Diabetic retinopathy (DR), Retinopathy of Pre-maturity (ROP), and Age-related Macular Degeneratio... more Diabetic retinopathy (DR), Retinopathy of Pre-maturity (ROP), and Age-related Macular Degeneration (AMD) are multifactorial manifestations associated with abnormal growth of blood vessels in the retina. These three diseases account for 5% of the total blindness and vision impairment in the US alone. The current treatment options involve heavily invasive techniques such as frequent intravitreal administration of anti-VEGF (vascular endothelial growth factor) antibodies, which pose serious risks of endophthalmitis, retinal detachment and a multitude of adverse effects stemming from the diverse physiological processes that involve VEGF. To overcome these limitations, this current study utilizes a micellar delivery vehicle (MC) decorated with an anti-angiogenic peptide (aANGP) that inhibits αvβ3 mediated neovascularization using primary endothelial cells (HUVEC). Stable incorporation of the peptide into the micelles (aANGP-MCs) for high valency surface display was achieved with a lipida...
Polyelectrolyte complexes are a fascinating class of soft materials that can span the full spectr... more Polyelectrolyte complexes are a fascinating class of soft materials that can span the full spectrum of mechanical properties from low-viscosity fluids to glassy solids. This spectrum can be accessed by modulating the extent of electrostatic association in these complexes. However, to realize the full potential of polyelectrolyte complexes as functional materials, their molecular level details need to be clearly correlated with their mechanical response. The present work demonstrates that by making simple amendments to the chain architecture, it is possible to affect the salt responsiveness of polyelectrolyte complexes in a systematic manner. This is achieved by quaternizing poly(4vinylpyridine) (QVP) with methyl, ethyl, and propyl substitu-entsthereby increasing the hydrophobicity with increasing side chain lengthand complexing them with a common anionic polyelectrolyte, poly(styrenesulfonate). The mechanical behavior of these complexes is compared to the more hydrophilic system of poly(styrenesulfonate) and poly(diallyldimethylammonium) by quantifying the swelling behavior in response to salt stimuli. More hydrophobic complexes are found to be more resistant to doping by salt, yet the mechanical properties of the complex remain contingent on the overall swelling ratio of the complex itself, following near universal swelling−modulus master curves that are quantified in this work. The rheological behaviors of QVP complex coacervates are found to be approximately the same, only requiring higher salt concentrations to overcome strong hydrophobic interactions, demonstrating that hydrophobicity can be used as an important parameter for tuning the stability of polyelectrolyte complexes in general, while still preserving the ability to be processed "saloplastically".
Nanocarrier administration has primarily been restricted to intermittent bolus injections with li... more Nanocarrier administration has primarily been restricted to intermittent bolus injections with limited available options for sustained delivery in vivo. Here, we demonstrate that cylinder-to-sphere transitions of self-assembled filomicelle (FM) scaffolds can be employed for sustained delivery of monodisperse micellar nanocarriers with improved bioresorptive capacity and modularity for customization. Modular assembly of FMs from diverse block copolymer (BCP) chemistries allows in situ gelation into hydrogel scaffolds following subcutaneous injection into mice. Upon photo-oxidation or physiological oxidation, molecular payloads within FMs transfer to micellar vehicles during the morphological transition, as verified in vitro by electron microscopy and in vivo by flow cytometry. FMs composed of multiple distinct BCP fluorescent conjugates permit multimodal analysis of the scaffold's non-inflammatory bioresorption and micellar delivery to immune cell populations for one month. These...
and mechanical response. We have investigated the mechanical properties of these materials with l... more and mechanical response. We have investigated the mechanical properties of these materials with low-amplitude oscillatory shear rheological measurements. The solvent used here, 2-ethyl-1-hexanol, is a poor solvent for PMMA (A) at low temperatures but a good solvent for PnBA (B) over the entire temperature range studied. The solubility of the PMMA blocks in 2-ethyl-1-hexanol drives the formation of an elastic gel in the ABA triblock copolymer at low temperatures. In these SWCNT/copolymer materials the storage and loss moduli have been observed to increase with time at fixed temperature. When triblock copolymer gels are used as the matrix, we find that the aging effect is erased by cycling the temperature through the gel transition. An increase in storage modulus is observed upon cooling before the gel formation. However, the moduli revert back to lower values when the gel dissolves on heating. We believe this is a result of semi-permanent nanotube junctions being pulled apart when the gel forms. This reversibility is not observed when the nanotubes are dispersed in solutions of diblock copolymer, which do not form gels.
Membranes formed by mixing high molecular weight hyluronic acid (HA) and oppositely charged pepti... more Membranes formed by mixing high molecular weight hyluronic acid (HA) and oppositely charged peptide amphiphiles (PAs) have been shown to have a unique hierarchically ordered structure which consists of three regions: an amorphous biopolymer layer, a narrow region of PA fibers parallel to the interface and a layer of fibers perpendicular to the interface. Understanding of the structure-property relationships in
Model physically associating solutions of acrylic triblock copolymer molecules in a midblock-sele... more Model physically associating solutions of acrylic triblock copolymer molecules in a midblock-selective solvent displayed non-linear strain-stiffening behavior which transitioned to rapid strain softening during shear start-up experiments at reduced rates spanning almost four orders of magnitude. Softening was believed to result from the shear-induced formation of highly localized regions of deformation in the macromolecular network. This behavior was accurately captured by a model that incorporated the strain energy and relaxation behavior of individual network strands in the solution. Flow curves predicted from the model were nonmonotonic, consistent with the onset of flow instabilities at high shear rates. The non-linear stress response reported here, coupled with the wide range of accessible relaxation times of these thermoreversible solutions, makes them ideal model systems for studies of failure-mode transitions in physically associating solutions and gels.
Flory-Huggins interaction parameters (AB, AC, and BC) and the statistical segment lengths (lA, lB... more Flory-Huggins interaction parameters (AB, AC, and BC) and the statistical segment lengths (lA, lB, and lC) were all determined experimentally by fitting the random phase approximation (RPA) to small-angle neutron scattering (SANS) data from the three binary homopolymer blends. These parameters were successfully used to predict the scattering from concentration fluctuations in a homogeneous A/B/A-C blend using multicomponent RPA. These same binary parameters were also used as the only inputs to self-consistent field theory (SCFT) calculations of ordered multicomponent polymer blends. The SCFT calculations enabled quantitative interpretation of the SANS profiles from microphase separated A/B/A-C blends. The phase separation temperatures predicted by theory for the blends were within the experimental error, and the theoretical domain spacings were within 10% of the experimental values.
A quantitative mean-field treatment, based on the representation of polymer chain statistics by p... more A quantitative mean-field treatment, based on the representation of polymer chain statistics by probability distribution functions, is applied to lamellar blends of an AB diblock copolymer with an A homopolymer. We work in the strong segregation limit, where the interface between A and B microdomains is narrow and the A homopolymer segregates exclusively to the A domain. Copolymer blocks are modeled as end-adsorbed 'brushes" which are anchored to opposing sides of a microdomain. Numerical solution of the mean-field equations can be used to determine homopolymer and copolymer profiles for all blend compositions. For pure diblock copolymers we obtain a simple analytic expression for the width of the overlap region between opposing copolymer brushes. Similar analytic expressions are obtained for the homopolymer distribution in blends for which the homopolymer molecular weight is larger than the molecular weight of the corresponding copolymer block. Numerical solutions to the mean-field equations are given for a set of experimentally studied copolymerlhomopolymer blends. Fundamental aspects of lamellar copolymer/ homopolymer blends, including those with low molecular weight homopolymers, are discussed in the context of this well-characterized model system.
Submitted for the MAR10 Meeting of The American Physical Society Behavior of gradient copolymers ... more Submitted for the MAR10 Meeting of The American Physical Society Behavior of gradient copolymers at liquid/liquid interfaces WA YUAN, MICHELLE MOK, JOHN TORKELSON, SONBINH NGUYEN, KEN-NETH SHULL, Northwestern University-The behavior of styrene/acrylic acid gradient and diblock copolymers at liquid/liquid interfaces was investigated using drop shape analysis. Copolymers were dissolved in chloroform, and pendant drops of these solutions were created in water. Molecular conformations at the interface were inferred by measuring changes in the interfacial tension as the interface was contracted and expanded through the control of drop volume. In our experiments, we find that gradient copolymers are much more effective interfacial modifiers than diblock copolymers with similar molecular weights and overall compositions, a result that is ascribed to the reduction of kinetic barriers associated with the micellar aggregates that form in the chloroform phase. Structural evolution of the adsorbed layer can be characterized by trajectories on an interfacial map that includes two independent parameters, interfacial pressure and the area modulus. The area modulus of the adsorbed layer is dependent on the processing history and not directly related to the interfacial pressure. This result is attributed to a local segmental desorption process where portions of the molecules reversibly desorb while the number of copolymer molecules at the interface remains fixed.
Submitted for the MAR05 Meeting of The American Physical Society Adhesive Transfer of Thin Viscoe... more Submitted for the MAR05 Meeting of The American Physical Society Adhesive Transfer of Thin Viscoelastic Films KENNETH SHULL, RACHEL MCSWAIN, Northwestern University-Micellar suspensions of acrylic diblock copolymers are excellent model materials for studying the adhesive transfer of viscoelastic solids. The micellar structure is maintained in films with a variety of thicknesses, giving films with a well-defined structure and viscoelastic character. Thin films were cast onto elastomeric silicone substrates from micellar suspensions in butanol, and the adhesive interactions between these coated elastomeric substrates and a rigid indenter were quantified. By controlling the adhesive properties of the film/indenter and film/substrate interfaces we were able to obtain very clean transfer of the film from the substrate to the portion of the glass indenter with which the film was in contact. Adhesive failure at the film/substrate begins with the nucleation of a cavity at the film/substrate interface, followed by complete delamination of this interface. The final stage in the transfer process involves the failure of the film that bridges the indenter and the elastomeric substrate at the periphery of the contact area. This film is remarkably robust, and is extended to three times its original length prior to failure. Failure of this film occurs at the periphery of the indenter, giving a transferred film that conforms to the original contact area between the indenter and the coated substrate.
Submitted for the MAR05 Meeting of The American Physical Society Contact Mechanics Studies with t... more Submitted for the MAR05 Meeting of The American Physical Society Contact Mechanics Studies with the Quartz Crystal Microbalance F. NELSON NUNALEE, KENNETH R. SHULL, Northwestern University-The mechanism of adhesion between two surfaces that are immersed in a liquid medium is a problem of critical scientific and industrial importance. Practical applications range from targeted drug delivery systems to coatings that are designed to resist fouling by marine organisms. However, quantitative measurement of adhesion in liquids is often complicated by difficulties in determining the true nature of the contact between the two surfaces. In some cases a lack of optical contrast makes it difficult to visualize the contact area, whereas in other cases the optically determined contact may not represent a region of true mechanical contact. We have utilized the quartz crystal microbalance (QCM) in contact mechanics experiments because its response is coupled to the surface rheological properties of the materials that are pressed against it. We have shown that when a hemispherical polymer gel is brought into contact with the electrode surface of the QCM, changes in both the resonant frequency and the dissipation are proportional to the gel/QCM contact area. The actual proportionality constants are determined by the high frequency rheological response of the gel. As a result we have been able to calibrate the QCM for use as a highly sensitive contact sensor for fundamental studies of adhesion of polymer gels.
The binding of streptavidin to biotin located at the terminal ends of poly(ethylene oxide) tether... more The binding of streptavidin to biotin located at the terminal ends of poly(ethylene oxide) tethered to a planar surface is studied using molecular theory. The theoretical model is applied to mimic experiments (Langmuir 2008, 24, 2472) performed using drop-shape analysis to study receptorligand binding at the oil/water interface. Our theoretical predictions show very good agreements with the experimental results. Furthermore, the theory enables us to study the thermodynamic and structural behavior of the PEO-biotin+streptavidin layer. The interfacial structure, shown by the volume fraction profiles of bound proteins and polymers, indicates that the proteins form a thick layer supported by stretched polymers, where the distribution of bound proteins is greater than the thickness of the height of one layer of proteins. When the polymer spacer is composed of PEO (3000), a thick layer with multi-layers of proteins is formed, supported by the stretched polymer chains. It was found that thick multi-layers of proteins are formed when long spacers are present or at very high protein surface coverages on short spacers. This shows that the flexibility of the polymer spacer plays an important role in determining the structure of the bound proteins due to their ability to accommodate highly distorted conformations to optimize binding and protein interactions. Protein domains are predicted when the amount of bound proteins is small due to the existence of streptavidinstreptavidin attractive interactions. As the number of proteins is increased, the competition between attractive interactions and steric repulsions determines the stability and structure of the bound layer. The theory predicts that the competition between these two forces leads to a phase separation at higher protein concentrations. The point where this transition happens depends on both spacer length and protein surface coverage and is an important consideration for practical applications of these and other similar systems. If the goal is to maximize protein binding, it is favorable to be above the layer transition, as multiple layers can accommodate greater bound protein densities. On the other hand, if the goal is to use these bound proteins as a linker group to build more complex structures, such as when avidin or streptavidin serves as a linker between two biotinylated polymers or proteins, the optimum is to be below the layer transition such that all bound linker proteins are available for further binding.
Graphite oxide sheet, now called graphene oxide (GO), is the product of chemical exfoliation of g... more Graphite oxide sheet, now called graphene oxide (GO), is the product of chemical exfoliation of graphite and has been known for more than a century. GO has been largely viewed as hydrophilic, presumably due to its excellent colloidal stability in water. Here we report that GO is an amphiphile with hydrophilic edges and a more hydrophobic basal plane. GO can act like a surfactant, as measured by its ability to adsorb on interfaces and lower the surface or interfacial tension. Since the degree of ionization of the edge-COOH groups is affected by pH, GO's amphiphilicity can be tuned by pH. In addition, sizedependent amphiphilicity of GO sheets is observed. Since each GO sheet is a single molecule as well as a colloidal particle, the molecule-colloid duality makes it behave like both a molecular and a colloidal surfactant. For example, GO is capable of creating highly stable Pickering emulsions of organic solvents like solid particles. It can also act as a molecular dispersing agent to process insoluble materials such as graphite and carbon nanotubes in water. The ease of its conversion to chemically modified graphene could enable new opportunities in solution processing of functional materials.
APS Bulletin of the American Physical Society. 2007 APS March Meeting Volume 52, Number 1. Monday... more APS Bulletin of the American Physical Society. 2007 APS March Meeting Volume 52, Number 1. MondayFriday, March 59, 2007; Denver, Colorado. ...
In our laboratory we have developed a highly sensitive experiment which allows the measurement of... more In our laboratory we have developed a highly sensitive experiment which allows the measurement of very weak adhesive interactions between interfaces joined together by specific bonds. In the experiment, we place a membrane across a glass tube and use a ...
We report here on the formation of a bioactive hierarchically structured membrane by selfassembly... more We report here on the formation of a bioactive hierarchically structured membrane by selfassembly. The membrane is formed with hyaluronic acid and peptide amphiphiles with binding affinity for heparin, and its hierarchical structure contains both an amorphous zone and a layer of fibrils oriented perpendicular to the membrane plane. The design of bioactivity is based on the potential ability to bind and slowly release heparin-binding growth factors. Human mesenchymal stem cells seeded on these membranes attached and remained viable. Basic fibroblast growth factor (FGF2) and vascular endothelial growth factor (VEGF) were incorporated within the membrane structure prior to self-assembly and released into media over a prolonged period of time (14 days). Using the chicken chorioallantoic membrane (CAM) assay, we also found that these membranes induced a significant and rapid enhancement of angiogenesis relative to controls.
Diabetic retinopathy (DR), Retinopathy of Pre-maturity (ROP), and Age-related Macular Degeneratio... more Diabetic retinopathy (DR), Retinopathy of Pre-maturity (ROP), and Age-related Macular Degeneration (AMD) are multifactorial manifestations associated with abnormal growth of blood vessels in the retina. These three diseases account for 5% of the total blindness and vision impairment in the US alone. The current treatment options involve heavily invasive techniques such as frequent intravitreal administration of anti-VEGF (vascular endothelial growth factor) antibodies, which pose serious risks of endophthalmitis, retinal detachment and a multitude of adverse effects stemming from the diverse physiological processes that involve VEGF. To overcome these limitations, this current study utilizes a micellar delivery vehicle (MC) decorated with an anti-angiogenic peptide (aANGP) that inhibits αvβ3 mediated neovascularization using primary endothelial cells (HUVEC). Stable incorporation of the peptide into the micelles (aANGP-MCs) for high valency surface display was achieved with a lipida...
Polyelectrolyte complexes are a fascinating class of soft materials that can span the full spectr... more Polyelectrolyte complexes are a fascinating class of soft materials that can span the full spectrum of mechanical properties from low-viscosity fluids to glassy solids. This spectrum can be accessed by modulating the extent of electrostatic association in these complexes. However, to realize the full potential of polyelectrolyte complexes as functional materials, their molecular level details need to be clearly correlated with their mechanical response. The present work demonstrates that by making simple amendments to the chain architecture, it is possible to affect the salt responsiveness of polyelectrolyte complexes in a systematic manner. This is achieved by quaternizing poly(4vinylpyridine) (QVP) with methyl, ethyl, and propyl substitu-entsthereby increasing the hydrophobicity with increasing side chain lengthand complexing them with a common anionic polyelectrolyte, poly(styrenesulfonate). The mechanical behavior of these complexes is compared to the more hydrophilic system of poly(styrenesulfonate) and poly(diallyldimethylammonium) by quantifying the swelling behavior in response to salt stimuli. More hydrophobic complexes are found to be more resistant to doping by salt, yet the mechanical properties of the complex remain contingent on the overall swelling ratio of the complex itself, following near universal swelling−modulus master curves that are quantified in this work. The rheological behaviors of QVP complex coacervates are found to be approximately the same, only requiring higher salt concentrations to overcome strong hydrophobic interactions, demonstrating that hydrophobicity can be used as an important parameter for tuning the stability of polyelectrolyte complexes in general, while still preserving the ability to be processed "saloplastically".
Nanocarrier administration has primarily been restricted to intermittent bolus injections with li... more Nanocarrier administration has primarily been restricted to intermittent bolus injections with limited available options for sustained delivery in vivo. Here, we demonstrate that cylinder-to-sphere transitions of self-assembled filomicelle (FM) scaffolds can be employed for sustained delivery of monodisperse micellar nanocarriers with improved bioresorptive capacity and modularity for customization. Modular assembly of FMs from diverse block copolymer (BCP) chemistries allows in situ gelation into hydrogel scaffolds following subcutaneous injection into mice. Upon photo-oxidation or physiological oxidation, molecular payloads within FMs transfer to micellar vehicles during the morphological transition, as verified in vitro by electron microscopy and in vivo by flow cytometry. FMs composed of multiple distinct BCP fluorescent conjugates permit multimodal analysis of the scaffold's non-inflammatory bioresorption and micellar delivery to immune cell populations for one month. These...
and mechanical response. We have investigated the mechanical properties of these materials with l... more and mechanical response. We have investigated the mechanical properties of these materials with low-amplitude oscillatory shear rheological measurements. The solvent used here, 2-ethyl-1-hexanol, is a poor solvent for PMMA (A) at low temperatures but a good solvent for PnBA (B) over the entire temperature range studied. The solubility of the PMMA blocks in 2-ethyl-1-hexanol drives the formation of an elastic gel in the ABA triblock copolymer at low temperatures. In these SWCNT/copolymer materials the storage and loss moduli have been observed to increase with time at fixed temperature. When triblock copolymer gels are used as the matrix, we find that the aging effect is erased by cycling the temperature through the gel transition. An increase in storage modulus is observed upon cooling before the gel formation. However, the moduli revert back to lower values when the gel dissolves on heating. We believe this is a result of semi-permanent nanotube junctions being pulled apart when the gel forms. This reversibility is not observed when the nanotubes are dispersed in solutions of diblock copolymer, which do not form gels.
Membranes formed by mixing high molecular weight hyluronic acid (HA) and oppositely charged pepti... more Membranes formed by mixing high molecular weight hyluronic acid (HA) and oppositely charged peptide amphiphiles (PAs) have been shown to have a unique hierarchically ordered structure which consists of three regions: an amorphous biopolymer layer, a narrow region of PA fibers parallel to the interface and a layer of fibers perpendicular to the interface. Understanding of the structure-property relationships in
Model physically associating solutions of acrylic triblock copolymer molecules in a midblock-sele... more Model physically associating solutions of acrylic triblock copolymer molecules in a midblock-selective solvent displayed non-linear strain-stiffening behavior which transitioned to rapid strain softening during shear start-up experiments at reduced rates spanning almost four orders of magnitude. Softening was believed to result from the shear-induced formation of highly localized regions of deformation in the macromolecular network. This behavior was accurately captured by a model that incorporated the strain energy and relaxation behavior of individual network strands in the solution. Flow curves predicted from the model were nonmonotonic, consistent with the onset of flow instabilities at high shear rates. The non-linear stress response reported here, coupled with the wide range of accessible relaxation times of these thermoreversible solutions, makes them ideal model systems for studies of failure-mode transitions in physically associating solutions and gels.
Flory-Huggins interaction parameters (AB, AC, and BC) and the statistical segment lengths (lA, lB... more Flory-Huggins interaction parameters (AB, AC, and BC) and the statistical segment lengths (lA, lB, and lC) were all determined experimentally by fitting the random phase approximation (RPA) to small-angle neutron scattering (SANS) data from the three binary homopolymer blends. These parameters were successfully used to predict the scattering from concentration fluctuations in a homogeneous A/B/A-C blend using multicomponent RPA. These same binary parameters were also used as the only inputs to self-consistent field theory (SCFT) calculations of ordered multicomponent polymer blends. The SCFT calculations enabled quantitative interpretation of the SANS profiles from microphase separated A/B/A-C blends. The phase separation temperatures predicted by theory for the blends were within the experimental error, and the theoretical domain spacings were within 10% of the experimental values.
A quantitative mean-field treatment, based on the representation of polymer chain statistics by p... more A quantitative mean-field treatment, based on the representation of polymer chain statistics by probability distribution functions, is applied to lamellar blends of an AB diblock copolymer with an A homopolymer. We work in the strong segregation limit, where the interface between A and B microdomains is narrow and the A homopolymer segregates exclusively to the A domain. Copolymer blocks are modeled as end-adsorbed 'brushes" which are anchored to opposing sides of a microdomain. Numerical solution of the mean-field equations can be used to determine homopolymer and copolymer profiles for all blend compositions. For pure diblock copolymers we obtain a simple analytic expression for the width of the overlap region between opposing copolymer brushes. Similar analytic expressions are obtained for the homopolymer distribution in blends for which the homopolymer molecular weight is larger than the molecular weight of the corresponding copolymer block. Numerical solutions to the mean-field equations are given for a set of experimentally studied copolymerlhomopolymer blends. Fundamental aspects of lamellar copolymer/ homopolymer blends, including those with low molecular weight homopolymers, are discussed in the context of this well-characterized model system.
Submitted for the MAR10 Meeting of The American Physical Society Behavior of gradient copolymers ... more Submitted for the MAR10 Meeting of The American Physical Society Behavior of gradient copolymers at liquid/liquid interfaces WA YUAN, MICHELLE MOK, JOHN TORKELSON, SONBINH NGUYEN, KEN-NETH SHULL, Northwestern University-The behavior of styrene/acrylic acid gradient and diblock copolymers at liquid/liquid interfaces was investigated using drop shape analysis. Copolymers were dissolved in chloroform, and pendant drops of these solutions were created in water. Molecular conformations at the interface were inferred by measuring changes in the interfacial tension as the interface was contracted and expanded through the control of drop volume. In our experiments, we find that gradient copolymers are much more effective interfacial modifiers than diblock copolymers with similar molecular weights and overall compositions, a result that is ascribed to the reduction of kinetic barriers associated with the micellar aggregates that form in the chloroform phase. Structural evolution of the adsorbed layer can be characterized by trajectories on an interfacial map that includes two independent parameters, interfacial pressure and the area modulus. The area modulus of the adsorbed layer is dependent on the processing history and not directly related to the interfacial pressure. This result is attributed to a local segmental desorption process where portions of the molecules reversibly desorb while the number of copolymer molecules at the interface remains fixed.
Submitted for the MAR05 Meeting of The American Physical Society Adhesive Transfer of Thin Viscoe... more Submitted for the MAR05 Meeting of The American Physical Society Adhesive Transfer of Thin Viscoelastic Films KENNETH SHULL, RACHEL MCSWAIN, Northwestern University-Micellar suspensions of acrylic diblock copolymers are excellent model materials for studying the adhesive transfer of viscoelastic solids. The micellar structure is maintained in films with a variety of thicknesses, giving films with a well-defined structure and viscoelastic character. Thin films were cast onto elastomeric silicone substrates from micellar suspensions in butanol, and the adhesive interactions between these coated elastomeric substrates and a rigid indenter were quantified. By controlling the adhesive properties of the film/indenter and film/substrate interfaces we were able to obtain very clean transfer of the film from the substrate to the portion of the glass indenter with which the film was in contact. Adhesive failure at the film/substrate begins with the nucleation of a cavity at the film/substrate interface, followed by complete delamination of this interface. The final stage in the transfer process involves the failure of the film that bridges the indenter and the elastomeric substrate at the periphery of the contact area. This film is remarkably robust, and is extended to three times its original length prior to failure. Failure of this film occurs at the periphery of the indenter, giving a transferred film that conforms to the original contact area between the indenter and the coated substrate.
Submitted for the MAR05 Meeting of The American Physical Society Contact Mechanics Studies with t... more Submitted for the MAR05 Meeting of The American Physical Society Contact Mechanics Studies with the Quartz Crystal Microbalance F. NELSON NUNALEE, KENNETH R. SHULL, Northwestern University-The mechanism of adhesion between two surfaces that are immersed in a liquid medium is a problem of critical scientific and industrial importance. Practical applications range from targeted drug delivery systems to coatings that are designed to resist fouling by marine organisms. However, quantitative measurement of adhesion in liquids is often complicated by difficulties in determining the true nature of the contact between the two surfaces. In some cases a lack of optical contrast makes it difficult to visualize the contact area, whereas in other cases the optically determined contact may not represent a region of true mechanical contact. We have utilized the quartz crystal microbalance (QCM) in contact mechanics experiments because its response is coupled to the surface rheological properties of the materials that are pressed against it. We have shown that when a hemispherical polymer gel is brought into contact with the electrode surface of the QCM, changes in both the resonant frequency and the dissipation are proportional to the gel/QCM contact area. The actual proportionality constants are determined by the high frequency rheological response of the gel. As a result we have been able to calibrate the QCM for use as a highly sensitive contact sensor for fundamental studies of adhesion of polymer gels.
The binding of streptavidin to biotin located at the terminal ends of poly(ethylene oxide) tether... more The binding of streptavidin to biotin located at the terminal ends of poly(ethylene oxide) tethered to a planar surface is studied using molecular theory. The theoretical model is applied to mimic experiments (Langmuir 2008, 24, 2472) performed using drop-shape analysis to study receptorligand binding at the oil/water interface. Our theoretical predictions show very good agreements with the experimental results. Furthermore, the theory enables us to study the thermodynamic and structural behavior of the PEO-biotin+streptavidin layer. The interfacial structure, shown by the volume fraction profiles of bound proteins and polymers, indicates that the proteins form a thick layer supported by stretched polymers, where the distribution of bound proteins is greater than the thickness of the height of one layer of proteins. When the polymer spacer is composed of PEO (3000), a thick layer with multi-layers of proteins is formed, supported by the stretched polymer chains. It was found that thick multi-layers of proteins are formed when long spacers are present or at very high protein surface coverages on short spacers. This shows that the flexibility of the polymer spacer plays an important role in determining the structure of the bound proteins due to their ability to accommodate highly distorted conformations to optimize binding and protein interactions. Protein domains are predicted when the amount of bound proteins is small due to the existence of streptavidinstreptavidin attractive interactions. As the number of proteins is increased, the competition between attractive interactions and steric repulsions determines the stability and structure of the bound layer. The theory predicts that the competition between these two forces leads to a phase separation at higher protein concentrations. The point where this transition happens depends on both spacer length and protein surface coverage and is an important consideration for practical applications of these and other similar systems. If the goal is to maximize protein binding, it is favorable to be above the layer transition, as multiple layers can accommodate greater bound protein densities. On the other hand, if the goal is to use these bound proteins as a linker group to build more complex structures, such as when avidin or streptavidin serves as a linker between two biotinylated polymers or proteins, the optimum is to be below the layer transition such that all bound linker proteins are available for further binding.
Graphite oxide sheet, now called graphene oxide (GO), is the product of chemical exfoliation of g... more Graphite oxide sheet, now called graphene oxide (GO), is the product of chemical exfoliation of graphite and has been known for more than a century. GO has been largely viewed as hydrophilic, presumably due to its excellent colloidal stability in water. Here we report that GO is an amphiphile with hydrophilic edges and a more hydrophobic basal plane. GO can act like a surfactant, as measured by its ability to adsorb on interfaces and lower the surface or interfacial tension. Since the degree of ionization of the edge-COOH groups is affected by pH, GO's amphiphilicity can be tuned by pH. In addition, sizedependent amphiphilicity of GO sheets is observed. Since each GO sheet is a single molecule as well as a colloidal particle, the molecule-colloid duality makes it behave like both a molecular and a colloidal surfactant. For example, GO is capable of creating highly stable Pickering emulsions of organic solvents like solid particles. It can also act as a molecular dispersing agent to process insoluble materials such as graphite and carbon nanotubes in water. The ease of its conversion to chemically modified graphene could enable new opportunities in solution processing of functional materials.
APS Bulletin of the American Physical Society. 2007 APS March Meeting Volume 52, Number 1. Monday... more APS Bulletin of the American Physical Society. 2007 APS March Meeting Volume 52, Number 1. MondayFriday, March 59, 2007; Denver, Colorado. ...
In our laboratory we have developed a highly sensitive experiment which allows the measurement of... more In our laboratory we have developed a highly sensitive experiment which allows the measurement of very weak adhesive interactions between interfaces joined together by specific bonds. In the experiment, we place a membrane across a glass tube and use a ...
We report here on the formation of a bioactive hierarchically structured membrane by selfassembly... more We report here on the formation of a bioactive hierarchically structured membrane by selfassembly. The membrane is formed with hyaluronic acid and peptide amphiphiles with binding affinity for heparin, and its hierarchical structure contains both an amorphous zone and a layer of fibrils oriented perpendicular to the membrane plane. The design of bioactivity is based on the potential ability to bind and slowly release heparin-binding growth factors. Human mesenchymal stem cells seeded on these membranes attached and remained viable. Basic fibroblast growth factor (FGF2) and vascular endothelial growth factor (VEGF) were incorporated within the membrane structure prior to self-assembly and released into media over a prolonged period of time (14 days). Using the chicken chorioallantoic membrane (CAM) assay, we also found that these membranes induced a significant and rapid enhancement of angiogenesis relative to controls.
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Papers by Kenneth Shull