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MEL 456Tribology

Topic: Ocular Tribology

A Presentation by: Sajeed Mahaboob

2011ME1111

Instructor: Dr. Harpreet Singh

09-10-2014

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Introduction

What is Ocular Tribology…

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Ocular Tribology is concerned with the mechanisms of contact lens lubrication.

There are three major driving forces in contact lens design and development… a. Cost b. Convenience c. Comfort

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Introduction

It is a lens simply placed in the eye. They are considered as medical devices and can be worn

to correct vision, for cosmetic or therapeutic reasons. In 2010, worldwide contact lens market was estimated at

$6.1 billion. The average age of contact lens wearers globally was 31

years old as of 2010.

09-10-2014 Source: http://www.daviddarling.info/encyclopedia/C/contact_lenses.html

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Uses of materials in Contact

lenses

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Nowadays generally used soft contacts are made of pliable hydrophilic (water-loving) plastics called hydrogels.

Because of this hydrophilic characteristics, the water content of various hydrogel contact lenses can range from approximately 38% to 75%.

The water content of soft lenses allows oxygen to pass through the lenses and keep the cornea healthy during contact lens wear.

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Availability of hydrogel soft lenses (Based on their water

content) 1. Low water content (Less than 40%) 2. Medium water content (50%) 3. High water content (more than 60%)

Trending soft contact lenses nowadays called silicone hydrogel lenses are becoming popular very fast these days.

These lenses include silicone within the hydrogel material to increase the oxygen transmissibility of the lenses.

Continued…

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Tribological Issues You will feel itchiness if the friction between

eyelid and eyeball is very high, If friction between eyelid and eyeball will be

low, lens might slip out. Dynamic friction/static friction ratios plays very

important role to design new lenses. Friction changes when the lens starts to dry out. Lens property changes when its dipped in

contact lens solution overnight. we can’t wear lens and sleep at night

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Lens will be more comfortable if….

1. The lens has the ability to maintain moisture (wetting) at the surface.

2. The lens should remains lubricated so that the mechanical friction and the associated pressures from the eyelid do not impact the lens and cornea negatively.

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Wetting and Lubrication of Lenses

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Micro scale:

Micro-tribometry is a technique that uses delicate flexures to resolve forces to levels approaching 10µN.

Such low force resolution is particularly important in measuring frictional forces on hydrogels because of their softness and the low contact pressures(3-5 kPa) that they experience when being worn.

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Measurement of surface friction

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Measurement of surface friction

Customized micro-tribometer instrument, used for contact lens friction analysis

Source: http://www.clspectrum.com/articleviewer.aspx?articleid=106075

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Four commercially available soft contact lenses were tested:

Acuvue Advance, Acuvue Oasys, O2 Optix, and Pure Vision.

The lenses were prepared by cycling. each lens cycles 6 times through hydration and dehydration cycles and then mechanically secured onto a soft probe.

The lens-coated probes were submerged in either a HydraGlyde Moisture Matrix solution or in a saline control solution, and lightly loaded against a highly polished SiO2 surface to average contact pressures below 5 kPa.

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Measurement of surface friction

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Nano scale:Atomic force microscopy (AFM) is mainly used in the topographic analysis of material surfaces on the nanometer scale.

This technique works by detecting the deflection of a weak cantilever, which supports a probe tip, as function of the programed motion across a sample surface.

AFM measurements detected frictional differences between each of the neat lens, as well as additional evidence for the uptake of the HydraGlyde Moisture Matrix through a reduction in friction following solution treatment.

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Measurement of surface friction

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We can measure the lens moisture using

Lipophilic diffusion.

Lenses are placed in a concentrated dye solution (Sudan IV in silicone oil solution) for either 30 minutes or 16 hours.

The length of time that the lenses are exposed to the hydrophobic dye environment is meant to simulate an exaggerated hydrophobic lens wear environment.

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Moisture measurement

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Moisture and Dye diffusion uptake

Image source: http://www.clspectrum.com/articleviewer.aspx?articleid=106075

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• Laboratory-based techniques to evaluate contact angles

include the sessile drop and the captive bubble techniques.

• The sessile drop method measures the contact angle between a drop of water and a contact lens surface exposed to air.

• Dynamic captive bubble technique measures the contact angle between an air bubble and a contact lens surface in an aqueous environment.

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Measurement of contact angles

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Tight lens syndrome

This may be because of the tight fitting of lenses, or it may be related to increase in dryness of the lens with the eye.

Corneal ulcerOver wear of lenses, improper cleaning of lenses, extended wear use of lenses, and overly tight lenses may increase the risk of developing the surface breakdown.

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Problems with contact lenses

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Corneal swelling

Essentially, the cornea becomes smothered by the lens.Sleeping in contact lenses, as with extended wear lenses, greatly increases the risk of corneal swelling.

Eye rednessLens allergy, lens solution allergy, or allergy to protein build-up on lenses.

Lens over wear with corneal edema, with corneal drying or a tight contact lens syndrome.

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Continued…

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Interaction of bacterial toxins trapped beneath

the contact lens leading to corneal irritation. Corneal ulcer. Giant papillary conjunctivitis. Poorly fitting or defective contact lenses

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Continued…

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References• http://en.wikipedia.org/wiki/Contact_lens• http://www.daviddarling.info/encyclopedia/C/contact_lenses.html• http://www.allaboutvision.com/contacts/faq/contact- materials.htm• http://eprints.aston.ac.uk/10925/• http://www.tribologyblog.com/2009/04/contact-lens-• tribology.html• Biotribology By: Anne Jacobson• http://www.clspectrum.com/articleviewer.aspx?articleid=106• 075• Disscussion and some help with friends and seniors

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27-06-2014

So…gentlemen

Thanks for your Patience.