BE 220 Study Guide - Midterm Guide: Targeted Drug Delivery, Mass Spectrometry, Amphiphile
BE 220 Exam 3 Material
Biological environment
- Body relatively mild- neutral pH, constant temperature
- Many ions- metals generally more susceptible that ceramics
METALS
Corrosion = degradation of metals, electrochemical process
- Leaching of ions from metallic surface into surroundings
- Redox reaction- reduction at cathode (consume electron, deposition of metal ions), oxidation at
anode (lose electrons, metal dissolves into ion, anodic dissolution)
- EMF table- ranks favorability to disintegration (reduction potential)
o Dependent on temp/concentration- described by Nernst Equation
- Galvanic corrosion- metals electrically coupled when placed in body, physiological fluid becomes the
salt bridge
Pourbaix Diagram and passivation- metals at particular pH and potential
- Other factors that affect corrosion
o Anything that disrupts passivating film: pitting, flaws/cracks introduced during fabrication
o Anything that causes presence of different microstructures- changes localized ion
concentrations
▪ Crevice corrosion: depletion of oxygen within crevice forces anodic reactions to
deepen the crevice, remainder of metal acts as cathode → increase in pH
o Intergranular corrosion- grain boundaries at heightened energy state, represent more active
regions of material (anodic)
o Fatigues corrosion- continued loading and unloading may disrupt the passivating film and
expose underlying surface → more corrosion
- Three main regions
o Corrosion: energetically favorable, region in which
greater than 10-6M of the etal’s ios ae foud i
solution at equilibrium
o Immune: corrosion not energetically favorable, cathodic
protection (metal cannot act as anode)
o Passivation: corrosion energetically favorable, but largely
impeded by formation of a stable solid film (usually oxide
or hydroxide) coating the metal surface through surface
oxidation
- Dashed lines represent stability of water- corrosion occurs in
environments between dashed lines
o Outside dashed lines- water breaks down, not metal
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o Biological environment factors- anything that changes passivation layer or electrochemical
potential
▪ Recruitment of inflammatory cells: cause drastic changes in chemistry surrounding
implant (drop in pH, release of strong oxidizing agents), can promote corrosion or
growth of passive layer- depends on system of interest
▪ Attachment of proteins:
• Creates barrier that reduces oxygen diffusion to surface→ reduced stability
of oxide layer formed on implant, shifts cell parameters so they fall outside
passive region on Pourbaix
• Certain proteins require metal ions to function- often consume a product of
corrosion reaction, can favor dissolution by shifting equilibrium
▪ Bacteria: by products of bacterial metabolism can change regional pH to affect
stability of the passive layer, consume hydrogen (often found at cathode) to change
the reaction equilibrium and encourage anodic dissolution
Corrosion control
- Alloy metals to form passive oxide coatings
- Cooling rate to affect grain size- slower cooling rate = larger grains = fewer grain boundaries
- Add coatings- pre-treat to form passive layer before implantation, use ceramic/polymeric surface
coatings to provide barrier
- Galvanic corrosion- prevented by selection of combination of metals that are close together, use of
non-reactive (cathodic) metals (gold, silver, platinum)
POLYMERS
Two main mechanisms of degradation
- Swelling/dissolution
o Polymer with hydrophilic domains absorb water (small enough to fit b/w macromolecular
chains) → swelling, long chains in tact but pushed further apart (affecting VDW, reduces
secondary bonds)
o Affects mechanical properties, thermal transition
o Extreme case (few covalent bonds)- polymer dissolves
- Chain scission = chemical breakdown
o Break primary bonds → separate chain segments via hydrolysis/oxidation, decrease in mw
o Oxidation- highly reactive species (free radicals) attack/break covalent bonds
▪ Extent and rate depends
• Amount of susceptible chemical domains
• Original MW (lower MW = increased rate b/c less bonds to break)
• Crosslinking density
▪ Ex. Metal-catalyzed oxidation: polymer in contact with metal → fissures form at
contact (metal corrodes causing strong oxidizing agents to attack the polymer) →
brittle fracture of polymer
o Hydrolysis- water cleaves susceptible primary bonds
▪ Extent and rate depend on
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• Reactivity of groups in polymer backbone
• Extent of interchain bonding (crystallinity)
• Amount of water available
• Chemical properties (hydrophobicity)
Enzymes - affinity for certain chemical groups, act as catalysts, hard to predict in human body
BIODEGRADABLE MATERIALS
Bioerosion- breakdown mediated by physiological environment (general umbrella term), includes
chemical and physical dissolution and processes in which bond cleavage is not required (swelling)
- Biodegradation = chemical breakdown of material
Synthetic polymers can be modified to undergo hydrolytic or enzymatic degradation (natural polymers
only enzymatic cleavage)
- Enzymatic degradation- good for localized degradation at target tissues (targeted drug delivery)
- Hydrolytic degradation- easier to control between patients, influenced by degree of crosslinking,
chemical group reactivity
Biodegradation via hydrolysis
- Bulk degradation- rate of water into polymer greater than rate polymer degrades, polymer
dimensions remain about the same
o Disadvantage- rapid decrease in mechanical properties, unable to sustain loads
- Surface degradation- rate of water into material less than rate of hydrolysis, need hydrophobic
region to keep water out, decrease in thickness but maintains mechanical integrity
o Disadvantage- surface constantly degrading, difficult for tissue/cells/proteins to adhere to
o Better for sustained drug delivery- gradual release of the drug (bulk = burst effect, peak of
drug delivery at very beginning)
Porosity- increases surface area, more space for cleavage, additional stress raisers
CERAMICS
Breakdown of ceramic materials = degradation
- More stable than metals- ceramics have ionic character, takes much more energy to break
Ceramic bioerosion- mainly due to interactions with water (similar to polymer hydrolysis)
- Rate depends on chemical susceptibility, amount of crystallinity, amount of water available, material
surface area to volume ratio
SURFACE PROPERTIES
Protein adsoption- adhesion of molecules to surface
- Cells interact with biomaterial via proteins- adsorbate can be ions, water, proteins
“ufae = plaa defet, sufae tesio eeg γ due to ioplete odig
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Document Summary
Body relatively mild- neutral ph, constant temperature. Many ions- metals generally more susceptible that ceramics. Leaching of ions from metallic surface into surroundings. Redox reaction- reduction at cathode (consume electron, deposition of metal ions), oxidation at anode (lose electrons, metal dissolves into ion, anodic dissolution) Emf table- ranks favorability to disintegration (reduction potential: dependent on temp/concentration- described by nernst equation. Galvanic corrosion- metals electrically coupled when placed in body, physiological fluid becomes the salt bridge. Pourbaix diagram and passivation- metals at particular ph and potential. Three main regions: corrosion: energetically favorable, region in which greater than 10-6m of the (cid:373)etal"s io(cid:374)s a(cid:396)e fou(cid:374)d i(cid:374) solution at equilibrium. Immune: corrosion not energetically favorable, cathodic protection (metal cannot act as anode: passivation: corrosion energetically favorable, but largely impeded by formation of a stable solid film (usually oxide or hydroxide) coating the metal surface through surface oxidation.
