Using energy-saving electrochromic (EC) displays in smart devices for augmented reality makes cost-effective, easily producible, and efficiently operable devices for specific applications possible. Prussian blue (PB) is a metal-organic coordinated compound with unique EC properties that limit EC display applications due to the difficulty in PB micro-patterning. This work presents a novel micro-printing strategy for PB patterns using localized crystallization of FeFe(CN)6 on a substrate confined by the acidic-ferric-ferricyanide ink meniscus, followed by thermal reduction at 120 °C, thereby forming PB. Uniform PB patterns can be obtained by manipulating printing parameters, such as the concentration of FeCl3·K3Fe(CN)6, printing speed, and pipette inner diameter. Using a 0.1 M KCl (pH 4) electrolyte, the printed PB pattern is consistently and reversibly converted to Prussian white (CV potential range: −0.2–0.5 V) with 200 CV cycles. The PB-based EC display with a navigation function integrated into a smart contact lens is able to display directions to a destination to a user by receiving GPS coordinates in real time. This facile method for forming PB micro-patterns could be used for advanced EC displays and various functional devices.
With the advent of the Metaverse era, there have been growing expectations that virtual reality (VR) and augmented reality (AR) technologies will likely to enhance convenience in everyday life, as well as industry productivity performance.
A joint research team, affiliated with UNIST has introduced core technology for smart contact lenses that can implement AR-based navigation through a 3D printing process. According to the research team, the new smart contact lenses can be worn inside the eye of a person, like a normal contact lens.
Published in the February 2023 issue of Advanced Science, this breakthrough has been jointly led by Professor Im Doo Jung in the Department of Mechanical Engineering at UNIST and Dr. Seung Kwon Seol from the Smart 3D Printing Research Team at Korea Electrotechnology Research Institute (KERI).
Figure 1. Meniscus-guided micro-printing of Prussian blue (PB).
Some of the disadvantages of existing AR devices include high price, experimental technology, and bulky appearance, making them difficult to get access to the market. Smart contact lenses, on the other hand, have the advantages of being affordable and convenient, as it can be worn inside the eye of a person. Leading companies throughout the world, such as Google are currently working on creating smart contact lenses, capable of implementing AR. Still, there exists barriers that impede the effective and efficient commercialization of research due to severe technical challenges.
In implementing AR with smart contact lenses, energy-saving electrochromic (EC) displays that can be driven with low power are suitable. Prussian blue (PB) has been regarded as one of the attractive EC material due to its uniform coloration, fast kinetics, high optical contrast, multiple color states (blue, white, green), environmental friendliness, and cost competitiveness. However, this method has limitations in displaying words or images that are needed for a display on AR smart contact lenses because of the difficulty of micro-patterning PB on the contact lens, noted the research team.
Figure 2. Optical micrographs presenting the dependence of the printed line width on inner diameter (ID).
The joint research team studied how to not use an electroplating process, and as a result, they developed simple and effective printing strategy to produce micro-patterns of PB using the meniscus-guided printing of an acidic-ferric-ferricyanide ink composed of FeCl3, K3Fe(CN)6, and HCl. The key to this is the meniscus of the acidic-ferric-ferricyanide ink.
As with the conventional electroplating approach, the substrate used must be a conductor when voltage is applied. However, with the meniscus phenomenon, there is no restriction on the substrate that can be used because crystallization occurs by natural evaporation of the solvent. Our micro-pattern technology is very fine (7.2 micrometers) that can be applied to smart contact lens displays for AR, and the color is continuous and uniform.
The role of smart contact lens is most anticipated in fields, like navigation. Through experiments, researchers successfully demonstrated PB-based EC displays in a smart contact lens with a navigation function. The device was able to display directions to the destination to the user on the EC display by receiving GPS coordinates in real time, noted the research team.
Figure 3. Electrochromic (EC) display for navigation system embedded in a contact lens.
“Although thin glass ITO was used for the EC display in this study, it can be further developed as a method of patterning transparent electrodes, such as graphene on flexible materials and printing EC materials,” noted the research team. “We believe that our novel strategy will serve as an attractive method for realizing PB-based EC displays as well as diverse functional devices with micro PB patterns.”
In recognition of its excellence, the related research results have been featured as the back cover of Advanced Science (IF 17.521/JCR 4.71%), a world-renowned academic journal in the field of materials science.
Je Hyeong Kim, Seobin Park, Jinhyuck Ahn, et al., “Meniscus-Guided Micro-Printing of Prussian Blue for Smart Electrochromic Display,” Advanced Science (2023).