Week 2 - Contact lens materials Flashcards
What would the properties of an ideal contact lens do?
• 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
What are the different types of materials used for CLs?
Rigid
• PMMA
• RGP
Soft Lenses
• pHEMA
• Other hydrogels
• Silicone Hydrogels
What are the basic polymer chemistry?
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
Why is silicone polymer chemistry important?
• 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.
What happens when you combine two monomers?
• 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
Why is changing the chemical nature of a polymer significant?
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.
What were the first rigid lenses, and how did they come about?
• 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
So what is PMMA?
• 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
What is silicone rubber and its value for contact lenses?
• 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
What monomers comprise a Rigid gas permeability lens?
• Copolymer of PMMA and Silicone Rubber
- Tris (trimethyl-siloxy)-methacyloxy-propylsilane (TRIS)
So what are the properties of RGP for a contact lens?
• 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
How are RGP Lenses produced?
Polymer buttons are cut using a computer controlled lathe then polished
Are RGP’s still being fitted today?
• Although initially successful, their new fitting has been declining rapidly.
What are the 10 reasons to the demise of RGP lenses?
- Initial discomfort comfort compared to soft lenses
- Intractable RGP - induced corneal & lid pathology
- Soft lens advertising
- Superior soft lens fitting logistics
- Lack of RGP clinical training opportunities
- RGP problem solver function redundant
- Improved soft toric and bifocal/varifocal lenses
- Limited uptake of orthokeratology
- Lack of investment in RGP lens technology
- Emergence of aberration control soft lenses
What are the soft contact lens materials?
• poly-HEMA
• other hydrogels
• silicone hydrogels
Who, how and what are pHEMA lenses?
• 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
What is the chemistry of pHEMA lenses?
• Washing line polymers
- Greater physical stability is achieved by fastening the “washing-lines” together at intervals by the use of cross-links.
Why is EGDMA added to pHEMA?
• HEMA is cross-linked with EGDMA to improve mechanical rigidity
who is Otto Wichterle?
• 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!
Why is pHEMA being a washing like polymer significant for polymers?
• 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
What different types of polymer can be added to hydrogel lenses?
• NVP
• MMA
• MAA
• GMA
• DMA
are pHEMA lenses useable for contact lenses?
• 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
What are the added affects on pHEMA when NVP, MMA/VP, MAA/HEMA and GMA are added?
• 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
What work was done on silicone hydrogels?
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
What were the two successful approaches to silicone hydrogels that were discovered?
- Alter TRIS by adding a polar group this allows it to mix with hydrogel monomers
- 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
What did these techniques give rise too?
• 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
How does conventional hydrogels oxygen transportation work?
• 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
Whats the difference between Dk values and water content between conventional hydrogels, and silicone hydrogels?
• 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
How are second generation SiHy lenses different from first generation?
• Have increased water content
• Reduced moduli
• Do not need to be surface treated
• Still have high O2 permeability
What is the data on order of contact lens and refits 2014?
• 68%: Silicone hydrogel
• 24%: Hydrogel
• 6%: Gas Permeable
• 1%: PMMA
•1%: Hybrid
What are the different types of soft contact lens manufacture techniques?
• Lathe cut
• Spin cast
• Cast moulding
What are the steps of Lathe cutting a soft lens? (12)
- Polymer button + Polymer rod
- Back surface is lathed
- Button transferred to front surface lathe
- Front surface is lathed
- Anhydrous lens edge polished
- Anhydrous lens is inspected
- Lens hydrated in saline (and swelling)
- Hydrated lens is inspected
- Lens is inserted into glass vial - containing saline
- Glass vial containing lens is sealed and labelled
- Glass vial and lens are autoclaved
- Vials containing lenses are despatched individually
What are the steps of Spin casting a soft lens?
- Make tool
- Female mould
- Monomers introduced into spinning mould
- Ultraviolet radiation is applied,
- Anhydrous lens is removed from mould and the edge may be polished
- Anhydrous lens is inspected
- Lens hydrated in saline (and swelling)
- Hydrated lens is inspected
- Lens is inserted into blister pack containing saline
- Blister pack containing lens is sealed and labelled
- Blister pack and lens are autoclaved
- Blister pack containing lenses are boxed and despatched
What are the steps of Cast moulding a soft lens?
- Male and female tool
- Male and female mould applied
- Monomers introduced into female mould
- Male and female mould compress over monomer
- Excess monomer squeezed out
- UV applied to dry
- Anhydrous lens is removed from mould
- Steps 6-12 from spin casting are performed
What are the different properties of contact lenses?
• O2 Transmission
• Wetability
• Flexibility/rigidity
• Refractive index
What are Oxygen permeability and Dk values?
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
what is Wettability?
• 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
Why is angle important for wetability?
• 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
What are the different flexibility and rigidity of the different types of lenses?
• 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
How is refractive index affected in hydrogels?
• 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
