Polymer Biomaterials

1
Polymer Biomaterials

• There are a large number of uses for polymers in
  the biomaterials field. These arise due to the
  wide variety of properties possible.
• OBJECTIVES
• to introduce some fundamental polymer properties
  and the factors that influence them
• to provide an overview of the uses of polymers as
  biomaterials

2
POLYMERS

• Polymers - long chain molecules of high molecular
  weight
• -(CH2)n-

3
Definitions

• monomer
• oligomer
• backbone
• thermoplastic
• thermoset
• elastomer
• gel

4
Polymers In Specific Applications
5
Molecular weight

• synthetic polymers possess a molecular weight
  distribution

Ni
Mi
6
The Bulk State Solid

• Polymers can be either amorphous or
  semi-crystalline, or can exist in a glassy state.
• amorphous glassy state
• hard, brittle
• no melting point
• semi-crystalline glassy state
• hard, brittle
• crystal formation when cooled
• exhibit a melting point

7
Effect of Temperature on Polymer Properties

• amorphous

viscous liquid
rubbery
Tg
T
glassy
Mw
8
Effect of Temperature

• semi-crystalline

Rubber
Liquid
Viscous Liquid
Tm
tough plastic
T
Tg
semi-crystalline plastic
crystalline solid
10
1000
100000
1000000
molecular weight (g/mol)
9
Crosslinked Networks

• crosslinks
• covalent H-bonding entanglements
• crosslinking
• increased molecular weight
• swell in solvents
• organogel
• hydrogel

10
Temperature Effects
Tg
Tm
semicrystalline
log(Modulus)
crosslinked
Tf
linear amorphous
Temperature
11
Viscoelasticity

• The response of polymeric materials to an imposed
  stress may under certain conditions resemble the
  behavior of a solid or a liquid.

Stress
Strain
12
Mechanical Properties
13
Thermal Properties
14
Diffusion in Polymers

• Polymers can also act as solvents for low
  molecular weight compounds. The diffusion of
  small molecular weight components in polymers is
  important in a number of biomedical applications
• purification of gases by membrane separation
• dialysis
• prevention of moisture loss in drugs (packaging)
• controlled drug delivery (transdermal patches,
  Ocusert)
• polymer degradation in vivo

15
Diffusion in Polymers

• Flux is dependent on
• solubility of component in polymer
• diffusivity of component in polymer
• These in turn depend on
• nature of polymer
• temperature
• nature of component
• interaction of component with polymer

16
Solubility Estimation

• From Hildebrand, the interaction parameter, c, is
  defined as
• The solubility parameter, d, reflects the
  cohesive energy density of a material, or the
  energy of vapourization per unit volume.
• While a precise prediction of solubility requires
  an exact knowledge of the Gibbs energy of mixing,
  solubility parameters are frequently used as a
  rough estimator.
• In general, a polymer will dissolve a given
  solvent if the absolute value of the difference
  in d between the materials is less than 1
  (cal/cm3)1/2.

17
Diffusivity

• experimental observations
• effect of T vs Tg

18
Diffusivity

• effect of permeant size

19
Diffusivity Effect of Crystallinity

• solutes
• do not penetrate crystals readily
• take path of least resistance
• through amorphous regions
• increased path length

D1,c diffusivity in semi-crystalline
polymer D1,a diffusivity in amorphous
polymer fc volume fraction of crystals x
shape factor (2 for spheres) (Mathematics of
Diffusion)
20
Example of Undesirable Absorption

• poppet-style heart valve
• poppet is composed of PDMS
• in small of patients the poppet jammed or
  escaped
• recovered poppets were yellow, smelled, and had
  strut grooves

21
Leaching - Undesirable

• polymers often contain contaminants as a result
  of their synthesis/manufacturing
  procedure/equipment
• may also contain plasticizers, antioxidants and
  so on
• these contaminants are a frequent cause of a
  polymers observed incompatibility

22
Drug Delivery
Ocusert
TD - Scopolamine
23
In Vivo Degradation of Polymers

• no polymer is impervious to chemical and physical
  actions of the body

Mechanisms causing degradation
24
Hydrolytic Degradation

• hydrolysis
• the scission of chemical functional groups by
  reaction with water
• some polymers are very stable to hydrolysis
• there are a variety of hydrolyzable polymeric
  materials

25
Hydrolytic Degradation

• degradation rate dependent on
• hydrophobicity
• crystallinity
• Tg
• impurities
• initial molecular weight, polydispersity
• degree of crosslinking
• manufacturing procedure
• geometry
• site of implantation

26
Hydrolytic Degradation

• bulk erosion (homogeneous)
• uniform degradation throughout polymer
• process
• random hydrolytic cleavage (auto-catalytic)
• diffusion of oligomers and fragmentation of
  device
• surface erosion (heterogeneous)
• polymer degrades only at polymer-water interface
• degrading life-saver

27
Polyesters
fractional change in molecular weight
28
Oxidative Degradation

• usually involves the abstraction of an H to yield
  an ion or a radical
• there are 2 general categories of oxidative
  biodegradation
• direct oxidation by host and/or device
• release of superoxide anion and hydrogen peroxide
  by neutrophils and macrophages
• release of metal ions from metal components of
  device
• oxidation induced by the external environment
• photo-oxidation

29
Poly(Carbonates)
PEC in vivo
M. Acemoglu, In. J. Pharm. 277 (2004) 133-139
30
Enzymatic Degradation

• Natural polymers degrade primarily via enzyme
  action
• collagen by collagenases, lysozyme
• glycosaminoglycans by hyaluronidase, lysozyme
• There is also evidence that degradation of
  synthetic polymers is due to or enhanced by
  enzymes.
• poly(e-caprolactone) elastomers

C.G. Pitt et al., J. Control. Rel. 1(1984) 3-14