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Glaucoma patients often exhibit an increase in their intraocular pressure (IOP) levels during their sleep, limiting the ability of clinicians to accurately diagnose this disease in many potential glaucoma patients. Several novel contact lens sensing devices that monitor IOP-related changes in an eye over 24 hours have been developed.
The Need to Monitor Intraocular Pressure
Glaucoma is an asymptomatic group of eye conditions that cause damage to the optic nerve. As the leading worldwide cause of irreversible blindness, glaucoma is primarily diagnosed through tonometric screenings for high intraocular pressure (IOP) in eye exams.
Although IOP screening can effectively reduce vision losses in up to half of all glaucoma patients, peak IOP often occurs at night during sleep, limiting the efficacy of detecting this condition during in-person office visits.
Since the early detection of glaucoma is directly related to the likelihood of preventing vision loss, it is crucial to obtain an accurate glaucoma diagnosis as soon as possible. Therefore, the limitations associated with monitoring IOP levels during wake hours have led to the generation of glaucoma screening methods that are capable of measuring IOP profiles 24 hours a day.
Non-Invasive IOP Sensors
IOP measurements are typically obtained through applanation, which involves the use of telemetric pressure sensors that are invasively implanted into the eye.
Comparatively, IOP can also be measured through non-invasive approaches, such as Contact Lens Tonometry (CLT), which is achieved by measuring the curvature of the patient’s corneas.
When using the CLT approach, high IOP levels cause the local strain in the contact lens to shift. Whereas this change in the local strain levels will be detected by the piezoresistive sensing element, any deformation to the cornea is instead sensed by a rigid metallic sensing element within the sensor. The metallic sensing element can only sense local deformation after being mechanically attenuated by the soft lens.
Chip-Less Contact Lens Sensor
To amplify and transmit any strain that has been detected by the contact lens sensor (CLS), a silicon chip has traditionally been used. Despite its usefulness for wireless signal transmission, the incorporation of an encapsulated chip into CLSs increases their total thickness to as much as 583 micrometers (µm). This added thickness is required to ensure the appropriate encapsulation of the chip into the sensor. However, it can also increase the mechanical attenuation of the sensor and ultimately degrade its strain sensitivity. For reference, the thickness of commercial contact lenses that are used to correct vision is typically within the range of 50 to 100 µm.
Several alternative sensing elements have been investigated for their potential to replace the silicon chip and ultimately reduce the thickness of an IOP CLS.
Although certain flexible non-metallic conductive film elements have been proposed and demonstrated enhanced sensitivity capabilities, their use in these sensors was not found to significantly reduce the thickness.
Another material that has recently been proposed as a replacement sensing element is a position-sensitive dyed glycerol-filled micro-chamber. As corneal curvature is detected, the dye within the micro-chamber moves with the curvature, creating a trackable movement that can be captured by a camera. Unfortunately, since the camera must be used when the patient is awake, this other material is not suitable for measuring peak IOP levels, as this typically occurs when the patient is asleep.
Over the past several years, researchers worldwide searched for solutions that could overcome many of the challenges associated with the early prototypes for IOP CLSs.
As of March 4, 2016, the United States Food and Drug Administration (FDA) approved clinical use of the Triggerfish CLS that has been developed by Swiss company Sensimed.
At a diameter of approximately 14.1 millimeters (mm) and a thickness of 585 µm, the Triggerfish CLS is a silicone soft CLS that can remain on the surface of the eyes for 24 continuous hours.
Within the Triggerfish CLS are two strain gauges, which detect any changes that arise in the shape of the cornea, as well as a microprocessor and an antenna.
The contact lens transmits all detected strain gauge information to the adhesive antenna, which powers the contact lens and is directly attached to the patient’s orbit.
All information received by the antenna is then sent to a portable recorder device worn by the patient. Every five minutes, the Triggerfish CLS obtains 300 strain gauge readings over a 30-second period. Over a 24-hour period, that equates to a total of 86,400 data points that can subsequently be analyzed by the treating clinician.
Although the Triggerfish CLS does not directly measure IOP, it provides the clinician with precise information on exactly what time of the day the patient’s strain gauge is at its peak.
References and Further Reading
Karunaratne, I. K., Lee, C. H. C., Or, P. W., et al. (2021). Wearable Dual-Element Intraocular Pressure Contact Lens Sensor. Sensors and Actuators A: Physical. doi:10.1016/j.sna.2021.112580
Chen, G., Chan, I., Leung, L. K. K., & Lam, D. C. C. (2014). Soft wearable contact lens sensor for continuous intraocular pressure monitoring. Medical Engineering & Physics 36(9); 1134-1139. doi:10.1016/j.medengphy.2014.06.005
Dunbar, G. E., Shen, B. Y., & Aref, A. A. (2017). The Sensimed Triggerfish contact lens sensor: efficacy, safety and patient perspectives. Clinical Ophthalmology 11; 875-882. doi:10.2147/OPTH.S109708
U.S. Food and Drug Administration. FDA permits marketing of device that senses optimal time to check patient’s eye pressure. [Online] Available at: https://www.fda.gov/news-events/press-announcements/fda-permits-marketing-device-senses-optimal-time-check-patients-eye-pressure