Joseph D. Andrade

Protein-surface interactions in the presence of polyethylene oxide

Abstract The protein resistance character of polyethylene oxide (PEO) chains terminally attached to a hydrophobic solid substrate is theoretically studied. Steric repulsion, van der Waals attraction, and hydrophobic interaction free energies are considered. The results are dependent on the chain length and surface density of PEO. The protein approaches the PEO surface by diffusion and is affected by the van der Waals attraction between the PEO surface and protein through water. Further approach of the protein initiates the compression of PEO chains, which induces a steric repulsion effect; an additional van der Waals attraction becomes important between the substrate and protein through the water solvated PEO layer. The van der Waals component with the substrate decreases with increasing surface density and chain length of terminally attached PEO chains. Other synthetic polymers were also studied, indicating that the protein resistance character is related to the refractive index, with PEO having the lowest refractive index of the common water-soluble synthetic polymers. The osmotic and elastic constants of steric repulsion for terminally attached PEO were estimated as ∼0.007 and 0.02, respectively, from literature data for PEO adsorbed to mica. The steric repulsion free energy and the combined steric repulsion and hydrophobic interaction free energies were calculated as a function of surface density and chain length of PEO. The free energy calculations as a function of surface density and chain length of PEO reveal that a high surface density and long chain length of terminally attached PEO should exhibit optimal protein resistance, with the attainment of high surface density of PEO being more important than long chain length. These theoretical results should be helpful in the design and development of materials resistant to protein adsorption.

Protein-surface interactions in the presence of polyethylene oxide. II, Effect of protein size

Abstract Polyethylene oxide (PEO) surfaces exhibit low protein adsorption. PEO surface—protein interactions are examined theoretically as a function of surface density and chain length of PEO and variation in the size of the protein (assumed to be a sphere). Recent studies suggest that the PEO surface may have a small hydrophobic character. We study the effect of surface density of PEO and protein size and deduce the PEO surface density conditions for optimal protein resistance. For small proteins ( R ∼ 20 A ), D should be small (∼ 10 A), while for large proteins ( R ∼ 60–80 A ), D should be larger (∼ 15 A), where R is the protein radius and D is the average distance between end-attached PEO chains. These results evolve from the trade-offs between steric repulsion and the assumed weak hydrophobic interaction between the PEO layer and the protein. The longest chain length of PEO at optimum surface density appears best for protein resistance. As a number of assumptions and estimates are involved in the model, the results can be taken only as qualitative trends at this time. The trends should be helpful in the design and evaluation of surfaces resistant to protein adsorption.

Flat plate streaming potential investigations: Hydrodynamics and electrokinetic equivalency

Abstract The accurate measurement of streaming potentials in either capillaries or flat plate systems requires Poiseuille flow, i.e., flow must be steady, incompressible, laminar, and established. The established flow stipulation is rarely addressed yet it is of critical importance. Our findings suggest that while the onset of turbulence causes no abrupt change in the streaming potential, flow must be established throughout at least 90% of the flow field for accurate streaming potential measurement. The development of a flat plate flow system based on (1 × 25 × 75) mm plates is discussed in light of the hydrodynamic requirements. The electrokinetic equivalency between plates and capillaries of the same material is discussed and the small discrepancy is attributed to surface roughness and possible differences in surface chemical composition. The flat plate system offers substantial advantages over capillaries in that both surface treatments and analyses via a variety of quantitative techniques are greatly facilitated.

Surface properties of copolymers of alkyl methacrylates with, methoxy (polyethylene oxide) metiiacrylates and their application as protein-resistant coatings

New polymeric surfactants, copolymers of alkyl methacrylates with methoxy (polyethylene oxide) methacrylates, were synthesized and characterized by gel permeation chromatography. They were studied as possible means to produce polyethylene oxide-rich surfaces by a simple coating treatment on common hydrophobic medical materials. They were further studied as cleaners for the removal of proteins preadsorbed on hydrophobic surfaces. The surface properties of the copolymers such as the adsorption properties of the copolymer on a hydrophobic surface, low density polyethylene, the protein-resistant character of the prepared polyethylene oxide surfaces and the effectiveness of the copolymers for removal of proteins pre-adsorbed on the surface, were investigated by X-ray photoelectron spectroscopy and by using 125I-labelled copolymers and 125I-labelled proteins. The surface properties of the synthesized copolymers were compared with those of commercially available polyethylene oxide containing block copolymer surfactants.

Contact angles at the solid—water interface

Abstract The study of polymer—water interfaces by contact angle methods can be accomplished directly at the polymer—water interface. Using two water-immiscible liquids or a liquid and a vapor, one can deduce the dispersion and polar components of the hydrated solid surface free energy and the solid—water interfacial free energy. The theory is presented and a numerical analysis procedure is developed to solve the equations in the general case. The special case of n -octane and air is also presented. Data and results are given for poly(hydroxyethyl methacrylate-methoxyethyl methacrylate) copolymers of varying composition and equilibrium water contents. The results show that the hydrophilic component dominates the polymer—water interfacial properties, even at relatively low hydrophilic component compositions. The method presented should be useful for the study of polymer—water interfaces, particularly for hydratable or mobile polymers which can reorient to equilibrate differently with a water environment than with the air or vapor environment commonly used in contact angle studies.

Polymer Surface Dynamics

Classical surface chemistry assumes that solid surfaces are rigid, immobile, and at equilibrium. These assumptions allow one to probe adsorption and wetting or contact angle processes purely from the point of view of the liquid phase, because one assumes that the solid phase does not in any way respond, reorient, or otherwise change in the different liquid environments. Although such assumptions may be partially correct for truly rigid solids, they are generally inappropriate for polymers (see also Chapter 7).

Nature of water in synthetic hydrogels. I. Dilatometry, specific conductivity, and differential scanning calorimetry of polyhydroxyethyl methacrylate

Abstract Three classes of water may exist in certain hydrogels. We have previously labeled these as X water (bulk water), Z water (bound water) and Y water (intermediate forms we call interfacial water). Bulk gel conductivity data for poly (2-hydroxyethyl methacrylate) (PHEMA) were obtained. The activation energy for specific conduction was calculated. A plot of the activation energy versus wt percent of water in the gel clearly indicated three different zones, showing three possible classes of water in the gels. These results were confirmed by thermal expansion measurements. The high water content gels (50%) demonstrated an extremely sharp volume change at 0°C, indicating the presence of normal bulk water. Lower water content gels (20%) showed no anomalous change in thermal expansion, indicating that the water is bound. The medium water content gels exhibited intermediate behavior. A semiquantitative analysis of the three classes of water is presented. A further verification of these results was obtained by differential scanning calorimetry (DSC) studies. The low water content gel (20%) consists mainly of bound water, which exhibited no phase transitions over the range −15 to 24°C. The high water content gels showed phase transitions near 0°C. The medium water content gels show gradual shifts of the phase transition temperatures near 0°C.

Remote fiber-optic biosensors based on evanescent-excited fluoro-immunoassay: Concept and progress

A homogeneous silica optical fiber with its coating and cladding removed will interact with its surroundings via evanescent-wave modes at the interface. Antibody or antigen molecules can be covalently linked to the silica-fiber surface. The immobilized biomolecules will bind complementary antigen or antibody from a surrounding solution. If the bound antigen or antibody is fluorescent, a fluoro-immunoassay can be performed. The sensitivity of such a sensor is enhanced in a competitive immunoassay mode, where a fluorescently labeled antigen or antibody competes for the binding sites on the immobilized bio-molecules, thereby providing a competitive binding fluoro-immunoassay. Such sensors have the potential for remote unattended pseudocontinuous monitoring of biomolecules.

The Contact Angle and Interface Energetics

Information on the outermost few angstroms of solid surfaces is very difficult to obtain. One of the most sensitive methods known for obtaining true surface information is solid/liquid/vapor (S/L/V) or solid/liquid/liquid (S/L/L) contact angles. These methods are unique in that the equipment required is relatively simple and inexpensive. Although interpretation of the results obtained is dependent on a number of assumptions, each of which is somewhat controversial, a first-order interpretation is possible and has proven to be very useful in practically all areas of surface science and engineering. Most of the surface science texts briefly referred to in Chapter 1 contain one or more chapters on surface tension, capillarity, or contact angle methods. In addition, a number of the monographs and review serials cited in Chapter 1 also contain chapters on the contact angle technique.

X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is generally regarded as an important and key technique for the surface characterization and analysis of biomedical polymers.(1) This technique, also called ESCA (Electron Spectroscopy for Chemical Analysis), provides a total elemental analysis, except for hydrogen and helium, of the top 10–200 A (depending on the sample and instrumental conditions) of any solid surface which is vacuum stable or can be made vacuum stable by cooling. Chemical bonding information is also provided. Of all the presently available instrumental techniques for surface analysis, XPS is generally regarded as being the most quantitative, the most readily interpretable, and the most informative with regard to chemical information. For these reasons it has been highly recommended and used by biomedical researchers for the analysis of medical polymers. The basic advantages and disadvantages of the technique are given in Table 1. Although the method requires relatively sophisticated instrumentation, many universities, industrial R and D groups, and commercial service laboratories provide access to their instruments on a collaborative or fee for service basis.