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Stuck Wading Through the Wave of Artificial Tears? Try Surfing Through Rheology!

(I-MED Pharma)

One of the first steps in managing dry eye disease, a multifactorial condition of the ocular surface, is with tear replacement therapy through the use of artificial tears.[1] The aim is to supplement an unhealthy or unstable tear film to protect and coat the ocular surface in order to maintain visual acuity and eye health. While there are a myriad of eye drops to choose from, each boasting different ingredients and formulations, determining which drop is best for your patients’ needs can be challenging. One approach to take when deciding the best artificial tear to use for meibomian and lacrimal gland supplementation is through scientific analysis, specifically through the branch of science called rheology. This post will demystify the challenges in choosing the ideal artificial tear by explaining how the application of rheology can simplify the best drop choice for your patients’ dry eye needs.

To begin, what is rheology and how does it apply to eye drops?

Rheology is the study of flow and deformation of matter under the influence of external forces, especially at the flow behaviour of non-Newtonian liquids and their properties.[2] Since rheology explains how materials deform and flow under the influence of external forces, the science can be used to understand the changes that occur to the tear film from eyelid motions, specifically from blinking.[3] While the best artificial tears attempt to mimic natural, healthy tears as closely as possible,[4] comparing the rheology of natural tears to specific artificial tears shows which formulations more closely imitate the natural tear film’s fluidic behaviours.

Newtonian vs non-Newtonian

First, a definition of a Newtonian fluid is needed, which is a liquid whose viscosity is not affected by the application of external forces and which lacks viscoelastic (as in having both viscous and elastic characteristics when being deformed) properties.[5] This means that the Newtonian liquid has a constant viscosity independent of stress. A classic example of a Newtonian fluid is water. Now a non-Newtonian fluid has more complex rheological properties since an application of external forces results in changes in the liquid’s viscosity.[6] In other words, the fluid’s viscosity changes when under force and the fluid become more liquid (called shear thinning) or more solid (called shear thickening). Human tears are shear thinning fluids, meaning the viscosity decreases as the shear rate increases.

Put another way, tears become more liquid as there is more force placed on it (i.e., from blinking). Many synthetic and natural liquids are non-Newtonian, with human tears, paint, and ketchup, being examples of shear-thinning non-Newtonian fluids.

What do tears do?

Tears serve an important purpose, and a healthy tear film is a vital component in the ocular system. The tear film must effectively spread across the surface of the eye to keep out dust, debris, and bacteria, while also focusing our vision.[7] In terms of rheology, human tears (which are non-Newtonian shear thinning fluids) prolong the contact time on open eyes and decreases viscosity during the blink cycle in order to protect the ocular surface.[8] The quality and quantity of our tear film has a direct impact on both our level of comfort and quality of vision.

The ideal artificial tear

Artificial tears can be categorized into two major rheological categories: Newtonian (behaving more like water) and non-Newtonian (behaving more like natural human tears).[9] Artificial tears that are non-Newtonian have a high viscosity when the eye is open in order to resist drainage and tear film break-up, creating a stable fluid ideally without producing any visual blurriness. While blinking, the eyelid puts increasing stress on the tear film, and the artificial tears reduce viscosity (become more liquid) in order to spread over the ocular surface and avoid damaging the epithelial surface. Since non-Newtonian artificial tears mimic the rheology of healthy, natural human tears, they should provide better comfort and longer-lasting symptomatic relief for dry eye disease.[10]

How the I-DROP® line of products can suit every dry eye need

In a recent article publication which measured and sorted artificial tears based on their rheology, I-DROP® MGD and I-DROP® PUR GEL were found to exhibit significant non-Newtonian shear thinning behaviour.[11] What does that mean in terms of dry eye management and relief? That in addition to showing rheological behaviours which mimic the rheology of natural tears, I-DROP® MGD, with its inclusion of superior osmoprotectants and bioprotectants, is truly the most advanced eye drop for hydrating and lubricating the cornea, making it ideal for relieving symptoms associated with chronic evaporation.

I-DROP® PUR GEL, also exhibiting significant non-Newtonian shear thinning behaviour, is recommended for moderate to severe chronic dry eye due to its moisturizing, lubricating, and non-blurring attributes as well as its rheological properties.

Finally, I-DROP® PUR, which exhibits moderate non-Newtonian shear thinning behaviour, is recommended for patients with mild to moderate chronic dry eye disease and is also contact lens compatible.

While there are many considerations in choosing the right eye drop for your patients, looking at the rheology of an artificial tear can be eye opening. I-MED Pharma is committed to advancing the science of dry eye and providing doctors and patients with superior quality products in the fight against the prevalent and chronic symptoms associated with dry eye disease.

[1] Lemp, Michael A. “The Definition and Classification of Dry Eye Disease: Report of the Definition and Classification Subcommittee of the International Dry Eye Workshop.” The Ocular Surface 5, no. 2 (2007): 75-92.

[2] “Rheology.” A Dictionary of Mechanical Engineering, 2nd ed.. Oxford University Press (2019).

[3] Arshinoff, Steve, Ilan Hofmann, and Hemi Nae. “Role of rheology in tears and artificial tears.” Journal of Cataract and Refractive Surgery Published Ahead of Print (2020).

[4] Arshinoff, Hofmann, and Nae,, “Role of rheology,”.

[5] Yañez-Soto, Bernardo, Mark Mannis, Ivan Schwab, Jennifer Li, Brian Leonard, Nicholas Abbott, and Christopher Murphy. “Interfacial Phenomena and the Ocular Surface.” The Ocular Surface 12, no. 3 (2014): 178-201.

[6] “Non-Newtonian Fluid.” A Dictionary of Mechanical Engineering, 2nd ed., Oxford University Press (2019).

[7] Arshinoff, Hofmann, and Nae, “Role of rheology,”.

[8] Arshinoff, Hofmann, and Nae, “Role of rheology,”.

[9] Arshinoff, Hofmann, and Nae, “Role of rheology,”.

[10] Arshinoff, Steve, Ilan Hofmann, and Hemi Nae. “Rheological behavior of commercial artificial tear solutions.” Journal of Cataract and Refractive Surgery Published Ahead of Print (2020).

[11] Arshinoff, Hofmann, and Nae, “Rheological behavior,”.