Kiyotaka Yamada

Confocal Raman spectroscopic analysis of cross-linked ultra-high molecular weight polyethylene for application in artificial hip joints

Confocal spectroscopic techniques are applied to selected Raman bands to study the microscopic features of acetabular cups made of ultra-high molecular weight polyethylene (UHMWPE) before and after implantation in vivo. The micrometric lateral resolution of a laser beam focused on the polymeric surface (or subsurface) enables a highly resolved visualization of 2-D conformational population patterns, including crystalline, amorphous, orthorhombic phase fractions, and oxidation index. An optimized confocal probe configuration, aided by a computational deconvolution of the optical probe, allows minimization of the probe size along the in-depth direction and a nondestructive evaluation of microstructural properties along the material subsurface. Computational deconvolution is also attempted, based on an experimental assessment of the probe response function of the polyethylene Raman spectrum, according to a defocusing technique. A statistical set of high-resolution microstructural data are collected on a fully 3-D level on gamma-ray irradiated UHMWPE acetabular cups both as-received from the maker and after retrieval from a human body. Microstructural properties reveal significant gradients along the immediate material subsurface and distinct differences are found due to the loading history in vivo, which cannot be revealed by conventional optical spectroscopy. The applicability of the confocal spectroscopic technique is valid beyond the particular retrieval cases examined in this study, and can be easily extended to evaluate in-vitro tested components or to quality control of new polyethylene brands. Confocal Raman spectroscopy may also contribute to rationalize the complex effects of gamma-ray irradiation on the surface of medical grade UHMWPE for total joint replacement and, ultimately, to predict their actual lifetime in vivo.

Fluorescence spectroscopic analysis of surface and subsurface residual stress fields in alumina hip joints

We aim to establish a confocal spectroscopic technique able to study the features of fluorescence spectra arising from native Cr3+ impurity in polycrystalline alumina (Al2O3) as a biomaterial and to use their emission lines as microscopic probes for the characterization of residual stress fields stored in artificial hip prostheses during their implantation in vivo. As an application of the technique, we report for the first time concerning the evolution of microscopic (residual) stress fields stored on the surface and in the subsurface of N=7 retrieved Al2O3 hip joints after exposure in the human body from a few months to 19 yr. The micrometric diameter of the laser beam waist impinging on the joint surface (typically about 1 microm in lateral resolution) enables us to estimate the patterns and magnitude of residual stress with high spatial resolution, at least comparable with the grain size of the material. In addition, a selected confocal configuration for the optical probe enables minimization of the probe size along the in-depth direction. According to a statistical collection of data on the microscopic level for retrieved femoral heads in toto, a residual stress field arising from loading history in vivo during the lifetime of the Al2O3 femoral head can be revealed. Finally, an interpretation is given of microscopic wear mechanisms in Al2O3 artificial hip joints consistent with the observed evolution of surface residual stress fields on elapsed time in vivo.

Fracture Mechanics and Toughening Mechanisms Analysis of Ce-TZP/Al2O3 Nanocomposite for Biomedical Applications

Zirconia ceramics were introduced in the seventhies for use as structural biomaterials after laboratory tests and simulator studies. However, nowadays concerns remain about their reliability in vivo, despite published clinical studies have already established the safety and the good tribological performance of these materials. It is still unclear what level of reliability can be achieved in ceramic biomaterials and how much their toughness level can be enhanced by microstructural design. The polycrystalline nature of ceramic materials may make both the observed properties and performance very scattered. In particular, the grain size and other microstructural features likely play a fundamental role in the mechanical behavior of the material. In this paper, we propose a set of fracture mechanics assessments, aimed to establish the quantitative amount of toughness achievable in a zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite), and in situ confocal Raman spectroscopy to visualize toughening mechanisms, including polymorph transformation and residual stress fields stored around the crack path.

Confocal Raman Spectroscopic Analysis of Phase Transformation and Residual Stresses in Ce-TZP/Al2O3 Nanocomposite for Biomedical Applications

Zirconia ceramics have been widely used for new generation of bearing materials in biomedical applications. In this context, a zirconia-matrix, stabilized with cerium oxide and dispersed with fine alumina particles (Ce-TZP/Al2O3 nanocomposite) was recently developed and this material experienced significant improvements in both fracture toughness and strength above the standard mechanical performance of monolithic zirconia. In this paper, we used confocal Raman spectroscopy to provide quantitative assessments with high spatial resolution of phase structure and residual stress fields developed in the Ce-TZP/Al2O3 nanocomposite. According to confocal Raman spectroscopy, we have directly visualized patterns of phase transformation and residual stress fields stored on the very surface of the material around an indentation print. These spectroscopic assessments may open a perspective in understanding the micromechanical behavior of the Ce-TZP/Al2O3 nanocomposite when subjected to local surface impingement and shocks.

Microscopic Mechanisms behind the Toughening Behavior of Ceria Stabilized Tetragonal Zirconia/Alumina Nanocomposite for Biomedical Applications

With elongation of average human life, problem such as bone embrittlement and osteoporosis call for quick solution and the expectation for artificial biomaterials heightens. Many ceramics widely used as artificial biomaterials are limited by their poor reliability characteristics. A CeO2 stabilized tetragonal zirconia polycrystalline (Ce-TZP) matrix incorporating nanometer sized Al2O3 particles within the zirconia grains (Ce-TZP/Al2O3) was recently developed. This material experienced significant improvements in both fracture toughness and strength above the standard mechanical performance of monolithic zirconia. In this paper, we performed a macro/microscopic fracture mechanics assessment of this developed Ce-TZP/Al2O3 nanocomposite, in comparison with a 3 mol% Y-TZP according to advanced in situ confocal Raman spectroscopy techniques.

Environmental Phase Stability of Ceramics Composite for Hip Prostheses in Presence of Surface Damage

Alumina matrix composite (AMC) has been widely used for artificial hip and knee joints because of its phase stability in human body and its excellent wear resistance. The excellent mechanical properties of strength and fracture toughness of zirconia materials are well known to be closely related to stress-induced transformation from the tetragonal to the monoclinic phase, which is accompanied with 4% volume increase of the zirconia crystal cell. However, it is also to be considered that the material is prone to low temperature aging degradation (LTAD) under hydrothermal environment, like in the human body. This LTAD is influenced by the tetragonal to the monoclinic (t-m) phase transformation. T-m transformation also induces the formation of microcracks at the material surface, and an increase in surface. Microcracking leads to a decrease of mechanical properties, and could explain the failure of implants after some years in vivo [1, 2] .Therefore, it is very important to study how to prevent phase transformation in zirconia components. Transformed monoclinic zirconia percentage can be experimentally measured by Raman spectroscopy and the residual stress distribution, which is related to phase transformation, can be determined by a non-destructive piezo-spectroscopic analysis. In this paper, we attempted to evaluate it from both stress and mechanical properties points of view by confocal Raman and fluorescence spectroscopy.

Electro-Stimulated Piezo-Spectroscopy for Measuring Nano-Scale Residual Stress Fields in Ceramics

Electro-stimulated piezo-spectroscopy (PS) can be quantitatively used for obtaining information about applied and residual stress fields piled up in ceramic materials and devices. PS experiments can be conducted in a field-emission-gun scanning electron microscope (FEG-SEM) equipped with a high spectral resolution cathodoluminescence (CL) spectrometer. Micromechanical information can be thus added to the microscopic crystallographic and chemical information already available in conventional SEM devices. Independent of the physical mechanisms behind CL emission, the spectral position of selected bands in ceramics is shown to possess high stress sensitivity. In addition, given the high scanning flexibility and spatial resolution of the electron beam, residual stress assessments can be performed on relatively large areas with significantly improved spatial resolution as compared with the more popular photo-stimulated PS approach (i.e., using a laser beam as the excitation source). In this paper, we first quantitatively characterize the stress dependence of the spectroscopic bands observed in ruby. Then, based on this knowledge, an application is shown of bi-dimensional residual stress mapping around an indentation print.

Non-Destructive Stress Evaluation on Ce-TZP/Al2O3 Nanocomposite-VINTAGE ZR T-Glass Systems by Confocal Fluorescence Piezo-Spectroscopy

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.

Development of Piezo-Spectroscopic Techniques for Nano-Scale Stress Analysis in the Scanning Electron Microscope of Zirconia Bioceramics Based on Rare-Earth Fluorescence

The electro-stimulated luminescence spectrum of a rare-earth ion added to zirconia (ZrO2) lattice was investigated with the aim of using it as a sensor for nano-scale stress (fluorescence piezo-spectroscopy) and phase transformation assessments in a field emission scanning electron microscope (FE-SEM). In this paper, the selected rare-earth fluorescent ion Eu, added to ZrO2 as a raw oxide powder (Eu2O3) before sintering (in the amount of 1.0 wt. %). Spectroscopic results indicated that the spectral shift of some fluorescent band of the selected rare-earth ion was sensitive to residual stress and that the electron-stimulated spectra of Eu2O3-doped ZrO2 in its tetragonal and monoclinic polymorphs were different to each other. Based on these findings, the luminescent substance can be useful as a “stress and phase transformation sensor”, in order to clarify the elementary mechanisms behind synthetic ZrO2.