Question 3. Over 8% of people wear contact lenses for vision correction. The most common type is made from soft hydrogel

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Question 3 Over 8 Of People Wear Contact Lenses For Vision Correction The Most Common Type Is Made From Soft Hydrogel 1 (118.06 KiB) Viewed 6 times

Question 3 Over 8 Of People Wear Contact Lenses For Vision Correction The Most Common Type Is Made From Soft Hydrogel 2 (152.25 KiB) Viewed 6 times

Question 3. Over 8% of people wear contact lenses for vision correction. The most common type is made from soft hydrogels, which are oxygen- permeable. Many factors can affect the hydration in the lens and therefore the perceived comfort level, including environmental elements such as tem- perature. A cross-sectional schematic of how the lens sits over the cornea is shown in the figure below. Contact lens (L) contact lens retina R₁ R₂ cornea (a) Air-conditioned offices are a challenging environment for contact wear- ers due to the dry air and the direct breeze in the eye, though it is unclear whether the colder temperature itself is also a contributing fac- tor to the discomfort. In the following, you will estimate the temperature at the outer surface of the cornea (R = R₁). It can be assumed that at the interior surface of the cornea (R= Ro) the temperature is constant T(Ro) To, while on the outer surface of the lens, the temperature is equal to the ambient air T(R₂) = T₂. i. Assuming a spherical symmetry and other necessary simplifica- tions, reduce the appropriate form of the heat diffusion equation and find the general expression for the temperature distribution. ii. The thermal conductivity in the corneal (kc) and the lens (*) layers are distinct, since they are made of different materials. Write down the two equations that describe the temperature in the lens-cornea layers. iii. How many unknown coefficients must be found in order to fully Cornea (C) Re
determine the temperature profile? iv. List all the boundary conditions necessary to solve the system. v. Calculate the temperature at the corneal surface; i.e. T(R₁) = T₁. (b) The assumption of spherical symmetry in this case is, of course, an unrealistic one given that only a part of the eye is exposed to the air (or covered by the contact lens). Discuss how this might have led to errors in the estimation of T₁. Is the actual cornea surface temperature likelv to be higher or lower, and why? (c) Oxygen permeability is a key measure in clinical contact lens perfor- mance, where decreased oxygen at the cornea may lead to oedema and neovascularisation. Manufacturers provide standard data of their products, as shown in the table below. 1-DAY Proclear DAILIES Aqua AIR OPTIX Aqua Soflens Daily Disposable ACUVUE OASYS ACUVUE 1 day Comfort MOIST Plus CIBA Johnson & Manufacturer CooperVision Bausch & Lomb CIBA Vision Vision Johnson Johnson & Johnson Vision Care Vision Care nelfilcon A ll 2 etafilcon A IV 2 hilafilcon B II 2 Polymer omafilcon All 2 lotrafilcon B15 senofilcon A15 69% 58% 60% 59% 33% 38% Water content 26 28 25 22 110 103 Oxygen permeability (Dkx10-11)
i. Commmercially available hydrogel lenses vary in their water con- tent. It is generally said that a higher level of hydration would lead to an increased capacity for oxygen transmission. Does the data support this? Briefly discuss. ii. The oxygen permeability is defined as DH, where D is the diffusion coefficient. Using the appropriate form of Fick's law and by making simplifying assumptions, show what role D plays in determining the steady state oxygen profile across the lens. Clearly state your conclusion. iii. The oxygen transport to the eye is dependent on the concentration of oxygen at the lens surface, i.e. the boundary condition. The process by which oxygen dissolves into the lens at the air-lens in- terface is described via Henry's law C = Hp, where C denotes the dissolved oxygen concentration, and p the partial pressure of oxy- gen in air. H is known as the Henry solubility (measured in mol), and is the second parameter in the oxygen permeability. Pa Derive the units for the oxygen permeability and thus define what it measures as clearly as you can. iv. Although currently there is no technology available to directly mea- sure the cornea oxygen concentration under the lens, it is known that the oxygen uptake rate by the cornea can vary depending on the lens worn, further complicating the product comparison. Can you suggest a more universal indicator for comparing oxygen per- formance between lens types?