Contact lenses are primarily used to correct eyesight issues such as myopia (nearsightedness) and hyperopia (farsightedness), but have you ever wondered if they have any further uses beyond correcting eyesight? Intraocular pressure (IOP) is the fluid pressure inside of the eye, and measuring IOP is a routine task in ophthalmic medicine, however despite the importance of these measurements, there remains no way for patients to monitor their IOP using on-demand products. With that being known, there is an increasing demand for on-demand IOP monitoring. To address this concern, a group of researchers from Nanjing Univeristy in China sought to develop a hydrogel-based smart contact lens for highly sensitive personal IOP monitoring.
If you recall back to the discussion of artificial muscles and IUPAC’s top 10 emerging technologies in chemistry we had two articles ago, these contact lenses happen to fall into another one of the other top 10 emerging technologies - wearable sensors.
Developing contact lenses that function as biosensors is difficult, because factors such as flexibility, view window, hydrophilicity, and oxygen permeability all need to be considered in order to make these contacts consumer safe. With all of this in mind, the Xu Group created an on-demand IOP measurement system using a hydrogel with poly(2-hydroxyethyl methacrylate) (pHEMA) (Figure 1) as the primary polymer for the contact lenses.
Figure 1: Structure of poly(2-hydroxyethyl methacrylate) (pHEMA)
The decision to use pHEMA as the polymer in the hydrogel seemed ideal, since like many methacrylate-based polymers pHEMA is highly tissue compatible and is the most widely used commercial contact lens material; however, the issue with pHEMA is that it is difficult to integrate sensors directly into the hydrogel, so they had to resort to creating the IOP device scaffold on the exterior surface.
The external system on the contacts is a complex sensor that consists of three unique layers integrated with micropyramid elastomers that allow for the measurement of IOP. When the IOP increases, the cornea of the eye expands which in turn elongates and stretches the pHEMA hydrogel layer of the contact lens. The amount of stretching of the hydrogel is then detected by the sensor and is converted into a signal that can be picked up by a nearby computer.
Figure 2: Layout of the structure of the smart contact lens. It consists of a micro pyramid elastomer integrated into three unique layers.
In order to improve the portability of this device, they developed a pair of glasses that have a tunable reader for remote signal measurement that allows users to monitor their IOP on the go.
Figure 3: This an example of the respective pair of glasses that are embedded with a small computer chip and can be worn with the smart contact lenses.
After testing their device on a porcine (pig) model, their results found that they were successfully able to develop and integrate IOP-monitoring sensors in order to create a smart contact lens. The device was found to have great sensitivity, as well as reliability. Sensitivity for this device is especially important as the hydrogel deformation is microscopic, expanding or contracting only 0.03% per change in mmHg, the measured unit of IOP. Regarding reliability, the mean deviation and standard deviation between measurements were 0.30 and 0.77 mmHg, respectively, which showcases the excellent reliability this device had. It was also found that the computer-embedded glasses functioned best at 14 mm from the contact lens, making the glasses a convenient way to analyze personal IOP.
The excellent performance of this smart contact lens has the potential to bridge the gap between clinical and personal IOP measurements, and given the importance of self-monitoring IOP, this device shows hope towards the next-generation of devices that promote daily ocular health management.
The finding of this work has been published in ACS Sensors: Zhu, H.; Yang, H.; Zhan, L.; Chen, Y.; Wang, J.; Xu, F. Hydrogel-Based Smart Contact Lens for Highly Sensitive Wireless Intraocular Pressure Monitoring. ACS Sens. 2022, 7 (10), 3014-3022.
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