ABSTRACT
In the past decade, substantial advances in biochemistry and materials science, including surface science, have occurred, allowing a comprehensive description of processes and mechanisms on the molecular scale and, thus, a deeper understanding of the interactions between artificial implants and the host human body. From the manufacturing viewpoint, an integrated approach by physicists, chemists, materials scientists, biologists, and physicians has successfully led to the development of a large variety of devices implantable in the human body that use many diff erent materials, including metals, polymers, and ceramics. An entirely new generation of biomaterials has thus been developed, which shows suitable physical properties, including acceptably high responses in terms of biocompatibility and overall structural reliability. However, while extensive research has so far detailed substantial improvements in biocompatibility, this enhanced property has been strongly correlated with surface topography and yet less with surface chemistry and mechanics. An increased capability to measure surface roughness down to the nanometer scale, as produced by tribomechanical processes occurring during
frictional exposure of artificial joints in vivo, has not yet been sufficiently matched with the necessary links to a full clarification of the elementary chemical and physical processes behind its occurrence. Therefore, it is now expected that the major emphasis on wear-resistance issues given so far in the field of load-bearing biomaterials should naturally result in a shift toward further understanding of the complex boundary conditions imposed by the living environment. When reviewing, for example, the development of ceramic biomaterials, the most commonly employed one is found to be alumina, which can certainly be regarded as a highly bioinert material. However, the alumina microstructure is highly dependent on the applied manufacturing process (e.g., maximum temperature during sintering, duration of the thermal steps, purity of the raw powder, size, and distribution of the grains, and porosity) and this might have a clear and direct impact on both mechanical and biological properties. It is also well known that both wear and friction behaviors of alumina ceramics are significantly better in water than in a dry atmosphere. However, the eff ect of the pH of the aqueous media on wear and friction behavior is also known to play a key role in reciprocating sliding tests. It has been found that the wear resistance can vary up to one order of magnitude and that the coefficient of friction increases by a threefold factor, depending on the pH environmental conditions [101]. Furthermore, significantly diff erent wear surfaces can be generated for diff erent pH values, and the cumulative development of these surfaces in turn exerts a diverse eff ect on long-term wear and friction behavior. Clearly, chemical and electrochemical eff ects associated with the selected tribological conditions should be invoked in order to explain the observed behavior since, by varying the pH of the sliding environment, one might obtain low-wear or high-wear counterparts with exactly the same alumina ceramics. Despite such evidence, there have been so far no attempts to classify the pH environment naturally existing in human joints, nor has been the pH chemical eff ect introduced among the variables of in vitro wear simulation in artificial joints. Since all the human-made biomaterials are unequivocally employed at the interface between organic and inorganic functions, it is quite intuitive that surface properties impact enormously on the success or failure of a selected biomaterial. Accordingly, the
search for new instrumental approaches is quite demanding and advanced analytical techniques of surface analysis are continuously updated in the field of biomaterials [102]. There is also a great need to provide quick and non-destructive detection of biomaterial damage at their early stage and to quantify degradation at the micro-structural level in order to both assess residual life in vivo and as an aid to material development. In this latter context, analytical techniques have been developed and employed for measuring surface properties, including chemical and phase structures, bioinertness, morphology, and topography, and for correlating those properties to the macroscopic joint performance. Methods of surface analysis can be also systematically used for the verification of intentionally produced surface modifications in vitro, as well as for diff erentiating sliding responses of diff erent bearing couples upon exposure in body environment. Surface analyses also play key roles in understanding the eff ects of diff erent lubricants, especially useful in characterizing the intrinsic diff erences between in vivo and in vitro exposure. Since even slight stoichiometric changes and/or topological modifications can greatly alter the performance of a joint bearing surface, the characterization tools should be highly sensitive and eff ective in correlating any of such modifications with changes in biological and frictional performance. Accordingly, the detailed information gained may allow one tailoring biomaterial microstructures that can significantly elongate the joint lifetime or even favor specific biological responses [103-106]. A combination of several surface characterization methods is usually necessary to provide the comprehensive information necessary for judging about the performance of a biomaterial to be used in artificial joints. The most basic assessment is that of the morphology and of the topography of the load-bearing surface (i.e., surface roughness, planarity, etc.), which can be measured with diff erent resolution and degrees of precision by optical (or stylus) profilometry [107-109] or by atomic force microscopy (AFM) [110, 111]. The choice of the analytical instrument should be made depending on the scale on which surface roughness has to be measured (i.e., on the micrometer and on the nanometer scale for optical profilometry and AFM, respectively). Spectroscopic techniques such as Raman, infrared (IR), and luminescence
spectroscopy are often used to broaden knowledge about oxidation [91, 112-116], phase composition [117-119], elastic or plastic state of strain [92, 116, 120-123], and texture on the molecular scale [123, 124]. Nowadays, the ease, rapidity and non-destructiveness with which spectroscopic techniques can be applied make them more and more frequently used in combination with optical microscopy methods. Nevertheless, care should be taken in interpreting spectroscopic data when steep gradients are present along the immediate subsurface of the biomaterial. As a matter of fact, many biomaterials are quite transparent to visible light and a laser beam focused on the biomaterial surface might probe significantly deep portions of materials. Under such circumstances, the spectroscopic output becomes an average between the surface and the bulk state of the biomaterial, thus making it difficult understanding the actual state of the biomaterial surface. Substantial improvements of in-depth resolution can be obtained by means of confocal pinholes placed in the optical circuit of the measurement device [92, 125-127]. The pinhole aperture can be modulated to allow a controlled depth, in the scale of the single micron, to be monitored. However, when nanometer-scale shallowness is required for the measurement the choice of the probe unavoidably shifts to electrons. The success of using electron microscopy methods (especially in terms of spatial resolution) to characterize biomaterial surfaces is well established [122]. A microscopy technique historically well established and most frequently found in the biomaterials literature is scanning electron microscopy (SEM) [128-136]. This technique makes use of a primary beam of electrons that interact with the specimen of interest, in a vacuum environment, resulting in diff erent types of electrons and electromagnetic waves being emitted. The secondary electrons discharged from the specimen surface are collected and displayed to provide a high-resolution micrograph. Unlike optical microscopy methods, SEM involves a sample preparation procedure for coating the (usually) electrically non-conductive biomaterial with a metal prior to data collection. The thin metallic coating, which can be applied by sputter coating, is typically kept to an extent of few nanometers in thickness. Common conductive metals used for coating include gold, platinum, or gold/palladium alloys. Upon insertion of the sample into the SEM, acquisition of the micrographs can be quickly
done allowing for a series of images to be obtained with increasing resolution. Modern SEM devices can easily resolve features in the nanometer range, while computerized image acquisition provides a permanent record of them. In addition to imaging the surface morphology of biomaterials, the SEM can be combined with other analytical methods such as energy dispersive X-ray analysis (EDX) to determine the elemental distribution on the biomaterial surface [137, 138], electron backscattering diff raction (EBSD) for highly localized crystallographic assessments [139-141], and cathodoluminescence (CL) spectroscopy for monitoring surface modifications in terms of stoichiometry, lattice defects and elastic strain [71, 121, 142-144]. EDX, EBSD, and CL thus provide a set of complementary data of various nature that are typically obtained from sampling depths ranging from few nanometers to few micrometers, depending on the acceleration voltage used for generating the primary electrons. In other words, an increase in acceleration voltage can be seen as a means for collecting data that are more representative of the bulk material rather than the surface and vice versa [145, 146]. Obviously, the choice of surface characterization methods can be influenced by numerous factors, including not only the type of measurement required but also the extent of the analyzed surface region, the required spatial resolution, precision and accuracy, the influence, and the eventual damage induced by the selected probe on the biomaterial surface, namely, the bias that such probe can introduce in the output of the measurement itself. Further criteria for the choice can be the degree of manipulation to which the sample must be subjected before being analyzed and the ease of use of the analytical instrument. The remainder of this chapter is conceived to provide a succinct yet comprehensive overview of emerging characterization techniques in the field of biomaterials for artificial joints. For reasons of available space, only a limited selection of information can be given here, but emphasis is placed on those new aspects that have recently come to the forefront of biomaterials research, including the applications and limitations of each technique. While the main aim of this chapter is to provide a brief synopsis of some of those less known (but newly developed or newly extended) analytical techniques, a number of references are also listed that detail how those technique are specifically applied to the characterization of biomaterials. In addition, some discussion is
also included to more common characterization methods such as fracture mechanics [147-149] that have been long performed in biomaterials research but require updating. A subchapter is also dedicated to methods of surface tribology characterization [150, 151] updated with the use of in situ laser spectroscopy in a specially designed tribometer. These latter two characterization methods are important because they provide direct outputs about the intrinsic structural resistance of joint biomaterials when loaded under extreme environmental conditions.
