43 terms

Polymer Degradation

Midterm 2
STUDY
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T or F: all polymers will degrade eventually?
True
What amount of time is considered biodegradable?
Less than two years
What properties do synthetic polymers have that natural doesnt
no immunotoxicity
reproducible
wide range of properties defined by user
main applications of synthetic polymers
sutures, tissue engineering, drug delivery
Michaelis-Menten kinetics
typically used to describe enzyme kinetics,
enzymatic degradation of biomaterials
zero order reaction
has a rate that is independent of the concentration of the reactants
- usually found when a material that is required for reaction is saturated by the reactants
what are the two most common types of degradation?
hydrolytic and enzymatic degradation
do hydrophobic or hydrolytic polymers degrade faster?
hydrolytic because it allows water in
do amorphous or semicrystalline polymers degrade faster?
amorphous
do polymers with a higher Tg or lower Tg degrade faster?
lower Tg
T or F: the types of chemical bond in the backbone of a biodegradable polymer plays major role in dictating the rate of bond cleavage
True
polyanhydrides
- typically hydrophobic which limits water penetration -> fast hydrolysis leads to surface erosion
- non toxic acid degradation products
- zero order drug release
- app: short term drug release
poly(ortho esters)
- more hydrophobic than polyanhdrides -> slower degradation rates
- degradation products > esters> acids and alcohols
- app: controlled drug release devices
poly(a-hydroxy esters)
-PLA, PGA,PLGA
-hydrolysis occurs by the ester bonds in the polymer backbone and is thus determined by water accessibility to these bonds
Does PLA or PGA degrade more slowly?
PLA because bulky hydrophobic metal groups
Why does PLGA degrade faster than homopolymers PLA or PGA?
it is amorphous compared to PLA and PGA which are semi-crystalline
Thioether-esters
-an ester bond next to a thioether group is much more susceptible to degradation than an unmodified ester
-app: drug delivery, Tissue engineering
-degradation rates controlled by altering charge of neighboring groups or length of sulfide containing linker
What are the two modes of degradation?
surface erosion and bulk degradation
surface erosion
-mainly occurs with hydrophobic polymers
-the rate of bond cleavage is faster than water diffusion into the gel
-bulk device structure remains unchanged until complete degradation
-release properties correlate with mass loss
-ZERO ORDER
bulk degradation
-bond cleavage throughout the polymer
-decrease in MW of polymer
- more water infiltration
- dramatic change in mechanical properties
- release properties depend on mass loss and diffusion
how does water affect degradation rate of a polymer?
polymer hydrophilicity,
water diffusivity,
accessibility of water to chemical bonds
hydrolytic surface erosion
-starts at surface and proceeds gradually to the interior
-release rate correlated with mass loss
- ZERO ORDER KINETICS for thin slab geometry
hydrolytic bulk degradation
-random polymer chain scission through the polymer
-auto-accelerated by increasing carboxylic acid groups
-starts off as second order but then because the concentration of water is thought of to be constant turns into a PSEUDO FIRST ORDER RATE
enzymatic degradation
- all natural polymers and some synthetic
- surface erosion or bulk degradation based on location and stability of acting enzyme
enzymatic surface erosion
hydrophobic polymers
where rate of enzymatic bond cleavage is faster than enzyme diffusion
ZERO ORDER KINETICS
enzymatic bulk degradation
hydrophilic polymers
1) enzyme infiltrates and is distributed evenly throughout the polymer bulk
2) rate of enzymatic bond cleavage is slower than enzyme infiltration
MICHAELIS-MENTEN KINETICS
enzymatic degradation of peptide substrates in hydrogels
- encapsulation of enzyme secreting cells > gel degradation occurs only in present of cells and in the vicinity of cells
-kinetics might be described by Michaelis-Menten but are several important differences
Three main mechanisms for drug delivery?
surface erosion mediated release, bulk degradation mediated release, pendant chain cleavage mediated release
surface erosion mediated release
1) polymers degrade via surface erosion
2) drug diffusion is not significant until degradation occurs
-hydrophobic polymers such as PCL and polyanhydrides are typically used as base
-release rates correlate with mass loss
bulk degradation mediated release
both drug diffusion and degradation play a role in release
pendant chain cleavage mediated release
- release of drug attached to a polymer network through a cleavable linker (S-S bond)
- affected by cleavage kinetics and drug diffusion
how is the moduli of a scaffold affected during degradation
at first increases due to shrinkage, but then decreases dramatically due to MW of polymer chains
main applications of biodegradable polymers
suture, tissue engineering, and drug delivery
rate of first order kinetics
k[A]
depends on one concentration
rate of second order kinetics
k[A]^2
depends on two concentrations
examples of synthetic polymers that degrade enzymatically
-PCL
-plymers containing oligopeptide crosslinkers
-polymers with S-S bonds
biodegradation
when degradation involves biological processes, such as bodily fluids, cellular activities and enzymatic reactions
degradation rates of a polymer are influenced by what
biological environment
- pH, cellular activity and enzymes
hydrolytically degradable polymer bonds
have ester bonds which are easily degraded
- pH, MW, copolymer composition, etc also affect degradation rate
hydrophilic degradable polymers
PEG, PVA, pHEMA, pNIPAM
hydrophobic degradable polymers
polyanhydrides
poly(ortho esters)
polyesters
What are the kinetics of biodegradation determined by?
-type of chemical bond
-hydrophobicity of the monomer
- steric hindrance of the side group
- copolymer composition
-Tg
-crystallinity
-pH of local evironment
bulk degradation in hydrogels
degradation depends on concentration of degradable units
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