M. Amaral

Si3N4-bioglass composites stimulate the proliferation of MG63 osteoblast-like cells and support the osteogenic differentiation of human bone marrow cells☆

The invitro osteocompatibility of a n ovel Si 3N4-bioglass composite (70–30% weight proportion) with improved mechanical properties (fracture toughness=4.4 MPa m 1/2 ; bending strength=383747 MPa) is reported. Immersionof the composite samples in culture medium (30 min to 7 days) resulted in rapid protein adsorption to the surface and, also, dissolution of the intergranular phase of bioglass (time-dependent process) with the formation of different size cavities. ‘‘As-received’’ and pre-treated material samples presented a similar behaviour concerning the proliferation of MG63 osteoblast-like cells, evaluated during a 5-day culture period. Seeded materials showed a higher cell growth rate as compared to cultures performed onthe standard plastic culture plates. To assess the osteogenic potential of the composite, ‘‘as-received’’ material samples were seeded with human bone marrow cells and cultured for 35 days in experimental conditions that favour the development of the osteoblastic phenotype. The cell adhesion process was similar to that observed in control cultures. Cells successfully adapted to the irregularities of the surface and were able to grow towards inside the cavities; in addition, osteogenic differentiation occurred with the formation of abundant cell-mediated mineralised deposits. Results suggest that this Si3N4-bioglass composite seems to be a promising candidate for high-stress medical applications.

Densification route and mechanical properties of Si3N4–bioglass biocomposites

The processing route and the final microstructural and mechanical characteristics of a novel biomaterial composite are described. This new material is composed of 70 wt% Si3N4 ceramic phase and 30 wt% bioglass, the later performing as a liquid sintering aid system and simultaneously providing bioactivity characteristics to the composite. The conditions for fabrication of an almost fully dense material (approximately 98% of relative density) were pursued. Optimised parameters were 1350 degrees C-40 min-30 MPa by hot-pressing technique. The very fast densification rate of the process avoided the crystallisation of the bioglass intergranular phase and therefore its intrinsic properties were maintained. Also, the large amount of glassy phase assured the densification by liquid phase assisted grain rearrangement without Si3N4 phase transformation. The final mechanical properties of the Si3N4 bioglass were as follows: fracture toughness, K(IC) = 4.4 MPa m(1/2); Vickers hardness, Hv = 10.3 GPa; Youngs modulus, E = 197 GPa; bending strength, sigma(g) = 383 MPa; Weibull modulus, m = 8.3. These values provide an attractive set of properties among other bioactive materials, namely by upgrading the main drawback of bioceramcs and bioglasses for high-load medical applications, which is the lack of satisfactory fracture toughness.

Cytotoxicity evaluation of nanocrystalline diamond coatings by fibroblast cell cultures

The cytotoxicity profile of nanocrystalline diamond (NCD) coatings on a Si(3)N(4) ceramic was investigated. This material is envisaged to have biomedical dental applications such as burrs and surgical instruments. Two fibroblast cell culture systems were used to address the cytotoxicity of NCD-coated samples: L929 cells (a mouse permanent cell line) and human gingival fibroblasts. Cell behavior was evaluated in terms of cell adhesion, cell viability/proliferation (mitochondrial function, MTT assay) and the pattern of cell growth. Fibroblast cell behavior on standard polystyrene culture plates was used as control, as Si(3)N(4) substrates have previously been shown to be biocompatible. NCD coatings provided a suitable surface for cell attachment, spreading and proliferation. Human gingival cells showed a homogeneous cytoplasm spreading, a flattened elongated morphology and a typical parallel alignment on confluent cultures. In comparison, L929 cells denoted a lower cytoplasm expansion, a heterogeneous spreading but a higher proliferation rate. For both cells, after few days, the NCD coating was completely covered with continuous cell layers. As compared to standard polystyrene culture plates, no deleterious or cytotoxic responses were observed with L929 and human fibroblast cell cultures, and in both a slight enhancement in cell proliferation was observed. In addition, the seeded NCD film allowed reproduction of the typical features of the two cell culture systems tested, further suggesting the lack of cytotoxicity of this coating.

Wettability and surface charge of Si3N4–bioglass composites in contact with simulated physiological liquids

Wettability and surface charge studies were performed on a novel Si3N4-30wt% bioglass biocomposite. Contact angle and surface tension variation with time were determined at 25 degrees C, respectively, by the sessile and pendant drop techniques, for distinct testing liquids: water, diiodomethane, simulated body fluid (SBF) and bovine serum albumin (BSA) dissolved in SBF solution. This biocomposite revealed a hydrophilic character (theta = 26.6 +/- 2.0 degrees) and a surface tension value (66.6 mJ m(-2)) comparatively higher than those of the most common bioceramics. An important characteristic is the high work of adhesion towards SBF + BSA (96.4 +/- 0.2mJ m(-2)) that was measured. The Si3N4-bioglass material is negatively surface charged above the pH(IEP) = 2.5 in aqueous SBF + BSA solution, as a result of the presence at the surface of unsaturated Si-O bonds and Si-OH groups. The very high negative zeta potential at pH approximately 7 (-58.6 +/- 5.5mV) influenced albumin adsorption and mechanisms are discussed in terms of entropy and enthalpy gains from conformational unfolding and cations coadsorption.

Three‐dimensional printed PCL‐hydroxyapatite scaffolds filled with CNTs for bone cell growth stimulation

A three-phase [nanocrystalline hydroxyapatite (HA), carbon nanotubes (CNT), mixed in a polymeric matrix of polycaprolactone (PCL)] composite scaffold produced by 3D printing is presented. The CNT content varied between 0 and 10 wt % in a 50 wt % PCL matrix, with HA being the balance. With the combination of three well-known materials, these scaffolds aimed at bringing together the properties of all into a unique material to be used in tissue engineering as support for cell growth. The 3D printing technique allows producing composite scaffolds having an interconnected network of square pores in the range of 450-700 μm. The 2 wt % CNT scaffold offers the best combination of mechanical behaviour and electrical conductivity. Its compressive strength of ∼4 MPa is compatible with the trabecular bone. The composites show typical hydroxyapatite bioactivity, good cell adhesion and spreading at the scaffolds surface, this combination of properties indicating that the produced 3D, three-phase, scaffolds are promising materials in the field of bone regenerative medicine.

Nanocrystalline diamond as a coating for joint implants: cytotoxicity and biocompatibility assessment

Nanocrystalline diamond (NCD) coatings combine a very low surface roughness with the outstanding diamond properties, such as superlative hardness, low self-friction coefficient, high wear and corrosion resistance, and biotolerance, which are ideal features for applications in medicine (knee and hip replacement) and surgical tools. The present work presents a comprehensive study of the cytotoxicity and biocompatibility of NCD films grown by hot-filament chemical vapour deposition (HFCVD) technique, aiming such future applications. Cytotoxicity was evaluated in vitro by seeding human gingival fibroblasts on the NCD surface for 14 days, while specific biocompatibility was assessed on samples seeded with human bone marrow-derived osteoblasts during 21 days. The NCD coatings proved to be noncytotoxic in the preliminary human gingival fibroblast cell cultures, as denoted by a notable sequence of cell attachment, spreading, and proliferation events. In the specific biocompatibility assay envisaging bone tissue applications, NCD coatings induced human osteoblast proliferation and the stimulation of differentiation markers, compared to standard polystyrene tissue culture plates.

Tailored Si3N4 Ceramic Substrates for CVD Diamond Coating

Abstract A review is presented of chemical vapour deposition (CVD) diamond coating of silicon nitride (Si3N4) materials. Microcrystalline and nanocrystalline diamond films were grown using microwave plasma (MPCVD) and hot filament (HFCVD) reactors, respectively. Scanning electron and atomic force microscopy, μ-Raman spectroscopy, low incident angle and classical X-ray diffraction, acoustic emission assisted Brale indentation and thermal conductivity measurements were employed for the full characterisation of the diamond/ Si3N4 system. Using these techniques, the nucleation and growth stages as a function of substrate composition and surface pretreatment were characterised, as well as the diamond quality, the existence of residual stresses and the adhesion between the diamond film and the substrate. Based on this study, a tailored material was developed and tested in the machining of hardmetal workpieces with encouraging results.

Residual stress minimum in nanocrystalline diamond films

Nanocrystalline diamond films have been deposited on silicon nitride substrates by hot filament chemical vapor deposition. Gas mixtures of CH4–H2–Ar were used with variation of the Ar∕H2 ratio in order to study the influence of the Ar content on the formation of nondiamond phases at the grain boundaries and thus in the film residual stress assessed by x-ray diffraction techniques. By varying this ratio it is possible to optimize conditions, decreasing the film’s residual stress to a minimum of 0.09GPa.

The High performance of nanocrystalline CVD diamond coated hip joints in wear simulator test.

The superior biotribological performance of nanocrystalline diamond (NCD) coatings grown by a chemical vapor deposition (CVD) method was already shown to demonstrate high wear resistance in ball on plate experiments under physiological liquid lubrication. However, tests with a close-to-real approach were missing and this constitutes the aim of the present work. Hip joint wear simulator tests were performed with cups and heads made of silicon nitride coated with NCD of ~10 μm in thickness. Five million testing cycles (Mc) were run, which represent nearly five years of hip joint implant activity in a patient. For the wear analysis, gravimetry, profilometry, scanning electron microscopy and Raman spectroscopy techniques were used. After 0.5 Mc of wear test, truncation of the protruded regions of the NCD film happened as a result of a fine-scale abrasive wear mechanism, evolving to extensive plateau regions and highly polished surface condition (Ra<10nm). Such surface modification took place without any catastrophic features as cracking, grain pullouts or delamination of the coatings. A steady state volumetric wear rate of 0.02 mm(3)/Mc, equivalent to a linear wear of 0.27 μm/Mc favorably compares with the best performance reported in the literature for the fourth generation alumina ceramic (0.05 mm(3)/Mc). Also, squeaking, quite common phenomenon in hard-on-hard systems, was absent in the present all-NCD system.

Artifact level produced by different femoral head prostheses in CT imaging: diamond coated silicon nitride as total hip replacement material

Commercial femoral head prostheses (cobalt–chromium alloy, yttria partially stabilized zirconia (Y-PSZ) and alumina) and new silicon nitride ceramic ones (nanocrystalline diamond coated and uncoated) were compared in terms of artifact level production by computed tomography (CT). Pelvis examination by CT allows the correct diagnosis of some pathologies (e.g. prostate and colon cancer) and the evaluation of the prosthesis-bone interface in post-operative joint surgery. Artifact quantification is rarely seen in literature despite having a great potential to grade biomaterials according to their imaging properties. Materials’ characteristics (density and effective atomic number), size and geometry of the prostheses can cause more or less artifact. A quantification procedure based on the calculation of four statistical parameters for the Hounsfield pixel values (mean, standard deviation, mean squared error and worst case error) is presented. CT sequential and helical scanning modes were performed. Results prove the artifact reproducibility and indicate that the cobalt–chromium and Y-PSZ are the most artifact-inducing materials, while alumina and silicon nitride (diamond coated and uncoated) ceramic ones present a low level of artifact. Considering the excellent biocompatibility and biotribological behaviour reported in earlier works, combined with the high medical imaging quality here assessed, diamond coated silicon nitride ceramics are arising as new materials for joint replacement.