High-Performance Photochromic Hydrogels for Rewritable Information Record

Reversible fluorescence/photochromic switching of repeated-response cellulose-based hydrogels for information encryption

The rapid development of anti-counterfeiting technology has brought new challenges to the repeatability and stability of reversible fluorescence/photochromic switching hydrogels. To address this issue, a series of chemical cross-linked cellulose-based intelligent responsive hydrogels were synthesized by free-radical graft copolymerization in a hydrothermal process. This strategy allows for the creation of a chemical cross-linked three-dimensional structure that anchors photochromic ammonium molybdate and fluorescent carbon dots together, resulting in enhanced stability and mechanical properties. Especially, the tensile and compressive strength of hydrogel reached a maximum value of 280 kPa and 560 kPa, respectively, which far exceeds that of some reported hydrogels. The resultant hydrogels exhibited desired reversible fluorescence/photochromic switching, reversible printing and erasing of patterns, and information encryption/decryption. Notably, the change of photochromism from yellow to green can be realized, and the self-fading process can be shortened to 25 min at 60 °C instead of 6 h at room temperature. More importantly, the fluorescence quenching phenomenon of the hydrogel occurs gradually after 2 min of continuous irradiation, and it can be recovered by selective treatment with ethanol. Overall, this study provides a simple strategy for the preparation of environmentally friendly reversible fluorescence/photochromic switching cellulose-based hydrogels for information encryption.

Multiresponsive Color-Changing and Tough Hydrogels Enabled by Self-Assembled Epoxy Oligomer Microspheres

The fabrication process of hydrogels often incorporates various strategies to achieve multiple responses and enhance strength, which always make the procedure complex and even hinder the incorporation. Here, we develop a facile and flexible method to simultaneously achieve multiresponsive color-changing and tough properties in hydrogels by introducing epoxy oligomer microspheres (DEPMS) to hydrophobic association (HA) hydrogels. DEPMS is responsive to both pH and solvents, showing color changes due to conversion to a conjugated structure. The obtained DEPMS composite hydrogels could demonstrate diverse color-changing patterns by simply adjusting the components and pH of the solvents. Meanwhile, amphiphilic DEPMS helps to disperse hydrophobic regions of the HA hydrogel, resulting in more uniform cross-linking and thereby contributing to the enhanced mechanical properties. The tensile strength and toughness of the composite hydrogels could be easily adjusted and reach 1.00 MPa and 11.18 MJ m-3, respectively. This work provides an approach to the design of multiple responsive and tough hydrogels while offering insights into the recycling of waste epoxy resins.

A gelatin-based ionogel with anti-swelling properties for underwater human physiological signal detection

A hydrogel is an ideal matrix material for flexible electronic devices, electronic skin and health detection devices due to its outstanding flexibility and stretchability. However, hydrogel-based flexible electronic devices swell once they are placed in a high humidity or underwater environment. The swelling behavior could damage the internal structure of hydrogels, ultimately leading to the reduction or complete loss of mechanical properties, electrical conductivity and sensing function. In order to resolve the above problems, a double network ionogel with remarkable anti-swelling behavior, stretchability and conductive properties was prepared. The ionogel consisted of gelatin (G) and copolymerization of acrylic acid (AA), 2-hydroxyethyl methacrylate (HEMA), butyl acrylate (BA), dimethylaminoethyl methacrylate maleate (D) and N,N'-methylene-bis-acrylamide (MBAA). Due to the dense crosslinking network and hydrophobic interaction, the ionogel exhibited remarkable anti-swelling properties (7.64% of the 30-day equilibrium swelling ratio in deionized water). D and MBAA were simultaneously introduced into the ionogel system as cross-linking agents to provide a large number of cross-linking points, improving the cross-linking density of the ionogel. Importantly, the introduction of D avoided ionic leakage by free radical copolymerization. Furthermore, the ionogel maintained stable mechanical properties and conductivity after being submerged in deionized water owing to remarkable anti-swelling performance. The mechanical properties of the ionogel retained 89.75% of the initial mechanical properties after a 5-day immersion in deionized water. Therefore, this ionogel could be employed as an underwater flexible wearable sensor for high humidity or underwater motion monitoring.

Recent Development of Photochromic Polymer Systems: Mechanism, Materials, and Applications

Photochromic polymer is defined as a series of materials based on photochromic units in polymer chains, which produces reversible color changes under irradiation with a particular wavelength. Currently, as the research progresses, it shows increasing potential applications in various fields, such as anti-counterfeiting, information storage, super-resolution imaging, and logic gates. However, there is a paucity of published reviews on the topic of photochromic polymers. Herein, this review discusses and summarizes the research progress and prospects of such materials, mainly summarizing the basic mechanisms, classification, and applications of azobenzene, spiropyran, and diarylethene photochromic polymers. Moreover, 3-dimensional (3D) printable photochromic polymers are worthy to be summarized specifically because of its innovative approach for practical application; meanwhile, the developing 3D printing technology has shown increasing potential opportunities for better applications. Finally, the current challenges and future directions of photochromic polymer materials are summarized.

Polyacrylamide/sodium alginate/sodium chloride photochromic hydrogel with high conductivity, anti-freezing property and fast response for information storage and electronic skin

Photochromic hydrogels have promising prospects in areas such as wearable device, information encryption technology, optoelectronic display technology, and electronic skin. However, there are strict requirements for the properties of photochromic hydrogels in practical engineering applications, especially in some extreme application environments. The preparation of photochromic hydrogels with high transparency, high toughness, fast response, colour reversibility, excellent electrical conductivity, and anti-freezing property remains a challenge. In this study, a novel photochromic hydrogel (PAAm/SA/NaCl-Mo7) was prepared by loading ammonium molybdate (Mo7) and sodium chloride (NaCl) into a dual-network hydrogel of polyacrylamide (PAAm) and sodium alginate (SA) using a simple one-pot method. PAAm/SA/NaCl-Mo7 hydrogel has excellent conductivity (175.9 S/cm), water retention capacity and anti-freezing properties, which can work normally at a low temperature of -28.4 °C. In addition, the prepared PAAm/SA/NaCl-Mo7 hydrogel exhibits fast response (<15 s), high transparency (>70 %), good toughness (maximum elongation up to 1500 %), good cyclic compression properties at high compressive strains (60 %), good biocompatibility (78.5 %), stable reversible discolouration and excellent sensing properties, which can be used for photoelectric display, information storage and motion monitoring. This work provides a new inspiration for the development of flexible electronic skin devices.

Ion-Cross-Linked Hybrid Photochromic Hydrogels with Enhanced Mechanical Properties and Shape Memory Behaviour

Shape-shifting polymers usually require not only reversible stimuli-responsive ability, but also strong mechanical properties. A novel shape-shifting photochromic hydrogel system was designed and fabricated by embedding hydrophobic spiropyran (SP) into double polymeric network (DN) through micellar copolymerisation. Here, sodium alginate (Alg) and poly acrylate-co-methyl acrylate-co-spiropyran (P(SA-co-MA-co-SPMA)) were employed as the first network and the second network, respectively, to realise high mechanical strength. After being soaked in the CaCl2 solution, the carboxyl groups in the system underwent metal complexation with Ca2+ to enhance the hydrogel. Moreover, after the hydrogel was exposed to UV-light, the closed isomer of spiropyran in the hydrogel network could be converted into an open zwitterionic isomer merocyanine (MC), which was considered to interact with Ca2+ ions. Interestingly, Ca2+ and UV-light responsive programmable shape of the copolymer hydrogel could recover to its original form via immersion in pure water. Given its excellent metal ion and UV light stimuli-responsive and mechanical properties, the hydrogel has potential applications in the field of soft actuators.

Preparation of high-strength photochromic alginate fibers based on the study of flame-retardant properties

Color-changing fibers have attracted much attention for their wide applications in camouflage, security warnings, and anti-counterfeiting. The inorganic color-changing material tungsten trioxide (WO3) has been widely investigated for its good stability, controllability, and ease of synthesis. In this study, photochromic alginate fibers (WO3@Ca-Alg) were prepared by incorporating UV-responsive hybrid tungsten trioxide nanoparticles in the fiber production process. The prepared photochromic alginate fibers changed from white to dark blue after 30 min of UV irradiation and returned to their original color after 64 h. It can be seen that WO3@Ca-Alg has the advantage of long color duration. The strength of this fiber reached 2.61 cN/dtex and the limiting oxygen index (LOI) was 40.9 %, which indicates that the fiber exhibited mechanical resistance and flame-retardant properties. After the cross-linking of WO3@Ca-Alg by sodium tetraborate, a new core-shell structure was generated, which was able to encapsulate tungsten trioxide in it, thus reducing the amount of tungsten trioxide loss, and its salt and washing resistance was greatly improved. This photochromic alginate fiber can be mass produced and spun into yarn.

Designing Multistimuli-Responsive Anisotropic Bilayer Hydrogel Actuators by Integrating LCST Phase Transition and Photochromic Isomerization

Stimuli-responsive hydrogel actuators have attracted tremendous interest in switches and microrobots. Based on N-isopropylacrylamide (NIPAM) monomers with LCST phase separation and photochromic molecule spiropyran which can respond to ultraviolet light and H+, we develop a novel multistimuli-responsive co-polymer anisotropic bilayer hydrogel, which can undergo complex deformation behavior under environmental stimuli. Diverse bending angles were achieved based on inhomogeneous swelling. By controlling the environmental temperature, the bilayer hydrogels achieved bending angles of 83.4° and -162.4° below and above the critical temperature of PNIPAM. Stimulated by ultraviolet light and H+, the bilayer hydrogels showed bending angles of -19.4° and -17.3°, respectively. In addition, we designed a strategy to enhance the mechanical properties of the hydrogel via double network (DN). The mechanical properties and microscopic Fourier transform infrared (micro-FTIR) spectrum showed that the bilayer hydrogel can be well bonded at the interfaces of such bilayers. This work will inspire the design and fabrication of novel soft actuators with synergistic functions.

Recent Progress in Smart Polymeric Gel-Based Information Storage for Anti-Counterfeiting

Information security protection has a tremendous impact on human life, social stability and national security, leading to the rapid development of anti-counterfeiting materials and related techniques. However, the traditional stored information on hard or dry media is often static and lacks functions, which makes it challenging to deal with increasing and powerful counterfeiting technologies. Modified intelligent polymeric gels exhibit color changes and shape morphing under external stimuli, which give them great potential for applications in information storage. This paper provides an overview of the latest progress in polymeric gel-based information storage materials in relation to counterfeiting. Following a brief introduction of anti-counterfeiting materials, the preparation methods for intelligent gels with adjustable colors (e.g., chemical colors and physical colors) and various encryption/decryption modes involving dimensions and diverse colors are outlined. Finally, the challenges and prospects for information storage and anti-counterfeiting based on smart gels are discussed.

Data storage and encryption with a high security level based on molecular configurational isomers

Developing advanced materials for highly secure data-encryption is crucial but very challenging, as most data-encryption materials (the message area) are chemically different from the substrates (the background) on which they are being written, leading to high risks of data leakage by deciphering via sophisticated instrumental analysis. Additionally, most materials require only one stimulus for decryption, resulting in a low-level of data-security. Here, a three configurational isomer-based data-encryption method is developed (i.e., propylamine, isopropylamine, and cyclopropylamine). Their similar molecular formulae, elemental constitution, and physiochemical properties make them ideal date-encryption materials. On the other hand, the significant differences in lower critical solution temperatures (LCST) of the corresponding polyacrylamides, i.e., 10 °C for poly(N-propylacrylamide), 32 °C for poly(N-isopropylacrylamide), and 53 °C for poly(N-cyclopropylacrylamide), respectively, render an effective method for data decryption. Relying on the above features, the data written by three isomers are well-hidden under given conditions. And a specific temperature range, rather than a simple temperature increase or decrease, would be required for decryption. Furthermore, undesired temperatures give wrong outputs, which is highly deceptive to the hacker. Therefore, a high-level of data security can be achieved. This result opens a new door for designing advanced materials for improving the data-security level.

Bifunctional Phenol-enabled Sequential Polymerization Strategy for Printable Tough Hydrogels

Hydrogels are promising material candidates in engineering soft robotics, mechanical sensors, biomimetic regenerative medicine, etc. However, developing multinetwork hydrogels with high mechanical properties and excellent printability is still challenging. Here, we report a bifunctional phenol-enabled sequential polymerization (BPSP) strategy to fabricate high-performance multinetwork hydrogels under the orthogonal catalysis of efficient ruthenium photochemistry. Benefiting from this bifunctional design, phenols can sequentially polymerize with typical monomers and themselves to fabricate various phenol-containing polymers (Ph-Ps) and Ph-Ps-based multinetwork tough hydrogels, respectively. The as-prepared hydrogels have maximum stress of 0.75 MPa and toughness of 2.2 MJ/m³ under the critical strain of 800%. These property parameters are a maximum of 16 times higher than that of the phenol-postmodified and phenol-free hydrogels. Moreover, the rapid coupling polymerization of phenols can shorten the gelation times of hydrogels to as low as ∼4 s, which enables its printable property for customizable applications. As a proof of concept, a 3D scaffold-like structure is optimized as highly sensitive mechanical sensors for detecting various human motions. This article is protected by copyright. All rights reserved.