polymers (methylcelluloses) as emulsifiers; but Optive nice

[Rojzen]

The polymer-based products, like much of the Refresh line, have never been irritating to me, but I noted, early on, that they made my "menthol sensation" keener and more problematic. . .For me, that means tear break-up time has been accelerated. . .alas. . .

I later learned that methycellulose compounds are often used as emulsifiers. . .

I understand emulsifiers to be substances that can separate or break down fatty or oily substances. . .Could this mean that the polymer/methylcellulose in so many artificials tears could actually be breaking down the lipid layer that many of us need to build up?

Anway. . .my humble theory is that polymer products are likely to bring subjective relief to aqueous dry eye patients whose lipid films are THICKENED due to lack of fluid . . .but are, conversely, likely to speed up evaporation time for people with thinned/abnormal lipid films. . .

Still, I am eager to try Dakrina, Dwelle, and Nutratears. . .even if they contain polymers. . .

Moreover, Optive, which contains polymer, has not sped up my tear-break-up time, I suspect, since they feel benign, in my lipid-starved eyes. . .and so I clearly have much to learn . . .

[neil0502]

The polymer-based products, like much of the Refresh line, have never been irritating to me, but I noted, early on, that they made my "menthol sensation" keener and more problematic. . .For me, that means tear break-up time has been accelerated. . .alas. . .

I later learned that methycellulose compounds are often used as emulsifiers. . .

I understand emulsifiers to be substances that can separate or break down fatty or oily substances. . .Could this mean that the polymer/methylcellulose in so many artificials tears could actually be breaking down the lipid layer that many of us need to build up?

I'm surely no chemist, but I'm not sure I'd be quick to agree with your interpretation of the term "emulsifier" in this context. To cite Wikipedia:

An emulsifier (also known as an emulgent or surfactant) is a substance which stabilizes an emulsion. Examples of food emulsifiers are egg yolk (where the main emulsifying chemical is the phospholipid lecithin), and mustard, where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers; proteins and low-molecular weight emulsifiers are common as well. In some cases, particles can stabilise emulsions as well through a mechanism called Pickering stabilization. Both mayonnaise(willhuang) and hollandaise sauce are oil-in-water emulsions stabilized with egg yolk lecithin. Detergents are another class of surfactant, and will chemically interact with both oil and water, thus stabilising the interface between oil or water droplets in suspension. This principle is exploited in soap to remove grease for the purpose of cleaning. A wide variety of emulsifiers are used in pharmacy to prepare emulsions such as creams and lotions.

Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and on the type of emulsifier. Generally, the Bancroft rule applies: emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well; for example, proteins dissolve better in water than in oil and so tend to form oil-in-water emulsions (that is they promote the dispersion of oil droplets throughout a continuous phase of water).

I think the emulsification here is to promote the mixing of oil (lipid layer) and water (aqueous layer) of the tear film. Maybe somebody (Tony Barnes? One of the docs??) can weigh in on this....

[Tony Barnes]

To keep the difference phases of the tear film in check, they need to intereact with each other. Polar phospholipids are key in the lipid layer, as one end is water soluble (the phosphate) whilst the other is fat soluble (the fatty tail). This makes the molecule ampiphilic i.e. both "water loving" and "water hating".

Methylcellulose is a hydrophilic compound - i.e. it likes water. When combined with water it forms a gel structure. This is an emulsification role, yes.

Whilst it won't directly "break down" your lipid layer to my knowledge, it will do nothing to repair it, and it won't interact with it. As such you will have the cellulose gel trying to keep water in the eye.

For non-evaporative dry eye sufferers this may work. For evaporative dry eye, the same problem will continue, as the water will just evaporate from this gel instead (albeit it a probably a slightly reduced rate... hmm, unless the gel structure actually increases curface area for evaporation... dunno!!).

Triglyceride gels like Liposic look to repair the lipid layer, but fail to recognise the significance of the polarity. The lipid layer itself has 2 layers - a polar and a non-polar layer. The polar layer of ampiphilic molecules like phospholipids is what maintains the integrity against the aqueous layer. Triglycerides treatments enter the non-polar region, and as such don't manage the evaporation in the same way.

Hope enough of that made sense!

[Rojzen]

Liposic

For Tony -
Brilliant and extremely thoughtful presentation on the nature of the lipid layer, and on how the methylcellulose products work (though not to improve lipid composition/texture). . .Is Liposic, the triglyceride you mentioned, an opthalmic preparation? And do you follow the work of those researchers who have been trying, for so long, to mimic human meibum, towards a replacement therapy? Soothe was produced by one of these. . .I participated in a clinical trial of the phospholipid in Soothe. . .and personally found it unhelpful. . .Then, when the Soothe product added mineral oil to the mix, all hopes were dashed. . .Ugh. . .Anyway, do you use this Liposic now? and is it an OTC?
Rojzen

[Tony Barnes]

Hi Rojzen,

No worries - glad it made enough sense!

I'm not actually a perticular DE sufferer, I only get it when I've been drinking too much...

I work for a company who imports Clarymist (known as Tears Again in other countries), so need to know about how it works!

Liposic is a preperation that is used over here, though I'm not certain on how common it is?

It sounds like that Soothe product was trying to achieve something similar to Clarymist - a phospholipid approach. Clarymist only seems to work as well as it does because the phospholipids are in liposomal format (in any phospholipid solution this will happen to a degree I believe, but it is the stability and total amount of liposomes that is relevant) - hence why it's used on closed eyes, as it will work it's way to the problem area.

This sort of approach works for around 80% of DE sufferers - the other 20% have problems with tear production/clearance, and evaporation is not an issue for them. It's also a culmative effect - the longer it is used, the less it needs to be as the lipid layer starst to retain more stability.