CL Polymers & Properties Flashcards
What are the properties of the ideal CL material?
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
What are the common materials used for RGP & Soft CLs?
- Rigid:
o PMMA
o RGP - Soft:
o pHEMA
o Other hydrogels
o Silicone hydrogels
Describe Polymethyl Methacrylate (PMMA) material?
- 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
Describe silicone rubber material?
- 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
Describe RGP lenses?
- 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
Describe poly-HEMA material?
- 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
Describe Silicone Hydrogels (SiHy) material?
- 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
Describe 1st Generation SiHy?
- 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)
Describe oxygen, Dk & water content in conventional hydrogels?
-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
Describe oxygen, Dk & water content in silicone hydrogels?
-O2 transported in silicone segments & Dk ↑ as proportion of silicone polymer ↑ & water ↓
-Reverse to conventional hydrogels
Higher H2O content of SiHy, typically lower Dk
Describe 2nd generation SiHy?
- Dominate CL fits -> 70%
- Have ↑ water content
- ↓moduli (elasticity)
- Do not need to be surface treated
- Still have high O2 permeability
Describe soft CL manufacture: Lathe Cut?
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
Describe soft CL manufacture: Spin Cast?
o Make mould, mould spun, polymerisation is initiated by UV/heat, edge polishing, hydration, verification
Describe soft CL manufacture: Cast Moulding?
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
Describe oxygen permeability (Dk)?
- 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
