George L. Grobe

The relationship between contact lens surface charge and in-vitro protein deposition levels

The adsorption of lysozyme and human serum albumin (HSA) onto hydrogel contact lenses was investigated as a function of lens surface charge. Anionic, cationic and non-ionic contact lenses were deposited using single protein solutions of identical pH and osmolarity. Protein deposition was analyzed using matrix assisted laser desorption ionization mass spectrometry (MALDI-ToF MS) and compared to a direct UV protein analysis method, the bicinchoninic acid (BCA) assay. The results showed remarkable consistency between the two techniques. By inference of results from analyses of sample solutions, lysozyme, a positively charged protein at physiological pH, was only detected on the anionic surface charged contact lenses, presumably a result of electrostatic interactions. Neither the cationic nor the non-ionic lenses deposited lysozyme, possibly due to charge repulsion. HSA, a negatively charged protein at physiological pH, was detected on the cationic lenses, again as a result of electrostatic interactions. The fact that HSA was not observed on either the anionic or non-ionic charged species further demonstrates the effect of charge repulsion.

Comparative studies of poly(dimethyl siloxanes) using automated GPC-MALDI-TOF MS and on-line GPC-ESI-TOF MS

In this study we compare on-line gel permeation chromatography (GPC) electrospray ionization (ESI) time-of-flight (TOF) mass spectrometry (MS) to automated GPC matrix assisted laser desorption ionization (MALDI) TOF MS for poly (dimethylsiloxane) (PDMS) analysis. Average mass values for a hydroxyl-terminated PDMS (OH-PDMS) sample were obtained and compared to traditional GPC that was calibrated with narrow polystyrene standards, by direct ESI and MALDI MS analysis, by a summation of mass spectra of all GPC fractions, and also by the recalibration method determined by both mass spectrometric methods. Quantitatively, the difference noted here between these hyphenated techniques is that GPC-ESI-TOF MS effectively reports the low-mass oligomers and underestimates the high-mass oligomers, while GPC-MALDI-TOF MS effectively reports the high-mass oligomers and underestimates the low-mass oligomers. In the GPC-ESI-TOF MS experiments, ion current suppression was observed in the high molecular weight region. The suppression effect was confirmed by repeatable sample runs and by injecting different PDMS samples. Higher chromatographic resolution was observed for GPC-ESI-TOF MS compared to GPC-MALDI-TOF MS. In fact, truly mono-disperse oligomers were observed in the low molecular weight range from GPC-ESI MS experiments.

Detailed analysis of α,ω-bis(4-hydroxybutyl) poly(dimethylsiloxane) using GPC-MALDI TOF mass spectrometry

In this study the prepolymer α,ω-bis(4-hydroxybutyl) poly(dimethylsiloxane), used in the formulation of oxygen permeable films, is evaluated by gel permeation chromatography (GPC) combined with matrix assisted laser desorption ionization (MALDI) time of flight (TOF) mass spectrometry (MS). Two unexpected mass distributions are observed in the mass spectra. Reaction schemes for the formation of these distributions are proposed. A solution phase trimethylsilane end group modification was performed on the prepolymer to determine whether the unexpected mass distributions occur as impurities from synthesis or as artifacts from the MS process. Evaluation of the TMS modified prepolymer indicates the unexpected mass distributions indeed occur as impurities from the synthetic procedure. Average molecular weight values are determined by traditional GPC, direct MALDI-TOF MS, and GPC-MALDI-TOF MS methods and the results are compared.

Quantitative mass spectrometry of technical polymers: a comparison of several ionization methods

The development of soft ionization methods such as matrix-assisted laser desorption/ionization (MALDI), electrospray ionization (ESI) and secondary ion mass spectrometry (SIMS) has led to an increased use of mass spectrometry in characterizing technical (synthetic) polymers. In this paper, we compare the relative performance of these three ionization methods for characterizing the molecular weights, polydispersities and quantification of relative amounts of polymer components in mixtures. Two polymers used in biomaterials, poly(dimethylsiloxane) and poly(ethylene glycol), are employed as the model polymer systems for our survey because of their well-defined molecular weights and importance as surfactants in biomaterials and because many of their surface and solution-phase properties are well understood. Matrix-assisted laser desorption/ionization can be used to examine the surface and bulk composition of biomaterials, whereas secondary ion mass spectrometry is used for examining monolayer and submonolayer coverage of polymers on surfaces and electrospray ionization is suited for examination of extractables from biomaterials. Secondary ion mass spectrometry and electrospray show discriminate behavior against ionization of higher molecular weight oligomers, especially of poly(dimethylsiloxane). Matrix-assisted laser desorption/ionization appears to exhibit the best performance for reliable molecular weight determination at higher masses and polydispersity characterization as well as for quantification of components in polymer mixtures. The results are discussed within the context of the ionization mechanisms by which each soft ionization technique operates and by the attributes of the mass analyzers (time-of-flight and Fourier transform mass spectrometers) employed.

Characterization of silicone rubber extracts using gel- permeation chromatography, matrix-assisted laser desorption/ionization mass spectrometry, electrospray ionization mass spectrometry, Fourier transform infrared spectroscopy and gas chromatography/mass spectrometry

Extracts from a silicone rubber product were characterized by gel-permeation chromatography (GPC), matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry, Fourier transform infrared spectroscopy (FT-IR), electrospray time-of-flight (ESI-ToF) mass spectrometry and gas chromatography/mass spectrometry (GC/MS). The extracted sample was determined to be a complex mixture with a molecular weight ranging from 200 to 210,000 Da as determined from GPC measurements. GPC fractions of the extract sample were further analyzed by MALDI-ToF mass spectrometry, ESI-ToF mass spectrometry, FT-IR spectroscopy and GC/MS. The extracted products were determined to be macrocyclic poly(dimethylsiloxane) oligomers. Two additives known as bis(2-ethylhexyl) adipate and bis(2-ethylhexyl) phthalate were observed in the extract.

Characterization of Solution-Cast Extracts from Cardiothane-51 ® by FT-IR and ESCA:

Extracts from Cardiothane-51 ® have been cast from solvents of varying polarity. Cardiothane-51® can be described as block polyether-polyurethane copolymer with a 10% by weight addition of polydimethylsiloxane (DMS) that is partially copolymerized and partially blended to form a copolv mer/polv blend. FT-IR has been utilized to probe bulk and near-surface bonding and composition in the solution-cast extract films. Results show that Cardiothane-51 ® is similar in bulk composition, as measured by FT-IR, to Avcothane-51®. However, these two polymers are quite different in composition over a depth of microns. Both Cardiothane-51 ® and Avcothane-51 ® have surfaces, as measured by ESCA, which are dominated by their DMS component with a significant amount of the polyether present. Results suggest that the DMS in Cardiothane-51 ® is a blended phase, because of its ease of extraction.

Characterization of Solution-Cast Extracts from Biomer® by FT-IR and ESCA

Biomer® extracts have been cast from varying polarity solvents in an attempt to control surface composition and morphology. This polymer has previously been described as a block copolymer. FT-IR has been utilized to probe bulk and surface bonding and composition in these resulting extract cast films. FT-IR data was combined with x-ray photoelectron spectroscopic measurements to provide a detailed description of the surface composition. Results will show that a polydimethysiloxane contaminant in Biomer® can be preferentially segregated to the surface as a function of the Hildebrand parameter of the solvent that Biomer® extracts are cast from. Extracts of Biomer ® as solution-cast from varying polarity solvents had many different components. This suggests that Biomer® resembles a blend of copolymerized segments with different compositions and molecular weights, with a significant number of impurities including DMS.

In-Situ Surface Modification of Contact Lens Polymers

Soft contact lenses provide vision correction (myopia, hyperopia, presbyopia and astigmatism), cosmetic alterations such as color, or can be used as a therapeutic device. It has been proposed that the biocompatibility of the lens is determined in part by the wettability of the surface in the ocular environment.1 A nonwetting lens surface will cause discomfort and the potential for increased deposits. The deposits or biofiim on a contact lens affect vision, lens properties and ocular health.2,3 Thus, morphology and the surface chemistry of a contact lens can determine the resultant behavior witnessed by a practitioner in a clinical setting. Contact lens surfaces must remain clear and wetted, provide an adequate supply of atmospheric oxygen to and adequate expulsion of carbon dioxide from the cornea, maintain normal tear fluid flow and not abrade the ocular surface or eyelids.4 As research advances toward new materials that provide significant improvements in eye physiology, more attention is being focused on contact lens surface properties. Soft contact lens materials have bulk physical properties that dictate the type of polymer to be used. These properties include water content, refractive index, elasticity, light transmittance and tear strength. Often, polymeric materials with desired bulk properties are deficient in surface properties of wettability, lubricity and tear component deposition. Since the biocompatibility of a contact lens polymer is highly dependent upon the interactions of the polymer surface with ocular tissue and tear fluid, it may be necessary to modify the surface chemistry to achieve both the desired bulk and surface properties.