Artemis Stamboulis

Self-Assembled Growth, Microstructure, and Field-Emission High-Performance of Ultrathin Diamond Nanorods

We report the growth of ultrathin diamond nanorods (DNRs) by a microwave plasma assisted chemical vapor deposition method using a mixture gas of nitrogen and methane. DNRs have a diameter as thin as 2.1 nm, which is not only smaller than reported one-dimensional diamond nanostructures (4-300 nm) but also smaller than the theoretical value for energetically stable DNRs. The ultrathin DNR is encapsulated in tapered carbon nanotubes (CNTs) with an orientation relation of (111)diamond//(0002)graphite. Together with diamond nanoclusters and multilayer graphene nanowires/nano-onions, DNRs are self-assembled into isolated electron-emitting spherules and exhibit a low-threshold, high current-density (flat panel display threshold: 10 mA/cm2 at 2.9 V/microm) field emission performance, better than that of all other conventional (Mo and Si tips, etc.) and popular nanostructural (ZnO nanostructure and nanodiamond, etc.) field emitters except for oriented CNTs. The forming mechanism of DNRs is suggested based on a heterogeneous self-catalytic vapor-solid process. This novel DNRs-based integrated nanostructure has not only a theoretical significance but also has a potential for use as low-power cold cathodes.

Uptake of Sr 2+ and Co 2+ into biogenic hydroxyapatite: implications for biomineral ion exchange synthesis

Biomineral hydroxyapatite (Bio-HAp) produced by Serratia sp. has the potential to be a suitable material for the remediation of metal contaminated waters and as a radionuclide waste storage material. Varying the Bio-HAp manufacturing method was found to influence hydroxyapatite (HAp) properties and consequently the uptake of Sr(2+) and Co(2+). All the Bio-HAp tested in this study were more efficient than the commercially available hydroxyapatite (Com-HAp) for Sr(2+) and Co(2+) uptake. For Bio-HAp the uptake for Sr(2+) and Co(2+) ranged from 24 to 39 and 29 to 78 mmol per 100 g, respectively. Whereas, the uptake of Sr(2+) and Co(2+) by Com-HAp ranged from 3 to 11 and 4 to 18 mmol per 100 g, respectively. Properties that increased metal uptake were smaller crystallite size (<40 nm) and higher surface area (>70 m(2) g(-1)). Organic content which influences the structure (e.g., crystallite arrangement, size and surface area) and composition of Bio-HAp was also found to be important in Sr(2+) and Co(2+) uptake. Overall, Bio-HAp shows promise for the remediation of aqueous metal waste especially since Bio-HAp can be synthesized for optimal metal uptake properties.

Early osseointegration of a strontium containing glass ceramic in a rabbit model

The most important property of a bone cement or a bone substitute in load bearing orthopaedic implants is good integration with host bone with reduced bone resorption and increased bone regeneration at the implant interface. Long term implantation of metal-based joint replacements often results in corrosion and particle release, initiating chronic inflammation leading onto osteoporosis of host bone. An alternative solution is the coating of metal implants with hydroxyapatite (HA) or bioglass or the use of bulk bioglass or HA-based composites. In the above perspective, the present study reports the in vivo biocompatibility and bone healing of the strontium (Sr)-stabilized bulk glass ceramics with the nominal composition of 4.5SiO2-3Al2O3-1.5P2O5-3SrO-2SrF2 during short term implantation of up to 12 weeks in rabbit animal model. The progression of healing and bone regeneration was qualitatively and quantitatively assessed using fluorescence microscopy, histological analysis and micro-computed tomography. The overall assessment of the present study establishes that the investigated glass ceramic is biocompatible in vivo with regards to local effects after short term implantation in rabbit animal model. Excellent healing was observed, which is comparable to that seen in response to a commercially available implant of HA-based bioglass alone.

Effect of plasma surface modification on the biocompatibility of UHMWPE.

In this paper active screen plasma nitriding (ASPN) is used to chemically modify the surface of UHMWPE. This is an unexplored and new area of research. ASPN allows the homogeneous treatment of any shape or surface at low temperature; therefore, it was thought that ASPN would be an effective technique to modify organic polymer surfaces. ASPN experiments were carried out at 120 °C using a dc plasma nitriding unit with a 25% N(2) and 75% H(2) atmosphere at 2.5 mbar of pressure. UHMWPE samples treated for different time periods were characterized by nanoindentation, FTIR, XPS, interferometry and SEM. A 3T3 fibroblast cell line was used for in vitro cell culture experiments. Nanoindentation of UHMWPE showed that hardness and elastic modulus increased with ASPN treatment compared to the untreated material. FTIR spectra did not show significant differences between the untreated and treated samples; however, some changes were observed at 30 min of treatment in the range of 1500-1700 cm(-1) associated mainly with the presence of N-H groups. XPS studies showed that nitrogen was present on the surface and its amount increased with treatment time. Interferometry showed that no significant changes were observed on the surfaces after the treatment. Finally, cell culture experiments and SEM showed that fibroblasts attached and proliferated to a greater extent on the plasma-treated surfaces leading to the conclusion that ASPN surface treatment can potentially significantly improve the biocompatibility behaviour of polymeric materials.

A MAS-NMR and Combined Rietveldt Study of Mixed Calcium/Strontium Fluorapatite Glass-Ceramics

Strontium is one of the most common substituents in apatite crystals. The presence and behavior of Sr in apatite-group phases are of considerable significance in biology. The present paper investigates the substitution of strontium for calcium in a glass-ceramic of the following composition 4.5SiO23Al2O31.5P2O54CaO1CaF2. The glasses were characterized using Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), neutron diffraction (ND) and 19F Resonance Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). The all calcium glass crystallized to calcium fluorapatite (Ca5(PO4)3F). Substituting strontium partially for calcium resulted in the formation of a mixed strontium/calcium fluorapatites. In contrast complete substitution resulted in the formation of strontium fluorapatite. MAS-NMR showed the the F to be present as F-Ca(3) representing a fluoride ion surrounded by three Ca2+ ions in the all calcium glass and was present as F-Sr(3) in the all strontium glass. In the mixed glasses fluorine was present as FCa( 3), F-Ca(2)Sr, F-CaSr(2) and F-Sr(3). Ca had a higher tendency to occupy the F-M(3) sites than Sr which may reflect the higher charge to size ratio of Ca2+ relative to Sr2+ and its greater affinity for F- ions.

Peptide aptamers: Novel coatings for orthopaedic implants

Current processes for coating titanium implants with ceramics involve very high energy techniques with associated high cost and disadvantages such as heterogeneity of the coatings, phase transformations and inability to coat complex structures. In order to address the above problems, we propose a biomimetic hydroxyapatite coating process with the use of peptides that can bind both on titanium surfaces and hydroxyapatite. The peptides enabled homogeneous coating of a titanium surface with hydroxyapatite. The hydroxyapatite-peptide sandwich coating showed no adverse effects on cell number or collagen deposition. This makes the sandwich coated titanium a good candidate for titanium implants used in orthopaedics and dentistry.

A MAS NMR Study of the Crystallisation Process of Apatite-Mullite Glass-Ceramics

The crystallisation behaviour of 4.5SiO 2 .3Al 2 O 3 . 1.5P 2 O 5 . (5-z)CaO.zCaF 2 glasses with z between 0 and 3 were investigated using differential scanning calorimetry (DSC), x-ray diffraction analysis and 2 7 Al, 2 9 Si, 3 1 P and 1 9 F magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The glasses showed two exothermic peaks at Tpl and Tp2 due to crystallisation, which decreased in temperature with fluorine content. In the nonfluorine containing glass (z=0), whitlockite (Ca 3 (PO 4 ) 2 and anorthite (2SiO 2 .Al 2 O 3 .CaO) were the final crystallisation products. In the fluorine containing glasses (z=2 and 3), fluorapatite (FAP) (Ca 5 (PO 4 ) 3 , crystallised at Tpl and mullite (2SiO 2 .3Al 2 O 3 ) at Tp2. MAS-NMR results showed that a glass phase remained after crystallisation. The coordination state of the phosphate species in the glass drastically changed during crystallisation. Five coordinate Al, Al(V), in the glass containing fluoride disappeared after crystallisation of mullite. The 2 7 Al and 2 9 Si MAS-NMR spectra did not change significantly during fluorapatite crystallisation, but changed markedly on mullite formation. 1 9 F MAS-NMR demonstrated the presence of FAP in the heat treated glasses. In the glasses with a higher fluorine content than that required to convert all the Ca and P to FAPa Al-F-Ca(n) type species remained in the residual glass phase.

Solid State MAS-NMR and FTIR Study of Barium Containing Alumino-Silicate Glasses

The glass based on a 1.5SiO2-Al2O3-0.5P2O5-CaO-0.67CaF2 composition was produced and substituted gradually by barium. The structure of the glasses was studied by multinuclear Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) and Fourier Transform Infrared Spectroscopy (FTIR). It was indicated by 29Si and 31P MAS-NMR spectra that silicon was present as Q4 (4Al) and Q3 (3Al) species and phosphorus was in a Q1 pyrophosphate environment. 29Al MAS-NMR spectra showed that four fold coordinated aluminum Al (IV) was the dominant species with a second peak assigned to octahedral aluminum Al (VI). The 19F spectra suggested that the barium addition caused the formation of Al-F-Ba(n) and F-Ba(n) species. Furthermore, a distribution of silicate network including Si-O-Si stretching (Q4 and Q3) and Si-O-[NBO] (Q3) per SiO4 was reflected by the FTIR study.

Antimicrobial peptide coatings for hydroxyapatite: electrostatic and covalent attachment of antimicrobial peptides to surfaces

The interface between implanted devices and their host tissue is complex and is often optimized for maximal integration and cell adhesion. However, this also gives a surface suitable for bacterial colonization. We have developed a novel method of modifying the surface at the material–tissue interface with an antimicrobial peptide (AMP) coating to allow cell attachment while inhibiting bacterial colonization. The technology reported here is a dual AMP coating. The dual coating consists of AMPs covalently bonded to the hydroxyapatite surface, followed by deposition of electrostatically bound AMPs. The dual approach gives an efficacious coating which is stable for over 12 months and can prevent colonization of the surface by both Gram-positive and Gram-negative bacteria.

Nanoclay addition to a conventional glass ionomer cements: Influence on physical properties

Objective: The objective of the present study is to investigate the reinforcement effect of  polymer-grade montmorillonite (PGN nanoclay) on physical properties of glass ionomer cement (GIC). Materials and Methods: The PGN nanoclay was dispersed in the liquid portion of GIC (HiFi, Advanced Healthcare, Kent, UK) at 1%, 2% and 4% (w/w). Fourier-transform infrared (FTIR) spectroscopy was used to quantify the polymer liquid of GICs after dispersion of nanoclay. The molecular weight (M w ) of HiFi liquid was determined by gel permeation chromatography. The compressive strength (CS), diametral-tensile strength, flexural strength (FS) and flexural modulus (E f ) of cements (n = 20) were measured after storage for 1 day, 1 week and 1 month. Fractured surface was analyzed by scanning electron microscopy. The working and setting time (WT and ST) of cements was measured by a modified Wilson′s rheometer. Results: The FTIR results showed a new peak at 1041 cm−1 which increased in intensity with an increase in the nanoclay content and was related to the Si-O stretching mode in PGN nanoclay. The M w of poly (acrylic acid) used to form cement was in the range of 53,000 g/mol. The nanoclay reinforced GICs containing <2% nanoclays exhibited higher CS and FS. The E f cement with 1% nanoclays was significantly higher. The WT and ST of 1% nanoclay reinforced cement were similar to the control cement but were reduced with 2% and 4% nanoclay addition. Conclusion: The dispersion of nanoclays in GICs was achieved, and GIC containing 2 wt% nanoclay is a promising restorative materials with improved physical properties.