Contact Lens Materials

Material Dk Hardness Shaping technique

Wettability Elasticity Advantages Disadvantages Examples and Other Info

Thermoplastic polymers

PMMA (Poly [Methyl Methacrylate])

0.5 Hard, TG=130° Lathe or heat and pressure moulding

Wettable for tears, not water

Rigid Optical clarity Processability Toughness Ease of sterilisation Non-bioreactive

Impermeable to O2

Rigid (poor comfort)Original corneal RGP material

CAB (Cellulose Acetate Butyrate)

4-8 Soft, scratches easily

Lathed (poor dimensional stability) or moulded for better control

Good Low modulus of elasticity – lens warpage

Less rigid, better comfort

Less brittle Better Dk

Lens warpage due to poor dimensional stability

Still q. poor Dk Lipid deposition Poor reproducibility

between batches due variable exchange of acetyl and butyril groups

RGP material: Persecon and GPII

Siloxane Methacrylates

12-60 Softer and more flexible than PMMA, q. easily scratched

- Siloxane – poor, but copolymerised with methyl methacrylate OK.

OK Much better O2 permeability (↑Dk)

Less warpage than CAB Better comfort (softer

and more flexible) than PMMA

Protein and lipid deposition

Higher Dk material prone to crazing due to oxidisation

Q. easily scratched

Bond in siloxane allow rotation in backbone, facilitating O2 transport.Polycon I, HDK, Boston II and IV, Paraperm O2 (EW lens), Menicon O2

Alkyl-styrene copolymers

24 - - Reduced Resistant to flexure

High n – good for high Rx (↓t, ↑Dk/t)

Can’t use chlorhexidine solutions (main RGP sterilisation solution!)

t-butyl styrene – no silicone. Low SG – floats!Wesley-Jensen Airlens

Fluro-siloxane methacrylates

40-100+ Copolymerised with methyl-methacrylate for strength, scratch easily

- Copolymerised with NVP for good wettability

Vulnerable to flexure but good dimensional stability

High O2 permeability Resistant to deposition Dimensional stability

similar to silicoxane methacrylates

Break more easily Vulnerable to flexure and

scratching

Fluorine loves O2 and is good for wettability and deposition but hard to get into polymer.Boston XO, Fluoroperm, Quantum II, Persecon

Dk: Describes oxygen permeability, product of D (diffusion coefficient – speed of dissolved O2 movement) and k

(solubility coefficient – number of O2 molecules dissolved)

TG: Glass transition temperature – temperature at which there is sufficient energy for movement of the backbone of

the polymer and polymer becomes soft rather than glass-like

Plasticisers in polymers act as internal lubricants, allowing backbone molecules to move; make the

polymer softer and more flexible. Water acts as the plasticiser in hydrogels, cuasing the material to swell

and reducing TG. Also affects elasticity.

Cross-linking: Individual polymer chains joined by cross-links. Number of cross links = cross-link density, determines

strength and provide elastic properties of the material. Cross links hold the 3D structure together.

Material Dk Hardness Shaping technique

Wettability Elasticity Advantages Disadvantages Examples and Other Info

Synthetic Elastomers

Silicone Rubber 140 Low TG – high chain mobility

- Very poor - Brilliant O2 transmission

Poor wettability causes increased friction in eye (poor comfort and corneal adhesion!)

Surface treatments for wettability impermanent and ↓clarity

Good for aphakic infants as they have a nice thick lipid layer to their tear films.

Hydrogels HEMA - Poly (2-Hydroxyethyl methacrylate)

Varies with water content, temp. etc.

Soft lens material

Moulding, lathing, spin casting or combination

Hydrophillic, but specific affinity (i.e. limit to amount of H2O taken on)

- Lovely and comfy! Beginnings of SCL era

and all that came with it – disposables etc

No double bonds, resistant to acids and alkalis

Takes up more polar molecules (e.g. alcohols) preferentially due to increased van der Waals

? not as good vision etc

Similar to PMMA, additional pendant hydroxyl group. The more hydroxyl groups, the greater the van der Waals forces and the more water taken on.

Polymer Chemistry and Lens Performance

Dk – Oxygen transmissibility to the eye. In the open eye, the cornea receives oxygen from the atmosphere and the tear film. In the closed eye, oxygen is supplied by the palpebral conjunctiva, limbus and aqueous. With RGPs, tear exchange is essential. With SCLs, oxygen diffuses through the lens. In hydrogels, Dk depends solely on water content (although water content is determined by polymer structure).

Dk: Describes oxygen permeability, product of D (diffusion coefficient – speed of dissolved O2 movement) and k

(solubility coefficient – number of O2 molecules dissolved)

Plasticisers in polymers act as internal lubricants, allowing backbone molecules to move; make the

polymer softer and more flexible. Water acts as the plasticiser in hydrogels, cuasing the material to swell

and reducing TG. Also affects elasticity.

Oxygen is required for metabolic processes such as cellular division, endothelial pump functioning, cell junction formation, synthesis of proteins, lipids etc. and cell division (repair, maturation).

CL oxygen performance can be measured by:

EOP (equivalent oxygen potential): The only in-vivo method – cornea with and without CL exposed to atmosphere (goggles) of known [O2] and uptake is measured using a sensor.

Dk: Fatt units vs hectopascal. Dk/t takes into account thickness. Fatt method or measuring Dk has issues which need correcting for – Boundary effect (stagnant saline traps O2 on lens surfaces) and Edge effect (electrode picks up O2 from edges as well as area being measured) (used for hydrogels Dk<100). Coulometric technique doesn’t have these problems (used for RGPs Dk>100)

Oxygen flux: amount of O2 which reaches cornea per unit time – takes into account O2 tension across lens. Tension in front of open eye is 159 mmHg, closed eye 59 mmHg. Behind lens depends on Dk.

The Holden-Mertz criterion state that closed eye corneal oedema should not exceed 4% (similar to normal ON oedema without lens), which required a Dk/t of 87. Obviously this depends on lens thickness (i.e. Rx) as well as Dk!! Compromise criterion of returning to normal oedema on waking gives Dk/t of around 34 mmHg.

SiH lenses are much better at providing oxygen and avoid the associated issues (see CL induced pathology notes).

Physical properties – see properties lecture

Water content – Determined by number of hydroxyl groups (range 38-85%) – see properties lecture

Ionicity – Ionic vs. Non-ionic. Determines wettability (more ionic = more wettable), deposition resistance (more ionic = more deposition) and pH resistance.

(FDA Classification system)

Classification Ionicity Hydration Deposition ExamplesType I Non-ionic Low Large Mr, sticky, fairly low

charge proteins (albumin, lactoferrin) – hard to remove

pHEMA, Tetrafilcon, Hefilcon

Type 2 Non-ionic High Lipids, also hard to remove Hilafilcon A, Alphafilcon A, Lidofilcon

Type 3 Ionic Low Smaller Mr, mobile proteins with large positive charge (lysozyme) – easy to clean

Etafilcon, Deltafilcon, Ocufilcon

Type 4 Ionic High Deposit more by volume than other types, but more wettable

Perfilcon, Etafilcon A, Ocufilcon C