Multi-stimuli responsive hydrogels with shape memory and self-healing properties for information encryption

A rewritable and shape memory hydrogel doped with fluorescein-functionalized ZIF-8 for information storage and fluorescent anti-counterfeiting

The emergence of stimuli-responsive fluorescence anti-counterfeiting technology has garnered increasing attention in the era of intelligent internet. Smart fluorescent hydrogels combine the characteristics of luminous materials with the unique structure of hydrogels, offering the potential for dynamic reversible erasing and multi-tiered data encryption. In this work, a fluorescent hydrogel was constructed by zeolitic imidazolate framework-8 loaded with fluorescein and then mixed with polyvinyl alcohol hydrogel, sodium carboxymethyl cellulose and borax, which could be used for image hiding in visible light. The reversible bonds cross-linked fluorescent hydrogel was stretchable and self-healing with a three-dimensional network structure. The hydrogel presented bright green fluorescence under 365 nm UV light, which was quenched by adding copper ions. Meanwhile, the imprint of the hydrogel could be cleared by L-Cysteine and repeatedly recorded information many times. The alkali-induced shape memory capability was further utilized to achieve multi-tiered data encryption by deforming it to a 3D-specific shape through folding. The rewritable and multi-dimensional encrypted hydrogel is expected to improve data security and reduce resource consumption.

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.

A Shape Memory Hydrogel with Excellent Mechanical Properties, Water Retention Capacity, and Tunable Fluorescence for Dual Encryption

Information encryption platforms with reliable encryption performance, excellent mechanical performance, and high water retention capacity are highly desired. In this study, a tough double-network hydrogel is designed using the first network of a polyion complex containing lanthanide complexes via one-pot polymerization and the second network of a poly (N-hydroxyethyl acrylamide) (PHEAA) obtained by deep eutectic solvent (DES)-assisted introduction and subsequent photopolymerization. In this system, the pH-induced shape memory function and pH-/wavelength-dependent fluorescence allow the use of the prepared hydrogel as a dual-encryption platform. Owing to its high response reversibility, the hydrogel-based platform exhibits both a high security level and the advantages of rewritability, reprogrammability, and reusability. Additionally, the excellent mechanical properties and water retention capacity owing to the solvent exchange process involving the low-volatility solvent DES and the resulting introduction of the second network of PHEAA offer high practical application value for the hydrogel-based dual encryption platform, demonstrating its potential for information security protection.

Multi-stimuli-responsive Hydrogels for Therapeutic Systems: An Overview on Emerging Materials, Devices, and Drugs

The rising interest in hydrogels nowadays is due to their usefulness in physiological conditions as multi-stimuli-responsive hydrogels. To reply to the prearranged stimuli, including chemical triggers, light, magnetic field, electric field, ionic strength, temperature, pH, and glucose levels, dual/multi-stimuli-sensitive gels/hydrogels display controllable variations in mechanical characteristics and swelling. Recent attention has focused on injectable hydrogel-based drug delivery systems (DDS) because of its promise to offer regulated, controlled, and targeted medication release to the tumor site. These technologies have great potential to improve treatment outcomes and lessen side effects from prolonged chemotherapy exposure.

Fabrication of Thermo-Responsive Controllable Shape-Changing Hydrogel

Temperature response double network (DN) hydrogels comprising a network formed by polymerization of methacrylic acid (MA) modified PVA, N,N'-methylene bis(acrylamide), N-isopropylacryl amide (NIPAM), and one formed from crystalline polyvinyl alcohol (PVA) are prepared in a 3D printed tailor-made mold. The (PVA-MA)-g-PNIPAAm thermoset intermediate is formed in water by a radical, photo-initiated process, and in the presence of dissolved PVA polymers. A subsequent freezing-thawing sequence induces the crystallization of the PVA network, which forms a second network inside the thermoset NIPAM polymer. The prepared hydrogel is thermoresponsive by the phase transition of PNIPAAm segments (T ≈ 32 °C) and has good mechanical properties (tensile strength 1.23 MPa, compressive strength 1.47 MPa). Thermal cycling between room temperature at 40 or 50 °C shows the product converses from a virgin-state to a steady-state, which most likely involves the reorganization of PVA crystals. The swelling-deswelling cycles remain clear at a length change of about 13%.

Hofmeister effect in gelatin-based hydrogels with shape memory properties

The soaking strategy with the Hofmeister effect has been proposed to fabricate gelatin- based hydrogels with excellent properties. However, the modulation mechanism of hydrogels lacks in-depth study. In this work, we studied in detail the effects of Hofmeister ions on the structural, thermal, viscoelastic and mechanical properties of gelatin hydrogels. The results showed that kosmotropic anions (Cit^3-, SO₄^2-, H₂PO₄^- and S₂O₃^2-) enhanced hydrogen bonds and hydrophobic interactions between gelatin molecules, resulting in increases in the length and content of triple helices and thus improving the properties of gelatin hydrogels. In contrast, chaotropic anions (I^- and SCN^-) weakened the interactions between gelatin molecules, and thus attenuated the properties. Based on the Hofmeister effect, we successfully fabricated gelatin poly N-methylolacrylamide (PNMA) double network hydrogels with shape memory properties. The Hofmeister effect provides an excellent route for the rational design and fabrication of functional gelatin-based hydrogels.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Multi-level information security realized in ortho-stannic acid magnesium with a single activator of Tb3+

In this work, we designed and successfully synthesized a material Mg₂SnO₄:Tb³⁺, which fully prove that the security of information has been greatly improved.