Anne M. Mayes

Epidermal growth factor (EGF)-like repeats of human tenascin-C as ligands for EGF receptor.

Signaling through growth factor receptors controls such diverse cell functions as proliferation, migration, and differentiation. A critical question has been how the activation of these receptors is regulated. Most, if not all, of the known ligands for these receptors are soluble factors. However, as matrix components are highly tissue-specific and change during development and pathology, it has been suggested that select growth factor receptors might be stimulated by binding to matrix components. Herein, we describe a new class of ligand for the epidermal growth factor (EGF) receptor (EGFR) found within the EGF-like repeats of tenascin-C, an antiadhesive matrix component present during organogenesis, development, and wound repair. Select EGF-like repeats of tenascin-C elicited mitogenesis and EGFR autophosphorylation in an EGFR-dependent manner. Micromolar concentrations of EGF-like repeats induced EGFR autophosphorylation and activated extracellular signal–regulated, mitogen-activated protein kinase to levels comparable to those induced by subsaturating levels of known EGFR ligands. EGFR-dependent adhesion was noted when the ligands were tethered to inert beads, simulating the physiologically relevant presentation of tenascin-C as hexabrachion, and suggesting an increase in avidity similar to that seen for integrin ligands upon surface binding. Specific binding to EGFR was further established by immunofluorescence detection of EGF-like repeats bound to cells and cross-linking of EGFR with the repeats. Both of these interactions were abolished upon competition by EGF and enhanced by dimerization of the EGF-like repeat. Such low affinity behavior would be expected for a matrix-“tethered” ligand; i.e., a ligand which acts from the matrix, presented continuously to cell surface EGF receptors, because it can neither diffuse away nor be internalized and degraded. These data identify a new class of “insoluble” growth factor ligands and a novel mode of activation for growth factor receptors.

Rubbery Block Copolymer Electrolytes for Solid‐State Rechargeable Lithium Batteries

For nearly 20 years, poly(ethylene oxide)-based materials have been researched for use as electrolytes in solid-state rechargeab le lithium batteries. Technical obstacles to commercialization derive from the inability to satisfy simultaneously the electrical and mechanical performance requirements: high ionic conductivity along with resistance to flow. Herein, the synthesis and characterization of a series of poly(lauryl methacrylate)- b-poly[oligo(oxyethylene) methacrylate]-based block copolymer electrolytes (BCEs) are reported. With both blocks in the rubbery state (i.e., having glass transition temperatures well below room temperatu re) these materials exhibit improved conductivities over those of glassy-rubbery block copolymer systems. Dynamic rheological testing verifies that these materials are dimensionally stable, whereas cyclic voltammetry shows them to be electrochemically stable over a wide potential window, i.e., up to 5 V at 55 8C. A solid-state rechargeable lithium battery was constructed by laminating lithium metal, BCE, and a composite cathode composed of particles of LiAl0.25Mn0.75O2 (monoclinic), carbon black, and graphite in a BCE binder. Cycle testing showed the Li/BCE/LiAl0.25Mn0.75O2 battery to have a high reversible capacity and good capacity

Design and performance of foul-resistant poly(vinylidene fluoride) membranes prepared in a single-step by surface segregation

Immersion precipitated membranes with enhanced fouling resistance are prepared from blends of poly(vinylidene fluoride) (PVDF) and a free-radically synthesized amphiphilic comb polymer having a methacrylate backbone and poly(ethylene oxide) side chains. X-ray photoelectron spectroscopy analysis indicates substantial surface segregation of the comb polymer during membrane coagulation, providing an integrated near-surface coverage of up to 50 vol.% comb for a membrane containing 10 wt.% (14.6 vol.%). The surface coverage increases with comb molecular weight, providing hydrophilic surfaces with excellent stability and substantiating a proposed mechanism for surface localization. Separation surface porosities for comb-modified membranes are up to an order of magnitude higher than PVDF controls. With the combined benefits of fouling resistance and increased porosity, a membrane containing 10 wt.% comb is over 20 times as permeable as a PVDF-only membrane with equivalent separation characteristics after 3 h of filtration of a foulant protein solution.

Melt-Formable Block Copolymer Electrolytes for Lithium Rechargeable Batteries

Microphase separated block copolymers consisting of an amorphous poly(ethylene oxide) (PEO)-based polymer covalently bound to a second polymer offer a highly attractive avenue to achieving both dimensional stability and high ionic conductivity in polymer electrolytes for solid-state rechargeable lithium batteries. However, due to the strong thermodynamic incompatibility typically found for most polymer pairs, the disordered, liquid state of the copolymer can rarely he achieved without the incorporation of a solyent, which complicates processing, Herein, we report the design of new block copolymer electrolytes based on poly(methyl methacrylate), PMMA, and poly(oligo oxycthylene methacrylate), POEM, which are segmentally mixed at elevated temperatures appropriate for melt processing, while exhibiting a mierophase separated (ordered) morphology at ambient temperature. Although pure PMMA-b-POEM is segmentally mixed at all temperatures, it is shown that mierophase separation in these materials can be induced in a controlled manner by the incorporation of even limited amounts of lithium trifluoromethane sulfonate (LiCF 3 SO 3 ), a salt commonly employed to render PEO ionically conductive. Such salt-induced microphase separation suggests a simple method for designing new solid polymer electrolytes combining high ionic conductivities with excellent dimensional stability and improved processing flexibility.

Simulations of Cell-Surface Integrin Binding to Nanoscale-Clustered Adhesion Ligands

Clustering of ligated integrins strongly influences integrin signaling and mechanical linkages between integrins and intracellular structures. Extracellular spatial organization of integrin ligands in clusters may facilitate clustering of bound integrins and thus potentially regulate cellular responses to a defined average amount of ligand in the extracellular environment. The possible role of such ligand clustering effects in controlling overall receptor occupancy is studied here using a simple mass-action equilibrium model as well as a two-dimensional Monte Carlo lattice description of the cell-substrate interface, where cell surface receptors are free to diffuse in the plane of the interface and interact with the substrate-immobilized ligand. Results from the analytical treatment and simulation data indicate that for a single-state model in which receptor-ligand binding equilibria are not influenced by neighboring complexes, clustering of ligand does not enhance total receptor binding. However, if receptor binding energy increases in the presence of neighboring ligated receptors, strong ligand spatial distribution effects arise. Nonlinear responses to increasing ligand density are also observed even in the case of random ligand placement due to stochastic juxtaposition of ligand molecules. These results describe how spatial distribution of ligand presented by the extracellular matrix or by synthetic biomimetic materials might control cell responses to external ligands, and suggest a feedback mechanism by which focal contact formation might be initiated.

Microphase separation in multiblock copolymer melts

The microphase separation transition for asymmetric triblock and (A–B)m star copolymer melts is investigated following a mean‐field approach proposed by Leibler for diblock copolymer melts. Continuous transitions are found in all triblock architectures and in stars for any m at some composition fc. The phase diagram for the equilibrium morphologies is notably altered by varying the architecture of triblock copolymers.

Effect of Counter Ion Placement on Conductivity in Single-Ion Conducting Block Copolymer Electrolytes

Single-ion conducting block copolymer electrolytes were prepared in which counter ions were tethered to the polymer backbone to achieve a lithium transference number of unity. Through tailored anionic synthesis, the influence of counter ion placement on conductivity was investigated. Incorporating the anions outside the ion-conducting @poly~ethylene oxide!-based# block, such as in poly~lauryl methacrylate!-block-poly~lithium methacrylate!-block-poly@(oxyethylene) 9 methacrylate#, known as PLMA-bPLiMA-b-POEM, and P~LMA-r-LiMA!-b-POEM, caused lithium ions to dissociate from the carboxylate counter ions upon microphase separation of the POEM and PLMA blocks, yielding conductivities of 10 25 S/cm at 70°C. In contrast, incorporating anions into the conducting block, as in PLMA-b-P~LiMA-r-OEM!, rendered the majority of lithium ions immobile, resulting in conductivities one to two orders of magnitude lower over the range of temperatures studied for equivalent stoichiometries. Converting the carboxylate anion to one that effectively delocalized charge through complexation with the Lewis acid BF3 raised the conductivity of the latter system to values comparable to those of the other electrolyte architectures. Ion dissociation could thus be equivalently achieved by using a low charge density counter ion (COOBF3 2 ) or by spatially isolating the counter ion from the ion-conducting domains by microphase separation.

Polymer latexes for cell-resistant and cell-interactive surfaces

Novel polymer latexes were prepared that can be applied in several ways for the control and study of cell behavior on surfaces. Acrylic latexes with glass transitions ranging from -30 to 100 degrees C were synthesized by dispersion polymerization in a water and alcohol solution using an amphiphilic comb copolymer as a stabilizing agent. The comb had a poly(methyl methacrylate) backbone and hydrophilic poly(ethylene glycol) (PEG) side chains, which served to stabilize the dispersion and create a robust hydrophilic coating on the final latex particles. The end groups of the comb stabilizer can be selectively functionalized to obtain latex particles with a controlled density of ligands tethered to their surfaces. Latexes were prepared with adhesion peptides (RGD) linked to the surface of the acrylic beads to induce attachment and spreading of cells. Coalesced films obtained from the RGD-bearing latex particles promoted attachment of WT NR6 fibroblasts, while films from unmodified latex particles were resistant to these cells. Additionally, RGD-linked beads were embedded in cell-resistant comb polymer films to create cell-interactive surfaces with discrete clustered-ligand domains. Cell attachment and morphology were seen to vary with the surface density of the RGD-bearing latex beads.

Rubbery Graft Copolymer Electrolytes for Solid-State, Thin-Film Lithium Batteries

Graft copolymer electrolytes (GCEs) of poly[(oxyethylene)9 methacrylate]-g-poly(dimethyl siloxane) (POEM-g-PDMS) (70:30) have been synthesized by simple free radical polymerization using a macromonomer route. Differentialscanning calorimetry, transmission electron microscopy, and small angle neutron scattering confirmed the material to be microphase-separated with a domain periodicity of ∼25 nm. Over the temperature range 290 200 cycles) at a discharge rate of 2/3 C and could be cycled (charged and discharged) at subambient temperature (0°C).

Ultrafiltration Membranes Incorporating Amphiphilic Comb Copolymer Additives Prevent Irreversible Adhesion of Bacteria

We examined the resistance to bacterial adhesion of a novel polyacrylonitrile (PAN) ultrafiltration membrane incorporating the amphiphilic comb copolymer additive, polyacrylonitrile-graft-polyethylene oxide (PAN-g-PEO). The adhesion of bacteria (E. coli K12) and the reversibility of adhered bacteria were tested with the novel membrane, and the behavior was compared to a commercial PAN ultrafiltration membrane. Under static (no flow) bacterial adhesion tests, we observed no bacterial adhesion to the PAN/PAN-g-PEO membrane at all ionic strengths tested, even with the addition of calcium ions. In contrast, significant adhesion of bacterial cells was observed on the commercial PAN membrane, with increased cell adhesion at higher ionic strengths and in the presence of calcium ions. Under crossflow filtration conditions, initial bacterial deposition rate increased with ionic strength and with addition of calcium ions for both membranes, with generally lower bacterial deposition rate with the PAN/PAN-g-PEO membrane. However, deposited bacteria were readily removed (between 97 and 100%) from the surface of the PAN/PAN-g-PEO membrane upon increasing the crossflow and eliminating the permeate flow (i.e., no applied transmembrane pressure), suggesting reversible adhesion of bacteria. In contrast, bacterial adhesion on the commercial PAN membrane was irreversible, with approximately 50% removal of adhered bacteria at moderate ionic strengths (10 and 30 mM) and less than 25% removal at high ionic strength (100 mM). The resistance to bacterial adhesion of the PAN/PAN-g-PEO membrane was further analyzed via measurement of interaction forces with atomic force microscopy (AFM). No adhesion forces were detected between a carboxylated colloid probe and the PAN/PAN-g-PEO membrane, while the probe exhibited strong adhesion to the commercial PAN membrane, consistent with the bacterial adhesion tests. The exceptional resistance of the PAN/PAN-g-PEO membrane to bacterial adhesion is attributable to steric repulsion imparted by the dense brush layer of polyethylene oxide (PEO) chains.