Ch 5 - Biomaterial Degradation Flashcards
Metallic degradation
• “Corrosion” = leaching of ions from metallic surface into surroundings
* Metals more suscep. (in vivo) than ceramics
Redox reactions
- OIL (of e-) → dissolution at \anode
* RIG (of e-) → deposition at \cathode
Nernst equation
ΔE=( E_2 - E_1 ) − (RT/nF) ∗ ln( [M_1^n] / [M_2^] )
Galvanic corrosion
• 2 metals electrically coupled in the body, connected by physiological fluid (\salt bridge)
• More active/anodic metal will dissolve at accelerated pace (dissolution), generates e-
* Redox rates must equal s.t. overall corrosion is rate-limited by slower one
** Mitigated by non-reactive/cathodic metals OR w/ passive oxide coatings
Pourbaix diagram
• Potential (V) v. pH
1 . \corrosion = +10^-6 M ions in sol’n (at equil.)
2. \immunity = “cathodic protection”
3. \passivation = stable solid film
* dashed lines = stability of water (WANT B/W LINES)
** CANNOT predict rate of rxns
Cathodic protection
Not energetically favorable to corrode/dissolve
Passivation
- Surface oxidation leads to formation of stable solid film that coats surface of metal
- Can slow or stop corrosion (e- transfer), even if energetically favorable
Corrosion by processing parameters
- ANY change in microstructure (processing, mech loading, proteins/bacteria) → change localized ion conc ∴ corrosion ↑
• Mech stress → higher energy state, # microcracks ↑
Crevice corrosion
- Depletion of O_2 in crevice (neces. for OH passive layer) → anodic rxn → pH ↓
- Frees H+ ∴ corrosion
Pitting corrosion
• Flaw disrupts passivation film
• Small anode, large cathode (signif. dissolution of anode)
* Dangerous b/c may be undetected until sudden device failure
Intergranular corrosion
• GB = heightened energy state
∴ more active/anodic susc.
* can lead to intergranular attack (corrosion of passivating layer)
** Mitigated by heat treating
Mech corrosion: stress corrosion cracking
• Metal under tension AND subjected to corrosive envir.
• Small cracks form ⊥ to direction of applied stress
• \crack propag, brittle fracture
* Dangerous b/c can occur at low loads and normally tolerated sol’ns
** Mitigated by design w/ min. stress raisers
Fatigue corrosion
- Con’t bending, loading or motion around implant may disrupt passivating film on surface → corrosion of local area
- ↓ max stress at failure as N cycles ↑ (fatigue life ↓)
Fretting corrosion
Removal of passivating layer by mechanical motion near implant
Corrosion by biological envir
- Inflam. cells: strong oxidizing agents, ↓ pH
- Proteins: scavenge metals (alter equil → further dissolution)
- Bacteria: infect device (affect passive layer by consuming H+ from cathode) → equil change, anodic dissolution
Ceramic degradation
• Breakdown of ceramic mat’ls
• Ceramics = passive layer (on metals) b/c more stable in physio. envir. (ionic bonds stronger)
* e.g. porosity/stress raisers → elev. energy → cracks → SA for rxn → water penetration → degradation
Polymer degradation
- \swelling/dissolution (breaking 2 ° chains)
* \chain scission (breaking 1 ° chains)
Swelling/dissolution
- Polymers w/ hydrophilic domains swell in physiologic envir. → water penetrates hydrophilic polymers (reduces 2 ° bonds, more ductile)
- Not enough interchain bonding ∴ eventually falls apart
Chain scission
- Separation in chain segments at point of bond rupture ∴ overall ↓ MW
- \hydrolysis or \oxidation
Hydrolysis
- Cleavage of crosslinks/hydrophobic side chains b/w chains via water molec’s → low MW, water-soluble products (to be cleared by body’s natural processes)
• PROMOTED by:
1 . ↑ Reactivity/# of groups in polymer backbone
2. ↓ Interchain bonding (MW, 1 ° bonds)
3. ↑ Amount media(water)/SA to penetrate polymer
• REDUCED by:
– Physical prop’s e.g. X-talinity (2 ° bonds) ↑ ∴ slows water
– Chem prop’s e.g. hydrophobicity ↑ ∴ slows water
– Water penetration ↓ ∝ degradation ↓
Oxidation
• Highly reactive species (i.e. free radicals) attack and break covalent bonds in suscep. chem. groups within polymer backbone
• Radicals can either cause crosslinking or attack polymer chain (good/bad)
• Can reduce oxidation by: heating to allow radicals to recombine or adding radical scavengers e.g. vitamin E
* \initiation (homo-/heterolysis), \propagation and \termination
* Most often due to active agents released by inflam.
Metal-catalyzed oxidation
Caused by corrosion of metal (interior) → formation of strong oxidizing agent/free radicals → attack polymer coating (from inside) → brittle fracture
Environmental stress cracking
Polymer under sufficient tensile stresses in biological envir → exterior of implant develops deep cracks ⊥ to primary loading axis
Enzyme-catalyzed degradation
Catalysts w/ affinity for polymer chemical groups → cleavage of crosslinks/hydrophobic side chains b/w chains → low MW, water-soluble products (to be cleared by body’s natural processes)
* Differs person to person (hard to predict extent)
** Rate depends on:
1 . amount of enzyme at implant site
2. # cleavable moieties
Biodegradation
- Chem breakdown of mat’l mediated by any compon. of physiological envir. e.g. water, ions, cells, proteins, bacteria
- \Synthetic: typ. by hydrolysis (b/c more consistent patient to patient, v. enzymes)
- \Natural: typ. by enzymes (allows more localized delivery)
Biodegradable ceramics
• Typ. composed of CaPO_4 (orthopedics/bone tissue)
• Dissolution/disintegration influenced by:
1 . Chem suscep. (hydrated = faster erosion)
2. ↑ % crystalinity (less suscep. b/c tight packing)
3. Amount media/water
4. SA/vol ratio (porous = ↑ SA for dissolution)
Biodegradable polymers
- \bulk degrad: water in > rate of cleavage
* \surface degrad: water in < polymer hydrolysis
Bulk degradation
- Rate of water ingress > rate at which polymer is cleaved/converted into water-soluble degrad. products e.g. sutures
- Often implant develops cracks and fissures before complete degrad.
- Disadv: rapid ↓ mech prop’s/MW → collapse of implant (limited potential app., esp. in load-bearing app)
Surface degradation
- Rate of water penetr. into mat’l < rate of polymer hydrolysis e.g. implantable birth control
- Implant ↓ thickness, but maintains mech prop’s/MW
- Must possess highly hydrolytically labile (alterable) bonds and hydrophobic moieties (groups) to prevent signif. water penetr. into interior of device
- Disadv: con’t turnover of implant surface → harder to est. good integr. w/ surrounding tissue
- Often used for constant drug release (b/c rate of degrad. determined by geometry)
Passive layer
Insulating layer made of hydroxides that form spontaneously on the surface of some metals
