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— A fluorescence quenching technique has been used to determine the absolute partition coefficients of a set of n-doxyl stearates which partition between the aqueous and lipid phases of a phospholipid dispersion, and it is shown that the values of the coefficients vary in a systematic way depending on the position of the doxyl group on the acyl chain of the fatty acid. The n-doxyl stearates quench the fluorescence of n-(9-anthroyloxy) fatty acids in lipid bilayers, and consideration of the absolute partition coefficients of the quencher and of the microviscosity of the bilayer is necessary to extract information about the proximity of the fluorophore and quencher. In addition, quenching of the fluorescent fatty acids with a quencher which is small relative to the width of the lipid bilayer allows determination of both the absolute and local partition coefficients, the latter referring to a subvolume of the bilayer within which quenching occurs. The relationship between absolute and local partition coefficients is defined for micelles and bilayers and a method for determining the relative transverse positions of the anthroyloxy rings in these structures is described.

Identification of the biomolecules that form the first adsorbed monolayer, which thus effect “interface conversion”, in competitive adsorption from multicomponent biological solutions can be challenging because of limitations in mass resolution and sensitivity of established techniques. In this study matrix-assisted laser desorption ionization (MALDI) time of flight mass spectrometry is developed and applied as a novel surface analytical method to enable analysis of adsorbed multicomponent biomolecule layers directly on the biomaterial surfaces. We show that proteins adsorbed in vivo (on human eyes) on contact lenses can be detected rapidly and conveniently by the diagnostic highly resolved mass signals recorded by MALDI mass spectrometry. This new approach allows detection of minor (and major) proteinaceous constituents of biofouled layers at levels substantially below monolayer coverage. Identification is done by comparison with molecular masses of known proteins. Specifically, it is shown that in addition to lysozyme, other low molecular weight proteins adsorb from human tear fluid onto contact lenses; these proteins had not been detected in earlier studies using other techniques. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res, 49, 36–42, 2000.

Fine elastomeric sutures intended for cardiovascular surgery can exhibit “stick-slip” behavior as they are pulled through tissue; the resulting oscillatory force can damage delicate tissue or cause sutures to snap. To eliminate this undesirable effect, sutures were surface-modified using a radiofrequency glow discharge in a vapor of either hexamethyldisiloxane or hexamethyldisilazane, to produce a thin polymeric coating on the suture. The same coatings were also deposited onto aluminized tape to facilitate their characterization by measurement of air/water contact angles and by X-ray photoelectron spectroscopy. Coatings from both monomers were found to be very hydrophobic. The hexamethyldisiloxane glow discharge coatings underwent negligible oxidation when stored in air, and thus remained stable over a shelf-life period akin to what may be required of sutures. The hexamethyldisilazane glow discharge coatings, in contrast, incorporated substantial amounts of oxygen over a 3-month period. The coatings did not measurably alter the tensile properties of the sutures. The frictional properties of coated sutures were assessed by measuring the dynamic friction between the suture and ovine myocardium. Both coatings were effective in removing the inherent stick-slip behavior of polybutester sutures in this model. The coatings remained intact after several passes and proved to be robust and efficacious under various strain regimes. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 53: 235–243, 2000

Approaches are described for the analysis of XPS signals from samples comprising thin conformal coatings on substrates with non-ideal surface topography. In particular, attention is focused on arbitrarily shaped rough or curved substrates. Relations are derived for the relative x-ray photoelectron intensities emitted from various elements in such non-ideal samples. The surface topography of arbitrarily rough samples is analysed by atomic force microscopy to provide a frequency histogram of the local slopes, which is incorporated into a global, multi-element analysis method for interpretation of the observed XPS elemental intensities. An analogous approach is used for the analysis of curved samples. Our algorithms also enable simultaneous analysis of successively coated multilayer structures. When the elemental composition of the individual layers is known, the fit procedure enables determination of the thickness values of single-layer and multilayer coatings. The range of validity of the theory is discussed. © 1997 by John Wiley & Sons, Ltd.

In the course of plasma deposition of organic-polymeric thin films, radicals are incorporated into the growing film. These radicals initiate spontaneous oxidation reactions that continue over many weeks when the plasma polymers are stored in air. The resultant changes to the composition with time were monitored by XPS for ethylene diamine plasma polymer samples in order to improve understanding of the products of the oxidative reactions. The broadness of all the peaks, and the multitude of functional groups expected to be present, precluded the obtainment of detailed compositional information by curve fitting for components, but shifts in the binding energies (BEs) of the N 1s and O 1s peaks with time provided useful evidence of compositional changes. As the oxygen content of the plasma polymer increased upon oxidation, the binding energy of the N 1s photoelectrons increased from 399.1 to 399.8 eV. Concurrently, the amount of nitrogen in relation to carbon (N/C) decreased from 0.42 to 0.34. As the nitrogen content of the plasma polymer decreased, the binding energy of the O 1s photoelectrons increased from 531.1 to 531.8 eV. The BE values and their shifts with time/compositional changes suggested that the oxidation process predominantly caused oxidation of the carbon atoms that had amine groups attached to them, leading to the formation of amide groups and perhaps also some imides.

A protocol for global analysis of x-ray photoelectron spectra of plasma-deposited polymer films is presented. These materials are difficult to analyse because of the multitude of different chemical groups present. The combination of different primary and secondary binding energy shifts results in relatively broad, featureless photoelectron peaks. The protocol is based on fitting a series of spectra obtained by monitoring the surface composition of a plasma polymer film over extended periods of time after deposition. Information obtained from this first round of curve-fitting is used in the form of additional constraints for a second round of fitting. This leads to a self-consistent procedure which is akin to a global approach to curve-fitting. To illustrate application of this method, results of a study of ann-heptylamine plasma polymer are described. The spectral changes on ageing provided clear evidence for radical-initiated oxidation reactions. These reactions generated additional radicals, which sustained the oxidation process for many months and not only led to substantial incorporation of oxygen into the material (forming a variety of carbon–oxygen functionalities) but also to conversion of most of the existing carbon–nitrogen functionalities to an oxidized form.

Treatment of Pellethane 2363-80A--a medical-grade poly(tetramethylene oxide)-based polyurethane elastomer--with 25% (w/w) hydrogen peroxide at 100 degrees C for times ranging from 24 h to 336 h led to significant decreases in ultimate tensile properties and decreases in molecular weight, both at the surface and in the bulk. IR spectral changes were similar to those observed after degradation in vivo. Differential scanning calorimetry showed that hydrogen-peroxide-induced degradation was associated with greater order in the hard domain and greater mobility in the soft domain. Studies conducted with low-molecular-weight model compounds for the hard and soft segments confirmed that methylene groups adjacent to oxygen were susceptible toward oxidation. The extent of degradation of a series of commercial polyurethanes on treatment with hydrogen peroxide (25%, 24 h, 100 degrees C) correlated well with their reported susceptibility to environmental stress cracking in vivo.

A series of novel polyetherurethaneurea elastomers based on m-TMXDI, polyether macrodiols containing 4, 6, 8, or 10 methylene groups between the ether linkages, and aliphatic and aromatic diamines was prepared by two-step solution polymerization. All the materials were thermally processable at 120–160°C except one that was solvent-castable. Mechanical properties and hydrolytic stability (at neutral pH) were found to depend on the macrodiol and chain extender. Poly(tetramethylene oxide)-based polyetherurethaneureas that were chain-extended with primary aliphatic diamines, other than 1,3-propanediamine, exhibited significantly better mechanical properties than those chain-extended with secondary aliphatic or aromatic diamines. The mechanical properties and hydrolytic stability of polyetherurethaneureas containing poly(hexa-, octa-, or decamethylene oxide) and 1,6-hexamethylenediamine were considerably better than those containing 1,2-ethylenediamine. The hydrolytic stability increased with the increasing C/O ratio of the macrodiol. With poly(hexamethylene oxide)-based materials, the phase separation between soft and hard domains was higher than that of the other materials as determined by DSC. © 1993 John Wiley & Sons, Inc.

Novel polyurethane elastomers based on 4,4′-diphenylmethanediisocyanate, 1,4-butanediol, and polyether macrodiols that have 6, 8, or 10 methylene groups between the ether oxygens were prepared by two-step solution polymerization. Their mechanical properties and degradation resistance were compared with poly(tetramethylene oxide)-based polyurethanes. The new polyurethanes were more resistant to hydrolysis and to hydrogen peroxide-induced oxidation than were the poly(tetramethylene oxide) -based materials. The poly(octamethylene oxide) -based polyurethane displayed good resistance to sodium hypochlorite. 1,2-Diaminoethane chain-extended polyurethane–ureas were also prepared using the macrodiols poly(hexamethylene oxide) and poly(octamethylene oxide). They were not thermally processable or solvent-castable. Bulk synthesis of novel polyether polyurethanes yielded, in some cases, materials with improved mechanical properties compared with their solution-polymerized counterparts. The outcome of the bulk syntheses was highly dependent, however, on reaction conditions and stoichiometry.

Hydroxy-terminated poly(alkylene oxide)s with molecular weight M n in the range 700-2400 depending on the reaction time and temperature and hydroxy functionality of 2.0 were obtained by acid-catalysed polycondensation of 1,6-hexanediol, 1,8-octanediol and 1,10-decandiol in the 150-190°C temperature range in the presence of H 2 SO 4 or Nafion-H resin.

Relations describing sedimentation equilibrium in solutions containing two macromolecular solute components are derived for the following cases: (1) two nonassociating proteins at arbitrary concentration, (2) one dilute self-associating protein in the presence of a second inert protein at arbitrary concentration, and (3) two proteins at arbitrary concentration that can associate to form a single heterocomplex of arbitrary composition. As in earlier work (R. C. Chatelier and A. P. Minton (1987) Biopolymers, 26, 507–524), the relations are obtained by using scaled particle theory to calculate the thermodynamic activity of each species present at a given radial distance in the centrifuge. The results of numerical simulations of sedimentation equilibrium are presented as the dependence of apparent molecular weights, or apparent weight-average molecular weights, upon solution composition. Semiempirical methods are presented, by means of which the weight-average molecular weights of self- and heteroassociating proteins in highly nonideal solutions may be estimated from experimental data. It is found that the semiempirical methods yield reasonably accurate estimates of the true weight-average molecular weight over a broad range of experimental conditions, providing that the partial specific volumes of two components in a heteroassociating system do not differ by more than about 0.05 mL/g.

To extract ligand-cell binding parameters from flow cytometric fluorescence data, one needs a method of converting the measured cellular fluorescence to the actual number of bound molecules producing that fluorescence. Bead standards containing a known number of fluorophores do not necessarily provide the correct conversion factor. We therefore present a method for calibrating flow cytometric bead standards. The technique uses the isoparametric analysis (Chatelier et al: EMBO J 5:1181--1186, 1986) to construct a plot of fluorescence per cell versus the number of bound ligands per cell, thus allowing a direct comparison of the quantum yields of the bead-associated fluorophore with that of the cell-bound fluorophore. The potential of this analysis is demonstrated by contrasting the fluorescent brightness of fluorescein isothiocyanate associated with thymocyte nuclei and synthetic polymer beads with that of fluoresceinated epidermal growth factor bound to A431 cells. When the standards are improperly calibrated, then the number of ligand-binding sites is incorrectly reported, and the derived Scatchard plot may exhibit an apparent positive or negative cooperativity as well as a strong dependence on cell concentration.