Topic 2 Biomaterials_2017.ppt

Topic 2 Biomaterials_2017.ppt - BioE 201 Biomaterials...

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1 BioE 201 Biomaterials: Properties and Characterization
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2
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3 Biomaterials Definition-A biomaterial is a nonviable material used in a medical device, intended to interact with biological systems (Williams, 1987). Metals Ceramics Polymers Synthetic Naturally derived
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4 Selection Criteria Availability / cost effectiveness Ability to be processed into desired form Mechanical properties fatigue strength wear resistance Non-toxic (leachables-degradation products) corrosion resistance Biocompatible Sterilizable Stability / Degradability
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5 Metals Metals are (mostly) crystalline solids composed of elemental, positively charged ions in a cloud of electrons High strength and stiffness High conductivity Isotropic properties-mechanical properties are independent of the direction of testing Properties and fabrication well known Relatively bioinert / stable
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6 Biomedical Applications of Metals Artificial Joints stainless steel cobalt chromium titanium alloys Heart Valves stainless steel mounts Vascular Stents stainless steel nitinol All metals used as biomaterials are fabricated from alloys that possess increased corrosion resistance relative to base metals Alloy -a mixture of two metallic elements
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7 Metal Corrosion Metals are rarely found in nature in pure form, usually as oxides (raw ores) Purification requires tremendous energy input Corrosion-degradation of metals through electrochemical reactions, reproducing metallic oxides Corrosion is energetically favorable because it transforms the pure metal back to the lower energy oxidized form
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8 Corrosion Formation of rust: Water, oxygen and metal essential Overall Reaction 2: 4Fe +2 + O 2 + 2H 2 O 4Fe +3 + 4OH - 4Fe +3 + 12OH - 4Fe(OH) 3 Overall Reaction 1: 4Fe +2 +O 2 +8H 2 O 2Fe 2 O 3 . H 2 O (s) + 8H + RUST
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9 Corrosion Problems Release of metal ions Allergic reactions-particularly nickel Deterioration of mechanical properties
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10 Solution-Noble Metals A few metals such as gold and platinum have limited electrochemical reactivity and hence little corrosion Occur in nature in pure forms Dental implants Limitation-Expense, poor mechanical properties
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11 Solution-Alloys-Passivation Passivation -the spontaneous formation of a stable oxide layer that minimizes corrosion Biomaterial alloys are tailored to include metals such as titanium and chromium that support passivation Never completely prevented Pitting corrosion -defect in surface oxide layer creates a site for the initiation of corrosion
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