Power-Free Electrochromic System on Smart Contact Lens for Glucose Sensing

Recent advances in wearable electronics incorporating the Internet of Things (IoT) have enabled developments in noninvasive and continuous health monitoring. By real-time analysis of changes in vital signs such as temperature, heart rate, and metabolites including glucose and lactate, early disease diagnosis can be realized. As tear fluid consists of numerous detectable biomarkers, smart contact lenses have been considered one of the most promising wearable devices and have attracted substantial interest in scientific research. It has been reported that tear fluid compositions such as glucose have a close relationship with blood because of the plasma leakage from blood to tear through blood-tear barriers. Compared with the conventional finger-pricking method using a glucometer for diabetes diagnosis, smart contact lenses reduce possible pains and contaminants during blood sample collection. Although great progress has been achieved in investigations on tear glucose monitoring, most of the reported studies mainly focus on electronics like sensors and integrated chips, leading to the necessity of power supplies. Despite the high specific capacity and excellent long-term cyclic performances, conventional lithium-ion batteries cannot be applied to smart contact lenses because of the extremely high safety requirements of eye environments. As an alternative to batteries, inductive power transmission has been widely developed. However, an additional transmitter within an effective distance is always required during the operation, which is inconvenient in particle applications and not applicable for continuous monitoring as well. Here, an electrochromic power-free system for biomonitoring is proposed to reduce the requirements of power supply. Prussian blue (PB), known as one of the promising and biocompatible electrochromic electrodes, was applied to the sensor. Glucose oxidase (GOx) was coated on the surface of the PB electrode for selective detection of glucose. Initially, PB electrodes are at reduced states known as Prussian white (PW), which is transparent. After observing glucose, reactions between glucose and GOx take place, generating the oxidizing agent hydrogen peroxide (H2O2), which oxidizes PW to PB accompanied by an observable color change from transparent to blue. The degree of color change depends on both glucose concentrations and PB thicknesses. Therefore, to enable a wide range of glucose level detection, the sensor with multi-electrode in different thicknesses was designed, with thinner electrodes indicating lower glucose concentration while thicker electrodes working for higher glucose situations. The glucose level can be roughly estimated by the combination of color changes in different electrodes using bare eyes, and more accurate information is accessible by machine learning and image processing. This direct color information thus reduces the requirements of power supplies. Due to inclusions of all required ions for the above redox reactions, tears are able to perform as electrolytes to support normal operations of the electrochromic sensor, eliminating potential risks caused by applications of additional electrolytes. The proposed sensor achieved glucose detection in both the ordinary range (0.16−0.5 mM) and high concentrations (0.9 mM), with a correlation coefficient r = 0.99543 between the predicted results and real concentrations. Lower level glucose detection down to 0.05 mM was also demonstrated. In addition, the recyclability of the sensor was achieved by the implementation of the reduction container, which electrochemically reduced PB to PW in tear-based solutions. The standard deviations of glucose sensing results after 4 repeated detections and several days’ storage were 0.0462 and 0.025, respectively, indicating the consistency of the sensor. We believe that the reported electrochromic power-free sensor has great potential for daily health monitoring and the working principle is not limited to only glucose detection but also suitable for other various metabolites for disease diagnosis. Acknowledgment The content of the presentation has been published as Reference [1]. Reference [1] Z. Li, J. Yun, X. Li, M. Kim, J. Li, D. Lee, A. Wu, S. W. Lee, Power-Free Contact Lens for Glucose Sensing. Adv. Funct. Mater. 2023, 33, 2304647. https://doi.org/10.1002/adfm.202304647 Figure 1