Kinetics of Sorption in Hygroscopic Hydrogels

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.

Robustly and Intrinsically Stretchable Ionic Gel-Based Moisture-Enabled Power Generator with High Human Body Conformality

Direct harvesting of energy from moist air will be a promising route to supply electricity for booming wearable and distributed electronics, with the recent rapid development of the moisture-enabled electricity generator (MEG). However, the easy spatial distortion of rigid MEG materials under severe deformation extremely inconveniences the human body with intense physical activity, seriously hindering the desirable applications. Here, an intrinsically stretchable moisture-enabled electricity generator (s-MEG) is developed based on a well-fabricated stretchable functional ionic gel (SIG) with a flexible double-network structure and reversible cross-linking interactions, demonstrating stable electricity output performance even when stretched up to 150% strain and high human body conformality. This SIG exhibits ultrahigh tensile strain (∼600%), and a 1 cm × 1 cm SIG film-based s-MEG can generate a voltage of ∼0.4 V and a current of ∼5.7 μA when absorbing water from humidity air. Based on the strong adhesion and the excellent interface combination of SIG and rough fabric electrodes induced by the fabrication process, s-MEG is able to realize bending or twisting deformation and shows outstanding electricity output stability with ∼90% performance retention after 5000 cycles of bending tests. By connecting s-MEG units in series or parallel, an integrated device of "moisture-powered wristband" is developed to wear on the wrist of humans and drive a flexible sensor for tracking finger motions. Additionally, a comfortable "moisture-powered sheath" based on s-MEGs is created, which can be worn like clothing on human arms to generate energy while walking and flexing the elbow, which is promising in the field of wearable electronics.

An Injectable, Self-Healing, Adhesive Multifunctional Hydrogel Promotes Bacteria-Infected Wound Healing

Bacterial infections have a serious impact on public health. It is urgent to develop antibacterial hydrogels with good biocompatibility to reduce the use of antibiotics. In this study, poly(lipoic acid-co-sodium lipoate)-phytic acid (P(LA-SL)-PA) hydrogels are prepared by a simple mixture of the natural small molecules lipoic acid (LA) and phytic acid (PA) in a mild and green reaction environment. The crosslinking network is constructed through the connection of covalent disulfide bonds as well as the hydrogen bonds, which endow the injectable and self-healing properties. The P(LA-SL)-PA hydrogels exhibit an adjustable compression modulus and adhesion. The in vitro agar plates assay indicates that the antibacterial rate of hydrogels against Escherichia coli and Staphylococcus aureus is close to 95%. In the rat-infected wound model, the P(LA-SL)-PA hydrogels adhere closely to the tissue and promote epithelialization and collagen deposition with a significant effect on wound healing. These results prove that the P(LA-SL)-PA hydrogels could act as effective wound dressings for promoting the healing of infected wounds.

Entangled Mesh Hydrogels with Macroporous Topologies via Cryogelation for Rapid Atmospheric Water Harvesting

Sorption-based atmospheric water harvesting (SAWH) is a promising technology to alleviate freshwater scarcity. Recently, hygroscopic salt-hydrogel composites (HSHCs) have emerged as attractive candidates with their high water uptake, versatile designability, and scale-up fabrication. However, achieving high-performance SAWH applications for HSHCs has been challenging because of their sluggish kinetics, attributed to their limited mass transport properties. Herein, a universal network engineering of hydrogels using a cryogelation method is presented, significantly improving the SAWH kinetics of HSHCs. As a result of the entangled mesh confinements formed during cryogelation, a stable macroporous topology is attained and maintained within the obtained entangled-mesh hydrogels (EMHs), leading to significantly enhanced mass transport properties compared to conventional dense hydrogels (CDHs). With it, corresponding hygroscopic EMHs (HEMHs) simultaneously exhibit faster moisture sorption and solar-driven water desorption. Consequently, a rapid-cycling HEMHs-based harvester delivers a practical freshwater production of 2.85 Lwater kgsorbents -1 day-1 via continuous eight sorption/desorption cycles, outperforming other state-of-the-art hydrogel-based sorbents. Significantly, the generalizability of this strategy is validated by extending it to other hydrogels used in HSHCs. Overall, this work offers a new approach to efficiently address long-standing challenges of sluggish kinetics in current HSHCs, promoting them toward the next-generation SAWH applications.

Macroporous, Highly Hygroscopic, and Leakage-Free Composites for Efficient Atmospheric Water Harvesting

Hygroscopic composites based on hygroscopic salts and hydrogels are promising for atmospheric water harvesting (AWH), but their relatively low water production and possible salt leakage hinder real applications. Here, we report highly hygroscopic and leakage-free composites from loading LiCl into emulsion-templated sodium alginate and poly(vinyl alcohol) hydrogels with macroporous structures and interpenetrating polymer networks. The resulting composites exhibited an enhanced moisture uptake (up to 3.4 g g-1) and leakage-free behavior even at an extremely high relative humidity (RH) of 90%. Moreover, the composites showed accelerated adsorption, with high adsorption (0.803 g g-1 water at 25 °C and 90% RH within 1 h) and desorption due to the emulsion-templated, highly interconnected macropores. The hygroscopic composites obtained 1.12 g g-1 water per adsorption-desorption collection cycle and showed high reusability, without obvious deterioration in adsorption, desorption, and collection after 10 cycles. With the presence of carbon nanotubes, solar-driven AWH could be realized, without the requirement of additional energy. The highly hygroscopic and leakage-free composites with enhanced and accelerated adsorption and desorption are excellent candidates for efficient AWH.

Intrinsic Water Transport in Moisture-Capturing Hydrogels

Moisture-capturing hydrogels have emerged as attractive sorbent materials capable of converting ambient humidity into liquid water. Recent works have demonstrated exceptional water capture capabilities of hydrogels while simultaneously exploring different strategies to accelerate water capture and release. However, on the material level, an understanding of the intrinsic transport properties of moisture-capturing hydrogels is currently missing, which hinders their rational design. In this work, we combine absorption and desorption experiments of macroscopic hydrogel samples in pure vapor with models of water diffusion in the hydrogels to demonstrate the first measurements of the intrinsic water diffusion coefficient in hydrogel-salt composites. Based on these insights, we pattern hydrogels with micropores to significantly decrease the required absorption and desorption times by 19% and 72%, respectively, while reducing the total water capacity of the hydrogel by only 4%. Thereby, we provide an effective strategy toward hydrogel material optimization, with a particular significance in pure-vapor environments.

Hierarchical Engineering of Sorption-Based Atmospheric Water Harvesters

Harvesting water from air in sorption-based devices is a promising solution to decentralized water production, aiming for providing potable water anywhere, anytime. This technology involves a series of coupled processes occurring at distinct length scales, ranging from nanometer to meter and even larger, including water sorption/desorption at the nanoscale, condensation at the mesoscale, device development at the macroscale and water scarcity assessment at the global scale. Comprehensive understanding and bespoke designs at every scale are thus needed to improve the water-harvesting performance. For this purpose, a brief introduction of the global water crisis and its key characteristics is provided to clarify the impact potential and design criteria of water harvesters. Next the latest molecular-level optimizations of sorbents for efficient moisture capture and release are discussed. Then, novel microstructuring of surfaces to enhance dropwise condensation, which is favorable for atmospheric water generation, is shown. After that, system-level optimizations of sorbent-assisted water harvesters to achieve high-yield, energy-efficient, and low-cost water harvesting are highlighted. Finally, future directions toward practical sorption-based atmospheric water harvesting are outlined.

Biopolymers for Hygroscopic Material Development

The effective management of atmospheric water will create huge value for mankind. Diversified and sustainable biopolymers that are derived from organisms provide rich building blocks for various hygroscopic materials. Here, a comprehensive review of recent advances in developing biopolymers for hygroscopic materials is provided. It is begun with a brief introduction of species diversity and the processes of obtaining various biopolymer materials from organisms. The fabrication of hygroscopic materials is then illustrated, with a specific focus on the use of biopolymer-derived materials as substrates to produce composites and the use of biopolymers as building blocks to fabricate composite gels. Next, the representative applications of biopolymer-derived hygroscopic materials for dehumidification, atmospheric water harvesting, and power generation are systematically presented. An outlook on future challenges and key issues worthy of attention are finally provided.

Extreme Water Uptake of Hygroscopic Hydrogels through Maximized Swelling-Induced Salt Loading

Hygroscopic hydrogels are emerging as scalable and low-cost sorbents for atmospheric water harvesting, dehumidification, passive cooling, and thermal energy storage. However, devices using these materials still exhibit insufficient performance, partly due to the limited water vapor uptake of the hydrogels. Here, the swelling dynamics of hydrogels in aqueous lithiumchloride solutions, the implications on hydrogel salt loading, and the resulting vapor uptake of the synthesized hydrogel-salt composites are characterized. By tuning the salt concentration of the swelling solutions and the cross-linking properties of the gels, hygroscopic hydrogels with extremely high salt loadings are synthesized, which enable unprecedented water uptakes of 1.79 and 3.86 gg-1 at relative humidity (RH) of 30% and 70%, respectively. At 30% RH, this exceeds previously reported water uptakes of metal-organic frameworks by over 100% and of hydrogels by 15%, bringing the uptake within 93% of the fundamental limit of hygroscopic salts while avoiding leakage problems common in salt solutions. By modeling the salt-vapor equilibria, the maximum leakage-free RH is elucidated as a function of hydrogel uptake and swelling ratio. These insights guide the design of hydrogels with exceptional hygroscopicity that enable sorption-based devices to tackle water scarcity and the global energy crisis.

Hydrovoltaic Electricity Generator with Hygroscopic Materials: A Review and New Perspective

The global energy crisis caused by the overconsumption of nonrenewable fuels has prompted researchers to develop alternative strategies for producing electrical energy. In this review, a fascinating strategy that simply utilizes water, an abundant natural substance throughout the globe and even in air as moisture, as a power source is introduced. The concept of the hydrovoltaic electricity generator (HEG) proposed herein involves generating an electrical potential gradient by exposing the two ends of the HEG device to dissimilar physicochemical environments, which leads to the production of an electrical current through the active material. HEGs, with a large variety of viable active materials, have much potential for expansion toward diverse applications including permanent and/or emergency power sources. In this review, representative HEGs that generate electricity by the mechanisms of diffusion, streaming, and capacitance as case studies for building a fundamental understanding of the electricity generation process are discussed. In particular, by comparing the use and absence of hygroscopic materials, HEG mechanism studies to establish active material design principles are meticulously elucidated. The review with future perspectives on electrode design using conducting nanomaterials, considerations for high performance device construction, and potential impacts of the HEG technology in improving the livelihoods are reviewed.

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.

Phospholipid Bilayer Inspired Sandwich Structural Nanofibrous Membrane for Atmospheric Water Harvesting and Selective Release

Atmospheric water harvesting (AWH) has been broadly exploited to meet the challenge of water shortage. Despite the significant achievements of AWH, the leakage of hydroscopic salt during the AWH process hinders its practical applications. Herein, inspired by the unique selective permeability of the phospholipid bilayer, a sandwich structural (hydrophobic-hydrophilic-hydrophobic) polyacrylonitrile nanofibrous membrane (San-PAN) was fabricated for AWH. The hydrophilic inner layer loaded with LiCl could capture water from the air. The hydrophobic microchannels in the outer layer could selectively allow the free transmission of gaseous water molecules but confine the hydroscopic salt solution in the hydrophilic layer, achieving continuous and recyclable water sorption/desorption. As demonstrated, the as-prepared AWH devices presented high-efficient adsorption kinetics from 1.66 to 4.08 g g-1 at 30% to 90% relative humidity. Thus, this work strengthens the understanding of the water transmission process along microchannels and provides insight into the practical applications of AWH.

Hygroscopy as an Indicator of Specific Surface Area in Polymer Materials

Specific surface area (SSA) is an integral characteristic of the interfacial surface in poly-disperse systems, widely used for the assessment of technological properties in polymer materials and composites. Hygroscopic water content (Wh) is an obligate indicator of dispersed materials prior to any analysis of their chemical composition. This study links both indicators for the purpose of the express assessment of SSA using widely available Wh data, on the example of natural (starch, cellulose) and synthetic (acrylic hydrogels) polymer materials. The standard BET analysis of SSA using water vapor desorption was chosen as a reference method. In contrast to the known empirical correlations, this study is based on the fundamental thermodynamic theory of the disjoining water pressure for the connection of the analyzed quantities. The statistical processing of the results for the new methodology and the standard BET method showed their good compliance in a wide range of SSA from 200 to 900 m2/g. The most important methodological conclusion is the possibility of an accurate physically based calculation of hydrophilic SSA in polymer materials using their Wh data at a known relative humidity in the laboratory.

Bridging the Gap-Thermofluidic Designs for Precision Bioelectronics

Bioelectronics, the merging of biology and electronics, can monitor and modulate biological behaviors across length and time scales with unprecedented capability. Current bioelectronics research largely focuses on devices' mechanical properties and electronic designs. However, the thermofluidic control is often overlooked, which is noteworthy given the discipline's importance in almost all bioelectronics processes. It is believed that integrating thermofluidic designs into bioelectronics is essential to align device precision with the complexity of biofluids and biological structures. This perspective serves as a mini roadmap for researchers in both fields to introduce key principles, applications, and challenges in both bioelectronics and thermofluids domains. Important interdisciplinary opportunities for the development of future healthcare devices and precise bioelectronics will also be discussed.

Scalable Biomass-Derived Hydrogels for Sustainable Carbon Dioxide Capture

Carbon capture and sequestration are promising emissions mitigation technologies to counteract ongoing climate change. The aqueous amine scrubbing process is industrially mature but suffers from low energy efficiency and inferior stability. Solid sorbent-based carbon capture systems present a potentially advantageous alternative. However, practical implementation remains challenging due to limited CO2 uptake at dilute concentrations and difficulty in regeneration. Here, we develop sustainable carbon-capture hydrogels (SCCH) with an excellent CO2 uptake of 3.6 mmol g-1 (400 ppm) at room temperature. The biomass gel network consists of konjac glucomannan and hydroxypropyl cellulose, facilitating hierarchically porous structures for active CO2 transport and capture. Precaptured moisture significantly enhances CO2 binding by forming water molecule-stabilized zwitterions to improve the amine utilization efficiency. The thermoresponsive SCCH exhibits a notable advantage of low regeneration temperature at 60 °C, enabling solar-powered regeneration and highlighting the potential for sustainable carbon capture to meet global decarbonization targets.

Hydratable Core-Shell Polymer Networks for Atmospheric Water Harvesting Powered by Sunlight

The development of technologies to enable fresh water harvesting from atmospheric moisture could help overcome the problem of potable water scarcity. Here, an atmospheric water harvesting (AWH) device is assembled in a core-shell structure, with the core consisting of networks of alginate (Alg) and polyaniline (PANI) and the outer layer consisting of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) modified with sulfonic acid groups (SPNIPAM) to increase the water adsorption at low relative humidity. The resulting hydrogel, modified with lithium chloride (LiCl) for increased water storage capacity (SPNIPAM-Li-PANIAlg), displays a similar lower critical solution temperature to pristine PNIPAM (32 °C) while affording a 15-fold higher water capture ratio, and releases water upon exposure to sunlight at intensities less than 1 kW m-2 . The developed AWH system is capable of harvesting 6.5 L of water per kilogram in a single daily absorption/desorption cycle under sunlight and can operate at relative humidity levels as low as 17% with no additional external energy input. The thermo-responsive hydrogel SPNIPAM-Li-PANIAlg exhibits excellent stability during natural sunlight-driven absorption/desorption cycles for at least 30 days, and allows sustainable harvesting of over 28.3 L kg-1 from a moisture-rich environment by means of multiple absorption/desorption cycles.

Hydrogel Electrolyte Enabled High-Performance Flexible Aqueous Zinc Ion Energy Storage Systems toward Wearable Electronics

To cater to the swift advance of flexible wearable electronics, there is growing demand for flexible energy storage system (ESS). Aqueous zinc ion energy storage systems (AZIESSs), characterizing safety and low cost, are competitive candidates for flexible energy storage. Hydrogels, as quasi-solid substances, are the appropriate and burgeoning electrolytes that enable high-performance flexible AZIESSs. However, challenges still remain in designing suitable and comprehensive hydrogel electrolyte, which provides flexible AZIESSs with high reversibility and versatility. Hence, the application of hydrogel electrolyte-based AZIESSs in wearable electronics is restricted. A thorough review is required for hydrogel electrolyte design to pave the way for high-performance flexible AZIESSs. This review delves into the engineering of desirable hydrogel electrolytes for flexible AZIESSs from the perspective of electrolyte designers. Detailed descriptions of hydrogel electrolytes in basic characteristics, Zn anode, and cathode stabilization effects as well as their functional properties are provided. Moreover, the application of hydrogel electrolyte-based flexible AZIESSs in wearable electronics is discussed, expecting to accelerate their strides toward lives. Finally, the corresponding challenges and future development trends are also presented, with the hope of inspiring readers.

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.

Ca-MOF-Derived Porous Sorbents for High-Yield Solar-Driven Atmosphere Water Harvesting

The development of high-yield, metal-organic framework (MOF)-based water harvesters in arid areas remains challenging due to the absence of effective strategies for enhancing water sorption capacity and kinetics. Herein, we presented a novel strategy for in situ fabrication of calcium chloride (CaCl2) decorated MOF-derived porous sorbents (PCC-42) through pyrolysis Ca-MOF and subsequently hydrochloric acid (HCl) vapor treatment process. The resulting PCC-42 sorbents exhibited a high water adsorption capacity of 3.04 g g-1 at 100% relative humidity (RH), outstanding photothermal performance, and rapid water uptake-release kinetics, surpassing most reported MOFs adsorbents. At 20, 30, 40, and 50% RH, PCC-42 demonstrated water uptake capacity of 0.45, 0.59, 0.76, and 0.9 g g-1, which represented an increase of 421 and 940% (at 20% RH) and 333 and 351% (at 30% RH) compared to Ca-MOF and CaCl2·2H2O, respectively. Approximately 80% of the adsorbed water in PCC-42 could be released under one sun within 50 min. Indoor water harvesting experiments demonstrated that PCC-42 is a promising adsorbent for various humidity environments. Additionally, outdoor solar-driven atmospheric water harvesting (AWH) tests revealed a high daily water production of 1.13 L/kgadsorbent under typical arid conditions (30-60% RH). The proposed strategy helps the design of high-performance adsorbents for solar-driven AWH in arid environments.

All-Day Multicyclic Atmospheric Water Harvesting Enabled by Polyelectrolyte Hydrogel with Hybrid Desorption Mode

Sorption-based atmospheric water harvesting (AWH) is a promising approach for mitigating worldwide water scarcity. However, reliable water supply driven by sustainable energy regardless of diurnal variation and weather remains a long-standing challenge. To address this issue, a polyelectrolyte hydrogel sorbent with an optimal hybrid-desorption multicyclic-operation strategy is proposed, achieving all-day AWH and a significant increase in daily water production. The polyelectrolyte hydrogel possesses a large interior osmotic pressure of 659 atm, which refreshes sorption sites by continuously migrating the sorbed water within its interior, and thus enhancing sorption kinetics. The charged polymeric chains coordinate with hygroscopic salt ions, anchoring the salts and preventing agglomeration and leakage, thereby enhancing cyclic stability. The hybrid desorption mode, which couples solar energy and simulated waste heat, introduces a uniform and adjustable sorbent temperature for achieving all-day ultrafast water release. With rapid sorption-desorption kinetics, an optimization model suggests that eight moisture capture-release cycles are capable of achieving high water yield of 2410 mLwater kgsorbent -1 day-1 , up to 3.5 times that of single-cyclic non-hybrid modes. The polyelectrolyte hydrogel sorbent and the coupling with sustainable energy driven desorption mode pave the way for the next-generation AWH systems, significantly bringing freshwater on a multi-kilogram scale closer.

Unusual Temperature Dependence of Water Sorption in Semicrystalline Hydrogels

Water vapor sorption is a ubiquitous phenomenon in nature and plays an important role in various applications, including humidity regulation, energy storage, thermal management, and water harvesting. In particular, capturing moisture at elevated temperatures is highly desirable to prevent dehydration and to enlarge the tunability of water uptake. However, owing to the thermodynamic limit of conventional materials, sorbents inevitably tend to capture less water vapor at higher temperatures, impeding their broad applications. Here, an inverse temperature dependence of water sorption in poly(ethylene glycol) (PEG) hydrogels, where their water uptake can be doubled with increasing temperature from 25 to 50 °C, is reported. With mechanistic modeling of water-polymer interactions, this unusual water sorption is attributed to the first-order phase transformation of PEG structures, and the key parameters for a more generalized strategy in materials development are identified. This work elucidates a new regime of water sorption with an unusual temperature dependence, enabling a promising engineering space for harnessing moisture and heat.

Sustainable, Insoluble, and Photonic Cellulose Nanocrystal Patches for Calcium Ion Sensing in Sweat

Self-assembly of cellulose nanocrystals (CNCs) is invaluable for the development of sustainable optics and photonics. However, the functional failure of CNC-derived materials in humid or liquid environments inevitably impairs their development in biomedicine, membrane separation, environmental monitoring, and wearable devices. Here, a facile and robust method to fabricate insoluble hydrogels in a self-assembled CNC-polyvinyl alcohol (PVA) system is reported. Due to the reconstruction of inter- or intra-molecular hydrogen bond interactions, thermal dehydration makes an optimized CNC/PVA photonic film form a stable hydrogel network in an aqueous solution rather than dissolve. Notably, the resulting hydrogel exhibits superb mechanical performance (stress up to 3.3 Mpa and tough up to 0.73 MJ m^-3 ) and reversible conversion between dry and wet states, enabling it convenient for specific functionalization. Sodium alginate (SA) can be adsorbed into the CNC photonic structure by swelling dry CNC/PVA film in a SA solution. The prepared hydrogel showcases the comprehensive properties of freezing resistance (-20°C), strong adhesion, satisfactory biocompatibility, and highly sensitive and selective Ca²⁺ sensing. The material could act as a portable wearable patch on the skin for the continuous analysis of calcium trends during different physical exercises, facilitating their development in precision nutrition and health monitoring.

Recent progress of antibacterial hydrogels in wound dressings

Hydrogels are essential biomaterials due to their favorable biocompatibility, mechanical properties similar to human soft tissue extracellular matrix, and tissue repair properties. In skin wound repair, hydrogels with antibacterial functions are especially suitable for dressing applications, so novel antibacterial hydrogel wound dressings have attracted widespread attention, including the design of components, optimization of preparation methods, strategies to reduce bacterial resistance, etc. In this review, we discuss the fabrication of antibacterial hydrogel wound dressings and the challenges associated with the crosslinking methods and chemistry of the materials. We have investigated the advantages and limitations (antibacterial effects and antibacterial mechanisms) of different antibacterial components in the hydrogels to achieve good antibacterial properties, and the response of hydrogels to stimuli such as light, sound, and electricity to reduce bacterial resistance. Conclusively, we provide a systematic summary of antibacterial hydrogel wound dressings findings (crosslinking methods, antibacterial components, antibacterial methods) and an outlook on long-lasting antibacterial effects, a broader antibacterial spectrum, diversified hydrogel forms, and the future development prospects of the field.

Pressure Drop Performance of Porous Composites Based on Cotton Cellulose Nanofiber and Aramid Nanofiber for Cigarette Filter Rod

Porous composites have been widely used in the adsorption and catalysis field due to their special structure, abundant sites, and light weight. In this work, an environmentally friendly porous composite was successfully prepared via a facile freeze-drying method, in which cotton cellulose nanofiber (CCNF) was adopted as the main framework to construct the connected flue structure, and aramid nanofiber (ANF) was used as a reinforcer to enhance its thermal property. As-prepared porous materials retained a regulated inter-connected hole structure and controllable porosity after ice template evolution and possessed improved resistance to thermal collapse with the introduction of a small amount of aramid nanofiber, as evaluated and verified by FTIR, SEM, and TGA measurements. With the increased addition of cotton cellulose nanofiber and aramid nanofiber, the porous composites exhibited decreased porosity and increased pressure drop performance. For the CCNF/ANF-5 sample, the pressure drop was 1867 Pa with a porosity of 7.46 cm³/g, which best met the required pressure drop value of 1870 Pa. As-prepared porous composite with adjustable interior structure and enhanced thermal property could be a promising candidate in the tobacco field.

Fe-Co controlled super-hygroscopic hydrogels toward efficient atmospheric water harvesting

Extracting atmospheric moisture for freshwater production is an appealing way to mitigate the global water crisis. However, the low moisture sorption capacity and high desorption temperature are the major bottlenecks for efficient atmospheric water harvesting. Herein, we develop a transition metal super-hygroscopic hydrogel by an economical strategy, which is constructed through a facile coordination between metal salts and ethanolamine. When the empty electron orbital of the metal ion is coordinated with the lone electron pair of nitrogen or oxygen atom, the water active sorption site is formed. A single water layer is bonded on the sites by a coordination effect, followed by physical interaction with water to form multi-layer structures. The Fe and Co ions in the hydrogel function as dual sorption sites to capture moisture, which can harvest additional water by the synergistic effect of bimetals. As a result, the bimetal hydrogel contributes to a high water uptake of 5.22 g g^-1 at 95% RH, triggering the desorption process by one solar intensity due to its low desorption temperature (≤50 °C).

Pathways to Energy-efficient Water Production from the Atmosphere

Atmospheric water harvesting (AWH) provides a fascinating chance to facilitate a sustainable water supply, which obtains considerable attention recently. However, ignoring the energy efficiency of AWH leads to high energy consumption in current prototypes (ca. 10¹ to 10²  MJ kg^-1 ), misfitting with the high-strung and complicated water-energy nexus. In this perspective, a robust evaluation of existing AWHs is conducted and a detailed way to high-efficiency AWH is paved. The results suggest that using cooling-assisted adsorption will weaken the bounds of climate to sorbent selections and have the potential to improve efficiency by more than 50%. For device design, the authors deeply elucidate how to perfect heat/mass transfer to narrow the gap between lab and practices. Reducing heat loss, recovering heat and structured sorbent are the main paths to improve efficiency on the device scale, which is more significant for a large-scale AWH. Besides efficiency, the techno-economic evaluation reveals that developing a cost-effective AWH is also crucial for sustainability, which can be contributed by green synthesis routes and biomass-based sorbents. These analyses provide a uniform platform to guide the next-generation AWH to mitigate the global water crisis.

Hygroscopic Porous Polymer for Sorption-Based Atmospheric Water Harvesting

Sorption-based atmospheric water harvesting (SAWH) holds huge potential due to its freshwater capabilities for alleviating water scarcity stress. The two essential parts, sorbent material and system structure, dominate the water sorption-desorption performance and the total water productivity for SAWH system together. Attributed to the superiorities in aspects of sorption-desorption performance, scalability, and compatibility in practical SAWH devices, hygroscopic porous polymers (HPPs) as next-generation sorbents are recently going through a vast surge. However, as HPPs' sorption mechanism, performance, and applied potential lack comprehensive and accurate guidelines, SAWH's subsequent development is restricted. To address the aforementioned problems, this review introduces HPPs' recent development related to mechanism, performance, and application. Furthermore, corresponding optimized strategies for both HPP-based sorbent bed and coupling structural design are proposed. Finally, original research routes are directed to develop next-generation HPP-based SAWH systems. The presented guidelines and insights can influence and inspire the future development of SAWH technology, further achieving SAWH's practical applications.

Exceptional water production yield enabled by batch-processed portable water harvester in semi-arid climate

Sorption-based atmospheric water harvesting has the potential to realize water production anytime, anywhere, but reaching a hundred-gram high water yield in semi-arid climates is still challenging, although state-of-the-art sorbents have been used. Here, we report a portable and modularized water harvester with scalable, low-cost, and lightweight LiCl-based hygroscopic composite (Li-SHC) sorbents. Li-SHC achieves water uptake capacity of 1.18, 1.79, and 2.93 g g−1 at 15%, 30%, and 60% RH, respectively. Importantly, considering the large mismatch between water capture and release rates, a rationally designed batch processing mode is proposed to pursue maximum water yield in a single diurnal cycle. Together with the advanced thermal design, the water harvester shows an exceptional water yield of 311.69 g day−1 and 1.09 g gsorbent−1 day−1 in the semi-arid climate with the extremely low RH of ~15%, demonstrating the adaptability and possibility of achieving large-scale and reliable water production in real scenarios.

Studies on the Impact of the Photoinitiator Amount Used during the PVP-Based Hydrogels' Synthesis on Their Physicochemical Properties

In recent times, a great interest is directed to developing biomaterials incorporated with various therapeutical substances which may enhance them with new properties and thus increase their application potential. In this work, polyvinylpyrrolidone (PVP)-based hydrogels modified with Aloe vera juice and vitamin C and differing in the amount of the photoinitiator used during their synthesis were developed. Analysis of hydrogels included characterization of their chemical structure via FT-IR spectroscopy, sorption properties, wettability, surface morphology, behavior in simulated physiological liquids and mechanical properties. Finally, hydrogels' cytotoxicity towards L929 murine fibroblasts using MTT reduction assay was additionally verified. It was demonstrated that as the amount of the photoinitiator used during the synthesis of hydrogels increased, the smoother their surface and the higher their hydrophilicity. Next, the greater the amount of the photoinitiator, the lower is the percentage elongation of the hydrogel and the greater the hardness. In turn, the swelling ability of hydrogels depended strongly on the type of the absorbed liquid-swelling ratios of samples in distilled water were 24% higher than in SBF, 18% higher than in Ringer liquid, and 32% higher than in hemoglobin wherein the amount of the photoinitiator did not affect this property. Additionally, hydrogels were stable and did not degrade in simulated physiological liquids. The only changes in pH of the incubation media were probably caused by the active substances release from hydrogels which was also confirmed via a lesser intensity of the absorption band on FT-IR spectra corresponding to the functional group occurring in compounds included in Aloe vera juice. Importantly, the viability of fibroblasts incubated with developed materials was at least 86%. Thus the hydrogels, due to their properties, seem to show application potential to be used for biomedical purposes, e.g., as innovative dressing materials.

Review of Flexible Wearable Sensor Devices for Biomedical Application

With the development of cross-fertilisation in various disciplines, flexible wearable sensing technologies have emerged, bringing together many disciplines, such as biomedicine, materials science, control science, and communication technology. Over the past few years, the development of multiple types of flexible wearable devices that are widely used for the detection of human physiological signals has proven that flexible wearable devices have strong biocompatibility and a great potential for further development. These include electronic skin patches, soft robots, bio-batteries, and personalised medical devices. In this review, we present an updated overview of emerging flexible wearable sensor devices for biomedical applications and a comprehensive summary of the research progress and potential of flexible sensors. First, we describe the selection and fabrication of flexible materials and their excellent electrochemical properties. We evaluate the mechanisms by which these sensor devices work, and then we categorise and compare the unique advantages of a variety of sensor devices from the perspective of in vitro and in vivo sensing, as well as some exciting applications in the human body. Finally, we summarise the opportunities and challenges in the field of flexible wearable devices.

Cellulose-Based Cryogel Microspheres with Nanoporous and Controllable Wrinkled Morphologies for Rapid Hemostasis

First-aid hemostatic agents for acute bleeding can save lives in emergency situations. However, rapid hemostasis remains challenging when uncontrolled hemorrhage occurs on lethal noncompressible and irregular wounds. Herein, cellulose-based cryogel microspheres with deliberately customized micromorphologies for ultrafast water transportation and diffusion, including the shark skin riblet-inspired wrinkled surface with low fluid drag and the hydrophilic nanoporous 3D networks, are developed to deal with the acute noncompressible bleeding within seconds. These cryogel microspheres can rapidly absorb a large amount of blood over 6 times their own weight in 10 s and form a robust barrier to seal a bleeding wound without applying pressure. Remarkably, massive bleeding from a cardiac penetrating hole is effectively stopped using the microspheres within 20 s and no blood leakage is observed after 30 min. Additionally, these microspheres could be readily removed without rebleeding and capillary thrombus, which is highly favorable to rapid hemostasis in emergency rescue.

Fatigue-free artificial ionic skin toughened by self-healable elastic nanomesh

Robust ionic sensing materials that are both fatigue-resistant and self-healable like human skin are essential for soft electronics and robotics with extended service life. However, most existing self-healable artificial ionic skins produced on the basis of network reconfiguration suffer from a low fatigue threshold due to the easy fracture of low-energy amorphous polymer chains with susceptible crack propagation. Here we engineer a fatigue-free yet fully healable hybrid ionic skin toughened by a high-energy, self-healable elastic nanomesh, resembling the repairable nanofibrous interwoven structure of human skin. Such a design affords a superhigh fatigue threshold of 2950 J m⁻² while maintaining skin-like compliance, stretchability, and strain-adaptive stiffening response. Moreover, nanofiber tension-induced moisture breathing of ionic matrix leads to a record-high strain-sensing gauge factor of 66.8, far exceeding previous intrinsically stretchable ionic conductors. This concept creates opportunities for designing durable ion-conducting materials that replicate the unparalleled combinatory properties of natural skins more precisely.

Developing robust skin-like sensing materials is essential for soft electronics and robotics with extended service life. Here, inspired by the repairable nanofibrous structure of human skin, the authors engineer a fatigue-resistant artificial ionic skin toughened by self-healable elastic nanomesh.

Scalable super hygroscopic polymer films for sustainable moisture harvesting in arid environments

Extracting ubiquitous atmospheric water is a sustainable strategy to enable decentralized access to safely managed water but remains challenging due to its limited daily water output at low relative humidity (≤30% RH). Here, we report super hygroscopic polymer films (SHPFs) composed of renewable biomasses and hygroscopic salt, exhibiting high water uptake of 0.64–0.96 g g⁻¹ at 15–30% RH. Konjac glucomannan facilitates the highly porous structures with enlarged air-polymer interfaces for active moisture capture and water vapor transport. Thermoresponsive hydroxypropyl cellulose enables phase transition at a low temperature to assist the release of collected water via hydrophobic interactions. With rapid sorption-desorption kinetics, SHPFs operate 14–24 cycles per day in arid environments, equivalent to a water yield of 5.8–13.3 L kg⁻¹. Synthesized via a simple casting method using sustainable raw materials, SHPFs highlight the potential for low-cost and scalable atmospheric water harvesting technology to mitigate the global water crisis.

Extracting atmospheric water is a sustainable strategy to enable decentralized access to safely managed water but remains impractical due to its limited daily water output at low relative humidity. Here, the authors demonstrate a hygroscopic polymer composed of renewable biomass which allows high water uptake at low relative humidity