Chemistries and materials for atmospheric water harvesting

Atmospheric water harvesting (AWH) is recognized as a crucial strategy to address the global challenge of water scarcity by tapping into the vast reserves of atmospheric moisture for potable water supply. Within this domain, sorbents lie in the core of AWH technologies as they possess broad adaptability across a wide spectrum of humidity levels, underpinned by the cyclic sorption and desorption processes of sorbents, necessitating a multi-scale viewpoint regarding the rational material and chemical selection and design. This Invited Review delves into the essential sorption mechanisms observed across various classes of sorbent systems, emphasizing the water-sorbent interactions and the progression of water networks. A special focus is placed on the insights derived from isotherm profiles, which elucidate sorbent structures and sorption dynamics. From these foundational principles, we derive material and chemical design guidelines and identify key tuning factors from a structural-functional perspective across multiple material systems, addressing their fundamental chemistries and unique attributes. The review further navigates through system-level design considerations to optimize water production efficiency. This review aims to equip researchers in the field of AWH with a thorough understanding of the water-sorbent interactions, material design principles, and system-level considerations essential for advancing this technology.

Ultrafast Self-Healing, Superstretchable, and Ultra-Strong Polymer Cluster-Based Adhesive Based on Aromatic Acid Cross-Linkers for Excellent Hydrogel Strain Sensors

Synthetic hydrogel strain sensors rarely exhibit a comprehensive combination of mechanical properties such as ultra-stretchability, ultrafast self-healing, and high sensitivity. Herein, seven small molecule enhanced mechanical behaviors of polymer-cluster based hydrogels are demonstrated. The oxidized polyethyleneimine/polymeric acrylic acid (ohPEI/PAA) hydrogels with aromatic formic acids as supramolecular cross-linkers are prepared by simultaneous formation of ohPEI polymer clusters and PAA upon the addition of ammonium persulfate. The optimized hydrogel adhesive exhibits comprehensive excellent properties, such as high extensibility (up to 12 298%), real-time mechanical self-healing capability (<1 s, 93% efficiency), high uniformity, underwater adhesivity, and water-sealing ability. The proper binding strength of hydrogel and skin (47 kPa) allows the hydrogel to be utilized as highly sensitive (gauge factor:16.08), highly conductive (2.58 mS cm-1), and underwater strain sensors. Specially, the adhesive strength of the adhesive to wood after dehydration is extremely high, reaching up to 29.59 MPa. Additionally, when glycerol is introduced, the obtained gel maintains the physical properties even at harsh-temperature conditions (-40 to 80 °C). It presents that multiple and hierarchical non-covalent interactions including multiple hydrogen bonding interactions, π-π stacking, electrostatic interactions, and dipole-dipole interactions of polymer clusters, allow for the energy dissipation and contribute to the excellent performance of the hydrogel.

Hygroscopic-Microgels-Enabled Rapid Water Extraction from Arid Air

Sorbent-based atmospheric water harvesting (AWH) has emerged as a promising decentralized water-production technology to mitigate the freshwater crisis in arid areas. Hydrogels have been regarded as attractive sorbents due to their high water retention and tailorable polymer-water interactions. Yet, the kinetics of water sorption and desorption at low relative humidity (RH) shall be improved for their practical implementation. Here, hygroscopic microgels (HMGs) composed of hydroxypropyl cellulose (HPC) and hygroscopic salt are reported, which achieve a water uptake of ca. 0.5-0.8 g g-1 at 15-30% RH. HMGs enable rapid sorption-desorption kinetics owing to the short-distance diffusion in the microgels and hydrophilicity-hydrophobicity switching of the thermoresponsive HPC. To validate the feasibility of HMGs for moisture extraction, a potential daily water collection of up to equivalent 7.9-19.1 L kg-1 at low RH is demonstrated, enabled by 24-36 operation cycles per day based on the material-level experiments. With renewable raw materials and superior performance, HMGs provide a sustainable approach for rapid moisture extraction in arid climates.

Recent advances in double network hydrogels based on naturally-derived polymers: synthesis, properties, and biological applications

Hydrogels composed of naturally-derived biopolymers have garnered significant research interest due to the bioavailability and biocompatibility of starting materials. However, translating these advantages to practical use is challenged by limitations of mechanical properties and stability of the resulting materials. The development of double network (DN) hydrogels has led to greatly enhanced mechanical properties and shows promise toward broadening the applications of conventional synthetic or natural hydrogels. This review highlights recently developed protein-based and polysaccharide-based DN hydrogels. For each biopolymer, we focus on a subset of DN hydrogels centered around a theme related to synthetic design or applications. Network structures and crosslinking mechanisms that endow enhanced mechanical properties and performance to the materials are discussed. Important applications, including tissue engineering, drug delivery, bioadhesives, wound healing, and wearable sensors, that arise from the inherent properties of the natural polymer or its combination with other materials are also emphasized. Finally, we discuss ongoing challenges to stimulate the discovery of new design principles for the future of DN hydrogels based on naturally-derived polymers for biological applications.

Sorption-Based Atmospheric Water Harvesting: Materials, Components, Systems, and Applications

Freshwater scarcity is a global challenge posing threats to the lives and daily activities of humankind such that two-thirds of the global population currently experience water shortages. Atmospheric water, irrespective of geographical location, is considered as an alternative water source. Sorption-based atmospheric water harvesting (SAWH) has recently emerged as an efficient strategy for decentralized water production. SAWH thus opens up a self-sustaining source of freshwater that can potentially support the global population for various applications. In this review, the state-of-the-art of SAWH, considering its operation principle, thermodynamic analysis, energy assessment, materials, components, different designs, productivity improvement, scale-up, and application for drinking water, is first extensively explored. Thereafter, the practical integration and potential application of SAWH, beyond drinking water, for wide range of utilities in agriculture, fuel/electricity production, thermal management in building services, electronic devices, and textile are comprehensively discussed. The various strategies to reduce human reliance on natural water resources by integrating SAWH into existing technologies, particularly in underdeveloped countries, in order to satisfy the interconnected needs for food, energy, and water are also examined. This study further highlights the urgent need and future research directions to intensify the design and development of hybrid-SAWH systems for sustainability and diverse applications.

Bile Acid Sequestrants Based on Natural and Synthetic Gels

Bile acid sequestrants (BASs) are non-systemic therapeutic agents used for the management of hypercholesterolemia. They are generally safe and not associated with serious systemic adverse effects. Usually, BASs are cationic polymeric gels that have the ability to bind bile salts in the small intestine and eliminate them by excretion of the non-absorbable polymer-bile salt complex. This review gives a general presentation of bile acids and the characteristics and mechanisms of action of BASs. The chemical structures and methods of synthesis are shown for commercial BASs of first- (cholestyramine, colextran, and colestipol) and second-generation (colesevelam and colestilan) and potential BASs. The latter are based on either synthetic polymers such as poly((meth)acrylates/acrylamides), poly(alkylamines), poly(allylamines) and vinyl benzyl amino polymers or biopolymers, such as cellulose, dextran, pullulan, methylan, and poly(cyclodextrins). A separate section is dedicated to molecular imprinting polymers (MIPs) because of their great selectivity and affinity for the template molecules used in the imprinting technique. Focus is given to the understanding of the relationships between the chemical structure of these cross-linked polymers and their potential to bind bile salts. The synthetic pathways used in obtaining BASs and their in vitro and in vivo hypolipidemic activities are also introduced.

Synthesis of Gallic Acid-Loaded Chitosan-Grafted-2-Acrylamido-2-Methylpropane Sulfonic Acid Hydrogels for Oral Controlled Drug Delivery: In Vitro Biodegradation, Antioxidant, and Antibacterial Effects

In this study, chitosan (CS) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS)-based hydrogels were formulated by the free radical polymerization technique for the controlled release of gallic acid. Fourier transform infrared spectroscopy (FTIR) confirmed the successful preparation and loading of gallic acid within the hydrogel network. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) confirmed the increased thermal stability of the hydrogels following the crosslinking and polymerization of chitosan and AMPS. In X-ray diffraction analysis (XRD), the crystallinity of the raw materials decreased, indicating strong crosslinking of the reagents and the formation of a new polymeric network of hydrogels. Scanning electron microscopy (SEM) revealed that the hydrogel had a rough, dense, and porous surface, which is consistent with the highly polymerized composition of the hydrogel. After 48 h, the hydrogels exhibited higher swelling at pH 1.2 (swelling ratio of 19.93%) than at pH 7.4 (swelling ratio of 15.65%). The drug release was analyzed using ultraviolet-visible (UV-Vis) spectrophotometer and demonstrated that after 48 h, gallic acid release was maximum at pH 1.2 (85.27%) compared to pH 7.4 (75.19%). The percent porosity (78.36%) and drug loading increased with the increasing concentration of chitosan and AMPS, while a decrease was observed with the increasing concentration of ethylene glycol dimethyl methacrylate (EGDMA). Crosslinking of the hydrogels increased with concentrations of chitosan and EGDMA but decreased with AMPS. In vitro studies demonstrated that the developed hydrogels were biodegradable (8.6% degradation/week) and had antimicrobial (zone of inhibition of 21 and 16 mm against Gram-positive bacteria Escherichia coli and Staphylococcus aureus as well as 13 mm against Gram-negative bacteria Pseudomonas aeruginosa, respectively) and antioxidant (73% DPPH and 70% ABTS) properties. Therefore, the prepared hydrogels could be used as an effective controlled drug delivery system.

Feedback-controlled topological reconfiguration of molecular assemblies for programming supramolecular structures

In biology, nonequilibrium assembly is characterized by fuel-driven switching between associating and nonassociating states of biomolecules. This dynamic assembly model has been used routinely to describe the nonequilibrium processes in synthetic systems. Here, we present a G-quartet-based nonequilibrium system based on fuel-driven co-assembly of guanosine 5'-monophosphate disodium salt hydrate and urease. Addition of lanthanum(III) ions to the system caused macroscopic dynamic switching between precipitates and hydrogels. Interestingly, combined analyses of the nonequilibrium systems demonstrated that molecules could switch between two distinct associating states without undergoing a nonassociating state. This finding suggested a nonequilibrium assembly mechanism of topological reconfiguration of molecular assemblies. We detailed quantitatively the nonequilibrium assembly mechanism to precisely control the phase behaviors of the active materials; thus, we were able to use the materials for transient-gel-templated polymerization and transient circuit connection. This work presents a new nonequilibrium system with unusual phase behaviors, and the resultant active hydrogels hold promise in applications such as fluid confinements and transient electronics.

Super Water-Extracting Gels for Solar-Powered Volatile Organic Compounds Management in the Hydrological Cycle

Water-soluble volatile organic compounds (VOCs) are widely spread in the natural hydrological cycle, contaminating potential water sources, and leading to unexpected ecological hazards. However, water-purification technologies toward VOCs are energy-intensive and present unsatisfactory purity of the obtained water. The fundamental challenge is to differentiate the motion of water and VOC molecules by separators. Here, the concept of a super water-extracting gel (SWEG) for VOC-management and water purification via direct solar distillation is proposed. The strong hydrogen bonding effect in the hypercrosslinked hydrophilic polymeric networks enables the SWEG to extract water from VOC-containing water, which rejects the VOC solutes while allowing water through for interfacial evaporation. The obtained SWEG achieves a VOCs removal ratio up to 99.99% by solar distillation under 1 sun. A solar water-purification system is also demonstrated to produce clean water, which surpasses other competitive technologies based on electricity.

Kinetics of Sorption in Hygroscopic Hydrogels

Hygroscopic hydrogels hold significant promise for high-performance atmospheric water harvesting, passive cooling, and thermal management. However, a mechanistic understanding of the sorption kinetics of hygroscopic hydrogels remains elusive, impeding an optimized design and broad adoption. Here, we develop a generalized two-concentration model (TCM) to describe the sorption kinetics of hygroscopic hydrogels, where vapor transport in hydrogel micropores and liquid transport in polymer nanopores are coupled through the sorption at the interface. We show that the liquid transport due to the chemical potential gradient in the hydrogel plays an important role in the fast kinetics. The high water uptake is attributed to the expansion of hydrogel during liquid transport. Moreover, we identify key design parameters governing the kinetics, including the initial porosity, hydrogel thickness, and shear modulus. This work provides a generic framework of sorption kinetics, which bridges the knowledge gap between the fundamental transport and practical design of hygroscopic hydrogels.

Highly Elastic Interconnected Porous Hydrogels through Self-Assembled Templating for Solar Water Purification

Interfacial evaporation using porous hydrogels has demonstrated highly effective solar evaporation performance under natural sunlight to ensure an affordable clean water supply. However, it remains challenging to realize scalable and ready-to-use hydrogel materials with durable mechanical properties. Here, self-assembled templating (SAT) is developed as a simple yet effective method to fabricate large-scale elastic hydrogel evaporators with excellent desalination performance. The highly interconnected porous structure of the hydrogels with low tortuosity and tunable pore size enables high level of tunability on the water transport rate. With superior elasticity, the porous hydrogels are easy to process with a rapid shape recovery after being rolled, folded, and twisted over hundred times, and exhibit highly effective and stable evaporation with an evaporation rate of ≈2.8 kg m^-2  h^-1 and ≈90 % solar-to-vapor efficiency. It is anticipated that this SAT strategy, without the typical need for freeze-drying, will accelerate the industrialization of hydrogel solar evaporators for practical applications.