MSE 536- SP2025_Biomaterials Degradation
MSE 536: Advanced Biomaterials
Biomaterials Degradation and Corrosion
Introduction
Focus on the degradation and corrosion of biomaterials within the body.
Biomaterial Degradation
Uncontrolled Material Degradation:
Leads to structural breakdown and premature failure of devices.
Controlled Degradation:
Occurs under expected parameters to:
Serve a desired purpose (e.g., controlled release of bioactive factors).
Local Reactions:
Can change local chemistry post-implantation, encouraging degradation.
Involves specific inflammatory cells that lower tissue pH and generate strong oxidizing agents.
Leaching Ions:
Metallic surfaces can leach ions, having biological consequences over time.
Degradation in Biological Environment
Body Environment:
Mild with neutral pH and constant temperature.
Corrosion Testing:
In vivo assessments are necessary for proposed implants.
Symptoms of Corrosion:
Local pain/swelling without infection.
X-ray evidence of cracking/flaking.
Surgical observations of tissue discoloration.
Degradative Properties
Longevity and Stability:
Key factors for biomaterials in biological environments.
Influencing Factors:
Proteins, cells, bacteria.
Degradation Rate:
Should match tissue healing/regeneration paces.
Affected by pH, temperature, enzymes.
Biodegradable Materials:
Designed for safe breakdown, e.g., PLA and PGA used in sutures.
Corrosion/Degradation of Biomaterials
Metal Requirements:
Metals must be noble or corrosion resistant to body environments.
Types of Corrosion:
General corrosion, pitting, crevice corrosion, stress-corrosion cracking, fatigue corrosion, intergranular corrosion.
Ceramics vs. Metals:
Ceramics show less corrosion than metals but will degrade in vivo.
Corrosion of Metallic Implants
Definition:
Unwanted metal reaction with the environment raising biocompatibility concerns.
Tissue Fluid Composition:
Contains water, oxygen, proteins, ions, creating an aggressive environment.
Electrochemical Properties of Metallic Biomaterials
Gold: Used in dental restorations.
Titanium: Superior corrosion resistance but less rigid than steel.
Cobalt-chromium Alloys: Passive in the body, not prone to pitting corrosion.
Stainless Steel: Only the most resistant types (316, 316L, 317) are suitable for implants.
Electrochemical Cell Corrosion
Components of an Electrochemical Cell
Anode: Where oxidation occurs and electrons are lost.
Cathode: Where reduction occurs and electrons are gained.
Electrolyte: A conductive liquid that connects anode and cathode.
Electrical Connection: Enables electron flow.
Half-Cell Reactions
Oxidation Reaction: Metal loses electrons.
Reduction Reaction: Electrons transfer to another species (e.g., hydrogen ions).
Standard Electromotive Force (EMF) Series
Rankings of metals based on their corrosion tendency.
Comparison to Standard Hydrogen Electrode (SHE) to visualize corrosion potential.
Electrode Potentials
Driving Force: Electric potential difference driving corrosion reaction.
Galvanic Cell Example: Copper and zinc connected electrically produce a measurable potential (+1.1 V).
Nernst Equation
Addressing the impact of concentration and temperature on cell potential.
Used to calculate electrochemical cell potential under varied conditions.
Galvanic Series
Relative stabilities of metals/alloys in seawater.
Stability in physiological fluids considered for implants.
Pourbaix Diagram
Represents corrosion activity based on pH and potential.
Regions: Corrosion, immunity, and passivation.
Contribution of pH and Ion Concentration
Variations in body pH affect performance of metallic implants.
Additional factors influencing corrosion include:
Metal processing/handling techniques.
Mechanical loading characteristics.
Presence of biological elements.
Contribution of Mechanical Environment
Location Impact: Affects time to corrosion occurrence.
Stress locations lead to higher corrosion rates due to microcrack formation.
Galvanic Corrosion
Corrosion between noble and less noble metals in aggressive environments.
Rate dependent on surface area ratios of anode/cathode.
Crevice Corrosion
Causes: Occurs in tight spaces with stagnation, leading to ion concentration differences.
Accelerated By: Sodium chloride solutions.
Pitting Corrosion
Highly localized corrosion leading to critical vulnerabilities.
Initiated by surface damage, creates pits quickly that propagate.
Fretting Corrosion
Type: Occurs at touching metal components under vibration.
Effects: Deterioration of passivation layer, accelerated corrosion in contact areas.
Intergranular Corrosion (IGC)
Specific to grain boundaries, exacerbated by temperature and improper heating.
Solutions include proper heat treatment to mitigate attacks.
Stress Corrosion Cracking (SCC)
Caused by tensile stress and corrosive effects, leading to rapid crack formation.
Corrosion Fatigue
Occurs when corrosion and cyclic loading interact, accelerating failure risks.
Contribution of Biological Environment
Effects: Localized changes in biochemistry alter corrosion dynamics post-implantation.
Protein Attachment and Corrosion
Proteins can modify passive layers, impacting corrosion outcomes.
Corrosion Control Methods
Enhancements: Surface treatments like passivation and anodization improve resistance.
Proper material selection and design can mitigate corrosion risks.
Application-Specific Corrosion Issues
Cardiovascular Implants
Unique biocompatibility required to avoid rejection. Common materials include:
Stents and composites from stainless steel, cobalt-chrome alloys.
Dental Implants
Highly variable environment with significant corrosion incidentally due to pH and food interactions.
Orthopedic Implants
Temporary and permanent implants face corrosive wear, particularly crevice types due to wear and movement.
Degradation of Ceramics and Polymers
Differences in ionic stability lead ceramics to degrade mainly via dissolution; polymers can degrade through various mechanisms including hydrolysis and enzymatic actions.