4. Soft Contact Lens Material Properties - Contact Lenses I Summer 2026

Repeating short units (monomers) joined to form long chains

polymers

Characteristics of Polymers

-long chains

-high molecular weight

-linked by covalent bonds (chemical, not ionic)

What is a homopolymer?

All the same monomer, linear link

What is an example of a homopolymer?

Polymethyl methacrylate - PMMA

What is a copolymer?

two or more different monomers within the polymer

All modern materials are (homopolymer/copolymer)

copolymers

More cross linking in a polymer structure will increase _______ and decrease ______

durability, Dk

PMMA (rigid) + hydroxyl group (OH) =

pHEMA (soft)

What is the function of adding -OH to PMMA?

to increase hydrophilicity

What is a xerogel?

dry state for a hydrogel material/soft contact lens

What is a hydrogel?

wet state of a hydrogel material/soft contact lens

Increase H2O imbibition = (increased/decreased) H2O

increased

Increase H2O imbibition = (increased/decreased) dissolved O2

increased

Hydrogels are _______% water

38-74%

Hydrogel Material (Pic)

Hydrogel Material (Pic)

Is there both free water and bound water in a hydrogel material?

Yes

Will bound water dehydrate in a hydrogel material?

No

Will free water dehydrate in a hydrogel material?

Yes -- leads to increased levels of dryness over time

What are the Monomers that are used in CLs chemistry?

-Methacrylic Acid (MA)

-Methyl methacrylate (MMA)

-Hydroxyethyl methacrylate (HEMA)

-Glycerol methacrylate (GMA)

-Vinyl Pyrrolidone (VP)

-Vinyl alcohol (VA)

-Phosphorylchlorine (PC)

-Siloxane

What does ionicity refer to?

electrical charge of the material

Describe an ionic material

-negative surface changes

-attracts positively charged ions and lipids

Describe a non-ionic material

-neutral surface charge

-repels charged proteins and lipids

Is a non-ionic soft CLs easy to achieve?

No -- very hard

What was the 1st soft lens material?

polyHEMA

What are the advantages of polyHEMA?

-cheap

-easily machinable

-long life

-dimensionally stable

What are the disadvantages of polyHEMA?

-low Dk

-Increased corneal edema

-Increased vascularization

-Reduced thickness

-Poor handling

What was the first SOFT lens materials on the market that can be manufactured with a lathe or molding?

Hydrogel materials

True or False:

All modern materials have monomers to prevent water evaporation

true

Which is more expensive?

Hydrogel or silicone hydrogel

silicone hydrogel

What is hydrogel material used for?

most dailies and myopia control lenses

What are the advantages to SILICONE hydrogel mateirals (SiHy)?

-improved Dk

-Reduced hypoxic complications

-Less dehydration

What are the disadvantages to SILICONE hydrogel mateirals (SiHy)?

-Not inherently wettable (needs a wetting strategy)

-More mechanical complications

-Still has deposition

A (smaller/larger) angle is always better wetability

smaller

What are the 3 tests that are used in labs to test wetability?

-Contact Angle in Air

-Captive Bubble

-Wilhelmy Plate

Surface Wetability Tests (Pic)

Surface Wetability Tests (Pic)

Why are labratatory tests poor indicators of on-eye wetability?

-Not only dependent on the type of polymer used

-Surface chemistry changes on the eye or with certain solutions

-Binding to eye, proteins, lipids

-Surface treatments (plasma) change the wetability

Strategies for Increased Silicone Wetability

-plasma treatment

-monomers within the copolymer that provide internal wetting agent

-permanent water surfaces

What is plasma?

-a state of matter consisting of electrons

-fourth state of matter

-highly excited ionized gas

How are materials treated with plasma?

treated with cold plasma in a reaction chamber

What is the purpose of plasma treatment for silicone?

-increase wetability

-permanent alteration to the surface (more in RGPs)

What is the purpose of an internal wetting agent in a silicone material?

monomer within the polymer will make a silicone more wettable

What is an example of monomer added to a polymer to make a silicone more wettable? (internal wetting agents)

Polyvinyl pyrrolidone (PVP)

Hyaluronic acid (HA)

What are examples of lenses with internal wetting agents?

Acuvue Oasys

Precision 7

Infuse

Biofinity

What is a permanent water surface on a silicone lens?

Layer of non-silicone polymer over a core material of silicone creating a higher water content at the surface of the lens

What are examples of lenses with a permanent water surface?

Dailies Total 1

Total 30

Precision 1

Alcon Water Gradient (Pic)

Alcon Water Gradient (Pic)

Silicone Core of Alcon Lenses have a _____% water content

33

On the surface of Alcon Lenses there is a hydrophilic surface gel with a brush like structure. What is the water content of this outer surface?

80+%

What is the equation for water content of a soft CLs?

-Water content = (wet weight-dry weight) / wet weight

-Water content = 100% -solid content

How to measure solid content of a CLs?

Hand held refractometer

A higher FREE WATER content = more (hydration/dehydration) of the ocular surface

dehydration

Why does a higher free water content lead to dehydration?

As the water dehydrates from the lens to the atmosphere, the lens will pull water from the tears and worsen dry eye symptoms

HIGH Water content = (increased/decreased) Dk

increased

HIGH Water content = (increased/decreased) thickness

increased

HIGH Water content = (increased/decreased) stability dimensionally

decreased

HIGH Water content = (increased/decreased) difficulty in care

increased

HIGH Water content = (increased/decreased) flexibility

increased

HIGH Water content = (increased/decreased) dehydration

increased

HIGH Water content = (increased/decreased) deposition

increased

LOW Water content = (increased/decreased) Dk

decreased

LOW Water content = (increased/decreased) thickness

decreased

LOW Water content = (increased/decreased) stability

increased

LOW Water content = (increased/decreased) ease of care

increased

LOW Water content = (increased/decreased) rigidity

increased

LOW Water content = (increased/decreased) dehydration

decreased

LOW Water content = (increased/decreased) deposition

decreased

Why are silicone hydrogel lenses their own FDA category?

Opposite relationship of water content & Dk compared to hydrogels

Low water, high Dk

What is Dk of a soft CLs?

permeability to O2

What is transmissibility of a soft CLs?

-DK/L = DK/t

-accounts for thickness of a lens

What happens to Dk with a higher plus/minus lens?

Much less O2 to the cornea d/t the thickness of the lens. Plus > Minus

Do you want patients with high Plus/minus lenses to sleep in their CLs ever?

No -- very low O2 transmissibility the way it is

Minimum Dk value to avoid corneal swelling?

25

What is the normal % of corneal swelling in a normal patient w/o overnight Cls use?

4%

To limit overnight corneal edema, what is the recommended Dk value for a CLs?

85-135

If a patient is going to sleep in their CLs, we want to put them in a lens with a (high/low) Dk

high

Dk for Overnight Wear (Pic)

Dk for Overnight Wear (Pic)

What is the equivalent oxygen percentage (EOP)?

compare open eye oxygen uptake of cornea to oxygen uptake after CLs exposure

How is EOP measured?

with an electrode

Higher Dk = (higher/lower) EOP

higher

Contact Lenses and Oxygen -- EOP (Pic)

Contact Lenses and Oxygen -- EOP (Pic)

How do we increase Dk in a soft lens?

-Increase water content

-Increase silicone content

Water Content & Dk Relationship (Pic)

Water Content & Dk Relationship (Pic)

In standard hydrogel materials, Dk was always increased by (increasing/decreasing) water content

increasing

What are the disadvantages to increasing water content of a standard hydrogel material in a soft CLs?

-dehydration on eye if free water

-hypoxia complications

-protein deposition

Dk by Brand (Pic)

Dk by Brand (Pic)

Durability is a function of what characteristics of a CL?

1) Hardness (RGPs and sclerals only)

2) Modulus

3) Toughness (RGPs and sclerals only)

What is the modulus of a soft Cls?

slope of the stress-strain curve or the stiffness of a lens

Modulus is important in the ability to mask _______

astigmatism

Does modulus mask astigmatism in a soft Cls?

No

What is modulus determined by?

polymer chemistry

More cross linking in the polymer of a soft Cls = (improved/worse) durability

Improved

More cross linking in the polymer of a soft Cls = (more/less) O2 transmission

less

More modulus = (more/less) O2 transmission

less

Higher modulus = _______ lens

stiffer

Stress/Strain Testing Curve on Soft Cls (Pic)

Stress/Strain Testing Curve on Soft Cls (Pic)

Stress/Strain Testing Curve on a Soft and Weak Lens (Pic)

Stress/Strain Testing Curve on a Soft and Weak Lens (Pic)

Stress/Strain Testing Curve on a Soft and Tough Lens (Pic)

Stress/Strain Testing Curve on a Soft and Tough Lens (Pic)

What is modulus determined by in soft Cls?

polymer chemistry and water content of the lens

More water in a soft Cls = (less/more) modulus

less

More water in a soft Cls = (less/more) stiffness

less