Dr. Gregory W. DeNaeyer from the United States recently stated, “Wouldn’t it be great if we had a lens with the benefits of both hydrogel and silicone hydrogel materials?”1 Now researchers in the United States and Europe have created a unique contact lens material – a water gradient contact lens. This material is so different that Tyler’s Quarterly Soft Contact Lens Parameter Guide now has a new water gradient category.2 Practitioners across the United States and Europe are describing it as a new era in comfort.
It is widely acknowledged that despite innovation in lens materials over the past two decades, discomfort is still the biggest issue faced by contact lenses wearers.3 While many oxygen-related issues have been overcome with contact lenses, combining exceptional comfort in a silicone hydrogel lens with the convenience of the daily disposable modality has remained a challenge.
The focus of recent innovation in contact lens technology has been to try to resolve these issues. When we consider the properties we desire in an ideal contact lens there is often a compromise between those desirable at the core of a lens and those at the surface. This can present challenges to meeting the needs of both health and comfort.
The core of a lens needs to provide high oxygen transmissibility as well as mechanical properties such as for handling and lens fit. At the surface, however, biocompatibility with the tear film is more important. The lens surface needs to be highly wettable, have a low coefficient of friction, and maintain a stable tear film. Until now soft contact lenses have utilised the same material composition and water content throughout the contact lens.4 The introduction of a water gradient concept enables independent optimisation of the contact lens core and surface in order to achieve ideal properties without compromise. In most cases there is an inverse relationship between oxygen transmissibility and water content. However, an optimal situation is to have both high oxygen transmission and high water content for wettability.
This has created the first ever water gradient contact lens featuring an increase from 33 per cent to over 80 per cent†water content from core to surface…
Figure 1. Cross-section of Dailies Total1 water gradient contact lens
Dailies Total1 Water Gradient Contact Lenses
By combining a heritage of silicone hydrogel innovation with the manufacturing platform LightStream Lens technology, research scientists have created a unique lens material called delefilcon A, the building block of Dailies Total1 water gradient contact lenses.
A measurable change can be demonstrated in the lens material, the water content and the modulus of Dailies Total1 lenses from core to surface. This has created the first ever water gradient contact lens featuring an increase from 33 per cent to over 80 per cent† water content from core to surface. To enable this water gradient, the material changes from a highly breathable silicone hydrogel material at the core to a non-silicone hydrophilic polymer structure at the surface, (Figure 1). This enables a lens with a Dk/t of 156 (at the centre of a -3.00D) combined with a surface water content of over 80 per cent and approaching 100 per cent at the outer surface.† A transition occurs between the two areas, in which the water content rapidly increases and the materials change from a high silicone material to one with essentially no silicone.5 The thickness of the surface structure makes up about 10 per cent of the total lens thickness.
Figure 2. AFM image of cross-section of Dailies Total1 contact lenses
Several laboratory techniques can and have been used to measure this change in lens material properties, including atomic force microscopy (AFM), neutron reflectometry, and fluorescent laser confocal microscopy.6 These methods show the gradient in modulus, adhesive properties and topography across the lens cross section, not seen in other lens materials. Figure 2 shows an AFM image of a lens cross-section, demonstrating the change in the lens structure. The colour change represents the change in composition and water content across a cross section of the lens. The modulus of the lenses also changes with the material becoming much softer at the surface of the lens.6
Most breathable daily disposable lens
A water gradient enables a highly breathable lens core and creation of the most breathable daily disposable contact lenses on the market with a central Dk/t of 156 @-3.00D. It might be questioned whether a high Dk/t for daily disposable wear is needed. However, there are several factors to be remembered. Stated Dk/t values (and even theoretical flux or equivalent oxygen estimations) are normally only given for the centre of a -3.00D lens. In minus powers the peripheral lens thickness means oxygen transmissibility is lower, whereas in plus powers the central oxygen transmission values will be much lower than the stated value for -3.00D. Figure 3 illustrates this through the use of colour oxygen maps across the whole lens in different materials and powers. The blue end of the spectrum is used to indicate high Dk/t, whereas red shows areas of lower Dk/t. It is clear that a Dk/t of 156 is just the starting point at the centre of a -3.00D in Dailies Total1 lenses. The stated Dk/t values for the other lenses shown are also only valid at the centre of a -3.00D lens and the peripheral oxygen transmissibility is generally much lower as seen in the coloured maps. Research has shown that peripheral oxygen transmission is just as important to ocular health as that in the centre.7
In addition, individual patients will have different oxygen demands that may not be predictable at routine examination, even with the slit lamp.9 So starting with the highest available central Dk/t value is the best way to avoid hypoxic concerns and meet the needs of patient lifestyles. Oxygen transmissibility in Dailies Total1 contact lenses is largely a property of the core of the lens, as this makes up most of the lens thickness. Of particular interest are the properties of the surface; as we will see, these are key to a comfortable lens-wearing experience.
Importance of Surface Lubricity
The comfort of a contact lens can be influenced by several factors ranging from the modulus of the material, lens thickness, water content to lens design and parameter. A factor shown to have a high correlation with lens comfort scores is the surface co-efficient of friction, or lubricity.10
Lubricity is the inverse of friction and, for a contact lens, can be described as how easily the ocular surface such as the palpebral conjunctiva can slide across the lens surface. Given that we blink on average 14,000 times per day,11 and with each blink the superior lid needs to slide down and up over the lens surface, it makes sense that this is such a strong determinant of lens comfort.
(Click here to view Figure 3. full image)
Assuming a distance of 1cm down and up with each blink, it can be estimated that the top eyelid travels almost 300 metres per day over the surface of the eye or contact lens. Lubricity can be detected with the fingers as a slippery feeling and it can be measured by using either an inclined plane or a micro tribometer.
Whichever method is used for measuring contact lenses, it is important that the pressures used reflect those found in the ocular environment (matching those exerted by the eyelid against the lens on eye). This is especially important when measuring the lubricity of Dailies Total1 contact lenses, as the extremely soft water gradient surface structures can be artificially crushed if measured at pressures that far exceed those found in the eye, thereby giving erroneous lubricity results at high testing pressures. Dailies Total1 water gradient contact lenses have been shown to have the highest lubricity (or lowest surface friction) compared
to competitor contact lenses tested, including both hydrogel and silicone hydrogel lens materials.‡12
In some cases, manufacturers add lubricating agents to the packaging saline of lenses to aid comfort on insertion. For all measurements in this case, lenses were rinsed in saline to remove the effect of any blister saline additives. Additionally the lubricity of Dailies Total1 lenses has been tested after a day of wear (average 14 hours) and they were shown to maintain 100 per cent of their initial lubricity,13 indicating the lasting effect of the water gradient surface structure throughout a day of wear.
Exceptional Comfort from Beginning to End of Day
The outcome of this superior lubricity is outstanding wearer comfort from the beginning to the end of the day. Figure 4 shows results of a clinical trial showing comfort scores in excess of nine out of 10 for the entire wearing period.14 In fact, clinical trials have shown that four out of five wearers report not feeling the lens
on the eye at all.15
Figure 4. Dailies Total1 contact lenses provide exceptional comfort from the beginning to the end of day13
A new contact lens with different core and surface properties means detailed research is needed to optimise the chosen design parameters. Numerous studies were done to co-optimise the base curve, diameter and lens design. Ultimately, the combination of an 8.5mm base curve with a 14.1mm diameter were selected to give optimal centration and lens movement. This one base curve and diameter combination provides a successful fit in at least 97 per cent of subjects across numerous clinical trials.16
In one clinical study with over 100 participants having a range of corneal curvatures from 39.37D to 48.00D (8.57mm to 7.03mm), Dailies Total1 lenses achieved 97 per cent fit success.17 The full technical specifications and range of parameters can be seen in Table 1.
Two tables above ( referred to as Table 1.) Technical specifications and range of parameters
The Start of a New Era
According to Bo Lauenborg, an optometrist in Denmark where the first launch took place: “This is a revolution. My patients tell me this opens a whole new world to them when it comes to lens comfort at the end of the day. When I first apply the lens I have customers telling me they think something is wrong, because they cannot feel anything in their eye.”
Since soft contact lenses were first introduced, there have been numerous incremental changes to materials to improve water retention and wearer comfort. When Ciba Vision introduced the first silicone hydrogel contact lens in 1998 it was the start of a new era in lens material technology that has brought enormous benefits in oxygen transmission to patients worldwide.
The creation of the first water gradient contact lens, featuring an increase from 33 per cent to over 80 per cent† water content from core to surface also promises to deliver a new era in comfort for contact lens wearers across the globe. The ultimate test for any contact lens is the real-world experience of eye care professionals trying it in their own practices on real customers. It is now up to you to judge for yourself. Mark Draper has a Bachelor of Optometry from the University of NSW and a Master’s Degree in Business Administration at Imperial College in London. He has worked in independent practice – with a special interest in contact lenses and ocular therapeutics – in Australia and the UK. He joined Ciba Vision in 2005 and is now the Global Manager of Dailies Total1 contact lenses at Alcon.
Dr. Inma Pérez-Gómez has a Bachelor of Optometry from the University of Catalonia, Spain, a Master Degree in Investigative Ophthalmology and Vision Sciences and a PhD from the University of Manchester. She is Head of Professional Affairs for Alcon in Europe, Middle East and Africa.
Dr. John Pruitt has a PhD in chemical engineering. He joined Ciba Vision in 2005 and has held several key roles in the development of Dailies contact lenses.
This article has been modified from an article which was originally published in Optician April 2013. It has been reproduced with kind permission from Optician magazine.
References:
1. DeNaeyer GW. Soft lens material choices and selection. Contact Lens Spectrum May 2012;27:38-40.
2. Tyler’s Quarterly December 2013 – Professional Edition
3. Rumpakis J. New data on contact lens dropouts: an alternative perspective. Review of Optometry 2010: 37-40.
4. ACLM Yearbook, 2011.
5. Alcon data on file, 2011.
6. Thekvali S et al. Structure-property relationship of Delfilcon A lenses. BCLA Annual Clinical Conference 2012.
7. Papas E and Willcox M. Reducing the consequences of hypoxia: the ocular redness response. Contact Lens Spectrum 2006:32-37.
8. Alcon data on file 2010. Based on the ratio of lens oxygen transmissibilities among daily disposable lenses.
9. Bonanno JA et al. Can variability in corneal metabolism explain the variability in corneal swelling? Eye & Contact Lens 2003;29(Suppl 1):S7-9; discussion S26-9, S192-4.
among daily disposable lenses.
10. Brennan NA. Contact lens-based correlates of soft lens wearing comfort. Opt Vis Sci Nov 2009: Abstract 90957.
11. Inoue K. Blinking and superficial punctate keratopathy in patients with diabetes mellitus. Eye 2005;19:418-421.
12. Alcon data on file 2011 & 2013. Based on the critical coefficient of friction measured by inclined plate method. Significance demonstrated at the 0.05 level.
13. Alcon data on file 2011. 100% lubricity maintained after one day of wear.
14. Alcon data on file 2011. In a randomised, subject-masked clinical study.
15. Alcon data on file 2011. In a clinical study with 80 patients.
16. Alcon data on file 2011. In 5 clinical studies with 236 patients.
17. Alcon data on file 2011. In a randomised, double-masked clinical study with 104 patients.
†In vitro measurement of unworn lenses, Alcon data on file, 2011.
‡Biofinity*, Biotrue ONEday*, 1-Day Acuvue* Moist*, Acuvue* Oasys*, MyDay* and Clariti* 1 Day.
*Trademarks are the property of their respective owners. Alcon Laboratories (Australia) Pty. Ltd. ABN 88 000 740 830 10/25 Frenchs Forest Rd East, Frenchs Forest NSW 2086. Phone: 1800 025 032. Dailies Total1® is a trademark of Novartis AG. 02/14 ALC0105 PCLC 14105
