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Novel Wireless Sensor to Keep an Eye on the Diabetic Eye

Researchers have developed a wireless theranostic contact lens (WTCL) sensor for long-term and continuous tracking of IOP fluctuations in a real-time manner. Such smart sensors could benefit diabetics and other populations at high risk of glaucoma. In addition to sensing IOP changes, the device can also deliver anti-glaucoma drugs to the eye.

The link between diabetes and glaucoma

Glaucoma can cause irreversible optic nerve damage and loss of vision, with elevated intraocular pressures (IOP) due to abnormal circulation in the eye’s aqueous humor. There are two types of glaucoma: open-angle and closed-angle. Open-angle glaucoma is the most common type in which the IOP builds gradually and vision loss occurs slowly. Closed-angle glaucoma accounts for 10 percent of cases, in which symptoms occur suddenly.

Diabetic retinopathy, a complication of diabetes can increase the risk of open-angle glaucoma. The risk of diabetic retinopathy is higher among diabetics with increased age, poor control of blood glucose, and those with hypertension or high blood pressure. With diabetic retinopathy, changes in blood glucose levels can weaken and damage the retinal blood vessels This can eventually lead to glaucoma. Therefore, the Centers for Disease Control and Prevention (CDC) has stated that people with diabetes should get a diabetic retinopathy check-up every year.

WTCL sensor fabrication and working principle

The sensor uses a sensing modulus with the inductor-capacitor-resistor (LCR) circuit for IOP monitoring and multiple delivery moduli with the wireless power transfer (WPT) circuit for ocular drug delivery.

The sensor comprises a soft lens that can conformally interface with the cornea of the eye. It can efficiently deform to transduce corneal limbus expansion to the LCR circuit when IOP rises. The device also exerts external electrical and chemical stimulations locally on the cornea. The double-layer design of the lens provides compactness. It also incorporate multiple electric drug delivery moduli and the LCR circuit into the curved (and limited) surface of the lens. The linear range of the sensor was 5 to 50 mmHg, indicative of the wide breadth of IOP sensing by the device.

The air film sandwiched between the contact lens layers was combined with the LCR circuit into a cantilever configuration. This formed the transducer for detecting IOP changes and transmitting them in a wireless manner by producing signals at the resonant frequency. The zero viscoelasticity and ultra-low elasticity modulus of the air film facilitates ultrasensitive detection of IOP changes.

The LCR circuit has a snowflake-shaped design, in which six capacitive sensing plates are folded along with the reference plates to yield a cantilever structure. This design increases the sensitivity, reliability, and repeatability of the WTCL sensor. The corneal curvature deformation occurs when on IOP elevation and compresses the air film thickness. This results in capacitance rise (CSR) and decreases the LCR resonant frequency. Such changes can be recorded wirelessly by the reading coils of the integrated antenna of the WTCL sensor

The WPT circuit enables in situ delivery of anti-glaucoma drugs into the aqueous chamber via iontophoresis. This circuit has a flower-shaped design which enables optimal mechanical interlocking of the WPT circuit with the lower layer of the lens.

The sensing modulus is comprised of copper and has a nickel/aluminum coating. The delivery moduli comprise the same substances patterned into coils to form the WPT receiver along with an additional copper layer connected to the coils through holes. Capacitors are soldered onto the circuit for tuning the WPT frequency.

Ex vivo and in vivo performance of the WTCL sensor

The performance of the WTCL sensor was assessed ex vivo using porcine eyeballs. IOP was increased stepwise using saline injections and the WTCL resonance frequency at each IOP was recorded. The results showed that increased IOP was associated with decreased resonant frequency and that the sensor could detect IOP fluctuations sensitively and reliably.

The drug-delivery performance of the sensor was tested ex vivo by drop-casting a brimonidine-loaded hydrogel layer into the porcine eyes via iontophoresis. On wireless power transmission, the WPT receiver generated alternating voltages, which led to the electric migration of the positively charged brimonidine drug molecule into the aqueous chamber of the porcine eyes. The results showed that the sensor could effectively deliver anti-glaucoma drugs such as brimonidine.

The in vivo performance of the sensor was assessed using rabbits’ eyes. After incorporating the sensor, the researchers measured the IOP which was then compared to that by Tonopen (commercial tonometer). Additionally, brimonidine was delivered by the sensor and by eyedrops and the results were comparatively assessed. The analyses showed that the sensor could detect IOP changes rapidly.

The study was published on May 16, 2022, in the Nature Communications journal.

Author:

 Dr. Pooja Toshniwal Paharia is a Consultant Oral and Maxillofacial Physician and Radiologist, M.DS (Oral Medicine and Radiology) from Mumbai. She strongly believes in evidence-based radiodiagnosis and therapeutic regimens for benign, potentially malignant, or malignant lesions and conditions either arising from the oral and maxillofacial structures or manifesting in the associated regions. 

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