CL Polymers & Properties

Question 1

What are the properties of the ideal CL material?

Answer 1

Lens would:
* Meet cornea’s O2 requirements
* Physiologically inert – so body doesn’t react to it – want it to be biocompatible (not elicit any response or any response is not detrimental)
* Excellent in vivo wetting – on eye covered by tear film
o Wettability – how easily a liquid spreads over the surface of a CL
* Resist spoilation (build-up of biological materials):
o Maintains good optical performance
o Helps maintain comfort
o ↓risk of CL-associated papillary conjunctivitis (CLAP-C)
o ↓risk of microbial keratitis

Question 2

What are the common materials used for RGP & Soft CLs?

Answer 2

• Rigid:
  o PMMA
  o RGP
• Soft:
  o pHEMA
  o Other hydrogels
  o Silicone hydrogels

Question 3

Describe Polymethyl Methacrylate (PMMA) material?

Answer 3

• Rigid lens material
• Perspex, Plexi-glass
• 1st suggested as CL material during WWII – 1st introduced in 1940s
• Fighter pilots w/ fragments in eye -> well tolerated, no reaction
• Polymerisation gives rise to a glassy material
• Good optical properties, ease of manufacture & to sterilise
• BUT virtually impermeable to O2
• Now little used as CL material

Question 4

Describe silicone rubber material?

Answer 4

• Rigid lens material
• Widely used in range of applications – medical, industrial, household
• Has v high O2 permeability
• BUT highly hydrophobic & elastic nature means they grip cornea w/ every blink – causes lens binding
• Never used successfully as CL material

Question 5

Describe RGP lenses?

Answer 5

• Rigid Gas Permeable
• Has higher O2 permeability than PMMA
• Better wettability than silicone rubber – did not have elastic modulus problem
• Further develops have been made:
  o Incorporation of fluorocarbons to further ↑O2 transmission
  o Use of hydrophilic monomers e.g. methacrylic acid to improve wettability
• RGP Lens Production:
  o Polymer buttons are cut using controlled lathe then polished
• New RGP lens fitting ↓since soft lenses - ~6%
  o Initial discomfort vs soft lenses -> induced corneal & lid pathology

Question 6

Describe poly-HEMA material?

Answer 6

• Soft lens material
• Not ideal material
• Polymer made of HEMA – hydroxyethyl methacrylate
• When fully hydrated, has 40% water
  o Water bound by OH group
• Poor O2 transmission (transmission is via the H2O which binds
• Good wettability & biocompatibility (body tolerates it well)
• Good comfort – gel like mechanical properties
• Other monomers have been used to ↑water content (up to 85%) & alter mechanical (become less rigid) & surface (become more charged) properties
• Wearing pHEMA lenses causes corneal hypoxia & suffer from spoilation (binding tear components)
• 2 possible solns to O2 transmission:
  o Develop v thin lenses
  o ↑water content & therefor O2 transmission

Question 7

Describe Silicone Hydrogels (SiHy) material?

Answer 7

• Soft lenses material
• Work done to incorporate desirable high O2 transport of silicones into hydrogels
• Problem: TRIS won’t mix w/ hydrogel monomers due to hydrophobicity, gives rise to opaque phase-separated polymers
• 2 successful approaches discovered:
  1) Alter TRIS by adding a polar group, allows it to mix w/ hydrogel monomers
  2) Utilise macromers (large monomers formed by preassembly of structural elements that confer desired properties on polymer)
   Then use these macromers w/ hydrogel monomers give a biphasic polymer (block co-polymer) w/ each contributing their desired property

Question 8

Describe 1st Generation SiHy?

Answer 8

• Have high O2 permeability
• Have poor wettability – need to be surface treated using gas plasma to make wettable
• Have greater elastic moduli than pHEMA  easy to handle but causes discomfort & other problems
• 1st to be licensed:
  o Pure Vision Lens – Bausch&Lomb
  o Air Optrix Night & Day (CIBA vision)

Question 9

Describe oxygen, Dk & water content in conventional hydrogels?

Answer 9

-O2 is transported in aqueous phase, Dk ↑ as H2O content ↑
-O2 has little affinity for the carbon backbone
Higher H2O content of conventional hydrogels, ↑Dk

Question 10

Describe oxygen, Dk & water content in silicone hydrogels?

Answer 10

-O2 transported in silicone segments & Dk ↑ as proportion of silicone polymer ↑ & water ↓
-Reverse to conventional hydrogels
Higher H2O content of SiHy, typically lower Dk

Question 11

Describe 2nd generation SiHy?

Answer 11

• Dominate CL fits -> 70%
• Have ↑ water content
• ↓moduli (elasticity)
• Do not need to be surface treated
• Still have high O2 permeability

Question 12

Describe soft CL manufacture: Lathe Cut?

Answer 12

o Make rod of dry polymer, cut it to button, mounting it on a lathe, cutting back surface, turn round & cut front surface, polish edges so smooth, hydrate lens, verify & sterilise

Question 13

Describe soft CL manufacture: Spin Cast?

Answer 13

o Make mould, mould spun, polymerisation is initiated by UV/heat, edge polishing, hydration, verification

Question 14

Describe soft CL manufacture: Cast Moulding?

Answer 14

o Produce mould for both front & rear surface (normally in 2 halves), drop of monomer is put into mould & top mould (to form back surface) is put on top, squeezed together, polarisation, edge polishing, hydration, verification
o Majority made this way – especially dailies

Question 15

Describe oxygen permeability (Dk)?

Answer 15

• D is diffusion of O2 through polymer -> how fast O2 can meander through polymer
• k is solubility of O2 in polymer -> how much O2 material can dissolve
• Dk is product of the 2 – a measure of permeability
• Dk/t is Dk per unit thickness of lens
• Dk values v dependent on how measured – makes comparison difficult

Question 16

Describe wettability?

Answer 16

• Ability of drop of liquid to adhere to a solid surface
• Superior wettability enhances spread of liquid over a surface
• Contact angle is measure of hydrophilicity of surface
  o Lower contact angle, better wettability
  o Silicone rubber – v high angle – poor wettability
  o Hydrogels – low angle – good wettability
  o 1st generation SiHy – surfaces need treatment to give good wettability – don’t need to as much for later materials

Question 17

Describe flexibility/rigidity of materials?

Answer 17

• PMMA – hard glassy material
• Silicone rubbers – too elastic
• RGP – limited flexure
• Hydrogels – gel like propreties – low modulus
• SiHy – 1st generation more rigid than other hydrogels
• General rule on hydrogels: ↑ water content, ↓physical strength

Question 18

Describe refractive index in CLs?

Answer 18

• In hydrogels RI ↓ w/ ↑ water content
• 1.46 -1.48 at 20% water content
• 1.37-1.38 at 75% water content
• Means approx. water content can estimated by measuring RI