Ch 5 - Biomaterial Degradation

Question 1

Metallic degradation

Answer 1

• “Corrosion” = leaching of ions from metallic surface into surroundings
* Metals more suscep. (in vivo) than ceramics

Question 2

Redox reactions

Answer 2

• OIL (of e-) → dissolution at \anode

* RIG (of e-) → deposition at \cathode

Question 3

Nernst equation

Answer 3

ΔE=( E_2 - E_1 ) − (RT/nF) ∗ ln⁡( [M_1^n] / [M_2^] )

Question 4

Galvanic corrosion

Answer 4

• 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

Question 5

Pourbaix diagram

Answer 5

• 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

Question 6

Cathodic protection

Answer 6

Not energetically favorable to corrode/dissolve

Question 7

Passivation

Answer 7

• 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

Question 8

Corrosion by processing parameters

Answer 8

• ANY change in microstructure (processing, mech loading, proteins/bacteria) → change localized ion conc ∴ corrosion ↑
  • Mech stress → higher energy state, # microcracks ↑

Question 9

Crevice corrosion

Answer 9

• Depletion of O_2 in crevice (neces. for OH passive layer) → anodic rxn → pH ↓
• Frees H+ ∴ corrosion

Question 10

Pitting corrosion

Answer 10

• Flaw disrupts passivation film
• Small anode, large cathode (signif. dissolution of anode)
* Dangerous b/c may be undetected until sudden device failure

Question 11

Intergranular corrosion

Answer 11

• GB = heightened energy state
∴ more active/anodic susc.
* can lead to intergranular attack (corrosion of passivating layer)
** Mitigated by heat treating

Question 12

Mech corrosion: stress corrosion cracking

Answer 12

• 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

Question 13

Fatigue corrosion

Answer 13

• 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 ↓)

Question 14

Fretting corrosion

Answer 14

Removal of passivating layer by mechanical motion near implant

Question 15

Corrosion by biological envir

Answer 15

• 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

Question 16

Ceramic degradation

Answer 16

• 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

Question 17

Polymer degradation

Answer 17

• \swelling/dissolution (breaking 2 ° chains)

* \chain scission (breaking 1 ° chains)

Question 18

Swelling/dissolution

Answer 18

• Polymers w/ hydrophilic domains swell in physiologic envir. → water penetrates hydrophilic polymers (reduces 2 ° bonds, more ductile)
• Not enough interchain bonding ∴ eventually falls apart

Question 19

Chain scission

Answer 19

• Separation in chain segments at point of bond rupture ∴ overall ↓ MW
• \hydrolysis or \oxidation

Question 20

Hydrolysis

Answer 20

• 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 ↓

Question 21

Oxidation

Answer 21

• 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.

Question 22

Metal-catalyzed oxidation

Answer 22

Caused by corrosion of metal (interior) → formation of strong oxidizing agent/free radicals → attack polymer coating (from inside) → brittle fracture

Question 23

Environmental stress cracking

Answer 23

Polymer under sufficient tensile stresses in biological envir → exterior of implant develops deep cracks ⊥ to primary loading axis

Question 24

Enzyme-catalyzed degradation

Answer 24

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

Question 25

Biodegradation

Answer 25

• 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)

Question 26

Biodegradable ceramics

Answer 26

• 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)

Question 27

Biodegradable polymers

Answer 27

• \bulk degrad: water in > rate of cleavage

* \surface degrad: water in < polymer hydrolysis

Question 28

Bulk degradation

Answer 28

• 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)

Question 29

Surface degradation

Answer 29

• 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)

Question 30

Passive layer

Answer 30

Insulating layer made of hydroxides that form spontaneously on the surface of some metals