Virginia M. Ayres

Increased FGF-2 secretion and ability to support neurite outgrowth by astrocytes cultured on polyamide nanofibrillar matrices

An electrospun nonwoven matrix of polyamide nanofibers was employed as a new model for the capillary basement membrane at the blood-brain barrier (BBB). The basement membrane separates astrocytes from endothelial cells and is associated with growth factors, such as fibroblast growth factor-2 (FGF-2). FGF-2 is produced by astrocytes and induces specialized functions in endothelial cells, but also has actions on astrocytes. To investigate potential autocrine actions of FGF-2 at the BBB, astrocytes were cultured on unmodified nanofibers or nanofibers covalently modified with FGF-2. The former assumed an in vivo-like stellate morphology that was enhanced in the presence of cross-linked FGF-2. Furthermore, astrocyte monolayers established on unmodified nanofibers were more permissive for neurite outgrowth when cultured with an overlay of neurons than similar monolayers established on standard tissue culture surfaces, while astrocytes cultured on FGF-2-modifed nanofibers were yet more permissive. The observed differences were due in part to progressively increasing amounts of FGF-2 secreted by the astrocytes into the medium; hence FGF-2 increases its own expression in astrocytes to modulate astrocyte-neuron interactions. Soluble FGF-2 was unable to replicate the effects of cross-linked FGF-2. Nanofibers alone up-regulated FGF-2, albeit to a lesser extent than nanofibers covalently modified with FGF-2. These results underscore the importance of both surface topography and growth factor presentation on cellular function. Moreover, these results indicate that FGF-2-modified nanofibrillar scaffolds may demonstrate utility in tissue engineering applications for replacement and regeneration of lost tissue following central nervous system (CNS) injury or disease.

Nanopipes in Gallium Nitride Nanowires and Rods

Gallium nitride nanowires and rods synthesized by a catalyst-free vapor-solid growth method were analyzed with cross section high-resolution transmission electron microscopy. The cross section studies revealed hollow core screw dislocations, or nanopipes, in the nanowires and rods. The hollow cores were located at or near the center of the nanowires and rods, along the axis of a screw dislocation. The formation of the hollow cores is consistent with effect of screw dislocations with giant Burgers vector predicted by Frank.

Electronic and Structural Characteristics of Zinc-Blende Wurtzite Biphasic Homostructure GaN Nanowires

We report a new biphasic crystalline wurtzite/zinc-blende homostructure in gallium nitride nanowires. Cathodoluminescence was used to quantitatively measure the wurtzite and zinc-blende band gaps. High-resolution transmission electron microscopy was used to identify distinct wurtzite and zinc-blende crystalline phases within single nanowires through the use of selected area electron diffraction, electron dispersive spectroscopy, electron energy loss spectroscopy, and fast Fourier transform techniques. A mechanism for growth is identified.

Precision micromachining of CVD diamond films

Abstract The laser ablation technique has been extensively recognized to be a unique method for the micromachining and designing of micro components. Chemical vapor deposited (CVD) diamond films have been processed by various types of pulsed lasers for this purpose. Nanosecond pulsed excimer lasers, and recently femtosecond pulsed lasers have been used for the micromachining of diamond films. Even though the interaction between the laser and the material is limited to the nanosecond to femtosecond range, plasma, induced by ablative material ejection, extends to tens of microseconds. Formation and expansion of the plasma give rise to thermal damage, material ejection and re-deposition, resulting in spoiled interfaces, recast layers, and rippled surfaces. The plasma generated thermal damage has been a major obstruction to micron and submicron micromachining of diamond films. To implement the laser ablation technique to the level of precision, a systematic investigation is required on the plasma interaction. This investigation is mainly focused on a novel approach to compensate for the deleterious plasma–diamond interaction. CVD diamond films were subject to various processing environments, such as atmospheric condition, vacuum condition and gas stream condition. Observed damage depends on behavior of the plasma in the different processing environments. In the gas stream condition, it has been clearly shown that proper dissipation of the high-temperature plasma leads to precisely irradiated surfaces, which are almost free from thermal damage. An emphasis of this research is to achieve maximum dissipation of the high-temperature plasma and proper quenching of the target surface. Based on a nanosecond pulsed excimer laser, technical and fundamental investigations for the precision micromachining are performed. Further investigations of the properties of the laser-irradiated films are also currently in progress, including investigations of laser-induced stress, surface morphologies, surface free energy and electronic properties.

Nanofibrillar scaffolds induce preferential activation of Rho GTPases in cerebral cortical astrocytes

Cerebral cortical astrocyte responses to polyamide nanofibrillar scaffolds versus poly-L-lysine (PLL)-functionalized planar glass, unfunctionalized planar Aclar coverslips, and PLL-functionalized planar Aclar surfaces were investigated by atomic force microscopy and immunocytochemistry. The physical properties of the cell culture environments were evaluated using contact angle and surface roughness measurements and compared. Astrocyte morphological responses, including filopodia, lamellipodia, and stress fiber formation, and stellation were imaged using atomic force microscopy and phalloidin staining for F-actin. Activation of the corresponding Rho GTPase regulators was investigated using immunolabeling with Cdc42, Rac1, and RhoA. Astrocytes cultured on the nanofibrillar scaffolds showed a unique response that included stellation, cell–cell interactions by stellate processes, and evidence of depression of RhoA. The results support the hypothesis that the extracellular environment can trigger preferential activation of members of the Rho GTPase family, with demonstrable morphological consequences for cerebral cortical astrocytes.

Deconvolution of atomic force microscopy data for cellular and molecular imaging

The restoration of image features in cellular and molecular images is a crucial problem in nanobiological investigations. Scanning probe microscopy (SPM) offers the potential for direct investigative capability at nanometer resolution necessary for imaging biological units and macromolecular protein control blocks. The distortion of the measured image due to tip-sample interaction is a major challenge for nanoscale metrology, and signal processing solutions are needed for increasing the accuracy and reliability of the data. Two candidate approaches have been described in detail in this article for modeling the tip-sample interaction from a topographical perspective, which is then used for reconstructing the sample surface from known tip geometry. When the aspect ratio of a feature is comparable with that of the tip, the two methods produce similar results, but when the aspect ratio is larger than that of the tip, the MM method produces a sharper estimate than the LT method. When the tip geometry is not known, blind-tip estimations methods are needed for iterative estimations of tip and sample surfaces

The effect of nitrogen on competitive growth mechanisms of diamond thin films

The correlation between surface morphology and texture (distribution of crystal orientations) has been analyzed to determine how nitrogen provides a competitive growth advantage for the formation of 001 texture in diamond films. The film morphologies were measured using TappingMode@ atomic force microscopy. Corresponding texture studies were performed using X-ray diVraction with a four-circle goniometer to measure (111), (220) and (400) pole figures. Orientation distributions were generated from the pole figures, from which inverse pole figures were made to quantify how texture was aVected by process variables. After 2 h of growth, with and without nitrogen, similar textures were observed. With increasing time, nitrogen stimulates the growth of orientations between 001, 114, and 104. Without nitrogen, the majority orientation is 101 with a secondary component near 113. Examination of fracture surfaces indicates correlations with the observed texture and morphology changes.

Electron transport in zinc-blende wurtzite biphasic gallium nitride nanowires and GaNFETs

Two-point and four-point probe electrical measurements of a biphasic gallium nitride nanowire and current–voltage characteristics of a gallium nitride nanowire based field effect transistor are reported. The biphasic gallium nitride nanowires have a crystalline homostructure consisting of wurtzite and zinc-blende phases that grow simultaneously in the longitudinal direction. There is a sharp transition of one to a few atomic layers between each phase. All measurements showed high current densities. Evidence of single-phase current transport in the biphasic nanowire structure is discussed.

Infra-red emission characterization of polycrystalline diamond films

Abstract Infra-red emission has been used to characterize polycrystalline diamond films of differing qualities, produced by de arcjet chemical vapor deposition. Intrinsic and defect-induced bands were observed from samples heated in a low emissivity cell using Fourier Transform Infra-red spectroscopy (FTIR). In addition, the IR emission and absorption spectra of polished Ila and Ib single crystal diamonds were measured to compare the intrinsic and nitrogen-induced absorption and emission bands. Emission features observed included the intrinsic two phonon bands, defect-induced one phonon bands (from symmetry breaking), nitrogen-related bands, and CH stretching bands. Complementary information on the quality and defects in the samples was provided using photoluminescence and Raman spectroscopies.

Differentiation of reactive-like astrocytes cultured on nanofibrillar and comparative culture surfaces

AIM To investigate the directive importance of nanophysical properties on the morphological and protein expression responses of dibutyryladenosine cyclic monophosphate (dBcAMP)-treated cerebral cortical astrocytes in vitro. MATERIALS & METHODS Elasticity and work of adhesion characterizations of culture surfaces were performed using atomic force microscopy and combined with previous surface roughness and polarity results. The morphological and biochemical differentiation of dBcAMP-treated astrocytes cultured on promising nanofibrillar scaffolds and comparative culture surfaces were investigated by immunocytochemistry, colocalization, super resolution microscopy and atomic force microscopy. The dBcAMP-treated astrocyte responses were further compared with untreated astrocyte responses. RESULTS & CONCLUSION Nanofibrillar scaffold properties were shown to reduce immunoreactivity responses while poly-L-lysine-functionalized Aclar® (Ted Pella Inc., CA, USA) properties were shown to induce responses reminiscent of glial scar formation. The comparison study indicated that directive cues may differ in wound-healing versus quiescent situations.