INTRODUCTION

The concept of a contact lens was first attributed to Leonardo da Vinci in 1508, but the first wearable contact lens to correct vision was only developed in 1888 by Adolf Fick. There have been many advances in contact lens designs and materials since the original glass lens that covered the entire eye. Evolution of materials changed from polymethyl methacrylate (PMMA) to gas permeable (GP) materials, while the early 1960’s heralded a change with the introduction of soft contact lenses using hydrogel materials. Silicone hydrogel materials were launched in the 1990’s, and currently enjoy popularity owing to its comparative advantages of better oxygen transmissibility and modulus properties. Traditionally contact lenses were used to correct refractive errors, and for therapeutic or cosmetic purposes. However, there are some novel uses and interesting proposals for future development which will be described in this article.

1. MYOPIA CONTROL

It is predicted that 49% of the global population will have myopia by 2050, seven and half times more than in 2000. There are concerns about the implications of this and much research is being conducted in this field. Two methods involving contact lenses have been proposed in myopia control:

i) Centre distance multifocal contact lenses

Studies have shown that use of this design of contact lens in young myopes can halt or slow progression of myopia by about forty-eight percent.

ii) Corneal Reshaping

The United States, Food and Drug Administration (FDA), approved orthokeratology for up to -6.00 dioptres of myopia. Modern orthokeratology technology uses RGP materials and custom-made reverse geometry contact lens designs to temporarily modify the curvature of the cornea. This allows for more significant and rapid central corneal flattening to correct myopia and control its progression. Some studies have shown that orthokeratology can reduce myopia progression by forty to forty-five percent.

2. CORNEAL REPAIR

Corneal health is dependent on limbal stem cells and a deficiency thereof affects the corneal healing process. Researchers have been able to cultivate a patient’s stem cells on the concave surface of a silicone hydrogel contact lens for use in patients with such a deficiency. Using limbal stem cells from the superior limbus, along with the patient’s blood serum, it takes approximately sixteen days for the limbal cells to colonise and cover the concave surface of the lens. After removal of any scar tissue, this is then transferred to the patient’s cornea.

3. DRUG DELIVERY

Topical ophthalmic solutions are currently a commonly used method of ocular drug distribution. Administered by pulse delivery, it is characterised by a transient initial overdose followed by a relatively short period of effective therapeutic concentration, and thereafter a prolonged period of an insufficient dose concentration. Each drop is diluted and washed away by reflex tearing and an amount is either spilled over the eyelids or drained through the nasolacrimal system. The cornea only absorbs a small amount of the topical dose due to its surface area and its short contact time with topical drops. Patient compliance is of concern with ophthalmic drops with the rate of noncompliance in patients with glaucoma estimated to be between twenty-four and fifty-nine percent, even in long-term users.

The design criteria for a drug eluting contact lens include comfort, biocompatibility, and ideally, zero-order kinetics (that is, the release of a constant amount of drug per day) for an extended period.

Research has centred on the use of colloidal nanoparticles to create imprinted pockets in the lens structure that provides the lens with a high affinity for the drug. This is achieved by a process of molecular imprinting in which sub-micron-sized particles, either coated with, or encapsulating a drug’s molecules,  is used in the modification of the contact lens material during manufacture .

Researchers have developed a biodegradable polymer nanowafer, designed to dissolve in the eye while releasing safe, sustained doses of a drug. Currently in prototype stage is a contact lens that dispenses latanoprost for patients with glaucoma. Drug eluting lenses that are in clinical trial stage include a lens with ketotifen for patients with allergic conjunctivitis and containing econazole, which is used to treat fungal keratitis and another with alginic acid for contact lens wearers with severe dry eye condition.

4. SELF-ADJUSTING CONTACT LENSES FOR PRESBYOPIA

Two contact lenses have been described to correct presbyopia:

i) Electronic contact lenses

These have been designed, using liquid crystal technology, such that the vertex power is adjustable. The variation in optical power is triggered by a change in the refractive index of the liquid crystal, induced by an applied voltage possibly using wireless technology.

ii) Auto-focus on distance and near

A prototype contact lens is being developed in which researchers have drawn inspiration from the retina of the tropical elephant nose fish. The lens contains thousands of very small finger-like glass protrusions coated with aluminium, which directs the incoming light onto reflective sidewalls to help focus the light.  In an autofocus camera, a small infra-red beam focuses on the desired object and a second beam captures the image and analyses the sharpness of the borders of the images to determine if the image is in or out of focus. Based on this design, the contact lens will need to be equipped with an extremely small and thin power source in order to change focus. Researchers are working on a tiny embedded solar cell that simultaneously harvests electrons from sunlight, converting them into electricity, and stores energy within a network of nanostructures. It works much the way a conventional solar panel does and a working prototype may be available in less than a decade.

5. BIOSENSORS

It is possible for sensors to be embedded in contact lenses to monitor tear fluid composition and translate this information into signals that can be read optically by an observer or by an instrument. Tear fluid contains many biomarkers that are found in blood, such as glucose, cholesterol and sodium. Biosensor technology has been used in contact lens designs for the following conditions:

i) Glucose monitoring in Diabetes

The global prevalence of diabetes is estimated at 8.8% and predicted to rise to 10.4% by 2040. Management of diabetes centres on maintaining normal blood sugar levels by frequent glucose monitoring along with the correct dosage and timing of medications. Continuous glucose monitoring can help early diagnosis and ensure effective control of diabetes complications. The most commonly used method of blood glucose assessment is the finger-prick test, which is an invasive, often inconvenient test procedure, and requires patient compliance.

An alternative glucose assessment has been proposed by using tear fluid. The glucose level in the tear film is reported to be in the range of 0.1–0.6 millimoles per liter, which is about ten times lower than the levels in blood.

Fluorescent, holographic, colloidal crystal array, and electrochemical sensors have been used to measure the glucose concentration in tear fluid in vivo.

Two different types of contact lens designs have been proposed to provide real-time continuous monitoring of glucose levels.

a) A concept patented by Google and being developed by their company Verily, has been described as having a biosensor on the lens surface which is in contact with the tear layer (see image 1). It detects the tear glucose levels and wirelessly transmits the results to an external device (a reader or cell phone). When glucose level in the tears reaches a certain threshold, the app will notify the user to take immediate action.

b) A second design is a fluorescence-based glucose-sensing contact lens which has probes comprising fluorophores of boronic acid. These emit colour upon excitation and depending on the tear glucose level, the clear contact lens develops a colour so the wearer or an observer will be able to see when an increase in tear glucose occurs. There is also a possibility of incorporating this design into a coloured contact lens, and the level of hyperglycaemia may be determined by comparing a pre-calibrated colour strip to the colour of lens being worn.

These designs are still in concept phase and years away from clinical trials. Studies have shown a weak correlation between blood glucose and glucose levels in the tears. So there is uncertainty as to whether these devices will accurately monitor glucose levels.

ii) Monitoring of Intraocular Pressure in Glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide. Worldwide prevalence was estimated to be 3.54% in 2013 and projections suggest that globally the number of people with glaucoma will increase by seventy-four percent from 2013 to 2040. Early diagnosis and management of glaucoma is essential but traditional testing of intra-ocular pressure (IOP) involves invasive testing and requires specialised instruments which is conducted in-office. The proposed silicone hydrogel contact lens has two independent layers: an outer reference layer and an inner sensing layer. The sensing layer can detect changes in the curvature of the lens (relative to the reference layer), which is related to changes in IOP. As the curvature of the lens changes, the resonance frequency of the inductor-capacitor circuit also changes, which can be measured and correlated to IOP.

 To adequately personalise treatment for glaucoma a twenty-four hour profile of the cornea and IOP is required. The IOP fluctuates with the individual’s circadian rhythm with the highest pressure often while the individual is asleep, therefore a true IOP profile may be achieved with consecutive twenty-four hours monitoring of IOP readings.

The Sensimed Triggerfish CLS^® ( image 2) has been approved by the American FDA and is currently in use. It is a silicone soft contact lens designed to remain on the eye surface for twenty-four continuous hours. Embedded within the contact lens is a micro-machined electro-mechanical systems microsensor, that wirelessly transmits data to an adhesive antenna that sends information to a portable recorder worn by the patient. At the end of the recording period the data is transmitted to a computer via Bluetooth technology for analysis. According to Sensimed, up to fifty percent of IOP peaks occur outside of office hours.

The Sensimed Triggerfish CLS^® has also been evaluated as a monitoring device in thyroid eye disease patients.

iii) Lactic acid and cancer biomarkers

Human tears contain biomarkers which could indicate the general health of a patient.

a) an electronic enzymatic l-lactate sensor on a polymersubstrate was moulded into a contact lens shape for potential in situmonitoring of l-lactate levels in tear fluid. Increased lactic-acid levels may indicate heart failure, liver disease, and lung disease. 

b) Tears also contain a chemical called lacryglobin, which may be elevated in human tears in patients with breast, lung, colon, prostrate or ovarian cancer.

The technology for the lactic acid and lacryglobin sensing contact lens would likely be similar in nature to the glucose monitoring contact lens design.

6. NIGHT VISION TECHNOLOGY

Night vision technology makes it possible to see light that is imperceptible to human eyes, and heat radiating from the bodies of people and animals in the dark. Researchers have developed a sensor by combining two layers of graphene, which is a lightweight and super-strong form of carbon, and an insulator that can detect the full infrared spectrum, along with visible and ultraviolet light. Researchers plan to integrate this technology into a night vision contact lens. It is reported that the US Military is interested in this development as a replacement for night vision goggles. The technology could also be used to identify gas leaks, help doctors monitor blood flow without having to move a patient or subject them to any scans. It can also be used by art historians to examine layers of paint underneath the surface of artwork.

7. AUGMENTED REALITY

Eye tracking technology follows the movements of a user’s eyes, storing these movements as information. In augmented or virtual reality there is an overlay of information on the normal field of view. The information is projected in front of the eyes, not at the plane of the contact lens, using eye tracking features. Many uses for this technology have been envisioned including language translation, gaming industry, military, and travel industry.

Sony has a patent for a magnetised and reflective contact lens that could be used for gaming and viewing 3D information on a television. This design intends using contact lenses to browse the menu of computer applications, control virtual characters of video games, select-drag-manipulate objects, and perform other learned actions in response to the user’s eye movement or gaze.

Innovega have designed a concept called Emacula. Using a combination of embedded optics contact lenses and spectacles with mounted micro display units, the screen merges with the outside world. This provides simultaneous vision of background and foreground, with high resolution and a wide field of view (see Image 3). Amongst others, it is anticipated to be used by surgeons who can access digital data while performing surgery, and by athletes to monitor heart rate and performance during an activity.

8. CAMERA AND VIDEO FUNCTION

Researchers have developed a prototype contact lens with a built-in light-emitting diode (LED) display. Designed by using quantum dot technology, it has an embedded antenna allowing the lens to work wirelessly.  Although it can currently display one pixel of information, researchers have indicated this lens will in future be able to display more data and allow access to emails. The contact lens will display the same information as a smartphone, which only the wearer will be able to see.

Samsung has a patent to develop a contact lens that blends augmented reality into real life seamlessly, without the use of a headset. The contact lenses have a built-in camera, motion sensors and a transmitter that can be controlled simply by blinking. Content is sent to your smartphone through embedded antennas and users will be able to take photos by simply blinking and can also view them immediately.

Sony also has a patent for a contact lens that captures images and can record video footage. It is anticipated that the user could share live images. Photo and video capture will be initiated with conscious blink patterns and will be powered wirelessly.  The lenses would work with a connected phone, allowing users to take photos and overlay information onto the real world. This is in the prototype phase and it could be years before the start of clinical trials.

Concerns have been raised about privacy and security control when using these smart contact lenses, as it is virtually undetectable and may result in unconsented images or recordings and surveillance issues.

9. LOW VISION REHABILITATION

Telescopic contact lenses that can magnify vision by approximately three times for visually impaired individuals is still in the design phase. With an almost 1.55mm thickness, these scleral lenses have a thin reflective telescope made of mirrors and filters (Image 4). When light enters the eye it reflects off the series of mirrors and increases the perceived size of an object in view.  When used in combination with battery-powered liquid crystal display glasses, these contact lenses allow users to switch between normal and magnified vision with a wink. That is, wink with the right eye for magnification, and wink with the left eye for normal distance vision. Ordinary blinking will not create any change. The lens prototype is manufactured in PMMA, but a new design is being created using a high oxygen transmissibility GP material. The telescopic lens is not yet ready for human trials. It is hoped that the lens will improve the sight of people with age-related macular degeneration, which is the third leading cause of blindness globally.

Google also has a patent for a contact lens designed with more than one embedded camera to assist individuals with low vision.  The cameras on the lens would scan the surroundings and send data to the user’s smartphone which will provide them with an audio alert, warning them of obstacles around them. This is a patent application and it is unsure if the idea will be further developed.

10. FORENSICS USE

There has been a report of a soft contact lens being used as a ‘material witness’ in a murder trial. Orbital remnants from an exhumed body was proven to be the fragments of contact lens which was worn by the deceased at time of death. This evidence provided factual proof contesting the defendant’s testimony in the murder trial. This forensic case study represents the first published documentation of a contact lens from an exhumed body being used in a murder investigation and developed a protocol for future forensic contact lens examinations.

CONCLUSION

There are some concerns regarding wireless powering of contact lens sensors. Currently achieved using electromagnetic radiation with a high frequency, this may have a negative effect on health. While Bluetooth connectivity is a convenient method to transmit readouts wirelessly, security and health concerns are potential drawbacks of this technology. The security vulnerabilities of Bluetooth systems and the manipulation of the readout to the consumer could prove harmful. To counteract this, LEDs have been incorporated into contact lenses and may serve as an alternative readout method. A power source is required to use the features in the smart lens prototypes described above. Research is being conducted on making the contact lenses self-powering. Developments include use of solar power, a biofuel cell, that runs on the ascorbate and oxygen naturally present in tears, or converting the vibrations caused by eye movements into electrical power. Samsung also has a patent to change components present in tears into electricity to help power a smart lens. Other researchers are working on a contact lens storage case that will recharge electronic contact lenses as well as transfer data from the lenses.

Although the primary application of contact lenses is vision correction, their scope of use is growing. The not-so-humble contact lens can have many applications and technological advances may allow for the development of contact lenses into what was previously considered to be the realm of science-fiction.

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