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Title: Characteristics, surface modification, and in vivo biocompatibility of porous NiTi shape memory alloys for bone implants fabricated via capsule-free hot isostatic pressing
Other Titles: Bu feng zhuang re deng jing ya fa zhi bei de gu yi zhi cai liao duo kong nie tai he jin de xing neng, biao mian gai xing yi ji zai dong wu ti nei de sheng wu xiang rong xing
不封裝熱等靜压法製備的骨移植材料多孔鎳鈦合金的性能, 表面改性以及在动物体内的生物相容性
Authors: Wu, Shuilin (吴水林)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Bone Transplantation
Bone substitutes
Isostatic pressing
Nickel-titanium alloys
Shape memory alloys
Notes: CityU Call Number: TA480.N63 W8 2007
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2007
xviii, 244 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: Biometals such as Ti-based alloys and NiTi shape memory alloys (SMA) have commercial success due to their higher strength and excellent biocompatibility. However, the Young’s moduli of most biometals are much higher than those of human bones. This mismatch has hampered wider commercial applications such as surgical implants because only a low stiffness close to that of nature bones can provide a good load transfer whereby effective new bone formation can be realized. In recent years, porous NiTi SMAs are becoming one of the most promising biomaterials for bone grafts because they have several advantages over currently used biomaterials. For example, they have good mechanical properties and lower Young’s modulus relative to dense NiTi, Ti, and Ti alloys. Porous NiTi SMAs are relatively easy to machine when compare with porous ceramics such as hydroxyapatite and calcium phosphate that tend exhibit brittle failure. The porous structure with interconnecting open pores can also allow tissue in-growth and favors bone osseointegration. In addition, porous NiTi alloys remain exhibiting good shape memory effect (SME) and superelasticity (SE) similar to dense NiTi alloys. However, porous NiTi SMAs fabricated by the current powder metallurgical (PM) processes, such as elemental powder sintering (EPS), combustion synthesis (CS), Self-propagating high temperature synthesis (SHS), spark plasma sintering (SPS), and hot isostatic pressing (HIP) usually exhibit one of the following deficiencies: brittle failure, low compressive strength, no superelasticity, and uncontrollable porous structure. The aim of this investigation is to prepare porous NiTi with adjustable porous structure, good mechanical properties and excellent superelasticity via capsule-free hot isostatic pressing (CF-HIP). The space holder, ammonium acid carbonate (NH4HCO3), was also used in CF-HIP pressing. The effects of the PM processing parameters such as sintering temperatures, cold compaction pressures, hot pressures during CF-HIP, and volume fraction of NH4HCO3 on the properties of porous NiTi alloys prepared by CF-HIP were systematically investigated. Two different foaming models, namely foaming at solid state and foaming at semi-solid state, are used to explain the pore expansion mechanism during the sintering process that involves slow and continuous reduction of the argon pressure at high temperatures. Using the CF-HIP method, porous NiTi SMAs with 20-48% porosity and adjustable pore size, pore shape and pore distribution had been achieved by with space holder addition. Furthermore, these porous SMAs exhibited excellent super-elastic behavior as well as lower Young’s moduli that match the Young’s moduli of human bones. Nickel ions released from nickel-containing alloys is considered one of the common causes for allergic contact dermatitis. The wider applications of NiTi SMAs were thus limited by the toxicity and hypersensitivity of the by-products released from the corroded surfaces of porous NiTi SMAs in human body. In addition, the larger surface area of porous NiTi SMAs would likely increase the possibility of out-leaching of Ni ions. The complex surface morphology would also make it very difficult to modify the entire surface by common line-of-sight coating or surface engineering techniques. In this work, different non-line-of-sight surface modification techniques, such as oxygen plasma immersion ion implantation (O-PIII), air oxidation, and chemical modification were used to modify the surface of porous NiTi SMAs. The related modification mechanisms were also studied in the thesis. The effects of these modification processes on the properties of porous NiTi, such as surface characterization, nickel release behavior, superelasticity, phase transformation temperature, compression strength, bioactivity and cytocompatibility were systematically investigated using various materials characterization techniques. Electrochemical impedance spectroscopy (EIS) was used to monitor the status and charge transition at the interfaces between the exposed surface of porous NiTi SMAs and simulated body fluids (SBF). This analysis aimed at evaluating the corrosion behavior and the influence of the passive films (or coatings) due to different surface modification techniques on the corrosion resistance of the porous alloys. Relevant equivalent circuits involving porous models are proposed to describe the experimental results. The in vivo biocompatibility and biomechanics of porous NiTi fabricated by CF-HIP were preliminarily investigated by implanting 42% and 48% porosity samples in rabbits’ thigh bones for 3.5 months. As a control, the porous titanium samples prepared by CF-HIP with the same porosity were also implanted. Our results revealed that bone tissue can grow smoothly into the inner pores and has no apparent venenous response to porous NiTi alloys. In summary, this project explores the fabrication process and sintering mechanism, phase transformation behaviors, surface modification, EIS characteristics, cytocompatibility, and in vivo biocompatibility of porous NiTi SMAs fabricated by selected powder metallurgy methods. The achievement attained through this study has contributed some significant findings to the development of future clinical application of porous NiTi SMAs in human body implantation.
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