Light and Humidity Dual-Responsive Anti-Counterfeiting Films Based on Hydrogen-Bonded Cholesteric Liquid Crystal Polymers with Spiropyran

Structural Colors Go Active

Structural colors find wide applications for color printing, intelligent display, filtering imaging, etc., owing to their benefits, including high resolution, stable properties, and dynamic tunability. This review first illustrates the mechanisms of structural color generation, such as surface plasmon resonances, localized surface plasmon resonances, Fabry-Perot resonances, Mie resonances, etc. It then proposes the recent technological strategies employed to realize dynamic structural colors. The integration of structural colors with functional materials like phase-change, along with the development of color dynamic control mechanisms such as microfluidic chips, micro-electro-mechanical system drivers, and microheaters, represents key approaches for spectrum regulation. Furthermore, the review assesses the performance, advantages, and limitations of various technologies for dynamic structural colors. Finally, this review concluded with a section on the future challenges and prospects in large-area fabrication, practical applications, and performance improvement. It explains the current typical applications, including smart windows, adaptive camouflage, sensors, etc., and explores the processing methods that can achieve large-area, high-fidelity preparation of structural colors, such as nanoimprint, deep ultraviolet lithography, immersion lithography, laser printing, etc. This field promises advancements in high-density data storage, information encryption, and broader market applications.

Cholesteric Liquid Crystal Network Composite Film with Structure, Dual-Band Responsive Luminescent Color for Multiple Anti-Counterfeiting and Information Encryption

Anti-counterfeiting technology plays an indispensable role in the high-tech field and various critical application areas. However, traditional anti-counterfeiting approaches currently in widespread use are too simplistic and easily replicated or forged, while advanced technologies with multiple anti-counterfeiting functions remain in the developmental stage. This study presents a novel multiple anti-counterfeiting technique. By employing a patterned cholesteric liquid crystal polymer network (CLCN) film as a template, a composite film endowed with multiple anti-counterfeiting capabilities was successfully fabricated by filling the hollow patterned areas with a light-cured acrylate monomer mixed with commercial infrared and ultraviolet phosphors. The prepared composite film exhibits high transmittance and excellent flexibility, enabling its application across various applications. By utilizing the thermochromic and angular chromic properties of the CLCN film, along with ultraviolet-excited colorful fluorescent materials and infrared-excited upconversion luminescent materials incorporated within the acrylate film in the CLCN patterned area, a film featuring multiple anti-counterfeiting attributes is achieved. Furthermore, by modifying the mask, adjusting the content of the chiral dopant, and selecting the type of phosphors, a multiple anti-counterfeiting color image can be produced, which has significant implications for enhanced anti-counterfeiting measures and information encryption storage.

Multiple Stimuli-Responsive Color-Changing Polymer Materials for Reversible Writing and Anti-Counterfeiting

Polymer materials with multiple stimuli-responsive properties have demonstrated many potential and practical applications. By covalently introducing spiropyran (SP1) and spirothiopyran (STP) into the polyurethane backbone, photochromic, mechanochromic, and thermally discolored polymer materials have been prepared. In this work, we report for the first time that white light (violet, blue, and green light) above a certain intensity can activate STP to green color. Based on the above discovery, the polyurethane with SP1 and STP can exhibit reversible three-color changes (brown, green, and purple) in response to four stimuli: ultraviolet irradiation, white light irradiation, mechanical stress, and heat. The color-changing polymer materials have high color contrast and excellent reversibility, and can be used for reversible writing, anticounterfeiting and information encryption, etc.

Drop-on-Demand Pyro-Electrohydrodynamic Printing of Nematic Liquid Crystal Microlenses

Inkjet printing of liquid crystal (LC) microlens arrays is particularly appealing for the development of switchable 2D/3D organic light-emitting diode (OLED) displays, as the printing process ensures that the lenses can be deposited directly and on-demand onto the pixelated OLED layer without the need for additional steps, thus simplifying fabrication complexity. Even if different fabrication technologies have been employed and good results in LC direct printing have already been achieved, all the systems used require costly equipment and heated nozzles to reduce the LC solution's viscosity. Here, we present the direct printing of a nematic LC (NLC) lens by a Drop-on-Demand (DoD) inkjet printing by a pyro-electrohydrodynamic effect for the first time. The method works at ambient temperature and avoids dispensing nozzles, thus offering a noncontact manipulation approach of liquid with high resolution and good repeatability on different kinds of substrates. NLC microlenses are printed on different substrates and fully characterized. Polarization properties are evaluated for various samples, i.e., NLC lenses on unaligned and indium-tin oxide (ITO) aligned. Moreover, an in-depth characterization of the NLC lenses is reported by polarized optical microscopy and by analyzing the birefringence in digital holographic microscopy.