# Contact Lens-Based Microchannel Rings for Detecting Ocular Hypertension
## 1 Introduction  
Glaucoma is one of the main causes of permanent blindness, affecting over 65 million people globally. Almost half of the patients suffering from glaucoma do not recognize that they have this disease in the early stages [1, 2]. Increasing of intraocular pressure (IOP) is the main risk factor for developing glaucoma. In these cases, IOP exceeds 21 mmHg, and the individual is considered at risk of developing glaucoma disease [3]. Although the IOP fluctuates during the day in glaucoma patients, most of the eye-care clinicians depend mainly on one time or few IOP assessments within regular office hours. Recent studies have indicated that glaucoma patients experience the peak level of IOP at night, during out of office hours [3, 4, 5]. IOP was found to fluctuate over a 24-h period, recording average and maximum peaks of 4.9 and 12 mmHg, respectively [6, 7, 8, 9, 10]. Therefore, continuous detection of IOP is essential to monitor glaucoma progression, which is quite beneficial for prescribing the ideal treatment. In addition, continuous monitoring may assist opticians to evaluate the effectiveness of the medication.  
Goldmann explanation tonometry (GAT) has been the most known standard test for measuring intraocular pressure (IOP) since 1940 [11]. GAT measurements are based on law of Imbert-Fick. The law assumes that the eyeball is an ideal sphere and the cornea is thin and elastic [5, 12]. GAT works by flattening the cornea over a defined area with a diameter of 3.06 mm and IOP estimation is provided by the explanation pressure. However, the central thickness of the cornea, scleral rigidity, and mechanical properties of the cornea affect GAT performance [13, 14]. Currently, the most popular method for 24 h monitoring of IOPfluctuations is the diurnal tension curve (DTC), which records several IOP measurements at different times during working hours or in a sleep laboratory [15, 16]. Both GAT and DTC offer limited IOP values and are inconvenient and expensive. Patients are required to be awake during both approaches, which may cause stress issues [17]. In recent years, self-tonometry has been recommended; however, its use is a challenge for elderly patients and does not provide IOP measurements during sleep time [18]. In order to tackle these issues, the Swiss company SENSIMED Triggerfish(r) has developed a contact lens sensor to continuously detect IOP changes [19]. The sensor is designed based on the idea of that IOP fluctuations induce changes in the cornea's curvature [20]. The sensor involves a soft silicone contact lens embedded with two pressure gauges, antenna, and a microprocessor. The gauges monitor the modifications in the cornea's shape to record IOP values. It is a safe and well-tolerated device for 24 h measuring of IOP for healthy and glaucomatous users. Although the device is minimally invasive, safe, and has limited incompliance, it has some side effects such as blurry vision as the contact lens has no vision correction option [19, 21]. Other reported side effects include dry eye, eye irritation, and the eye-feels scratchy.  
Here, commercial contact lenses were laser-processed to allow for detecting the intraocular pressure (IOP). A ring-couple was engraved on the surface of commercial contact lenses (ACUVUE TrueEye) using a carbon dioxide laser. Fabrication of the rings on the contact lenses was carried out by using a simple, easy, rapid, cost-effective, and applicable mass-production method. The interspace between the ring-couples changed upon IOP fluctuations and was detected by a smartphone camera.