I.N. Mihailescu

Strontium-substituted hydroxyapatite coatings synthesized by pulsed-laser deposition: In vitro osteoblast and osteoclast response

The increasing interest in strontium incorporation into biomaterials for hard tissue repair is justified by the growing evidence of its beneficial effect on bone. We successfully synthesized hydroxyapatite (HA) thin films with different extents of strontium substitution for calcium (0, 1, 3 or 7 at.%) by pulsed-laser deposition. The coatings displayed a granular surface and a good degree of crystallinity, which slightly diminished as strontium content increased. Osteoblast-like MG63 cells and human osteoclasts were cultured on the thin films up to 21 days. MG63 cells grown on the strontium-doped HA coatings displayed normal morphology, good proliferation and increased values of the differentiation parameters, whereas the number of osteoclasts was negatively influenced by the presence of strontium. The positive effect of the ion on bone cells was particularly evident in the case of coatings deposited from HA at relatively high strontium contents (3-7%), where significantly increased values of alkaline phosphatase activity, osteocalcin, type I collagen and osteoprotegerin/TNF-related activation-induced cytokine receptor ratio, and considerably reduced values of osteoclast proliferation, were observed.

Microstructure and mechanical properties of hydroxyapatite thin films grown by RF magnetron sputtering

Hydroxyapatite (HA) thin films for applications in bone prosthesis fabrication were obtained by the radio-frequency magnetron sputtering technique. The depositions were performed from pure HA targets on Si and Ti–5Al–2.5Fe alloy substrates. In some experiments a buffer layer of TiN was introduced by pulsed laser deposition before HA coating. The films were deposited in low-pressure argon or Ar–O2 mixtures at substrate temperatures ranging from 70 to 550 °C. We observed that the films grown at temperatures below 300 °C were prevalently amorphous and contain a small amount of crystalline material. The initial crystalline structure of the target is reconstructed after 1 h films annealing at 550 °C in ambient air. Both the films directly deposited at 550 °C and the ones obtained at room temperature and after that annealed at this temperature—mostly contain the HA phase and exhibit good mechanical characteristics. Weaker peaks of CaO secondary phase are visible in the X-ray diffraction patterns of the films directly grown at 550 °C. Traces of TiO2 were detected at the interface with the metallic substrate in case of structures grown without the TiN interlayer. All films were smooth, with an average surface roughness of 50 nm. The films grown on TiN interlayer are harder, have higher elasticity modulus values and an increased mechanical resistance at the indenter penetration.

Plasma diagnostics in pulsed laser TiN layer deposition

Time‐ and space‐resolved emission and laser‐induced fluorescence spectroscopic measurements were performed to investigate vaporization and plasma formation resulting from excimer laser irradiation of titanium targets in a low‐pressure nitrogen atmosphere. Measurement series have been done by varying the laser intensity from the vaporization threshold at 25 MW cm−2 up to values of about 500 MW cm−2 typically applied in pulsed laser deposition processing of titanium nitride films. Thus, the transition from thermal evaporation to the high‐density plasma formation process, leading to the production of reactive species and high‐energy ions, was evidenced. An interesting result for the comprehension of the reactive deposition process was the observation of a quantity of dissociated and ionized nitrogen, which is transported with the plasma front in the direction of the substrate.

Pulsed laser deposition of hydroxyapatite thin films on Ti-5Al-2.5Fe substrates with and without buffer layers

We present a method for processing hydroxyapatite (HA) thin films on Ti-5Al-2.5Fe substrates. The films were grown by pulsed laser deposition (PLD) in vacuum at room temperature, using a KrF∗ excimer laser. The amorphous as-deposited HA films were recrystallized in ambient air by a thermal treatment at 550°C. The best results have been obtained when inserting a buffer layer of ceramic materials (TiN, ZrO2 or Al2O3). The films were characterized by complementary techniques: grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), cross-section transmission electron microscopy (XTEM), SAED, energy dispersive X-ray spectroscopy (EDS) and nanoindentation. The samples with buffer interlayer preserve the stoichiometry are completely recrystallized and present better mechanical characteristics as compared with that without buffer interlayer.

Biofunctional alendronate-Hydroxyapatite thin films deposited by Matrix Assisted Pulsed Laser Evaporation

We applied Matrix Assisted Pulsed Laser Evaporation (MAPLE) in order to synthesize alendronate-hydroxyapatite thin films on titanium substrates. Alendronate-hydroxyapatite composite nanocrystals with increasing bisphosphonate content (0, 3.9, 7.1%wt) were synthesized in aqueous medium. Then, they were suspended in deionised water, frozen at liquid nitrogen temperature and used as targets for MAPLE experiments. The depositions were conducted with a KrF* excimer laser source (l=248nm, t(FWHM)=25ns) in mild conditions of temperature and pressure. The obtained thin films had a good crystallinity, which slightly decreases with the increase of alendronate content, and exhibited a porous-like structure. Osteoblast-like MG63 cells and human osteoclasts were cultured on the thin films up to 14 days. In the presence of alendronate, MG63 cells displayed a normal morphology, increased proliferation and higher values of differentiation parameters, namely type I collagen, osteocalcin, and osteoprotegerin/TNF-related activation-induced cytokine receptor ratio. In contrast, osteoclasts showed significantly reduced proliferation, and increased level of Caspase 3. Moreover, the coatings synthesized from hydroxyapatite at relatively high bisphosphonate content (7.1% wt) displayed a reduced production of Tumour Necrosis Factor alpha (TNF-alpha) and Interleukin 6 (IL-6), suggesting a down-regulatory role of alendronate on the inflammatory reaction. The successful deposition of alendronate modified hydroxyapatite thin films yields coatings with enhanced bioactivity, able to promote osteoblast differentiation and to inhibit osteoclast proliferation.

Magnesium and strontium doped octacalcium phosphate thin films by matrix assisted pulsed laser evaporation

Octacalcium phosphate (OCP) is a promising alternative to hydroxyapatite as biomaterial for hard tissue repair. In this study we successfully applied Matrix Assisted Pulsed Laser Evaporation (MAPLE) to deposit Mg and Sr doped OCP (MgOCP and SrOCP), as well as OCP, thin films on titanium substrates. OCP, Mg-substituted and Sr-substituted OCP were synthesized in aqueous medium, then were suspended in deionised water, frozen at liquid nitrogen temperature and used as targets for MAPLE experiments. The depositions were carried out using a KrF* excimer laser source (λ=248 nm, τ(FWHM)=25 ns) in mild conditions of temperature and pressure. The results of X-ray diffraction, infrared spectroscopy, scanning electron microscopy and energy dispersive spectroscopy investigations revealed that the OCP thin films are deposited in the form of cauliflower-like aggregates and droplets, as well as crystal fragments, with a homogeneous distribution of magnesium and strontium on the surface of the coatings. Human osteoblast-like MG-63 cells were cultured on the different biomaterials up to 14days. MgOCP and SrOCP coatings promote osteoblast proliferation and differentiation with respect to OCP. Under these experimental conditions, the production of procollagen-type I, transforming growth factor-β1, alkaline phosphatase and osteocalcin indicated that the level of differentiation of the cells grown on the different coatings increased in the order OCP

Levan Nanostructured Thin Films by MAPLE Assembling

Synthesis of nanostructured thin films of pure and oxidized levan exopolysaccharide by matrix-assisted pulsed laser evaporation is reported. Solutions of pure exopolysaccharides in dimethyl sulfoxide were frozen in liquid nitrogen to obtain solid cryogenic pellets that have been used as targets in pulsed laser evaporation experiments with a KrF* excimer source. The expulsed material was collected and assembled onto glass slides and Si wafers. The contact angle studies evidenced a higher hydrophilic behavior in the case of oxidized levan structures because of the presence of acidic aldehyde-hydrogen bonds of the coating formed after oxidation. The obtained films preserved the base material composition as confirmed by Fourier transform infrared spectroscopy. They were compact with high specific surface areas, as demonstrated by scanning electron and atomic force microscopy investigations. In vitro colorimetric assays revealed a high potential for cell proliferation for all coatings with certain predominance for oxidized levan.

Deposition of high quality TiN films by excimer laser ablation in reactive gas

A new laser method is proposed for the deposition of high purity, hard fcc TiN layers of unlimited thickness. The film thickness can be very finely controlled mainly through the intermediary of the number of applied laser pulses as the deposition rate is of only 0.02–0.05 nm/pulse. The ablation is promoted from a Ti target by high intensity multipulse excimer laser irradiation in a low pressure N2 ambient gas while the forming compound is collected on a Si single‐crystalline wafer. The best results have been obtained for an ambient pressure of p=10–30 mTorr and a distance between the target and support of d=10 mm. It is shown that the formation of a liquid phase within the irradiated zone, maintained even after the end of a laser pulse, is the most important requisite for TiN formation. TiN is then ablated as a stoichio‐ metric phase.

Structural and optical characterization of WO3 thin films for gas sensor applications

The structure, chemical composition, and optical properties of tungsten trioxide thin films grown by pulsed laser deposition were investigated. An ultraviolet KrF* excimer laser (λ=248nm, τFWHM≅20ns, ν=2Hz) was used for irradiation of tungsten trioxide targets in oxygen atmosphere. Our research focused on the effect of the ambient gas pressure and substrate temperature on the chemical composition, crystalline status, and optical properties of the obtained thin films. To this end, the films were studied by x-ray diffractometry Raman spectroscopy, and energy dispersive x-ray spectroscopy. Optical transmittance measurements were performed with a double beam spectrometer within the 400–1200nm range. The films deposited at oxygen pressure values higher than 10Pa and substrate temperatures above 300°C consist of crystalline tungsten trioxide. Their average transmittance in the visible-infrared spectral region reaches about 85% appropriate for the envisaged applications.

Multistage plasma initiation process by pulsed CO2 laser irradiation of a Ti sample in an ambient gas (He, Ar, or N2)

New experimental results are reported on plasma initiation in front of a titanium sample irradiated by ir (λ=10.6 μm) laser pulses in an ambient gas (He, Ar, and N2) at pressures ranging from several Torr up to the atmosphere. The plasma is studied by space‐ and time‐resolved emission spectroscopy, while sample vaporization is probed by laser‐induced fluorescence spectroscopy. Threshold laser intensities leading to the formation of a plasma in the vapor and in the ambient gases are determined. Experimental results support the model of a vaporization mechanism for the plasma initiation (vaporization‐initiated plasma breakdown). The plasma initiation is described by simple numerical criteria based on a two‐stage process. Theoretical predictions are found to be in a reasonable agreement with the experiment. This study provides also a clear explanation of the influence of the ambient gas on the laser beam‐metal surface energy transfer. Laser irradiation always causes an important vaporization when performed i...