Polymer Composites 3 Flashcards
Define a degradable polymer?
Designed to degrade in the body after performing their functions
What chemical process does degradation refer to?
Chemical process resulting in cleavage of covalent bonds, either by hydrolysis or by enzymatic action into non-toxic products
What are the advantages of biodegradable devices?
- Useful in short term applications, they do not need to be surgically removed
- No issues w long term safety of permanent implanted devices
- Do not elicit chronic foreign body reactions due to gradual degradation
What are examples of poly-alpha-hydroxyacid esters?
Polyglycolic acid, polylactic acid, and their copolymers
Define poly alpha-hydroxyacid ester
Degrading polymers where the repeating unit is based on (-O-CHR-CO-)n derived from monomers that are alpha hydroxy acids (HO-CHR-COOH) [R = H or CH3]
Which polyesters can be hydrolysed?
Polyesters containing the group -CO-O- in the polymer backbone can be hydrolyzed causing chain scission
What are the important performance criteria of degradable polymers?
- Mechanical properties
- Biological performance
- Diffusion characteristics
What is an important mechanical property of degradable polymers?
Rate at which they are lost due to degradation after implantation
What is an important criteria of biological performance for degradable polymers?
Rate of absorption and tissue reaction profile
When are diffusion characteristics important for degradable polymers?
In controlled delivery devices
What are the medical applications of degradable polymers?
- Sutures
- Orthopedic fixation devices
- Drug delivery devices
- Scaffolds for Tissue Engineering/Regeneration
What are the observations for sutures?
Earliest successful application of synthetic, degradable polymers in human medicine
What is an important property of degradable sutures and what does it depend on?
Tensile strength is paramount and is dependent on the amount of molecular orientation imparted during processesing
Examples of degradable sutures on the market?
- Dexan (PGA suture, lose mechanical strength rapidly after 2-4 weeks)
- Vicryl (random copolymer 90:10 PGA PLA)
How would you alter the degradation rate of a random copolymer composed of PLA and PGA?
- High PGA: Semi-crystalline, medium degradation (2-4 months)
- 50:50: Amorphous, fastest degradation
- High PLA: Semi crystalline, slowest degradation (2 years)
[LOOK AT GRAPH]
What are the observations about orthopedic fixation devices?
Requires polymers of exceptionally high mechanical properties, often polymers form the matrix of composites - filled with bio(active) ceramic particles
Analyze the tensile strength and elastic modulus of cancellous bone, HAPEX, AW/PE, BG/PE, BG/PS, cortical bone, 4585 Bioglass, HA, Titanium, Stainless Steel, Co-Cr, and Alumina?
[LOOK AT GRAPH]
- Polymers and ceramics seem to have a much wider range of tensile strengths and elastic moduli, more similar to bone
What are the observations noted for drug delivery devices?
One of the most widely investigated medical applications for degradable polymers, commonly used for delivery of chemotherapeutics in cancer and contraceptives.
Describe the systemic vs local rebase of therapeutic agents?
[LOOK AT GRAPH: Dose of Drug vs. Time]
Systemic Delivery: sinusoidal (high toxic levels, low non-effective levels)
Local Release: logarithmic (optimum target dosage, in between upper and lower limits)
Compare the degradation times of PGA, PLA, and PCL
Degradation times:
PGA (months) < PLA (months - few years) «_space;PCL (2-5 years)
What are the observations noted for scaffolds in TE?
Attempts to recreate or improve native tissue function using degradable scaffolds. Cells may be seeded before implantation. Bioactive materials, as well as growth factors and peptides are sometimes added to modulate cell response.
How does a solid polymeric implant chemically change while degrading?
Polymers that are produced by condensation polymerization are prone to hydrolysis, which is a reaction with water to produce -OH bonds
(polymers are hydrolyzed)
How does a solid polymeric implant physically change while degrading?
- Swelling
- Deformation
- Structural Disintegration
- Weight loss
- Eventual loss of function
Describe the PLA hydrolysis mechanism
Stage 1: H20 attack, random chain scission at labile groups
Stage 2: Further chain scission
Stage 3: Lactic acid + lower molecular weight fragments
