OPT506 - L16 CL Materials (University of Plymouth) PDF

Contact Lens Material Dr. Asma Zahidi GH-KC-JX Code: EU-DB-CG Learning outcomes Understanding Ideal Lens Properties Evaluating Material Properties Recognizing the Role of Oxygen Permeability (Dk) Analyzing Wettability Distinguishing...

Contact Lens Material Dr. Asma Zahidi GH-KC-JX Code: EU-DB-CG Learning outcomes Understanding Ideal Lens Properties Evaluating Material Properties Recognizing the Role of Oxygen Permeability (Dk) Analyzing Wettability Distinguishing Between Material Types Understanding Material Classification Codes Assessing Manufacturing Techniques Code: EU-DB-CG Ideal lens material: Meets corneas O2 requirements Durable Inert, physiologically Transparent, optically Excellent in vivo wetting Be optically regular Allow high quality surfaces Resists spoilage Requires minimal patient care Stable, dimensionally Easily machinable/latheable/moldable Code: EU-DB-CG Important CL Material Properties: O2 permeability Wettability Scratch resistance Rigidity (GP CLs) Good stability Deposit resistance Durability Flexibility (SCLs) Code: EU-DB-CG Material properties Oxygen permeability (Dk) property of material Oxygen transmissibility (Dk/t) property of lens (often at -3.00DS) http://www.no7contactlenses.com/icon.html Code: EU-DB-CG Oxygen permeability (Dk) D is how fast oxygen molecules pass through a material (diffusion) k is how many molecules of oxygen there are dissolved in the material (solubility) Affected by temperature and pressure Measured in FATT units or BARRERS 10-11 cm2/s ml O2 /ml mmHg Or if ISO unit of pressure used (hectopascal) 10-11 cm2/s ml O2 /ml hPa To convert traditional values of Dk to ISO units, multiply by 0.75 Code: EU-DB-CG Impact of Low O2 Transmissibility Epithelial microcysts Endothelial polymegethism Microcysts ↓ corneal pH (due largely to CO2 retention) Oedema Endothelial mosaic Endothelial blebs (pH ∆s ←hypoxia) And more…. Endothelial blebs Code: EU-DB-CG CL Wettability Ability of a drop of liquid to adhere to a solid surface Measured with contact angles Low contact angle = good wettability Different measurement methods give different results Property of base material and can be enhanced by surface treatment also Plasma treatment Bombard with oxygen ions to produce hydrophilic surface Graft polymerisation Graft hydrophilic surface onto lens Code: EU-DB-CG CL Wettability In vitro: Wetting angle Sessile drop Wilhelmy plate Captive bubble In vivo: Tear coverage (Tear) Break-Up Time (BUT or TBUT) Drying time Code: EU-DB-CG CL Wettability: Sessile Drop q advancing q receding q Drop of q water Material A Material A Code: EU-DB-CG CL Wettability: Wilhelmy Plate Material A Material A Advancing Receding WATER q q q advancing > q receding Code: EU-DB-CG CL Wettability: Captive Bubble (Air-in-Water) CL mount CL being tested Tangent to CL surface q WATER Tangent to bubble Air bubble at point of contact Controlled air supply NOTE: In this technique, q advancing < q receding This is the REVERSE of the other methods (because expanding air bubble is meeting previously wetted surface) Code: EU-DB-CG CL Wettability: Non wetting CL Surface Code: EU-DB-CG Contact Lens Material Code: EU-DB-CG The contact lens material PMMA Poly(methyl methacrylate) HEMA (hydroxy ethyl methacrylate) polyHEMA Code: EU-DB-CG Contact lens materials pHEMA Made by polymerising 2-hydroxyethyl methacrylate monomer With a cross-linker such as ethylene glycol dimethacrylate (EGDMA) Most of the hydrophilic behaviour of PHEMA is due to the hydroxyl group (OH) Contact lenses made from PHEMA contain approximately 38–40% water in the fully hydrated state. Code: EU-DB-CG Classification of material Code: EU-DB-CG Material classification Classified by a 6-part code: a. Prefix b. Stem c. Series suffix d. Group suffix e. Dk range f. Surface modification code Code: EU-DB-CG Material classification Prefix / stem / series / group / Dk range / surface modification code a. Prefix Optional for all countries except USA b. Stem Filcon for soft lenses (hydrogel) Focon for rigid lenses c. Series suffix A capital letter added to the stem to indicate the revision level of the chemical formula (A, B, C,...) Code: EU-DB-CG d. Group suffix (RGP) Prefix / stem / series / group / Dk range / surface modification code (Stone ) focon stem Code: EU-DB-CG d. Group suffix (SCL) GROUP H2O CONTENT IONICITY I < 50% Non-ionic II > 50% Non-ionic filcon stem III < 50% Ionic IV > 50% Ionic V Enhanced oxygen permeable materials (SiHy) Code: EU-DB-CG d. Group suffix (SCL) Silicone Hydrogels: FDA Groups - Proposed Proposed addition of further FDA group specific to SiHy CLs Group V-A: Low water content, non-ionic, & surface treated Group V-B1: Low water content, non-ionic, non-surface treated, & hydrophilic monomer containing Group V-B2: Low water content, non-ionic, non-surface treated, & semi-interpenetrating network-containing Group V-C: High water content & non-ionic Group V-D: Ionic, both low & high-water content Code: EU-DB-CG Material and lens deposition Ionic hydrogels Non ionic hydrogels Negatively charged Surface treatment surface to remove negative Sensitive to charges charge in pH and osmolarity More likely to attract tear proteins (lysozyme) Code: EU-DB-CG Material classification Prefix / stem / series / group / Dk range / surface modification code Dk range (oxygen permeability) Numerical code which identifies the permeability in ranges which are considered significant in CL wear. Dk is expressed in ISO units (cm2/s) [ml O2/(ml* hPa)] Code: EU-DB-CG Material classification e. Dk range (oxygen permeability) 0 < 1Dk unit Numerical code which identifies the permeability in 1 1-15 Dk units ranges which are considered 2 16-30 Dk units significant in CL wear. 3 31-60 Dk units Dk is expressed in ISO units: 4 61-100 Dk units (cm2/s) [ml O2/(ml* hPa)] 5 101-150 Dk units 6 151-200 Dk units 7+… In increments of 50 Dk units Code: EU-DB-CG Material classification Prefix / stem / series / group / Dk range / surface modification code f. Modification code A lower-case m which denotes that the surface of the lens is modified, having different chemical characteristics from the bulk material Code: EU-DB-CG Examples Paflufocon B III 3 Paflu USAN prefix Focon stem indicates that this is a rigid lens material B USAN series suffix, B indicates that this is the second formulation of this polymer III Group suffix, in this case indicated that the material contains both silicone and fluorine 3 Dk range is 31 to 60 ISO units Code: EU-DB-CG Examples Etafilcon A IV 1 Eta USAN prefix Filcon Stem indicates that this material contains >10% water by mass A USAN series suffix, indicates that this is the first formulation of the polymer IV Water content is >50%, material is ionic 1 Dk range is 1 to 15 ISO units Code: EU-DB-CG RGP Material Code: EU-DB-CG Common RGP materials used Cellulose acetate butyrate (CAB) plastic polymer with a cellulose base. infrequently fitted now. might be considered where a low Dk is acceptable, and where CLIPC (contact lens induced papillary conjunctivitis) and/or 3 and 9 ‘clock staining have been a problem with other RGP materials. thermoplastic that can be heated to allow for moulding without affecting the chemical composition traditional method of manufacture was lathing Silicone acrylates (SA) Copolymers of acrylate (the rigid bit) and silicone (the oxygen bit) Also included are things to improve wettability (Methacrylic Acid) Fluorosilicone acrylates (FA) Newer generation Added fluorine Improves wettability, oxygen performance, deposit resistance Higher Dks can be achieved – even suited for extended wear Sometimes thicker due to instability Greater lens flexure and scratch risk Code: EU-DB-CG Cellulose acetate butyrate (CAB) Advantages Disadvantages Low Dk Moulding necessary for Good wettability dimensional stability Relatively inert Limited range of lens Does not attract protein designs Low breakage rate Scratches easily Very low incidence of Attracts lipids from tears CLPC Corneal adhesion in Relatively good for 3&9 some cases o’clock staining Lens flexure and distortion on toric corneas with tight lids Code: EU-DB-CG Silicone acrylates (siloxanes) Advantages Disadvantages Good range of materials available Wide range of designs Attract protein from the with practitioner control tears Low to medium Dks Some materials are available brittle with a breakage Good dimensional problem stability 3&9 o’clock staining Good vision with limited Some incidence of CLPC lens flexure Good scratch resistance Code: EU-DB-CG Silicon acrylates (siloxanes) Code: EU-DB-CG Fluorosilicone acrylates (FA) Advantages Disadvantages Brittle if too thin Very high Dks possible Less stable/more flexure Suitable for flexible Require careful extended wear manufacture Better wettability Dimensional stability Fewer deposit problems depends on material and Lower incidence of CLPC manufacture Modification possible Corneal adhesion in some cases Code: EU-DB-CG Fluorosilicone acrylates Code: EU-DB-CG Material group suffix Code: EU-DB-CG SCL Material Code: EU-DB-CG SCL Materials: Physical Compatibility Must allow CL movement Must be flexible especially in thicker CLs Code: EU-DB-CG SCL Materials: Optical Quality Depends on surface quality after hydration Shape regularity after hydration BVP within tolerance No unwanted toricity Accurate cylinder axis if CL is toric Code: EU-DB-CG Important Physical/Chemical Properties of SCL O2 permeability/transmissibility (Dk) Water content Elasticity Ionicity Deposit resistance Refractive index (n) Durability Environmental susceptibility Code: EU-DB-CG 1. Influential factors of O2 Permeability Water content Chemistry of the polymer Material’s method(s) of water retention Temperature pH Tonicity Code: EU-DB-CG 2. H2O Content Influences: O2 permeability (Dk) Refractive index (n) Rigidity (handling) Durability Minimum thickness to prevent pervaporation Environment susceptibility including spoilage CL care system choice Code: EU-DB-CG Hydrogels: Low Water Content Advantages Disadvantages Less susceptible to Lower Dk environment influences Less flexible More stable parameters Thin CLs more difficult to Higher rigidity, easier to handle handle Higher refractive index Any CL care product Ease of manufacture Higher reproducibility Higher wettability Pervaporation staining is less likely Code: EU-DB-CG Hydrogels: High Water Content Advantages Disadvantages Higher Dk Fragile Less flexible More deposit prone Faster restoration of shape More susceptible to the after deformation environment Lower refractive index Less stable parameters, lower reproducibility Thermal disinfection not recommended (trial CLs) More difficult to manufacture Cannot be made too thin – pervaporation staining Code: EU-DB-CG 3. Physical Properties - Elasticity Elastic limit should be large Should be strong (high Young’s Modulus of Elasticity) Combination of above should → a durable CL Shape recovery should be rapid Code: EU-DB-CG Modulus in soft CL materials Material Young’s modulus (109 Pa) PMMA ~ 2,000 CAB ~1,500 Lotrafilcon A 1.5 Balafilcon A 1.1 Lotrafilcon B 1.0 Silicone rubber 0.80 Senofilcon A 0.72 Vifilcon A ~0.51 PHEMA ~0.50 Omafilcon A 0~.49 Galyfilcon A 0.43 Etafilcon A ~0.3 Code: EU-DB-CG 4. Ionicity Hydraulic and ionic permeability are necessary in order to maintain adequate lens movement Allowing the post –tear film, to reform between blinks Pore diameter determines what substances penetrate hydrogels Lactate - accumulate in many hydrogels and make environment acidic Lysozyme - may penetrate high water content lenses Fluorescein - penetrates all hydrogels - very difficult to remove Na+ permeability is important. Transport is impeded with H2O