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Biomaterials > Polymer Composites 3 > Flashcards

Polymer Composites 3 Flashcards

1
Q

Define a degradable polymer?

A

Designed to degrade in the body after performing their functions

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2
Q

What chemical process does degradation refer to?

A

Chemical process resulting in cleavage of covalent bonds, either by hydrolysis or by enzymatic action into non-toxic products

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3
Q

What are the advantages of biodegradable devices?

A
  • 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
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4
Q

What are examples of poly-alpha-hydroxyacid esters?

A

Polyglycolic acid, polylactic acid, and their copolymers

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5
Q

Define poly alpha-hydroxyacid ester

A

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]

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6
Q

Which polyesters can be hydrolysed?

A

Polyesters containing the group -CO-O- in the polymer backbone can be hydrolyzed causing chain scission

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7
Q

What are the important performance criteria of degradable polymers?

A
  • Mechanical properties
  • Biological performance
  • Diffusion characteristics
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8
Q

What is an important mechanical property of degradable polymers?

A

Rate at which they are lost due to degradation after implantation

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9
Q

What is an important criteria of biological performance for degradable polymers?

A

Rate of absorption and tissue reaction profile

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10
Q

When are diffusion characteristics important for degradable polymers?

A

In controlled delivery devices

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11
Q

What are the medical applications of degradable polymers?

A
  • Sutures
  • Orthopedic fixation devices
  • Drug delivery devices
  • Scaffolds for Tissue Engineering/Regeneration
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12
Q

What are the observations for sutures?

A

Earliest successful application of synthetic, degradable polymers in human medicine

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13
Q

What is an important property of degradable sutures and what does it depend on?

A

Tensile strength is paramount and is dependent on the amount of molecular orientation imparted during processesing

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14
Q

Examples of degradable sutures on the market?

A
  • Dexan (PGA suture, lose mechanical strength rapidly after 2-4 weeks)
  • Vicryl (random copolymer 90:10 PGA PLA)
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15
Q

How would you alter the degradation rate of a random copolymer composed of PLA and PGA?

A
  • 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]

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16
Q

What are the observations about orthopedic fixation devices?

A

Requires polymers of exceptionally high mechanical properties, often polymers form the matrix of composites - filled with bio(active) ceramic particles

17
Q

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?

A

[LOOK AT GRAPH]

- Polymers and ceramics seem to have a much wider range of tensile strengths and elastic moduli, more similar to bone

18
Q

What are the observations noted for drug delivery devices?

A

One of the most widely investigated medical applications for degradable polymers, commonly used for delivery of chemotherapeutics in cancer and contraceptives.

19
Q

Describe the systemic vs local rebase of therapeutic agents?

A

[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)

20
Q

Compare the degradation times of PGA, PLA, and PCL

A

Degradation times:

PGA (months) < PLA (months - few years) &laquo_space;PCL (2-5 years)

21
Q

What are the observations noted for scaffolds in TE?

A

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.

22
Q

How does a solid polymeric implant chemically change while degrading?

A

Polymers that are produced by condensation polymerization are prone to hydrolysis, which is a reaction with water to produce -OH bonds

(polymers are hydrolyzed)

23
Q

How does a solid polymeric implant physically change while degrading?

A
  • Swelling
  • Deformation
  • Structural Disintegration
  • Weight loss
  • Eventual loss of function
24
Q

Describe the PLA hydrolysis mechanism

A

Stage 1: H20 attack, random chain scission at labile groups
Stage 2: Further chain scission
Stage 3: Lactic acid + lower molecular weight fragments

25
Q

What are the two main types of degradation?

A

1) Bulk degradation

2) Surface degradation

26
Q

Describe bulk degredation?

A
  • The rate of water uptake into device exceeds rate at which it is transformed into water soluble materials
  • The water uptake is followed by erosion
  • Cracks and crevices form, leading to rapid breakdown into smaller segments
27
Q

What are the two types of surface degradation?

A
  1. Hydrophobic

2. Enzymatic

28
Q

Describe hydrophobic surface degradation?

A
  • Hydrophobic polymers impede water intake

- Device becomes thinner throughout time while maintaining structural integrity

29
Q

Describe enzymatic surface degradation?

A
  • Enzymes are not able to get into the solid polymer
  • Results in enzyme-mediated surface erosion
  • Varies from patient to patient due to diff enzyme activities
30
Q

What are factors that affect the degradation?

A
  • Chemistry of the polymer backbone
  • Hydrophilic/hydrophobic character of repeat unit
  • Morphology of polymer
  • Temperature
  • Device fabrication process
  • Geometry of implant
31
Q

What is the effect of morphology on degradation?

A
  • As the degree of crystallization increases, the degradation rate decreases
  • Glassy polymers absorb less water than the same polymer in the rubbery state
32
Q

What is the effect of water uptake on Tg of an amorphous polymer?

A

There is a shift in Tg to a lower transition temperature due to water uptake

33
Q

In autocatalytic degradation of PGA and PLA, do thicker or thinner samples degrade faster?

A

Degradation of thicker samples may be faster than thinner ones due to local build up of low pH and formation of lactic acid

34
Q

What causes autocatalysis?

A

Chain scission can result in the rapid release of lactic acid and oligomers of lactides causing autocatalysis

35
Q

Why/How does autocatalysis occur?

A

Water and oligomeric degradation products can’t escape and carboxylic acids catalyze the reaction, which leads to a shell and burst acid release.

36
Q

What does the reduction of molar mass in autocatalysis result in?

A
  • The hydrolysis occurs randomly along the chain and decreases molar mass
  • Reduction in molar mass results in reduction in fracture toughness and strength
37
Q

What is the relationship between mechanical properties and time after implantation?

A

As time passes, mechanical properties decrease
For PDLLA and PLLA: flat, and then sudden decrease
Surface erodible polymers: Linear

Biomaterials (18 decks)

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