Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 27

Metals are composed of single elements (such as Ti) or a combination of elements, forming alloys such as brass, which is an alloy of copper and zinc. Most metals show excellent mechanical properties, such as high strength, high stiffness, high ductility, and good fatigue resistance. Strength refers to the ability of a material to support an applied load (or mechanical stress) without breaking. While stiffness refers to the ability to resist deformation when subjected to an applied load, the elastic modulus is a more effective and more widely used measure of a material’s stiffness. Ductility refers to the ability to deform rather than shatter catastrophically, particularly when the applied stress becomes sufficiently high. Fatigue resistance refers to the ability to withstand repeated cyclic loading without fracturing. Most metals generally show moderate hardness and moderate resistance to abrasion or wear, somewhere between ceramics and polymers. The majority of metals have a high density, higher than ceramics and much higher than polymers. The excellent electrical and thermal conductivity of metals is well known.

Except for the noble metals such as gold, silver, and platinum, most pure metals corrode in an aqueous environment, such as the physiological environment. Consequently, most metals cannot be used as implantable biomaterials. On the other hand, a protective oxide surface layer forms rapidly on some metals upon exposure to an oxidizing environment, which passivates them from corrosion. These passivated metals, such as Ti, certain Ti alloys, and stainless steel, have a high resistance to corrosion in the normal physiological environment. Because of their excellent mechanical properties and corrosion resistance, they find considerable use in a variety of orthopedic and dental applications, such as fracture fixation plates, total joint replacement, and dental implants.

Ease of fabrication, as noted earlier, is also an important factor in the selection of a material for use as a biomaterial. Metals can be formed with reasonable ease into 3D objects, coatings and films using conventional fabrication methods that are widely used in the metallurgical industry. Additive manufacturing, also referred to as 3D printing, now provides another method to produce metals with the requisite external shape and microstructure for use as biomaterials.

Overall, metals are normally selected for use as biomaterials when excellent mechanical properties, high electrical conductivity, or a combination of both must be guaranteed. Suitable metals have a high resistance to corrosion in the physiological environment, such as certain noble metals or metals passivated by a protective oxide surface layer.