8 The outer lipid layer can maintain the thickness of the film by reducing the rate of tear evaporation. Only with these structures can the tear film be firmly attached to the eye surface. Tear secretion is a carefully controlled process [4]. When sensory afferent nerves of the cornea and conjunctiva detect dryness and irritants [5], they will signal the efferent parasympathetic and sympathetic nerves connected to the lacrimal gland (Figure 1), to induce secretion of electrolytes, water, and proteins to the eye surface [4]. Notably, the sensory input can be modulated by the lacrimal nucleus of the brain, which integrates input from other centers as well, including emotional input, to produce a graded output. A stronger integrated input can induce the secretion of a greater volume of tear by the lacrimal gland. This can explain why tears overflow during emotional episodes, or in response to environmental irritants to flush away deleterious substances. In fact, low levels of nerve stimulation are already enough to produce basal tear to maintain the normal thickness of the tear film. Artificial Tears The uncomfortable sensations of eye dryness can be distressing. Common causes of dry eye include eye strain from prolonged computer use, specific medical conditions, and exposure to smoky or windy environments [6]. To alleviate this discomfort, lubricating eye drops, often referred to as artificial tears, can be beneficial [7]. Most artificial tears consist of aqueous solutions with thickeners such as carboxymethyl cellulose, hyaluronic acid, hydroxypropyl guar, and polyethene glycol to enhance lubrication and prolong their stay on the eye. Natural tears are a non-Newtonian fluid whose viscosity temporarily reduces during each blink to protect the eye surface. Because of the resemblance to natural tears in terms of physical properties, hyaluronic acid is now under extensive research as a promising viscosityenhancing agent. Other ingredients of artificial tears include electrolytes, pH buffers, antioxidants, and preservatives. It is also worth noting that such aqueous-based artificial tears work by replenishing the aqueous layer of the tear film. However, lipid-based drops also become increasingly common as they can target the outer lipid layer, relieving dry eye symptoms in individuals whose meibomian gland (Figure 1) cannot properly secrete lipids to maintain the layer [7]. A Cry for Help: Tears in Disease Screening and Health Analysis As a peripheral body fluid that can be collected in an easy and non-invasive manner, tears have been studied for their potential use in disease screening. By analyzing tear composition, it could be possible to diagnose a disease by quantifying certain biomarkers, Figure 1 Lacrimal gland and meibomian glands.
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