New research published in Physics of Fluids assessed human tears at a micron level to determine a potential new way to customize artificial tears for addressing specific symptoms of dry eye disease (DED).
So what’s the difference between these tears?
Artificial tears are a synthetic formulation customarily used to supplement a person’s naturally-produced tears for increased eye lubrication by mimicking characteristics of the tear film.
Human tears, on the other hand, are significantly more complex liquids with components such as lipids, carbohydrates, proteins, salt, immunological molecules (secretory immunoglobulin A and G [IgA, IgG]), antimicrobial molecules (lysozyme, lactoferrin, etc), and water.
To note, the only similarity between the two is their water content.
Gotcha. So why assess them?
Aside from customizing artificial tears for DED, researchers were also looking to gain a more comprehensive understanding of human tear composition and how the tears’ behavior might also be used to study other ocular pathogens and biological fluids.
Now talk about this research.
Investigators collected healthy human tears and tested 10 different artificial tear formulations to determine properties like:
- Viscosity
- Elasticity
- Stability
- Effects of different concentrations of components in the liquids
- Behavior of liquids under stress (eye blinking)
Expand on these artificial tears that were used.
All artificial tears included in the study were commercial formulations with water and hyaluronic acid (HA) as the main components (along with inorganic salts, stabilizers, buffers, and preservatives), with biopolymer at a 1.0 to 4.0 mg/mL concentration range.
Click here to see the complete list of commercial brands, which include Lubristil Intense, Vismed, and Eyestil PF.
And the human tears?
Multiple sessions were conducted to collect tears from healthy volunteers, all while minimizing cell contamination and reflex tearing.
The tear samples were stored at -80°C (providing rapid freezing and preservation of compounds) until extracted for study.
How were tears tested?
Researchers used microrheology methods to monitor the movement of micron-sized particles in liquids via dynamic light scattering (DLS)—a technique that “measures how light reflects off particles suspended in liquid to reveal how the liquid behaves in different scenarios.”
And the findings?
The artificial tears’ viscoelastic properties, which include Newtonian viscosity and relaxation time, were scaled to the concentration and macromolecular size of HA and resembled “unentangled semidiluted solutions,'' according to the study authors.
To note, semidiluted solutions are typically characterized by large fluctuations in concentration.
What about human tears?
The viscosity of human tears was found to be 50% greater than that of pure water—comparable to artificial tears containing 0.1% HA—but with a relaxation time (which is the time required for a viscous substance to recover from stress) that was one order of magnitude longer than ophthalmic solutions.
The study authors reported this to be due to the tears’ intricate composition.
Author input?
Per the study authors, the data suggests that tear viscosity is not solely dependent on the level of high-molecular weight mucins (glycosylated protein).
Further, tear lipocalin (a tear major protein) was found to significantly contribute to tear viscosity via “hetero-protein interactions involving lipid-binding-induced structural changes.”
And why is this significant?
Because these findings challenge today’s theories of tear structure and performance, the study group stressed the need for further research.
Take home.
The authors noted that, with the use of an extremely small sample size and microrheology to measure human tears’ properties, “these findings are of significant value as they pave the way for future research on small volumes of tears from ophthalmic patients.”