Week 2 - Contact lens materials

Question 1

What would the properties of an ideal contact lens do?

Answer 1

• Meet cornea’s oxygen requirements
• Physiological inert: Biocompatible
• Excellent in vivo wetting: Comfort
• Resist spoilation
- Maintains good optical performance
- Helps to maintain comfort
- Reduces risk of CL-associated Papillary Conjunctivitis (CLAPC)
- Reduces risk of Microbial Keratitis

Question 2

What are the different types of materials used for CLs?

Answer 2

Rigid
• PMMA
• RGP
Soft Lenses
• pHEMA
• Other hydrogels
• Silicone Hydrogels

Question 3

What are the basic polymer chemistry?

Answer 3

Polymers’ unique properties arise from the ability of certain atoms to link together to form long chains;
• Carbon (C)
Can link with four (C) atoms or
(H) Hydrogen
(0) Oxygen
(N) Nitrogen
(S) Sulphur
(CI) Chlorine
(FI) Fluorine
(Si) Silicone

Question 4

Why is silicone polymer chemistry important?

Answer 4

• Silicone has a similar ability to Carbon in bond formation but different inherent chemical properties.
• These differences are exploited in the formation of a new generation of contact lens materials.

Question 5

What happens when you combine two monomers?

Answer 5

• 2 monomers are polymerised creating a CO-POLYMER
- e.g. styrene and methyl-methacrylate monomers
Give a styrene methyl-methacrylate polymer.
• Most CL materials are Co-polymers

Question 6

Why is changing the chemical nature of a polymer significant?

Answer 6

By changing the chemical nature of the polymer chain we can change the physical properties and obtain e.g. either flexible, elastomeric behaviour such a silicone rubber, or hard glassy behaviour such as PMMA.

Question 7

What were the first rigid lenses, and how did they come about?

Answer 7

• Polymeric contact lenses made from Polymethyl Methacrylate (PMMA): PlexiGlass
- First suggested as Contact lenses as result of WW2 pilots having shattered cockpit screens embedded in eye - with good reaction

Question 8

So what is PMMA?

Answer 8

• Free radical polymerisation turns Methyl Methacrylate into PolyMethyl methacrylate
- Good optical properties, ease of manufacture and to sterilise
- Virtually impermeable to oxygen
- Now little used as a CL material

Question 9

What is silicone rubber and its value for contact lenses?

Answer 9

• Widely used in a range of applications - medical, industrial& household
• Has very high oxygen permeability
• BUT highly hydrophobic and elastic nature means they grip the cornea with every blink - causes lens binding
• Never used successfully as a contact lens

Question 10

What monomers comprise a Rigid gas permeability lens?

Answer 10

• Copolymer of PMMA and Silicone Rubber
- Tris (trimethyl-siloxy)-methacyloxy-propylsilane (TRIS)

Question 11

So what are the properties of RGP for a contact lens?

Answer 11

• Has high oxygen permeability than PMMA
• Better wetability than Silicone Rubber

Further developments have been made
- Incorporation of Fluorocarbons to further increase 02 transmission
- Use of hydrophilic monomers such as methacrylic acid to improve wetability

Question 12

How are RGP Lenses produced?

Answer 12

Polymer buttons are cut using a computer controlled lathe then polished

Question 13

Are RGP’s still being fitted today?

Answer 13

• Although initially successful, their new fitting has been declining rapidly.

Question 14

What are the 10 reasons to the demise of RGP lenses?

Answer 14

1. Initial discomfort comfort compared to soft lenses
2. Intractable RGP - induced corneal & lid pathology
3. Soft lens advertising
4. Superior soft lens fitting logistics
5. Lack of RGP clinical training opportunities
6. RGP problem solver function redundant
7. Improved soft toric and bifocal/varifocal lenses
8. Limited uptake of orthokeratology
9. Lack of investment in RGP lens technology
10. Emergence of aberration control soft lenses

Question 15

What are the soft contact lens materials?

Answer 15

• poly-HEMA
• other hydrogels
• silicone hydrogels

Question 16

Who, how and what are pHEMA lenses?

Answer 16

• Czechoslovakia in the 1950’s and 1960’s
• Commercialised USA in the 1970’s
• Polymer made of HEMA
• When fully hydrated has 40% water
• Water is bound by the OH group
• Poor O, transmission (transmission is via the HO)
• Good wetability and biocompatibility
• Good comfort - gel like mechanical properties

Question 17

What is the chemistry of pHEMA lenses?

Answer 17

• Washing line polymers
- Greater physical stability is achieved by fastening the “washing-lines” together at intervals by the use of cross-links.

Question 18

Why is EGDMA added to pHEMA?

Answer 18

• HEMA is cross-linked with EGDMA to improve mechanical rigidity

Question 19

who is Otto Wichterle?

Answer 19

• Made the first pHEMA but found that could not make pHEMA lenses by cast moulding
• Developed a spin-casting technique. Used a childrens’ mechanical construction kit!

Question 20

Why is pHEMA being a washing like polymer significant for polymers?

Answer 20

• Hydrogels have a long “washing-line” backbone, where monomers can be added
• Added Functional groups attract and bind water, and can also change other properties of pHEMMA
• Example, other monomer

Question 21

What different types of polymer can be added to hydrogel lenses?

Answer 21

• NVP
• MMA
• MAA
• GMA
• DMA

Question 22

are pHEMA lenses useable for contact lenses?

Answer 22

• Wearing pHEMA lenses causes corneal hypoxia also suffers from spoilation (binding tear components)
Two possible solutions to oxygen transmission
• Develop very thin lenses
• Increase water content and therefore 02 transmission

Question 23

What are the added affects on pHEMA when NVP, MMA/VP, MAA/HEMA and GMA are added?

Answer 23

• Co polymers with NVP increased H2O but more rubbery feel reduces comfort
• MMA/VP copolymers have H2O content of 60-85%. MMA gives improved mechanical strength but MMA is very hydrophobic
• MAA/HEMA co polymers have improved H0 content of 50-60% compared to pHEMA
• GMA is also more hydrophilic than HEMA, so H20 content of 70% possible

Question 24

What work was done on silicone hydrogels?

Answer 24

Little success until late 1990’s
• Much work carried out to incorporate the high O2 transport of silicone into Hydrogels
• However TRIS will not mix with hydydrogel monomer due to hydrophobicity

Question 25

What were the two successful approaches to silicone hydrogels that were discovered?

Answer 25

1. Alter TRIS by adding a polar group this allows it to mix with hydrogel monomers 2. Utilise macromers with desired properties on the polymer. Then use these macomers with the hydrogel monomers give a biphasic polymer with each contributing their desired property

Question 26

What did these techniques give rise too?

Answer 26

• First Generation SiHy • Have high O2 permeability • Have poor wetability- need to be surface treated using gas plasma to make wetable • Have greater elastic moduli than pHEMA - Easy to handle but causes discomfort and other problems • First generation were Pure vision lens, and Air Optrix night and day

Question 27

How does conventional hydrogels oxygen transportation work?

Answer 27

• Oxygen is transported in aqueous phase Dk increases as H2O increases - O2 has little affinity for carbon backbone • Silicone Hydrogel, o2 is transported in silicone segments, and Dk increases as proportion of silicone polymer increases, and water decreases

Question 28

Whats the difference between Dk values and water content between conventional hydrogels, and silicone hydrogels?

Answer 28

• Conventional hydrogels start with low Dk values, and go up in Dk value with increasing water content • Silicone hydrogels start with high Dk value, and go down in Dk with increasing water content, until 60% whereby they go back up again

Question 29

How are second generation SiHy lenses different from first generation?

Answer 29

• Have increased water content • Reduced moduli • Do not need to be surface treated • Still have high O2 permeability

Question 30

What is the data on order of contact lens and refits 2014?

Answer 30

• 68%: Silicone hydrogel • 24%: Hydrogel • 6%: Gas Permeable • 1%: PMMA •1%: Hybrid

Question 31

What are the different types of soft contact lens manufacture techniques?

Answer 31

• Lathe cut • Spin cast • Cast moulding

Question 32

What are the steps of Lathe cutting a soft lens? (12)

Answer 32

1. Polymer button + Polymer rod 2. Back surface is lathed 3. Button transferred to front surface lathe 4. Front surface is lathed 5. Anhydrous lens edge polished 6. Anhydrous lens is inspected 7. Lens hydrated in saline (and swelling) 8. Hydrated lens is inspected 9. Lens is inserted into glass vial - containing saline 10. Glass vial containing lens is sealed and labelled 11. Glass vial and lens are autoclaved 12. Vials containing lenses are despatched individually

Question 33

What are the steps of Spin casting a soft lens?

Answer 33

1. Make tool 2. Female mould 3. Monomers introduced into spinning mould 4. Ultraviolet radiation is applied, 5. Anhydrous lens is removed from mould and the edge may be polished 6. Anhydrous lens is inspected 7. Lens hydrated in saline (and swelling) 8. Hydrated lens is inspected 9. Lens is inserted into blister pack containing saline 10. Blister pack containing lens is sealed and labelled 11. Blister pack and lens are autoclaved 12. Blister pack containing lenses are boxed and despatched

Question 34

What are the steps of Cast moulding a soft lens?

Answer 34

1. Male and female tool 2. Male and female mould applied 3. Monomers introduced into female mould 4. Male and female mould compress over monomer 5. Excess monomer squeezed out 6. UV applied to dry 7. Anhydrous lens is removed from mould 8. Steps 6-12 from spin casting are performed

Question 35

What are the different properties of contact lenses?

Answer 35

• O2 Transmission • Wetability • Flexibility/rigidity • Refractive index

Question 36

What are Oxygen permeability and Dk values?

Answer 36

DK: where D is the diffusion of oxygen through the polymer • how fast oxygen can meander through the polymer k is the solubility of oxygen in the polymer • how much oxygen that the material can dissolve - Dk is the product of the two - a measure of permeability - DK/t is the Dk per unit thickness of the lens - Dk values very dependent on how measured - makes comparison difficult (GOU

Question 37

what is Wettability?

Answer 37

• The ability of a drop of liquid to adhere to a solid surface. Superior wettability enhances the spread of liquid over a surface. • Contact angle is a measure of the hydrophilicity of a surface

Question 38

Why is angle important for wetability?

Answer 38

• The lower the angle, the more wettable the surface • Silicone rubber very high angle - poor wettability • Hydrogels, low angle - good wettability • 1st generation Silly-surfaces need treatment to give good wettability

Question 39

What are the different flexibility and rigidity of the different types of lenses?

Answer 39

• PMMA - hard glassy material • Silicone rubbers - too elastic • RGP - limited flexure • Hydrogels - gel like properties - low modulus • SiHy- 1st generation more rigid than other hydrogels General rule on hydrogels increased water content reduces physical strength

Question 40

How is refractive index affected in hydrogels?

Answer 40

• In hydrogels RI decreases with increasing water content - Example: 1.46-1.48 at 20% water content - Example: 1.37-1.38 at 75% water content Therefore approx water content can be estimated by measuring RI