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

1. Anode: Where oxidation occurs and electrons are lost.

2. Cathode: Where reduction occurs and electrons are gained.

3. Electrolyte: A conductive liquid that connects anode and cathode.

4. 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.