Under pressure: Hydrogel swelling in a granular medium

Injectable and in situ foaming shape-adaptive porous Bio-based polyurethane scaffold used for cartilage regeneration

Irregular articular cartilage injury is a common type of joint trauma, often resulting from intense impacts and other factors that lead to irregularly shaped wounds, the limited regenerative capacity of cartilage and the mismatched shape of the scaffods have contributed to unsatisfactory therapeutic outcomes. While injectable materials are a traditional solution to adapt to irregular cartilage defects, they have limitations, and injectable materials often lack the porous microstructures favorable for the rapid proliferation of cartilage cells. In this study, an injectable porous polyurethane scaffold named PU-BDO-Gelatin-Foam (PUBGF) was prepared. After injection into cartilage defects, PUBGF forms in situ at the site of the defect and exhibits a dynamic microstructure during the initial two weeks. This dynamic microstructure endows the scaffold with the ability to retain substances within its interior, thereby enhancing its capacity to promote chondrogenesis. Furthermore, the chondral repair efficacy of PUBGF was validated by directly injecting it into rat articular cartilage injury sites. The injectable PUBGF scaffold demonstrates a superior potential for promoting the repair of cartilage defects when compared to traditional porous polyurethane scaffolds. The substance retention ability of this injectable porous scaffold makes it a promising option for clinical applications.

Gelatin/carboxymethyl chitosan/aloe juice hydrogels with skin-like endurance and quick recovery: Preparation, characterization, and properties

Gelatin-based hydrogels have gained considerable attention due to their resemblance to the extracellular matrix and hydrophilic three-dimensional network structure. Apart from providing an air-permeable and moist environment, these hydrogels optimize the inflammatory microenvironment of the wounds. These properties make gelatin-based hydrogels highly competitive in the field of wound dressings. In this study, a series of composite hydrogels were prepared using gelatin (Gel) and carboxymethyl chitosan (CMCh) as primary materials, glutaraldehyde as a crosslinker, and aloe vera juice as an anti-inflammatory component. The properties of the hydrogel, including its rheological properties, microscopic structures, mechanical properties, swelling ratios, thermal stability, antibacterial properties, and biocompatibility, were investigated. The results demonstrate that the gelatin-based hydrogels exhibit good elasticity and rapid self-healing ability. The hydrogels exhibited slight shear behavior, which is advantageous for skin care applications. Furthermore, the inclusion of aloe vera juice into the hydrogel resulted in a dense structure, improved mechanical properties and enhanced swelling ratio. The Gel/CMCh/Aloe hydrogels tolerate a compressive strength similar to that of human skin. Moreover, the hydrogels displayed excellent cytocompatibility with HFF-1 cells, and exhibited antibacterial activity against E. coli and S. aureus. Lomefloxacin was used as a model drug to study the releasing behavior of the Gel/CMCh/aloe hydrogels. The results showed that the drug was released rapidly at the initial stage, and could continue to be released for 12 h, the maximum releasing rate exceeded 20 %. These findings suggest that the gelatin-based hydrogels hold great promise as effective wound dressings.

Dielectric Barrier Discharge Plasma-Assisted Preparation of Chitosan-Based Hydrogels

Chitosan is widely used in the production of various hydrogels due to its non-biological toxicity, good biocompatibility, and strong biodegradability. However, chitosan-based hydrogels have not been widely used in tissue engineering due to their poor mechanical strength, poor stability and high biotoxicity of cross-linking agents. As a green technology, low temperature plasma is rich in active groups that can be involved in various chemical reactions, such as replacing the components on the chitosan chain, contributing to the cross-linking of chitosan. In this study, a plasma-assisted preparation method of chitosan-based hydrogels was developed and the properties, including mechanics, water absorption, and degradation (or stability), were characterized and analyzed. It is proved that plasma treatment plays a significant role in improving the mechanical strength and stability of hydrogels.

Obstructed swelling and fracture of hydrogels

Obstructions influence the growth and expansion of bodies in a wide range of settings-but isolating and understanding their impact can be difficult in complex environments. Here, we study obstructed growth/expansion in a model system accessible to experiments, simulations, and theory: hydrogels swelling around fixed cylindrical obstacles with varying geometries. When the obstacles are large and widely-spaced, hydrogels swell around them and remain intact. In contrast, our experiments reveal that when the obstacles are narrow and closely-spaced, hydrogels fracture as they swell. We use finite element simulations to map the magnitude and spatial distribution of stresses that build up during swelling at equilibrium in a 2D model, providing a route toward predicting when this phenomenon of self-fracturing is likely to arise. Applying lessons from indentation theory, poroelasticity, and nonlinear continuum mechanics, we also develop a theoretical framework for understanding how the maximum principal tensile and compressive stresses that develop during swelling are controlled by obstacle geometry and material parameters. These results thus help to shed light on the mechanical principles underlying growth/expansion in environments with obstructions.

Programmed microalgae-gel promotes chronic wound healing in diabetes

Chronic diabetic wounds are at lifelong risk of developing diabetic foot ulcers owing to severe hypoxia, excessive reactive oxygen species (ROS), a complex inflammatory microenvironment, and the potential for bacterial infection. Here we develop a programmed treatment strategy employing live Haematococcus (HEA). By modulating light intensity, HEA can be programmed to perform a variety of functions, such as antibacterial activity, oxygen supply, ROS scavenging, and immune regulation, suggesting its potential for use in programmed therapy. Under high light intensity (658 nm, 0.5 W/cm2), green HEA (GHEA) with efficient photothermal conversion mediate wound surface disinfection. By decreasing the light intensity (658 nm, 0.1 W/cm2), the photosynthetic system of GHEA can continuously produce oxygen, effectively resolving the problems of hypoxia and promoting vascular regeneration. Continuous light irradiation induces astaxanthin (AST) accumulation in HEA cells, resulting in a gradual transformation from a green to red hue (RHEA). RHEA effectively scavenges excess ROS, enhances the expression of intracellular antioxidant enzymes, and directs polarization to M2 macrophages by secreting AST vesicles via exosomes. The living HEA hydrogel can sterilize and enhance cell proliferation and migration and promote neoangiogenesis, which could improve infected diabetic wound healing in female mice.

Masking Strategy Constructed Metal Coordination Hydrogels with Improved Mechanical Properties for Flexible Electronic Sensors

Metal coordination hydrogels (MC-HGs) that introduce dynamically coordinate bonds together with metal ionic conduction have attracted considerable attention in flexible electronics. However, the traditional soaking method alleged to have technical scalability faces the challenge of forming MC-HGs with a "core-shell" structure, which undoubtedly reduces the whole mechanical properties and ionic stimulation responsiveness required for flexible electronics materials. Herein, a novel strategy referred to as "masking" has been proposed based on the theory of the valence bond and coordination chemistry. By regulating the masking agents and their concentrations as well as pairing mode with the metal ions, the whole mechanical properties of the resulting composites (MC-HGsMasking) show nearly double the values of their traditional soaking samples (MC-HGsSoaking). For example, the fracture stress and toughness of Fe-HGsMasking(SA, 5.0 g/L) are 1.55 MPa and 2.14 MJ/m3, almost twice those of Fe-HGsSoaking (0.83 MPa and 0.93 MJ/m3, respectively). Microstructure characterization combined with finite element analysis, molecular dynamics, and first-principles simulations demonstrates that the masking strategy first facilitating interfacial permeation of metal complexes and then effective coordination with functional ligands (carboxylates) of the hydrogels is the mechanism to strengthen the mechanical properties of composites MC-HGsMasking, which has the potential to break through the limitations of current MC-HGs in flexible electronic sensor applications.

3D Printable Active Hydrogels with Supramolecular Additive-Driven Adaptiveness

Smart hydrogels are a promising candidate for the development of next-generation soft materials due to their stimuli-responsiveness, deformability, and biocompatibility. However, it remains challenging to enable hydrogels to actively adapt to various environmental conditions like living organisms. In this work, supramolecular additives are introduced to the hydrogel matrix to confer environmental adaptiveness. Specifically, their microstructures, swelling behaviors, mechanical properties, and transparency can adapt to external environmental conditions. Moreover, the presence of hydrogen bonding provides the hydrogel with applicable rheological properties for 3D extrusion printing, thus allowing for the facile preparation of thickness-dependent camouflage and multistimuli responsive complex. The environmentally adaptive hydrogel developed in this study offers new approaches for manipulating supramolecular interactions and broadens the capability of smart hydrogels in information security and multifunctional integrated actuation.

Characterization of drying-induced changes in moduli and internal stresses in a constrained gel using laser vibrometry

Hydrogels, water-saturated polymer networks find widespread use in soft robotics, biomedical, pharmaceutical and food industries. Both solid and water constituents of hydrogels are sensitive to external stimuli such as temperature, humidity, osmolarity, and light. For instance, common hydrogels swell or shrink in the presence of chemical potential gradient between the sample and surrounding environment. Corresponding changes in internal water content lead to significant changes in mechanical properties of hydrogels. Besides, internal stresses build up if the gel samples are constrained during swelling or dehydration. In the present research, we utilize modal analyses technique on drying hydrogels to identify dehydration-induced changes in elastic moduli and internal stresses. In particular, natural frequencies and damping ratios of the first two axisymmetric transverse vibration modes are measured on clamped gelatin disks using non-contact laser vibrometry at various water loss states. Experimental modal frequencies are then compared to the predictions of a pre-stressed thick plate model. The evolutions of elastic moduli and internal stresses for water losses up to 80% are identified. The broadband loss capacity of gelatin is also determined from the measured modal damping ratios. Highly transient mechanical response observed on the gelatin disks further demonstrates the need for non-contact and rapid mechanical characterization of hydrogels. As illustrated in this work, vibration and wave-based techniques are promising candidates to fulfill that need.

Dynamics of underwater microparticle adhesion to soft hydrated surfaces: Modeling and analysis by time-dependent AFM force spectroscopy

HYPOTHESIS

Understanding the attachment and detachment of microparticles and living cells to surfaces is crucial for developing antifouling strategies. Hydrogel coatings have shown promise in reducing fouling and particle adhesion due to their softness and high water content, yet the mechanisms involved are dynamic and complex, and relevant parameters are not easily accessible. AFM-based force spectroscopy (FS) experiments with colloidal probe particles is a direct way of evaluating adhesive and mechanical relaxational dynamics, yet their interpretation and modeling has been challenging. The present study proposes and examines several dynamic models, suitable for quantitative analysis of FS results with model probe particle on hydrogels surfaces.

EXPERIMENTS

FS were performed using polyethylene glycol (PEG) hydrogels and polystyrene microspheres including particle attachement to the hydrogel surface (loading), holding the particle on the surface with a constant force for variable times (dwell) and pulling the particle away from the surface (unloading) FINDINGS: It was found that a viscoelastic extension of the classical JKR model with energy of adhesion unevenly distributed over the contact area and vanishing at its circumferences accurately described all FS experiments and yielded physically consistent viscoelastic and adhesive dynamic parameters, steadily changing with dwell time and applied force. The observed time evolution and force dependence were rationalized as combination of osmotic and osmo-mechnical relaxation in the contact region.

Ionization Engineering of Hydrogels Enables Highly Efficient Salt-Impeded Solar Evaporation and Night-Time Electricity Harvesting

Interfacial solar evaporation holds immense potential for brine desalination with low carbon footprints and high energy utilization. Hydrogels, as a tunable material platform from the molecular level to the macroscopic scale, have been considered the most promising candidate for solar evaporation. However, the simultaneous achievement of high evaporation efficiency and satisfactory tolerance to salt ions in brine remains a challenging scientific bottleneck, restricting the widespread application. Herein, we report ionization engineering, which endows polymer chains of hydrogels with electronegativity for impeding salt ions and activating water molecules, fundamentally overcoming the hydrogel salt-impeded challenge and dramatically expediting water evaporating in brine. The sodium dodecyl benzene sulfonate-modified carbon black is chosen as the solar absorbers. The hydrogel reaches a ground-breaking evaporation rate of 2.9 kg m-2 h-1 in 20 wt% brine with 95.6% efficiency under one sun irradiation, surpassing most of the reported literature. More notably, such a hydrogel-based evaporator enables extracting clean water from oversaturated salt solutions and maintains durability under different high-strength deformation or a 15-day continuous operation. Meantime, on the basis of the cation selectivity induced by the electronegativity, we first propose an all-day system that evaporates during the day and generates salinity-gradient electricity using waste-evaporated brine at night, anticipating pioneer a new opportunity for all-day resource-generating systems in fields of freshwater and electricity.

High-Performance Nanogel-in-Oils as Emulsion Evolution Controller for Displacement Enhancement in Porous Media

We designed and synthesized high-performance nanogel-in-oils with intermediate properties between solid particles and liquid droplets for multiphase flow control in porous media. The ultrasmall polymeric nanogels prepared via inverse emulsion polymerization were efficiently encapsulated in micrometer-sized oil droplets with the aid of surfactants during transfer from the oil phase to the aqueous phase. The composite colloidal system exhibited high loading capacity, unimodal size distribution, and long-term kinetic stability in suspension. The colloidal behaviors of nanogel-in-oils and the corresponding interfacial evolution during displacement in porous media were investigated via microfluidic experiments. In situ emulsification was observed with a state contrary to that of static characterizations. The spontaneous and sustainable formation of foam-like water-in-oil macroemulsions originated from aqueous phase breakup and oil film development, both enhanced by nanogel-in-oils. Sweeping efficiency enhancement by invasion events and residual oil transport in macroemulsion phases yielded exceptional displacement performances. Flow field fluctuations and emulsion state variations can be manipulated by adjusting nanogel-in-oil concentrations. The nanogel-in-oil suspension was found to exhibit optimal performance among the tested dispersed systems.

An energy-optimization method to study gel-swelling in confinement

We recast the problem of hydrogel swelling under physical constraints as an energy optimization problem. We apply this approach to compute equilibrium shapes of hydrogel spheres confined within a jammed matrix of rigid beads and interpret the results to determine how confinement modifies the mechanics of swollen hydrogels. In contrast to the unconfined case, we find a spatial separation of strains within the bulk of the hydrogel as the strain becomes localized to an outer region. We also explore the contact mechanics of the gel, finding a transition from Hertzian behavior to non-Hertzian behavior as a function of swelling. Our model, implemented in the Morpho shape optimization environment and validated against an experimentally demonstrated prototypical scenario, can be applied in any dimension, readily adapted to diverse swelling scenarios and extended to use other energies in conjunction.

Weakly Cross-Linked Anionic Copolymers: Kinetics of Swelling and Water-Retaining Properties of Hydrogels

Six cross-linked copolymers consisting of sodium acrylate, N-acrylamide, starch fragments and a cross-linker were synthesized, potentially suitable for use in agriculture as superabsorbents. The copolymers had the same content of carboxyl groups equal to 6.2 mmoles per 1 g of copolymer and the content of cross-linker (Q) varied from 0.04 up to 1 wt.%. The copolymers swelled in a pH 6.5 aqueous buffer solution thus giving hydrogel particles, which were characterized by a set of methods including gravimetry, rheometry, swelling pressure analysis, equilibrium centrifugation and water retention analysis with the following main conclusions. An increase in Q decreases the equilibrium degree of swelling. When swelling in a solid substrate, sand or soil, the equilibrium degree of swelling shows the maximum at Q = 0.14 wt.%. The cross-linking degree controls the swelling pressure of hydrogels and water-retaining properties of solid substrates with embedded hydrogels; in both cases, the maximum effects are observed at Q = 0.14 wt.%. These extreme dependences set the algorithm for synthesis of polymeric superabsorbents and optimization of their operational characteristics.

pH-Sensitive Hydrogel Bilayers: Investigation on Transient Swelling-Induced Bending through Analytical and FEM Approaches

pH-responsive hydrogels are recognized as versatile sensors and actuators due to their unique time-dependent properties. Specifically, pH-sensitive hydrogel-based bilayers exhibit remarkable bending capabilities when exposed to pH-triggered swelling. This study introduces a semi-analytical technique that combines non-linear solid mechanics with ionic species transport to investigate the bending behavior of such bilayers. The technique is validated through numerical simulations, exploring the influence of kinetic and geometric properties on bilayer behavior. The results highlight the significance of the interfacial region, particularly in configurations with lower hydrogel geometric ratios, which are susceptible to rupture. The study also uncovers the benefits of a lower hydrogel layer ratio in improving the swelling rate and final deflection, with a stronger effect observed in the presence of a buffer solution. Additionally, the compressibility of the elastomer contributes to the durability of the final bent shape. These findings enhance our understanding of pH-sensitive hydrogel-based bilayers and offer valuable insights for their design and optimization in diverse applications.

Aloe vera-Based Hydrogels for Wound Healing: Properties and Therapeutic Effects

Aloe vera-based hydrogels have emerged as promising platforms for the delivery of therapeutic agents in wound dressings due to their biocompatibility and unique wound-healing properties. The present study provides a comprehensive overview of recent advances in the application of Aloe vera-based hydrogels for wound healing. The synthesis methods, structural characteristics, and properties of Aloe vera-based hydrogels are discussed. Mechanisms of therapeutic agents released from Aloe vera-based hydrogels, including diffusion, swelling, and degradation, are also analyzed. In addition, the therapeutic effects of Aloe vera-based hydrogels on wound healing, as well as the reduction of inflammation, antimicrobial activity, and tissue regeneration, are highlighted. The incorporation of various therapeutic agents, such as antimicrobial and anti-inflammatory ones, into Aloe vera-based hydrogels is reviewed in detail. Furthermore, challenges and future prospects of Aloe vera-based hydrogels for wound dressing applications are considered. This review provides valuable information on the current status of Aloe vera-based hydrogels for the delivery of therapeutic agents in wound dressings and highlights their potential to improve wound healing outcomes.

Osmotically Rupturing Phagosomes in Macrophages Using PNIPAM Microparticles

The rupture of macrophage phagosomes has been implicated in various human diseases and plays a critical role in immunity. However, the mechanisms underlying this process are complex and not yet fully understood. This study describes the development of a robust engineering method for rupturing phagosomes based on a well-defined mechanism. The method utilizes microfabricated microparticles composed of uncrosslinked linear poly(N-isopropylacrylamide) (PNIPAM) as phagocytic objects. These microparticles are internalized into phagosomes at 37 °C. By exposing the cells to a cold shock at 0 °C, the vast majority of the microparticle-containing phagosomes rupture. The percentage of phagosomal rupture decreases with the increase of the cold-shock temperature. The osmotic pressure in the phagosomes and the tension in the phagosomal membrane are calculated using the Flory-Huggins theory and the Young-Laplace equation. The modeling results indicate that the osmotic pressure generated by dissolved microparticles is probably responsible for phagosomal rupture, are consistent with the experimentally observed dependence of phagosomal rupture on the cold-shock temperature, and suggest the existence of a cellular mechanism for resisting phagosomal rupture. Moreover, the effects of various factors including hypotonic shock, chloroquine, tetrandrine, colchicine, and l-leucyl-l-leucine O-methyl ester (LLOMe) on phagosomal rupture have been studied with this method. The results further support that the osmotic pressure generated by the dissolved microparticles causes phagosomal rupture and demonstrated usefulness of this method for studying phagosomal rupture. This method can be further developed, ultimately leading to a deeper understanding of phagosomal rupture.

Experimental Demonstration of Compact Polymer Mass Transfer Device Manufactured by Additive Manufacturing with Hydrogel Integration to Bio-Mimic the Liver Functions

In this paper, we designed and demonstrated a stimuli-responsive hydrogel that mimics the mass diffusion function of the liver. We have controlled the release mechanism using temperature and pH variations. Additive manufacturing technology was used to fabricate the device with nylon (PA-12), using selective laser sintering (SLS). The device has two compartment sections: the lower section handles the thermal management, and feeds temperature-regulated water into the mass transfer section of the upper compartment. The upper chamber has a two-layered serpentine concentric tube; the inner tube carries the temperature-regulated water to the hydrogel using the given pores. Here, the hydrogel is present in order to facilitate the release of the loaded methylene blue (MB) into the fluid. By adjusting the fluid’s pH, flow rate, and temperature, the deswelling properties of the hydrogel were examined. The weight of the hydrogel was maximum at 10 mL/min and decreased by 25.29% to 10.12 g for the flow rate of 50 mL/min. The cumulative MB release at 30 °C increased to 47% for the lower flow rate of 10 mL/min, and the cumulative release at 40 °C climbed to 55%, which is 44.7% more than at 30 °C. The MB release rates considerably increased when the pH dropped from 12 to 8, showing that the lower pH had a major impact on the release of MB from the hydrogel. Only 19% of the MB was released at pH 12 after 50 min, and after that, the release rate remained nearly constant. At higher fluid temperatures, the hydrogels lost approximately 80% of their water in just 20 min, compared to a loss of 50% of their water at room temperature. The outcomes of this study may contribute to further developments in artificial organ design.

Convenient tumor 3D spheroid arrays manufacturing via acoustic excited bubbles for in situ drug screening

The quick and convenient fabrication of in vitro tumor spheroids models has been pursued for clinical drug discovery and personalized therapy. Here, uniform three-dimensional (3D) tumor spheroids are quickly constructed by acoustically excited bubble arrays in a microfluidic chip and performed drug response testing in situ. In detail, bubble oscillation excited by acoustic waves induces second radiation force, resulting in the cells rotating and aggregating into tumor spheroids, which obtain controllable sizes ranging from 30 to 300 μm. These spherical tumor models are located in microfluidic networks, where drug solutions with gradient concentrations are generated from 0 to 18 mg mL^-1, so that the cell spheroids response to drugs can be monitored conveniently and efficiently. This one-step tumor spheroids manufacturing method significantly reduces the model construction time to less than 15 s and increases efficiency by eliminating additional transfer processes. These significant advantages of convenience and high-throughput manufacturing make the tumor models promising for use in tumor treatment and point-of-care diagnosis.

The Rising Aerogel Fibers: Status, Challenges, and Opportunities

Aerogel fibers garner tremendous scientific interest due to their unique properties such as ultrahigh porosity, large specific surface area, and ultralow thermal conductivity, enabling diverse potential applications in textile, environment, energy conversion and storage, and high-tech areas. Here, the fabrication methodologies to construct the aerogel fibers starting from nanoscale building blocks are overviewed, and the spinning thermodynamics and spinning kinetics associated with each technology are revealed. The huge pool of material choices that can be assembled into aerogel fibers is discussed. Furthermore, the fascinating properties of aerogel fibers, including mechanical, thermal, sorptive, optical, and fire-retardant properties are elaborated on. Next, the nano-confining functionalization strategy for aerogel fibers is particularly highlighted, touching upon the driving force for liquid encapsulation, solid-liquid interface adhesion, and interfacial stability. In addition, emerging applications in thermal management, smart wearable fabrics, water harvest, shielding, heat transfer devices, artificial muscles, and information storage, are discussed. Last, the existing challenges in the development of aerogel fibers are pointed out and light is shed on the opportunities in this burgeoning field.

Effects of pH and Crosslinking Agent in the Evaluation of Hydrogels as Potential Nitrate-Controlled Release Systems

Water scarcity and the loss of fertilizer from agricultural soils through runoff, which also leads to contamination of other areas, are increasingly common problems in agriculture. To mitigate nitrate water pollution, the technology of controlled release formulations (CRFs) provides a promising alternative for improving the management of nutrient supply and decreasing environmental pollution while maintaining good quality and high crop yields. This study describes the influence of pH and crosslinking agent, ethylene glycol dimethacrylate (EGDMA) or N,N'-methylenebis (acrylamide) (NMBA), on the behavior of polymeric materials in swelling and nitrate release kinetics. The characterization of hydrogels and CRFs was performed by FTIR, SEM, and swelling properties. Kinetic results were adjusted to Fick, Schott, and a novel equation proposed by the authors. Fixed-bed experiments were carried out by using the NMBA systems, coconut fiber, and commercial KNO₃. Results showed that on the one hand, no significant differences were observed in nitrate release kinetics for any system in the selected pH range, this fact allowing to apply these hydrogels to any type of soil. On the other hand, nitrate release from SLC-NMBA was found to be a slower and longer process versus commercial potassium nitrate. These features indicate that the NMBA polymeric system could potentially be applied as a controlled release fertilizer suitable for a wide variety of soil typologies.

Orbital fat swelling: A biomechanical theory and supporting model for spaceflight-associated neuro-ocular syndrome (SANS)

Spaceflight-Associated Neuro-ocular Syndrome (SANS) is a descriptor of several ocular and visual signs and symptoms which commonly afflicts those exposed to microgravity. We propose a new theory for the driving force leading to the development of Spaceflight-Associated Neuro-ocular Syndrome which is described via a finite element model of the eye and orbit. Our simulations suggest that the anteriorly directed force produced by orbital fat swelling is a unifying explanatory mechanism for Spaceflight-Associated Neuro-ocular Syndrome, as well as producing a larger effect than that generated by elevation in intracranial pressure. Hallmarks of this new theory include broad flattening of the posterior globe, loss of tension in the peripapillary choroid, decreased axial length, consistent with findings in astronauts. A geometric sensitivity study suggests several anatomical dimensions may be protective against Spaceflight-Associated Neuro-ocular Syndrome.

Recent advances in conductive hydrogels: classifications, properties, and applications

Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Hydrogels improved parsley (Petroselinium crispum(Mill.) Nyman) growth and development under water deficit stress

Water scarcity is one of the most pressing problems facing countries in the semi-arid and arid regions of the world. Data predicts that by 2030, global water consumption will increase by 50%, leading to severe water shortages. Today, agricultural production consumes more than 70% of fresh water in many parts of the world, increasing the pressure on water scarcity. For these reasons, agricultural production models and approaches should be developed to reduce water consumption. One developed approach is the use of hydrogel to reduce water consumption and have a positive effect on plant growth. This study investigated the use of hydrogels as chemical components that can be used in water shortage conditions and against the expected water scarcity. Parsley was used as the model organism. The method used was as follows: two different water treatments (50% and 100%) and four different hydrogel concentrations (0%, 50%, 75%, and 100%) were applied, and root width and length, leaf width and length, main stem length, and the number of tillers were measured. According to the results, while no improvement was observed in the plants with 100% hydrogel concentration, the best results were obtained from 50% hydrogel application. The results obtained from 75% hydrogel application were found to be higher than those of 100% hydrogel but lower than 0% hydrogel application. With 50% hydrogel (water-restricted), all plant growth parameters were higher compared to the plants with 100% (full irrigation) water application. It was determined that the average value of the I1 (50%) irrigation was the highest (3.6), and the average value of the I2 (100%) irrigation (2.4) was the lowest. It was determined that the highest average value (6.2) in all measured traits was the average value of the H1 (50%) application, and the lowest average value (0.0) was in the H3 hydrogel applications (100%). In conclusion, this study suggested that hydrogel application is beneficial on a large scale, can optimize water resource management for higher yields in agriculture, and has a positive effect on agricultural yield under water deficit stress.

Gel-Forming Soil Conditioners of Combined Action: Field Trials in Agriculture and Urban Landscaping

The article summarizes multivariate field trials of gel-forming soil conditioners for agriculture and urban landscaping in various climatic conditions from arid (O.A.E., Uzbekistan) to humid (Moscow region, Russia). The field test program included environmental monitoring of weather data, temperature, water-air regimes, salinity, alkalinity, and biological activity of various soils (sandy and loamy sandy Arenosols, Retisols, loamy Serozems), productivity and yield of plants (lawns, vegetables) and their quality, including pathogen infestation. The evolutionary line of polymer superabsorbents from radiation-crosslinked polyacrylamide (1995) to the patented "Aquapastus" material (2014-2020) with amphiphilic fillers and biocidal additives demonstrated not only success, but also the main problems of using hydrogels in soils (biodegradation, osmotic collapse, etc.), as well as their technological solutions. Along with innovative materials, our know-how consisted in the intelligent soil design of capillary barriers for water accumulation and antipathogenic and antielectrolyte protection of the rhizosphere. Gel-forming polymer conditioners and new technologies of their application increase the productivity of plant crops and the quality of biomass by 30-50%, with a 1.3-2-fold saving of water resources and reliable protection of the topsoil from pathogens and secondary salinization. The results can be useful to a wide range of specialists from chemical technologists to agronomists and landscapers.

The Impacts of Bio-Based and Synthetic Hydrogels on Soil Hydraulic Properties: A Review

Soil hydraulic properties are important for the movement and distribution of water in agricultural soils. The ability of plants to easily extract water from soil can be limited by the texture and structure of the soil, and types of soil amendments applied to the soil. Superabsorbent polymers (hydrogels) have been researched as potential soil amendments that could help improve soil hydraulic properties and make water more available to crops, especially in their critical growing stages. However, a lack of a comprehensive literature review on the impacts of hydrogels on soil hydraulic properties makes it difficult to recommend specific types of hydrogels that positively impact soil hydraulic properties. In addition, findings from previous research suggest contrasting effects of hydrogels on soil hydraulic properties. This review surveys the published literature from 2000 to 2020 and: (i) synthesizes the impacts of bio-based and synthetic hydrogels on soil hydraulic properties (i.e., water retention, soil hydraulic conductivity, soil water infiltration, and evaporation); (ii) critically discusses the link between the source of the bio-based and synthetic hydrogels and their impacts as soil amendments; and (iii) identifies potential research directions. Both synthetic and bio-based hydrogels increased water retention in soil compared to unamended soil with decreasing soil water pressure head. The application of bio-based and synthetic hydrogels both decreased saturated hydraulic conductivity, reduced infiltration, and decreased soil evaporation. Hybrid hydrogels (i.e., a blend of bio-based and synthetic backbone materials) may be needed to prolong the benefit of repeated water absorption in soil for the duration of the crop growing season.

Graphene Oxide-alginate Hydrogel for Drawing Water through an Osmotic Membrane

We report the preparation and evaluation of graphene oxide (GO)-enhanced alginate hydrogels for drawing water across an osmotic desalination membrane. GO-incorporated calcium alginate hydrogels (GO-HG) and pure calcium alginate hydrogels (P-HG) were synthesized for this study. Environmental scanning electron microscopy, water contact angle, and water uptake tests showed both samples to be strongly hydrophilic. The synthesized hydrogels demonstrated the ability to successfully and continuously draw water through a selective osmotic membrane in experiments. This was driven by the surface energy gradient-induced negative pressure between the more hydrophilic hydrogel and less hydrophilic membrane surface. The GO-HG was found to draw 21.2% more water than the P-HG, owing to the flexible GO nanosheets, which can be easily incorporated into the hydrogel framework. The GO nanosheets not only offer more hydrophilic functional sites but also enhance the connectivity within the alginate hydrogel framework so as to enhance the water production performance. The average amount of water drawn through the membrane by the GO-HG and the P-HG is 23.4 ± 0.9 g and 19.3 ± 1.8 g, respectively. It was found that no external stimuli were needed as water flows through the hydrogel due to gravitational force. The GO-enhanced alginate hydrogel, combined with the osmotic membrane, is a promising surface energy gradient-driven functional material for water purification and desalination without applying external pressure.

Quantifying the trade-off between stiffness and permeability in hydrogels

Hydrogels have a distinct combination of mechanical and water-transport behaviors. As hydrogels stiffen when they de-swell, they become less permeable. Here, we combine de Gennes' semi-dilute polymer theory with the Kozeny-Carman equation to develop a simple, succinct scaling law describing the relationship between mechanical stiffness and hydraulic permeability where permeability scales with stiffness to the -8/9 power. We find a remarkably close agreement between the scaling law and experimental results across four different polymer families with varied crosslinkings. This inverse relationship establishes a fundamental trade-off between permeability and stiffness.

Negatively Charged Composite Nanofibrous Hydrogel Membranes for High-Performance Protein Adsorption

Nanofibrous materials are considered as promising candidates for fabricating high-efficiency chromatography media, which are urgently needed in protein pharmaceuticals purification and biological research, yet still face several bottlenecks. Herein, novel negatively charged composite nanofibrous hydrogel membranes (NHMs) are obtained by a facile combination of electrospinning and surface coating modification. The resulting NHMs exhibit controllable morphologies and chemical structures. Benefitting from the combined effect of the stable framework of silicon dioxide (SiO₂) nanofiber and the function layer of negatively charged hydrogel, as well as good pore connectivity among nanofibers, NHMs exhibit a high protein adsorption capacity of around 1000 mg g^-1, and are superior to the commercial cellulose fibrous adsorbent (Sartobind^®) and the reported nanofibrous membranous adsorbents. Moreover, due to their relatively stable physicochemical and mechanical properties, NHMs possess comprehensive adsorption performance, favorable resistance to acid and solvents, good selectivity, and excellent regenerability. The designed NHMs composite adsorbents are expected to supply a new protein chromatography platform for effective protein purification in biopharmaceuticals and biochemical reagents.

Mechanism of temperature-induced asymmetric swelling and shrinking kinetics in self-healing hydrogels

Understanding the physical principle that governs the stimuli-induced swelling and shrinking kinetics of hydrogels is indispensable for their applications. Here, we show that the shrinking and swelling kinetics of self-healing hydrogels could be intrinsically asymmetric. The structure frustration, formed by the large difference in the heat and solvent diffusions, remarkably slows down the shrinking kinetics. The plateau modulus of viscoelastic gels is found to be a key parameter governing the formation of structure frustration and, in turn, the asymmetric swelling and shrinking kinetics. This work provides fundamental understandings on the temperature-triggered transient structure formation in self-healing hydrogels. Our findings will find broad use in diverse applications of self-healing hydrogels, where cooperative diffusion of water and gel network is involved. Our findings should also give insight into the molecular diffusion in biological systems that possess macromolecular crowding environments similar to self-healing hydrogels.

Ultrasensitive and rapid colorimetric detection of urotropin boosted by effective electrostatic probing and non-covalent sampling

Leakage and contamination of hazardous chemical substances have been widely recognized as the critical issue in ensuring human health, maintaining environmental sustainability, and safeguarding public security. Urotropin as a crucial raw material in industrial holds a potential threat to aquatic/atmospheric environment with refractory degradation problem, hence, there remains a severe challenge to effectively and on-site monitor urotropin. Here, a general design with all-in-one strategy was presented, in which a highly integrated "pocket sensing chip" combining a sampling unit and a detecting unit together endows a rapid and ultrasensitive colorimetric detection without dead-zone towards urotropin. By loading fast blue B as sensing reagent in the detecting unit, a moderate and sensitive detection towards urotropin via electrostatic interaction was achieved with detection limits of 9 μM for liquid and 17.19 ng for particulates. Furthermore, an expandable sensing chip for the purpose of simultaneously screening on multi-target exhibited remarkable applicability for examining suspicious objects with all sorts of surface in real scenes, being unacted on environmental complexity. We expect this design would provide a universal strategy and the high referential value to propel the development of handy and portable sensing device to efficiently screen the environmental relevant critical substance on-site.

Harnessing osmotic swelling stress for robust hydrogel actuators

The volumetric expansion of hydrogels driven by osmotic swelling stress has enabled hydrogel actuators for myriad applications. However, most existing studies disregard optimizing the osmotic swelling stress for powerful actuation and simply utilize the osmotic swelling stress to trigger certain modes of actuation. In this work, we probe the osmotic swelling stress of hydrogels using polyacrylamide as a model system. We design and perform constrained swelling experiments to measure the osmotic swelling stresses at different levels of constraint and compare the results to the theoretical predictions based on the Flory-Huggins model. We optimize the osmotic swelling properties by tuning the constituents and structures of the hydrogel and achieve an enhancement of the magnitude of actuation stress from ∼180 kPa to ∼400 kPa. As a proof of concept, we demonstrate a robust hydrogel jack that can lift a weight 2000 times its own weight by harnessing the high osmotic swelling stress. The feasibility and limits of harnessing the osmotic swelling stress of hydrogels for actuation are discussed.

The Characteristics of Time-Dependent Changes of Coefficient of Permeability for Superabsorbent Polymer-Soil Mixtures

Water uptake dynamics of superabsorbent polymers (SAP) in soil is of key importance for the optimum application of these materials in environmental engineering and agriculture, so goal of this paper is to determine time dependent values of coefficient of permeability for various SAP-soil mixtures. Retaining water in soil is a key requirement in critical zones to support plant growth. There is an urgent need for technologies that can increase soil water retention, given the increasing prevalence of droughts and scarcity of clean water as the climate changes, combined with the rising demand for food by a growing world population. SAPs are materials that can absorb significant amounts of water, and thus have tremendous potential to help increase water retention in soil. However, while some studies have characterized the equilibrium swelling behavior of SAPs in soil, how their addition influences the time-dependent flow of water through soil remains poorly understood. Here, we address this gap in knowledge by directly measuring the coefficient of permeability of SAP-soil mixtures, testing different soil grain sizes, SAP grain sizes, and different SAP-soil ratios. We find that SAP addition can dramatically hinder the flow rate of water through soil—reducing the permeability by several orders of magnitude, and in some cases causing complete blockage of water infiltration, at mass fractions as small as 1%. In this scenario coefficient of permeability of 1.23 × 10⁻⁴ m/s dropped by a factor of ~10 after 14 min, a factor of ~100 after 36 min, and by nearly a factor of ~1000 after 63 min, eventually causing complete blockage of infiltration after 67 min. Authors concluded that in this particular situation the size and quantity of SAP particles was enough to nearly completely fill the available pore space resulting in rendering the soil column almost completely impermeable. Moreover, we demonstrate that these effects are well-described by a simple hydraulic model of the mutual interactions between SAP and soil grains, providing more generally-applicable and quantitative principles to model SAP-soil permeability in applications. Ultimately, this work could help evaluate the optimal proportions and grain sizes of SAPs to use for a given soil to simultaneously achieve a desirable permeability along with increased water holding capacity in the plant root zone.

Creep Control in Soft Particle Packings

Granular packings display a wealth of mechanical features that are of widespread significance. One of these features is creep: the slow deformation under applied stress. Creep is common for many other amorphous materials such as many metals and polymers. The slow motion of creep is challenging to understand, probe, and control. We probe the creep properties of packings of soft spheres with a sinking ball viscometer. We find that in our granular packings, creep persists up to large strains and has a power law form, with diffusive dynamics. The creep amplitude is exponentially dependent on both applied stress and the concentration of hydrogel, suggesting that a competition between driving and confinement determines the dynamics. Our results provide insights into the mechanical properties of soft solids and the scaling laws provide a clear benchmark for new theory that explains creep, and provide the tantalizing prospect that creep can be controlled by a boundary stress.

Evaluation of Injectable Hyaluronic Acid-Based Hydrogels for Endodontic Tissue Regeneration

Dental pulp tissue engineering (TE) endeavors to regenerate dentin/pulp complex by combining a suitable supporting matrix, stem cells, and biochemical stimuli. Such procedures foresee a matrix that can be easily introduced into the root canal system (RCS) and tightly adhere to dentin walls to assure the dentin surface’s proper colonization with progenitor cells capable of restoring the dentin/pulp complex. Herein was investigated an injectable self-setting hyaluronic acid-based (HA) hydrogel system, formed by aldehyde-modified (a-HA) with hydrazide-modified (ADH), enriched with platelet lysate (PL), for endodontic regeneration. The hydrogels’ working (wT) and setting (sT) times, the adhesion to the dentine walls, the hydrogel’s microstructure, and the delivery of human dental pulp cells (DPCs) were studied in vitro. Hydrogels incorporating PL showed a suitable wT and sT and a porous microstructure. The tensile tests showed that the breaking point occurs after 4.3106 ± 1.8677 mm deformation, while in the indentation test after 1.4056 ± 0.3065 mm deformation. Both breaking points occur in the hydrogel extension. The HA/PL hydrogels exhibited supportive properties and promoted cell migration toward dentin surfaces in vitro. Overall, these results support using PL-laden HA injectable hydrogels (HA/PL) as a biomaterial for DPCs encapsulation, thereby displaying great clinical potential towards endodontic regenerative therapies.

Scaling laws to predict humidity-induced swelling and stiffness in hydrogels

From pasta to biological tissues to contact lenses, gel and gel-like materials inherently soften as they swell with water. In dry, low-relative-humidity environments, these materials stiffen as they de-swell with water. Here, we use semi-dilute polymer theory to develop a simple power-law relationship between hydrogel elastic modulus and swelling. From this relationship, we predict hydrogel stiffness or swelling at arbitrary relative humidities. Our close predictions of properties of hydrogels across three different polymer mesh families at varying crosslinking densities and relative humidities demonstrate the validity and generality of our understanding. This predictive capability enables more rapid material discovery and selection for hydrogel applications in varying humidity environments.

Poroelastic shape relaxation of hydrogel particles

Hydrogels are commonly used in research and energy, manufacturing, agriculture, and biomedical applications. These uses typically require hydrogel mechanics and internal water transport, described by the poroelastic diffusion coefficient, to be characterized. Sophisticated indentation-based approaches are typically used for this purpose, but they require expensive instrumentation and are often limited to planar samples. Here, we present Shape Relaxation (SHARE), an alternative way to assess the poroelastic diffusion coefficient of hydrogel particles that is cost-effective, straightforward, and versatile. This approach relies on first indenting a hydrogel particle via swelling within a granular packing, and then monitoring how the indented shape of the hydrogel relaxes after it is removed from the packing. We validate this approach using experiments in packings with varying grain sizes and confining stresses; these yield measurements of the poroelastic diffusion coefficient of polyacrylamide hydrogels that are in good agreement with those previously obtained using indentation approaches. We therefore anticipate that the SHARE approach will find broad use in a range of applications of hydrogels and other swellable soft materials.