Co-reporter:C. J. Radke;S. Bowers;C. Cerretani;C.-C. Peng;M. C. Lin;Y. Li;S. Shahsavarani
Industrial & Engineering Chemistry Research November 26, 2014 Volume 53(Issue 47) pp:18130-18139
Publication Date(Web):2017-2-22
DOI:10.1021/ie5030497
A novel in vivo flow evaporimeter is developed to measure human tear-evaporation rates. The flow evaporimeter relies on a well-defined flow field to the eye with known and adjustable flow rates and relative humidities, and quantitatively reproduces evaporation rates for pure water. Mass-transfer analysis of the evaporimeter data elucidates, for the first time, the resistance of the human tear-film lipid layer (TFLL) toward minimizing tear loss to the environment. A pilot study on human subjects validates the feasibility of the flow evaporimeter to obtain the tear-film evaporation rates in vivo. Resistance of the TFLL against tear evaporation is found subject specific. Our flow evaporimeter offers an accurate, safe, and convenient diagnostic tool for clinical evaluation of dry-eye-related maladies.
Co-reporter:J. Chen, N. Razdan, T. Field, D.E. Liu, P. Wolski, X. Cao, J.M. Prausnitz, C.J. Radke
Journal of Membrane Science 2017 Volume 528(Volume 528) pp:
Publication Date(Web):15 April 2017
DOI:10.1016/j.memsci.2017.01.018
•Membrane vapor extraction (MVE) recovers biosolutes from water into a non-volatile, water-insoluble solvent. In MVE, a semi-volatile aqueous solute vaporizes at the upstream side of a omniphobic membrane, diffuses as a vapor through the membrane pores, and subsequently condenses and dissolves into a high-boiling nonpolar solvent, favorable to the solute but not to water.•MVE features minimal energy requirements, non-permselective membranes, non-contact of feed and solvent, high membrane transport rates, minimal water carry-over, and high separation factors.•The MVE process is validated by extraction of 2-wt% aqueous butanol into dodecane or mesitylene at 25 or 40 °C through a Versapor®200 R omniphobic membrane housed in a laboratory-scale plate-and-frame channeled flow cell. Excellent agreement of experiment is achieved with a priori mass-transfer theory.A novel, nearly isothermal, nonselective-membrane separation process, membrane vapor extraction (MVE), efficiently recovers butanol from a dilute aqueous solution, for example, from a fermentation broth (Liu et al., 2015). In MVE, feed and solvent liquids are not in contact; they are separated by vapor. Therefore, compared to conventional extraction, MVE avoids formation of difficult-to-separate emulsions.In MVE, a semi-volatile aqueous solute (e.g., butanol) vaporizes at the upstream side of a membrane, diffuses as a vapor through the membrane pores, and subsequently condenses and dissolves into a high-boiling nonpolar solvent, favorable to the solute but not to water. Design analysis of a 1.5-m long, 30-m2 membrane-area countercurrent MVE unit for processing 2-wt% aqueous butanol by dodecane solvent at 40 °C indicates over 90% recovery of the feed butanol with essentially no water loss and with very low energy requirement (Liu et al., 2015). The separation factor is over 1500. However, the published design study gives no experimental evidence for the calculated MVE separation.Here, we present experimental data to validate the MVE process. We use an omniphobic (i.e., hydrophobic and oleophobic), 0.2-µm pore-diameter Versapor®200 R membrane (Pall Corporation, Exton, PA) housed in a 6-cm wide by 10-cm long plate-and-frame channeled flow cell with 0.8-cm gap thickness. Membrane transfer area is 28 cm2. The membrane flow cell is designed for minimal axial concentration change and is operated in the transient mode between two recirculating flow loops.2-wt% aqueous butanol is extracted into dodecane or mesitylene at 25 or 40 °C. Also, 1.5-wt% furfural is extracted into dodecane at 40 °C. Since vapor transport across the membrane contributes minimal resistance, MVE performance is governed by mass transfer through feed and solvent boundary layers. Mass-transfer coefficients are determined from the Graetz-Lévêque analysis of laminar thin-slit flow (Bird et al., 2002). Predicted extraction performance agrees well with experiment using no adjustable parameters. Consistent with the initial multistage-design analysis (Liu et al., 2015), our new bench-scale experimental results confirm that MVE is a viable separation process to recover dilute semi-volatile biosolutes from water with minimal energy requirement. Preliminary analysis of downstream solute recovery from the extract via distillation is more efficient than that for pervaporation because of insignificant water carry over through the MVE membrane.Extraction of butanol from water into a nonvolatile, water-insoluble solvent using countercurrent membrane vapor extraction (MVE). Butanol in the aqueous feed vaporizes, diffuses through the pores of an omniphobic membrane, and absorbs into the solvent.Download high-res image (184KB)Download full-size image
Co-reporter:R. Telles, W. Li, T.J. Dursch, M.C. Lin, C.J. Radke
Colloids and Surfaces A: Physicochemical and Engineering Aspects 2017 Volume 521(Volume 521) pp:
Publication Date(Web):20 May 2017
DOI:10.1016/j.colsurfa.2016.08.027
•Human-eye health, specifically dry eye, is diagnosed by inserting a Schirmer strip into the lower fornix and measuring the wetting length after 5 min.•Wicking theory is applied to predict wetting dynamics of a Schirmer strip. Agreement of theory with clinical observation is excellent.•A linear-length time regime is predicted in which the wetted strip length varies linearly in time. The slope of wetting length in this regime is directly proportion to tear production rate. A new clinical procedure is suggested to obtain quantitative lacrimal-production rates from the Schirmer tear test (STT).A Schirmer tear test (STT) is commonly used to gauge human tear production, especially when dry-eye symptoms present. In an STT, the rounded tip of a standardized paper strip is inserted into the lower fornix of the eye, and the wetted length extending out from the lower lid is recorded after 5 min of eye closure. Longer wetted lengths suggest higher tear production rates. To date, however, there is no methodology to transform STT transient wetting lengths into basal tear- production rates. We develop a physical model to elucidate wetting kinetics in a Schirmer strip. Tear evaporation from the exposed portion of the strip and gravity are accounted for. Careful consideration of the initial depletion of tear in the closed-eye tear prism reveals an initial fast increase in wetted length followed by slower growth. Excellent agreement of the proposed model is achieved with experimental observation. When evaporation is negligible, the slow-growth regime exhibits a linear increase of wetted length in time. The linear-length-growth time regime permits simple calculation of quantitative tear-production rates. We suggest measuring several dynamic wetting lengths along a sheathed Schirmer strip and near the 5-min insertion duration followed by fitting to a straight line. The slope of the length-versus-time data gives the basal lacrimal-supply rate.Download high-res image (115KB)Download full-size image
Co-reporter:D.E. Liu, T.J. Dursch, N.O. Taylor, S.Y. Chan, D.T. Bregante, C.J. Radke
Journal of Controlled Release 2016 Volume 239() pp:242-248
Publication Date(Web):10 October 2016
DOI:10.1016/j.jconrel.2016.08.025
We measure and, for the first time, theoretically predict four prototypical aqueous-drug diffusion coefficients in five soft-contact-lens material hydrogels where solute-specific adsorption is pronounced. Two-photon fluorescence confocal microscopy and UV/Vis-absorption spectrophotometry assess transient solute concentration profiles and concentration histories, respectively. Diffusion coefficients are obtained for acetazolamide, riboflavin, sodium fluorescein, and theophylline in 2-hydroxyethyl methacrylate/methacrylic acid (HEMA/MAA) copolymer hydrogels as functions of composition, equilibrium water content (30–90%), and aqueous pH (2 and 7.4). At pH 2, MAA chains are nonionic, whereas at pH 7.4, MAA chains are anionic (pKa ≈ 5.2). All studied prototypical drugs specifically interact with HEMA and nonionic MAA (at pH 2) moieties. Conversely, none of the prototypical drugs adsorb specifically to anionic MAA (at pH 7.4) chains. As expected, diffusivities of adsorbing solutes are significantly diminished by specific interactions with hydrogel strands. Despite similar solute size, relative diffusion coefficients in the hydrogels span several orders of magnitude because of varying degrees of solute interactions with hydrogel-polymer chains. To provide a theoretical framework for the new diffusion data, we apply an effective-medium model extended for solute-specific interactions with hydrogel copolymer strands. Sorptive-diffusion kinetics is successfully described by local equilibrium and Henry's law. All necessary parameters are determined independently. Predicted diffusivities are in good agreement with experiment.
Co-reporter:C.J. Radke
Advances in Colloid and Interface Science 2015 Volume 222() pp:600-614
Publication Date(Web):August 2015
DOI:10.1016/j.cis.2014.01.001
Highlights
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Gibbs adsorption equation (GAE) is a cornerstone of surface science, yet sometimes controversial.
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Invariants are the key to understanding the Gibbs adsorption equation.
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The Guggenheim finite-volume surface phase and the Gibbs zero-volume surface phase give identical results for the GAE.
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Application is made to electrified interfaces, surface complexation, molecular thermodynamics, and molecular simulation.
Co-reporter:T.J. Dursch, D.E. Liu, Y. Oh, C.J. Radke
Acta Biomaterialia 2015 Volume 15() pp:48-54
Publication Date(Web):15 March 2015
DOI:10.1016/j.actbio.2014.11.046
Abstract
Partitioning of aqueous packaging, wetting, and care-solution agents into and out of soft contact lenses (SCLs) is important for improving wear comfort and also for characterizing lens physico-chemical properties. We illustrate both features of partitioning by application of fluorescent-solute partitioning into DAILIES TOTAL1® (delefilcon A) water-gradient SCLs, which exhibit a layered structure of a silicone–hydrogel (SiHy) core sandwiched between thin surface-gel layers. Two-photon fluorescence confocal laser-scanning microscopy and attenuated total-reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) characterize the lens and assess uptake profiles of six prototypical fluorescent solutes. Comparison of solute uptake in a SiHy-core prototype lens (i.e., O2OPTIXTM) validates the core SiHy structure of DAILIESTOTAL1®. To establish surface-layer charge, partition coefficients and water contents are obtained for aqueous pH values of 4 and 7.4. Solute fluorescence-intensity profiles clearly confirm a layered structure for the DAILIES TOTAL1® lenses. In all cases, aqueous solute partition coefficients are greater in the surface layers than in the SiHy core, signifying higher water in the surface gels. ATR-FTIR confirms surface-layer mass water contents of 82 ± 3%. Water uptake and hydrophilic-solute uptake at pH 4 compared with that at pH 7.4 reveal that the surface-gel layers are anionic at physiologic pH 7.4, whereas both the SiHy core and O2OPTIX™ (lotrafilcon B) are nonionic. We successfully confirm the layered structure of DAILIES TOTAL1®, consisting of an 80-μm-thick SiHy core surrounded by 10-μm-thick polyelectrolyte surface-gel layers of significantly greater water content and aqueous solute uptake compared with the core. Accordingly, fluorescent-solute partitioning in SCLs provides information on gel structure and composition, in addition to quantifying uptake and release amounts and rates.
Co-reporter:D.E. Liu, T.J. Dursch, Y. Oh, D.T. Bregante, S.Y. Chan, C.J. Radke
Acta Biomaterialia 2015 Volume 18() pp:112-117
Publication Date(Web):May 2015
DOI:10.1016/j.actbio.2015.02.019
Abstract
Equilibrium water content of and solute partitioning in silicone hydrogels (SiHys) are investigated using gravimetric analysis, fluorescence confocal laser-scanning microscopy (FCLSM), and back extraction with UV/Vis-absorption spectrophotometry. Synthesized silicone hydrogels consist of silicone monomer, hydrophilic monomer, cross-linking agent, and triblock-copolymer macromer used as an amphiphilic compatibilizer to prevent macrophase separation. In all cases, immiscibility of the silicone and hydrophilic polymers results in microphase-separated morphologies. To investigate solute uptake in each of the SiHy microphases, equilibrium partition coefficients are obtained for two hydrophilic solutes (i.e., theophylline and caffeine dissolved in aqueous phosphate-buffered saline) and two oleophilic solutes (i.e., Nile Red and Bodipy Green dissolved in silicone oil), respectively. Measured water contents and aqueous-solute partition coefficients increase linearly with increasing solvent-free hydrophilic-polymer volume fraction. Conversely, oleophilic-solute partition coefficients decrease linearly with rising solvent-free hydrophilic-polymer volume fraction (i.e., decreasing hydrophobic silicone-polymer fraction). We quantitatively predict equilibrium SiHy water and solute uptake assuming that water and aqueous solutes reside only in hydrophilic microdomains, whereas oleophilic solutes partition predominately into silicone microdomains. Predicted water contents and solute partition coefficients are in excellent agreement with experiment. Our new procedure permits a priori estimation of SiHy water contents and solute partition coefficients based solely on properties of silicone and hydrophilic homopolymer hydrogels, eliminating the need for further mixed-polymer-hydrogel experiments.
Co-reporter:Cheng-Chun Peng, Colin Cerretani, Richard J. Braun, C.J. Radke
Advances in Colloid and Interface Science 2014 Volume 206() pp:250-264
Publication Date(Web):April 2014
DOI:10.1016/j.cis.2013.06.001
Highlights
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Lipid-layer instabilities lead to evaporation-driven breakup of the human tear film.
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A physically consistent explanation is provided for tear-film breakup.
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Environmental conditions strongly affect tear-film stability.
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Locally elevated evaporation leads to hyperosmolar spots in the tear film.
Co-reporter:Thomas J. Dursch, Nicole O. Taylor, David E. Liu, Rong Y. Wu, John M. Prausnitz, Clayton J. Radke
Biomaterials 2014 35(2) pp: 620-629
Publication Date(Web):
DOI:10.1016/j.biomaterials.2013.09.109
Co-reporter:Colin F. Cerretani, Nghia H. Ho, C.J. Radke
Advances in Colloid and Interface Science 2013 Volumes 197–198() pp:33-57
Publication Date(Web):September 2013
DOI:10.1016/j.cis.2013.03.007
Highlights
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Duplex films reduce water evaporation by a dissolution-diffusion mechanism.
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Uniform duplex films are necessary for effective evaporation reduction.
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Temperature, humidity, and airflow affect evaporation rates.
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100-nm layers of meibum do not reduce water evaporation significantly in vitro.
Co-reporter:D. E. Liu, C. Kotsmar, F. Nguyen, T. Sells, N. O. Taylor, J. M. Prausnitz, and C. J. Radke
Industrial & Engineering Chemistry Research 2013 Volume 52(Issue 50) pp:18109
Publication Date(Web):November 14, 2013
DOI:10.1021/ie402148u
Transient solute absorption and desorption concentration profiles were measured in a 70 wt % hydroxyethyl methacrylate (HEMA)/30 wt % methacrylic acid (MAA) anionic hydrogel using two-photon confocal microscopy. Dilute aqueous solutes included fluorescently labeled dextrans with molecular masses of 4, 10, and 20 kDa, and fluorescently labeled cationic avidin protein. Cross-linking densities with ethylene glycol dimethacrylate (EGDMA) varied from 0 to 1 wt % with polymer volume fractions increasing from 0.15 to 0.25. Average gel mesh sizes, determined from zero-frequency oscillatory shear storage moduli, ranged from about 3.6 to 8.4 nm over the cross-link ratios studied. All solutes exhibit Stokes–Einstein hydrodynamic radii obtained from measured free diffusion coefficients, Do, comparable to or larger than the average gel mesh size. In spite of considerable size exclusion, the studied solutes penetrate the gels indicating a range of mesh sizes available for transport. Transient uptake and release concentration profiles for FITC-dextrans follow simple diffusion theory with diffusion coefficients, D, essentially independent of loading or release characteristic of reversible absorption. Although strongly size-excluded, these solutes do not interact specifically with the polymer network. Diffusivities are accordingly predicted from a large-pore effective-medium (LPEM) model developed to account for solute size, hydrodynamic drag, and distribution of mesh sizes available for transport in the polymer network. For this class of solute, and using no adjustable parameters, diffusivities predicted from the new effective-medium model demonstrate good agreement with experiment. For the specific-interacting cationic protein, avidin, gel loading is 3 orders of magnitude slower than that of dextran of similar hydrodynamic radius. Desorption of avidin is not complete even after 2 weeks of extraction. On the basis of size alone, avidin is strongly size-excluded, yet it exhibits a partition coefficient of over 20. For the positively charged protein, we observed specific ion binding on the negatively charged carboxylate groups of MAA-decorated polymer strands in the larger mesh spaces. Simple linear sorption kinetics gives an adsorption time constant of 5 min and a desorption time constant of about 20 days, suggesting nearly irreversible uptake of cationic avidin on the anionic gel matrix.
Co-reporter:S. A. Maurer, C. N. Bedbrook, and C. J. Radke
Industrial & Engineering Chemistry Research 2012 Volume 51(Issue 35) pp:11389
Publication Date(Web):July 30, 2012
DOI:10.1021/ie3008538
Enzymatic deconstruction of cellulose occurs at the aqueous/cellulose interface. Most assays to explore cellulase activity, however, are performed in bulk solution and, hence, fail to elucidate surface-reaction kinetics. We use flow ellipsometry to quantify the adsorption and surface reactivity of aqueous cellulase on a model cellulose film substrate. The rate of cellulose digestion at the aqueous/solid interface increases with increasing bulk concentration of enzyme, but only up to a plateau corresponding to the maximum adsorption density of cellulase. Kinetic data are analyzed according to a modified Langmuir–Michaelis–Menten framework including both reversible adsorption of cellulase to the cellulose surface and complexation of surface cellulose chains with adsorbed cellulase. At ambient temperature, the molar turnover number is 0.57 ± 0.08 s–1, commensurate with literature values, and the Langmuir adsorption equilibrium constant, characterizing the binding strength of the cellulase, is 0.086 ± 0.026 ppm–1. The rate-determining step in the surface-reaction sequence is complexation of adsorbed cellulase with the solid-cellulose surface. Simultaneous knowledge of sorption and digestion kinetics is necessary to quantify cellulose deconstruction.
Co-reporter:T. J. Dursch, M. A. Ciontea, C. J. Radke, and A. Z. Weber
Langmuir 2012 Volume 28(Issue 2) pp:1222-1234
Publication Date(Web):December 1, 2011
DOI:10.1021/la2033737
Nucleation and growth of ice in the fibrous gas-diffusion layer (GDL) of a proton-exchange membrane fuel cell (PEMFC) are investigated using isothermal differential scanning calorimetry (DSC). Isothermal crystallization rates and pseudo-steady-state nucleation rates are obtained as a function of subcooling from heat-flow and induction-time measurements. Kinetics of ice nucleation and growth are studied at two polytetrafluoroethylene (PTFE) loadings (0 and 10 wt %) in a commercial GDL for temperatures between 240 and 273 K. A nonlinear ice-crystallization rate expression is developed using Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory, in which the heat-transfer-limited growth rate is determined from the moving-boundary Stefan problem. Induction times follow a Poisson distribution and increase upon addition of PTFE, indicating that nucleation occurs more slowly on a hydrophobic fiber than on a hydrophilic fiber. The determined nucleation rates and induction times follow expected trends from classical nucleation theory. A validated rate expression is now available for predicting ice-crystallization kinetics in GDLs.
Co-reporter:Csaba Kotsmar, Teresa Sells, Nicole Taylor, David E. Liu, J. M. Prausnitz, and C. J. Radke
Macromolecules 2012 Volume 45(Issue 22) pp:9177-9187
Publication Date(Web):November 9, 2012
DOI:10.1021/ma3018487
Using two-photon confocal microscopy, equilibrium partition coefficients, k, were measured for aqueous Na-fluorescein, fluorescently labeled dextrans with molecular masses ranging from 4 to 20 kDa, two fluorescently labeled proteins with opposite charges, anionic bovine serum albumin (BSA), and cationic avidin in anionic 70 wt % hydroxyethyl methacrylate (HEMA)/30 wt % methacrylic acid (MAA) gels saturated with aqueous phosphate buffer solution. Cross-linking density with ethylene glycol–dimethacrylate (EGDMA) ranged from 0 to 1 wt %. All partition coefficients, except for avidin, were considerably less than unity and diminished strongly with increasing Stokes–Einstein diameter of the free aqueous solute. The average mesh size of the wet gels, obtained from the zero-frequency oscillatory shear-storage gel modulus, ranged from 3.6 to 8.3 nm over the cross-link ratios studied. Except for Na-fluorescein, solute hydrodynamic diameters were larger than the smallest average gel mesh size. Yet, all solutes permeated the gels but with small partition coefficients less than about 0.001 for the largest diameter solutes in the small mesh size gels. To express deviation from ideal partitioning, we define an enhancement (or exclusion) factor, E ≡ k/(1 – φ), where φ is the polymer volume fraction in the gel and E is unity for point solutes. A hard-sphere excluded-volume Ogston mesh size distribution is adopted to predict a priori the measured enhancement factors as a function of average gel mesh size for those solutes that do not interact specifically with the anionic gel (i.e., for solutes with E < 1). Agreement between the extended Ogston distribution and experiment is qualitative for both enhancement factors and water content of the gels. The cationic protein, Fl-avidin, exhibits a large enhancement factor in the anionic gels due to strong specific interaction with the charged carboxylate groups of MAA. In this case, consideration must be given to both hard-sphere size exclusion and specific complexation with the polymer strands.
Co-reporter:Samuel A. Maurer, Claire N. Bedbrook, and Clayton J. Radke
Langmuir 2012 Volume 28(Issue 41) pp:14598-14608
Publication Date(Web):September 11, 2012
DOI:10.1021/la3024524
For the first time, the competitive adsorption of inhibited cellobiohydrolase I (Cel7A, an exoglucanase) and endoglucanase I (Cel7B) from T. longibrachiatum is studied on cellulose. Using quartz crystal microgravimetry (QCM), sorption histories are measured for individual types of cellulases and their mixtures adsorbing to and desorbing from a model cellulose surface. We find that Cel7A has a higher adsorptive affinity for cellulose than does Cel7B. The adsorption of both cellulases becomes irreversible on time scales of 30–60 min, which are much shorter than those typically used for industrial cellulose hydrolysis. A multicomponent Langmuir kinetic model including first-order irreversible binding is proposed. Although adsorption and desorption rate constants differ between the two enzymes, the rate at which each surface enzyme irreversibly binds is identical. Because of the higher affinity of Cel7A for the cellulose surface, when Cel7A and Cel7B compete for surface sites, a significantly higher bulk concentration of Cel7B is required to achieve comparable surface enzyme concentrations. Because cellulose deconstruction benefits significantly from the cooperative activity of endoglucanases and cellobiohydrolases on the cellulose surface, accounting for competitive adsorption is crucial to developing effective cellulase mixtures.
Co-reporter:Victoria B. Tran, Ye Suel Sung, Suzanne M.J. Fleiszig, David J. Evans, C.J. Radke
Journal of Colloid and Interface Science 2011 Volume 362(Issue 1) pp:58-66
Publication Date(Web):1 October 2011
DOI:10.1016/j.jcis.2011.06.012
Binding of bacteria to solid surfaces is complex with many aspects incompletely understood. We investigate Pseudomonas aeruginosa uptake kinetics onto hydrogel surfaces representative of soft-contact lenses made of nonionic poly(2-hydroxyethylmethacrylate) (p-HEMA), anionic poly(methacrylic acid) (p-MAA), and anionic poly(acrylic acid) (p-AA). Using a parallel-plate flow cell under phase-contrast microscopy, we document a kinetic “burst” at the anionic hydrogel surface: dilute aqueous P. aeruginosa first rapidly accumulates and then rapidly depletes. Upon continuing flow, divalent cations in the suspending solution sorb into the hydrogel network causing the previously surface-accumulated bacteria to desorb. The number of bacteria eventually bound to the surface is low compared to the nonionic p-HEMA hydrogel. We propose that the kinetic burst is due to reversible divalent-cation bridging between the anionic bacteria and the negatively charged hydrogel surface. The number of surface bridging sites diminishes as divalent cations impregnate into and collapse the gel. P. aeruginosa association with the surface then falls. Low eventual binding of P. aeruginosa to the anionic hydrogel is ascribed to increased surface hydrophilicity compared to the counterpart nonionic p-HEMA hydrogel.Graphical abstractPAK bacteria exhibit an accumulation burst on an anionic hydrogel surface when immersed in: (a) aqueous MgCl2 and (b) aqueous CaCl2.Highlights► PAK strain of Pseudomonas aeruginosa exhibits burst association on anionic hydrogel membranes. ► Bursting arises from reversible aqueous divalent-cation bridging. ► Divalent cations collapse the anionic gel and destroy surface binding sites for the bacteria. ► The few strongly bound bacteria on the gel are likely due to increased hydrophilicity of the anionic gel surface. ► This study emphasizes a delicate balance of physical and chemical forces between bacteria and substrate surfaces.
Co-reporter:L. Guan;M. E. González Jiménez;C. Walowski;A. Boushehri;J. M. Prausnitz;C. J. Radke
Journal of Applied Polymer Science 2011 Volume 122( Issue 3) pp:1457-1471
Publication Date(Web):
DOI:10.1002/app.33336
Abstract
Transport of physiologic saline through soft contact lenses is important to on-eye behavior. Using a specially designed Stokes-diaphragm cell, we measure aqueous NaCl permeabilities through commercial soft contact lenses at 35°C. The permeabilities increase exponentially with the water content of the lenses spanning a range from 10−7 to 10−5 cm2/s. Equilibrium partition coefficients are obtained by the back-extraction of lenses initially immersed in 1M aqueous NaCl. Partition coefficients also increase with lens water content but over a smaller range, from 0.1 to 0.7. Because the partition coefficient values are smaller than the water content of the lenses, ideal theory is not followed. Donnan exclusion, bound water, and excluded volume are proposed explanations. The diffusion coefficients of aqueous NaCl through soft contact lenses increase with increasing lens water content following free-volume theory. Aqueous NaCl diffusivities in the lower water-content lenses are smaller than the diffusion coefficient of NaCl in water by factors up to 100 indicating very tortuous diffusion paths. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
Co-reporter:Ali Boushehri, Darren Tang, K.J. Shieh, John Prausnitz, C.J. Radke
Journal of Membrane Science 2010 Volume 362(1–2) pp:529-534
Publication Date(Web):15 October 2010
DOI:10.1016/j.memsci.2010.07.003
On-eye movement of commercial soft contact lenses (SCL) is crucial to the health of the cornea. Comfort and safety of a SCL lens depends on both the water content of and the water flux through the lens membrane. To acquire SCL water-permeability data, a newly designed fan-evaporation cell (FEC) is constructed. The fan-evaporation cell uses a more simple design compared to the previously used vacuum-evaporation cell for flat membranes [16]. The new cell accommodates both commercial SCLs and flat-sheet membranes. After correcting for membrane thickness, measured water fluxes in the FEC agree with those obtained in the vacuum-evaporation cell (VEC) validating the new FEC. Ambient-temperature (23.5 °C) and on-eye (35 °C) gradient-driven water fluxes are reported for 10 commercial SCLs including both HEMA and silicone-hydrogel materials. For relative humidities less than about 75%, effective Fickian diffusivities of water in the hydrogels are about 3 × 10−7 cm2/s, essentially independent of lens material, saturated water content, and temperature. The effect of salt, mucin, and lysozyme on water-transport rates is minimal through Biomedics® 38, PureVision™, and Focus® DAILIES® lenses. However, lysozyme significantly reduces water flux through Acuvue 2 lenses.
Co-reporter:V. A. Andreev, J. M. Prausnitz, and C. J. Radke
Industrial & Engineering Chemistry Research 2010 Volume 49(Issue 24) pp:12461-12470
Publication Date(Web):October 27, 2010
DOI:10.1021/ie1012954
New experimental data collected at Lam Research Corporation and theoretical analyses are presented for aqueous-foam cleaning of silicon wafers contaminated with strongly adhered 90 nm Si3N4 particles (Freer et al. 2010). We analyze the distribution of contaminant removal along the wafer surface and the influence of foam quality in a vertical rectangular slot upon wafer immersion/withdrawal. At zero foam quality, particle removal along the wafer surface is uniform. Increased foam quality leads to improved overall removal. Removal, however, is no longer uniform with larger detachment rates toward the bottom of the wafer. To explain the observed nonuniform particle removal, we adopt a binary-collision model that demands a linear dependence of removal rate on the surface shear rate. Perturbation analysis provides the distribution of the wall shear rate along the wafer surface in an unfoamed solution. Calculations show that the wall shear rate on the wafer surface is strongly peaked in the meniscus just above the liquid-filled slot. Thus, with no foam present, removal in the meniscus zone dominates the overall removal process. Because the time of exposure to this high shear is the same for all parts of the surface, we obtain uniform cleaning. With foam bubbles present, the wall shear rate in the slot is enhanced, leading to significant removal in the bulk of the slot. Because the residence time of a wafer in the bulk cleaning solution varies for different parts of the wafer, contaminant removal in the bulk of the slot depends on the vertical position. Combined particle removal in the meniscus zone and in the slot leads to the observed nonuniform distribution of contaminant particles remaining on the wafer surface. Increasing foam quality increases the slot wall shear rate and, hence, the removal rate inside the immersion/withdrawal cell.
Co-reporter:Mahendra Chhabra;John M. Prausnitz;C. J. Radke
Journal of Biomedical Materials Research Part B: Applied Biomaterials 2009 Volume 90B( Issue 1) pp:202-209
Publication Date(Web):
DOI:10.1002/jbm.b.31274
Abstract
The rate of oxygen consumption is an important parameter to assess the physiology of the human cornea. Metabolism of oxygen in the cornea is influenced by contact-lens-induced hypoxia, diseases such as diabetes, surgery, and drug treatment. Therefore, estimation of in vivo corneal oxygen-consumption rate is essential for gauging adequate oxygen supply to the cornea. Phosphorescence quenching of a dye coated on the posterior of a soft contact lens provides a powerful technique to measure tear-film oxygen tension (Harvitt and Bonanno, Invest Ophthalmol Vis Sci 1996;37:1026–1036; Bonanno et al., Invest Ophthalmol Vis Sci 2002;43:371–376). Unfortunately, previous work in establishing oxygen-consumption kinetics from transient postlens tear-film oxygen tensions relies on the simplistic assumption of a constant corneal-consumption rate. A more realistic model of corneal metabolism is needed to obtain reliable oxygen-consumption kinetics. Here, physiologically relevant nonlinear Monod kinetics is adopted for describing the local oxygen-consumption rate, thus avoiding aphysical negative oxygen tensions in the cornea. We incorporate Monod kinetics in an unsteady-state reactive-diffusion model for the cornea contact-lens system to determine tear-film oxygen tension as a function of time when changing from closed-eye to open-eye condition. The model was fit to available experimental data of in vivo human postlens tear-film oxygen tension to determine the corneal oxygen-consumption rate. Reliance on corneal oxygen diffusivity and solubility data obtained from rabbits is no longer requisite. Excellent agreement is obtained between the proposed model and experiment. We calculate the spatial-averaged in vivo human maximum corneal oxygen-consumption rate as Q = 1.05 × 10−4 mL/(cm3 s). The calculated Monod constant is Km = 2.2 mmHg. © 2008 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2009
Co-reporter:Francesco Fornasiero, Florian Krull, John M. Prausnitz, Clayton J. Radke
Biomaterials 2005 Volume 26(Issue 28) pp:5704-5716
Publication Date(Web):October 2005
DOI:10.1016/j.biomaterials.2005.02.028
Water transport through soft contact lenses (SCL) is important for acceptable performance on the human eye. Chemical-potential gradient-driven diffusion rates of water through SCL materials are measured with an evaporation-cell technique. Water is evaporated from the bottom surface of a lens membrane by impinging air at controlled flow rate and humidity. The resulting weight loss of a water reservoir covering the top surface of the contact-lens material is recorded as a function of time.New results are reported for a conventional hydrogel material (SofLens™ One Day, hilafilcon A, water content at saturation w10=70w10=70 weight %) and a silicone hydrogel material (PureVision™, balafilcon A, w10=36%w10=36%), with and without surface oxygen plasma treatment. Also, previously reported data for a conventional 2-hydroxyethyl methacrylate (HEMA)-SCL (w10=38%)(w10=38%) hydrogel are reexamined and compared with those for SofLens™ One Day and PureVision™ hydrogels. Measured steady-state water fluxes are largest for SofLens™ One Day, followed by PureVision™ and HEMA. In some cases, the measured steady-state water fluxes increase with rising relative air humidity. This increase, due to an apparent mass-transfer resistance at the surface (trapping skinning), is associated with formation of a glassy skin at the air/membrane interface when the relative humidity is below 55–75%.Steady-state water fluxes are interpreted through an extended Maxwell–Stefan diffusion model for a mixture of species starkly different in size. Thermodynamic nonideality is considered through Flory–Rehner polymer-solution theory. Shrinking/swelling is self-consistently modeled by conservation of the total polymer mass. Fitted Maxwell–Stefan diffusivities increase significantly with water concentration in the contact lens.
Co-reporter:B.K. Leung, J.A. Bonanno, C.J. Radke
Progress in Retinal and Eye Research (November 2011) Volume 30(Issue 6) pp:471-492
Publication Date(Web):1 November 2011
DOI:10.1016/j.preteyeres.2011.07.001
Wear of low-oxygen-transmissible soft contact lenses swells the cornea significantly, even during open eye. Although oxygen-deficient corneal edema is well-documented, a self-consistent quantitative prediction based on the underlying metabolic reactions is not available. We present a biochemical description of the human cornea that quantifies hypoxic swelling through the coupled transport of water, salt, and respiratory metabolites. Aerobic and anaerobic consumption of glucose, as well as acidosis and pH buffering, are incorporated in a seven-layer corneal model (anterior chamber, endothelium, stroma, epithelium, postlens tear film, contact lens, and prelens tear film). Corneal swelling is predicted from coupled transport of water, dissolved salts, and especially metabolites, along with membrane-transport resistances at the endothelium and epithelium. At the endothelium, the Na+/K+ - ATPase electrogenic channel actively transports bicarbonate ion from the stroma into the anterior chamber. As captured by the Kedem–Katchalsky membrane-transport formalism, the active bicarbonate-ion flux provides the driving force for corneal fluid pump-out needed to match the leak-in tendency of the stroma. Increased lactate-ion production during hypoxia osmotically lowers the pump-out rate requiring the stroma to swell to higher water content. Concentration profiles are predicted for glucose, water, oxygen, carbon dioxide, and hydronium, lactate, bicarbonate, sodium, and chloride ions, along with electrostatic potential and pressure profiles. Although the active bicarbonate-ion pump at the endothelium drives bicarbonate into the aqueous humor, we find a net flux of bicarbonate ion into the cornea that safeguards against acidosis. For the first time, we predict corneal swelling upon soft-contact-lens wear from fundamental biophysico-chemical principles. We also successfully predict that hypertonic tear alleviates contact-lens-induced edema.
Co-reporter:Ladan L. Foose, Harvey W. Blanch, Clayton J. Radke
Journal of Biotechnology (15 October 2007) Volume 132(Issue 1) pp:32-37
Publication Date(Web):15 October 2007
DOI:10.1016/j.jbiotec.2007.07.954
Enzymatic cleavage of protein substrates at solid surfaces is important in the food and detergent industries, and in biomedical applications. Creation of a reproducible protein substrate to study surface proteolysis is difficult as protein monolayers may not necessarily provide complete coverage of the surface, and protein multilayer systems are often unstable and nonuniform. We present a method to form a reproducible, immobilized, multilayer protein substrate. A 100-nm ovalbumin protein film is spin-cast onto an amine-functionalized silicon wafer and chemically cross-linked using glutaraldehyde to create a multilayer film. This protein film is stable in the presence of non-protease components such as detergents, and can be tailored to include different proteins and their mixtures, and varying degrees of susceptibility to proteolysis. Ellipsometry was used to measure the protein-film thickness as the substrate is cleaved by the protease subtilisin Carlsberg. The decrease in film thickness over time was found to be linear, indicating the depth-homogeneity of the model substrates. Lateral-homogeneity of the substrates was corroborated by atomic force microscopy (AFM) and by the reproducibility of the ellipsometric film thickness measured across different spots on the sample substrates. AFM of the multilayer protein surface before and after exposure to enzyme suggests uniform areal surface cleavage by the protease.
Co-reporter:D.E. Liu, T.J. Dursch, N.O. Taylor, S.Y. Chan, D.T. Bregante, C.J. Radke
Journal of Controlled Release (10 March 2017) Volume 249() pp:
Publication Date(Web):10 March 2017
DOI:10.1016/j.jconrel.2017.01.033
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