6 - GP material selection

What is the Dk of a lens?

permeability of O₂

What is the D of Dk?

ability of the material to allow gas through

What is the k of Dk?

degree to which oxygen is solubilized (stays within plastic)

What is Dk/t (aka Dk/L)?

oxygen transmissibility

What does Dk/t depend on?

thickness of the lens (L or t)

Manufactures usually report Dk/t for which power of lens?

-3.00 lens → plano shape

How does high minus or plus power affect Dk/t?

lowers Dk/t because lens is thicker

What is the minimum Dk/t for daily wear RGPs if you don't want corneal swelling?

25 Dk/t

What is the minimum Dk/t for overnight wear if you want to keep the overnight corneal swelling to normal level (4%)?

85-135 Dk/t

Why are GPs a good option for overnight wear?

less swelling d/t tear lens and allow tear turnover

What is the equivalent oxygen percentage (EOP)?

comparison of open eye oxygen uptake of cornea to oxygen uptake after CL exposure

How does Dk relate to EOP?

higher Dk → higher EOP

direct relationship

What are the 3 components of durability that are looked at in an RGP?

1. hardness

2. modulus

3. toughness

What does the hardness of a GP indicate?

scratch ability → most GPs are scratchable usually on front surface

what measures hardness of GP?

Rockwell indentation method

What is modulus? And what equation represents modulus?

bending on the eye → aka stiffness or flexible

= slope of the stress - strain curve

What is modulus (stiffness) measured in?

mega pascals (MPa)

What molecular property affects modulus?

by polymer chemistry

more cross-linking → more modulus/durability

How does polymer cross linking affect the Dk of a lens?

decreases Dk/t (O₂ transmission)

What durability quality is important in the ability of a GP to mask astigmatism (esp irreg astigmatism)?

modulus

"may bend too much and cause tear lens to wrap as well → cause unwanted astigmatism"

What does a higher modulus mean?

stiffer lens (less likely to warp)

What is modulus measured with?

stress-strain curve → modulus is the slope [stress/strain]

What is toughness of a RGP?

resistance of material to breakage

How do you increase toughness/reduce breakage of an RGP?

1. increase thickness

2. choose material with more cross linking

What is the relationship between Dk and toughness?

more Dk → less toughness

What should you do if you see a chip in a GP?

replace immediately

What GP material is flexible but weak (low toughness)?

silicone

but has high Dk

What are the 3 tests for surface wettability?

1. contact angle in air

2. captive bubble

3. Wilhelmy plate (surface tension angle)

"don't correlate well with on eye wettability"

What test results for the 3 wettability tests means good wettability?

small angle!

Why are lab tests poor indicators of surface wettability?

1. not only dependent on polymer

2. surface chemistry changes (in air, water, CL soln)

3. binding to eye, protein, lipids

4. surface treatments

different environment!

What are the 3 general forces acting on a GP lens?

1. surface tension

2. lid interactions

3. gravitational

What does surface tension do?

suctions the lens to the eye

What is surface tension?

interaction of water molecules on lens and cornea

what does surface tension cause to form on the edge of the lens?

form a tear meniscus

How does the upper lid force affect a GP?

lid attached fit

1. can keep a CP lens up

2. moves lens in vertical direction during blink

How does the lower lid force affect a GP?

can push up on bottom of a GP lens

What is the importance of the center of mass or gravity (CoM/CoG) of a lens?

influences the position on eye and stability of fit → balancing point

What is the ideal CoG/CoM for a GP?

CoM further behind the lens helps lens position better

What happens if the CoM/CoG of a lens is closer to the back surface of the lens?

make the lens drop

What factors lead to a CoM further back from back surface of lens (ideal position)?

1. larger lens (largest effect) - refers to larger diameter

2. more minus power

3. thinner → lenticulars

Which lens is more likely to drop, a plus or minus lens?

a plus lens

What factor of CoM is the most important to location?

lens diameter → 4-7x more important than thickness or power (size of lenses)

How does BCR affect the CoM?

steeper BCR → further back CoM

What is the equation of lens mass?

= volume × specific gravity

When altering lens mass, what is the first thing you try and change?

specific gravity

What is specific gravity?

ratio of the mass of a solid to an equal volume of water

How should you adjust specific gravity if a lens is dropping too low on the cornea?

order a lower specific gravity to decrease lens mass

How should you adjust specific gravity of a lens is riding too high on the cornea?

order a higher specific gravity to increase lens mass

How does light transmission in CL compare to spectacles?

light transmission is higher in CL than specs

What is the index of refraction of different GP materials?

they are all very similar and don't really make a difference (all around 1.45)

What are polymers?

repeating monomers joined together to form long chains

What are the characteristics of polymers?

1. long chains with high molecular weight

2. linked by chemical covalent bonds

What are the types of polymers?

1. Homopolymer

2. Copolymer

What is a homopolymer?

polymer with a chain of the same monomer

What is an example of a homopoymer?

Polymethyl methacrylate (PMMA)

What is a copolymer?

polymer made up of 2 or more monomers

what polymer is in all modern materials?

copolymers

What is the monomer backbone for most GPs?

methyl methacrylate (MMA)

What is polymer cross-linking?

long main polymer chains cross linked to other polymer chains

what effect does more cross-linking have on durability and Dk?

↑ durability

↓ Dk

What are the advantages to PMMA?

1. readily machined

2. low cost

3. fairly wettable

4. easy to care for

What is the main disadvantage to PMMA?

zero O₂ transmissibility

What is the purpose of silicone acrylate in GPs?

added to PMMA to increase O₂ transmissibility

What is the Dk of silicone acrylate?

low-medium (12-60)

what are the molecular characteristics of silicone acrylate?

1. surface negatively charged

2. hydrophobic (lipophilic)

What are the advantages to silicone acrylate?

1. higher Dk than PMMA alone

2. reduced rigidity (stiffness)

What are the disadvantages to silicon acrylates?

1. more lipid deposit prone

2. surface easily scratched

3. higher breakage rate

4. flexure problems → too flexible

5. parameter instability → warps over time

what effect (and by how much) does adding fluorine monomer to silicon acrylate have?

1. lowers surface charge

2. increases Dk → 40-100+ (med-high)

used in paragon lenses

What are the advantages to fluorosilicone acrylates?

1. more wettable

2. less lipid deposits

3. higher Dk

What are the disadvantages to fluorosilicone acrylate?

1. easily scratchable

2. too much lens flexure

Menicon Z material is made up of what?

fluorosilcione acrylates + silystyrene

What is the advantage of Menicon Z?

very high Dk (163); Dk/t ~ 125

approved for up to 30 nights wear