Biomedical Engineering – From Theory to Applications

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414 Biomedical Engineering – From Theory to Applications solutions or intermetallic compounds that depend on the alloying composition and temperature. A phase is defined as a homogenous portion in a material that has its own physical and chemical characteristics and properties. Every pure metal is considered as a phase, as also is every solid solution and intermetallic compound. Alloying a metal with finely dispersed particles as a second-phase is one of the important method of strengthening alloys and enhancing their properties. The second-phase particles present as obstacles to the movement of dislocations thus increase the overall strength and hardness of the alloys. 3.2 Physical and mechanical properties of metals One important criterion in metals selection is the consideration of their physical properties, such as density, melting point, specific heat, thermal conductivity, thermal expansion and corrosion. Density of a metal plays a significant role on the specific strength and specific stiffness, which are the ratio of strength-to-weight and stiffness-toweight, respectively. For many applications, one of the most important considerations is their deterioration by corrosion. Corrosion of metal depends on the metals composition and the corrosive media in the surrounding environment. The most common and easiest way of preventing corrosion is the careful selection of metals once the corrosion environment has been characterized. Nonferrous metals, stainless steel, and non-metallic materials generally have high corrosion resistance due to the presence of protective passive layer. Titanium develops a film of titanium oxide, TiO2. A similar phenomenon also occurs in stainless steels due to the presence of chromium in the alloy that develops chromium oxide layer on the surfaces. If the protective film is broken and exposes the metal underneath, a new oxide film begins to form for further protection. Unlike the physical properties, mechanical properties of metal are the behaviour of metals that measured under the effect of external forces. Tension test is the most common method to determine the mechanical properties of materials, such as strength, ductility, toughness, elastic modulus, and strain hardening capability. The specimen used in this test usually is prepared according to ASTM specifications. Another important mechanical property of metal is the hardness which gives a general indication of its resistance to localize plastic deformation. Several test methods which use different indenter materials and shapes have been developed to measure the hardness of metals. Table 2 shows mechanical properties of some alloys used for implant. It also shows chemical composition of the alloy which is a determined factor for the formation of microstructure and phases, thus their properties, i.e. mechanical properties. For example, the addition of Al and V into pure Ti greatly increase its tensile strength. Beside composition, metallurgical state and synthesis process of the metals change their mechanical properties, i.e. annealed condition has better ductility than that of cold worked and cast metal implants usually possess lower strength than those made by forging. Different from the breakdown in the tension test where the specimen is subjected under gradual increase of loading until fractures, failure of a component practically occurs after a lengthy period of repeated stress or strain cycling. This phenomenon is called fatigue failure and responsible for the majority of failures in many mechanical components. To avoid this kind of failure, the stress level should be reduced to a level which the material can be subjected without fatigue failure, regardless of the number of cycle. The maximum level of loading stress is known as the endurance limit or fatigue limit.
Metals for Biomedical Applications Metals Main alloying composition (wt%) Stainless steel: Fe; 16-18.5Cr; 10-14Ni; 316L type (ASTM, 2003) 2-3Mo;
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BIOMEDICAL ENGINEERING - FROM THEOR
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free online editions of InTech Book
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VI Contents Chapter 9 Targeted Magn
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X Preface stand-alone and readers c
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2 Biomedical Engineering - From The
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4 Fig. 2. Process for retrieval inf
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6 Biomedical search engine Scientif
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120 6. Conclusion Biomedical Engine
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150 5. Conclusions Biomedical Engin
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162 1 st phase : half year 4 2 nd p
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166 7. Innovative active training B
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172 12. Maturing projects’ story
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180 16. Acknowledgments Biomedical
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190 R* M M M M MR*M O O O O M O R*
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196 Fig. 9. Chemical structure of 5
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198 Relative Intensity (AU) Biomedi
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204 2. Preparation of IO nanopartic
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