Solvent Effects on the Formation of Nanoparticles and Multilayered Coatings Based on Hydrogen-Bonded Interpolymer Complexes of Poly(acrylic acid) with Homo- and Copolymers of N-Vinyl Pyrrolidone

The formation of hydrogen-bonded interpolymer complexes between poly(acrylic acid) and poly(N-vinyl pyrrolidone) as well as amphiphilic copolymers of N-vinyl pyrrolidone with vinyl propyl ether has been studied in aqueous and organic solutions. It was demonstrated that introduction of vinyl propyl ether units into the macromolecules of the nonionic polymer enhances their ability to form complexes in aqueous solutions due to more significant contribution of hydrophobic effects. The complexation was found to be a multistage process that involves the formation of primary polycomplex particles, which further aggregate to form spherical nanoparticles. Depending on the environmental factors (pH, solvent nature), these nanoparticles may either form stable colloidal solutions or undergo further aggregation, resulting in precipitation of interpolymer complexes. In organic solvents, the intensity of complex formation increases in the following order: methanol < ethanol < isopropanol < dioxane. The multilayered coatings were developed using layer-by-layer deposition of interpolymer complexes on glass surfaces. It was demonstrated that the solvent nature affects the efficiency of coating deposition.

Multilayered micro/nanocrystalline CVD diamond coatings for biotribology

In the present work multilayered micro/nanocrystalline (MCD/NCD) diamond coatings were developed by Hot Filament Chemical Vapour Deposition (HFCVD). The aim was to minimize the surface roughness with a top NCD layer, to maximize adhesion onto the Si3N4 ceramic substrates with a starting MCD coating and to improve the mechanical resistance by the presence of MCD/NCD interfaces in these composite coatings. This set of features assures high wear resistance and low friction coefficients which, combined to diamond biocompatibility, set this material as ideal for biotribological applications. The deposition parameters of MCD were optimized using the Taguchi method, and two varieties of NCD were used: NCD-1, grown in a methane rich gas phase, and NCD-2 where a third gas, Argon, was added to the gas mixture. The best combination of surface pre-treatments in the Si3N4 substrates is obtained by polishing the substrates with a 15 μm diamond slurry, further dry etching with CF4 plasma for 10 minutes and final ultrasonic seeding in a diamond powder suspension in ethanol for 1 hour. The interfaces of the multilayered CVD diamond films were characterized with high detail using HRTEM, STEM-EDX and EELS. The results show that at the transition from MCD to NCD a thin precursor graphitic film is formed. On the contrary, the transition of the NCD to MCD grade is free of carbon structures other than diamond, as a result of the richer atomic hydrogen content and of the higher substrate temperature for MCD deposition. At those transitions, WC nanoparticles were found due to contamination from the filament, being also present at the first interface of the MCD layer with the silicon nitride substrate. In order to study the adhesion and mechanical resistance of the diamond coatings, indentation and particle jet blasting tests were conducted, as well as tribological experiments with homologous pairs. Indentation tests proved the superior behaviour of the multilayered coatings that attained a load of 800 N without delamination, when compared to the mono and bilayered ones. The multilayered diamond coatings also reveal the best solid particle erosion resistance, due to the MCD/NCD interfaces that act as crack deflectors. These results were confirmed by an analytical model on the stress field distribution based on the von Mises criterion. Regarding the tribological testing under dry sliding, multilayered coatings also exhibit the highest critical load values (200N for Multilayers with NCD-2). Low friction coefficient values in the range μ=0.02- 0.09 and wear coefficient values in the order of ~10-7 mm3 N-1 m-1 were obtained for the ball and flat specimens indicating a mild wear regime. Under lubrication with physiological fluids (HBSS e FBS), lower wear coefficient values ~10-9-10-8 mm3 N-1 m-1) were achieved, governed by the initial surface roughness and the effective contact pressure.

Increasing the wear resistance of molds for injection of glass fiber reinforced plastics

Abrasion by glass fibers during injection molding of fiber reinforced plastics raises new challenges to the wear performance of the molds. In the last few decades, a large number of PVD and CVD coatings have been developed with the aim of minimizing abrasion problems. In this work, two different coatings were tested in order to increase the wear resistance of the surface of a mold used for glass fiber reinforced plastics: TiAlSiN and CrN/CrCN/DLC. TiAlSiN was deposited as a graded monolayer coating while CrN/CrCN/DLC was a nanostructured coating consisting of three distinct layers. Both coatings were produced by PVD unbalanced magnetron sputtering and were characterized using scanning electron microscopy (SEM) provided with energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), micro hardness (MH) and scratch test analysis. Coating morphology, thickness, roughness, chemical composition and structure, hardness and adhesion to the substrate were investigated. Wear resistance was characterized through industrial tests with coated samples and an uncoated reference sample inserted in a feed channel of a plastic injection mold working with 30 wt.% glass fiber reinforced polypropylene. Results after 45,000 injection cycles indicate that the wear resistance of the mold was increased by a factor of 25 and 58, by the TiAlSiN and CrN/CrCN/DLC coatings, respectively, over the uncoated mold steel.

Hydrogen-bonding-driven self-assembly of PEGylated organosilica nanoparticles with poly(acrylic acid) in aqueous solutions and in layer-by-layer deposition at solid surfaces

PEGylated organosilica nanoparticles have been synthesized through self-condensation of (3-mercaptopropyl)trimethoxysilane in dimethyl sulfoxide into thiolated nanoparticles with their subsequent reaction with methoxypoly(ethylene glycol) maleimide. The PEGylated nanoparticles showed excellent colloidal stability over a wide range of pH in contrast to the parent thiolated nanoparticles, which have a tendency to aggregate irreversibly under acidic conditions (pH < 3.0). Due to the presence of a poly(ethylene glycol)-based corona, the PEGylated nanoparticles are capable of forming hydrogen-bonded interpolymer complexes with poly(acrylic acid) in aqueous solutions under acidic conditions, resulting in larger aggregates. The use of hydrogen-bonding interactions allows more efficient attachment of the nanoparticles to surfaces. The alternating deposition of PEGylated nanoparticles and poly(acrylic acid) on silicon wafer surfaces in a layer-by-layer fashion leads to multilayered coatings. The self-assembly of PEGylated nanoparticles with poly(acrylic acid) in aqueous solutions and at solid surfaces was compared to the behavior of linear poly(ethylene glycol). The nanoparticle system creates thicker layers than the poly(ethylene glycol), and a thicker layer is obtained on a poly(acrylic acid) surface than on a silica surface, because of the effects of hydrogen bonding. Some implications of these hydrogen-bonding-driven interactions between PEGylated nanoparticles and poly(acrylic acid) for pharmaceutical formulations are discussed.

Optimizing layer-by-layer deposition of interpolymer complexes on solid substrates using Biacore

The layer-by-layer deposition of polymers onto surfaces allows the fabrication of multilayered materials for a wide range of applications, from drug delivery to biosensors. This work describes the analysis of complex formation between poly(acrylic acid) and methylcellulose in aqueous solutions using Biacore, a surface plasmon resonance analytical technique, traditionally used to examine biological interactions. This technique characterized the layer-by-layer deposition of these polymers on the surface of a Biacore sensor chip. The results were subsequently used to optimize the experimental conditions for sequential layer deposition on glass slides. The role of the solution pH and poly(acrylic acid) molecular weight on the formation of interpolymer multilayered coatings was researched, and showed that the optimal deposition of the polymer complexes was achieved at pHs ≤2.5 with a poly(acrylic acid) molecular weight of 450 kDa.

Multilayered hydrogel coatings covalently-linked to glass surfaces showing a potential to mimic mucosal tissues

Multilayered hydrogel coatings can be developed on the surface of glass slides via layer-by-layer deposition of hydrogen-bonded interpolymer complexes formed by poly(acrylic acid) and methylcellulose. Chemical modification of the glass surface with (3-aminopropyl)triethoxysilane with subsequent layer-by-layer deposition and cross-linking of interpolymer complexes by thermal treatment allows fabrication of ultrathin hydrogel coatings, not detachable from the substrate. The thickness of these coatings is directly related to the number of deposition cycles and cross-linking conditions. An unusual dependence of the hydrogel swelling properties on the sample thickness is observed and can be interpreted by gradual transitions between two- and three-dimensional networks. The hydrogels exhibit pH-responsive swelling behaviour, achieving higher swelling degrees at pH > 6.0. These coatings can be used as model substrates to study the adhesive properties of pharmaceutical tablets and can potentially mimic the total work of adhesion observed for the detachment of mucoadhesives from porcine buccal mucosa but fail to exhibit identical detachment profiles.

Exploitation of multilayer coatings for infrared surface plasmon resonance fiber sensors

We demonstrate surface plasmon resonance (SPR) fiber devices based upon ultraviolet inscription of a grating-type structure into both single-layered and multilayered thin films deposited on the flat side of a lapped D-shaped fiber. The single-layered devices were fabricated from germanium, while the multilayered ones comprised layers of germanium, silica, and silver. Some of the devices operated in air with high coupling efficiency in excess of 40 dB and an estimated index sensitivity of Delta lambda/Delta n = 90 mn from 1 to 1.15 index range, while others provided an index sensitivity of Delta lambda/Delta n = 6790 mn for refractive indices from 1.33 to 1.37. (C) 2009 Optical Society of America

Multilayered coated infra-red surface plasmon resonance fibre sensors for aqueous chemical sensing

A series of surface plasmonic fibre devices were fabricated by depositing multiple thin coatings on a lapped section of a standard single mode telecoms fibre forming a D-shaped section and then inscribing a grating-type structure using UV light. The coatings consisted of base coatings of semi-conductor (germanium) and dielectric (silicon dioxide) materials, followed by different metals. These fibre devices showed high spectral refractive index sensitivity with high coupling efficiency in excess of 40 dB for indices in the aqueous regime and below, with estimated index sensitivities of Lambda lambda/Lambda n = 90-800 nm from 1 to 1.15 index range and Lambda lambda/Lambda n = 1200-4000 nm for refractive indices from 1.33 to 1.39. (C) 2009 Elsevier Inc. All rights reserved.

Porous polyurethane coatings bonded onto surface-modified titanium substrates for the growth of an endothelial cell layer

Cardiovascular diseases refer to the class of diseases that involve the heart or blood vessels (arteries and veins). Examples of medical devices for treating the cardiovascular diseases include ventricular assist devices (VADs), artificial heart valves and stents. Metallic biomaterials such as titanium and its alloy are commonly used for ventricular assist devices. However, titanium and its alloy show unacceptable thrombosis, which represents a major obstacle to be overcome. Polyurethane (PU) polymer has better blood compatibility and has been used widely in cardiovascular devices. Thus one aim of the project was to coat a PU polymer onto a titanium substrate by increasing the surface roughness, and surface functionality. Since the endothelium of a blood vessel has the most ideal non-thrombogenic properties, it was the target of this research project to grow an endothelial cell layer as a biological coating based on the tissue engineering strategy. However, seeding endothelial cells on the smooth PU coating surfaces is problematic due to the quick loss of seeded cells which do not adhere to the PU surface. Thus it was another aim of the project to create a porous PU top layer on the dense PU pre-layer-coated titanium substrate. The method of preparing the porous PU layer was based on the solvent casting/particulate leaching (SCPL) modified with centrifugation. Without the step of centrifugation, the distribution of the salt particles was not uniform within the polymer solution, and the degree of interconnection between the salt particles was not well controlled. Using the centrifugal treatment, the pore distribution became uniform and the pore interconnectivity was improved even at a high polymer solution concentration (20%) as the maximal salt weight was added in the polymer solution. The titanium surfaces were modified by alkli and heat treatment, followed by functionlisation using hydrogen peroxide. A silane coupling agent was coated before the application of the dense PU pre-layer and the porous PU top layer. The ability of the porous top layer to grow and retain the endothelial cells was also assessed through cell culture techniques. The bonding strengths of the PU coatings to the modified titanium substrates were measured and related to the surface morphologies. The outcome of the project is that it has laid a foundation to achieve the strategy of endothelialisation for the blood compatibility of medical devices. This thesis is divided into seven chapters. Chapter 2 describes the current state of the art in the field of surface modification in cardiovascular devices such as ventricular assist devices (VADs). It also analyses the pros and cons of the existing coatings, particularly in the context of this research. The surface coatings for VADs have evolved from early organic/ inorganic (passive) coatings, to bioactive coatings (e.g. biomolecules), and to cell-based coatings. Based on the commercial applications and the potential of the coatings, the relevant review is focused on the following six types of coatings: (1) titanium nitride (TiN) coatings, (2) diamond-like carbon (DLC) coatings, (3) 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer coatings, (4) heparin coatings, (5) textured surfaces, and (6) endothelial cell lining. Chapter 3 reviews the polymer scaffolds and one relevant fabrication method. In tissue engineering, the function of a polymeric material is to provide a 3-dimensional architecture (scaffold) which is typically used to accommodate transplanted cells and to guide their growth and the regeneration of tissue. The success of these systems is dependent on the design of the tissue engineering scaffolds. Chapter 4 describes chemical surface treatments for titanium and titanium alloys to increase the bond strength to polymer by altering the substrate surface, for example, by increasing surface roughness or changing surface chemistry. The nature of the surface treatment prior to bonding is found to be a major factor controlling the bonding strength. By increasing surface roughness, an increase in surface area occurs, which allows the adhesive to flow in and around the irregularities on the surface to form a mechanical bond. Changing surface chemistry also results in the formation of a chemical bond. Chapter 5 shows that bond strengths between titanium and polyurethane could be significantly improved by surface treating the titanium prior to bonding. Alkaline heat treatment and H2O2 treatment were applied to change the surface roughness and the surface chemistry of titanium. Surface treatment increases the bond strength by altering the substrate surface in a number of ways, including increasing the surface roughness and changing the surface chemistry. Chapter 6 deals with the characterization of the polyurethane scaffolds, which were fabricated using an enhanced solvent casting/particulate (salt) leaching (SCPL) method developed for preparing three-dimensional porous scaffolds for cardiac tissue engineering. The enhanced method involves the combination of a conventional SCPL method and a step of centrifugation, with the centrifugation being employed to improve the pore uniformity and interconnectivity of the scaffolds. It is shown that the enhanced SCPL method and a collagen coating resulted in a spatially uniform distribution of cells throughout the collagen-coated PU scaffolds.In Chapter 7, the enhanced SCPL method is used to form porous features on the polyurethane-coated titanium substrate. The cavities anchored the endothelial cells to remain on the blood contacting surfaces. It is shown that the surface porosities created by the enhanced SCPL may be useful in forming a stable endothelial layer upon the blood contacting surface. Chapter 8 finally summarises the entire work performed on the fabrication and analysis of the polymer-Ti bonding, the enhanced SCPL method and the PU microporous surface on the metallic substrate. It then outlines the possibilities for future work and research in this area.