Materials Science Forum Vols. 539-543

Abstract: To exploit the impressive electronic, mechanical and thermal properties of Carbon Nanotubes (CNTs) in the nanoelectronics technology, the development of deposition methods enabling the synthesis of well ordered, properly located and reproducible CNTs structures, is strongly recommended. We have been developing catalytic CVD synthesis of CNTs in order to get aligned nanotubes for applications ranging from nuclear particle-position detectors and cold cathode emitters for storage devices, to interconnects, vias and CNT-FETs. In this paper, the significant achievements gained on CVD growth processes for the CNTs deposition are presented. Ni and Fe catalyst nanoparticles have been obtained starting from thin films evaporated on silicon based substrates. The growth of vertically aligned carpets of MWNTs and horizontally aligned SWNTs, having a diameter of about 1 nm and bridging between patterned catalyst islands, has been accomplished. The SEM, TEM, Raman spectroscopy and AFM characterizations are discussed.

Abstract: Since preferential orientation of c-axis of biological apatite (BAp) crystallites depends strongly on the shape of hard tissue, closely relating to the in vivo stress distribution, it is a useful parameter to judge the bone quality. In this study, preferential alignment of BAp crystallites in original and regenerated hard tissues were analyzed by the micro-beam X-ray diffractometer (μ-XRD) with a beam spot of 50 or 100 μm in diameter. Regenerating processes of bone defects introduced artificially in the rabbit ulna or skull were healed by inserting a biodegradable gelatin hydrogel incorporating basic fibroblast growth factor-2 (FGF-2). Recovery of BAp orientation alignment depends strongly on the regenerated portion and period, which is insufficient to recover the original level, while bone mineral density (BMD) is almost improved to the original level. This means that BMD recovers prior to improvement of the BAp orientation and the related mechanical function in the regenerated tissues. Thus, reloading on the regenerated portion caused by BMD restoration is suggested to accelerate to produce the appropriate BAp preferential alignment due to the remodeling process. The BAp orientation was finally concluded to be one of the most important indices to check the regenerative degree and process in the regenerated bone under the tissue engineering technique.

Abstract: Fretting fatigue is a form of adhesive wear damage caused due to tangential micro motion of two contact bodies under normal pressure and cyclic load. Biomedical implants such as hip joints and bone plates undergo fretting fatigue damage leading to premature in-vivo failure and revision surgeries. Surface modification of implants delays the process of fretting and thereby improves the life of these medical devices. This work involves investigation of fretting fatigue damage of surface treated titanium alloys couple. The surface treatment involves PVD TiN coating, Plasma nitriding, Ion Implantation, Laser nitriding and thermal oxidation. Fretting of all surface treated alloys have shown both adhesive and abrasive mode of contact damage. Friction coefficient of all the surface treated pairs is less compared to uncoated alloys. Plasma nitrided pairs have shown the best performance in terms of fretting fatigue life and friction coefficient compared to all other coatings. Ion implanted pairs have shown little improvement in fretting fatigue lives due to shallow modified layer. PVD TiN coated pairs have irregular friction pattern due to abrasive particles at contact. Thermal oxidation and Laser nitriding have shown poor fretting fatigue performance due to high case thickness.

Abstract: Hydroxyapatite (Ca10(PO4)6(OH)2, HAp), carbonated HAp and titanium oxide are of interest for bone-interfacing implant applications, because of their demonstrated osteoconductive properties. They were coated on the titanium implants and investigated the in vitro and in vivo performance. HAp coatings were performed by the thermal substrate method in aqueous solutions. Titanium oxide film was also formed on the titanium implants by gaseous oxidation, or by anodizing in the acidic solution. All the specimens covered with HAp, carbonated HAp or TiO2 (rutile or anatase). were characterized by XRD, EDX, FT-IR and SEM. In the in vitro testing, the mouse osteoblast-like cells (MC3T3-E1) were cultured on the coated and non-coated specimens for up to 30 days. Moreover, the osseointegration was evaluated from the rod specimens implanted in rats femoral for up to 8 weeks. In in vivo evaluations two weeks postimplantation, new bone formed on the coated and non-coated titanium rods in the cancellous bone and cortical bone, respectively. Bone-implant contact ratio, in order to evaluate of new bone formation, was significantly depended on the compound formed on the titanium implant.

Abstract: A brief description of the uses and clinical applications of synthetic cyanoacrylate polymer adhesive/glues that have been cleared and/or approved as medical devices by FDA since the Medical Device Amendments of 1976 were enacted. This includes cyanoacrylate Class I devices (Exempt and not Exempt devices), Class II cyanoacrylate devices such as Dental Cements and Orthodontic Bracket Adhesives, and Class III (PMA) devices such as Dermabond™, Indermil™ Tissue Adhesive, and Trufill® n-Butyl Cyanoacrylate Embolic Agent. By citing an example of recently FDA approved Class III (PMA) devices in the Cyanoacrylate technology, the author provides a brief discussion of the FDA approval process of medical devices. It includes the FDA issues regarding the published guidance document for "Cyanoacrylate Topical Tissue Adhesives" that will provide guidance to regulatory personnel and manufacturers in the preparation of IDE applications and in the development of valid scientific evidence to support PMA applications for cyanocrylate tissue adhesives intended for topical approximation of skin and others. Also, the author provides a short regulatory description of US FDA; under what laws its operates, how FDA evaluates new devices for marketing, and how the device regulatory system works, for example, Class I, Class II, and Class III cyanoacrylate medical devices.

Abstract: Metal alloys containing chromium (Cr), primarily stainless steels and CoCr alloys, are used in a wide variety of implantable medical devices. These alloys are exposed to chloride containing environments with varying oxidizing potential and complexing agents. These corrosion assisted environmental effects may result in metal ions going into solution. The toxicity of Cr is dependent on valence state. Hexavalent Cr ions are recognized to be more toxic than trivalent Cr. This paper discusses the state of knowledge regarding Cr release, the chemical and mechanical factors that most significantly affect Cr release, and the potential toxicity of Cr as it applies to longterm implantable medical devices.

Abstract: Hydroxyapatite (HAp) coatings were formed on cp titanium plates and rods by the thermal substrate method in an aqueous solution that included 0.3 mM Ca(H2PO4)2 and 0.7 mM CaCl2. The coating experiments were conducted at 40-140 oC and pH = 8 for 15 or 30 min. The properties for the coated samples were studied using XRD, EDX, FT-IR, and SEM. All the specimens were covered with HAp, which had different surface morphologies such as net-like, plate-like and needle-like. After cleaning and sterilization, all the coated specimens were subjected to in vivo and vitro testing. In the in vitro testing, the mouse osteoblast-like cells (MC3T3-E1) were cultured on the coated and non-coated specimens for up to 30 days. Moreover, the specimens (φ2 x 5 mm) were implanted in rats femoral for up to 8 weeks, the osseoinductivity on them were evaluated. In in vitro evaluations, there were not significant differences between the different surface morphologies. In in vivo evaluations, however, two weeks postimplantation, new bone formed on both the HAp coated and non-coated titanium rods in the cancellous and cortical bone. The bone-implant contact ratio, which was used for the evaluation of new bone formation, was significantly dependent on the surface morphology of the HAp, and the results demonstrated that the needle-like coating appears to promote rapid bone formation.

Abstract: Formation of crystallographically orientaed hydroxyapatite (HAp) is one of the promising ways to utilize their anisotropic nature of chemical and biological properties. On the other hand, the development of super conducting magnet technology enables to introduce a high magnetic field which can control crystal orientation of non-magnetic materials with magnetic anisotropy. In this study, a high magnetic field and sample rotation are simultaneously imposed on the hydroxyapatite during a slip casting process in order to align its c-plane within a horizontal plane. From X-ray diffraction, it has been found that the HAp crystals in the sample treated with the magnetic field and the sample rotation were oriented to a particular direction in the slip casting process and it was enhanced by the subsequent sintering process, while the c-axis crystal orientation of the sample treated without the magnetic field and with the sample rotation was not observed before and after the sintering.

Abstract: Highly porous titanium and titanium alloys with an open cell structure are promising implant materials due to their low elastic modulus, excellent bioactivity, biocompatibility and the ability for bone regeneration. However, the mechanical strength of the porous titanium decreases dramatically with increasing porosity, which is a prerequisite for the ingrowth of new bone tissues and vascularization. In the present study, porous titanium with porosity gradients, i.e. solid core with highly porous outer shell was successfully fabricated using a powder metallurgy approach. Satisfactory mechanical properties derived from the solid core and osseointegration capacity derived from the outer shell can be achieved simultaneously through the design of the porosity gradients of the porous titanium. The outer shell of porous titanium exhibited a porous architecture very close to that of natural bone, i.e. a porosity of 70% and pore size distribution in the range of 200 - 500 μm. The peak stress and the elastic modulus of the porous titanium with a porosity gradient (an overall porosity 63%) under compression were approximately 152 MPa and 4 GPa, respectively. These properties are very close to those of natural bone. For comparison, porous titanium with a uniform porosity of 63% was also prepared and characterised in the present study. The peak stress and the elastic modulus were 109 MPa and 4 GPa, respectively. The topography of the porous titanium affected the mechanical properties significantly.