Bio

Bio


The properties of ultrathin polymer films are often different from their bulk counterparts. We use spin casting, Langmuir-Blodgett deposition, and surface grafting to fabricate ultrathin films in the range of 100 to 1000 Angstroms thick. Macromolecular amphiphiles are examined at the air-water interface by surface pressure, Brewster angle microscopy, and interfacial shear measurements and on solid substrates by atomic force microscopy, FTIR, and ellipsometry. A vapor-deposition-polymerization process has been developed for covalent grafting of poly(amino acids) from solid substrates. FTIR measurements permit study of secondary structures (right and left-handed alpha helices, parallel and anti-parallel beta sheets) as a function of temperature and environment.

A broadly interdisciplinary collaboration has been established with the Department of Ophthalmology in the Stanford School of Medicine. We have designed and synthesized a fully interpenetrating network of two different hydrogel materials that have properties consistent with application as a substitute for the human cornea: high water swellability up to 85%,tensile strength comparable to the cornea, high glucose permeability comparable to the cornea, and sufficient tear strength to permit suturing. We have developed a technique for surface modification with adhesion peptides that allows binding of collagen and subsequent growth of epithelial cells. Broad questions on the relationships among molecular structure, processing protocol, and biomedical device application are being pursued.

Academic Appointments


Honors & Awards


  • Principal Investigator, National Science Foundation
  • CMA Stine Award, American Institute of Chemical Engineers
  • Fellow, American Physical Society

Professional Education


  • PhD, University of Illinois (1972)

Teaching

2013-14 Courses


Publications

Journal Articles


  • Biodegradable Polyesters from Renewable Resources ANNUAL REVIEW OF CHEMICAL AND BIOMOLECULAR ENGINEERING, VOL 4 Tsui, A., Wright, Z. C., Frank, C. W. 2013; 4: 143-?

    Abstract

    Environmental concerns have led to the development of biorenewable polymers with the ambition to utilize them at an industrial scale. Poly(lactic acid) and poly(hydroxyalkanoates) are semicrystalline, biorenewable polymers that have been identified as the most promising alternatives to conventional plastics. However, both are inherently susceptible to brittleness and degradation during thermal processing; we discuss several approaches to overcome these problems to create a balance between durability and biodegradability. For example, copolymers and blends can increase ductility and the thermal-processing window. Furthermore, chain modifications (e.g., branching/crosslinking), processing techniques (fiber drawing/annealing), or additives (plasticizers/nucleating agents) can improve mechanical properties and prevent thermal degradation during processing. Finally, we examine the impacts of morphology on end-of-life degradation to complete the picture for the most common renewable polymers.

    View details for DOI 10.1146/annurev-chembioeng-061312-103323

    View details for Web of Science ID 000321740100008

    View details for PubMedID 23540287

  • Diffusion of Protein through the Human Cornea OPHTHALMIC RESEARCH Charalel, R. A., Engberg, K., Noolandi, J., Cochran, J. R., Frank, C., Ta, C. N. 2012; 48 (1): 50-55

    Abstract

    To determine the rate of diffusion of myoglobin and bovine serum albumin (BSA) through the human cornea. These small proteins have hydrodynamic diameters of approximately 4.4 and 7.2 nm, and molecular weights of 16.7 and 66 kDa, for myoglobin and BSA, respectively.Diffusion coefficients were measured using a diffusion chamber where the protein of interest and balanced salt solution were in different chambers separated by an ex vivo human cornea. Protein concentrations in the balanced salt solution chamber were measured over time. Diffusion coefficients were calculated using equations derived from Fick's law and conservation of mass in a closed system.Our experiments demonstrate that the diffusion coefficient of myoglobin is 5.5 ± 0.9 × 10(-8) cm(2)/s (n = 8; SD = 1.3 × 10(-8) cm(2)/s; 95% CI: 4.6 × 10(-8) to 6.4 × 10(-8) cm(2)/s) and the diffusion coefficient of BSA is 3.1 ± 1.0 × 10(-8) cm(2)/s (n = 8; SD = 1.4 × 10(-8) cm(2)/s; 95% CI: 2.1 × 10(-8) to 4.1 × 10(-8) cm(2)/s).Our study suggests that molecules as large as 7.2 nm may be able to passively diffuse through the human cornea. With applications in pharmacotherapy and the development of an artificial cornea, further experiments are warranted to fully understand the limits of human corneal diffusion and its clinical relevance.

    View details for DOI 10.1159/000329794

    View details for Web of Science ID 000305551100009

    View details for PubMedID 22398578

  • Protein diffusion in photopolymerized poly(ethylene glycol) hydrogel networks BIOMEDICAL MATERIALS Engberg, K., Frank, C. W. 2011; 6 (5)

    Abstract

    In this study, protein diffusion through swollen hydrogel networks prepared from end-linked poly(ethylene glycol)-diacrylate (PEG-DA) was investigated. Hydrogels were prepared via photopolymerization from PEG-DA macromonomer solutions of two molecular weights, 4600 Da and 8000 Da, with three initial solid contents: 20, 33 and 50 wt/wt% PEG. Diffusion coefficients for myoglobin traveling across the hydrogel membrane were determined for all PEG network compositions. The diffusion coefficient depended on PEG molecular weight and initial solid content, with the slowest diffusion occurring through lower molecular weight, high-solid-content networks (D(gel) = 0.16 ± 0.02 × 10(-8) cm(2) s(-1)) and the fastest diffusion occurring through higher molecular weight, low-solid-content networks (D(gel) = 11.05 ± 0.43 × 10(-8) cm(2) s(-1)). Myoglobin diffusion coefficients increased linearly with the increase of water content within the hydrogels. The permeability of three larger model proteins (horseradish peroxidase, bovine serum albumin and immunoglobulin G) through PEG(8000) hydrogel membranes was also examined, with the observation that globular molecules as large as 10.7 nm in hydrodynamic diameter can diffuse through the PEG network. Protein diffusion coefficients within the PEG hydrogels ranged from one to two orders of magnitude lower than the diffusion coefficients in free water. Network defects were determined to be a significant contributing factor to the observed protein diffusion.

    View details for DOI 10.1088/1748-6041/6/5/055006

    View details for Web of Science ID 000295035500006

    View details for PubMedID 21873762

  • Toward the development of an artificial cornea: Improved stability of interpenetrating polymer networks JOURNAL OF BIOMEDICAL MATERIALS RESEARCH PART B-APPLIED BIOMATERIALS Hartmann, L., Watanabe, K., Zheng, L. L., Kim, C., Beck, S. E., Huie, P., Noolandi, J., Cochran, J. R., Ta, C. N., Frank, C. W. 2011; 98B (1): 8-17

    Abstract

    A novel interpenetrating network (IPN) based on poly(ethylene glycol) (PEG) and poly(acrylic acid) was developed and its use as an artificial cornea was evaluated in vivo. The in vivo results of a first set of corneal inlays based on PEG-diacrylate precursor showed inflammation of the treated eyes and haze in the corneas. The insufficient biocompatibility could be correlated to poor long-term stability of the implant caused by hydrolytic degradation over time. Adapting the hydrogel chemistry by replacing hydrolysable acrylate functionalities with stable acrylamide functionalities was shown to increase the long-term stability of the resulting IPNs under hydrolytic conditions. This new set of hydrogel implants now shows increased biocompatibility in vivo. Rabbits with corneal inlay implants are healthy and have clear cornea and non-inflamed eyes for up to 6 months after implantation.

    View details for DOI 10.1002/jbm.b.31806

    View details for Web of Science ID 000291598900002

  • pH-Driven Assembly of Various Supported Lipid Platforms: A Comparative Study on Silicon Oxide and Titanium Oxide LANGMUIR Cho, N., Jackman, J. A., Liu, M., Frank, C. W. 2011; 27 (7): 3739-3748

    Abstract

    Supported lipid platforms are versatile cell membrane mimics whose structural properties can be tailored to suit the application of interest. By identifying parameters that control the self-assembly of these platforms, there is potential to develop advanced biomimetic systems that overcome the surface specificity of lipid vesicle interactions under physiological conditions. In this work, we investigated the adsorption kinetics of vesicles onto silicon and titanium oxides as a function of pH. On each substrate, a planar bilayer and a layer of intact vesicles could be self-assembled in a pH-dependent manner, demonstrating the role of surface charge density in the self-assembly process. Under acidic pH conditions where both zwitterionic lipid vesicles and the oxide films possess near-neutral electric surface charges, vesicle rupture could occur, demonstrating that the process is driven by nonelectrostatic interactions. However, we observed that the initial rupturing process is insufficient for propagating bilayer formation. The role of electrostatic interactions for propagating bilayer formation differs for the two substrates; electrostatic attraction between vesicles and the substrate is necessary for complete bilayer formation on titanium oxide but is not necessary on silicon oxide. Conversely, in the high pH regime, repulsive electrostatic interactions can result in the irreversible adsorption of intact vesicles on silicon oxide and even a reversibly adsorbed vesicle layer on titanium oxide. Together, the results show that pH is an effective tool to modulate vesicle-substrate interactions in order to create various self-assembled lipid platforms on hydrophilic substrates.

    View details for DOI 10.1021/la104348f

    View details for Web of Science ID 000288970900068

    View details for PubMedID 21366275

  • Interfacial Biocatalysis on Charged and Immobilized Substrates: The Roles of Enzyme and Substrate Surface Charge LANGMUIR Feller, B. E., Kellis, J. T., Cascao-Pereira, L. G., Robertson, C. R., Frank, C. W. 2011; 27 (1): 250-263

    Abstract

    An enzyme charge ladder was used to examine the role of electrostatic interactions involved in biocatalysis at the solid-liquid interface. The reactive substrate consisted of an immobilized bovine serum albumin (BSA) multilayer prepared using a layer-by-layer technique. The zeta potential of the BSA substrate and each enzyme variant was measured to determine the absolute charge in solution. Enzyme adsorption and the rate of substrate surface hydrolysis were monitored for the enzyme charge ladder series to provide information regarding the strength of the enzyme-substrate interaction and the rate of interfacial biocatalysis. First, each variant of the charge ladder was examined at pH 8 for various solution ionic strengths. We found that for positively charged variants the adsorption increased with the magnitude of the charge until the surface became saturated. For higher ionic strength solutions, a greater positive enzyme charge was required to induce adsorption. Interestingly, the maximum catalytic rate was not achieved at enzyme saturation but at an invariable intermediate level of adsorption for each ionic strength value. Furthermore, the maximum achievable reaction rate for the charge ladder was larger for higher ionic strength values. We propose that diffusion plays an important role in interfacial biocatalysis, and for strong enzyme-substrate interaction, the rate of diffusion is reduced, leading to a decrease in the overall reaction rate. We investigated the effect of substrate charge by varying the solution pH from 6.1 to 8.7 and by examining multiple ionic strength values for each pH. The same intermediate level of adsorption was found to maximize the overall reaction rate. However, the ionic strength response of the maximum achievable rate was clearly dependent on the pH of the experiment. We propose that this observation is not a direct effect of pH but is caused by the change in substrate surface charge induced by changing the pH. To prove this hypothesis, BSA substrates were chemically modified to reduce the magnitude of the negative charge at pH 8. Chemical modification was accomplished by the amidation of aspartic and glutamic acids to asparagine and glutamine. The ionic strength response of the chemically modified substrate was considerably different than that for the native BSA substrate at an identical pH, consistent with the trend based on substrate surface charge. Consequently, for substrates with a low net surface charge, the maximum achievable catalytic rate of the charge ladder was relatively independent of the solution ionic strength over the range examined; however, at high net substrate surface charge, the maximum rate showed a considerable ionic strength dependence.

    View details for DOI 10.1021/la103079t

    View details for Web of Science ID 000285560400034

    View details for PubMedID 21128607

  • The Role of Electrostatic Interactions in Protease Surface Diffusion and the Consequence for Interfacial Biocatalysis LANGMUIR Feller, B. E., Kellis, J. T., Cascao-Pereira, L. G., Robertson, C. R., Frank, C. W. 2010; 26 (24): 18916-18925

    Abstract

    This study examines the influence of electrostatic interactions on enzyme surface diffusion and the contribution of diffusion to interfacial biocatalysis. Surface diffusion, adsorption, and reaction were investigated on an immobilized bovine serum albumin (BSA) multilayer substrate over a range of solution ionic strength values. Interfacial charge of the enzyme and substrate surface was maintained by performing the measurements at a fixed pH; therefore, electrostatic interactions were manipulated by changing the ionic strength. The interfacial processes were investigated using a combination of techniques: fluorescence recovery after photobleaching, surface plasmon resonance, and surface plasmon fluorescence spectroscopy. We used an enzyme charge ladder with a net charge ranging from -2 to +4 with respect to the parent to systematically probe the contribution of electrostatics in interfacial enzyme biocatalysis on a charged substrate. The correlation between reaction rate and adsorption was determined for each charge variant within the ladder, each of which displayed a maximum rate at an intermediate surface concentration. Both the maximum reaction rate and adsorption value at which this maximum rate occurs increased in magnitude for the more positive variants. In addition, the specific enzyme activity increased as the level of adsorption decreased, and for the lowest adsorption values, the specific enzyme activity was enhanced compared to the trend at higher surface concentrations. At a fixed level of adsorption, the specific enzyme activity increased with positive enzyme charge; however, this effect offers diminishing returns as the enzyme becomes more highly charged. We examined the effect of electrostatic interactions on surface diffusion. As the binding affinity was reduced by increasing the solution ionic strength, thus weakening electrostatic interaction, the rate of surface diffusion increased considerably. The enhancement in specific activity achieved at the lowest adsorption values is explained by the substantial rise in surface diffusion at high ionic strength due to decreased interactions with the surface. Overall, knowledge of the electrostatic interactions can be used to control surface parameters such as surface concentration and surface diffusion, which intimately correlate with surface biocatalysis. We propose that the maximum reaction rate results from a balance between adsorption and surface diffusion. The above finding suggests enzyme engineering and process design strategies for improving interfacial biocatalysis in industrial, pharmaceutical, and food applications.

    View details for DOI 10.1021/la103080a

    View details for Web of Science ID 000285217700047

    View details for PubMedID 21080656

  • Hydrophobic nanoparticles improve permeability of cell-encapsulating poly(ethylene glycol) hydrogels while maintaining patternability PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Lee, W., Cho, N., Xiong, A., Glenn, J. S., Frank, C. W. 2010; 107 (48): 20709-20714

    Abstract

    Cell encapsulating poly(ethylene glycol) hydrogels represent a promising approach for constructing 3D cultures designed to more closely approximate in vivo tissue environment. Improved strategies are needed, however, to optimally balance hydrogel permeability to support metabolic activities of encapsulated cells, while maintaining patternability to restore key aspects of tissue architecture. Herein, we have developed one such strategy incorporating hydrophobic nanoparticles to partially induce looser cross-linking density at the particle-hydrogel interface. Strikingly, our network design significantly increased hydrogel permeability, while only minimally affecting the matrix mechanical strength or prepolymer viscosity. This structural advantage improved viability and functions of encapsulated cells and permitted micron-scale structures to control over spatial distribution of incorporated cells. We expect that this design strategy holds promise for the development of more advanced artificial tissues that can promote high levels of cell metabolic activity and recapitulate key architectural features.

    View details for DOI 10.1073/pnas.1005211107

    View details for Web of Science ID 000284762400026

    View details for PubMedID 21071674

  • Fabrication of a Planar Zwitterionic Lipid Bilayer on Titanium Oxide LANGMUIR Cho, N., Frank, C. W. 2010; 26 (20): 15706-15710

    Abstract

    There is great demand to fabricate planar phospholipid bilayers on biocompatible materials. The preferred method of forming bilayers on these substrates is the spontaneous adsorption and rupture of phospholipid vesicles. However, in the case of titanium oxide, model vesicles composed solely of zwitterionic phospholipids do not follow this self-assembly pathway under physiological conditions, prompting the use of complex bilayer materials and less-facile methods. Herein, we report a novel pH-based strategy for fabricating zwitterionic bilayers on titanium oxide in a simple and robust manner. Depending on the pH conditions under which lipid vesicles adsorb onto titanium oxide, quartz crystal microbalance-dissipation (QCM-D) monitoring demonstrated that the self-assembly pathway can in fact result in planar bilayer formation. The pH of the solution could then be adjusted to physiological levels with no effect on the mass and viscoelastic properties of the bilayer. Moreover, fluorescence recovery after photobleaching (FRAP) measurements indicated a high degree of lateral lipid diffusivity within the bilayer at physiological pH, commensurate with its role as a cell membrane mimic. Compared to existing protocols, this strategy permits the fabrication of a more diverse array of planar bilayers on titanium oxide by tuning the self-assembly pathway of lipid vesicle adsorption onto solid substrates.

    View details for DOI 10.1021/1a101523f

    View details for Web of Science ID 000282936700002

    View details for PubMedID 20857902

  • Morphology of Photopolymerized End-Linked Poly(ethylene glycol) Hydrogels by Small-Angle X-ray Scattering MACROMOLECULES Waters, D. J., Engberg, K., Parke-Houben, R., Hartmann, L., Ta, C. N., Toney, M. F., Frank, C. W. 2010; 43 (16): 6861-6870

    Abstract

    Due to the biocompatibility of poly(ethylene glycol) (PEG), PEG-based hydrogels have attracted considerable interest for use as biomaterials in tissue engineering applications. In this work, we show that PEG-based hydrogels prepared by photopolymerization of PEG macromonomers functionalized with either acrylate or acrylamide end-groups generate networks with crosslink junctions of high functionality. Although the crosslink functionality is not well controlled, the resultant networks are sufficiently well ordered to generate a distinct correlation peak in the small angle x-ray scattering (SAXS) related to the distance between crosslink junctions within the PEG network. The crosslink spacing is a useful probe of the PEG chain conformation within the hydrogel and ranges from approximately 6 to 16 nm, dependent upon both the volume fraction of polymer and the molecular weight of the PEG macromonomers. The presence of a peak in the scattering of photopolymerized PEG networks is also correlated with an enhanced compressive modulus in comparison to PEG networks reported in the literature with much lower crosslink functionality that exhibit no scattering peak. This comparison demonstrates that the method used to link together PEG macromonomers has a critical impact on both the nanoscale structure and the macroscopic properties of the resultant hydrogel network.

    View details for DOI 10.1021/ma101070s

    View details for Web of Science ID 000280855000042

    View details for PubMedID 21403767

  • Type I Collagen-Functionalized Supported Lipid Bilayer as a Cell Culture Platform BIOMACROMOLECULES Huang, C., Cho, N., Hsu, C., Tseng, P., Frank, C. W., Chang, Y. 2010; 11 (5): 1231-1240

    Abstract

    The supported phospholipid bilayer serves as an important biomimetic model for the cell membrane in both basic and applied scientific research. We have constructed a biomimetic platform based on a supported phospholipid bilayer that is functionalized with type I collagen to serve as a substrate for cell culture. To create the type I collagen-functionalized lipid bilayer assembly, a simple chemical approach was employed: lipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(glutaryl) (DP-NGPE), a carboxylic acid-functionalized phospholipid, were prepared and then fused onto an SiO(2) substrate to form a supported lipid bilayer. Subsequently, type I collagen molecules were introduced to form stable collagen-lipid conjugates via amide linkages with activated DP-NGPE lipids. The binding kinetics of the conjugation process and the resultant changes in film thickness and viscoelasticity were followed using the quartz crystal microbalance with dissipation (QCM-D) monitoring. The morphology of the conjugated collagen adlayer was investigated with atomic force microscopy (AFM). We observed that the adsorbed collagen molecules tended to self-assemble into fibrillar structures. Fluorescence recovery after photobleaching (FRAP) was utilized to estimate lateral lipid mobility, which was reduced by up to 20% after the coupling of type I collagen to the underlying lipid bilayer. As a cell culture platform, the collagen-conjugated supported lipid bilayer showed promising results. Smooth muscle cells (A10) retained normal growth behavior on the collagen-functionalized platform, unlike the bare POPC lipid bilayer and the POPC/DG-NGPE bilayer without collagen. The biomimetic functionalized lipid system presented here is a simple, yet effective approach for constructing a cell culture platform to explore the interactions between extracellular matrix components and cells.

    View details for DOI 10.1021/bm901445r

    View details for Web of Science ID 000277355800013

    View details for PubMedID 20361729

  • Surface-Initiated Vapor Deposition Polymerization of Poly (gamma-benzyl-L-glutamate): Optimization and Mechanistic Studies LANGMUIR Zheng, W., Frank, C. W. 2010; 26 (6): 3929-3941

    Abstract

    Surface-initiated vapor deposition polymerization (SI-VDP) is a very effective approach to synthesize grafted poly(amino acids). In this study, we developed an SI-VDP system with pressure and temperature control and demonstrated highly efficient surface-grafting of poly(gamma-benzyl-L-glutamate) (PBLG) on a silicon wafer at pressure 1000 times larger than those in prior reports. More importantly, we developed new methods to quantitatively investigate mechanistic details of the SI-VDP process. First, we monitored the amount of vaporized monomer and developed a VDP reaction profile (VDPRP) method to study the major monomer reservoir processes. Next, we developed a quantitative Fourier transform infrared analysis of both as-deposited PBLG and chemisorbed PBLG films in addition to ellipsometric data to evaluate the major substrate surface processes. We observed two classes of characteristic features (pulses or two peaks) of VDPRPs, which depended upon the monomer temperature, and proposed possible mechanisms. We also found that the two peaks of VDPRPs can selectively track different reservoir processes in real time. For surface processes, we proposed possible mechanisms to obtain the surface-grafted PBLG that are expected to have either high packing density with mostly alpha-helix segments or low packing density with both random coil and alpha-helix segments.

    View details for DOI 10.1021/la9032628

    View details for Web of Science ID 000275226700029

    View details for PubMedID 19961195

  • Identification of a Class of HCV Inhibitors Directed Against the Nonstructural Protein NS4B SCIENCE TRANSLATIONAL MEDICINE Cho, N., Dvory-Sobol, H., Lee, C., Cho, S., Bryson, P., Masek, M., Elazar, M., Frank, C. W., Glenn, J. S. 2010; 2 (15)

    Abstract

    New classes of drugs are needed to combat hepatitis C virus (HCV), an important worldwide cause of liver disease. We describe an activity of a key domain, an amphipathic helix we termed 4BAH2, within a specific HCV nonstructural protein, NS4B. In addition to its proposed role in viral replication, we validate 4BAH2 as essential for HCV genome replication and identify first-generation small-molecule inhibitors of 4BAH2 that specifically prevent HCV replication within cells. Mechanistic studies reveal that the inhibitors target 4BAH2 function by preventing either 4BAH2 oligomerization or 4BAH2 membrane association. 4BAH2 inhibitors represent an additional class of compounds with potential to effectively treat HCV.

    View details for DOI 10.1126/scitranslmed.3000331

    View details for Web of Science ID 000277264100002

    View details for PubMedID 20371471

  • Quartz crystal microbalance with dissipation monitoring of supported lipid bilayers on various substrates NATURE PROTOCOLS Cho, N., Frank, C. W., Kasemo, B., Hook, F. 2010; 5 (6): 1096-1106

    Abstract

    Supported lipid bilayers (SLBs) mimic biological membranes and are a versatile platform for a wide range of biophysical research fields including lipid-protein interactions, protein-protein interactions and membrane-based biosensors. The quartz crystal microbalance with dissipation monitoring (QCM-D) has had a pivotal role in understanding SLB formation on various substrates. As shown by its real-time kinetic monitoring of SLB formation, QCM-D can probe the dynamics of biomacromolecular interactions. We present a protocol for constructing zwitterionic SLBs supported on silicon oxide and titanium oxide, and discuss technical issues that need to be considered when working with charged lipid compositions. Furthermore, we explain a recently developed strategy that uses an amphipathic, alpha-helical (AH) peptide to form SLBs on gold and titanium oxide substrates. The protocols can be completed in less than 3 h.

    View details for DOI 10.1038/nprot.2010.65

    View details for Web of Science ID 000278354700012

    View details for PubMedID 20539285

  • Hindered diffusion of oligosaccharides in high strength poly(ethylene glycol)/poly(acrylic acid) interpenetrating network hydrogels: Hydrodynamic vs. obstruction models POLYMER Waters, D. J., Frank, C. W. 2009; 50 (26): 6331-6339

    Abstract

    Diffusion coefficients of small oligosaccharides within high strength poly(ethylene glycol)/poly(acrylic acid) interpenetrating network (PEG/PAA IPN) hydrogels were measured by diffusion through hydrogel slabs. The ability of hindered diffusion models previously presented in the literature to fit the experimental data is examined. A model based solely on effects due to hydrodynamics is compared to a model based solely on solute obstruction. To examine the effect of polymer volume fraction on the observed diffusion coefficients, the equilibrium volume fraction of polymer in PEG/PAA IPNs was systematically varied by changing the initial PEG polymer concentration in hydrogel precursor solutions from 20 to 50 wt./wt.%. To examine the effect of solute radius on the observed diffusion coefficients, solute radii were varied from 3.3 to 5.1 Å by measuring diffusion coefficients of glucose, a monosaccharide; maltose, a disaccharide; and maltotriose, a trisaccharide. Both the hydrodynamic and obstruction models rely on scaling relationships to predict diffusion coefficients. The proper scaling relationship for each of the hindered diffusion models is evaluated based on fits to experimental data. The scaling relationship employed is found to have a greater significance for the hydrodynamic model than the obstruction model. Regardless of the scaling relationship employed, the obstruction model provides a better fit to our experimental data than the hydrodynamic model.

    View details for DOI 10.1016/j.polymer.2009.05.034

    View details for Web of Science ID 000272654100019

    View details for PubMedID 20514136

  • The reliable targeting of specific drug release profiles by integrating arrays of different albumin-encapsulated microsphere types BIOMATERIALS Lee, W., Wiseman, M. E., Cho, N., Glenn, J. S., Frank, C. W. 2009; 30 (34): 6648-6654

    Abstract

    Biodegradable polymer microspheres have been successfully utilized as a medium for controlled protein or peptide-based drug release. Because the release kinetics has been typically controlled by modulating physical or chemical properties of the medium, these parameters must be optimized to obtain a specific release profile. However, due to the complexity of the release mechanism and the complicated interplay between various design parameters of the release medium, detailed prediction of the resulting release profile is a challenge. Herein we suggest a simple method to target specific release profiles more efficiently by integrating release profiles for an array of different microsphere types. This scheme is based on our observation that the resulting release profile from a mixture of different samples can be predicted as the linear summation of the individually measured release profiles of each sample. Hence, by employing a linear equation at each time point and formulating them as a matrix equation, we could determine how much of each microsphere type to include in a mixture in order to have a specific release profile. In accordance with this method, several targeted release profiles were successfully obtained. We expect that the proposed method will allow us to overcome limitations in controlling complicated release mechanisms so that drug delivery systems can be reliably designed to satisfy clinical demands.

    View details for DOI 10.1016/j.biomaterials.2009.08.035

    View details for Web of Science ID 000271347900010

    View details for PubMedID 19775742

  • Mechanism of an Amphipathic alpha-Helical Peptide's Antiviral Activity Involves Size-Dependent Virus Particle Lysis ACS CHEMICAL BIOLOGY Cho, N., Dvory-Sobol, H., Xiong, A., Cho, S., Frank, C. W., Glenn, J. S. 2009; 4 (12): 1061-1067

    Abstract

    The N-terminal region of the hepatitis C virus (HCV) nonstructural protein NS5A contains an amphipathic alpha-helix that is necessary and sufficient for NS5A membrane association. A synthetic peptide (AH) comprising this amphipathic helix is able to lyse lipid vesicles that serve as a model system for virus particles. Based on quartz crystal microbalance-dissipation (QCM-D) experiments, the degree of vesicle rupturing was found to be inversely related to vesicle size, with maximal activity in the size range of several medically important viruses. In order to confirm and further study vesicle rupture, dynamic light scattering (DLS) and atomic force microscopy (AFM) experiments were also performed. The size dependence of vesicle rupturing helps explain the peptide's observed effect on the infectivity of a wide range of viruses. Further, in vitro studies demonstrated that AH peptide treatment significantly decreased the infectivity of HCV particles. Thus, the AH peptide might be used to rupture HCV particles extra-corporally (for HCV prevention) and within infected individuals (for HCV therapy).

    View details for DOI 10.1021/cb900149b

    View details for Web of Science ID 000272845900010

    View details for PubMedID 19928982

  • Biocompatibility of poly(ethylene glycol)/poly(acrylic acid) interpenetrating polymer network hydrogel particles in RAW 264.7 macrophage and MG-63 osteoblast cell lines. Journal of biomedical materials research. Part A Yim, E. S., Zhao, B., Myung, D., Kourtis, L. C., Frank, C. W., Carter, D., Smith, R. L., Goodman, S. B. 2009; 91 (3): 894-902

    Abstract

    Hydrogel polymers comprise a novel category of synthetic materials being investigated for use in cartilage replacement. One candidate compound, a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network (IPN), was developed for use in corneal prostheses and was recently engineered for potential orthopedic use. The current study examined the effects of particles of this compound on two cell lines (MG-63 osteoblast-like cells and RAW 264.7 macrophages) over a 48-h time course. To mimic the effects of wear debris, particles of the compound were generated and introduced to the cells. In the MG-63 cell line, the particles had no significant effect on cell viability measured by PicoGreen assay and trypan blue exclusion. In contrast, a significant decrease in cell viability was detected in the Raw 264.7 macrophage cells at the final timepoint with the highest concentration of hydrogel (3.0% v:v). A concentration- and time-dependent increase in TNF-alpha release characteristic of other known biocompatible materials was also detected in RAW 264.7 cells, but nitric oxide and interleukin (IL)-1beta showed no response. In addition, the MG-63 cell line demonstrated no IL-6 response. Particles of the PEG/PAA IPN thus seem to stimulate biological responses similar to those in other biocompatible materials.

    View details for DOI 10.1002/jbm.a.32311

    View details for PubMedID 19072924

  • Antibody Binding to a Tethered Vesicle Assembly Using QCM-D ANALYTICAL CHEMISTRY Patel, A. R., Kanazawa, K. K., Frank, C. W. 2009; 81 (15): 6021-6029

    Abstract

    The bilayer-tethered vesicle assembly has recently been proposed as a biomimetic model membrane platform for the analysis of integral membrane proteins. Here, we explore the binding of antibodies to membrane components of the vesicle assembly through the use of quartz crystal microbalance with dissipation monitoring (QCM-D). The technique provides a quantitative, label-free avenue to study binding processes at membrane surfaces. However, converting the signal generated upon binding to the actual amount of antibody bound has been a challenge for a viscoelastic system such as the tethered vesicle assembly. In this work, we first established an empirical relationship between the amount of bound antibody and the corresponding QCM-D response. Then, the results were examined in the context of an existing model describing the QCM-D response under a variety of theoretical loading conditions. As a model system, we investigated the binding of monoclonal antidinitrophenyl (DNP) IgG(1) to tethered vesicles displaying DNP hapten groups. The measured frequency and dissipation responses upon binding were compared to an independent measure of the amount of bound antibody obtained through the use of an in situ ELISA assay. At saturation, the surface mass density of bound antibody was approximately 900 ng/cm(2). Further, through the application of QCM-D models that describe the response of the quartz when loaded by either a single homogeneous viscoelastic film or by a two-layered viscoelastic film, we found that a homogeneous, one-layer model accurately predicts the amount of antibody bound to the tethered vesicles near antibody surface saturation, but a two-layer model must be invoked to accurately describe the kinetic response of the dissipation factor, which suggests that the binding of the antibody results in a stiffening of the top layer of the film.

    View details for DOI 10.1021/ac802756v

    View details for Web of Science ID 000268455600011

    View details for PubMedID 19580260

  • Bioactive interpenetrating polymer network hydrogels that support corneal epithelial wound healing. Journal of biomedical materials research. Part A Myung, D., Farooqui, N., Zheng, L. L., Koh, W., Gupta, S., Bakri, A., Noolandi, J., Cochran, J. R., Frank, C. W., Ta, C. N. 2009; 90 (1): 70-81

    Abstract

    The development and characterization of collagen-coupled poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network hydrogels is described. Quantitative amino acid analysis and FITC-labeling of collagen were used to determine the amount and distribution of collagen on the surface of the hydrogels. The bioactivity of the coupled collagen was detected by a conformation-specific antibody and was found to vary with the concentration of collagen reacted to the photochemically functionalized hydrogel surfaces. A wound healing assay based on an organ culture model demonstrated that this bioactive surface supports epithelial wound closure over the hydrogel but at a decreased rate relative to sham wounds. Implantation of the hydrogel into the corneas of live rabbits demonstrated that epithelial cell migration is supported by the material, although the rate of migration and morphology of the epithelium were not normal. The results from the study will be used as a guide toward the optimization of bioactive hydrogels with promise in corneal implant applications such as a corneal onlay and an artificial cornea.

    View details for DOI 10.1002/jbm.a.32056

    View details for PubMedID 18481785

  • Alpha-Helical Peptide-Induced Vesicle Rupture Revealing New Insight into the Vesicle Fusion Process As Monitored in Situ by Quartz Crystal Microbalance-Dissipation and Reflectometry ANALYTICAL CHEMISTRY Cho, N., Wang, G., Edvardsson, M., Glenn, J. S., Hook, F., Frank, C. W. 2009; 81 (12): 4752-4761

    Abstract

    We have used simultaneous quartz crystal microbalance-dissipation (QCM-D) monitoring and four-detector optical reflectometry to monitor in situ the structural transformation of intact vesicles to a lipid bilayer on a gold surface. The structural transformation of lipid vesicles to a bilayer was achieved by introducing a particular amphipathic, alpha-helical (AH) peptide. The combined experimental apparatus allows us to simultaneously follow the acoustic and optical property changes of the vesicle rupturing process upon interaction with AH peptides. While QCM-D and reflectometry have similar sensitivities in terms of mass and thickness resolution, there are unique advantages in operating these techniques simultaneously on the same substrate. These advantages permit us to (1) follow the complex interaction between AH peptides and intact vesicles with both acoustic and optical mass measurements, (2) calculate the amount of dynamically coupled water during the interaction between AH peptides and intact vesicles, (3) demonstrate that the unexpectedly large increase of both adsorbed mass and the film's energy dissipation is mainly caused by swelling of the vesicles during the binding interaction with AH peptides, and (4) permit us to understand the structural transformation from intact vesicles to a bilayer via the AH peptide interaction by monitoring viscoelastic properties, acoustic mass, optical mass, and thickness changes of both the binding and destabilization processes. From the deduced "hydration signature" we followed the complex transformation of lipid assemblies. On the basis of this information, a mechanism of this structural transformation is proposed that provides new insight into the process of vesicle fusion on solid substrates.

    View details for DOI 10.1021/ac900242s

    View details for Web of Science ID 000266969700012

    View details for PubMedID 19459601

  • Defect Generation Surrounding Nanoparticles in a Cross-Linked Hydrogel Network LANGMUIR Yanagioka, M., Frank, C. W. 2009; 25 (10): 5927-5939

    Abstract

    A detailed understanding of polymer-nanoparticle interactions is a key element in demystifying the reinforcement mechanism for nanocomposites. To decouple the effects of the polymer-nanoparticle interactions from the particle distribution, we utilized polymerized crystalline colloidal arrays based on a thermosensitive hydrogel, poly(N-isopropylacrylamide) (pNIPAAm). First, the hydrogel network structure in the vicinity of the nanoparticles was investigated by the deswelling behavior of particle-filled hydrogels. The addition of nanoparticles led to an increased rate of deswelling when the particle-filled hydrogel was heated beyond the lower critical solution temperature (32 degrees C). To interpret this observation, we have suggested that the polymer network has a significant increase in defects (e.g., dangling chain ends) in the vicinity of the nanoparticles. The apparent percolation threshold associated with the interaction of the nanoparticles was about 20 times smaller than the theoretical percolation threshold of spherical particles. As a consequence, we have determined the thickness of this defect zone to be about 85 nm. This is much larger than the size of the unperturbed linear pNIPAAm chains, suggesting that the polymers that play a role in the adsorption are not constrained segments of polymers bound between cross-link junctions but relatively free chains. This finding enabled us to emulate the adsorption behavior of pNIPAAm hydrogels on the particles by simply adding linear pNIPAAm chains to the particle suspensions. We then prepared silica and polystyrene suspensions with free pNIPAAm chains at a concentration much lower than the overlap concentration c*. A rheological study was conducted to determine the adsorption thickness of linear polymer chains on both silica and polystyrene nanoparticles. No significant adsorption was observed on silica, whereas the resultant thickness of the polymer was 8 nm on polystyrene.

    View details for DOI 10.1021/la804130m

    View details for Web of Science ID 000266081000069

    View details for PubMedID 19371046

  • Viral infection of human progenitor and liver-derived cells encapsulated in three-dimensional PEG-based hydrogel BIOMEDICAL MATERIALS Cho, N., Elazar, M., Xiong, A., Lee, W., Chiao, E., Baker, J., Frank, C. W., Glenn, J. S. 2009; 4 (1)

    Abstract

    We have studied the encapsulation of human progenitor cells into 3D PEG hydrogels. Replication-incompetent lentivirus promoter reporter vectors were found to efficiently detect the in vivo expression of human hepatic genes in hydrogel-encapsulated liver progenitor cells. Similarly, hydrogel-encapsulated cells could be efficiently infected with hepatitis C virus, and progeny infectious virus could be recovered from the media supernatants of the hydrogels. Provocatively, the diameters of these virus particles range from approximately 50 to 100 nm, while the calculated mesh size of the 8 k hydrogel is 44.6 +/- 1.7 A. To reconcile how viral particles can penetrate the hydrogels to infect the encapsulated cells, we propose that microfractures/defects of the hydrogel result in a functional pore size of up to 20 fold greater than predicted by theoretical mesh calculations. These results suggest a new model of hydrogel structure, and have exciting implications for tissue engineering and hepatitis virus studies.

    View details for DOI 10.1088/1748-6041/4/1/011001

    View details for Web of Science ID 000263071200001

    View details for PubMedID 18981544

  • Supramolecular Assembly of Block Copolypeptides with Semiconductor Nanocrystals LANGMUIR Atmaja, B., Cha, J. N., Marshall, A., Frank, C. W. 2009; 25 (2): 707-715

    Abstract

    We report the analogy between the self-assembly properties of amphiphilic phospholipids and the similar behavior observed for quantum dot (CdSe/CdS)-diblock copolypeptide hybrid systems, and the effect of the self-assembly on secondary structures of the polypeptides. At neutral pH, the diblock copolypeptide, poly(diethyleneglycol-l-lysine)-poly(l-lysine), comprises a positively charged poly-l-lysine (PLL) block and a hydrophilic and uncharged poly(diethyleneglycol-l-lysine) (PEGLL) block. By itself, the copolypeptide is not amphiphilic. However, when the polymers are mixed with water-soluble, negatively charged, citrate-functionalized quantum dots (QDs) in water, shell-like structures or dense aggregates are spontaneously formed. Electrostatic and hydrogen-bonding interactions between the positively charged PLL residues and the negatively charged ligands on the QDs lead to charge neutralization of the PLL block, while the PEGLL block remains hydrophilic. As a result, a pseudo "amphiphilic" molecular unit is formed in which the "hydrophobic" and hydrophilic sections constitute the charge-neutralized PLL residues together with the associating QD and the remaining polypeptide residues that are not neutralized, respectively. The generation of these "amphiphilic" molecular units in turn drives the formation of the QD-polypeptide assemblies. Support for this analogy comes from the observed transition in the shape of the assembly from a shell-like structure to a dense aggregate that is very much analogous to the vesicle-to-micelle transition observed in lipid systems. Furthermore, this shape transition can be explained qualitatively using a concept that is analogous to the surfactant number (N = a(hc)/a(hg)), which has been applied extensively in amphiphilic lipid systems. Specifically, as the ratio of the "hydrophobic" area (a(hc)) to the hydrophilic area (a(hg)) decreases, a shape transition from the shell-like structure to the dense aggregate occurs. In addition, the size of the shell-like structure changes as a function of the dimensions of the "amphiphilic" molecular unit in a manner that is similar to how the size of the lipid vesicle changes with the dimensions of the lipid molecule. Circular dichroism (CD) measurements have shown that the PEGLL-PLL molecule has a well-defined secondary structure (alpha-helical PEGLL block and random coil PLL block) that remains virtually unchanged after reacting with the QDs. This finding is consistent with the hypothesis that it is the electrostatic interaction between the amines on the PLL block and the citrate ligands on the QDs that drives the self-assembly.

    View details for DOI 10.1021/la801848d

    View details for Web of Science ID 000262431100013

    View details for PubMedID 19072205

  • Adsorbed alpha-Helical Diblock Copolypeptides: Molecular Organization, Structural Properties, and Interactions LANGMUIR Atmaja, B., Cha, J. N., Frank, C. W. 2009; 25 (2): 865-872

    Abstract

    In this work, we have developed 11-mercaptoundecanoic acid (MUA)-polypeptide "bilayer" systems by adsorbing poly(diethylene glycol-l-lysine)-poly(l-lysine) (PEGLL-PLL) diblock copolypeptide molecules of various architectures onto MUA-functionalized gold substrates. An objective of our present work is to use the PEGLL-PLL/MUA bilayer as a model system for studying the interfacial phenomena that occur when PEGLL-PLL molecules interact with carboxylic acid (COOH) moieties of nanoparticle ligands. Specifically, we have elucidated the nature of the interactions between the PEGLL-PLL and COOH moieties as well as the resulting polypeptide conformation and organization, using a combination of surface techniques-grazing-incidence IR spectroscopy, ellipsometry, and contact angle. We have also thoroughly characterized other film properties such as the packing and graft density of the polypeptide molecules as a function of the PEGLL-PLL architecture. From the IR data, the adsorption process occurs primarily by means of electrostatic interaction between the protonated PLL residues (pKa approximately 10.6) and carboxylate moieties of the MUA self-assembled monolayer (SAM) (pKa approximately 6) that is enhanced by H-bonding. The PLL block is thought to adopt a random-coil (extended) conformation, while the PEGLL block that is not interacting with the MUA molecules is found to adopt an alpha-helical conformation with an average tilt angle of -60 degrees. The PEGLL-PLL molecules have also been deduced to form a heterogeneous film and adopt liquidlike/disordered packing on the surface. The average contact angle of the MUA-polypeptide bilayer systems is -40 degrees, which implies that the diethylene glycol (EG2) side chains of the PEGLL residues may be oriented somewhat toward the surface normal. From ellipsometry measurements, it is found that PEGLLx-PLLy molecules with a longer alpha-helical block are associated with a lower graft density on the MUA surface compared to those with a shorter alpha-helical block. This observation may be attributed to the greater repulsion-steric and H-bonding effects-that is imposed by the EG2 side chains found on and projected area occupied by the longer PEGLL block. The bilayer systems have been found to be extremely stable over a 2-week period with no changes in the contact angle, thickness, polypeptide tilt angle, or conformation. Beyond that, there is a gradual decrease in the thickness and increase in the contact angle of the bilayer that could be attributed to the oxidation of the MUA SAM molecules.

    View details for DOI 10.1021/la801973x

    View details for Web of Science ID 000262431100033

    View details for PubMedID 19177647

  • Preparation and Characterization of Glycoacrylate-Based Polymer-Tethered Lipid Bilayers on Benzophenone-Modified Substrates LANGMUIR Hwang, L. Y., Goetz, H., Knoll, W., Hawker, C. J., Frank, C. W. 2008; 24 (24): 14088-14098

    Abstract

    Polymer-tethered lipid bilayers are promising models for biological membranes as they may provide a soft, lubricating environment with sufficient spacing between the substrate and bilayer for incorporating transmembrane proteins. We present such a system that uses a glycoacrylate-based telechelic lipopolymer in combination with a lipid analogue. Characterization of the mixed monolayers of lipopolymers and free lipids at the air-water interface is used to examine the molecular organization that dictates the final assembly properties. Isotherms indicate that the source of the dominating interactions, whether polymer interactions in the subphase or alkyl chain interactions, depends on both the tethering density and area per molecule. Moreover, a critical composition exists at which the alkyl chain interactions dominate the monolayer behavior regardless of the area per molecule. Isobaric creep and hysteresis experiments suggest that permanent states due to irreversible polymer-polymer interactions are not created as the monolayer is compressed. These data, combined with theoretical polymer predictions, are used to understand the organization of the monolayers at the air-water interface and, hence, the separation distance between the bottom of the bilayer and substrate in the water-swollen state of the final bilayer assembly. Atomic force microscopy is used to confirm that the measured separation distance of 11.2 nm is on the order of what would be predicted using a theoretical analysis for a representative 5 mol % lipopolymer-tethered bilayer. Next, the homogeneity of the final bilayer is probed at multiple scales. Fluorescence microscopy is used to demonstrate that homogeneous and continuous bilayers can be formed (within the optical resolution limit of 500 nm) with all polymer tethering densities used in this study. Atomic force microscopy studies demonstrate that homogeneity comparable to that of a solid-supported lipid bilayer can be achieved for a representative 5 mol % lipopolymer-tethered bilayer. Langmuir-Blodgett transfer conditions for depositing monolayers that can be used to create homogeneous, fluid bilayers are also discussed. Finally, the distal leaflet lateral mobility is measured using fluorescence recovery after photobleaching experiments and shown to be a function of the tethering density. A possible model for the mobility data is developed in which the tethered lipids in the proximal leaflet act as immobile lipid obstacles that couple to distal leaflet lipids.

    View details for DOI 10.1021/la8022997

    View details for Web of Science ID 000261631700040

    View details for PubMedID 19360958

  • Fluorescence Quantification for Surface Plasmon Excitation LANGMUIR Feller, B. E., Kellis, J. T., Cascao-Pereira, L. G., Knoll, W., Robertson, C. R., Frank, C. W. 2008; 24 (21): 12303-12311

    Abstract

    Surface plasmon resonance and surface plasmon fluorescence spectroscopy in combination have the potential to distinguish multicomponent surface processes. However, surface intensity variations from resonance angle shifts lead to a nonlinear response in the fluorescence intensity. We report a method to account for surface intensity variations using the experimentally measured relationship between fluorescence and reflectivity. We apply this method to monitor protease adsorption and proteolytic substrate degradation simultaneously. Multilayer protein substrates are prepared for these degradation studies using a layer-by-layer technique.

    View details for Web of Science ID 000260508800034

    View details for PubMedID 18844383

  • Progress in the development of interpenetrating polymer network hydrogels POLYMERS FOR ADVANCED TECHNOLOGIES Myung, D., Waters, D., Wiseman, M., Duhamel, P., Noolandi, J., Ta, C. N., Frank, C. W. 2008; 19 (6): 647-657

    Abstract

    Interpenetrating polymer networks (IPNs) have been the subject of extensive study since their advent in the 1960s. Hydrogel IPN systems have garnered significant attention in the last two decades due to their usefulness in biomedical applications. Of particular interest are the mechanical enhancements observed in "double network" IPN systems which exhibit nonlinear increases in fracture properties despite being composed of otherwise weak polymers. We have built upon pioneering work in this field as well as in responsive IPN systems to develop an IPN system based on end-linked poly-(ethylene glycol) (PEG) and loosely crosslinked poly(acrylic acid) (PAA) with hydrogen bond-reinforced strain-hardening behavior in water and high initial Young's moduli under physiologic buffer conditions through osmotically induced pre-stress. Uniaxial tensile tests and equilibrium swelling measurements were used to study PEG/PAA IPN hydrogels having second networks prepared with varying crosslinking and photoinitiator content, pH, solids content, and comonomers. Studies involving the addition of non-ionic comonomers and neutralization of the second network showed that template polymerization appears to be important in the formation of mechanically enhanced IPNs.

    View details for DOI 10.1002/pat.1134

    View details for Web of Science ID 000257014100025

    View details for PubMedID 19763189

  • Development of hydrogel-based keratoprostheses: A materials perspective BIOTECHNOLOGY PROGRESS Myung, D., Duhamel, P., Cochran, J. R., Noolandi, J., Ta, C. N., Frank, C. W. 2008; 24 (3): 735-741

    Abstract

    Research and development of artificial corneas (keratoprostheses) in recent years have evolved from the use of rigid hydrophobic materials such as plastics and rubbers to hydrophilic, water-swollen hydrogels engineered to support not only peripheral tissue integration but also glucose diffusion and surface epithelialization. The advent of the AlphaCor core-and-skirt hydrogel keratoprosthesis has paved the way for a host of new approaches based on hydrogels and other soft materials that encompass a variety of materials preparation strategies, from synthetic homopolymers and copolymers to collagen-based bio-copolymers and, finally, interpenetrating polymer networks. Each approach represents a unique strategy toward the same goal: to develop a new hydrogel that mimics the important properties of natural donor corneas. We provide a critical review of these approaches from a materials perspective and discuss recent experimental results. While formidable technical hurdles still need to be overcome, the rapid progress that has been made by investigators with these approaches is indicative that a synthetic donor cornea capable of surface epithelialization is now closer to becoming a clinical reality.

    View details for DOI 10.1021/bp070476n

    View details for Web of Science ID 000256593300033

    View details for PubMedID 18422366

  • Glucose-permeable interpenetrating polymer network hydrogels for corneal implant applications: A pilot study CURRENT EYE RESEARCH Myung, D., Farooqui, N., Waters, D., Schaber, S., Koh, W., Carrasco, M., Noolandi, J., Frank, C. W., Ta, C. N. 2008; 33 (1): 29-43

    Abstract

    Epithelialization of a keratoprosthesis requires that the implant material be sufficiently permeable to glucose. We have developed a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network (IPN) hydrogel that can provide adequate passage of glucose from the aqueous humor to the epithelium in vivo. A series of PEG/PAA IPNs with varying PEG macromonomer molecular weights were synthesized and evaluated through swelling studies to determine their water content and diffusion experiments to assess their permeability to glucose. One of the PEG/PAA hydrogels prepared in this study had a glucose diffusion coefficient nearly identical to that of the human cornea (approximately 2.5 x 10(-6) cm(2)/sec). When implanted intrastromally in rabbit corneas, this hydrogel was retained and well-tolerated in 9 out of 10 cases for a period of 14 days. The retained hydrogels stayed optically clear and the epithelium remained intact and multilayered, indicating that the material facilitated glucose transport from the aqueous humor to the anterior part of the eye. The results from these experiments indicate that PEG/PAA hydrogels are promising candidates for corneal implant applications such as keratoprostheses and intracorneal lenses, and that the PEG/PAA IPN system in general is useful for creating permeable substrates for ophthalmic and other biomedical applications.

    View details for DOI 10.1080/02713680701793930

    View details for Web of Science ID 000252582500005

    View details for PubMedID 18214741

  • Design and fabrication of an artificial cornea based on a photolithographically patterned hydrogel construct BIOMEDICAL MICRODEVICES Myung, D., Koh, W., Bakri, A., Zhang, F., Marshall, A., Ko, J., Noolandi, J., Carrasco, M., Cochran, J. R., Frank, C. W., Ta, C. N. 2007; 9 (6): 911-922

    Abstract

    We describe the design and fabrication of an artificial cornea based on a photolithographically patterned hydrogel construct, and demonstrate the adhesion of corneal epithelial and fibroblast cells to its central and peripheral components, respectively. The design consists of a central "core" optical component and a peripheral tissue-integrable "skirt." The core is composed of a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) double-network with high strength, high water content, and collagen type I tethered to its surface. Interpenetrating the periphery of the core is a microperforated, but resilient poly(hydroxyethyl acrylate) (PHEA) hydrogel skirt that is also surface-modified with collagen type I. The well-defined microperforations in the peripheral component were created by photolithography using a mask with radially arranged chrome discs. Surface modification of both the core and skirt elements was accomplished through the use of a photoreactive, heterobifunctional crosslinker. Primary corneal epithelial cells were cultured onto modified and unmodified PEG/PAA hydrogels to evaluate whether the central optic material could support epithelialization. Primary corneal fibroblasts were seeded onto the PHEA hydrogels to evaluate whether the peripheral skirt material could support the adhesion of corneal stromal cells. Cell growth in both cases was shown to be contingent on the covalent tethering of collagen. Successful demonstration of cell growth on the two engineered components was followed by fabrication of core-skirt constructs in which the central optic and peripheral skirt were synthesized in sequence and joined by an interpenetrating diffusion zone.

    View details for DOI 10.1007/s10544-006-9040-4

    View details for Web of Science ID 000250462200017

    View details for PubMedID 17237989

  • Enrichment of deuterium oxide at hydrophilic interfaces in aqueous solutions LANGMUIR Stalgren, J. J., Boschkova, K., Ericsson, J., Frank, C. W., Knoll, W., Satija, S., Toney, M. F. 2007; 23 (24): 11943-11946

    Abstract

    The structure of water at aqueous interfaces is of the utmost importance in biology, chemistry, and geology. We use neutron reflectivity and quartz crystal microbalance to probe an interface between hydrophilic quartz and bulk liquid solutions of H2O/D2O mixtures. We find that near the interface the neutron scattering length density is larger than in the bulk solution and there is an excess adsorbed mass. We interpret this as showing that there is a region adjacent to the quartz that is enriched in D2O and extends 5-10 nm into the solution. This suggests caution when interpreting results where D2O is substituted for H2O in aqueous interfacial chemistry.

    View details for DOI 10.1021/la700932s

    View details for Web of Science ID 000250976700001

    View details for PubMedID 17958382

  • New ground for organic catalysis: A ring-opening polymerization approach to hydrogels BIOMACROMOLECULES Nederberg, F., Trang, V., Pratt, R. C., Mason, A. F., Frank, C. W., Waymouth, R. M., Hedrick, J. L. 2007; 8 (11): 3294-3297

    Abstract

    Herein, we describe an organocatalytic living polymerization approach to network and subsequent hydrogel formation. Cyclic carbonate-functionalized macromolecules were ring-opened using an alcoholic initiator in the presence of an organic catalyst, amidine 1,8-diazabicyclo[5.4.0]undec-7-ene. A model reaction for the cross-linking identified monomer concentration-dependent reaction regimes, and enhanced kinetic control was demonstrated by introducing a co-monomer, trimethylene carbonate. The addition of the co-monomer facilitated near-quantitative conversion of monomer to polymer (>96%). Resulting poly(ethylene glycol) networks swell significantly in water, and an open co-continuous (water-gel) porous structure was observed by scanning electron microscopy. The organocatalytic ring-opening polymerization of cyclic carbonate functional macromonomers using alcoholic initiators provides a simple, efficient, and versatile approach to hydrogel networks.

    View details for DOI 10.1021/bm700895d

    View details for Web of Science ID 000250960300002

    View details for PubMedID 17994785

  • Quartz resonator signatures under Newtonian liquid loading for initial instrument check JOURNAL OF COLLOID AND INTERFACE SCIENCE Cho, N., D'Amour, J. N., Stalgren, J., Knoll, W., Kanazawa, K., Frank, C. W. 2007; 315 (1): 248-254

    Abstract

    The quartz crystal microbalance (QCM) has been increasingly utilized in the monitoring of the deposition of thin macromolecular films. Studies in the deposition of polymers, biomaterials, and interfacial reactions under electrochemical environment are some of the conditions for the study of these material and deposition properties at a lipid interface. Numerous studies have shown the difficulties in configuring an experimental setup for the QCM such that the recorded data reflect only the behavior of the quartz crystal and its load, and not some artifact. Such artifacts for use in liquids include mounting stress, surface properties such as hydrophobicity, surface roughness coupling to loading liquids, influence of compressional waves, and even problems with the electronic circuitry including the neglect of the quartz capacitance and the hysteretic effects of electronic components. It is thought useful to obtain a simple test by which the user could make a quick initial assessment of the instrument's performance. When a smooth quartz crystal resonator is immersed from air into a Newtonian liquid, the resonance and loss characteristics of the QCM are changed. A minimum of two experimental parameters is needed to characterize these changes. One of the changes is that of the resonant frequency. The second is characterized by either a change in the equivalent circuit resistance (DeltaR) or a change in the resonance dissipation (DeltaD). Two combinations of these observables, in terms of either Deltaf and DeltaR or Deltaf and DeltaD, which we define as Newtonian signatures of S(1) and S(2), are calculated to have fixed values and to be independent of the harmonic and of the physical values of the Newtonian liquid. We have experimentally determined the values of S(1) and S(2) using three different QCM systems. These are the standard oscillator, the network analyzer, and the QCM dissipation instrument. To test the sensitivity of these signatures to surface roughness, which is potential experimental artifact, we determined the values of S(1) and S(2) for roughened crystals and found that these signatures do reflect that experimental condition. Moreover, these results were qualitatively in accord with the roughness scaling factor described by Martin.

    View details for DOI 10.1016/j.jcis.2007.06.020

    View details for Web of Science ID 000249989500032

    View details for PubMedID 17706241

  • Creation of lipid partitions by deposition of amphipathic viral peptides LANGMUIR Cho, N., Cho, S., Hardesty, J. O., Glenn, J. S., Frank, C. W. 2007; 23 (21): 10855-10863

    Abstract

    Phospholipid vesicles exhibit a natural characteristic to fuse and reform into a continuous single bilayer membrane on hydrophilic solid substrates such as glass, mica, and silica. The resulting solid-supported bilayer mimics physiological tendencies such as lipid flip-flop and lateral mobility. The lateral mobility of fluorescently labeled lipids fused into solid-supported bilayers is found to change upon deposition on the membrane surface of an amphipathic alpha-helical peptide (AH) derived from the hepatitis C virus (HCV) NS5A protein. The binding of the AH peptide to a phospholipid bilayer, with the helical axis parallel to the bilayer, leads to immobilization of the bilayer. We used AFM to better understand the mechanistic details of this specific interaction, and determined that the diminished fluidity of the bilayer is due to membrane thinning. Utilizing this specific interaction between AH peptides and lipid molecules, we demonstrate a novel process for the creation of lipid partition by employing AH peptides as agents to immobilize lipid molecules, thus creating a patterned solid support with partition-defined areas of freely mobile lipid bilayers. This architecture could have a wide range of applications in novel sensing, biotechnology, high-throughput screening, and biomimetic strategies.

    View details for DOI 10.1021/1000640h

    View details for Web of Science ID 000249948700058

    View details for PubMedID 17803321

  • Employing two different quartz crystal microbalance models to study changes in viscoelastic behavior upon transformation of lipid vesicles to a bilayer on a gold surface ANALYTICAL CHEMISTRY Chot, N., Kanazawa, K. K., Glenn, J. S., Frank, C. W. 2007; 79 (18): 7027-7035

    Abstract

    By analyzing the viscoelastic properties of two distinct layers, a layer of "soft" vesicles and a "rigid" bilayer, we have created a model system to permit the study of film behavior in the region of nonlinear mass and frequency change (non-Sauerbrey). The structural transformation of lipid vesicles to a bilayer is shown to be accompanied by significant changes in their physical properties. After the adsorption and saturation of intact vesicles on gold surfaces, the adsorbed vesicle layer exhibits a soft, water-rich, viscoelastic state. The AH peptide, a vesicle-destabilizing agent, is then added to trigger the formation of a much thinner (approximately 5 nm), compact, and rigid bilayer. In this study, we used the quartz crystal microbalance with dissipation technique. Large non-Sauerbrey frequency and energy dissipation changes characterize the viscoelastic nature of adsorbed intact vesicle films thicker than approximately 10 nm. Once the transformation is complete, the frequency changes along with zero energy dissipation for sufficiently thin films (t approximately 5 nm) were effectively modeled with the Sauerbrey equation. Furthermore, we checked the validity of the Voigt-Voinova model in which the quartz substrate is treated as a Voigt element, which is beyond the Sauerbrey description. The calculations treating the film as having a constant viscosity agreed well with the Voigt-Voinova model. These results were compared to calculations done using the electromechanical (EM) model, which does not require a series expansion. The Voigt-Voinova results were in excellent agreement with the EM model, providing evidence that the expansion used in their study is quite accurate.

    View details for Web of Science ID 000249527700016

    View details for PubMedID 17685547

  • Kinetics of oligonucleotide hybridization to DNA probe Arrays on high-capacity porous silica substrates BIOPHYSICAL JOURNAL Glazer, M. I., Fidanza, J. A., McGall, G. H., Trulson, M. O., Forman, J. E., Frank, C. W. 2007; 93 (5): 1661-1676

    Abstract

    We have investigated the kinetics of DNA hybridization to oligonucleotide arrays on high-capacity porous silica films that were deposited by two techniques. Films created by spin coating pure colloidal silica suspensions onto a substrate had pores of approximately 23 nm, relatively low porosity (35%), and a surface area of 17 times flat glass (for a 0.3-microm film). In the second method, latex particles were codeposited with the silica by spin coating and then pyrolyzed, which resulted in larger pores (36 nm), higher porosity (65%), and higher surface area (26 times flat glass for a 0.3-microm film). As a result of these favorable properties, the templated silica hybridized more quickly and reached a higher adsorbed target density (11 vs. 8 times flat glass at 22 degrees C) than the pure silica. Adsorption of DNA onto the high-capacity films is controlled by traditional adsorption and desorption coefficients, as well as by morphology factors and transient binding interactions between the target and the probes. To describe these effects, we have developed a model based on the analogy to diffusion of a reactant in a porous catalyst. Adsorption values (k(a), k(d), and K) measured on planar arrays for the same probe/target system provide the parameters for the model and also provide an internally consistent comparison for the stability of the transient complexes. The interpretation of the model takes into account factors not previously considered for hybridization in three-dimensional films, including the potential effects of heterogeneous probe populations, partial probe/target complexes during diffusion, and non-1:1 binding structures. The transient complexes are much less stable than full duplexes (binding constants for full duplexes higher by three orders of magnitude or more), which may be a result of the unique probe density and distribution that is characteristic of the photolithographically patterned arrays. The behavior at 22 degrees C is described well by the predictive equations for morphology, whereas the behavior at 45 degrees C deviates from expectations and suggests that more complex phenomena may be occurring in that temperature regime.

    View details for DOI 10.1529/biophysj.106.103275

    View details for Web of Science ID 000248722200023

    View details for PubMedID 17496028

  • Employing an amphipathic viral peptide to create a lipid bilayer on Au and TiO2 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cho, N., Cho, S., Cheong, K. H., Glenn, J. S., Frank, C. W. 2007; 129 (33): 10050-?

    View details for DOI 10.1021/ja0701412

    View details for Web of Science ID 000248896400003

    View details for PubMedID 17661464

  • Binding dynamics of hepatitis C virus' NS5A amphipathic peptide to cell and model membranes JOURNAL OF VIROLOGY Cho, N., Cheong, K. H., Lee, C., Frank, C. W., Glenn, J. S. 2007; 81 (12): 6682-6689

    Abstract

    Membrane association of the hepatitis C virus NS5A protein is required for viral replication. This association is dependent on an N-terminal amphipathic helix (AH) within NS5A and is restricted to a subset of host cell intracellular membranes. The mechanism underlying this specificity is not known, but it may suggest a novel strategy for developing specific antiviral therapy. Here we have probed the mechanistic details of NS5A AH-mediated binding to both cell-derived and model membranes by use of biochemical membrane flotation and quartz crystal microbalance (QCM) with dissipation. With both assays, we observed AH-mediated binding to model lipid bilayers. When cell-derived membranes were coated on the quartz nanosensor, however, significantly more binding was detected, and the QCM-derived kinetic measurements suggested the existence of an interacting receptor in the target membranes. Biochemical flotation assays performed with trypsin-treated cell-derived membranes exhibited reduced AH-mediated membrane binding, while membrane binding of control cytochrome b5 remained unaffected. Similarly, trypsin treatment of the nanosensor coated with cellular membranes abolished AH peptide binding to the cellular membranes but did not affect the binding of a control lipid-binding peptide. These results therefore suggest that a protein plays a critical role in mediating and stabilizing the binding of NS5A's AH to its target membrane. These results also demonstrate the successful development of a new nanosensor technology ideal both for studying the interaction between a protein and its target membrane and for developing inhibitors of that interaction.

    View details for DOI 10.1128/JVI.02783-06

    View details for Web of Science ID 000246987500050

    View details for PubMedID 17428867

  • Glyco-acrylate copolymers for bilayer tethering on benzophenone-modified substrates COLLOIDS AND SURFACES B-BIOINTERFACES Hwang, L. Y., Goetz, H., Hawker, C. J., Frank, C. W. 2007; 54 (2): 127-135

    Abstract

    Model biological membranes are becoming increasingly important for studying fundamental biophysical phenomena and developing membrane-based devices. To address the anticipated problem of non-physiological interactions between membrane proteins and substrates seen in "solid-supported lipid bilayers" that are formed directly on hydrophilic substrates, we have developed a polymer-tethered lipid bilayer system based on a random copolymer with multiple lipid analogue anchors and a glyco-acrylate backbone. This system is targeted at applications that, most importantly, require stability and robustness since each copolymer has multiple lipid analogues that insert into the bilayer. We have combined this copolymer with a flexible photochemical coupling scheme that covalently attaches the copolymer to the substrate. The Langmuir isotherms of mixed copolymer/free lipid monolayers measured at the air-water interface indicate that the alkyl chains of the copolymer lipid analogues and the free lipids dominate the film behavior. In addition, no significant phase transitions are seen in the isotherms, while hysteresis experiments confirm that no irreversible states are formed during the monolayer compression. Isobaric creep experiments at the air-water interface and AFM experiments of the transferred monolayer are used to guide processing parameters for creating a fluid, homogeneous bilayer. Bilayer homogeneity and fluidity are monitored using fluorescence microscopy. Continuous bilayers with lateral diffusion coefficients of 0.6 microm(2)/s for both leaflets of the bilayer are observed for a 5% copolymer system.

    View details for DOI 10.1016/j.colsurfb.2006.08.010

    View details for Web of Science ID 000244473900001

    View details for PubMedID 17207977

  • Kinetics of oligonucleotide hybridization to photolithographically patterned DNA arrays ANALYTICAL BIOCHEMISTRY Glazer, M., Fidanza, J. A., McGall, G. H., Trulson, M. O., Forman, J. E., Suseno, A., Frank, C. W. 2006; 358 (2): 225-238

    Abstract

    The hybridization kinetics of oligonucleotide targets to oligonucleotide probe arrays synthesized using photolithographic fabrication methods developed by Affymetrix have been measured. Values for the fundamental adsorption parameters, k(a), k(d), and K, were determined at both room temperature and 45 degrees C by monitoring the hybridization of fluorescently labeled targets to the array. The values for these parameters and the adsorbed target density (

    View details for DOI 10.1016/j.ab.2006.07.042

    View details for Web of Science ID 000241681000008

    View details for PubMedID 16982029

  • Quantitative analysis of tethered vesicle assemblies by quartz crystal microbalance with dissipation monitoring: Binding dynamics and bound water content LANGMUIR Patel, A. R., Frank, C. W. 2006; 22 (18): 7587-7599

    Abstract

    To implement the molecular recognition properties of membrane proteins for applications including biosensors and diagnostic arrays, the construction of a biomimetic platform capable of maintaining protein structure and function is required. In this paper, we describe a tethered phospholipid vesicle assembly that overcomes the major limitations of planar supported lipid bilayers and alternative biomimetic membrane platforms and characterize it using quartz crystal microbalance with dissipation monitoring (QCM-D) and fluorescence microscopy. We provide evidence of a one-step mechanism for bilayer formation and monitor the subsequent adsorption and binding of streptavidin, vesicles, and streptavidin-coated microspheres. For all three species, we identify a critical surface density above which a significant amount of coupled interstitial water contributes to the response of the quartz resonator in a phenomenon similar to dynamic coupling due to surface roughness. A Sauerbrey-type analysis is sufficient to accurately interpret the QCM-D results for streptavidin binding if water is treated as an additional inertial mass, but viscoelastic models must be invoked for vesicle and microsphere binding. Additionally, we present evidence of vesicle flattening, possibly enhanced by a biotin-mediated membrane-membrane interaction.

    View details for DOI 10.1021/la0610452

    View details for Web of Science ID 000239921800020

    View details for PubMedID 16922537

  • Fluid supported lipid bilayers containing mono sialoganglioside GM1: A QCM-D and FRAP study COLLOIDS AND SURFACES B-BIOINTERFACES Weng, K. C., Kanter, J. L., Robinson, W. H., Frank, C. W. 2006; 50 (1): 76-84

    Abstract

    In an effort to use model fluid membranes for immunological studies, we compared the formation of planar phospholipid bilayers supported on silicon dioxide surfaces with and without incorporation of glycolipids as the antigen for in situ antibody binding. Dynamic light scattering measurements did not differentiate the hydrodynamic volumes of extruded small unilamellar vesicles (E-SUVs) containing physiologically relevant concentrations (0.5-5 mol%) of monosialoganglioside GM1 (GM1) from exclusive egg yolk L-alpha-phosphatidylcholine (egg PC) E-SUVs. However, quantifiable differences in deposition mass and dissipative energy loss emerged in the transformation of 5 mol% GM1/95 mol% egg PC E-SUVs to planar supported lipid bilayers (PSLBs) by vesicle fusion on thermally evaporated SiO2, as monitored by the quartz crystal microbalance with dissipation (QCM-D) technique. Compared to the 100 mol% egg PC bilayers on the same surface, E-SUVs containing 5 mol% GM1 reached a approximately 12% higher mass and a lower dissipative energy loss during bilayer transformation. PSLBs with 5 mol% GM1 are approximately 18% heavier than 100 mol% egg PC and approximately 11% smaller in projected area per lipid, indicating an increased rigidity and a tighter packing. Subsequent binding of polyclonal immunoglobulin G anti-GM1 to the PSLBs was performed in situ and showed specificity. The anti-GM1 to GM1 ratios at equilibrium were roughly proportional to the concentrations of anti-GM1 administered in the solution. Fluorescence recovery after photobleaching was utilized to verify the retained, albeit reduced lateral fluidity of the supported membranes. Five moles percentage of GM1 membranes (GM1 to PC ratio approximately 1:19) decorated with 1 mol% N-(Texas Red sulfonyl)-1,2-dihexadecanoyl-sn-glycerol-3-phosphoethanolamine (Texas Red DHPE) exhibited an approximately 16% lower diffusion coefficient of 1.32+/-0.06 microm2/s, compared to 1.58+/-0.04 microm2/s for egg PC membranes without GM1 (p<0.01). The changes in vesicle properties and membrane lateral fluidity are attributed to the interactions of GM1 with itself and GM1 with other membrane lipids. This system allows for molecules of interest such as GM1 to exist on a more biologically relevant surface than those used in conventional methods such as ELISA. Our analysis of rabbit serum antibodies binding to GM1 demonstrates this platform can be used to test for the presence of anti-lipid antibodies in serum.

    View details for DOI 10.1016/j.colsurfb.2006.03.010

    View details for Web of Science ID 000238515200012

    View details for PubMedID 16730958

  • Capillary aging of the contacts between glass spheres and a quartz resonator surface PHYSICAL REVIEW LETTERS D'Amour, J. N., Stalgren, J. J., Kanazawa, K. K., Frank, C. W., Rodahl, M., Johannsmann, D. 2006; 96 (5)

    Abstract

    The strength of the contacts between small glass spheres and the surface of a quartz crystal resonator has been probed based on the increase of resonance frequency induced upon sphere contact. The acoustic interaction between the sphere and the plate is modeled as a low-frequency coupled resonance; the dependence of the resonant parameters on overtone order lends support to this model. After exposing the sample to humid air and drying it again, the contact strength increases at least tenfold due to capillary forces--we observe a hysteretic form of the sand-castle effect. Repeated wet-dry cycles reveal logarithmic capillary aging with time. The experiments suggest that the drying of the liquid bridges leads to a contraction of small voids in the contact zone, subsequently increasing cohesion.

    View details for DOI 10.1103/PhysRevLett.96.058301

    View details for Web of Science ID 000235252200097

    View details for PubMedID 16487001

  • The dramatic effect of architecture on the self-assembly of block copolymers at interfaces LANGMUIR Kim, Y., Pyun, J., Frechet, J. M., Hawker, C. J., Frank, C. W. 2005; 21 (23): 10444-10458

    Abstract

    Dramatic morphological changes are observed in the Langmuir-Blodgett (LB) film assemblies of poly(ethylene glycol)-b-(styrene-r-benzocyclobutene) block copolymer (PEG-b-(S-r-BCB)) after intramolecular cross-linking of the S-r-BCB block to form a linear-nanoparticle structure. To isolate architectural effects and allow direct comparison, the linear block copolymer precursor and the linear-nanoparticle block copolymer resulting from selective intramolecular cross-linking of the BCB units were designed to have exactly the same molecular weight and chemical composition but different architecture. It was found that the effect of architecture is pronounced with these macromolecular isomers, which self-assemble into dramatically different surface aggregates. The linear block copolymer forms disklike surface assemblies over the range of compression states, while the linear-nanoparticle block copolymer exhibits long (>10 microm) wormlike aggregates whose length increases as a function of increasing cross-linking density. It is shown that the driving force behind the morphological change is a combination of the altered molecular geometry and the restricted degree of stretching of the nanoparticle block because of the intramolecular cross-linking. A modified approach to interpret the pi-A isotherm, which includes presence of the block copolymer aggregates, is also presented, while the surface rheological properties of the block copolymers at the air-water interface provide in-situ evidence of the aggregates' presence at the air-water interface.

    View details for DOI 10.1021/la047122f

    View details for Web of Science ID 000233129500037

    View details for PubMedID 16262305

  • Near-infrared optical-absorption behavior in high-beta nonlinear optical chromophore-polymer guest-host materials. II. Dye spacer length effects in an amorphous polycarbonate copolymer host JOURNAL OF CHEMICAL PHYSICS Barto, R. R., Frank, C. W., BEDWORTH, P. V., Ermer, S., Taylor, R. E. 2005; 122 (23)

    Abstract

    In the second of a three-part series, spectral absorption behavior of nonlinear optical (NLO) dyes incorporated into amorphous polycarbonate, comprised of a homologous series of dialkyl spacer groups extending from the midsection of the dye molecule, is characterized by UV-Vis and photothermal deflection spectroscopy. The dyes are structural analogs of the NLO dye FTC [2-(3-cyano-4-{2-[5-(2-{4-[ethyl-(2-methoxyethyl)amino]phenyl}vinyl)-3,4-diethylthiophen-2-yl]vinyl}-5,5-dimethyl-5H-furan-2-ylidene)malononitrile]. Previous Monte Carlo calculations [B. H. Robinson and L. R. Dalton, J. Phys. Chem. A 104, 4785 (2000)] predict a strong dependence of the macroscopic nonlinear optical susceptibility on the chromophore waist: length aspect ratio in electric-field-poled films arising from interactions between chromophores. It is expected that these interactions will play a role in the absorption characteristics of unpoled films, as well. The spacer groups range in length from diethyl to dihexyl, and each dye is studied over a wide range of concentrations. Among the four dyes studied, a universal dependence of near-IR loss on inhomogeneous broadening of the dye main absorption peak is found. The inhomogeneous width and its concentration dependence are seen to vary with spacer length in a manner characteristic of the near-IR loss-concentration slope at transmission wavelengths of 1.06 and 1.3 mum, but not at 1.55 mum. The lower wavelength loss behavior is assigned to purely Gaussian broadening, and is described by classical mixing thermodynamic quantities based on the Marcus theory of inhomogeneous broadening [R. A. Marcus, J. Chem. Phys. 43, 1261 (1965)], modeled as a convolution of dye-dye dipole broadening and dye-polymer van der Waals broadening. The Gaussian dipole interactions follow a Loring dipole-broadening description [R. F. Loring, J. Phys. Chem. 94, 513 (1990)] dominated by the excited-state dipole moment, and have a correlated homogeneous broadening contribution. The long-wavelength loss behavior has a non-Gaussian dye-dye dipole contribution which follows Kador's broadening analysis [L. Kador, J. Chem. Phys. 95, 5574 (1991)], with a net broadening described by a convolution of this term with a Gaussian van der Waals interaction given by Obata et al. [M. Obata, S. Machida, and K. Horie, J. Polym. Sci. B 37, 2173 (1999)], with each term governed by the dye spacer length. A minimum in broadening and loss-concentration slope at a spacer length of four carbons per alkyl at all wavelengths has important consequences for practical waveguide devices, and is of higher aspect ratio than the spherical limit shown by Robinson and Dalton to minimize dipole interactions under a poling field.

    View details for DOI 10.1063/1.1929742

    View details for Web of Science ID 000230091400064

    View details for PubMedID 16008487

  • Vesicle adsorption and lipid bilayer formation on glass studied by atomic force microscopy LANGMUIR Schonherr, H., Johnson, J. M., Lenz, P., Frank, C. W., Boxer, S. G. 2004; 20 (26): 11600-11606

    Abstract

    The adsorption of phosphatidylcholine (PC) vesicles (30, 50, and 100 nm nominal diameters) and of dye-labeled PC vesicles (labeled with 6% Texas Red fluorophore (TR) and encapsulated carboxy fluorescein (CF)) to glass surfaces was studied by contact mode atomic force microscopy in aqueous buffer. These studies were performed in part to unravel details of the previously observed isolated rupture of dye-labeled PC vesicles on glass (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), specifically to differentiate partial rupture, that is, pore formation and leakage of entrapped dye, from full rupture to form bilayer disks. In addition, the adhesion potential of PC vesicles on glass was calculated based upon the adhesion-driven flattening of adsorbed vesicles and a newly developed theoretical model. The vesicles were found to flatten considerably upon adsorption to glass (width-to-height ratio of approximately 5), which leads to an estimate for the adhesion potential and for the critical rupture radius of 1.5 x 10(-4) J/m2 and 250 nm, respectively. Independent of vesicle size and loading with dye molecules, the adsorption of intact vesicles was observed at all concentrations below a threshold concentration, above which the formation of smooth lipid bilayers occurred. In conjunction with previous work (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), these data show that 6% TR 20 mM CF vesicles adsorb to the surface intact but undergo partial rupture in which they exchange content with the external buffer.

    View details for DOI 10.1021/la049302v

    View details for Web of Science ID 000225816800046

    View details for PubMedID 15595789

  • In situ formation and characterization of poly(ethylene glycol)-supported lipid bilayers on gold surfaces LANGMUIR Munro, J. C., Frank, C. W. 2004; 20 (24): 10567-10575

    Abstract

    Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, mobile, tethered lipid bilayers were formed on a poly(ethylene glycol) (PEG) support via a two-step adsorption process. The PEG films were prepared by coadsorbing a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N-[3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) and a nonlipid functionalized PEG-PDP from an ethanol/water mixture, as described in a previous paper (Munro, J. C.; Frank, C. W. Langmuir 2004, 20, 3339-3349). Then a two-step lipid adsorption strategy was used. First, lipids were adsorbed onto the PEG support from a hexane solution. Second, vesicles were adsorbed and fused on the surface to create a bilayer in an aqueous environment. Fluorescence recovery after photobleaching experiments show that this process results in mobile bilayers with diffusion coefficients on the order of 2 microm2/s. The mobility of the bilayers is decreased slightly by increasing the density of tethered lipids. The formation of bilayers, and not multilayer structures, is also confirmed by surface plasmon resonance, which was used to determine in situ film thickness, and by fluorimetry, which was used to determine quantitatively the fluorescence intensity for each 18 by 18 mm sample. Unfortunately, fluorescence microscopy also shows that there are large defects on the samples, which limits the utility of this system.

    View details for DOI 10.1021/la048378o

    View details for Web of Science ID 000225207400029

    View details for PubMedID 15544386

  • Fluid biomembranes supported on nanoporous aerogel/xerogel substrates LANGMUIR Weng, K. C., Stalgren, J. J., Duval, D. J., Risbud, S. H., Frank, C. W. 2004; 20 (17): 7232-7239

    Abstract

    Planar supported lipid bilayers have attracted immense interest for their properties as model cell membranes and for potential applications in biosensors and lab-on-a-chip devices. We report the formation of fluid planar biomembranes on hydrophilic silica aerogels and xerogels. Scanning electron microscopy results showed the presence of interconnected silica beads of approximately 10-25 nm in diameter and nanoscale open pores of comparable size for the aerogel and grain size of approximately 36-104 nm with approximately 9-24 nm diameter pores for the xerogel. When the aerogel/xerogel was prehydrated and then allowed to incubate in l-alpha-phosphatidylcholine (egg yolk PC) unilamellar vesicle (approximately 30 nm diameter) solution, lipid bilayers were formed due to the favorable interaction of vesicles with the hydroxyl-abundant silica surface. Lateral mobility of labeled lipid N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine was retained in the membranes. A diffusion coefficient of 0.61 +/- 0.22 microm(2)/s was determined from fluorescence recovery after photobleaching analysis for membranes on aerogels, compared to 2.46 +/- 0.35 microm(2)/s on flat glass. Quartz crystal microbalance-dissipation was utilized to monitor the kinetics of the irreversible adsorption and fusion of vesicles into bilayers on xerogel thin films.

    View details for DOI 10.1021/la049940d

    View details for Web of Science ID 000223276400040

    View details for PubMedID 15301510

  • Langmuir and Langmuir-Blodgett films of amphiphilic bistable rotaxanes LANGMUIR Lee, I. C., Frank, C. W., Yamamoto, T., Tseng, H. R., Flood, A. H., Stoddart, J. F., Jeppesen, J. O. 2004; 20 (14): 5809-5828

    Abstract

    A series of amphiphilic bistable [2]rotaxanes--in which a ring-shaped component, the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), has been assembled around two recognition sites, a tetrathia-fulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system, situated apart at different strategic locations within the central polyether section of an amphiphilic dumbbell component that is terminated by a hydrophobic tetraarylmethane-based stopper (near the TTF unit) at one end and by a hydrophilic tetraarylmethane-based stopper (near the DNP ring system) at the other end--has been designed and synthesized. The effects of systematic changes in the constitutions of the three ethylene glycol tails (diethylene or tetraethylene glycol) and end groups (hydroxyl or methoxyl functions) attached to the hydrophilic stoppers on Langmuir film balance and surface rheology experiments at 20 degreesC were examined to determine the monolayer stabilities and co-conformations of the [2] rotaxanes and their free dumbbell counterparts. These experiments allow us to propose a model for the rotaxane's structures at different surface pressures. All the [2]rotaxanes form stable Langmuir films. These films typically pass from a liquid-expanded region to a liquid-condensed region. The transition between the two regions was either directly observed or ascertained using film stability experiments. Film balance and surface rheology experiments showed that the addition of the tetracationic cyclophane component and hydroxyl end groups markedly increased the stabilities and viscoelasticity of the films.

    View details for DOI 10.1021/la0361518

    View details for Web of Science ID 000222429200027

    View details for PubMedID 16459596

  • Adsorption of lipid-functionalized poly(ethylene glycol) to gold surfaces as a cushion for polymer-supported lipid bilayers LANGMUIR Munro, J. C., Frank, C. W. 2004; 20 (8): 3339-3349

    Abstract

    Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, the properties of a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N- [3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) adsorbed from ethanol and water solutions onto gold surfaces were studied using a variety of surface-sensitive techniques. X-ray photoelectron spectroscopy showed that the PEG molecules are tethered to the gold surface via thiolate bonds. When adsorbed from water, ethanol, or their mixtures, reflection-absorption infrared spectroscopy showed that amorphous PEG layers with disordered DSPE alkyl chains were formed, independent of adsorption time or solution concentration. On the basis of advancing and receding water and hexadecane contact angles on the lipopolymer films, the DSPE lipid groups appear to segregate from the PEG layer and become exposed at the surface of the polymer films. Swelling observed in surface plasmon resonance experiments and the large contact angle hysteresis observed indicate that highly swellable, mobile films capable of molecular rearrangements are formed. The self-assembling and amorphous properties of these PEG layers make them ideal candidates as polymer cushions for polymer-supported lipid bilayers. The DSPE surface concentration can be controlled, to a limited degree, by varying the adsorption time of DSPE-PEG-PDP from ethanol. A more effective strategy is to coadsorb DSPE-PEG-PDP with a non-lipid-functionalized PEG-PDP from an ethanol/water mixture, which allows the PEG thickness and density to remain constant while decreasing the density of DSPE groups.

    View details for DOI 10.1021/la036062v

    View details for Web of Science ID 000220750300050

    View details for PubMedID 15875867

  • The polymer-supported phospholipid bilayer: Tethering as a new approach to substrate-membrane stabilization BIOMACROMOLECULES Naumann, C. A., Prucker, O., Lehmann, T., Ruhe, J., Knoll, W., Frank, C. W. 2002; 3 (1): 27-35

    Abstract

    We present a new molecular engineering approach in which a polymer-supported phospholipid bilayer is vertically stabilized by controlled covalent tethering at both the polymer-substrate and polymer-bilayer interfaces. This approach is based on lipopolymer molecules, which not only form a polymer cushion between the phospholipid bilayer and a solid glass substrate but also act as covalent connections (tethers) between the bilayer and cushion. Our approach involves Langmuir-Blodgett transfer of a phospholipid-lipopolymer monolayer followed by Schaefer transfer of a pure phospholipid monolayer and is capable of varying the tethering density between the polymer layer and the phospholipid bilayer in a very controlled manner. Further stabilization is achieved if the glass substrate is surface-functionalized with a benzophenone silane. In this case, a photocross-linking reaction between the polymer and benzophenone group allows for the covalent attachment of the polymer cushion to the glass substrate. This approach is similar to that recently reported by Wagner and Tamm in which double tethering is achieved via lipopolymer silanes (Wagner, M. L.; Tamm, L. K. Biophys. J. 2000, 79, 1400). To obtain a deeper understanding of how the covalent tethering affects the lateral mobility of the bilayer, we performed fluorescence recovery after photobleaching (FRAP) experiments on polymer-tethered bilayers at different tethering densities (lipopolymer/phospholipid molar ratios). The FRAP data clearly indicate that the hydrophobic lipopolymer moieties act as rather immobile obstacles within the phospholipid bilayer, thereby leading to hindered diffusion of phospholipids. Whereas the high lateral diffusion coefficient of D = 17.7 mum(2)/s measured at low tethering density (5 mol % lipopolymer) indicates rather unrestricted motion within the bilayer, corresponding values at moderate (10 mol % lipopolymer) and high (30 mol % lipopolymer) tethering densities of D = 9.7 mum(2)/s and D = 1.1 mum(2)/s, respectively, show significant hindered diffusion. These results are contrary to the recent findings on similar membrane systems reported by Wagner and Tamm in which no significant change in phospholipid diffusion was found between 0 and 10 mol % lipopolymer. Our experimental report leads to a deeper understanding of the complex problem of interlayer coupling and offers a path toward a compromise between stability of the whole system and lateral mobility within the bilayer. Furthermore, the FRAP measurements show that polymer-tethered membranes are very interesting model systems for studying problems of restricted diffusion within two-dimensional fluids.

    View details for DOI 10.1021/bm0100211

    View details for Web of Science ID 000174256500005

    View details for PubMedID 11866552

  • Polymer-supported lipid bilayers on benzophenone-modified substrates BIOMACROMOLECULES Shen, W. W., Boxer, S. G., Knoll, W., Frank, C. W. 2001; 2 (1): 70-79

    Abstract

    Solid-supported lipid membranes are important for their roles in fundamental biophysical research as well as in applications such as biosensors. In our study, lipopolymers containing alkyl side chains were synthesized and a mixture of the lipopolymer and free lipids was preorganized at the air-water interface and then transferred to a solid substrate using the Langmuir-Blodgett technique. A photochemical reaction between a substrate-functionalized benzophenone and C-H bonds on the lipopolymer was used to attach the lipopolymers to the substrate. The final assembly of the membrane was completed by vesicle fusion. Langmuir film experiments at the air-water interface indicate tighter molecular packing for the lipopolymers with 28 mol % alkyl side chains than for the ones with 22 mol %. Atomic force microscopy images point to phase separation of lipopolymers on the substrates due to their dewetting from hydrophobic surfaces. However, a mixture of lipopolymers and free lipids formed a smooth film on the same substrate. After the addition of the second lipid layer on the lipopolymer/free lipid layer, the fluorescence images of the polymer-supported bilayer suggested that the distal lipid layer is homogeneous on the micrometer scale. The relaxation of the fluorescent probe lipids was analyzed after application of an electric field to determine their diffusion coefficient; the distal lipid layer was mobile with an average diffusion coefficient of approximately 0.1 microm(2)/s. Moreover, the immobile fraction of the lipids in the distal layer was estimated to be around 15%.

    View details for DOI 10.1021/bm005581z

    View details for Web of Science ID 000168755800011

    View details for PubMedID 11749157

  • Hindered diffusion in polymer-tethered membranes: A monolayer study at the air-water interface BIOMACROMOLECULES Naumann, C. A., Knoll, W., Frank, C. W. 2001; 2 (4): 1097-1103

    Abstract

    Polymer-tethered phospholipid bilayers, which are based on a phospholipid-lipopolymer mixture, represent a very promising approach to stabilize complex biomimicking composite membranes. Furthermore, they are interesting model systems to study problems of hindered diffusion in two-dimensional liquids. Here, we present fluorescence recovery after photobleaching experiments (FRAP) on mixed phospholipid-lipopolymer monolayers of DMPC and DSPE-EO(45) at the air-water interface. In contrast to recent polymer-tethered bilayer experiments where the hydrophobic lipopolymer anchors behaved as immobile obstacles within the fluid phospholipid matrix,(1) this paper investigates the influence of mobile lipopolymer obstacles on the lateral diffusion of phospholipids. We found that the lateral diffusion of phospholipids with D = 7.1 +/- 0.5 microm(2)/s is independent of the lipopolymer obstacle concentration if adjacent polymer chains do not interact with each other. However, the diffusion coefficient of nontethered phospholipids gradually decreases from D = 7.1 +/- 0.5 microm(2)/s to D = 3.4 +/- 0.1 microm(2)/s in the case of increasing polymer-polymer interactions based on frictional coupling. This can be understood by a slowing down of the obstacle mobility. While phospholipids still show a significant lateral diffusion as long as the polymer moieties interact with each other only via frictional coupling, they become rather immobile (D = 0.9 +/- 0.1 microm(2)/s) if lipopolymers form a two-dimensional physical network.

    View details for DOI 10.1021/bm010022t

    View details for Web of Science ID 000174256400006

    View details for PubMedID 11777379

  • Discrimination of DNA hybridization using chemical force microscopy BIOPHYSICAL JOURNAL Mazzola, L. T., Frank, C. W., Fodor, S. P., Mosher, C., Lartius, R., Henderson, E. 1999; 76 (6): 2922-2933

    Abstract

    Atomic force microscopy (AFM) can be used to probe the mechanics of molecular recognition between surfaces. In the application known as "chemical force" microscopy (CFM), a chemically modified AFM tip probes a surface through chemical recognition. When modified with a biological ligand or receptor, the AFM tip can discriminate between its biological binding partner and other molecules on a heterogeneous substrate. The strength of the interaction between the modified tip and the substrate is governed by the molecular affinity. We have used CFM to probe the interactions between short segments of single-strand DNA (oligonucleotides). First, a latex microparticle was modified with the sequence 3'-CAGTTCTACGATGGCAAGTC and epoxied to a standard AFM cantilever. This DNA-modified probe was then used to scan substrates containing the complementary sequence 5'-GTCAAGATGCTACCGTTCAG. These substrates consisted of micron-scale, patterned arrays of one or more distinct oligonucleotides. A strong friction interaction was measured between the modified tip and both elements of surface-bound DNA. Complementary oligonucleotides exhibited a stronger friction than the noncomplementary sequences within the patterned array. The friction force correlated with the measured strength of adhesion (rupture force) for the tip- and array-bound oligonucleotides. This result is consistent with the formation of a greater number of hydrogen bonds for the complementary sequence, suggesting that the friction arises from a sequence-specific interaction (hybridization) of the tip and surface DNA.

    View details for Web of Science ID 000080556700006

    View details for PubMedID 10354420

  • Adaptation of bulk constitutive equations to insoluble monolayer collapse at the air-water interface SCIENCE Kampf, J. P., Frank, C. W., Malmstrom, E. E., Hawker, C. J. 1999; 283 (5408): 1730-1733

    Abstract

    A constitutive equation based on stress-strain models of bulk solids was adapted to relate the surface pressure, compression rate, and temperature of an insoluble monolayer of monodendrons during collapse at the air-water interface. A power law relation between compression rate and surface pressure and an Arrhenius temperature dependence of the steady-state creep rate were observed in data from compression rate and creep experiments in the collapse region. These relations were combined into a single constitutive equation to calculate the temperature dependence of the collapse pressure with a maximum error of 5 percent for temperatures ranging from 10 degrees to 25 degrees C.

    View details for Web of Science ID 000079102800050

    View details for PubMedID 10073937

  • Structure in Thin and Ultrathin Spin-Cast Polymer Films Science (New York, N.Y.) Frank, C. W., Rao, V., Despotopoulou, M. M., Pease, R. F., Hinsberg, W. D., Miller, R. D., Rabolt, J. F. 1996; 273 (5277): 912-5

    Abstract

    The molecular organization in ultrathin polymer films (thicknesses less than 1000 angstroms) and thin polymer films (thicknesses between 1000 and 10,000 angstroms) may differ substantially from that of bulk polymers, which can lead to important differences in resulting thermophysical properties. Such constrained geometry films have been fabricated from amorphous poly(3-methyl-4-hydroxy styrene) (PMHS) and semicrystalline poly(di-n-hexyl silane) (PD6S) by means of spin-casting. The residual solvent content is substantially greater in ultrathin PMHS films, which suggests a higher glass transition temperature that results from a stronger hydrogen-bonded network as compared with that in thicker films. Crystallization of PD6S is substantially hindered in ultrathin films, in which a critical thickness of 150 angstroms is needed for crystalline morphology to exist and in which the rate of crystallization is initially slow but increases rapidly as the film approaches 500 angstroms in thickness.

    View details for PubMedID 8688068

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