Metal Corrosion in the Human Body: The Ultimate Bio-Corrosion Scenario by Douglas C. Hansen t is not something one usually thinks about until it becomes a personal Ihealth issue for most of us. That is, the need for the implantation of a medical device (Fig. 1) to improve our existing medical condition. The majority of these devices are made of a metal alloy (Table I), and for most of us, any concerns about corrosion implications from these implants are secondary to the relief we feel from the anticipated beneficial effect these devices are designed to have. As the global population increases in age, there is a parallel increase in the number of implantation procedures. One study reports that world-wide sales of orthopedic implants alone in 2003 was $8.7 billion and projected to increase at an annual growth rate of 12.5% and reach $17.9 billion by 2009.1 Clearly, as new devices and technologies are developed, there will be a continuing need for the FIG. 1. Knee and hip implant components. (Photo courtesy of Medcast Inc.) understanding and characterization of how metal surfaces of implants interact resulting in the rejection of the implant heat treatment, cold working, and surface with their surrounding physiological by the surrounding tissue, or both.3 In finishing, where surface treatments are environment. either case, explantation of the device is particularly important for corrosion Implant alloys are typically derived usually required to correct the situation. and wear properties. Since metals are from three materials systems: stainless The human body is not an inherently susceptible to corrosion, steels, cobalt-chromium based alloys and environment that one would consider implants are routinely pre-passivated titanium alloys.2 The question “Does hospitable for an implanted metal alloy: prior to final packaging using an acid corrosion of a metallic implant cause a highly oxygenated saline electrolyte bath or some other electrochemical a clinically relevant problem?” is one at a pH of around 7.4 and a temperature anodizing process (titanium alloys),4 or that probably only an electrochemist of 98.6°F (37°C). While it is well known an electropolishing method (stainless or materials engineer will ask when that chloride solutions are among the steel and cobalt alloys).5 Alloys specific confronted with the prospect of having most aggressive and corrosive to metals, for the intended uses of the implant are a metal device implanted into his or her the ionic composition and protein determined based upon whether they body. While numerous issues may arise concentration in body fluids complicate will be load bearing (wear and fretting with the implant following surgery, one the nascent understanding of biomedical resistant) or not. Finally, galvanic couples of the most fundamentally important is corrosion even further. Variations in are routinely encountered in static (i.e., the interaction between the surrounding alloy compositions can lead to subtle no relative motion) situations where the physiological environment and the sur- differences in mechanical, physical, or consideration of the potential difference face of the implant itself. This interaction electrochemical properties. However, between the metals involved is secondary can lead to either the failure of the these differences are minor compared to the required yield strength and implant to function as it was intended, with the potential variability caused by strength: weight ratio of the implant or have an adverse effect on the patient differences in fabrication methodology, device, such as stainless steel screws anchoring a titanium alloy bone fracture fixation plate. Table I. Major biomedical metals and alloys and their applications. The aim of this article, therefore, is to give the reader a broad overview of Material Major Application the different types of metals and alloys used, the corrosion of metals in the 316L Stainless Steel cranial plates, orthopedic fracture plates, human body, the different environments dental implants, spinal rods, joint encountered and how well these ma- replacement prostheses, stents, catheters terials resist degradation in the body. Cobalt-Chromium alloys orbit reconstruction, dental implants, orthopedic fracture plates, heart valves, Implant Materials spinal rods, joint replacement prostheses The fundamental requirement for Titanium, cranial plates, orbit reconstruction, choosing a metallic implant material is Nitinol, maxillofacial reconstruction, dental that it be biocompatible, that is, not Titanium alloys implants, dental wires, orthopedic fracture exhibiting any toxicity to the surroun- (Ti-6Al-4V, Ti-5AL-2.5 Fe, Ti-6Al-7Nb) plates, joint replacement prostheses, ding biological system. For more than stents, ablation catheters a hundred years, various metals have been investigated for implantation into The Electrochemical Society Interface • Summer 2008 31 Hansen (continued from previous page) Table II. Mechanical properties of implant alloys and human bone. Material Tensile Strength Yield Strength Vickers Hardness Young’s Modulus Fatigue Limit 2 2 2 2 (MN/m) (MN/m) (Hν) (GN/m) (GN/m) 316L SS 650 280 190 211 0.28 Wrought 1540 1050 450 541 0.49 Co-Cr Alloy Cast 690 490 300 241 0.30 Co-Cr Alloy Ti-6Al-4V 1000 970 --- 121 --- Human Bone 137.3 --- 26.3 30 --- the human body, such as aluminum, quantitative corrosion measurements of copper, zinc, iron and carbon steels, implants are made in vivo. However, to silver, nickel, and magnesium.3 All of maintain reproducibility and minimize these were discarded as being too reactive variables, very few in vitro studies involve in the body for long term implantation. simulated body fluids that contain When stainless steel was introduced into amino acids, proteins and ions at the general engineering as a new corrosion- proper temperature and pH, simply resistant material in the early 1900s, it due to the complexity of the system and was soon utilized in surgical applications. the inherent difficulty of reproducing However, the 18-8 stainless steel that that system in the laboratory. While this was initially used was found to exhibit approach may appear to be flawed, the intergranular corrosion due to high overall ranking of biomaterials in terms (0.08%) carbon content and gross pitting of corrosion resistance tested in vitro due to low molybdenum content. Of all does not change when compared to the the stainless steels, only the austenitic measurement of the same biomaterials molybdenum-bearing 316 was of any FIG. 2. Dental implants showing anchors and in vivo (although in quantitative terms, use, even though it was described as dental prostheses. (Image courtesy of BioHorizons, corrosion rates for each specific alloy may inherently corrodible.6 Movement toward Inc.) rise or fall).3 316L alloy, having a much lower carbon of the human body, this environment content (0.03%), greatly reduced the risk can contain water, complex organic Implant Corrosion Mechanisms of intergranular attack. compounds, dissolved oxygen, sodium, During the same period of time, chloride, bicarbonate, potassium, cal- The types of corrosion that are cobalt-chromium and cobalt-chromium- cium, magnesium, phosphate, amino pertinent to the currently used alloys are: molybdenum alloys were first introduced acids, proteins, plasma, lymph, saliva pitting, crevice, galvanic, intergranular, and utilized in dental and orthopedic etc. Upon implantation, the tissue stress-corrosion cracking, corrosion applications due to their corrosion environment is disturbed, disrupting fatigue, and fretting corrosion. These resistance. The most corrosion resistant blood supply to the surrounding tissue corrosion types will be discussed in of the implant materials presently em- and the ionic equilibrium. The initiation relation to the specific alloys and their ployed is titanium and its alloys. Titanium of corrosion can be the result of various occurrence. alloys were first used in the 1960s and conditions existing along the implant Titanium alloys.—The shape memory their use has been growing steadily since surface, whether it is the formation of alloy, Nitinol, is composed of near the mid-1970s and continues to increase. localized electrochemical cells resulting equi-atomic amounts of Nickel and Several titanium alloys (α & β phases), in pitting attack, or crevice corrosion Titanium. Since the early 1970s it such as Ti-6Al-4V, Ti-5Al-2.5Fe, and at the interface between a plate and a has found widespread clinical use as Ti-6Al-7Nb provide ideal strength and locking screw, or any one of the other corrosion resistance characteristics. The forms of corrosion that main advantage of titanium and its alloys can occur, which will be is the non-reactivity of the passive film discussed later. that is formed; the main disadvantages are its susceptibility to fretting as well Corrosion Testing as oxygen diffusion during fabrication, 7 causing embrittlement. The mechanical Numerous methods properties of the alloys discussed here are 8 have been used to evaluate presented in Table II. the corrosion resistance of implant materials in Biological Environment the laboratory, with the majority involving either When a metal device is implanted qualitative measurements of into the human body, it is continually implantation of devices into exposed to extracellular tissue fluid experimental animals (in (Fig. 2, for example). The exposed metal vivo) or quantitative electro- surface of the implant undergoes an chemical measurements electrochemical dissolution of material