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Transcript of metallic biomaterials

SAVIGNY Pascale & GIROUD Elodie

3rd December 2002

Metallic Biomaterials

Corrosion Titanium, Stainless steels, Chromium Cobalt alloys, Amalgam Orthopaedic & Dental Applications

Functional materials 4H1609 Course PM Version 1 Rolf Sandstrm

Metallic Biomaterials december 2002

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Summary

Introduction 1. Corrosion 1.1. Corrosion of metallic implant 1.2. Electrochemical aspects - mechanism 1.3. Pourbaix diagram 1.4. Rate of polarisation and polarization curves 1.5. Types of corrosion 1.6. Protection methods 1.7. Passivation of metallic materials 1.8. Conclusion on corrosion 2. Properties and applications of the most used metallic biomaterials 2.1. Stainless steels 2.2. CoCr Alloys 2.3. Ti Alloys 2.4. Dental metals 2.5. Other metals 2.6. Conclusion 3. Titanium and Ti alloys as Biomaterials 3.1. Background 3.2. Applications References

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IntroductionThe definition of a biomaterial covers a broad area. In fact, any natural or synthetic material that interfaces with living tissue and/or biological fluids may be classified as a biomaterial. However, certain physical, chemical, and mechanical characteristics render some materials more desirable than others for biological application, depending on its intended use in the body. For example, the material for a bone implant must exhibit great compressive strength, while the material for a ligament replacement must display far more flexibility and tensile strength. In all cases, however, a biomaterial must perform compatibly with the body. In other words, the biocompatibility and in some cases, bioactivity, of the material comprise key factors in determining whether a new graft or implant succeeds in the body. In order to define biocompatibility, it may be easier to define what it is not, rather than what it is. A biocompatible material disrupts normal body functions as little as possible. Therefore, the material causes no toxic or allergic inflammatory response when the material is placed in vivo. The material must not stimulate changes in plasma proteins and enzymes or cause an immunologic reaction, nor can it leads to carcinogenic or mutagenic effects. Bioactive materials play a more aggressive role in the body. While a biocompatible material should affect the equilibrium of the body as little as possible, a bioactive material recruits specific interactions between the material and surrounding tissue. For example, a bioactive material can encourage tissue integration to aid in the fixation of an implant in the body. Many total hip implants operations today rely partially on a porous coating of Hydroxyapatite (HA), a normal component of bone, to help permanently stabilize the stem of the implant in the bone. The coating encourages the ingrowth from the surrounding tissue that interlocks within the pores much like the pieces of a puzzle lock together. Although many current medical procedures call for inert biocompatible materials, the increasing understanding of tissue interaction promises many more applications for aggressive bioactive materials. The closely packed crystal structure and metallic bonding in metals or metal alloys render them valuable as load bearing implants as well as internal fixation devices in large part for orthopedic applications as well as dental implants. 316 L stainless steel, titanium alloys, and cobalt alloys when processed suitably contribute to high tensile, fatigue and yield

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strengths; low reactivity and good ductility to the stems of hip implant devices. Their properties depend on the processing method and purity of the metal, however, and the selection of the material must be made appropriate to its intended use. Metallic biomaterials are normally considered to be highly corrosion resistance due to the presence of an extremely thin passive oxide film that spontaneously forms on their surfaces. These films serve as a barrier to corrosion processes in alloy systems that would otherwise experience very high corrosion rates. That is, in the absence of passive films, the driving force for corrosion for typical implant alloys (e.g., titanium-based, cobalt chromium (CoCr)based, and stainless-steel alloys) is very high, and corrosion rates would also be high. The properties of these passive oxide films depend to a large extent on their structure and chemistry, which are themselves dependent on the substrate's prior thermal, mechanical, and electrochemical history.

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1. Corrosion1.1. Corrosion of metallic implant Corrosion is the unwanted chemical reaction of a metal with its environment, resulting in its continued degradation to oxides, hydroxides or other compounds. Biological fluids in the human body contains water, salt, dissolved oxygen, bacteria, proteins, and various ions such as chloride and hydroxide. As a result, the human body is a very aggressive environment for metals if we want to use them as biomaterials. Corrosion resistance of a metallic implant material is consequently an important aspect of its biocompatibility. 1.2. Electrochemical aspects - mechanism Corrosion occurs when a metal atom becomes ionized and goes into solution, or combine with oxygen or other species in solution to form a compound which flakes off or dissolves. The body environment is very aggressive in terms of corrosion since it contains chloride ions and proteins and many chemical reactions can occur. The electrolyte, which contains ions in solution, serves to complete the electrical circuit. Anions are negative ions that migrate toward the anode, and cations are positive ions that migrate toward the cathode. At the anode, or positive electrode, the metal oxidizes by losing valence electrons as in the following: M Mn+ + ne-. So the anode is always the one which corrodes and thus has to be protected. The tendency of metals to corrosion is based on the Standard Electrochemical Series of Nernst potentials, shown in the table 1, which are the potentials associated with the ionization of metal when one electrode is the standard hydrogen electrode. Reaction E0 (volts) Reaction E0 (volts) + 2+ LiLi -3,05 CuCu -0,34 + 2+ NaNa -2,71 CoCo -0,28 3+ 2+ AlAl -1,66 NiNi -0,23 TiTi3+ -1,63 H22H+ 0 2+ + CrCr -0,56 AgAg +0,80 2+ + FeFe -0,44 AuAu +1,68 Table 1 : Standard Electrochemical Series Figure 1 : Electrochemical cellV anode + anions cations electrolyte cathode -

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1.3. Pourbaix diagram The Pourbaix diagram is a plot of regions of corrosion, passivity and immunity as they depend on electrode potential and pH. The Pourbaix diagrams are derivated from the Nernst equation and from the solubility of the degradation products and the equilibrium constants of the reaction. Nernst equationE = E0 RT concentrat ion of reactants ln nF concentrat ion of products

Figures 2 et 3 : Immunity, Passivity, corrosion diagram (left) and Pourbaix diagram of Fe (right)

The corrosion region is set arbitrarily at the concentration of greater than 10-6 molar. Immunity, also called cathodic protection, is defined as equilibrium between metal and its ions at less than 10-6 molar. In this region, the corrosion is energetically impossible. In the passivity domain, the stable solid constituent is an oxide, hydroxide, hybrid, or slat of the metal. Passivity is defined as equilibrium between metal and its reaction products at a concentration less than 10-6 molar. There are two diagonal lines in the diagram. The top oxygen line represents the upper limit of the stability of water and is associated with oxygen rich solution or electrolytes near oxidizing materials. In the region above this line, oxygen is evolved according to 2H2O O2 + 4H+ + 4e-. In the human body, saliva, intracellular fluid, and interstitial fluid occupy regions near the oxygen line, since they are saturated with oxygen. The lower hydrogen diagonal line represents the lower limit of the stability of water. Hydrogen gas is evolved according to

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2H3O+ + 2e- H2 + 2H2O. Aqueous corrosion occurs in the region between these diagonal lines. In the human body, urine, bile, the lower gastrointestinal tract, and the secretions of ductless glands, occupy a region somewhat above the hydrogen line. Different parts of the body have different pH values and oxygen concentrations. Consequently, a metal which performs well in one part of the body may suffer an unacceptable amount of corrosion in another part. 1.4. Rate of polarisation and polarization curves The regions in the Pourbaix diagram specify whether corrosion will take place, but they do not determine the rate. The rate, expressed as an electric current density, depends upon electrode potential which can be seen in potential current curves (figure 4). From those curves, it is possible to calculate the number of ions per unit time liberated in the tissue, as well as the depth of Figure 4 : Potential current curves for different metals metal removed by corrosion in a given time. An alternative experiment is one in which the weight loss of a specimen of metal due to corrosion is measured as a function of time. The rate of corrosion also depends on the other factors such as mechanical stresses that are applied on the material. The stressed alloy failures occur due to the propagation of cracks in corrosive environments. But the main idea is to remind that the corrosion rate depends largely on the pH. 1.5. Types of corrosion We need distinguish two types of corrosion: endogenous (produced or growing from within