When trying to get thick tomato ketchup out of a bottle we often give it a good shake or a squeeze. This action causes the sauce to become runnier and allows it to leave the bottle more easily – although the change is not permanent.
The change in viscosity observed from high (thick) to low (flowing) is seen when you apply force to the ketchup, this is because ketchup is a non-Newtonian fluid – just like our natural tears. This term is used in the world of physics to describe substances which change their viscosity/structure when pressure is applied or released, the changes are reversable. Ketchup gets thinner when force is applied so is also known as a ‘shear thinning fluid’ – there are ‘shear thickening fluids’ too.
But what has this got to do with advanced dry eye drop technology?
Longer lasting relief
One of the biggest challenges for scientists developing dry eye drops is to formulate a solution which will stay in the eye for a longer period of time to give longer lasting symptom relief. This way someone suffering from more severe dry eye symptoms does not have to instil eye drops too many times during the day.
One way of addressing this has been to make the eye drops thicker or more viscous. This increases the time the drop stays in the eye, but with increased viscosity also comes the down side of more visual blurring. In fact, the very thickest or most viscous eye preparations are usually only recommended for night-time use due to the level of visual disturbance.
Non-Newtonian fluids to the rescue
Imagine, if you could have a more viscous eye drop which stayed in the eye but every time you blinked this force made the solution became thinner, spreading it over the surface of the eye to give greater lubrication and relief with less blur. AND when you open your eye again, removing the force, the viscosity once more increases to aid longer retention. This is pretty much what your natural tear film is doing throughout the day to protect your eyes and stay in place despite the ‘drag’ of our eye lids during blinking. The use of non-Newtonian fluids means that artificial tears can now get closer to the real thing!
These non-Newtonian properties are seen in advanced dry eye drop ingredients such as sodium hyaluronate or hyaluronic acid (HA) - this is why HA works so well as a natural lubricant , for example in our joints. Another example is tamarind seed polysaccharide (TSP) which because of these properties also provides ocular surface protection to help maintain comfort when blinking. Dry eye drops containing HA and TSP have been developed for the optimal balance between longer retention and minimal blurring. In fact some cleverly formulated eye drops combine these two ingredients to create a unique high performance bi-polymer.
What is happening to the sodium hyaluronate when it is in your eye?
HA molecules come in varying lengths, but typically when free from shearing forces (in between blinks) they form a mesh-like structure. The introduction of the shearing force (the eye lid moving over it during a blink) causes the molecules to become less viscous - spreading out and forming a lubricating shield over the surface of the eye. In between blinks the HA’s more viscous structure returns. Clever!
You can find more information about what else to look for in an eye drop and more reasons to choose sodium hyaluronate at Dry Eye Zone.
And don’t forget to visit Dry Eye Zone and subscribe to the regular free information updates.
If you are experiencing dry eye symptoms then you should ask the advice of your eye care professional. Why not book your next eye examination with an independent optician today.
Sources:
Lyndon Jones et al. TFOS DEWS II Management and Therapy Report. The Ocular Surface. http://dx.doi.org/10.1016/j.jtos.2017.05.006
Martin Pisárčik, Dušan Bakoš, Michal Čeppan,Non-Newtonian properties of hyaluronic acid aqueous solution,
Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 97, Issue 3,1995,Pages 197-202, ISSN 0927-7757, https://doi.org/10.1016/0927-7757(95)03097-W.
Properties of sodium hyaluronate. https://trbchemedica.co.uk/pages/hyaluronic-acid#:~:text=Hyaluronan%20solutions%20present%20non%2DNewtonian,is%20applied%20to%20hyaluronan%20solutions.
Hynnekleiv L, Magno M, Vernhardsdottir RR, Moschowits E, Tønseth KA, Dartt DA, Vehof J, Utheim TP. Hyaluronic acid in the treatment of dry eye disease. Acta Ophthalmol. 2022 Dec;100(8):844-860. doi: 10.1111/aos.15159. Epub 2022 May 5. PMID: 35514082; PMCID: PMC9790727.
Gouveia SM, Tiffany JM. Human tear viscosity: an interactive role for proteins and lipids. Biochim Biophys Acta. 2005 Dec 1;1753(2):155-63. doi: 10.1016/j.bbapap.2005.08.023. Epub 2005 Sep 21. PMID: 16236563.
Abusharha A, Pearce EI, Afsar T, Razak S. Protecting Tear-Film Stability under Adverse Environmental Conditions Using a Mucomimetic with a Non-Newtonian Viscosity Agent. Medicina (Kaunas). 2023 Oct 19;59(10):1862. doi: 10.3390/medicina59101862. PMID: 37893580; PMCID: PMC10608101. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10608101/
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