CHAPTER
5
Biomaterial
Degradation
5.1 Introduction: Degradation in the Biological Environment
Biomaterial degradation: uncontrolled vs. controlled degradation
Mild environment within body: neutral pH, temp, ions, specific reactions
in vivo testing
5.2 Corrosion/Degradation of Metals and Ceramics
Metal degradation: corrosion --- metal durability within body / biocompatibility
5.2.1. Fundamentals of Corrosion
(1) Oxidation-reduction reactions
corrosion: electrochemical process & electron transfer
Anode: oxidation reaction
Cathode: reduction reactions
1) in acidic milieu
2) oxygen in acidic solution
3) oxygen in neutral or basic solution
Battery: electrochemical (galvanic) cell
two metal strips
salt bridge
wire connection with voltmeter
(2) Half-cell potentials
standard reduction potential (electron affinity)
with standard hydrogen electrode (H2 gas through 1M HCl with platinum)
actual reduction potential: Nernst equation
galvanic series
metals towards the bottom of Table 5-3
: easy oxidation (unstable, degradation)
(3) Nernst equation
temp & conc. of metal ions
(4) Galvanic corrosion
two different types of metals in the body connected via physiological fluid
more active metal (oxidation, anodic)
complex corrosion process
oxidation rate = reduction rate
slow overall corrosion (passivation)
5.2.2. Pourbaix Diagrams and Passivation
Pourbaix diagram: regions of corrosion and non-corrosion
as a function of cell potential and pH
1) corrosion
2) immune: cathode protection
3) passivation: surface oxidation
--- stable solid film
(oxide or hydroxide)
no rate prediction
rate = current flow / surface area
other factors involved
5.2.3. Contribution of Processing Parameters
enhanced corrosion in vivo
microstructure w/in implant ---- change in localized ion conc. ---- corrosion rate
(1) Crevice corrosion
at a narrow, deep crack
O2 depletion
anodic reaction in the crevice
O2 reduction (pH increase)
Cl- influx
insoluble hydroxide & H+ liberation
pH decrease
(2) Pitting corrosion
flaw or disrupted passivation film
small anode and large cathode
1) inequality in surface area
2) rates of the redox reactions
dissolution of anodic regions
(3) Intergranular corrosion
grain boundaries --- high energy --- more active region
[intergranular attack]
5.2.4. Contribution of the Mechanical Environment
location of implants --- corrosion rate
(1) Stress & Galvanic corrosion
bending ---- tensile side (anodic) & compressed side
[galvanic corrosion]
(2) Stress corrosion cracking
metals under both tension and corrosive environment
small cracks ---- crack propagation and brittle fracture
(3) Fatigue corrosion
continuous loading --- disruption of passivating film --- surface exposure
[corrosion]
fatigue cycle, corrosion fatigue, premature device failure
(4) Fretting corrosion
motion near the implant (not by loading)
removal of the metal’s passivating layer
[nick on the surface]
5.2.5. Contribution of the Biological Environment
biological milieu ---- corrosion rate in vivo
(1) inflammatory cells
pH and strong oxidizing agents
growth of the passive layer
(2) proteins
1) protein binding to metal surface
2) proteins as electron carriers
3) metal binding proteins [equilibrium favoring metal dissolution]
(3) bacteria
device infection
by-products
H2 consumption and anodic dissolution
5.2.6. Means of Corrosion Control
(1) devices with few stress raisers
(2) combination of metals (located close in galvanic series)
(3) non-reactive metals and metals with passive oxide coating
(4) heat-treated stainless steel
(5) nitric acid pre-treatment
(6) surface coating
5.2.7. Ceramic Degradation
passivating layer 형성
more stable in physiological environment (ionic character)
inert / resorbable / controlled surface reactivity
ceramic degradation under mechanical environment
stress induced degradation
ceramic porosity [stress raiser & surface area 증가]
5.3. Degradation of Polymers
polymer degradation in the body
rate of polymer degradation
5.3.1. Primary Means of Polymer Degradation
(1) swelling/dissolution [plasticizer]
1) ductility
2) crystallinity
3) mechanical & thermal properties (Tg)
4) complete dissolution in aqueous environment
(2) chain scission
bond rupture ---- molecular weight 감소
hydrolysis / oxidation
5.3.2. Chain Scission by Hydrolysis
Hydrolysis
1) reactivity of groups of the polymer backbone
2) extent of interchain bonding
3) amount of media (water)
Susceptibility to hydrolysis
1) a large number of cleavable groups
2) hydrophilic domains for water influx
3) low initial mol. wt. and low X-link density
4) low or no crystallinity
5) Tg < body temp
6) high surface area to volume ratio
5.3.3. Chain Scission by Oxidation
(1) Oxidation via highly reactive species
homolysis vs. heterolysis
extent of oxidative degradation
1) number of susceptible chemical domains
2) lower mol wt polymers
3) less tight X-linking
(2) Metal-catalyzed oxidation
metal corrosion ---- strong oxidizing agents ---- attack polymer coating
5.3.4. Other Means of Degradation
(1) Environmental stress cracking
(stress corrosion cracking in metals)
polymer ---- tensile stress ---- deep crack on the exterior ---- fracture
(2) Enzyme-catalyzed degradation
enzymes --- affinity for certain chemical groups in polymer ---- catalysts
hydrolytic or oxidative degradation
5.3.5. Effects of Porosity
pores --- stress raiser (mechanically induced degradation)
--- surface area (more space for cleavage)
5.4. Biodegradable Materials
controlled biomaterial degradation
temporally nature of the material (tissue engineering, drug delivery)
biodegradation / bioerosion
5.4.1. Biodegradable ceramics
calcium phosphate --- calcium hydroxyapatite & tricalcium phosphate
(hydrated calcium sulfate or bioactive glasses)
(1) Erosion mechanisms
solubility, local pH, dissolution of grain boundaries
(2) Factors that influence degradation rate
1) chemical susceptibility of the material
2) crystallinity
3) amount of media (water)
4) surface area / volume ratio
5) high mechanical stress
6) pH drop
5.4.2. Biodegradable Polymers
(1) Introduction to biodegradable polymers and definitions
Hydrolytic degradation of polymers
1) bulk degradation
water penetration rate > polymer degradation rate [cracks and fissures]
2) surface degradation
water penetration rate < polymer degradation rate
[implant thickness 감소, with mechanical integrity maintained]
no good integration with the surrounding tissue in vivo
(2) Degradation mechanism
1) breaking X-linked bonds between water-soluble polymer chains
2) cleavage of hydrophobic side chains to reveal hydrophilic groups
3) cleavage of polymer backbone
(3) Factors that influence degradation rate
enzyme degradation
1) amount of enzyme
2) number of cleavable moieties
hydrolytic degradation
1) reactivity of chem groups
2) bonding between chains
3) amount of media
4) surface area
5.5 Techniques: Assays for Extent of Degradation
in vitro and in vivo testing
signs of degradation
1) mass loss
2) different physical and chemical properties
3) visual inspection --- color and crack formation
4) material surface with light and electron microscopy
corrosion test for metallic implants in vitro
1) electrochemical testing
IV-curve 분석