM chains as the temperature is increased beyond this value. No variation in layer height with temperature is detected for the entire range of grafting density for the lowest molecular weights. A slight contraction is detected for the higher molecular weight samples, but only at an intermediate grafting density. Further work in ongoing to examine much higher molecular weight samples and also layers prepared using the ``grafting from" method.

[L19.010] Analysis of Force Interactions Between Comb Stabilized Colloidal Particles

Polymer chains that are chemically attached to solid surfaces have important uses in colloidal stabilization. In this investigation, atomic force colloid probe microscopy and Langmuir trough compression have been employed to observe the interactions between colloidal particles stabilized by tethered polymer chains. Monodisperse latexes stabilized by hydrophilic side chains of surface- attached comb polymers are analyzed by the above methods in a water environment. Through the modification of the comb arcitecture to incorporate both hydrophilic and hydrophobic side chains, attractive wells are introduced into the force-distance profiles, in agreement with recent self-consistent mean field predictions. A theoretical model has been developed to explain the differing behaviors of the colloids when analyzed by the above methods.

[L19.011] Dynamics in Adsorbed Homopolymer Layers: Entanglements and Osmotic Effects

This work seeks the dynamic mechanism for the exchange of homopolymer chains between a dilute solution and a layer adsorbed at the solid-liquid interface. With the model system of polyethylene oxide (PEO) adsorbed onto silica from aqueous solution, it is shown that the behavior of saturated interfaces compared to starved layers reveals an interesting trend: The characteristic self exchange time is dependent only on coverage, not molecular weight, for chains of 100K or less. Therefore, it is concluded that classical entanglements do not play a role below 100K. For all molecular weights, when the coverage of 0.2 mg/m2 is exceeded, the interfacial dynamics become slow. At lower coverages, chains lie flat in train, with no loops or tails, and no lateral interactions either. The onset of slow dynamics at higher coverages may be a result of both surface crowding and the resistance of loops and tails to new chains approaching the layer.

[L19.012] Elasticity of Polymer-Grafted Membranes

Biomembranes containing anchored water-soluble polymers can be used as effective intravenous liposome drug carriers. This is due to the fact that the anchored polymers induce steric repulsive interactions with the antibodies, thereby preventing the vesicle from antibody attack. In this talk, I will present recent results from extensive Monte Carlo simulations of a continuum model of polymers grafted to a fluctuating membrane. I will show that when the polymers are in the brush regime, the membrane acquires an excess mean bending modulus with a scaling in terms of the polymerization index and surface grafting coverage which is in good agreement with self-consistent field results and scaling arguments. Furthermore, when polymers are grafted to one side of the membrane only, we found that the induced spontaneous curvature and mean bending modulus depend as well on the intrisic mean bending modulus of the membrane. In particular, for large spontaneous curvatures, the effective bending modulus is given by \tilde\kappa=(5/2)\kappa_0, where \kappa_0 is the bending modulus of the bare membrane.

Part L of program listing