Frequency-encoded eye tracking smart contact lens for human-machine interaction

Human-machine interaction (HMI) is revolutionized by wearable flexible devices, offering seamless connection between smart devices and the human body. Eye tracking, analyzing intention and cognition through attention detection, enables highly efficient and natural HMI. Existing methods, such as pupil center corneal reflection and electrooculography (EOG), have limitations including susceptibility to environmental light interference, awkward positioning, and low accuracy. A need exists for a wearable, imperceptible eye tracking device for diverse applications, including assistance for individuals with degenerative diseases, brain medical diagnosis, cognitive science research, product design, and driver fatigue detection. Miniaturized smart contact lenses (SCL) offer potential, with existing applications in augmented reality (AR) and healthcare. However, current scleral coil-based eye tracking SCLs are wired and require anesthesia, limiting user acceptance. This work presents a miniature, imperceptible, and wireless eye tracking SCL using a frequency encoding strategy and advanced spherical conformal preparation techniques.

Existing eye-tracking technologies, such as pupil center corneal reflection and electrooculography (EOG), suffer from limitations in accuracy, susceptibility to environmental interference, and user comfort. The need for a comfortable, imperceptible, and accurate eye-tracking system motivates the development of this smart contact lens. The paper reviews existing wearable technologies, focusing on flexible electronics and their application in HMI. Existing smart contact lenses are discussed, highlighting their roles in AR, healthcare monitoring, and medical treatment. The limitations of wired scleral coil-based eye-tracking systems are discussed, emphasizing the need for a wireless alternative.

The proposed eye-tracking SCL consists of four chip-less passive RF tags with different working frequencies, integrated into a silicone elastomer lens. A portable sweeping-frequency reader, installed on glasses, wirelessly collects the tags’ signals. Eye movement is tracked by analyzing variations in the received signal strength due to changes in coupling coefficients. The lens design includes a transparent central region for unobstructed vision, and hydrophilization treatment for long-term comfort. Comprehensive biocompatibility tests, including cytotoxicity and eye irritation assessments, are performed using human corneal cells (HCE-T) and in vivo rabbit models. A time-sequential eye-tracking algorithm, calibrated using an implicit swirling pattern, is employed to improve accuracy. The algorithm accounts for individual differences in the angle between the geometrical and vision axes and reduces common-mode drift. Various HMI applications are tested using the SCL, including eye-drawing, gaming, web interaction, camera control, and robot vehicle control, both in a model and with an in-vivo rabbit.

The frequency-encoded SCL achieves high angular accuracy (<0.5°), surpassing the visual range of the central fovea, using a time-sequential eye-tracking algorithm. The system demonstrates robustness to environmental light and RF interference. The lens successfully tracks eye movement and closure, distinguishing between the two via differential and common-mode signal analysis. Multiple HMI applications are successfully demonstrated including precise eye-calligraphy and painting, eye-controlled games (Gluttonous Snake), web interaction, pan-tilt-zoom (PTZ) camera control, and robot vehicle control, using both an eye movement model and in vivo rabbit tests. Biocompatibility testing demonstrates low cytotoxicity (cell viability above 90% after 72 hours) and low eye irritation in both in-vitro and in-vivo testing, even after prolonged wear. The system shows high tolerance to individual differences in corneal curvature and varying reading distances.

The high accuracy and robustness of the proposed frequency-encoded SCL, coupled with its biocompatibility, address the limitations of existing eye-tracking technologies. The ability to differentiate eye movement from closure is crucial for reliable HMI applications. The successful implementation of diverse HMI applications demonstrates the broad applicability of the technology. The in vivo validation in rabbits successfully demonstrates the feasibility of real-time wireless control of a robot vehicle through eye movements. The study's results provide a significant advancement in the field of eye-tracking, paving the way for more seamless and intuitive HMI. The use of an implicit calibration method simplifies the user experience and makes the technology more accessible.

This research presents a novel frequency-encoded eye-tracking smart contact lens exhibiting high accuracy, robustness, and biocompatibility. The successful demonstration of various HMI applications highlights the technology's potential to transform user interfaces. Future work may focus on enhancing flexibility and transparency, optimizing the reader and algorithm for calibration-free eye tracking, and integrating additional functionalities such as cameras and sensors to enable advanced applications in consumer research, virtual social interactions, and healthcare.

The study's scope is limited to specific applications. Further research is needed to assess the long-term effects of lens wear, evaluate performance across a wider range of users and conditions (e.g., different lighting conditions, varying degrees of corneal curvature), and explore additional applications of this technology. The accuracy of the swirling calibration method might be improved further to reduce the marginal error.