unit 2 contact lens, lens materials and wearing schedule.pptx

A comprehensive presentation on contact lens materials and it's wearing schedules

1. Contact Lens Material properties and Manufacturing Techniques NUDRAT IHSAN BVS,MPHIL Lecturer Optometry PICO,HMC Peshawar

2. Corneal Integrity Requires Oxygen 15 - 20.9% for regular function. Blink rate: approx. 15 blinks/min pH tolerance: pH of tear in the open eye: 7.34-7.43. Glucose supply: Main source: anterior chamber. Limited source: limbal vessels and tears.

3. Contact lenses act as a barrier to oxygen influx and carbon dioxide efflux . Open-eye, central cornea: 20.9%. Closed-eye, central cornea: 7.7%.

4. Preventing Oedema We know that oxygen is required for normal corneal metabolism. However, it has not been established with certainty just how much is actually required. Opinions vary widely. The following figures from Holden and Mertz, 1984 are widely accepted as being a useful guide: • Dk / t = 24 for Daily Wear (DW). • Dk / t = 87 for Extended Wear (EW). These values were determined using contact lenses as the test stimuli

5. Introduction to types of Contact lenses. Contact lenses can be classified in to two groups 1 - Corneal Lenses 2 - Scleral Lenses

6. Corneal Lenses Corneal lenses are rigid hard lenses, which stays in side limbal region. Corneal lenses are normally hydrophobic in nature. Newer generation of corneal lenses are rigid gas permeable lenses.

7. Scleral Lenses Scleral lenses are those which fit out side the limbal region. Scleral lenses now a days are normally soft in nature. These soft lenses are hydrophilic in nature.

8. Properties of Lens Materials Contact lens material used in the manufacturing should have following properties. Material stability. Flexibility. Intoxity reaction in eye. Wetting

9. Properties of Lens Materials Biologically inert. High oxygen permeability. Good optical quality. High surface tension. High refractive index. Low wetting angle. Good machine ability .

10. Properties of Contact Lens Materials Oxygen Permeability Oxygen Transmissibility Wettability Water Absorption

11. Oxygen Permeability Oxygen permeability (Dk) is that property of a material which allows the flow of atmospheric Oxygen by diffusion. Oxygen permeability (Dk) is an intrinsic physical property of the material from which a contact lens is fabricated.

12. Oxygen Permeability The diffusion coefficient (D): defines the speed of movement of the gas molecules within the material. The solubility coefficient (k): defines the number of oxygen molecules dissolved in the material .

13. Oxygen Permeability The permeability ( Dk ) varies directly with temperature, i.e. the higher the temperature, the greater the Dk. For most calculations a temperature of 34°C is used, as this approximates the corneal temperature in the open eye. When oxygen permeability of a material is to be measured.

14. Oxygen Permeability Units Fatt The unit of oxygen permeability is Fatt derived from the diffusion coefficient D and the solubility coefficient k. The exponent 10 -11 is appended to the unit of transmissibility

15. Oxygen Transmissibility Dk /t The oxygen transmissibility of a contact lens is a physical property of the actual lens, i.e. a property of both its material and its thickness. It is equal to the material’s oxygen permeability divided by the lens thickness, i.e. Dk / t . The exponent 10 -9 is appended to the unit of transmissibility because lens thickness is expressed in centimeters (cm), e.g. a thickness of 0.1 mm is 0.01cm.

16. Oxygen Transmissibility Dk /t It is normally specified at a specific temperature, usually 34°C. The permeability, and the transmissibility, are temperature dependent and are related directly, i.e. the higher the temperature, the higher the Dk and therefore the higher the Dk/ t . Clinically, oxygen transmissibility is more useful than oxygen permeability. It relates directly to on eye issues as it includes lens thickness.

17. Oxygen Transmissibility Dk /t Example: Average Oxygen permeability of a contact lens material = 34 x 10 –11 (cm 2 /sec)(mlO 2 /ml x mm Hg) If the center thickness of a C lens = 0.12mm i.e in cm it will be 0.012cm Dk/t= 34X 10 -11 / 0.012 = 28.3 X 10 -9

18. Wettability It is the property of the contact lens surface, define by the contact angle (wetting angle). Wettability of a lens tells us the amount of tear distribution on the lens surface. Wetting angle can be improved by. Increasing the surface tension of contact lens surface. Lowering the surface tension of liquid (contact lens solution).

19. Wettability Wettability of a contact lens depends upon the type of polymer used in making of lens, surface quality, polishing, cleanliness, tears, solution used. A measurement of wettability is given by the contact angle. The lower the contact angle better is wettability. Wettability of PMMA is best among rigid/ rigid gas permeable lenses. So if in extreme case of poor tears, a well design PMMA lens can be preferred over a RGP lens

20. Wettability A drop of pure water is placed on the test surface. The angle between the tangent to the drop’s surface at the point of contact and the horizontal test surface. a zero angle = completely wettable a low angle = somewhat wettable a large angle (especially > 90°) poorly wettable .

21. Poor Wettability of RGP

22. Water Absorption (Water Contents ) Water absorption or the water contents of a contact lens is the property of hydrophilllic polymer to absorb saline when hydrated from dry state. Polymers like HEMA or Poly HEMA are generally considered low water content materials (38% to 55%), where as poly Vinyl’s and other materials have higher water contents (55% to 75%).

23. Water Contents The higher water contents in one hand is a big advantage like greater oxygen permeability, more comfort, on the other hand the disadvantage is it makes the contact lens more fragile, more absorbent means that it will also absorb more debris and pollutants form the environment.

24. Water Contents effects on pH PH . As the pH of the lens environment decreases (becomes more acidic), so too does the water content. As it increases (becomes more basic or alkaline), the water content increases. The magnitude of the change may be dependent on the material’s chemistry as well as its water content. Low water content lens exhibits less environmental influences, especially pH. pH depends upon hypoxia and contact lens solutions

25. Water Contents Tonicity. The tonicity of the surrounding medium (tears or lens care products) can affect the water content. Hypertonic solutions decrease the water content, hypotonic solutions increase it. Oxygen permeability. Generally the higher the water content of the lens, the greater the oxygen permeability.

26. Water Contents Refractive index. Higher water content materials have a lower η . Rigidity (handling). Higher water content materials are generally less rigid and are more difficult to handle when thin. Durability. Higher water content materials are generally less durable. Higher water content are less durable because they absorb more debris and pollutants form the environment.

27. Low Water Contents Advantages Lower susceptibility to environmental influences. Greater rigidity provides easier handling. Higher refractive index allows a thinner lens to be made. Virtually any lens care product can be used. All methods of lens fabrication can be used. Generally more predictable behaviour and lower expansion on hydration results in greater reproducibility. Pervaporation staining is less likely because the bulk flow of water through such materials is more difficult.

28. Low Water Contents Disadvantages Low water content means lesser oxygen is available to the cornea. Because of low Dk , only the thinnest lenses will provide adequate oxygen for normal daily wearing. The greater rigidity of most of these materials results in less conformity to the topography of the anterior eye, which may result in lower comfort levels. Thin lenses (for adequate Dk / t ) are usually more difficult to handle, especially in lower BVPs.

29. Manufacturing Techniques Different methods used in making of Soft & Hard contact lenses .

30. Lathe Cut Lenses Polymer rods of 10 to 15mm in diameter with length of 20cm are cut in to small size buttons of 5 to 10 mm thick for lathe cutting operation. Conversion of button into lens involves first mounting the button in lathe to cut and polish the back curve, then reversing the button to make the front surface and edge of the lens. In the case of hard & RGP lenses the lens is now cleaned and disinfected for further use. In the case of soft lenses the lens is put in to a isotonic saline solution for proper hydration.

31. Spin Cast Lenses Wet casting. This technique used by Bausch & Lomb. The processes involve a mixture of liquid polymer is introduced in to a concave mould . The mould is rotated during the polymerization and the centrifugal force results in the direct formation of a lens form.

32. Spin Cast Lenses Dry casting is used by many companies like EDGE 111 processes involve is the same except that the dry mixture is added in the rotating mould and the lens is hydrated after wards.

33. Spin Cast

34. Injection Moulding This technique is used to make large numbers of lenses on commercial basis. Now a days mostly disposable lenses are made this way but most companies are also use this tech to make low cost lenses. Part moulded part late cut tech is also getting very popular.

35. Lathing vs Molding Lathing is the original method. Lathing is a well understood and longstanding method of fabricating anything that can be made symmetrical about an axis of rotation, e.g. a contact lens. While Molding contact lenses is a more recent adaptation of an old manufacturing technique in which the lens material enters a double-sided mold as a liquid and solidifies in situ as a result of polymerization. Once the mold is broken apart, the lens is in its final form and requires little or no secondary manufacturing or finishing.

36. Contact Lens Material Hard Lens Materials Soft Lens Materials

37. Hard Lens Materials Several materials have been used in the making of different types of lenses firstly blown glass then grinded glass was used, but in 1930s when plastic polymers were made for different usages, these polymers were later used to make hard lenses. In selecting the best material for contact lens some basic principal are followed.

38. Hard Lens Materials PMMA. PolyMetyhly MethAcrylate . PMMA is Rigid non gas permeable material with DK = 0 and low wetting angle high optical transparency, easy machine ability & the material is non toxic to the eye.

39. PMMA . PolyMetyhly MethAcrylate This is the basic compound for most of the contact lenses by changing the concentration of PMMA in a lens we can attain different properties. e.g if a lens has poor wetting angle then by increasing the concentration of PMMA wetting angle of the material can be improved

40. Hard Lens Materials CAB cellulose acetate butyrate. CAB was the initial material used in making of RGP rigid gas permeable lenses DK 4 low resistance to surface abrasion. Poor wetting angle Styrene. DK 14. Low specific gravity high ref. Index mainly used for hybrid lenses e,g soft perm. Main draw back is brittleness.

41. Hard Lens Materials Siloxane -methacrylate . Commonly know as gas permeable lenses High DK value low wetting properties. More Si-O-Si bonds higher will be the DK. The wetting component can be improved in theses lenses by adding Methacrylic acid but at the cost of decreasing DK.

42. Hard Lens Materials Fluro siloxane acrylate & Polymer, fluoroether are fluoro based components gives high DK value high deposit resistance. To improve wetting properties, and mechanical strength methyl- metha acrylate is added. To form Fluorosiloxane-methacracrylate lenses demonstrate excellent dimensional stability.

43. Soft lens materials About 70% of all contact lens fitted are hydrogel. They are more comfortable then rigid lenses because of the more flexible, softer nature of the hydrophilic material. Hydrogel lenses can be classified in many ways material, composition, water contents, diameter and mode of wear. They are composed of polymers capable of absorbing large amounts of water.

44. Soft lens materials HEMA Hydroxyethyl Methacrylate Low Water content material Vinyl Pyrrolidone . Poly vinyl materials are use to make High water content lens.

45. HEMA This is the most common plastics for hydrogel lenses. Cross-link (very stable covalent bonds) add stability to the lens and reduce water at saturation in the hydrogel. A cross linked polymer swells in a solvent (water) to from a gel, hence the term hydrogel lenses.

46. HEMA A polymer consist of many repeated units of monomers. Dehydrate hydrogel material is like a thin PMMA lens (hard and brittle); When hydrate the polymer chains move easily and become rubber like. The first hydrogel lenses were composed of Hydroxyethyl methacrylate (HEMA).

47. P HEMA When the monomers are identical the material is termed a homopolymer . With more then one kind of monomers, the material is termed copolymer. When HEMA is polymerized it become polyhydroxeythy methacrylate (PHEMA). Monomers can link up with units in adjacent chains, that is cross-links agents.

48. Vinyl Pyrrolidone Poly vinyl materials are use to make high water content lens 70% WC. Increased water content means more oxygen DK and flexibility, but a very fragile lens. PHEMA can be copolymerized with amount of hydrophilic monomer like vinyl pyrrolidone or acrylamide. There will be a loss in water Contents 55% but increases mechanical strength.

49. Soft lens Material Most soft lens material are combination of two or three polymers of the following monomers: PHEMA (poly( hydroxyethyl methacrylate). PVP (poly(vinyl pyrrolidone ). MA ( methacrylic acid). MMA (methyl methacrylate). GMA ( glyceryl methacrylate). DAA ( diacetone acrylamide). PVA (poly(vinyl alcohol). NVP ( N -vinyl pyrrolidone ). PC ( phosphorylcholine (use as coating to increase WC in HEMA & Silicone

50. Thank you!