Nanoscale Coatings Research Articles

Novel Vapor Phase biodegradable coatings for controlled drug delivery systems

AbstractThe synthesis of nano-functionalized drug particles or drug particles with nanoscale biodegradable polymeric coatings has wider ranging applications in sustained release drug delivery systems. An important area of application includes delivery of drugs, such as proteins, peptides, and glucocorticoids via the inhalation process. The advantages of this method include, rapid absorption by the body and lack of metabolic degradation effects which is typically by some drugs during oral delivery. Here we discuss the novel pulsed laser and pulsed electron method to synthesize nanoscale biodegradable coatings of poly lactic acid (PLA) onto antiasthmatic drugs. In pulsed laser method a pulsed excimer laser ablates a thin layer of polymer which is deposited onto 1 -5 micron drug particles which are fluidized in the gas chamber. The control of the thickness, conformation and the chemistry of the nanoscale coating process will be discussed in detail. Applications of this methodology to delivery of anti asthamtic drugs and insulin will be discussed. The effect of polymer coating chemistry, molecular weight, hydrophobicity, and thickness on the sustained release characteristics will be discussed. A unique method for depositing the PLA films, pulsed electron beam deposition (PED) was used to deposit the films. This technique was tried done to overcome the difficulties like low absorption co-efficient of UV-laser. Method proved to be promising for producing smooth films and offers a better thickness control of the deposition. The RMS roughness and other morphological characteristics of PED deposited films were superior over the conventional laser ablated films.

Cell/surface interactions on laser micro-textured titanium-coated silicon surfaces

This paper examines the effects of nano-scale titanium coatings, and micro-groove/micro-grid patterns on cell/surface interactions on silicon surfaces. The nature of the cellular attachment and adhesion to the coated/uncoated micro-textured surfaces was elucidated by the visualization of the cells and relevant cytoskeletal & focal adhesion proteins through scanning electron microscopy and immunofluorescence staining. Increased cell spreading and proliferation rates are observed on surfaces with 50 nm thick Ti coatings. The micro-groove geometries have been shown to promote contact guidance, which leads to reduced scar tissue formation. In contrast, smooth surfaces result in random cell orientations and the increased possibility of scar tissue formation. Immunofluorescence cell staining experiments also reveal that the actin stress fibers are aligned along the groove dimensions, with discrete focal adhesions occurring along the ridges, within the grooves and at the ends of the cell extensions. The implications of the observed cell/surface interactions are discussed for possible applications of silicon in implantable biomedical systems.

Thermal stability of nanostructured superhard coatings: A review

It is important to evaluate the thermal stability of hard coating because at high working temperatures the mechanical and tribological properties are deteriorated. The temperature operating on the cutting tool tip during work may reach temperatures as high as 1000 °C. Environmental considerations limiting the use of lubricants and coolant liquids, increase the necessity of finding coatings that can function at such high temperature. Coatings can be differentiated by their hardness, H, into three main categories: hard with H < 40 GPa; superhard with H > 40 GPa; and ultra-hard coatings with H > 80 GPa. There are two main reasons in the high hardness coatings: either high compressive stresses or nano-scale structure. The application of high biaxial compressive stress acts as a driving force for recovery, i.e. the higher the compressive stress, the lower is the thermal activation energy needed to initiate recovery. High biaxial compressive stress increases superhardness, but reduces the coating thermal stability. Dislocations increase the micro-scale compressive stress inside the coating and consequently, enhance recovery. In nano-scale coatings, the small nanometric scale grain size restricted grain growth and boundaries sliding, and therefore the thermal stability is enhanced. This study treats the thermal stability of several types of superhard materials, i.e. nanocomposite coatings and those consisting of a hard transition-metal nitride and a soft metal. It focuses on formation mechanisms, materials and phase composition.

Folic acid‐PEG conjugated superparamagnetic nanoparticles for targeted cellular uptake and detection by MRI

We report the development and in vitro study of a nanoconjugate serving as a targeted magnetic resonance imaging (MRI) contrast enhancement agent for detection of cancer cells overexpressing the folate receptor. The nanoconjugate was synthesized by coating superparamagnetic iron oxide nanoparticles with covalently bound bifunctional poly(ethylene glycol) (PEG), followed by conjugation with folic acid (FA). The specificity of the nanoconjugate targeting cancerous cells was demonstrated by comparative intracellular uptake of the nanoconjugate and PEG-/dextran-coated nanoparticles by human adenocarcinoma HeLa cells. Preferential targeting to cancerous cells was studied by comparing the uptake of the nanoconjugate by HeLa cells and by non-FR expressing osteosarcoma MG-63 cells. Uptake of the nanoconjugate by HeLa cells after 4 h incubation was found to be a 12-fold higher than that of PEG- or dextran-coated nanoparticles as quantified by inductively coupled plasma spectroscopy. A significant negative contrast enhancement was observed with magnetic resonance (MR) phantom imaging for HeLa cells over MG-63 cells, when both were cultured with the nanoconjugate. Specificity of the nanoconjugate for folate receptors was also verified with a competitive inhibition assay, in which HeLa cells were incubated with both NP-PEG-FA and free FA. The bifunctional PEG used has amide linkages within the PEG chains that can form interchain hydrogen bonding, leading to improved stability of the PEG coating. Self-assembled PEG can be controlled at the molecular level and are suitable for nanoscale coatings.

Dielectric relaxation in composites containing electrically isolated particles with thin semi-continuous metal coatings

The dielectric relaxation due to interfacial charges in metal particle-filled composites occurs in the optical (high frequency) regime. However, many applications would benefit from a shift of the relaxation to a much lower frequency regime (e.g. radio or microwave range). This could be achieved by using a filler with reduced conductivity, but there is no continuum of conductivity in naturally occurring materials to allow engineers to readily achieve this aim or to have complete design freedom. Recently, it was demonstrated, experimentally and theoretically, how insulating particles with nano-scale metal coatings enable this relaxation to be shifted to a lower frequency regime. The current work investigates the microstructure of the metal coating, highlighting a semi-continuous coating structure. Therefore, the relevance of a two-dimensional (2D) percolation-based model to describe the effective dielectric properties of the coating is explored. It is demonstrated that a model in which the metal coating is assumed to be near the 2D percolation threshold can provide a reasonable fit to the experimental data. An improved fit is achieved when a distribution of metal area coverage is permitted.

Fracture Toughness of Nanoscale Hydroxyapatite Coatings on Titanium Substrates

In the biomedical field, the surface modification of titanium aims to inhibit wear, reduce corrosion and ion release, and promote biocompatibility. Sol-gel-derived ceramic nanoscale coatings show promise due to their relative ease of production, ability to form a physically and chemically uniform coating over complex geometric shapes, and their potential to deliver exceptional mechanical properties due to their nanocrystalline structure. In this study hydroxyapatite coatings on titanium were investigated for their fracture toughness.

On the structure and oxidation mechanisms in nanoscale hard coatings

Thin-film structures consisting of alternating nanoscale multilayers show substantial hardness increases compared with monolithic coatings of the constituent materials. Coatings, such as TiAlN/VN are deposited using PVD with individual coating bi-layer thickness of ∼3nm. Since TiAlN and VN are isostructural and mutually soluble, mixing of the two layers during deposition is expected, which will inevitably affect properties. Energy filtered TEM using a field emission gun source allowed important information on layer structure, but failed to reveal details <1nm. Spherical aberration corrected scanning transmission electron microscopy (STEM) allowed the composition of individual atomic columns to be probed, which yielded a good match between experiment and numerical models of the layer mixing. For high speed machining operations, the oxidation behaviour of coatings becomes an important consideration. Static oxidation of TiAlN/VN films was studied in the range 550-700oC, and characterised by high temperature in-situ XRD and STEM/EDX/EELS of selected surface cross-sections. Oxidation of the TiAlN/VN coating started at ⩾550°C with the VN being the first component to oxidise. At temperatures >600°C, a duplex oxide structure was formed, with several phases observed, including V2O5, TiO2 and AlVO4, with V2O5 being the dominant oxide at the outer layer at 638°C. These coatings exhibit low friction in dry sliding which is believed to arise from the inherently low friction of V2O5. Focused ion beam studies of wear tests at 630°C confirmed that the contact surface comprised small (∼50nm), equiaxed and largely agglomerated V2O5 crystals, confirming the hypothesis.

Mode transition in high refractive index coated long period gratings

In this work, the numerical and experimental investigation of the cladding modes re-organization in high refractive index (HRI) coated Long Period Gratings (LPGs) is reported. Moreover, the effects of the cladding modes re-organization on the sensitivity to the surrounding medium refractive index (SRI) have been outlined. When azimuthally symmetric nano-scale HRI coatings are deposited along LPGs devices, a significant modification of the cladding modes distribution occurs, depending on the layer features (refractive index and thickness) and on the SRI. In particular, if layer parameters are properly chosen, the transition of the lowest order cladding mode into an overlay mode occurs. As a consequence, a cladding modes re-organization can be observed leading to relevant improvements in the SRI sensitivity in terms of wavelength shift and amplitude variations of the LPGs attenuation bands.

Nanoscale coatings of AuAlx and PtAlx and their mesoporous elemental derivatives

A method to produce nanoscale films of AuAlx and PtAlx, and their mesoporous elemental derivatives is described, and the morphology and optical properties of these coatings explored. The color of the AuAlx film is bright purple, in agreement with ab initio calculations and experimental observations for the compound AuAl2, but becomes black after de-alloying with NaOH. The film of PtAlx is silver-grey and changes to grey after being converted to mesoporous Pt by de-alloying. Both mesoporous coatings exhibit a very rough surface, with pores of between 2 and 20 nm in diameter, and display a significantly more absorptive optical characteristic compared to their ordinary elemental counterparts.

Chromium nitride/niobium nitride nano-scale multilayer coatings deposited at low temperature by the combined cathodic arc/unbalanced magnetron technique

CrN/NbN nano-scale multilayered coatings have been successfully grown at a deposition temperature of 250 °C. The coatings have been grown by the combined cathodic arc/unbalanced magnetron technique (Arc Bond Sputter) and were deposited at two different thicknesses namely ∼1.0 and ∼3.0 μm at bias voltages UB between −75 and −150 V. In all cases the stresses were compressive and increased from −4.4 GPa at UB=−75 V to −9.5 GPa at UB=−150 V. Coatings in all cases developed a pronounced {111} texture that tended to a maximum at a bias voltage of −95 V. The experiments indicated that the {111} texture developed by a competitive growth mechanism from a randomly oriented or {100} initial starting texture. Measurements of coefficients of friction and sliding wear show a decrease in the coefficients of friction and sliding wear with increasing bias voltage. The coefficient of friction decreases from μ=0.7 at UB=−75 V to μ=0.5 at UB=−150 V while in parallel the coefficient of sliding wear decreased by an order of magnitude from 1.3×10−14 at UB=−75 V to 2.8×10−15 at UB=−150 V. These decreases in coefficients of friction and sliding wear appear to mirror the increases in intensity of the {111} texture. The adhesion as measured by scratch test shows a distinct maximum of 72 N at a bias voltage UB of −95 V, which is a remarkably high value for a low temperature process in a coating with a compressive stress of −5.4 GPa.

Biomaterial Coatings by Stepwise Deposition of Silk Fibroin

A completely aqueous, stepwise deposition process with Bombyx mori silk fibroin for the assembly of nanoscale thin film coatings is reported the first time. The focus of this work was to develop an understanding of the control of this deposition process and to characterize the films formed from a physicochemical perspective. The deposition process was monitored by UV spectrophotometry and research quartz crystal microbalance. Both absorbance and film thickness correlated linearly with the number of silk fibroin layers deposited, analogous to multilayered materials fabricated from conventional polyelectrolytes. The polymer adsorption process was stable and reproducible, with control of a single layer thickness ranging from a few to tens of nanometers, determined by the concentrations of silk fibroin, salt concentration in the dipping solution, and method of rinsing. The driving force for the assembly of silk fibroin onto the substrate was primarily hydrophobic interactions, while some electrostatic interactions were also involved. The difference with this approach from traditional polyelectrolyte layer-by-layer techniques is that an intervening drying step is used to control the structure and stability of the self-assembled silk fibroin. The assembled films were stable under physiological conditions and supported human bone marrow stem cell adhesion, growth, and differentiation. This approach offers new options to engineer biomaterial coatings as well as bulk materials with control of both interfacial properties conducive to specific cellular or tissue responses and the potential to entrap and deliver labile molecules or other components due to the all-aqueous process described.

The role of the growth defects on the corrosion resistance of CrN/NbN superlattice coatings deposited at low temperatures

CrN/NbN nano-scale multilayered coatings have been successfully deposited at a temperature of 250 °C on 304 stainless steel substrates by the combined cathodic arc/unbalanced magnetron technique (Arc Bond Sputter) with a thickness typically 3.0 μm at bias voltages UB of −75 and −95 V. The microstructure and surface morphology of the coatings were examined in the as-deposited and corroded condition using an environmental scanning electron microscope (ESEM) in the gaseous secondary electron mode. The corrosion resistance of the coatings was determined by a potentiodynamic polarisation measurement in a 3% NaCl solution open to air at room temperature. In the ESEM images defects could be observed on the surface of the coatings and the number of these defects decreased with increasing bias voltage. ESEM images of the as-deposited coatings showed that microstructure adjacent to growth defects is under dense. These defects have been shown to be sites for preferential corrosive attack However, even with these drawbacks the corrosion resistance as-measured by potentiodynamic polarisation was shown to be superior to that of the uncoated 304 stainless steel substrate, whilst the corrosion resistance of the CrN/NbN nano-scale multilayered coatings increased with increasing bias voltage. In all cases the corrosion resistance was inferior to similar CrN/NbN nano-scale multilayered coatings deposited at higher temperatures of 450 °C.

Effect of hexafluoropropylene oxide plasma polymer particle coatings on the rheological properties of boron nitride/poly(dimethylsiloxane) composites

This work explores the use of conformal nanoscale plasma coatings on the surface of boron nitride (BN) powders to control the rheological properties of BN/poly(dimethylsiloxane) (PDMS) composites. BN particles are conformally coated with hexafluoropropylene oxide (HFPO) in a tumbling RF-plasma reactor. Following the HFPO plasma treatment, XPS evidence indicates the presence of thin coating on the surface of the particles having a F:C ratio of 1.77. Filled BN/PDMS composites are investigated using oscillatory shear rheometry in the concentration range of 0.09–0.41 vol% ( ϕ). The addition of the plasma treated BN particles to the PDMS matrix reduces the complex viscosity by 40–60% when compared with equally loaded control samples across a broad concentration range. The frequency dependence of the maximum packing fraction ( ϕ m = 0.38 – 0.42 ) is also observed for both treated and untreated particles. The maximum packing fraction does not appear to be significantly affected by the conformal plasma polymer treatment. The investigation has shown that the relative dynamic viscosity of the BN/PDMS can be described by the modified Mooney equation.

Properties of nano-multilayered hard coatings deposited by a new hybrid coating process: Combined cathodic arc and unbalanced magnetron sputtering

Nano-scale multilayered coatings of (Ti,Al)N/SiN, (Ti,Al)N/WN, and CrN/BCN were deposited by a new hybrid coating process that combined cathodic arc and unbalanced magnetron sputtering (UBM). Cross-sectional transmission electron microscopy (TEM) analysis of the coatings deposited by the hybrid process revealed that these coatings had a multilayer structure in that layers deposited by the arc source and the sputter source were stacked alternatively. The bilayer period of the multilayer system was the function of the substrate rotation speed and the input power to the each evaporation source, and it was typically in the range of 10–20 nm. From X-ray diffraction measurements, in case of Si or B4C target used as a sputter target, the grain size was drastically decreased from 30 to 10 nm as the evaporation rate of the sputter source was increased. We confirmed from TEM observation that the grain growth of the (Ti,Al)N or CrN layer was interrupted at the interface and this resulted in much smaller grain size compared to the conventional (Ti,Al)N coating. Mechanical and tribological properties of different nano-multilayered coatings are discussed.

Nano-scale, multi-functional coatings in the material system B–C–N–H

Protective and functional coatings for tools and components are nowadays required in an increasingly demanding and sophisticated profile with respect to the desired mechanical, tribological, physical or chemical properties. Frequently, the necessary requirements can only be realized by multi-functional coatings through a nano-scale thin-film design. CVD- and PVD-coatings in the material system B–C–N–H are known for their outstanding properties. Depending on the materials selection within this system and detailed coating design, extremely high hardness, low friction and excellent chemical stability can be obtained. Here it is shown that coatings with well tailored properties can be achieved according to special coating concepts, in particular through multilayers and nanocomposites, in conjunction with the control of surface processes. Several designs and optimization examples for anticipated nano-scale multi-functional coatings will be presented, showing the fundamental significance of the underlying knowledge, i.e. the interdependent relationships between the constitution and microstructure of the coating systems, the energy and momentum input of the processing particles during nucleation and growth of the coatings, the mechanical properties, and the operation performance.

Microstructure and properties of novel wear and corrosion resistant CrON/NbON nano-scale multilayer coatings

CrN/NbN nano-scale multilayer coatings have found use in a number of commercial applications where wear and corrosion resistance determine the service life of the components. To further improve their performance a novel CrON/NbON topcoat has been developed.The coatings were deposited using an industrial Hauzer HTC 1000/4 UBM-ABS coating machine utilising the Arc Bond Sputtering method where cathodic arc metal ion etching is used to prepare the interface prior to coating deposition by unbalanced magnetron (UBM) sputtering. The oxynitride process was performed in mixture of dry air and argon at bias voltages varying from UB=−75 V to −120 V at total pressures during deposition from 3.5*10−1 Pa to 4.9*10−1 Pa. The thickness of the oxynitride film varied between 1.6 and 2.3 μm, while the total coating thickness varied between 4.6 and 5.3 μm. The XTEM investigation revealed that the microstructure of the oxynitride layer was dense columnar with a pronounced nano-scale multilayer architecture. By X-ray diffraction (XRD), the coatings were identified as crystalline with mixed texture. As the pressure during the oxynitride deposition stage was increased the crystal structure of the top layer became increasingly amorphous/nano-crystalline. An increase in the bias voltage also caused a shift from {100} texture towards {111} texture.The best performing oxynitride coatings had similar low sliding wear rates as the reference standard coating without affecting their corrosion resistance. However, the wear rate against a 100Cr6 counter body was reduced by a factor of 10 and the friction coefficient from 0.57 to 0.49. The wear rate of both the coating and the counter body was reduced as the bias voltage was increased, while increasing the deposition pressure had adverse effects on the tribological properties. The wear behaviour can be related to the special nano-scale multilayer structure of the oxynitride layer as coating with best tribological properties exhibits a pronounced nano-scale multilayer structure parallel to the coating surface.

Dielectric relaxation in metal-coated particles: the dramatic role of nano-scale coatings

Insulating materials filled with conducting particles permit tailoring of electrical, electromagnetic and thermal properties of the resulting composite. When the filler particles are small and metallic, a dielectric relaxation due to interfacial polarization is commonly observed at optical or smaller wavelengths. Here, experimental results are presented in which the dielectric relaxation is shifted to microwave frequencies as a result of using metal-coated dielectric particles with a nano-scale coating thickness. The results are analysed in the context of effective medium theory adapted for multi-layer particles. Such a large shift in relaxation frequency, compared with that for a similar composite with solid metal filler particles, is shown to be a function of both the coating geometry and a thin-film-related reduction in the conductivity of the metal. The observed broadening of the relaxation peak is attributed to non-uniformity of the coating thickness and a consequent distribution of coating conductivity.

Nanoscale neuro-integrative coatings for neural implants

Silicon microelectrode arrays (Si MEAs) have great potential in enabling chronic in vivo recording of neural activity, but this potential has been hampered by scar tissue formation at the site of implantation. In this study, we report the fabrication and characterization of nanoscale coatings that have the potential of enhancing the biocompatibility of Si electrodes. We use electrostatic layer-by-layer (LbL) assembly to prepare nanoscale bioactive coatings on silicon substrates. We use the response of chick cortical neurons to these coatings to assess potential improvement in biocompatibility in vitro. The coatings are built on oxide covered silicon wafers by alternating polycations, polyethyleneimine (PEI) or chitosan (CH), with polyanions, either gelatin or laminin (LN). We use quartz crystal microbalance (QCM) to characterize the coatings. Our analysis confirms that we achieved ∼30–110 Å scale coatings via LbL assembly. In contrast to bare oxide covered silicon, coated substrates had significantly enhanced chick cortical neuron adhesion and differentiation, with multilayers of PEI–LN showing the greatest improvement. The multilayers of PEI–LN were stable for at least 7 days in physiological conditions, as determined by an enzyme-linked immunosorbent assay (ELISA). In addition, impedance spectroscopy confirmed that multilayers of PEI and LN did not increase the magnitude of impedance of Si MEAs at the biologically relevant frequency of 1 kHz. Our study demonstrates that electrostatic LbL assembly enables nanoscale bioactive coatings, and that PEI–LN multilayers significantly enhance cortical neuronal attachment and differentiation in vitro with no deleterious effects on impedance of the electrodes. Such well-controlled nanoscale coatings have the potential to significantly impact the compatibility and performance of Si MEAs in vivo.

Al2 O 3 Nanoencasulation of BaMgAl10 O 17 : Eu2 + Phosphors for Improved Aging Properties in Plasma Display Panels

We studied the aging characteristics of the plasma display panel (PDP) containing -coated blue phosphors and the thermal characteristics of coated phosphors. The nanoscale coatings on phosphors were obtained by the sol-gel method for an concentration of 2.0 wt %, pH values from 2 to 4, and a solution temperature of 50°C. Transmission electron microscopy measurements confirmed the formation of nano and uniform coatings on powders with the thickness of ∼8 nm. An optical enhancement (∼15%) was reported in the PDP containing -coated phosphors after a 3000 h longevity test of PDP (a discharge voltage of 170 V and an average current of 2.06 A). We proposed that this improvement originated from both the enhanced thermal stability of coated phosphors and the enhanced operating stability of PDP containing coated phosphors. © 2004 The Electrochemical Society. All rights reserved.

Tribological properties of unbalanced magnetron sputtered nano-scale multilayer coatings TiAlN/VN and TiAlCrYN deposited on plasma nitrided steels

Unbalanced magnetron sputtered multilayer coatings TiAlN/VN and TiAlCrYN grown on pulse plasma nitriding pre-treated low alloy steel P20 have been characterised by using X-ray diffraction (XRD), scanning electron microscope (SEM), micro-indentation, scratch and pin-on-disc wear tests. A 160-μm-thick nitrided case was formed on the steel surface containing a pure Fe 3N and Fe 4N compound layer and showing hardness up to 8.5 GPa, which led to improved load bearing ability and adhesion behaviour of the coating-substrate system. The coatings deposited on non-nitrided P20 showed poor adhesion and severe cracking and spalling wear. In contrast, the TiAlN/VN and TiAlCrYN deposited on nitrided substrates showed only mild polishing and oxidation wear mechanisms and extremely low wear coefficients in the scales of 10 −17 and 10 −16 m 3 N −1 m −1, respectively. In drilling solution treated 304 austenite stainless steel, the TiAlN/VN-coated drills showed a lifetime 50% longer than TiAlN-coated drills. Plasma nitriding pre-treatment led to further increases of lifetime by 33%.