Showing papers by "Stefan Zauscher published in 2009"

Friction Force Microscopy of Lubricin and Hyaluronic Acid between Hydrophobic and Hydrophilic Surfaces.

Abstract: Lubricin and hyaluronic acid (HA), molecular constituents of synovial fluid, have long been theorized to play a role in joint lubrication and wear protection. While lubricin has been shown to function as a boundary lubricant, conflicting evidence exists as to the boundary lubricating ability of hyaluronic acid. Here, we use colloidal force microscopy to explore the friction behavior of these two molecules on the microscale between chemically uniform hydrophilic (hydroxyl-terminated) and hydrophobic (methyl-terminated) surfaces in physiological buffer solution. Behaviors on both surfaces are physiologically relevant since the heterogeneous articular cartilage surface contains both hydrophilic and hydrophobic elements. Friction between hydrophobic surfaces was initially high (μ=1.1, at 100nN of applied normal load) and was significantly reduced by lubricin addition while friction between hydrophilic surfaces was initially low (μ=0.1) and was slightly increased by lubricin addition. At lubricin concentrations above 200 µg/ml, friction behavior on the two surfaces was similar (μ=0.2) indicating that nearly all interaction between the two surfaces was between adsorbed lubricin molecules rather than between the surfaces themselves. In contrast, addition of HA did not appreciably alter the frictional behavior between the model surfaces. No synergistic effect on friction behavior was seen in a physiological mixture of lubricin and HA. Lubricin can equally mediate the frictional response between both hydrophilic and hydrophobic surfaces, likely fully preventing direct surface-to-surface contact at sufficient concentrations, whereas HA provides considerably less boundary lubrication.

Versatile synthesis and micropatterning of nonfouling polymer brushes on the wafer scale

Abstract: In this article, the authors describe new approaches to synthesize and pattern surfaces with poly[oligo(ethylene glycol) methyl methacrylate] (POEGMA) polymer brushes synthesized by surface-initiated atom transfer radical polymerization. These patterned coatings confer “nonfouling” properties protein and cell resistance—to the surface in a biological milieu. The versatile routes for the synthesis of POEGMA demonstrated here offer clear advantages over other techniques previously used in terms of their simplicity, reliability, and ability to pattern large-area substrates. They also demonstrate that POEGMA polymer brushes can be patterned directly by photolithography, plasma ashing, and reactive ion etching to create patterns at the micro- and nanoscale over large areas with high throughput and repeatability, while preserving the protein and cell resistance of the POEGMA brush.

Mechanical properties and gene expression of chondrocytes on micropatterned substrates following dedifferentiation in monolayer

Abstract: Chondrocytes in articular cartilage normally exhibit high expression of collagen II and aggrecan but rapidly dedifferentiate to a fibroblastic phenotype if passaged in culture. Previous studies have suggested that the loss of chondrocyte phenotype is associated with changes in the structure of the F-actin cytoskeleton, which also controls cell mechanical properties. In this study, we examined how dedifferentiation in monolayer influences the mechanical properties of chondrocytes isolated from different zones of articular cartilage. Atomic force microscopy was used to measure the mechanical properties of superficial and middle/deep zone chondrocytes as they underwent serial passaging and subsequent growth on fibronectin-coated, micropatterned self-assembled monolayers that restored a rounded cell shape in 2D culture. Chondrocytes exhibited significant increases in elastic and viscoelastic moduli with dedifferentiation in culture. These changes were only partially ameliorated by the restoration of a rounded shape on micropatterned surfaces. Furthermore, intrinsic zonal differences in cell mechanical properties were rapidly lost with passage. These findings indicate that cell mechanical properties may provide additional measures of phenotypic expression of chondrocytes as they undergo dedifferentiation and possibly redifferentiation in culture.

Separation of peptides with polyionic nanosponges for MALDI-MS analysis.

Abstract: A polymer brush consisting of 70% poly(N-isopropylacrylamide) (PNIPAAM) and 30% polymethacrylic acid (PMAA) was synthesized from gold substrates with a "grafting from" AIBN-type free-radical initiator. Fractionation of two peptides, bradykinin and buccalin, was accomplished in less than 120 s by placing a 30 pM (pH approximately 6.2) droplet onto the polymer brush substrate. The eluant containing the anionic buccalin is pipetted away for MALDI analysis while the cationic bradykinin adsorbed to the swollen anionic brush and was subsequently released by adding a droplet of formic acid to the substrate. This caused the brush to collapse and release the bradykinin, much like squeezing a sponge; these nanosponge substrates exhibited very high loading capacity (>2.0 mg/mL) compared to plasma-polymer-modified MALDI substrates. Ellipsometric measurements showed that complementary peptides adsorb rapidly while those of the same charge do not, and MALDI-MS analysis of the two fractions showed separation of both peptides. The adsorption of bradykinin was monitored over time, and 85% of the peptide had been adsorbed to the nanosponge in 1 min from a 0.5 mg/mL aqueous solution.

Biological applications of polymer brushes

Abstract: Polymer brushes are polymer coatings in which polymer chains are tethered on one end to a surface at sufficiently high grafting density such that steric repulsion between the chains causes chain stretching. The polymer coating thickness is typically dictated by both the polymer chain length and surface graft density, and ranges from a few nanometers to several hundred nanometers. Thus, polymer brushes occupy a unique interfacial niche between self-assembled monolayers that are typically only a few nanometers thick and spin-cast polymer films that are typically thicker than several hundred nanometers. The synthesis of ultrathin, dense polymeric films and structures on surfaces has been a long standing challenge in interface science because packing polymer chains at high density onto a surface is difficult to achieve via methods that involve grafting polymers from the vapor or solution phase on to a surface, due to the intrinsic thermodynamic penalty for additional surface grafting that arises from size exclusion. The recent development of highly controlled polymer synthesis techniques largely based on controlled radical polymerizations that can be carried out from surface-tethered polymerization initiators, has allowed this thermodynamic constraint to be circumvented, and enables the facile, surface-initiated synthesis of polymer brushes with a broad range of chemical and structural properties. In reviewing the field, we have come to the conclusion and one that may excite raucous debate that the most promising technical advances in using polymer brushes have been associated with their biointerfacial applications, as defined by the interaction of these brushes with biological macromolecules, supramolecular assemblies, and cells. This InFocus section contains a set of papers that provides the reader with an idea of the breadth of the field associated with polymer brushes at biointerfaces, ranging from robust methods of brush synthesis and patterning, their use for protein imprinting, to the design of sophisticated macromolecular architectures to control protein and cell adhesion. Surprisingly, despite the wealth of approaches for polymer brush synthesis that are now available, the substrates used to grow polymer brushes have been largely confined to glass and gold. Similarly, methods to pattern polymer brushes over large, e.g., wafer-scale, areas are also scant. Hucknall et al. address these challenges, and demonstrate the synthesis of protein and cell-resistant poly oligo ethylene glycol methyl methacrylate POEGMA brushes over large areas on a range of substrate materials via simple and versatile methods. Furthermore, they present complementary approaches to pattern these polymer brushes with high fidelity and reliability on the wafer-scale for device production. There has been a proliferation of methods to grow stimulus-responsive polymer brush coatings that can be suitably functionalized to regulate biological function in response to an external stimulus. These methods enable the design of polymeric interfaces that can dynamically control the presentation of regulatory signals with applications that span the modulation of biomolecule activity, coatings that dynamically control drug permeation through nanoporous membranes, and substrates that control cell and protein adhesion. Here, Ionov et al. report on a novel approach that uses mixed polymer brushes with gradually changing charge composition to fabricate pH-sensitive surfaces that produce reversible gradients in protein adsorption. The paper by Arifuzzaman et al. describes amphiphilic polymer brush coatings whose chemistry and stimulus responsiveness can be tailored to protect against marine biofouling –a particularly vexing biointerfacial problem and one for which a solution has so far proved elusive. Their innovative approach, using amphiphilic copolymers comprising EG and fluorinated groups, stemmed from the crucial insight that a polymer brush that presents only a single chemical moiety was likely to be insufficient to prevent marine fouling. Control over cell adhesion is an important goal in the design of biomedical implants to improve their biocompatibility and integration with surrounding tissue. Raynor et al. show that titanium implants, modified with POEGMA brushes that present a bioactive peptide, promote osteoblast differentiation and enhance osseointegration. The function of bioanalytical devices, such as biosensors, heterogeneous immunoassays and microarrays, depends critically on generating a sufficiently large signal-to-noise ratio in response to the presence of an analyte of interest, so that minimizing nonspecific binding to reduce noise and maximizing specific binding between the analyte and a capture agent on the surface the receptor is a critical requirement. The immobilization of poly ethylene glycol PEG is the most commonly employed approach to render surfaces resistant to the adventitious and undesirable adsorption of proteins, cells and other biological components. However, synthetic polymer brushes, such as POEGMA as described by Hucknall et al., or polypeptoid brushes, such as described by Statz et al., have marked advantages over simple PEG coatings. In comparison with linear or branched PEGs that are grafted to a surface either by physisorption or covalent attachment and whose surface density cannot be increased

Development of an Adaptive Vapor Chamber With Thermoresponsive Polymer Coating

Abstract: We propose a novel concept for an adaptive vapor chamber using a thermoresponsive polymer coating to enhance heat transfer and reduce local thermal gradients. By coating the wick structures with stimulus-responsive polymer brushes with an upper critical solution temperature (UCST), the hotter surface becomes more wettable than the colder surface. The smaller contact angle at higher temperature generates larger capillary forces and promotes stronger return flow toward the hotspots. In this paper, we present our progress toward developing the adaptive vapor chamber. We have grafted poly(2-(meth-acryloyloxy)ethyl(dimethyl(3-sulfopropyl) ammonium hydroxide) (PMEDSAH) brushes on silica wafers, and the PMEDSAH polymer coating exhibits UCST properties with stable and tunable wettability. We have captured infrared images of the evaporator with steady and transient heating, and developed a thermographic technique that can be used to test the adaptive wick functionality in a vapor chamber.Copyright © 2009 by ASME