Temperature-switch-based light-responsive smart hydrogel for reversible message encryption and decryption

The use of stimulus-responsive hydrogels in intelligent gel anti-counterfeiting systems is a promising method to encrypt and decrypt information. External stimuli such as light, heat, electricity, magnetism and pH can alter the output information, while a single stimulus will limit higher levels of anti-counterfeiting. Therefore, multiple stimuli should be synergistically used to enhance information security. In this study, a highly transparent sodium alginate-polyacrylamide (SA-PAM) hydrogel is designed as the matrix. An inorganic color-changing material, ammonium molybdate (Mo7), is used to impart the gel with photochromic characteristics. Furthermore, poly-N-isopropylacrylamide (PNI-PAM), a temperature-sensitive material, makes the gel undergo a bulk phase transition at the lowest critical phase transition temperature (LCST), which in turn alters macroscopic state of the gel. The LCST can also be controlled by adjusting the ratio of hydrophilic and hydrophobic monomers in the gel system. By using ultraviolet (UV) light as the “ink”, the information is input, and temperature is used as the “switch” to encrypt/decrypt the information. Overall, the hydrogel exhibits excellent photochromic ability and can be colored within 10 s, while exhibiting a maximum UV light absorption rate of 96.23%. The hydrogel can rapidly reveal or conceal the information based on the response of the hydrogel to light and heat stimuli, thus ensuring that the information remains secure. This paper presents a novel approach of utilizing photochromic hydrogels in the field of information encryption.