4.2 Corrosion and degradation of metallic biomaterials
3 min read•august 16, 2024
Metallic biomaterials face unique challenges in the body. Corrosion and degradation can seriously impact their performance and safety. Understanding these processes is crucial for designing long-lasting, biocompatible implants.
This section dives into the types of corrosion, their effects on implants, and strategies to prevent them. We'll explore how the body's environment influences corrosion and what can be done to protect metallic biomaterials.
Corrosion and Degradation of Biomaterials
Electrochemical Processes and Types of Corrosion
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- Corrosion in metallic biomaterials involves oxidation of the metal (anodic reaction) and reduction of species in the surrounding environment (cathodic reaction)
- Uniform corrosion occurs evenly across the metal surface leading to gradual thinning of the material
- creates localized deep penetrations in the metal surface resulting in rapid material loss in specific areas
- occurs in confined spaces where electrolyte becomes stagnant accelerating corrosion in these areas (joint interfaces)
- happens when two dissimilar metals contact electrically in an electrolyte causing preferential corrosion of the less noble metal (titanium implant near surgical tools)
Advanced Corrosion Mechanisms
- (SCC) occurs due to combined action of mechanical stress and corrosive environment leading to sudden failure of implants
- degrades materials at contacting interfaces subjected to small amplitude oscillatory motion (modular hip implants)
- results from synergistic effects of wear and corrosion in articulating joints accelerating material degradation (artificial hip joints)
- occurs when biofilms form on implant surfaces creating localized environments that promote corrosion
Impact on Biomaterial Performance
Mechanical and Structural Effects
- Corrosion reduces mechanical properties including strength, fatigue resistance, and fracture toughness of metallic biomaterials
- Degradation can result in loosening, failure, or premature removal of implants necessitating revision surgeries
- Changes in surface properties affect and osseointegration of metallic implants
- Formation of wear debris due to corrosion leads to adverse tissue reactions (metallosis, pseudotumors)
Biological and Functional Consequences
- Release of metal ions and corrosion products causes local and systemic toxicity, allergic reactions, and inflammatory responses
- Electrochemical processes interfere with functionality of metallic biomaterials (neural electrodes, cardiac pacemakers)
- Long-term exposure to corrosion products may lead to chronic inflammation and potential carcinogenic effects in surrounding tissues
- Corrosion-induced changes in implant geometry can alter biomechanical load distribution leading to stress shielding or implant instability
Preventing Corrosion in Biomaterials
Surface Modification and Coating Techniques
- creates protective oxide layers enhancing corrosion resistance of stainless steel and
- Anodization forms thicker oxide films on titanium implants improving wear and corrosion resistance
- Plasma spraying deposits corrosion-resistant (hydroxyapatite) on implant surfaces
- Application of biocompatible ceramic (zirconia), polymer (PEEK), or composite coatings act as barriers against corrosive environments
Material Design and Protection Strategies
- Alloying with elements like chromium, molybdenum, or nitrogen improves inherent corrosion resistance (316L stainless steel)
- Cathodic protection techniques employ sacrificial anodes or impressed current systems to prevent corrosion
- Design considerations include proper material selection, avoiding dissimilar metal contacts, and minimizing crevices
- Surface treatments like shot peening or laser shock peening induce compressive residual stresses improving resistance to stress corrosion cracking and fatigue
Monitoring and Maintenance
- Implementation of corrosion monitoring systems detects issues before compromising implant performance
- Regular maintenance protocols include periodic imaging and blood tests to assess implant integrity and ion release
- Development of smart implants with integrated sensors allows real-time monitoring of corrosion processes
- Establishing standardized protocols for implant retrieval and analysis provides valuable data on long-term corrosion behavior
Environment vs Corrosion Behavior
Physiological Factors Influencing Corrosion
- in body fluids (7.4 in blood, 1-3 in stomach) affect corrosion rates of metallic biomaterials
- Temperature fluctuations in the body (fever, inflammation) can accelerate corrosion processes
- Oxygen concentration variations across implant surfaces lead to differential aeration cells promoting localized corrosion
- Presence of proteins and other biomolecules in physiological fluids influences corrosion behavior through adsorption and complexation
Mechanical and Chemical Interactions
- Cyclic loading in physiological environment leads to corrosion fatigue in load-bearing implants (hip stems, dental implants)
- Changes in local chemistry due to inflammatory responses or infections accelerate corrosion processes
- Chloride ions in bodily fluids (0.9% in blood plasma) promote breakdown of passive oxide layers initiating pitting corrosion
- Formation of biofilms on implant surfaces creates localized environments promoting crevice corrosion and microbially influenced corrosion
Key Terms to Review (21)
ASTM F1450: ASTM F1450 is a standard guide developed by ASTM International that provides criteria for assessing the corrosion resistance of metallic materials used in medical devices. This standard is crucial because it helps to ensure that these materials can withstand the harsh environments they may encounter within the human body, thereby preventing degradation and maintaining their structural integrity over time.
Bio-corrosion: Bio-corrosion is the process by which microorganisms, such as bacteria and fungi, contribute to the degradation and corrosion of materials, particularly metals. This phenomenon is significant in the context of metallic biomaterials, where the presence of biological environments can accelerate material failure, leading to compromised integrity and functionality of medical devices and implants.
Biocompatibility: Biocompatibility refers to the ability of a material to perform its desired function in a medical application without eliciting any adverse effects on the surrounding biological environment. This concept is critical because it directly influences the design and selection of materials for medical devices, drug delivery systems, and tissue engineering applications, ensuring that they integrate well with biological tissues while minimizing immune response or toxicity.
Coatings: Coatings refer to thin layers of material applied to a substrate to enhance its properties, such as biocompatibility, corrosion resistance, or wear resistance. In the context of biomaterials, coatings can significantly influence the performance and longevity of implants and devices by providing protective barriers or improving interactions with biological tissues.
Crevice Corrosion: Crevice corrosion is a localized form of corrosion that occurs in confined spaces or crevices where stagnant solutions can develop, leading to the breakdown of protective oxide layers. This type of corrosion is particularly concerning in metallic biomaterials, as it can occur in areas such as joints, under gaskets, and in any region where two surfaces come into contact. The lack of circulation in these tight spaces can create an environment that fosters aggressive chemical reactions, ultimately compromising the integrity of the biomaterial.
Electrochemical Impedance Spectroscopy: Electrochemical impedance spectroscopy (EIS) is an analytical technique used to study the electrochemical properties of materials by applying a small alternating current and measuring the resulting voltage response. This method provides insights into the kinetics of electrochemical reactions and the charge transfer processes occurring at interfaces, making it a powerful tool for understanding corrosion and degradation mechanisms in metallic biomaterials.
Electrolyte composition: Electrolyte composition refers to the specific arrangement and concentration of ions present in a solution, which can affect its electrical conductivity and reactivity. In the context of metallic biomaterials, understanding electrolyte composition is crucial as it directly influences corrosion processes and the degradation of these materials within biological environments, impacting their performance and longevity.
Fretting Corrosion: Fretting corrosion is a type of localized corrosion that occurs at the contact interface of two materials, typically metals, when there is small oscillatory motion or vibrations between them. This movement can lead to the breakdown of the protective oxide layer on the surfaces, exposing the underlying metal to corrosive environments, which can significantly weaken the integrity of metallic biomaterials used in medical devices.
Galvanic corrosion: Galvanic corrosion is an electrochemical process that occurs when two dissimilar metals are in electrical contact within a corrosive environment, leading to the accelerated deterioration of the more anodic metal. This type of corrosion is critical to understand in the context of metallic biomaterials, as it can significantly affect their longevity and functionality in medical applications. The phenomenon is driven by the potential difference between the metals, which results in the transfer of electrons and the consequent degradation of one of the metals involved.
Implant failure: Implant failure refers to the inability of a biomedical implant to function as intended, which can lead to complications, pain, or the need for additional surgical interventions. This failure can occur due to various reasons, including corrosion and degradation of the materials used in the implant, affecting its structural integrity and performance over time.
ISO 10271: ISO 10271 is an international standard that specifies the requirements and testing methods for the corrosion resistance of metallic biomaterials used in medical devices. This standard is crucial in ensuring that materials used in implants and other medical applications are safe and effective over time, particularly in physiological environments where degradation can lead to serious health issues.
Microbially influenced corrosion: Microbially influenced corrosion (MIC) refers to the degradation of metals due to the metabolic activities of microorganisms, leading to localized corrosion that can significantly impact the integrity of metallic biomaterials. This phenomenon occurs when bacteria, fungi, or other microorganisms interact with metal surfaces, altering the chemical environment and promoting corrosion processes that may not happen in their absence. The effects of MIC are particularly important in biomedical applications where metallic implants are exposed to biological environments.
Passivation: Passivation is the process by which a material, typically a metal, becomes less reactive due to the formation of a protective oxide layer on its surface. This phenomenon is crucial in enhancing the corrosion resistance of metallic biomaterials, particularly in biomedical applications where longevity and biocompatibility are essential. By limiting further oxidation and corrosion, passivation ensures that the integrity and functionality of the material are maintained over time.
PH levels: pH levels measure the acidity or alkalinity of a solution, indicating how many hydrogen ions are present. In the context of metallic biomaterials, pH levels significantly influence corrosion and degradation processes, as they affect the stability of metallic surfaces and their interactions with biological environments.
Pitting corrosion: Pitting corrosion is a localized form of corrosion that leads to the creation of small, deep holes or pits in a metal surface. This type of corrosion is particularly problematic for metallic biomaterials, as it can compromise the integrity and functionality of implants and devices. The localized nature of pitting means it can go undetected until significant damage has occurred, which makes understanding its mechanisms and prevention crucial in the field of biomaterials.
Polarization resistance: Polarization resistance refers to the opposition to the flow of electric current in an electrochemical cell, specifically related to the charge transfer at the electrode interface. This concept is crucial for understanding corrosion and degradation processes in metallic biomaterials, as it helps in assessing the stability and integrity of these materials when exposed to biological environments.
Stainless steel: Stainless steel is a group of iron-based alloys known for their corrosion resistance, achieved through the addition of chromium (at least 10.5%) and other alloying elements. This unique property makes stainless steel a popular choice for a variety of biomedical applications, especially in the design of orthopedic implants, cardiovascular devices, and various surgical instruments.
Stress corrosion cracking: Stress corrosion cracking is a failure mechanism that occurs when a material is subjected to tensile stress in a corrosive environment, leading to the formation of cracks. This phenomenon is particularly significant in metallic alloys used in biomedical applications, where mechanical stress and body fluids can interact, causing premature failure of implants and devices. Understanding this term is crucial for evaluating the integrity and longevity of biomaterials exposed to various physiological conditions.
Tissue reaction: Tissue reaction refers to the biological response of living tissues to foreign materials, such as metallic biomaterials, introduced into the body. This reaction can involve inflammation, healing processes, and potential integration or rejection of the material, influenced by factors like corrosion and degradation of the implant. Understanding tissue reaction is crucial for predicting how a biomaterial will perform and interact within a biological environment.
Titanium alloys: Titanium alloys are metallic materials composed primarily of titanium, combined with other elements such as aluminum, vanadium, or molybdenum to enhance their properties. These alloys are known for their excellent strength-to-weight ratio, corrosion resistance, and biocompatibility, making them ideal for various applications, particularly in the medical field, where they are used for implants and surgical instruments. The unique characteristics of titanium alloys also address challenges related to corrosion and degradation in biological environments.
Tribocorrosion: Tribocorrosion refers to the combined effect of mechanical wear and electrochemical corrosion occurring at the interface of materials, particularly metals, in the presence of a corrosive environment. This phenomenon is crucial for understanding the durability and longevity of metallic biomaterials, as it can significantly affect their performance and reliability in biological settings.