Papers by Author: Giuseppe Pezzotti

Abstract: The following research is aimed at understanding the influence of Zirconia-Toughened Alumina (ZTA) and Silicon Nitride (Si₃N₄) on Ultra-High Molecular Weight Polyethylene (UHMWPE) acetabular liners. Bioceramic femoral heads were systematically tested against UHMWPE in controlled environment according to static/load-free coupling in hydrothermal environment, pin-on-ball wear testing, and hip-simulator wear testing. In addition, a retrieved ZTA femoral head has been analyzed and results have been compared to the simulations. Experimental results from X-ray photoelectron (XPS), cathodoluminescence (CL), Raman and Fourier-Transformed Infrared spectroscopy suggest that, despite conventional notions imply that bioceramics are inert, the surface chemistry of bioceramics was relevant to the oxidation rate of polyethylene liners. Non-biointertness could either be advantageous or disadvantageous toward polyethylene oxidation. The main reason resides in the peculiar chemical interactions between polyethylene and different ceramics, and, more specifically, depends on the direction of oxygen flow at the interface between the ceramic and the polymer. ZTA femoral heads were found to release a non-negligible amount of oxygen moieties from their surfaces, thus accelerating oxidative degradation of polyethylene. Conversely, Si₃N₄ ceramics exerted a protective role towards the polyethylene liner by scavenging oxygen from the tribolayer. The results of this work provide new insights into the interaction between bioceramics and polymers, which should also be considered when designing the next generation artificial hip joints with significantly elongated lifetimes.

Abstract: Due to the favourable combination of mechanical strength and fracture toughness, silicon nitride has been applied as a load-bearing bioceramic, in particular for implants used in spinal fusion surgery. Only recently it has been observed that the supposedly inert surface of silicon nitride is actually bioactive: a slow, but not negligible, pH controlled, ionic exchange between nitrogen and oxygen leads to the formation and elution of silicic acid and ammonia groups, also resulting in an effective protection against bacteria colonization. These properties could be further modulated by chemical and mechanical treatments.

Abstract: The most advanced options nowadays available in joint arthroplasty rely on the application to diseased joints of human-made bearing surfaces consisting of microstructurally engineered polyethylene and ceramics as substitutes for the damaged joint cartilage and, partly, for bone. However, it is progressively becoming clear that, whatever superior the biomaterial designed for this purpose, owing to the quite severe structural requirements for human joints, including high contact stresses and aggressive environment at the load-bearing surface, it will have a necessarily limited service lifetime. Giving a quite critical but fundamentally true statement, one could say that, so far, no single product has yet been capable to meet all such severe requirements. Moreover, it is not clear if such a perfect biomaterial will ever exist. This is the main reason for pursuing repair (rather than replacement) of damaged cartilage. In this paper, we inquire about the present status and expected progress in healing osteoarthritis (OA) of chronically damaged joints, and surmise that such innovative procedures could sometime, in the near future, replace the current joint arthroplasty procedures, thus avoiding the unavoidably intrusive surgery associated with nowadays total joint replacements. After reviewing the state of art in the new field of joint cartilage healing, we shall stress the potential importance of vibrational spectroscopy both in diagnostics and in accelerating discoveries through the future developments of therapeutic approaches to cartilage diseases.

Abstract: Raman microprobe spectroscopy and atomic force microscopy (AFM) were systematically used to characterize the surface of an advanced alumina/zirconia composite (henceforth also referred to as zirconia toughened alumina, ZTA), in comparison with a commercially available femoral head made of monolithic zirconia. Experiments were conducted before and after in vitro exposure in water moist environment up to 100 h. Both materials contained zirconia partly stabilized with yttria. Tetragonal-to-monoclinic phase transformation, which was quantitatively characterized by confocal Raman spectroscopy as a function of grain size, showed significant difference between the investigated samples. Such difference was similarly found with respect to both roughness level and time needed for such topologic changes to occur. Variation of phase fractions and of topologic surface parameters were plotted as a function of in vitro exposure time and compared.

Abstract: In view of the imminent release on the Japanese market of hip prostheses made of an advanced alumina/zirconia, we performed morphologic and spectroscopic assessments of their surfaces with high spatial resolution. Femoral heads were characterized to a degree of statistical accuracy in the as-received state and after long-term exposures in vapor-moist environment. Surface and sub-surface screening was made by atomic force microscopy (AFM) and by confocal Raman spectroscopy, respectively. AFM scanning was systematically repeated with nanometer resolution on portions of surface as large as several tens of micrometers, randomly selected on the head surface, while in-depth scanning by the Raman probe allowed non-destructive analysis of phase structure in the sub-surface slab of material. Polymorphic transformation in zirconia was confined to the first few micrometers below the surface and involved no significant increase in surface roughness even after long-term environmental exposure. Surface roughness lied in a range <10 nm after environmental exposures up to 100 h, corresponding to an exposure time in vivo of several human lifetimes (i.e., according to experimentally derived thermal activation energy).

Abstract: A spatially resolved cathodoluminescence (CL) analysis is used as a means for chemical and mechanical analyses of the composite surface after environmental exposure. CL emission proves extremely efficient in concurrently monitoring the concentration of point defects (e.g., oxygen vacancies) on the material surface. Using CL, averaging effects from sub-surface parts of the material can be minimized, and the actual chemical state of the material surface is revealed. As a result, information about the stoichiometry of the material surface can be obtained directly from the lattices of the constituent phases, this enabling one to pattern relevant connections to the environmental resistance of oxide-based bioceramics. A highly fracture resistant alumina/zirconia composite represents the latest trend in ceramics for arthroplastic applications in alternative to monolithic alumina or zirconia ceramics. This composite material is designed from both chemical and microstructural viewpoints in order to prevent environmental degradation and fracture events in vivo, an important step forward in the full exploitation of ceramic materials in the field of arthroplasty. Systematically monitoring the optical activity of oxygen vacancies in both alumina and zirconia phase reveals the distinct role on the kinetics of polymorphic transformation. From the presented data an explicit role is evinced for oxygen vacancy formation in the alumina matrix in the complex cascade of mechanochemical events determining the environmental resistance of the composite.

Abstract: Photo- and electro-stimulated probes have been employed for quantitatively evaluating highly graded residual stress fields generated at the surface of alumina hip joints. Optical calibrations revealed large differences in probe size, which strongly affected the detected magnitude of residual stress. A comparison between the responses of Raman and fluorescence probes in polycrystalline alumina showed that the depth of those probes spread to an extent in the order of the tens of microns even with using a confocal probe configuration. On the other hand, the electro-stimulated luminescence emitted by oxygen vacancy sites (F+ center) in the alumina lattice represented a suitable choice for confining to a shallow volume the stress probe. The electron probe enabled confining the measurement depth to the order of the tens of nanometers. Maps of surface residual stress were collected on both main-wear and non-wear zones of an alumina femoral head. A comparison among stress maps taken at exactly the same location, but employing different probes, clarified the averaging probe effects on surface stress magnitude.

Abstract: With the proliferation of several types and classes of high performance ceramic materials, the screening, evaluation and integration of new materials into structures and devices require a new and more effective approach. Evaluation on the nano-scale of the mechanical characteristics of new ceramic materials requires multiple complementary metrology tools. We report here about an advanced metrology tool, cathodoluminescence (CL) spectroscopy, which has a potential to rapidly screen and evaluate residual stress characteristics in advanced ceramic materials and structures. Nano-scale stress measurements are made in situ into an integrated metrology vacuum chamber in a field-emission gun scanning electron microscope (FEG-SEM). Complementing this tool, we also describe a new image analysis based on CL emission for fast screening and ranking of domain structures in ferroelastic ceramics. The end result of this paper is to show how crystallographic and mechanical characteristics of ceramics can be quantitatively characterized in a hybrid device combining electro-stimulated imaging and spectroscopic outputs.

Abstract: Erbium-doped tellurite glasses show great potential for the fabrication of high-performance integrated optical amplifiers and lasers, thanks to their unique properties in terms of bandwidth and rare earth solubility. As a first step towards the development of smart multi-functional integrated optical circuits, the fabrication of multimode channel waveguides in a sodium-tungsten-tellurite glass, by using nitrogen ions implantation, has been recently demonstrated [1]. The effects of the ion implantation process, however, have not been fully clarified, and a deeper investigation would be necessary in order to optimize the process and to truly exploit the glass useful characteristics. We therefore report here the results of a broad optical, topographic, and structural characterization of tellurite samples irradiated with various doses of nitrogen ions, while keeping constant the beam energy at 1.5 MeV. Characterization techniques have included absorption and luminescence spectroscopy, modal (dark-line) spectroscopy, surface profilometry, scanning electron microscopy, cathodoluminescence spectroscopy and EDX analysis.

Abstract: In the area of dental treatment for crown and dental implants, ceramics restoration is becoming popular due to both aesthetic needs and release of metallic allergy. However, for the restoration of defected tooth, ceramics materials with higher reliability than that of conventional glass or alumina have been required, thus raising expectations for zirconia ceramics. Since residual stress play a significant role in the reliability of dental implants, in this paper, a non-destructive assessment of residual stress is presented for a zirconia-substrate/VINTAGE ZR T-Glass system using confocal fluorescence microprobe spectroscopy.