Smart Contact Lenses: High-Precision Eye Tracking Solution
17-06-2024 | By Liam Critchley
The rise of wearable flexible devices—driven by advancements in materials science, especially soft materials—has delivered a human-machine interface revolution. Materials can now be made with skin-like mechanical properties (low elastic modulus) and are enabling fully connected bridges to be built between the human body and smart devices.
Human-machine interfaces take many forms nowadays, from haptic sensing to speech recognition, gesture recognition and motion capture. There is a drive to create more vision-based human-machine interfaces because our vision accounts for 83% of all the information perceived from the outside world.
Eye tracking technology is now becoming a big technology—with products such as the Vision Pro goggles being brought to market by Apple—to detect user’s eye movements and to decode their attention and intentions. While some ocular human-machine interface devices have been successful so far, there is a drive to create better smart contact lenses that can users can wear without being noticeable and providing eye tracking functionality.
Key Things to Know
- New smart contact lenses using frequency-encoded RF tags provide precise eye movement detection without bulky equipment, enhancing user convenience.
- These lenses are highly biocompatible with low cytotoxicity and eye irritation, suitable for long-term wear in diverse environments.
- Applications range from continuous eye-painting and eye-controlled games to advanced medical diagnostics and consumer electronics.
- The versatility and robustness of these lenses make them ideal for both professional and recreational use, adapting to various lighting and electromagnetic conditions.
Existing Eye Tracking Technology
Many of the eye tracking technology available today rely on pupil centre corneal reflection techniques. However, these techniques are limited because they are highly susceptible to environmental light interface from the positioning of cameras and light sources, as well as often being obstructed by the user’s eyelids and eyelashes.
Another tracking technology based on electrooculography (EOG) uses electrodes to obtain the potential signals of the eye dipole, but muscle electrical signals can interfere with these measurements, giving them poor accuracy. Additionally, these are not as safe as other options because the electrode material can react with the skin. From an aesthetics perspective, they are not ideal for everyday social scenarios either, as the electrodes are obvious to anyone else in close proximity to the user.
There’s a growing interest for wearable and imperceptible eye tracking devices to further push eye tracking technology into different application and market sectors. Some areas that are being looked at include degenerative diseases, brain medical diagnosis, product human-factor design, consumer experience research, and driver fatigue detection.
The Rise of Smart Contact Lenses
Advancements in flexible optoelectronic technology have enabled the development of miniaturised and smart contact lenses. There is now the potential, with advancements in augmented reality (AR), to use smart contact lenses to directly project images onto the retina. In medical settings, smart contact lenses can dynamically monitor different physiological changes in real-time—such as intraocular pressure and tear glucose levels—as well as being able to integrate different medical treatments, including drug delivery, colour deficiency correction, and corneal cell repair.
In terms of smart contact lenses today, scleral coil-based eye-tracking technology is currently the gold standard. These lenses have a fast response time and high angular resolution, but they do have a big downside. Despite their performance, the lenses are wired, which requires the eye to be anesthetised to be implanted, and commonly slips during usage. They also have bulky measurement systems that limit the applications where they can be used. These factors limit the user acceptance of the lenses, so new wireless solutions have been sought.
Development of New Wireless Eye Tracking Smart Contact Lenses
Researchers have now created a miniature, imperceptible, and biocompatible smart contact lens that can be used for in-situ eye tracking in a wireless eye-machine interaction. The smart contact lens uses frequency encoding to monitor the ocular behaviour (eye movement and closure) of the eye. The lens is also chip- and battery-free and contains 4 chip-less radiofrequency (RF) tags that work at different frequencies. A portable sweeping-frequency reader is also installed at the framed glasses, and opposite to the user’s eyeball, to wirelessly detect any received signal strengths from the smart contact lens—collecting the signal from the RF tags.
Advantages of Frequency Encoding in Smart Contact Lenses
According to Xu et al., the use of frequency-encoded RF tags enables precise detection of eye movement without the need for bulky equipment. This innovation significantly reduces the complexity of traditional eye-tracking setups, making it more suitable for everyday use.
The smart contact lenses use a time-sequential eye tracking algorithm that track the gazing point of the eye and the eye movements and provides an ultra-high accuracy of the eye movement from the wearer. The whole eye tracking algorithm system has a simple structure and is lightweight, so it can be easily integrated into other wearable devices—including AR glasses and virtual reality (VR) headsets.
The integration of these smart contact lenses into AR and VR devices could enhance user experience by providing more intuitive and responsive interaction. Xu et al.'s research highlights that these lenses maintain high accuracy and reliability, even in varying lighting conditions and electromagnetic environments.
Key Benefits and Applications of New Smart Contact Lenses
There are a few key benefits of the new smart contact lenses that have been developed. Aside from being wireless, they have low cytotoxicity and eye irritation, making them highly biocompatible. They also possess robustness to light, electromagnetic interference, different wearing angles, corneal curvatures, and reading distances. They also have an angular accuracy of less than 0.5°, which is less than the vision range of the central fovea and can be used in multiple eye interaction modes. These modes include continuous eye-painting, eye-controlled games, web interaction, PTZ camera control, and robot vehicle drive.
The versatility of these lenses in different interaction modes, as demonstrated by Xu et al., not only broadens their application scope but also enhances their usability in both professional and recreational settings. The ability to switch seamlessly between tasks using eye movements alone is a significant advancement in human-machine interaction technology.
Enhanced Usability and Robustness in Diverse Conditions
Xu et al. further emphasise that the robustness of these lenses to various external factors, such as electromagnetic interference and varying corneal curvatures, makes them highly adaptable for diverse user needs and environmental conditions.
Two types of applications were proposed for the eye tracking smart contact lens. One is continuous eye-calligraphy and eye-painting on a virtual screen—such as for portraying letters and snake patterns. The second approach is the interaction with different hardware and software, such as in a gluttonous snake game, web, and PTZ camera, and for using user-defined eye commands. There is also the potential for the smart contact lens to detect eye movements in 3D to monitor rapid eye movement during sleep.
Xu et al.'s research indicates that the potential for 3D eye movement detection could open new avenues in medical diagnostics, particularly in monitoring sleep disorders and neurological conditions. The accuracy and non-intrusive nature of these lenses make them ideal for continuous monitoring in clinical settings.
Validation Tests
To ensure that the smart contact lenses are suitable for practical applications outside of the lab, the team performed different validation tests looking at the different properties and characteristics of the smart contact lenses.
The researchers implanted the smart contact lenses into a rabbit while it was placed in a robot vehicle that was wirelessly controlled using Bluetooth. During the test, the eye movement was measured in real-time. After continuous wear for 24 hours, or daily wear for 8 hours over a week, no abnormalities were found in the rabbit’s cornea. It was also found that the smart contact lenses are as safe as commercial contact lens based on cytotoxicity tests with human corneal cell lines.
Looking to the Future
While a lot of progress has been made, there are plans to improve the practical eye tracking applications. The researchers have stated that they’re looking towards highly conductive transparent electrodes (such as those made of silver nanowires) to improve the flexibility and transparency of the device.
There’s also an interest in developing more specialised eye-tracking systems using smart contact lenses, such as by integrating them into field cameras and sensors to create intelligent eye-tracking applications. While a lot of applications have already been targeted, looking towards more advanced tracking applications could open the door to using smart contact lenses in consumer behaviour research, detecting eye interactions in virtual social settings, sleep quality assessment, and a range of other, more complex, medical use cases—including visual function assessment, neurological disease diagnosis and treatment, and cognitive function assessment.
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