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The Visual Environment: Measurement and Design
Published in R. S. Bridger, Introduction to Human Factors and Ergonomics, 2017
Blinking is a reflex action that occurs every 2–10 s. It is also a voluntary, forced closure of the eye. The function of blinking is to stimulate tear production and flush out foreign objects (such as dust particles) from the surface of the eye. Tears, a dilute saline solution, lubricate eye movements and are mildly bactericidal. The eye is covered with a precorneal tear film (PTF) that protects it from the outer environment (Wolkoff et al., 2004). The PTF consists of an outer lipid layer covering a water layer, below which is a layer of mucin. Eye blinking and tear production are essential for the maintenance of the PTF. Blinking is divided into three categories: Voluntary blinking—a conscious decision to close the eyesInvoluntary blinking—at a frequency of approximately 12–20 blinks/min in subjects at restReflex blinking—in response to mainly visual and ocular stimuli
Unprecedented Innovations in Electrochemical Biosensing Approaches for Medical Applications
Published in Sibel A. Ozkan, Bengi Uslu, Mustafa Kemal Sezgintürk, Biosensors, 2023
Susana Campuzano, María Pedrero, Maria Gamella, Rebeca M. Torrente-Rodríguez, José M. Pingarrón
Tears are a biological fluid that can be used for monitoring physiological status since biomarkers diffuse from the blood and their levels can be correlated with concentrations in blood (64–65). Human tears are a complex fluid containing a variety of compounds such as proteins, peptides, lipids, metabolites, enzymes, electrolytes, and water (66). However, it contains fewer proteins than other biofluids due to the blood-tear barrier filtering process, which makes tears an attractive candidate fluid for non-invasive monitoring since sensor surface biofouling may be lower (45).
Computer and Human Vision Systems
Published in Sheila Anand, L. Priya, A Guide for Machine Vision in Quality Control, 2019
The next layer that protects the eye are the eyelids, which are movable folds of tissues. There are several muscles that help the eye to blink and help to keep our eyes open. When we blink our eye, tears are secreted, and the eyelids help to spread the tears evenly over the surface of the eye to keep it moist and lubricated. A simplified inner structure of the eye is shown in Figure 1.2.
A review of medical wearables: materials, power sources, sensors, and manufacturing aspects of human wearable technologies
Published in Journal of Medical Engineering & Technology, 2023
Mohammad Y. Al-Daraghmeh, Richard T. Stone
Tears are a complex extracellular fluid encapsulating the surface epithelial cells and forming the anterior component of the ocular surface. The tear fluid has earned significant recent consideration as a promising non-invasive body fluid for continuous monitoring of glucose, lactate, alcohol, intraocular pressure, vitamins, and other desired target molecules. Sempionatto et al. [98]. Contact lenses are a suitable and safe procedure to collect tears that are in contact directly with the basal tears [113]. For example, [114] developed a multifunctional, transparent, stretchable, and contact lens sensor based on a graphene-based Ag nanowire hybrid sensor to monitor glucose within tears, as well as intraocular pressure. Park et al. [115] reported a soft, stretchable, transparent, and smart contact lens occupied with glucose sensors and LED pixels to monitor and visualise the glucose concentration in tears. Ku et al. [116] described a soft, smart contact lens sensor based on graphene field-effect transistor for wireless real-time detection of the cortisol concentration in tears. And [117] fabricated a wearable contact lens intraocular pressure sensor based on a platinum (Pt) strain gauge by the micro-electro-mechanical (MEMS) process for continuous intraocular pressure monitoring.
Tear osmolarity is sensitive to exercise-induced fluid loss but is not associated with common hydration measures in a field setting
Published in Journal of Sports Sciences, 2018
Justin J. Holland, Michelle Ray, Christopher Irwin, Tina L. Skinner, Michael Leveritt, Ben Desbrow
An innovative method to assess hydration status in the field may lie in the measurement of tear osmolarity (Tosm) (Fortes et al., 2011; Sollanek et al., 2012). Various methods have been trialled to capture and analyse tear fluid, primarily for the diagnosis of dry eye disease. The most common method is the Schirmer test, which uses absorptive paper or glass capillary tubes to collect tear film for the diagnosis of dry-eye disease (Esmaeelpour, Cai, Watts, Boulton, & Murphy, 2008). Collection times, inadequate sample volume (≥1µL) and evaporation have the potential to impact Tosm. In combination, these variables may produce an intra-subject variability of Tosm to the degree of 9.5 ± 2.4% (Benjamin & Hill, 1983) when assessed by the typical freezing point and vapour pressure osmometry (Stahl, Willcox, & Stapleton, 2012). More importantly, the method in which tear film is collected and the use of laboratory osmometers is not practical in the field. Tosm as assessed by the TearLabTM system is a relatively new, non-invasive method of hydration assessment that requires only (50 nL) of tear fluid in comparison to traditional assessment which requires ≥1 µL. Unlike the traditional methods, the TearLabTM system can provide an output within 10 s of collection, reducing the potential for evaporation and contamination. Given the safety, speed, limited invasiveness and acceptable analytical variation (0.98%) of the TearLabTM system, it may prove ideal for practical field-based hydration assessment over absorptive paper and glass capillary tubes that may irritate and disturb the ocular surface.
Comparative assessment of blood glucose monitoring techniques: a review
Published in Journal of Medical Engineering & Technology, 2023
Nivad Ahmadian, Annamalai Manickavasagan, Amanat Ali
Tears are vital bio-fluid that serve the eyes as sanitiser and lubricant. Although the correlation of tears with the blood takes place through the blood-tear barrier, tears contain unique biomarkers, including glucose, proteins (enzymes and antibodies), sodium, potassium, and chloride ions [43]. Changes in tear composition can reveal possible diseases and allergies such as ocular rosacea, dry eye (xerophthalmia), glaucoma, and diabetes [44–47]. Lens-type sensors are attractive biosensors due to their easy attachment to the pupil, easy maintenance, flexibility, transparency, and continuous real-time monitoring [43].