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Objective To investigate the intraoral development and kinetics of low-temperature degradation (LTD) in second-generation 3 mol.% yttria-doped tetragonal zirconia polycrystal (3Y-TZP) monolithic prostheses, as well as the influence of masticatory mechanical stress and glaze layer on it. Methods A total of 101 posterior tooth elements were included in a prospective clinical study, which included ex vivo LTD monitoring (at baseline, 6 months, 1 year, and 2 years) using Raman spectroscopy (n = 2640 monoclinic phase measurement points per evaluation time) and SEM. Four types of areas (1–2 mm2 surface, 6 on molars, and 4 on premolars) were analyzed on each element surface: occlusal, axial, glazed, or unglazed. Raman depth mapping and high-resolution SEM were performed on the selected samples. Results LTD developed in 3Y-TZP monolithic restorations 6 months after intraoral placement and progressed with time. After two years, the tetragonal-to-monoclinic transformation was non-uniform, with the presence of localized clusters of transformed grains. In axial areas, the grain aspect was typical of the classical nucleation-growth process reported for LTD, which progresses from the surface to a depth of several tens of microns. However, in occlusal areas, tribological stress generated surface crushing and grain pull-out from the clusters, which induced an underestimation of the aging process when the evaluation was limited to monoclinic phase quantification. Glazing cannot be considered a protection against LTD. Significance If LTD occurs in dental prostheses in the same way as in orthopedic prostheses, its clinical impact is unknown and needs to be further studied.

Micro-extrusion-based 3D printing of complex geometrical and porous calcium phosphate (CaP) can improve treatment of bone defects through the production of personalized bone substitutes. However, achieving printing and post-printing shape stabilities for the efficient fabrication and application of rapid hardening protocol are still challenging. In this work, the coaxial printing of a self-setting CaP cement with water and ethanol mixtures aiming to increase the ink yield stress upon extrusion and the stability of fabricated structures was explored. Printing height of overhang structure was doubled when aqueous solvents were used and a 2 log increase of the stiffness was achieved post-printing. A standard and fast steam sterilization protocol applied as hardening step on the coaxial printed CaP cement (CPC) ink resulted in constructs with 4 to 5 times higher compressive moduli in comparison to extrusion process in the absence of solvent. This improved mechanical performance is likely due to rapid CPC setting, preventing cracks formation during hardening process. Thus, coaxial micro-extrusion-based 3D printing of a CPC ink with aqueous solvent enhances printability and allows the use of the widespread steam sterilization cycle as a standalone post-processing technique for production of 3D printed personalized CaP bone substitutes. Statement of significance Coaxial micro-extrusion-based 3D printing of a self-setting CaP cement with water:ethanol mixtures increased the ink yield stress upon extrusion and the stability of fabricated structures. Printing height of overhang structure was doubled when aqueous solvents were used, and a 2 orders of magnitude log increase of the stiffness was achieved post-printing. A fast hardening step consisting of a standard steam sterilization was applied. Four to 5 times higher compressive moduli was obtained for hardened coaxially printed constructs. This improved mechanical performance is likely due to rapid CPC setting in the coaxial printing, preventing cracks formation during hardening process.

3Y-TZP discs were coated using 5Y-PSZ and 12Ce-TZP suspensions. Samples were evaluated in terms of roughness (Ra, Rq, Rz and Rsk), B3B flexural resistance, hardness, Young's modulus by instrumented indentation and surface damage resistance and Vickers hardness by impress Vickers measurements. Coatings with thickness of ~7 μm were obtained. 3Y-TZP uncoated discs showed similar hardness and Young's modulus when compared to those coated with 5Y-PSZ. Discs coated with 12Ce-TZP had opposite behavior. Vickers and Berkovich hardness showed the same trend, with almost no difference observed between monolithic and the 5Y-PSZ coated samples, and with the 12Ce-TZP coated samples exhibiting lower hardness values. Coated samples presented lower flexural strength than uncoated ones. Correlation between roughness and B3B flexural strength was also performed, showing that surface quality achieved by the coating process is the one that rules the final resistance of the component, instead of the coating material.

Objectives The presentin vitro study evaluated the effect of LTD (Low Temperature Degradation) on microstructural properties, phase transformation and micro-crack formation of 5 commercially and non-commercially available Zirconia dental implant systems. Methods Accelerated ageing at 134 °C and 2 bar pressure for 30 h was completed. Focused Ion Beam-Scanning Electron Microscopy (FIB/SEM), X-ray diffraction (XRD) and cathodoluminescence quantified phase transformation and micro-crack formation. Results Transformation of the tetragonal grains towards the monoclinic symmetry was observed in all systems. The highest depth was measured in non-commercial TAV dental with the largest grain size (8.7 μm). A micro-cracked layer was associated with the transformation zone. The ageing-related micro-crack formation was parallel to the surface for all groups and was deepest for the non-commercial TAV dental with the largest grain size (7.4 μm). Conclusion LTD following in vitro ageing using an autoclave was minimal for all implant systems investigated.

To develop durable bone healing strategies through improved control of bone repair, it is of critical importance to understand the mechanisms of bone mechanical integrity when in contact with biomaterials and implants. Bone mechanical integrity is defined here as the adaptation of structural properties of remodeled bone in regard to an applied mechanical loading. Accordingly, the authors present why future investigations in bone repair and regeneration should emphasize osteocytes that surround matrices. Osteocytes are mechanosensitive cells considered as the orchestrators of bone remodeling, which is the biological process involved in bone homeostasis. These bone cells are trapped in an interconnected porous network, the lacunocanalicular network, which is embedded in a bone mineralized extracellular matrix. As a consequence of an applied mechanical loading, the bone deformation results in the deformation of this lacunocanalicular network inducing a shift in interstitial fluid pressure and velocity, thus resulting in osteocyte stimulation. The material environment surrounding each osteocyte, the so called perilacunar and pericellular matrices properties, define its mechanosensitivity. While this mechanical stimulation pathway is well known, the laws used to predict bone remodeling are based on strains developing at a tissue scale, suggesting that these strains are related to the shift in fluid pressure and velocity at the lacunocanalicular scale. While this relationship has been validated through observation in healthy bone, the fluid behavior at the bone-implant interface is more complex. The presence of the implant modifies fluid behavior, so that for the same strain at a tissue scale, the shift in fluid pressure and velocity will be different than in a healthy bone tissue. In that context, new markers for bone mechanical integrity, considering fluid behavior, have to be defined. The viewpoint exposed by the authors indicates that the properties of the pericellular and the perilacunar matrices have to be systematically investigated and used as structural markers of fluid behavior in the course of bone biomaterial development.

International audience; Objective. Healing of soft tissues and improvement of aesthetics have become major research objectives in implantology and renewed the interest for ceramics implants. The aim of this study was to evaluate the pre-clinical performance of screw-shaped sandblasted-etched implants processed from an innovative zirconia-based ceramic composite, in comparison to titanium. Methods. Twenty-four ceramic and twenty-four titanium screw-shaped sandblasted-etched dental implants were tested in a splitmouth design in six Beagle dogs. Surface topographies were investigated by confocal microscopy. Local tissue effects were evaluated at 4 and 13 weeks after implantation through histology. An ANOVA statistical analysis (5% risk; p

Poly(l-lactide-co-d,l-lactide) PDLA/45S5 Bioglass® (BG) composites for medical devices were developed using an original approach based on a thermal treatment of BG prior to processing. The aim of the present work is to gain a fundamental understanding of the relationships between the morphology, processing conditions and final properties of these biomaterials. A rheological study was performed to evaluate and model the PDLA/BG degradation during processing. The filler contents, as well as their thermal treatments, were investigated. The degradation of PDLA was also investigated by Fourier transform infrared (FTIR) spectroscopy, size-exclusion chromatography (SEC) and mechanical characterization. The results highlight the value of thermally treating the BG in order to control the degradation of the polymer during the process. The present work provides a guideline for obtaining composites with a well-controlled particle dispersion, optimized mechanical properties and limited degradation of the PDLA matrix.

Different kinds of interactions between the restorative material and mineralized dental tissues result in secondary caries around dental composites. Of these, the mechanical interactions have to be carefully investigated. Due to the elastic mismatch between dental tissues and the composite restoration, complex stresses and strains develop at their interface. This complex mechanical environment disturbs the demineralization-remineralization equilibrium of dental hard tissues. The fluid flow both over and within enamel and dentin, associated with their complex ultrastructure and mechanical behavior, is a key factor. It is known that external mechanical loading can indirectly promote the dissolution of enamel and dentin through a pumping action of cariogenic fluids in and out of microgaps at the interface between mineralized tissues and composite. Mechanical loading can also directly influence the physicochemical behavior of dental hard tissues by inducing complex strain and stress fields on the crystal scale. It is important to consider both the direct and indirect paths by which mechanical loading can influence the apatite dissolution kinetics. Therefore, a system atic approach should be used to investigate the mechanism of secondary caries formation considering the tooth-composite interface as a unique complex in which each element has an influence on the other.

Two-photon lithography is a potential route to produce high-resolution 3D ceramics. However, the large shrinkage due to the elimination of an important organic counterpart of the printed material during debinding/sintering remains a lock to further development of this technology. To limit this phenomenon, an original approach based on a composite resin incorporating 45 wt% ultrasmall (5 nm) zirconia stabilized nanoparticles into the zirconium acrylate precursor is proposed to process 3D zirconia microlattices and nanostructured optical surfaces. Interestingly, the nanoparticles are used both as seeds allowing control of the crystallographic phase formed during the calcination process and as structural stabilizing agent preventing important shrinkage of the printed ceramic. After 3D photolithography and pyrolysis, the weight and volume loss of the microstructures are drastically reduced as compared to similar systems processed with the reference resin without nanoparticles, and stable 3D microstructures of cubic zirconia are obtained with high spatial resolution. In the case of a patterned surface, the refractive index of 2.1 leads to a diffraction efficiency large enough to obtain microfocusing with linewidths of 0.1 µm, and the demonstration of a microlens array with a period as small as 0.8 µm.

Long-term, reliable operation of Solid Oxide fuel and electrolysis Cell (SOCs) necessitates cells with high strength and resistance to mechanical degradation. In this work, strength, Young’s modulus and Low-Temperature Degradation (LTD) of several zirconia-based SOCs supports are studied. The study shows that replacing 3YSZ with a tetragonal zirconia compound having lower stabilizer contents can improve the strength of porous supports. An enhancement up to 30 % over the-state-of-the-art support (NiO‒3YSZ) can be achieved for example with using NiO‒2.5YSZ. It is further evidenced that tetragonal zirconia-based SOC components (both Y-doped and Ce-Y co-doped) are susceptible to LTD in the studied range of grain sizes (i.e. ≈ 200−300 nm). Addition of small amount of alumina (0.5 wt%) is observed to increase the LTD resistance of porous supports. The susceptibility to LTD must be considered in designing materials for SOCs, also during materials processing, storage, handling and operation where a dry atmosphere is recommended.

The market share of zirconia (ZrO2) dental implants is steadily increasing. This material comprises a polymorphous character with three temperature-dependent crystalline structures, namely monoclinic (m), tetragonal (t) and cubic (c) phases. Special attention is given to the tetragonal phase when maintained in a metastable state at room temperature. Metastable tetragonal grains allow for the beneficial phenomenon of Phase Transformation Toughening (PTT), resulting in a high fracture resistance, but may lead to an undesired surface transformation to the monoclinic phase in a humid environment (low-temperature degradation, LTD, often referred to as ‘ageing’). Today, the clinical safety of zirconia dental implants by means of long-term stability is being addressed by two international ISO standards. These standards impose different experimental setups concerning the dynamic fatigue resistance of the final product (ISO 14801) or the ageing behavior of a standardized sample (ISO 13356) separately. However, when evaluating zirconia dental implants pre-clinically, oral environmental conditions should be simulated to the extent possible by combining a hydrothermal treatment and dynamic fatigue. For failure analysis, phase transformation might be quantified by non-destructive techniques, such as X-Ray Diffraction (XRD) or Raman spectroscopy, whereas Scanning Electron Microscopy (SEM) of cross-sections or Focused Ion Beam (FIB) sections might be used for visualization of the monoclinic layer growth in depth. Finally, a minimum load should be defined for static loading to fracture. The purpose of this communication is to contribute to the current discussion on how to optimize the aforementioned standards in order to guarantee clinical safety for the patients.

International audience; Ceramic-on-ceramic bearings in total hip replacement have shown the potential to provide low wear solutions in hip replacement. Assessing the tribological performance of these materials is important to predict their long-term performance in patients. In this study, a methodology was devised to assess the tribological in vitro behaviour of composite ceramics under combined adverse edge loading conditions and accelerated ageing. Two commercial ceramic composites were considered, namely Alumina-Toughened Zirconia (ATZ, ceramys®) and Zirconia-Toughened Alumina (ZTA, symarec®). The bearing couples were studied using the Leeds Mark II hip joint simulator for a total of eight million cycles, the first two million under normal gait (no edge loading) and the following six million cycles with the addition of edge loading conditions driven by medial-lateral separation. The bearing couples underwent hydrothermal ageing using an accelerated protocol in autoclave every million cycles. The influence of edge loading combined with ageing was significant for ATZ bearings, resulting in a slower overall ageing kinetics over the wear stripe than on the control heads. During the autoclave ageing steps, the monoclinic fraction increased more over the wear stripe area than over the unworn area. Both results thus indicated that the monoclinic phase was removed during shocks induced by edge loading. The wear performance of the two materials were similar exhibiting relatively low wear rates and low level of microstructural damage for these clinically relevant adverse conditions.

International audience; Osteomalacia is a pathological bone condition consisting in adeficient primary mineralization of the matrix, leading to an accumulation of osteoid tissue and reduced bone mechanical strength. The amounts, properties and organization of bone constituents at tissue level, are known to influence its mechanical properties. It is then important to investigate the relationship between mechanical behavior and tissue composition at this scale in order to provide a better understanding of bone fragility mechanisms associates with this pathology. Our purpose was to analyze the links between ultra-structural properties and the mechanical behavior of this pathological bone tissue (osteomalacia) at tissue level (mineral and osteoidseparately, or global). Four bone biopsies were taken from patients with osteomalacia, and subsequently embedded, sectioned, and polished.Then nanoindentation tests were performed to determine local elastic modulus E, contact hardness Hc and true hardness H for both mineralized and organic bone phases and for the global bone. The creep of the bone was also studied using a special indentation procedure in order to assess visco-elasto-plastic (creep) bone behavior. This allowed a detailed study of the rheological models adapted to the bone and to calculate the parameters associated to a Burgers model. Ultra-structural parameters were measured by Fourier Transform InfraRed Microspectroscopy (FTIRM) on the same position as the indents.The use of rheological models confirmed a significant contribution from the organic phase on the viscous character of bone tissue. The elastic E and the elasto-plastic Hc deformation were correlated to both collagen maturity and Mineral/Matrix. The pure plastic deformation H was only correlated to the mineral phase. Our data show that mineral phase greatly affects mechanical variables (moduli and viscosities) and that organic phase (as illustrated in osteoidtissue) may play an important role in the creep behavior of bone.In conclusion, this study brings mechanical and physicochemical values for osteoid and mineral phases

Transformation-induced plasticity in a ceria-stabilized-zirconia based composite was studied in four-point bending, with particular emphasis on nucleation and growth of tetragonal-to-monoclinic transformation bands. They initiated on the side in tension at a given value of stress and grow in size and number with increased applied load. The spacing between them was related to samples’ thickness: thicker samples led to a smaller number of larger/deeper bands. Residual stresses around indentations, placed on the tensile surface of some samples, triggered transformation bands nucleation but had no significant effect on the final total number of bands. A simple stress shielding model suggests that wide bands hinder adjacent thinner bands to propagate and expand because of stress shielding around them. Nucleation of a given transformation band is thus related to: (i) stress concentrations at the surface, (ii) geometry (stress-field inside the sample) and (iii) a shielding effect from the bands already present.

International audience; Two-photon lithography is a potential route to produce high-resolution 3D ceramics. However, the large shrinkage due to the elimination of an impor-tant organic counterpart of the printed material during debinding/sintering remains a lock to further development of this technology. To limit this phenomenon, an original approach based on a composite resin incorporating 45 wt% ultrasmall (5 nm) zirconia stabilized nanoparticles into the zirconium acrylate precursor is proposed to process 3D zirconia microlattices and nano-structured optical surfaces. Interestingly, the nanoparticles are used both as seeds allowing control of the crystallographic phase formed during the calcination process and as structural stabilizing agent preventing important shrinkage of the printed ceramic. After 3D photolithography and pyrolysis, the weight and volume loss of the microstructures are drastically reduced as compared to similar systems processed with the reference resin without nanoparticles, and stable 3D microstructures of cubic zirconia are obtained with high spatial resolution. In the case of a patterned surface, the refractive index of 2.1 leads to a diffraction efficiency large enough to obtain microfo-cusing with linewidths of 0.1 μm, and the demonstration of a microlens array with a period as small as 0.8 μm

International audience; This overview showcases the current state of the art in the fabrication, properties and applications of bioactive glass-polyester composites for dentistry, craniomaxillofacial surgery, orthopedics and bone tissue engineering. The combination of these materials is a successful strategy to simultaneously modulate and optimize the degradation rate, mechanical properties, cell response and osteostimulation of bone substitutes. Two major approaches can be identified: bone regeneration or bone repair. The first is performed using porous scaffolding materials, the second one by dense molded implants. For both strategies, the synthesis, processing and characterization of materials are presented based on a comprehensive review of the available literature. Relevant recent in vitro and in vivo studies are also covered. Current and potential future applications of this interesting family of biocomposites are discussed. The literature search revealed a considerable body of work investigating the biological performance of these composites, evidencing the interest on the topic. In particular, the use of polyester/BG composites is well-studied in terms of material fabrication, as well as characterization of physicochemical and in vitro biological properties. On the other hand, there is much less evidence of translational research efforts. It is apparent that future research will have to focus on the collection of more in vivo and clinical data to broaden the knowledge of the time dependent performance of these materials in realistic conditions

Magnesium oxide nanocubes are compressed along the [001] direction in situ in the transmission electron microscope. Incipient plasticity in the smaller samples is characterized by the nucleation of few $ \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/\kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} {\lt}110{\gt} \{{110} \} $ dislocations while a larger number of line defects is observed in larger nanocubes. Yield and flow stresses scattered stochastically above a minimum value varying as the inverse of the sample size. The upper bound is given by the reduced number of dislocation sources. Such size-dependent behaviour is justified by a detailed statistical analysis and is fully explained by the deformation mechanism.

International audience; Meso-porous γ-alumina ceramics used as catalyst supports are characterized by high porosity fraction (>70%) and specific surface area (>200 m²/g) targeted to ensure high catalytic performances, but at the expense of the mechanical strength. The aim of this work is to link the mechanical properties of three different catalyst supports with their microstructure. Instrumented indentation tests were performed at two different length scales to assess mechanical properties at both macroscopic scale (support) and at constituents scale (grains and matrices). The distribution of porosity fraction in grains and matrices is estimated with image processing on Scanning Electron Microscopy (SEM) pictures. The mechanical behavior of catalyst supports are strongly influenced by their microstructure. In spite of a larger porosity fraction, macro-and meso-porous grains exhibit higher mechanical properties than the matrices. Matrix of the three alumina supports have almost similar volume fraction of porosity but different mechanical properties. Finally, Focused Ion Beam (FIB) cross-sections of residual indents indicate a damageable behavior under contact loading, with densification of larger pores.

International audience; Thin, porous zirconia-based ceramic components are of high interest in energy application devices where they are used as structural ceramics. Mechanical reliability of such devices is not only dependent on the fracture toughness of the ceramic components, but also on their sensitivity to slow crack growth (SCG). In this work, the fracture toughness and SCG behavior of porous (4.5 to 45.5%) and thin (~ 0.25 mm) 3Y-TZP ceramics are investigated using the Double Torsion method. The analysis of the double torsion data, previously developed for dense materials, was here assessed and adapted. The compliance of the samples was observed to change linearly with crack length and the measured stress intensity factor was dependent on crack length, as for dense materials. This dependency decreased by increasing the sample porosity. For all materials, the ratio of the SCG threshold to fracture toughness was of 0.56 ± 0.06.

International audience; Individual platelets in nacre‐like ceramics are able to locally withstand stress levels many times larger than the strength of larger specimens made of the same materials. This size effect, usually reported as being relative to the size of pre‐existing defects, is described by considering both stress and energy failure conditions. We show that there is a characteristic length, defined by the material's Young's modulus, fracture energy, and strength, above which failure is governed by a stress criterion and below which energy considerations are dominant. Applying the coupled criterion to three‐point bending of a single alumina platelet allows the identification of the platelet strength and fracture energy. The proposed approach also allows capturing the decrease in failure stress due to a surface defect accounting for its shape and size.

In this work 3 mol.% yttria-stabilized zirconia (Y-TZP) disks were pressed and subsequently coated, aiming at producing reliable protective coatings to avoid Low Temperature Degradation (LTD) of Y-TZP. Suspensions of 12Ce-TZP or 5Y-PSZ (5 mol.% yttria stabilized zirconia) powders with different solid contents were used as coatings. Accelerated aging was conducted for times of 2, 6 and 18 hours at 134 °C. Specimens were analyzed using XRD, SEM and Colorimetry. Suspensions with solid content of 23 wt% prepared with 12 mol% ceria stabilized zirconia showed coarsening porosity, while all other suspensions achieved acceptable results for this property. Suspensions with 28 wt% Ce-TZP and 23 wt% Y-PSZ were the selected conditions for the LTD studies. They were selected among the remaining conditions, based on their enhanced color properties and surface density, displaying layers with ∼7 μm and 5-10 μm, respectively. Coatings enhanced resistance to LTD.

In this work, the effect of substituting Cu with small amounts of Sn and Si on the passive behavior and pitting resistance of the bulk metallic glass Ti40Zr10Cu36Pd14 was investigated. The Sn/Si substitution promoted homogenization of the solid solution and limited the presence of casting defects. In addition, unlike the heterogeneous passive film grown on TiZrCuPd, the passive films of the micro-alloyed materials were homogeneous, implying less sensitivity to film weakening and breakdown. Thus, alloying with small amounts of Sn and Si appears to be a viable approach to drastically improve the corrosion resistance of TiZrCuPd.

Further development and upscaling of the Solid Oxide fuel and electrolysis Cell (SOCs) technologies would significantly benefit from improvement of their mechanical robustness. In this work, microstructure, crystalline phase composition, fracture toughness and susceptibility to low- and high-temperature degradation of six different Ni(O)‒Zirconia fuel electrode supports, manufactured from six different stabilized zirconia compounds, are investigated. In the oxidized state, tetragonal zirconia-based supports have higher fracture toughness than cubic zirconia-based substrate, due to the transformation toughening effect and a finer grained microstructure. The NiO‒1.5CeO2 4.5YO1.5-SZ support exhibits the highest fracture toughness, showing a 30 and 10 % improvement compared to the state-of-the-art NiO‒5.8YO1.5-SZ support at room temperature and 800 °C, respectively. In the reduced state on the other hand, the tetragonal and cubic zirconia-based substrates have comparable fracture toughness. The Ceria-Yttria co-doped materials possess superior resistance to hydrothermal degradation due to the stabilizing effect of Ce3+ formed during reduction.