Degradation of Polymers and Metals Flashcards
Treatment/Processing affects end behavior by…
- Mechanical processes like extrusion, injection molding, etc. expose material to physical stresses
- Chemical processes like gamma irradiation sterilization can alter chemistry via free radical production
- Chain oxidation/scission/cross-linking can cause loss of strength and embrittlement
Protein and cellular element adsorption
- bulk material starts adsorbing water, ions, proteins, lipids that change polymer properties
- plasticization results in dimensional/mechanical property changes
- degradation, almost always involving hydrolysis or oxidation
Hydrolytic Biodegradation (how does it occur/factors that come into play)
- breaking of hydrolytic bonds
- can depend on bond susceptibility, chemical structure, morphology, and tissue microenvironment (list ways)
- often host-induced
General structure for hydrolytic bonds
- Carbonyls bonded to heterochain elements like oxygen, nitrogen, and sulfur
- In contrast, hydrolytically stable groups include hydrocarbons, halocarbons, and sulfones
What are the main host-induced hydrolytic processes?
- Ions in bodily fluids acting as catalysts
- Lipoproteins that transport catalytic ions into the polymer bulk
- Localized changes in pH (aggressive immune system response)
- Enzymes transferred onto substrates through cell contact (phagocytosis)
Ions acting as hydrolysis catalysts
- Hydrolysis rate of polyesters is increased tenfold when exposed to phosphate ions
- Very hydrophobic polymers adsorb negligible concentrations of ions directly from bodily fluids (less susceptible)
Enzyme-induced hydrolysis
- Proteases, esterases, lipases, and glycosidases can be highly specific catalyst for scission of groups
- Synthetic polymers are more resistant than natural polymers, but still susceptible
- Poly(ether urethane), etc. have general good resistance to hydrolysis, but show increased hydrolysis when exposed to certain enzymes (resemble lipids?)
Clinical/Preclinical examples:
- PET in fiber configurations often used for cardiovascular means, but observed to degrade long-term due to possible hydrolysis because of pH changes and oxidative agents (know statistics)
- Nylon 6 and 6, 6 can be degraded by ion-catalyzed surface and bulk hydrolysis, as tend to be rather hydrophilic
- Eye example… upon side chain degradation, polymers can be left with much more hydrophilic polymers which will swell
Homolytic v. Heterolytic:
- Homolytic: chemical bond dissociation where electrons are equally shared afterwards
- Heterolytic: chemical bond dissociation whether electron pair is split unevenly between the products
Oxidative Biodegradation
- Readily oxidized groups are site of the start of this degradation
- Abstraction of atom or ion can occur due to resonance stabilization of resultant radical/ion
- Oxidation resistance is based on the reactive group frequency, crystallinity, and hydrophobicity of polymer system
- Factors that regulate water penetration
- Host mediated factors like that of the FBR (PMNs and macrophages), which release powerful oxidants
- Foreign-body giant cells can launch persistent attack for months-years
Oxidative Biodegradation: clinical examples
- Poly(ether urethanes): with low polyether content, can resist hydrolysis, but leads made of these materials display cracks in their insulation due to oxidative stress cracking
Oxidative Stress Cracking
- Size of cracks are related to amount of residual stress and the ether (soft segment) content
- two types: surface oxidation leads to shallow/brittle tiny cracks, then propagation of cracks by body fluid components which enhance depth and width
- Generates rough-walled fissures, initiated by factors secreted from cells
- Know how to control!
Metal-ion Induced Oxidation
- Near corroded metallic components, initiates on enclosed inner surfaces
- Metal ions act as strong oxidants
- Smooth and random crack orientation on walls (or crack patterns that track the metal components)
- Know how to avoid!
Metallic Corrosion
1) Metal is placed into solution, and positively charged metal ions pass from the metal into the solution (anodic/oxidation reaction)
2) Left over electrons accumulate and produce surface negative charge, and then potential difference
3) Retards dissolution of metal ions, encourages ion deposition back to the metal (tries to reach equilibrium)
4) If ions or electrons can be removed from the system alternatively (cathodic/reduction), equilibrium will not be reached and corrosion ensues
- Study diagram!
Biological Environmental Factors affecting Metallic Corrosion
- metal ions can be removed by forming complexes with proteins and being transported away
- cyclic loading causes mixing at the interface, causing further imbalances in charge and corrosion
- metal alloys of different potentials in electric contact causes galvanic corrosion
- proteins/cells can be electrically active and interact, affecting electric potential, as well as causing pH changes
- availability of oxygen determines oxide layer stability
- bacterial consumption of hydrogen… further cathodic reactions
