CONTACT LENSES http://www.uic.edu/com/eye/LearningAboutVision/EyeFacts/SoftContactLenses.shtml

OPHTHALMIC APPLICATIONS Requirements: Good transmission of visible light High oxygen permeability No blood vessels Tear-film wettability (for comfort) Non-irritating Resistance to bacterial/lipid/mucus adhesion Chemical stability UV-blockers Ease of manufacturing and cost Ease of cleaning and disinfection Pigments (esthetics) Neither hydrophilic nor hydrophobic Surface properties Bulk properties Thickness Morphology The principal properties sought in contact lens materials, in addition to the required optical properties, chemical stability, and amenability to manufacture at reasonable cost, are high oxygen transmissibility (to meet the metabolic requirements of the cornea), tear-film wettability (for comfort), and resistance to accumulation on the lens surfaces of mucus/protein/lipid deposits from the tear film and other external sources. Contact lenses also must be easy to clean and disinfect (Kastl, 1995).

Oxygen permeability POLYMER: The oxygen permeability coefficient, P P = Dk, D : diffusivity, cm2/sec k : Henry’s law solubility coefficient, cm3(O2STP)/[cm3 (polymer) mm HG} CONTACT LENS: oxygen transmissibility (Dk/L) is more important than oxygen permeability coefficient of the material/average thickness of the lens (L, in cm)

Red eye Lipid deposition in the form of a heavy film on the lens surface of a silicone-hydrogel lens. Red eye Contact Lens and Anterior Eye (Supplement), 20, pp. S,5~10, 1997 http://www.siliconehydrogels.org/editorials/previous_editorial_jones_senchyna.asp#13

RIGID CONTACT LENSES More durable Less comfortable, longer adaptation period Fit loosely and move as one blinks Mechanically should tolerate the flex upon blinking PMMA First manufactured by Kevin Tuohey in 1948 lacks good oxygen permeability Small and floating

http://www.uic.edu/com/eye/LearningAboutVision/EyeFacts/SoftContactLenses.shtml

Rigid Gas Permeable Contact lenses (RGP) Oxygen permeable ones (RGD): Copolymers with siloxanyl methacrylates (hydrophobic for oxygen) and methacrylic acid (hydrophilic for wetting) + crosslinking agents :flexibility, stiffness and durability 1970s at Polycon Laboratories TRIS Oxygen permeability inversely proportional to density!

High-Dk Silicone Hydrogels decreases water content İncreases permeability oxygen is far more soluble in silicone than in water A theoretic 100 percent water lens' Dk could be no more than 80. highest practical water content for any material is about 80 percent, maximum permeability is approximately 40 Dk. High Dk RGP Contact lenses could have Dk > 100.

Rigid Gas Permeable Contact lenses (RGP) Perflurinated Perfluoroalkyl–siloxanylalkyl–methyl methacrylate hexafluroisopropylmethacrylate high oxygen permeability better surface properties but their Dk is not directly related to their density like that of the original RGP 7 ay wear

Rigid Gas Permeable Contact lenses (RGP) 3. Cellulose acetate butyrate (CAB) low oxygen low scratch resistance tends to wrap with humidity changes.

RGP Contact lenses 4. Siloxanyl alkylmethacrylates with styrene derivatives (Menicon) high enough oxygen transmissibility for extended wear (2002). Menicon SFP (Dk 134 to 138) Menicon Z (Dk 160 to 175). Advertised as hyper Dk lenses No microbial keratatis

Suffix GAME "filcon" (hydrophilic) for hydrogel materials "focon" (hydrophobic) for GP materials polymacon is the generic name of the original HEMA material

RGP Soft contact more expensive than RGP lenses more break in time less durable more replacements more likely to get eye infections Maintenance may require several cleaning and storing sterilization chemicals. more break in time Less comfortable if you discontinue wearing for a few days, it may be difficult to become used to wearing them again. Comfort over time: wear your lenses all the time. Hard Plastic Lenses (PMMA) Hard Plastic (PMMA) lenses do not allow oxygen to pass through to the cornea. They require a long adjustment period, and cannot be worn for more than 12 hours. These high-maintenance lenses were the first contact lenses on the market, but are now considered obsolete and are rarely used. http://www.1-save-on-lens.com/contactscategory.html

http://www.1-save-on-lens.com/contactscategory.html

SOFT HYDROGEL CONTACT LENSES FDA classifications Homogeneous hydrogels Poly(HEMA) based Heterogeneous hydrogels Hydrophobic siloxane-hydrogel High oxygen permeability

Homogenous Soft Hydrogel Contact lenses Supple / Fit snugly to corneal surface Slightly crosslinked p(HEMA) 40% water by weight Oxygen permeability increases Exponentially with the water content With decreasing thickness Otto Wichterle in 1961. Little tear exchange O2 should pass through the lens The original hydrogel contact lens material was poly(2-hydroxyethyl methacrylate) (PHEMA) (Wichterle and Lim, 1960); at equilibrium swelling in physiological saline solution it contains about 40% water of hydration. (Hydration of hydrogel contact lenses is customarily given as a percentage of water by weight, on a wet basis.) The soft hydrogel contact lenses (SCL) are supple and fit snugly on the corneal surface. Because there is little tear exchange under these lenses, most of the oxygen that reaches the cornea must permeate through the lens. The oxygen permeability coefficient of the standard hydrogel materials increases

Monomers in Contact lense Chemistry Methylmethacrylate (MMA), which contributes hardness and strength Silicone (SI), which increases flexibility and gas permeability through the material's silicon-oxygen bonds but has the disadvantage of poor wettability Fluorine (FL), which also adds a smaller degree of gas permeability and improves wettability and deposit resistance in silicone-containing lenses Hydroxyethyl-methacrylate (HEMA), the basic water-absorbing monomer of most soft lenses Methacrylic acid (MAA) and n vinyl pyrolidone (NVP) mono-mers, both of which absorb high amounts of water and are usually adjuncts to HEMA to increase lens water content Ethylene glycol dimethacrylate (EGDMA), a cross-linking agent that adds dimensional stability and stiffness but reduces water content

Copolymers 38 percent (low-water content) HEMA monomer + NVP or MAA Increase water content to medium (about 50 percent) or even high (about 70 percent) + MMA and/or EGDMA increases material durability, elasticity and stability but decreases water content.

Homogenous Soft Hydrogel Contact lenses Poly(vinyl alcohol) CIBA:daily disposable lens. Omafilcon A, Proclear, Biocompatibles Inc. 59% hydration better water retention than other conventional hydrogel lenses The original hydrogel contact lens material was poly(2-hydroxyethyl methacrylate) (PHEMA) (Wichterle and Lim, 1960); at equilibrium swelling in physiological saline solution it contains about 40% water of hydration. (Hydration of hydrogel contact lenses is customarily given as a percentage of water by weight, on a wet basis.) The soft hydrogel contact lenses (SCL) are supple and fit snugly on the corneal surface. Because there is little tear exchange under these lenses, most of the oxygen that reaches the cornea must permeate through the lens. The oxygen permeability coefficient of the standard hydrogel materials increases

Additives polymer chains to move more freely within the material may serve to block the transmission of ultraviolet light may help the material resist dehydration.

Anything dissolved in the water has the potential to enter into the polymer depending on its molecular size and the pore size in the material. The pore size of low-water content lenses is about 0.5μm and may be as high as 3.5μm with high-water lenses.

SOFT HYDROGEL CONTACT LENSES FDA classifications Homogeneous hydrogels Heterogeneous hydrogels Hydrophobic siloxane-hydrogel High oxygen permeability less likely to adsorb proteins Low water <50% non-ionic High water > 50% non-ionic Low water ionic High water ionic more likely to absorb lipids May absorb much higher amounts of protein prone to be sensitive to pH changes interact with some solutions

IONIC : methacrylic acid Hydration but, Protein binding HIGH WATER CONTENT Oxygen permeability Friability (fragile) Protein penetration Dehydrate faster (temp changes and water evaporation) Physiologically and optically best daily wear lens is ultrathin low-water content contact lens But thinner it is faster it dehydrates induce corneal epithelium injuries by a mechanism still unclear.

DAILY WEAR: open eye standard hydrogel contact lenses DAY AND NIGHT: Up to 1wk higher oxygen transmissibility

HETEREOGOUS SOFT GEL CONTACT LENSES

1. Siloxane-hydrogel Contact Lenses Goal: Comfort of a hydrogel Increased oxygen permeability Continues use of 1 month Solution: Copolymers of hydrophilic monomers with end functionalized siloxanes Siloxanes such as PDMS (silicone) or TRIS End product: Oxygen permeability mainly through siloxane Siloxane is on the surface RF-plasma reactor to make the surface hydrophilic Cause unstable surfaces-no long term advantage

Variation of oxygen permeability (Dk) with equilibrium water content for conventional and silicone-containing hydrogels. (Adapted from Silicone hydrogels -- the rebirth of continuous wear, Butterworth Heinneman, 2000)

Examples Purvision Balafilcon from Bausch & Lomb (36% water) O2 permeability : 110 × 10−11 cm2 (STP) cm2/(cm3 sec mm Hg) Dk of balafilcon A is 99 TRIS derivative + NVP :polar, hydrophilic First generation

PureVision surface treatment by plasma oxidation technique Forms hydrophilic glassy islands of inorganic silicate with exposed hydrophobic balafilcon material in between The gaps between the islands are so small that the wetting of the glassy silicate completely covers over the small hydrophobic interspaces, changing the surface chemistry from a hydrophobic silicone to hydrophilic silicate. The flexibility of the lens material does not adversely affect the surface integrity or the wettability of the lens surface. FDA Group III (low water, ionic). Atomic force microscopy (contrast mode 50 x 50μm) image of unused PureVision lens. http://www.clspectrum.com/articleviewer.aspx?articleid=101744

Focus Day&Night Lotrafilcon from Ciba (24% water) O2 permeability : 175 × 10−11., Dk of lotrafilcon A's is 140 Plasma treatment ultrathin (25nm) hydrophilic layer on the lens surface, hiding the TRIS structures underneath Biphasic with small domain size for optical transperency Fluoroether/silicone macromer + TRIS monomer + hydrophilic monomer DMA (N, N-dimethylacrylamide). fluorosiloxane phase allows for high oxygen permeability hydrogel phase transmits ions and water, allowing adequate lens moveme First generation

1st generation higer water content lower Dk Typical for silicone/siloxane

2nd generation more hydrophilic long-chain, high-molecular-weight polyvinyl pyrrolidone (PVP) Internal wetting agent lower modulus

3rd generation different chemistry, different trend long-chain siloxane macromer high oxygen permeability + high water + relatively low modulus inherently wettable

Second-Generation Silicone Hydrogels Galyfilcon A and senofilcon A more hydrophilic derivative of the TRIS monomer + siloxy macromer + hydrophilic monomers such as HEMA and DMA. no surface treatment is applied incorporate a long-chain, high-molecular-weight polyvinyl pyrrolidone (PVP) internal wetting agent hydrophilic humectant, meaning that it attracts and retains moisture with the intent of keeping the lenses hydrated throughout the wearing day.

Second-Generation Silicone Hydrogels Galyfilcon A and senofilcon A Lower modulus Mechanical issues related to higher modulus ones: epithelial arcuate lesions, mucin balls, contact lens papillary conjunctivitis, conjunctival epithelial flaps

Third-Generation Silicone Hydrogels Comfilcon A (Biofinity, CooperVision) Enfilcon A (Avaira, CooperVision) long-chain siloxane macromer high oxygen permeability relatively low modulus inherently wettable — no internal wetting agent or surface treatment is required have a fundamentally different chemistry

2. Flexible perfluoropolyether lenses OXYGEN PERMEABILITY LOW BACTERIAL ADHESION Copolymer with MMA (for rigidity) and VP (for wettability) flexible, nonhydrated

Proclear http://www.allaboutvision.com/products/proclear.htm