With the rapid increase in information technology, information recording and encryption/decryption functions are essential due to the prevalence of counterfeiting activities and information leakage in the current age. As a common information-storage medium, paper exhibits good mechanical properties and an ink-absorbing ability based

on its cellulose, packing, and pore structures. However, its limited functionality confines it as an intelligent responsive material related to information processing. At present, the emergence of information storage materials such as fluorescent materials, stimuli-responsive substrates, and rewritable substrates has alleviated these problems to a certain extent, but their information resolution is low and information security is still insufficient. The development of high-resolution information recording and multistage information protection systems to achieve high data security levels, such as self-erasing encrypted data and time-controlled data handling, remains limited.

 Inspired by the fiber and packing structures present in paper, researchers from Shanghai Jiao Tong University proposed a novel multifunctional nanofiber PNIPAM hydrogel (NCPN hydrogel) with improved mechanical properties and solvent-induced high-definition reversible information recording, self-encryption, and multidecryption capabilities. The related paper, entitled with “Paper-Structure Inspired Multiresponsive Hydrogels with Solvent-Induced Reversible Information Recording, Self-Encryption, and Multidecryption”, was published on Advanced Functional Materials. 

The paper link is as follows:

https://onlinelibrary.wiley.com/doi/10.1002/adfm.202201009

In the NCPN hydrogel matrix, nanofibers functioned as the reinforcement phase to achieve the desired mechanical properties and surface roughness. The PNIPAM hydrogel was applied as the packing phase in the information recording, self-encryption,

and multidecryption system, making the NCPN hydrogel responsive toward solvent and thermal stimuli based on its hydrophilic and hydrophobic structures. The designed hydrogel exhibited a relatively rapid transparent–opaque–transparent variation under the ethanol stimulus. These variations could be repeated in water to return the material to its original state. The transparent–opaque–transparent variations were repeated at least 20 times without any obvious fracture or structural changes. In addition, other polar solvents and compounds with similar structures were investigated. Similar transmittance changes were observed, showing the universal solvent-induced responsiveness of these systems. Furthermore, we designed several systems considering these transmittance variations related to information processing, achieving stable, adjustable, repeatable, and variable performances. Considering these findings, the designed NCPN hydrogels exhibit great potential as information-recognition systems with information recording, encryption, and decryption functions. The versatile stimuli methods and easy preparation of these hydrogels make them ideal materials for applications in the fields of biomedicine, multifunctional sensors, and artificial muscles.

Figure 1. Preparation and microscopic characterization of the NCPN hydrogels. (a) UV-induced polymerization and schematic structure of the NCPN hydrogels. (b) SEM images of the surface of the NC_0.1PN hydrogels. (c) SEM (left) and TEM (right) images of NC. (d) XRD patterns of the NCPN hydrogels with different NC contents. (e) Infrared spectra of NC, NIPAM, PNIPAM, and NC_0.1PN hydrogels.

Figure 2. (a) Tensile stress-strain curves of the NCPN hydrogels containing different NC contents. (b) Elastic moduli of the NCPN hydrogels containing different NC contents. (c) Tension–relaxation cycles (140% strain, 10 cycles) of the NC_0.1PN hydrogel. (d) Compressive stress−strain curves of the NCPN hydrogels containing different NC contents. (e) Compression−relaxation cycles (60% strain, 10 cycles) of the NC_0.1PN hydrogel. (f) Transparencies of the NCPN hydrogels with different NC contents.

Figure 3. (a) Transparent-opaque-transparent variation of the NC_0.1PN hydrogels induced by ethanol and corresponding SEM images. (b) Dependence of the amide Ⅰ (C=O) band frequency of PNIPAM on the ethanol molar ratio (mol%). (c) DSC curves for the NC_0.1PN hydrogels induced by ethanol with different molar ratios. (d) LCST changes of the NC_0.1PN hydrogels induced by ethanol with varied molar ratios.

Figure 4. (a) Transmittance variations of pure PNIPAM and NC_0.1PN hydrogels in ethanol. (b) Transmittance variation of PNIPAM and NC_0.1PN hydrogels as they become transparent due to the presence of ethanol in water. (c) Transparent-opaque-transparent cycles of the NC_0.1PN hydrogel in ethanol and water environments. (d) Transmittance variation of the NC_0.1PN hydrogels induced by ethanol solutions with different molar ratios. (e) Dynamic rheological tests of pure NC_0.1PN, NC_0.1PN turned opaque as induced by ethanol (NC_0.1PN-ethanol-O), NC_0.1PN turned transparent as induced by ethanol (NC_0.1PN-ethanol-T), NC_0.1PN turned transparent as induced by ethanol and then turned opaque as induced by water (NC_0.1PN-ethanol-T-H₂O-O).

Figure 5. (a) Schematic illustration of the information recording process using ethanol writing. (b) Optical images of recorded numbers using ethanol writing. (c) Schematic illustration of the information recorded through ethanol brushing, inspired by the woodblock printing technique. (d) Optical images of the different recording patterns on the surfaces of the NC_0.1PN hydrogels obtained by ethanol-brushing.

Figure 6. (a) Schematic illustration of the QR code encryption and decryption behaviors. (b) QR code information encryption and decryption behaviors achieved by ethanol- and water- stimulus, the scale bar is 1 cm. (c) QR code information encryption based on ethanol immersion and decryption using water and in an air environment, the scale bar is 1 cm.

Figure 7. (a) Schematic illustration of the information recording, self-encryption, and thermal decryption processes. (b) Decryption process of the encrypted pattern “8” under a thermal stimulus, the scale bar is 1 cm. (c) Gradual decryption process of the encrypted Chinese character “三” under a thermal stimulus, the scale bar is 1 cm.

Figure 8. (a) Optical images of the transmittance variation of the NC_0.1PN hydrogel in different polar solvents, the scale bar is 1 cm. (b) Optical images of the transmittance variation of the NC_0.1PN hydrogel in nonpolar solvents, the scale bar is 1 cm. (c) Optical images of the transmittance variation of the PEDA hydrogel in isopropanol solvents, the scale bar is 1 cm.