Sutures Flashcards
What are thermoplastics
Common polymers
Long thin string like molecules
Molecules are entangled together
Found in plastic bags, hip implants
What is the difference between solubility and degradation
Solubility - molecules separate and are dispersed. Longer molecules take longer to dissolve as they are more entangled
This is NOT degradation
Degradation- digestion by enzymes or hydrolysis
What are some common degradable polymers
Biodegradable polymers- Designed to break down within body after performing their function Degradation products should be non-toxic Materials can be hydrolysed by enzymes Eg. Polyglycolides, polylactides (Polyesters) Degradation products are lactic acid and glycolic acid
What is the degradation mechanism of polymers
Water diffuses into polymer Water attacks amorphous region Mw falls without loss of mechanical properties Chains start to be forced apart Oligomers created Carboxylic acid groups created pH locally decreases Critical Mw reached and oligomers start to leave Space created Water molecules diffuse Removal of more oligomers
What affects the rate of degradation
Rate of hydrolysis dependent on water absorption
Limited by diffusion through polymer
Related to Tg and Mw, hydrophilicity and degree of crystallisation
What does Tg measure
Reversible transition from brittle state to viscous or rubbery state as temperature increases
What is crystallinity
Influences strength, degradation
Crystalline regions do not take up much water
Outline the key features of polyglycolides PGA
R group = H High molecular weight Hard and tough crystalline Tg= 37 C same as body temp Water gets in and it becomes more amorphous = hydrolysis
Outline the key features of ploylactides PLLA
R group = CH3 More hydrophobic interactions Lower water uptake, lower hydrolysis rates Tg= 60C Semicrystalline Has a low degradation rate
Whats a problem with polyesters?
Hydrolysis is pH dependent
Hydrolysed faster under acidic conditions
Acid is produced when ester is hydrolysed
pH lowers
Accelerates degradation process
Whoosh effect - areas where pH has built up there will be rapid degradation
How can degradation rate of PLLA be made faster
Use enantiomer of molecule PDLLA More amorphous so water can enter Unable to form crystalline arrangement Lower tensile strength Tg= 55 C
Give examples if copolymers
PGA PLLA copolymers
90:10 eg. Surgical mesh
Monocryl suture Soft and hard segments 75% glycolide Low Mw and pliable 100% strength lost in 28 days
What other factors do polymers have to withstand?
Sterilisation:
Cannot heat because will degrade
Use gamma radiation or electron beam radiation
Some polymers sensitive to radiation so use ethylene oxide
Packaging:
Moisture in storage can degrade polymers
Normally double bagged in vacuum with dessicant
Stored in freezer
Shape:
Degradation of thick sections can occur faster than thin due to build up of low pH
