Contact Lens Polymers Justin Bergin Drawing by Leonardo DaVinci, from Salvatori, P.L. The Story of Contact Lenses. Obrig Laboratories: New York, Page 17.

Overview History Why use plastics? Why use contact lenses? What characteristics make a good contact lens? How are contact lenses made? Hard contact lenses Soft contact lenses Conclusion: the future of contact lens research

The History of Contact Lenses Leonardo DaVinci – 1508 –First conceptual drawing Rene Descartes – 1632 –Idea of the corneal contact lens Sir John F. W. Herschel – 1827 –Glass lens could be used to protect the eye Adolf Fick, Eugene Kale, August Miller – 1800’s –First corneoscleral shell contact lenes

History of Contact Lenses Continued William Feinbloom – 1948 –First plastic contact lens Kevin Tuohey –1948 –First corneal contact lens from Poly(methyl methacrylate) (PMMA) Otto Wichterle –1961 –Discovered poly(hydroxyethyl methacrylate) (HEMA) –Worlds first soft contact lens –Spin casting

Why Use Plastics?? Glass -Extremely Rigid -Brittle -Completely Impermeable -Difficult to Manufacture -Hard to Fit to Eye -Difficult to Wear -Damage to Eye Plastic -Can be Made Soft or Hard -High or Low Permeability -Easy to Manufacture -Cheap to Manufacture -Comfortable -Increased Biocompatibility -Lightweight

Contact Lenses Versus Others Contact Lenses -Aesthetic Reasons -Change eye color - Four eyes syndrome -Medical Reasons - Disorders and aliments -Physical Reasons -Size of image on cornea -Extreme conditions Glasses -Poor for severe myopia -Bulky to wear -Conditions where glasses cant be worn -No need to remove lenses -No risk of infection Laser Treatment -Not 100% successful -Convenient

Contact Lens Special Cases Severe Myopia –The size of the image on the cornea is decreased when glasses are used to correct severe myopia. Hamano, H., Kaufman, H.E. The Physiology of the Cornea and Contact Lens Applications. Churchill Livingstone: New York, Page 63.

What makes a good contact Scratch resistant Lightweight Easy to handle Cheap to manufacture Good specific gravity Hydrophilic Wetable Biocompatibility

Biocompatibility – Biofilm Tears consist of proteins, lipids, calcium, sodium, chlorine, carbonate, ect. –Contact lenses must not corrode –Must resist deposition of a biofilm Charge distribution on ionic polymers attract proteins Poor wetabilty encourages deposition of proteins and lipids Biofilms cause discomfort, infection, and contamination of lens

Biocompatibility - Permeability Eye does not receive adequate blood flow, needs atmospheric oxygen Diffusion measured in terms of DK –D = Diffusion Coefficient –K = Solubility Coefficient –Typically in Units of Barrers ( cm 3 O 2 (STP) cm/ cm 2 s cmHg) –Contacts need DK > 100 Barrers to be considered for extended wear Oxygen Transmissibility (DK/L) –Inversely proportional to lens thickness, L

Manufacturing Techniques Cast Molding –Monomer is injected into mold, mold is cast, and then initiated Lathe Cutting –Polymers are bulk polymerized into rods –Rods are cut into buttons, buttons are but by lathe into contact lenses Spin Molding –Monomer is placed in mold, mold is spun, and monomer is polymerized while rotating

Cast Molding Polymer is injected into molds Shape of anterior and posterior molds controls lens shape and thickness High yield Highly accurate Picture courtesy of Scientific

Lathe Cutting High precision lathe cutters are controlled by computers to give an accurate cut Moderate accuracy High yield Picture courtesy of Scientific Pictures courtesy of C & H Contact Lens,

Spin Casting Lens optics and shape can be varied by changing the shape of the mold and the speed of rotation High accuracy Lower yield Source: Aquavella, J. V., Rao, G. N. Contact Lenses. Lippincott Company: Philidelphia, Page 72.

Properties of All Contact Lenses Consist of a three dimensional amorphous matrix with cross-links Thermosets Ultraviolet or infrared initiated

Properties of Hard Contacts Below glass transition temperature Typically bulk polymerized into rods or buttons Cheap to manufacture due to bulk polymerization Formed into lenses via lathe cutting

Properties of Hard Contacts Continued Long lens life due to low permeability and/or low water content More difficult to fit to the eye due to rigidity Resistant to scratching Easy to handle High modulus of elasticity

Hard Contacts: Henry’s Law Henry’s law for Polymers Below T g C = K D p + C H (bp/(1+ bp)) –Can Be Simplified As Follows: C = K D p + C H bp (bp << 1) C = (K D + C H b)p C = K’ D p C=Concentration of penetrant gas dissolved in polymer K D = Solubility coefficient for penetrant gas p = Gas pressure at equilibrium C H = Languir mode concentration of sorbed gas b = Gas affinity parameter Singh, J.J., et al. ‘An Investigation of Micro Structural Characteristics of Contact Lens Polymers’. NASA Technical Paper 3034, 12/1990

Hard Contacts: Fick’s Law Fick’s Law for Glassy Polymers N = -D D (dC D /dx) – D H (dC H /dx) -Simplified as follows: N = -D’ D (d/dx)(C D + C H ) N = -D’ D (dC’/dx) N = Rate of gas transfer per unit area D D = Fick’s diffusion coefficient C D = Henry’s concentration of sorbed gas D H = Diffusion coefficient for gas trapped C H = Gas population (C H < C D ) Singh, J.J., et al. ‘An Investigation of Micro Structural Characteristics of Contact Lens Polymers’. NASA Technical Paper 3034, 12/1990

Hard Contacts - PMMA The first type of hard contact lens MMA only DK about 0.5 Moderately hydrophobic Limited wear time Extremely rigid Scratch resistant/durable Long life time

Hard Contacts: PMMA-TRIS Scientist Needed a Solution to Impermeability of PMMA –Sought to take advantage of silicone’s love for oxygen In 1970 Norman Gaylord copolymerized MMA with methacryloxypropyl tris(trimethysiloxysilane), (TRIS) Polymer had the strength of PMMA, but Oxygen permeability of silicone Silicone is extremely hydrophobic –Wetting agent methacrylic acid (MAA) was added to make it hydrophilic Highly successful, several lenses were approved by the FDA Still used today

Hard Contact Lenses: RGP Lenses The push for extended wear contacts, and PMMA-TRIS –PMMA-TRIS still hydrophobic –Permeability not suitable for extended wear Doping PMMA-TRIS with fluoromethacrylates –Addition of fluorine increases free volume fraction –Effectively increased permeability RGP lenses approved by FDA for extended wear –Approved for extended wear up to 7 days

Properties of Soft Contacts The most popular type of contact lens Most consist of polymer hydrogels Above their glass transition temperature Typically made via spin or cast molding Can have high, medium, or low water content

Properties of Soft Contacts Continued Easier to fit More comfortable Moves with the eye Low modulus of elasticity Can be difficult to handle

Soft Contact Lenses: HEMA HEMA alone or cross-linked with either ethylene dimethacrylate (EDMA) or ethylene glycol monogethacrylate (EGDMA) Future compositions added MAA or N vinyl pyrollidone (NPV) to make lens more hydrophilic

HEMA Continued Problems with MAA, and other surfactants Addition of glycerol methacrylate (GMA) Witch resists biofilm formation Typical permeability of barrers –This is 400% lower then RGP lenses Scientist must find ways to increase permeability

Soft Contacts: Silicone Hydrogels Once again scientist seek to use silicone –Some use of fluorine also noted Polydimethylsiloxane (PDMS) is a good candidate because of its permeability (600 Barrers) –Oxygen permeability decreases as water content increases Silicone is hydrophobic –Can’t blend with hydrophilic monomers –Wetability decreases as Water content decreases

Silicone Hydrogels Continued Increase wetability by polymerizing in polar plastic molds Increase wetability by grafting polyoxyethylene to the surface of the lens Grafting random copolymers of lauryl-, hexyl-, and methyl-, methacrylate shows promise Current siloxane lenses have permeability of Barrers No siloxane lenses have been approved by the FDA

In Conclusion: The Future of Contact Lens Research Focus is on creating extended wear lenses Increase permeability Decrease water content Increase Wetabilty Make resistant to biofilm Siloxane and Fluorine seem to be the answer chemically Answers also lie in manufacturing