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Biomaterials > Lecture 8 > Flashcards

Lecture 8 Flashcards

1
Q

What is a polymer?

A

Substances made of large molecules (macromolecules) composed of many repeated subunits (monomers).

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2
Q

Characteristic features of a macromolecule

A
  • Chemical composition of the monomers
  • Size of the macromolecule (number of monomers)
  • Microstructure of the macromolecule (composition, architecture, tacticity)
  • Macromolecular functionality
  • Structure/Conformation of the macromolecule
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3
Q

Degree of polymerization (DP):

A

the number N of repeating units. Usually 10^3 - 6

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4
Q

The size of a polymer

A

Bulk polymers (esp. synthetic) are often composed of a distribution of macromolecules of different lengths.

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5
Q

Dispersity in chain size

A 
# average mol. weight, weight average mol. weight. 
Disparity index = Mw/Mn
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6
Q

Trough the motion of its segments ___, a macromolecule can adopt different shapes ___

A

(monomers), (conformations)

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7
Q

The number and nature of macromolecule conformations are governed by __

A

interactions between monomers.

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8
Q

Biological tissues are composed of polymers, ex:

A

ECM: proteins (collagen, fibronectin, laminin)

pollsaccharides and proteoglycans

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9
Q

Macromolecules of biological tissues, proteins:

A

amino acids

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10
Q

The biological, chemical, physical activity of a macromolecule depends on its

A

conformation

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11
Q

Glycoproteins

A

Protein chain + grafted oligosaccharide (glycans) chains. Ex: fibronectin

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12
Q

Proteoglycans

A

Protein chain densely grafted by long linear glucose chains (glycosaminoglycans: GAG)

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13
Q

One macromolecule in a melt or solution have an extremely ___. Its shape is incessantly changing, exploring many possible configurations.

A

large number of degrees of freedom.

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14
Q

The persistance length Lp

A

If chain length = Nb < Lp it will be stiff. If chain length = Nb&raquo_space; Lp it will be flexible
Nb = # of monomers

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15
Q

In the absence of long range interactions between monomers

A

long polymer chains (L»lP) have a “random coil” conformation.

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16
Q

How to describe a long flexible chain ?

L»lP

A

Mean end-to-end distance (difficult), so radius of gyration

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17
Q

radius of gyration

A

The root mean square distance of chain segments from

the chain center of mass.

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18
Q

Probability distribution of obeys

A

Gaussian statistics

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19
Q

Validity of the ideal chain model

A
  1. In real chains, monomers are not freely articulated.

2. Two monomers cannot be on a same position

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20
Q

For chains much longer than the persistence length,

A

the characteristic chain size in a melt or solution is much smaller than the chain length.

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21
Q

Many biological macromolecules are not ideal chains because

A

their conformation is governed by interactions

between segments far from each other

22
Q

Stiffness of the spring

A
  • increases linearily with T

- decreases with increasing chain length

23
Q

Elastomers

A

Networks of cross-linked polymer melts.

Soft solids with an elastic behavior up to very large deformations (several times their size).

24
Q

What is the effect of solvent on a cross-linked macromolecular network ?

A

Swelling: polymer chains of the network diffuse into the solvent to allow solvatation and mixing. Swelling reaches a plateau.

25
Q

Swelling of polymer gels

A
  • a decrease in mixing energy

* an increase in stored elastic energy

26
Q

At high enough temperature, a bulk polymer material behaves like

A

very viscous liquid with a peculiar elasticity

27
Q

Polymer melts: Entanglements. Above a ___

A

Above a critical chain length, macromolecules are entangled.

28
Q

Ne : Number of monomer units between entanglements

A

N < Ne is not entangled.

N > Ne is entangled.

29
Q

N < Ne

A

A chain “swims” in a viscous medium formed by the other chains. (Rouse model)

30
Q

N > Ne

A

Disentanglement by diffusing in a “tube” (Reptation model). Much slower process than Rouse regime.

31
Q

In entangled polymer melts, entanglements have a characteristic

A

lifetime te (te depends on T)

32
Q

For t

A

the nodes of a temporary network

33
Q

Characteristics of elastomers

A
  • Macromolecular chains are permanently (covalently) attached.
  • The cross-linked network behaves like a very soft elastic solid.
  • Can withstand large deformations (x100%)
34
Q

Below a certain temperature (Tg), thermal agitation does not provide enough energy for

A

macromolecular rearrangements and equilibration. Transition from a liquid to a glass like behavior.

35
Q

Crystallization and semi-crystalline polymers. Before reaching Tg and at a critical temperature Tc

A

some macromolecules can crystallize (partially)

36
Q

Crystals are ___ than amorphous regions.

A

denser

37
Q

Crystals are much _____ than amorphous regions E ~ 100GPa (along chain axis)!

A

stiffer

38
Q

Crystalline regions act as __ and ___

A

cross-links and stiffening fillers.

39
Q

Features of semi-crystalline polymers

A

• high elastic modulus (~0.1-10GPa)
• often ductile (plasticity mechanisms in the crystals).
• often opaque (refraction index fluctuations in the semi-crystalline structure).
• better resistance to solvent than amorphous polymers (thanks to higher density of crystals)
• better thermal resistance than amorphous polymers (below melting T of crystals)
- Example: nylon, polyethylene

40
Q

Polymer behavior depends strongly on ___ and ___

A

time and temperature

41
Q

Terminal regime: chains can disentangle

A

(1/w > te).

The melt behaves like a very viscous liquid.

42
Q

G”

A

viscous modulus

43
Q

G’

A

elastic modulus

44
Q

Elastic or Rubbery plateau: chains cannot disentangle

A

(1/w < te). the melt behaves like a soft elastic solid.

45
Q

In many medical applications, great interest for biomaterials with limited in vivo residence time.

A
  • Surgical aids
  • Implants
  • Drug delivery devices
  • Scaffolds for tissue engineering
  • Biomarkers and contrast agents
46
Q

How do bioabsorbed materials exit the body?

A

Exhalation through lungs, addition to body as biomass, Excretion through kidneys

47
Q

Bioresorbability

A

is the ability to be eliminated in vivo through natural pathways either because of simple filtration of degradation by-products or after their metabolization.

48
Q

Design approaches to bioresorbable implants

A
  • Using dissolution in body fluids (bioabsorbability)
  • Using chemical degradation (corrosion, hydrolysis…)
  • Using cell-mediated degradation (biodegradability)
49
Q

Methods of breaking down bioresporbable materials

A

dissolution, chemical degradation, biodegradation

50
Q

Challenges in the design of bioresorbable devices

A
  • Resorption kinetics compatible with the application
  • Mechanisms and by-products non-toxic
  • Compatible with sterilization
51
Q

Strategies to tune biodegradability

A
  • Change the quantity or surface/volume ratio
  • Add/Remove defects
  • Change the density in degradable functions
  • Change the accessibility to degradable functions
52
Q

A paradigm for tissue engineering

A

Designing artificial matrices (scaffolds) with the appropriate bioactivity and biodegradability. [Ex: Nerve regeneration in collagen tubes, Decreasing the degradation speed of the collagen scaffold
(by increasing cross-linking)]

Biomaterials (10 decks)

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