Co-reporter:Kristin Engberg, Dale J. Waters, Shira Kelmanovich, Rachel Parke-Houben, Laura Hartmann, Michael F. Toney, Curtis W. Frank
Polymer 2016 Volume 84() pp:371-382
Publication Date(Web):10 February 2016
DOI:10.1016/j.polymer.2016.01.015
•Cholesterol was tethered into PEG networks via polymerization in an organic solvent.•Aqueous solvent exchange promoted cholesterol self-assembly in the hydrogels.•Self-assembled hydrogel morphologies depended on cholesterol concentration.•The cholesterol domains enhanced hydrogel elastic modulus and ductility.Cholesterol self-assembles into weakly ordered aggregates when tethered to a crosslinked hydrogel network of poly(ethylene glycol) (PEG). PEG-diacrylate and cholesterol-PEG-acrylamide (PEG-chol) were co-polymerized in organic solvent and transferred to water for equilibrium swelling. Small-angle x-ray scattering revealed self-assembled cholesterol structures not present during network synthesis. At lower ratios of PEG-tethered cholesterol to PEG (<12% cholesterol based on total solid content), cholesterol aggregates into the dense, weakly ordered crosslink junctions of the PEG network. The hydrogel networks exhibited classic affine behavior during compressive mechanical testing, and cholesterol aggregation enhanced the elastic modulus. At high PEG-chol to PEG ratios (12–20% cholesterol based on total solid content), cholesterol self-assembles into domains with lamellar-like meso-ordering. The structural transition causes network deswelling and significantly reduces material brittleness upon deformation.
Co-reporter:Steve S. He and Curtis W. Frank
Journal of Materials Chemistry A 2014 vol. 2(Issue 39) pp:16489-16497
Publication Date(Web):2014/08/05
DOI:10.1039/C4TA02942A
Alkaline exchange membranes (AEMs) are a promising class of polyelectrolytes whose alkaline operating environment enables the use of non-precious metal catalysts in low-temperature fuel cells. However, their poor ionic conductivities, which are often an order of magnitude lower than traditional acidic membranes (e.g., Nafion), have limited their practicality. The performance problem can partially be ascribed to the poorly-defined morphologies of typical random copolymer AEMs, leading to tortuous ion transport pathways. Here, we show the ability to form nanoscale (5 to 10 nm diameter) anion transport channels by grafting hydrophilic poly(ethylene glycol) side-chains along a model benzyltrimethylammonium polysulfone-based AEM. Concomitant with the structure formation is a 100% increase in the IEC-normalized hydroxide conductivity from 20.2 mS g cm−1 mmol−1 to 40.3 mS g cm−1 mmol−1 as well as a 50% increase in the peak power density from 118 mW cm−2 to 180 mW cm−2 when incorporated into a fuel cell.
Co-reporter:Beinn V. O. Muir, David Myung, Wolfgang Knoll, and Curtis W. Frank
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 2) pp:958
Publication Date(Web):December 23, 2013
DOI:10.1021/am404361v
The performance of medical implants and devices is dependent on the biocompatibility of the interfacial region between tissue and the implant material. Polymeric hydrogels are attractive materials for use as biocompatible surface coatings for metal implants. In such systems, a factor that is critically important for the longevity of an implant is the formation of a robust bond between the hydrogel layer and the implant metal surface and the ability for this assembly to withstand physiological conditions. Here, we describe the grafting of cross-linked hydrogel networks to titanium surfaces using grit-blasting and subsequent chemical functionalization using a silane-based adhesion promoter. Metal surface characterization was carried out using profilometry, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) analysis. Hydrogel layers composed of poly(ethylene glycol)-dimethacrylate (PEG-DMA), poly(2-hydroxyethylmethacrylate) (PHEMA), or poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) semi-interpenetrating polymer networks (semi-IPNs) have been prepared. The mechanical properties of these hydrogel-metal assemblies have been characterized using lap-shear measurements, and the surface morphology was studied by SEM and EDX. We have shown that both high surface roughness and chemical functionalization are critical for adhesion of the hydrogel layer to the titanium substrate.Keywords: cross-linked networks; grit-blasting; hydrogel grafting; lap-shear adhesion; medical implants; semi-interpenetrating networks; titanium; tribochemical modification;
Co-reporter:Shira G. Kelmanovich, Rachel Parke-Houben and Curtis W. Frank
Soft Matter 2012 vol. 8(Issue 31) pp:8137-8148
Publication Date(Web):02 May 2012
DOI:10.1039/C2SM25389E
Interpenetrating polymer network (IPN) hydrogels composed of a pH-sensitive poly(acrylic acid) (PAA) network and a temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) network were prepared via sequential, UV-initiated polymerization. The molecular interactions and responsive behavior of these hydrogels were investigated using swelling studies under various conditions of pH and temperature. Results indicate the existence of hydrogen-bonded complexation between the two polymer networks, which competes with polymer–water interactions, resulting in lower water content. The effect is most significant in hydrogels with an equimolar ratio of the polymer components. We have also found that ionization of the PAA network creates a swelling force that opposes heat-induced deswelling of the PNIPAAm network. We have shown that the use of ternary contour plots is an efficient way of analyzing the swelling behavior of these complex materials.
Co-reporter:Qi Liao;Amy Tsui;Sarah Billington
Polymer Engineering & Science 2012 Volume 52( Issue 7) pp:1495-1508
Publication Date(Web):
DOI:10.1002/pen.23087
Abstract
Extrusion foaming of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and two blends of PHBV with cellulose acetate butyrate (CAB) were studied using an azodicarbonamide (AZ) blowing agent and a single-screw extruder. The concentration of the blowing agent was systematically varied from 0 to 4.0 phr to achieve maximum density reduction reaching 41%, as well as to obtain information on the dependence of cell growth on blowing agent concentration. Extruded foams were characterized in terms of their bulk densities and cellular morphologies. Stereological and statistical methods permitted full characterization of the three-dimensional cell size distributions, assessing the average cell diameters (ranging from 58 to 290 μm) and cell densities (ranging from 650 to 180,000 cm−3). The variation in cellular morphology among foams consisting of different polymer matrix or blowing agent concentration was compared. The results were analyzed by considering the influence of viscoelastic properties of the polymer matrix on the bubble growth during foaming. Significantly higher melt viscosity and elasticity and reduced gas solubility of the PHBV/CAB blends are believed to retard cell coalescence and collapse during foam expansion, resulting in more uniform cell size distribution and better homogeneity of cellular morphology. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers
Co-reporter:Meredith E. Wiseman and Curtis W. Frank
Langmuir 2012 Volume 28(Issue 3) pp:1765-1774
Publication Date(Web):December 19, 2011
DOI:10.1021/la203095p
The orientation of a monoclonal, anti-streptavidin human IgG1 antibody on a model hydrophobic, CH3-terminated surface (1-dodecanethiol self-assembled monolayer on gold) was studied by monitoring the mechanical coupling between the adsorbed layer and the surface as well as the binding of molecular probes to the antibodies. In this study, the streptavidin antigen was used as a probe for the Fab portions of the antibody, while bacteria-derived Protein G′ was used as a probe for the Fc region. Bovine serum albumin (BSA) acted as a blocking protein. Monolayer coverage occurred around 468 ng/cm2. Below 100 ng/cm2, antibodies were found to adsorb flat-on, tightly coupled to the surface and unable to capture their antigen, whereas the Fc region was able to bind Protein G′. At half-monolayer coverage, there was a transition in the mechanism of adsorption to allow for vertically oriented antibodies, as evidenced by the binding of both Protein G′ and streptavidin as well as looser mechanical coupling with the surface. Monolayer coverage was characterized by a reduced level in probe binding per antibody and an even less rigid coupling to the surface.
Co-reporter:Margaret-Catherine Morse, Qi Liao, Craig S. Criddle, Curtis W. Frank
Polymer 2011 Volume 52(Issue 2) pp:547-556
Publication Date(Web):21 January 2011
DOI:10.1016/j.polymer.2010.11.024
Films of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P3HB-co-3HHx) containing 3.8–10 mol% of 3-hydroxyhexanoate (3HHx) comonomer were subjected to anaerobic biodegradation to explore the effects of copolymer composition, crystallinity, and morphology on biodegradation. As biodegradation proceeded, samples with higher HHx fraction tended to have faster weight loss; on Day 7 of the degradation experiment, P3HB-co-10 mol%-3HHx lost 80% of its original weight, while P3HB-co-3.8 mol%-3HHx lost only 28%. Scanning electron microscopy (SEM) images revealed that the anaerobic biodegradation proceeded at the surface of the samples, with preferential erosion of the amorphous regions, exposing the crystalline spherulites formed inside the copolymer films. It was observed that copolymers with higher HHx fraction had smaller diameter spherulites, ranging from roughly 40 μm for P3HB-co-3.8 mol%-3HHx to 10 μm for P3HB-co-10 mol%-3HHx. A banded spherulite morphology was observed for P3HB-co-6.9 mol%-3HHx and P3HB-co-10 mol%-3HHx, with much wider band spacing (2 μm) for the former than the latter (0.3 μm). Different thermal history seemed to affect the morphological properties and, thus, the biodegradability of the P3HB-co-3HHx samples as well. When comparing copolymers with the same copolymer composition, P3HB-co-3HHx annealed at 70 °C had 5–30% more weight loss after the same duration of incubation in active sludge compared to the quenched samples. We suggest that annealing of P3HB-co-3HHx likely induces void formation in the semi-crystalline structure, facilitating the movement of water or perhaps enzymes to a higher degree of penetration into the sample and subsequently enhancing microbial degradation.
Co-reporter:Laura Hartmann;Kenji Watanabe;Luo Luo Zheng;Chang-Yeon Kim;Stayce E. Beck;Philip Huie;Jaan Nooli;Jennifer R. Cochran;Christopher N. Ta
Journal of Biomedical Materials Research Part B: Applied Biomaterials 2011 Volume 98B( Issue 1) pp:8-17
Publication Date(Web):
DOI:10.1002/jbm.b.31806
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. © 2010 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2011.
Co-reporter:Jesús Alvarez-Sánchez, Angel Licea-Claveríe, José M. Cornejo-Bravo, Curt W. Frank
Reactive and Functional Polymers 2011 71(11) pp: 1077-1088
Publication Date(Web):November 2011
DOI:10.1016/j.reactfunctpolym.2011.08.003
Co-reporter:Dale J. Waters, Kristin Engberg, Rachel Parke-Houben, Christopher N. Ta, Andrew J. Jackson, Michael F. Toney, and Curtis W. Frank
Macromolecules 2011 Volume 44(Issue 14) pp:5776-5787
Publication Date(Web):June 27, 2011
DOI:10.1021/ma200693e
Hydrogels with high modulus and fracture strength are obtained by interpenetrating a tightly cross-linked poly(ethylene glycol) (PEG) network with a loosely cross-linked poly(acrylic acid) (PAA) network. Small-angle X-ray and neutron scattering (SAXS/SANS) are used in conjunction with swelling measurements to determine the structure of PEG/PAA interpenetrating polymer networks (IPNs) and to measure the average PEG chain extension within the IPN. At pH 7.4, the PEG chains within the IPN are extended to 45–70% of their maximum achievable length as a result of expansion of the ionized PAA network within the IPN. Near these high extension ratios, the force required to further strain the PEG chains is increased due to the entropic effects of finite chain extensibility. This leads to PEG/PAA IPN hydrogels with a 3-fold increase in both compressive modulus and fracture strength compared to PEG single networks with the same polymer volume fraction. The structure, mechanical properties, and mechanisms of strength enhancement for PEG/PAA IPN hydrogels are notably different than for the high toughness double-network hydrogels previously described by Gong et al.
Co-reporter:Bob E. Feller, James T. Kellis Jr., Luis G. Cascão-Pereira, Channing R. Robertson, and Curtis W. Frank
Langmuir 2011 Volume 27(Issue 1) pp:250-263
Publication Date(Web):December 3, 2010
DOI:10.1021/la103079t
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.
Co-reporter:Nam-Joon Cho, Joshua A. Jackman, Michael Liu, and Curtis W. Frank
Langmuir 2011 Volume 27(Issue 7) pp:3739-3748
Publication Date(Web):March 2, 2011
DOI:10.1021/la104348f
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.
Co-reporter:Wenwei Zheng and Curtis W. Frank
Langmuir 2010 Volume 26(Issue 6) pp:3929-3941
Publication Date(Web):December 4, 2009
DOI:10.1021/la9032628
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(γ-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 α-helix segments or low packing density with both random coil and α-helix segments.
Co-reporter:Nam-Joon Cho and Curtis W. Frank
Langmuir 2010 Volume 26(Issue 20) pp:15706-15710
Publication Date(Web):September 21, 2010
DOI:10.1021/la101523f
There is great demand to fabricate planar phosphlipid 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.
Co-reporter:Bob E. Feller, James T. Kellis Jr., Luis G. Cascão-Pereira, Channing R. Robertson, and Curtis W. Frank
Langmuir 2010 Volume 26(Issue 24) pp:18916-18925
Publication Date(Web):November 16, 2010
DOI:10.1021/la103080a
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.
Co-reporter:Joshua A. Jackman, Nam-Joon Cho, Randolph S. Duran and Curtis W. Frank
Langmuir 2010 Volume 26(Issue 6) pp:4103-4112
Publication Date(Web):December 18, 2009
DOI:10.1021/la903117x
Bee venom phospholipase A2 (bvPLA2) is part of the secretory phospholipase A2 (sPLA2) family whose members are active in biological processes such as signal transduction and lipid metabolism. While controlling sPLA2 activity is of pharmaceutical interest, the relationship between their mechanistic actions and physiological functions is not well understood. Therefore, we investigated the interfacial binding process of bvPLA2 to characterize its biophysical properties and gain insight into how membrane binding affects interfacial activation. Attention was focused on the role of membrane electrostatics in the binding process. Although dynamic light scattering experiments indicated that bvPLA2 does not lyse lipid vesicles, a novel, nonhydrolytic activity was discovered. We employed a supported lipid bilayer platform on the quartz crystal microbalance with dissipation sensor to characterize this bilayer-disrupting behavior and determined that membrane electrostatics influence this activity. The data suggest that (1) adsorption of bvPLA2 to model membranes is not primarily driven by electrostatic interactions; (2) lipid desorption can follow bvPLA2 adsorption, resulting in nonhydrolytic bilayer-disruption; and (3) this desorption is driven by electrostatic interactions. Taken together, these findings provide evidence that interfacial binding of bvPLA2 is a dynamic process, shedding light on how membrane electrostatics can modulate interfacial activation.
Co-reporter:Dale J. Waters, Kristin Engberg, Rachel Parke-Houben, Laura Hartmann, Christopher N. Ta, Michael F. Toney and Curtis W. Frank
Macromolecules 2010 Volume 43(Issue 16) pp:6861-6870
Publication Date(Web):July 22, 2010
DOI:10.1021/ma101070s
Because of 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 cross-link junctions of high functionality. Although the cross-link 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 cross-link junctions within the PEG network. The cross-link 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 cross-link 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.
Co-reporter:Wonjae Lee;Nam-Joon Cho;Anming Xiong;Jeffrey S. Glenn
PNAS 2010 Volume 107 (Issue 48 ) pp:20709-20714
Publication Date(Web):2010-11-30
DOI:10.1073/pnas.1005211107
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.
Co-reporter:Nam-Joon Cho, Guoliang Wang, Malin Edvardsson, Jeffrey S. Glenn, Fredrik Hook and Curtis W. Frank
Analytical Chemistry 2009 Volume 81(Issue 12) pp:4752
Publication Date(Web):May 21, 2009
DOI:10.1021/ac900242s
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 amphiphathic, α-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.
Co-reporter:Ankit R. Patel, Kay K. Kanazawa and Curtis W. Frank
Analytical Chemistry 2009 Volume 81(Issue 15) pp:6021
Publication Date(Web):July 6, 2009
DOI:10.1021/ac802756v
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) IgG1 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/cm2. 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.
Co-reporter:Masaki Yanagioka, Michael F. Toney and Curtis W. Frank
Macromolecules 2009 Volume 42(Issue 4) pp:1331-1343
Publication Date(Web):February 3, 2009
DOI:10.1021/ma802152s
The viscoelastic properties of nanocomposites are influenced by both the nanoparticle distribution and the nanoparticle−polymer affinity. These two parameters are closely coupled, and evaluation of individual contributions to the mechanical properties is a critical requirement for efficient development of nanocomposites. To decouple these two effects, we utilized charge repulsion among nanoparticles so that we could essentially eliminate particle agglomeration. We then investigated how the nanoparticle−polymer affinity relates to the mechanical properties of the nanocomposite by comparing silica and polystyrene nanoparticles. The surface roughness of the particles and the molecular conformation of the interfacial layer between the polymer and the nanoparticles were characterized by synchrotron small-angle X-ray scattering and quartz crystal microbalance, respectively. On polystyrene particles, the surface roughness was larger, and the polymer adsorbed strongly. Consequently, the mobility of the adsorbed polymer was reduced compared to that on silica particles. This reduced mobility explains a smaller viscoelastic loss for the polystyrene-filled nanocomposite compared to the silica-filled nanocomposite.
Co-reporter:Dale J. Waters, Curtis W. Frank
Polymer 2009 50(26) pp: 6331-6339
Publication Date(Web):
DOI:10.1016/j.polymer.2009.05.034
Co-reporter:Qi Liao, Isao Noda, Curtis W. Frank
Polymer 2009 50(25) pp: 6139-6148
Publication Date(Web):
DOI:10.1016/j.polymer.2009.10.049
Co-reporter:Bayu Atmaja, Jennifer N. Cha and Curtis W. Frank
Langmuir 2009 Volume 25(Issue 2) pp:865-872
Publication Date(Web):December 24, 2008
DOI:10.1021/la801973x
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 ≈ 10.6) and carboxylate moieties of the MUA self-assembled monolayer (SAM) (pKa ≈ 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 α-helical conformation with an average tilt angle of ∼60°. 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°, 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 α-helical block are associated with a lower graft density on the MUA surface compared to those with a shorter α-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.
Co-reporter:Bayu Atmaja, Jennifer N. Cha, Ann Marshall and Curtis W. Frank
Langmuir 2009 Volume 25(Issue 2) pp:707-715
Publication Date(Web):December 11, 2008
DOI:10.1021/la801848d
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 = ahc/ahg), which has been applied extensively in amphiphilic lipid systems. Specifically, as the ratio of the “hydrophobic” area (ahc) to the hydrophilic area (ahg) 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 (α-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.
Co-reporter:Masaki Yanagioka and Curtis W. Frank
Langmuir 2009 Volume 25(Issue 10) pp:5927-5939
Publication Date(Web):April 16, 2009
DOI:10.1021/la804130m
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 °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.
Co-reporter:David Myung;Dale Waters;Meredith Wiseman;Pierre-Emile Duhamel;Jaan Nooli;Christopher N. Ta
Polymers for Advanced Technologies 2008 Volume 19( Issue 6) pp:647-657
Publication Date(Web):
DOI:10.1002/pat.1134
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. Copyright © 2008 John Wiley & Sons, Ltd.
Co-reporter:Masaki Yanagioka and Curtis W. Frank
Macromolecules 2008 Volume 41(Issue 14) pp:5441-5450
Publication Date(Web):June 24, 2008
DOI:10.1021/ma8003778
Agglomeration of particles in composite polymeric materials is a fundamental issue, but the relationship between the particle distribution and the composite mechanical properties is not fully understood. The ultimate goal of this study is to evaluate the effect of particle agglomeration upon mechanical properties of the composite. To achieve this goal, a colloidal crystalline array was encapsulated within a polymer matrix to make a model composite that has a well-ordered particle distribution. We characterized the particle distribution within the polymer matrix experimentally using Bragg diffraction of visible light and compared it with the interaction potential calculated by the Derjaguin–Landau–Verwey–Overbeek theory. Then, apparent cross-link densities of the composites were characterized from both swelling and mechanical measurements. Finally, the dynamic mechanical behavior of the composites with different particle distributions was analyzed. These data suggest that the particle distribution in the polymeric matrix plays an important role in the composite mechanical properties.
Co-reporter:Bob E. Feller, James T. Kellis Jr., Luis G. Cascão-Pereira, Wolfgang Knoll, Channing R. Robertson and Curtis W. Frank
Langmuir 2008 Volume 24(Issue 21) pp:12303-12311
Publication Date(Web):October 10, 2008
DOI:10.1021/la8013943
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.
Co-reporter:Lisa Y. Hwang, Heide Götz, Wolfgang Knoll, Craig J. Hawker and Curtis W. Frank
Langmuir 2008 Volume 24(Issue 24) pp:14088-14098
Publication Date(Web):November 18, 2008
DOI:10.1021/la8022997
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.
Co-reporter:Jasper O. Hardesty, Luis Cascão-Pereira, James T. Kellis, Channing R. Robertson and Curtis W. Frank
Langmuir 2008 Volume 24(Issue 24) pp:13944-13956
Publication Date(Web):November 14, 2008
DOI:10.1021/la8020386
In this work, we studied the interactions of enzymes with model substrate surfaces using label-free techniques. Our model system was based on serine proteases (a class of enzymes that digests proteins) and surface-bound polypeptide substrates. While previous studies have focused on bulk media factors such as pH, ionic strength, and surfactants, this study focuses on the role of the surface-bound substrate itself. In particular, we assess how the substrate density of a polypeptide with an α-helical secondary structure influences surface reactivity. An α-helical secondary structure was chosen based on literature indicating that stable α-helices can resist enzymatic digestion. To investigate the protease resistance of a surface-bound α-helix, we designed an α-helical polypeptide (SS-polypeptide, where SS = disulfide), used it to form films of varying surface coverage and then measured responses of the films to enzymatic exposure. Using quartz-crystal microbalance with dissipation (QCM-D), angle-resolved X-ray photoelectron spectroscopy (AR-XPS), grazing-angle infrared spectroscopy (GAIRS), and other techniques, we characterized the degradation of films to determine how the lateral packing density of the surface-bound SS-polypeptide substrate affected surface proteolysis. Characterization of pure SS-polypeptide films indicated dense packing of helices that maintained their helical structure and were generally oriented normal to the surface. We found that films of pure SS-polypeptide significantly resisted enzymatic digestion, while incorporation of very minor amounts of a diluent in such films resulted in rapid digestion. In part, this may be due to the need for the enzyme to bind several peptides along the peptide substrate within the cleft for digestion to occur. Only SS-polypeptide films that were densely packed and did not permit catalytic access to multiple peptides (e.g., terminal peptides only) were resistant to enzymatic proteolysis.
Co-reporter:Nam-Joon Cho, J. Nelson D'Amour, Johan Stalgren, Wolfgang Knoll, Kay Kanazawa, Curtis W. Frank
Journal of Colloid and Interface Science 2007 Volume 315(Issue 1) pp:248-254
Publication Date(Web):1 November 2007
DOI:10.1016/j.jcis.2007.06.020
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 (ΔR) or a change in the resonance dissipation (ΔD). Two combinations of these observables, in terms of either Δf and ΔR or Δf and ΔD , which we define as Newtonian signatures of S1S1 and S2S2, 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 S1S1 and S2S2 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 S1S1 and S2S2 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.The topography of crystals was measured by AFM that had roughness of ∼2, ∼300, and ∼600 nm∼600 nm, respectively.
Co-reporter:Lisa Y. Hwang, Heide Götz, Craig J. Hawker, Curtis W. Frank
Colloids and Surfaces B: Biointerfaces 2007 Volume 54(Issue 2) pp:127-135
Publication Date(Web):15 February 2007
DOI:10.1016/j.colsurfb.2006.08.010
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 μm2/s for both leaflets of the bilayer are observed for a 5% copolymer system.
Co-reporter:Kevin C. Weng, Jennifer L. Kanter, William H. Robinson, Curtis W. Frank
Colloids and Surfaces B: Biointerfaces 2006 Volume 50(Issue 1) pp:76-84
Publication Date(Web):1 June 2006
DOI:10.1016/j.colsurfb.2006.03.010
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-α-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 ∼12% higher mass and a lower dissipative energy loss during bilayer transformation. PSLBs with 5 mol% GM1 are ∼18% heavier than 100 mol% egg PC and ∼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 ∼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 μm2/s, compared to 1.58 ± 0.04 μm2/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.
Co-reporter:Steve S. He and Curtis W. Frank
Journal of Materials Chemistry A 2014 - vol. 2(Issue 39) pp:NaN16497-16497
Publication Date(Web):2014/08/05
DOI:10.1039/C4TA02942A
Alkaline exchange membranes (AEMs) are a promising class of polyelectrolytes whose alkaline operating environment enables the use of non-precious metal catalysts in low-temperature fuel cells. However, their poor ionic conductivities, which are often an order of magnitude lower than traditional acidic membranes (e.g., Nafion), have limited their practicality. The performance problem can partially be ascribed to the poorly-defined morphologies of typical random copolymer AEMs, leading to tortuous ion transport pathways. Here, we show the ability to form nanoscale (5 to 10 nm diameter) anion transport channels by grafting hydrophilic poly(ethylene glycol) side-chains along a model benzyltrimethylammonium polysulfone-based AEM. Concomitant with the structure formation is a 100% increase in the IEC-normalized hydroxide conductivity from 20.2 mS g cm−1 mmol−1 to 40.3 mS g cm−1 mmol−1 as well as a 50% increase in the peak power density from 118 mW cm−2 to 180 mW cm−2 when incorporated into a fuel cell.
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