Surface modification to improve the corrosion resistance of Mg for the development of degradable orthopedic implants

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Surface modification to improve the corrosion resistance of Mg for the development of degradable orthopedic implants


Author: Ng, Wai-fong
Title: Surface modification to improve the corrosion resistance of Mg for the development of degradable orthopedic implants
Degree: M.Phil.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations
Bone plates (Orthopedics)
Magnesium alloys
Orthopedic implants
Department: Dept. of Applied Physics
Pages: 1 v. (various pagings) : ill. (some col.) ; 31 cm.
InnoPac Record:
Abstract: Bone plates are commonly employed for internal fixation in bone fracture healing. There are two types of bone plates: non-degradable and degradable. Non-degradable bone plates are currently made of metallic materials like stainless steels and titanium alloys. After bone healing, these plates might have to be removed by a second surgery, especially in young patients so as not to restrict bone growth. On the other hand, degradable bone plates do not require removal after bone healing because they degrade in the human body like resorbable sutures. Currently, degradable bone plates are made of polymeric materials and their low mechanical strength only qualify them as low load-bearing parts. Thus development of degradable metallic bone plates which combine high strength and degradability is interesting and important in bone fracture healing. Magnesium (Mg) is a promising candidate material for degradable metallic bone plates owing to a number of desirable characteristics: (i) Mg is non-toxic and biocompatible; (ii) Mg possesses mechanical properties close to those of human bones, thus minimizing the possibility of stress shielding; and (iii) Mg is degradable in the human body environment. However, the corrosion rate of Mg in body fluids is too high for Mg bone plates to maintain adequate mechanical support in the bone healing phase. Improvement of corrosion resistance is thus an essential step in the development of Mg bone plates. The present project aims at developing different coatings on Mg for decreasing the corrosion rate in the bone healing phase. In this connection four different coating systems have been attempted, including: (1) cerium oxide (CeO₂) coating on pure Mg by cathodic deposition in cerium nitrate solution followed by hydrothermal treatment, (2) sol-gel titanium oxide (TiO₂) coating on microarc oxidized Mg, (3) aluminum-alumina (Al-Al₂O₃) composite coating with aluminum coating fabricated by physical vapor deposition followed by anodization in sulfuric acid, and (4) stearic acid (CH₃(CH₂)₁₆COOH) coating on Mg pre-coated with hydroxide. The coatings formed were studied using various characterization techniques including scanning-electron microscopy (SEM), energy-dispersive X-ray analysis (EDS), X-ray diffractometry (XRD), and Fourier transform infrared spectroscopy (FTIR). The corrosion resistance of various coated Mg samples was studied in Hanks' solution (a simulated body fluid) at 37°C by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The corrosion behavior was also monitored in immersion tests over an extended period at different pH values of the Hanks' solution. The corrosion resistance of all the coated Mg samples was significantly higher than that of bare Mg, though their performance differed. Among various types of coatings, stearic acid coating was found to be the most effective, providing an increase in corrosion resistance which reached as high as 4 orders of magnitude in the initial period, and gradually dropped to about 40 times in the long run. The results of the present project thus demonstrate the possibility of significantly lowering the in vitro corrosion rate of Mg and hence contribute to the development of degradable Mg bone plates. A preliminary study of the effect of albumin on the corrosion rate of Mg samples was also attempted.

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