Strong, tough and conductive single-network hydrogels based on deswelling and the salting-out effect.

In recent years, the continuous attention given to increasing the fracture toughness and Young's modulus of polymeric gels has gradually shifted from toughening strategies on double-network (DN) gels to single-network (SN) gels. The salt-soaking method has been adopted to realize the toughening of SN gels through the salting-out effect and deswelling, constructing dense network structures with simultaneously precipitated polymer chains and cross-links. By comparing the mechanical properties between salt-treated hydrogels and air-dried hydrogels, the increased polymer chain concentration is proved to promote energy transfer by enlarging the dissipation region size due to the unwinding and slippage of coiled chains during stretching. The newly formed cross-link points in salt-treated hydrogels are considered to consume more deformation energy during stretching. The synergistic effect in energy transfer and dissipation arising from increases in polymer fraction and cross-linking plays an indispensable role in toughening SN hydrogels. In addition, the soaking process introduces abundant free ions to endow hydrogels with prominent conductivity. Thus, this salt-soaking method provides a general approach to synthesize strong, tough and conductive hydrogels with applications in flexible electrical devices.

Effects of different cellular and subcellular characteristics on the atmospheric Pb uptake, distribution and morphology in Tillandsia usneoides leaves.

Lead (Pb) is a widespread highly toxic and persistent environmental pollutant. Plant leaves play a key role in accumulating atmospheric Pb, but its distribution in different cells and subcellular structures and the factors affecting it have been little studied. Here, Tillandsia usneoides, an indicator plant for atmospheric heavy metals, was treated with an aerosol generation device to analyze Pb contents in different cells (three types of cells in leaf surface scales, epidermal cells, mesophyll cells, vascular bundle cells), subcellular structures (cell wall, cell membrane, vacuoles, and organelles) and cell wall components (pectin, hemicellulose 1 and 2, and cellulose). Results show the different cells of T. usneoides leaves play distinct roles in the process of Pb retention. The outermost wing cells are structures that capture external pollutants, while mesophyll cells, as the aggregation site after material transport, ring cells, disc cells, epidermal cells, and vascular cells are material transporters. Pb was only detected in the cell wall and pectin, indicating the cell wall was the dominant subcellular structure for Pb retention, while pectin was the main component affecting Pb retention. FTIR analysis of cell wall components indicated the esterified carboxyl (CO) functional group in pectin may function in absorbing Pb. Pb entered leaf cells mainly in the form of low toxicity and activity to enhance its resistance.

Strong and tough polysaccharide organohydrogels for strain, humidity and temperature sensors.

To avoid the potential toxicity of monomer residues in synthetic polymer based organohydrogels, natural polysaccharide-based organohydrogels are expected to be used in multi-functional wearable sensory systems, but most of them have unsatisfactory stiffness, strength and fracture toughness. Herein, a cooking and soaking strategy is proposed to prepare novel natural polysaccharide-based organohydrogels possessing outstanding stiffness, strength, toughness, freezing resistance, heating resistance and long-term durability. The agar organohydrogel exhibits a fracture stress of 3.3 MPa, a Young's modulus of 2.26 MPa and a fracture toughness of 14.8 kJ m-2, the κ-carrageenan organohydrogel exhibits a fracture stress of 3.3 MPa, a Young's modulus of 4.34 MPa and a fracture toughness of 11.0 kJ m-2, and the gellan organohydrogel exhibits a fracture stress of 1.2 MPa, a Young's modulus of 2.81 MPa and a fracture toughness of 5.4 kJ m-2. Furthermore, the agar organohydrogels are assembled into multi-functional wearable sensors by introducing NaCl as a conducting agent exhibiting responses to strain (5-150%), temperature (-15 to 60 °C) and humidity (11-97%), and possessing exceptional multi-sensory capabilities. Therefore, the developed strategy has shown a new pathway towards strengthening polysaccharide-based organohydrogels with potential for application in wearable sensory systems.

A physics-guided modelling method of artificial neural network for multiaxial fatigue life prediction under irregular loading.

To improve the generalization of the artificial neural network (ANN) model on the prediction of multiaxial irregular cases, a physics-guided modelling method is proposed with inspiration from the Basquin-Coffin-Manson equation. The method suggested using two neurons in the last hidden layer of the ANN model and constraining the sign of weight and bias value. In this way, the prior physical knowledge of fatigue life distribution is introduced into the ANN model, which resulted in a satisfactory performance on the life prediction of multiaxial loading cases and better extrapolation ability. Furthermore, the physics-guided ANN model can also provide satisfactory prediction on irregular cases with the training of only regular cases. Compared with the conventional model, the average relative error and root mean squared error (RMSE) of prediction decreased by 33.29% and 44.29%, respectively. It greatly broadens the application scenarios of neural networks on multiaxial fatigue life prediction. This article is part of the theme issue 'Physics-informed machine learning and its structural integrity applications (Part 1)'.

Stable isotopes in tree rings record physiological trends in Larix gmelinii after fires.

Fire is an important regulator of ecosystem dynamics in boreal forests, and in particular has a complicated association with growth and physiological processes of fire-tolerant tree species. Stable isotope ratios in tree rings are used extensively in eco-physiological studies for evaluating the impact of past environmental (e.g., drought and air pollution) factors on tree growth and physiological processes. Yet, such studies based on carbon (δ13C) and oxygen (δ18O) isotope ratios in tree rings are rarely conducted on fire effect, and are especially not well explored for fire-tolerant trees. In this study, we investigated variations in basal area increment and isotopes of Larix gmelinii (Rupr.) Rupr. before and after three moderate fires (different fire years) at three sites across the Great Xing'an Mountains, Northeastern China. We found that the radial growth of L. gmelinii trees has significantly declined after the fires across study sites. Following the fires, a simultaneous increase in δ13C and δ18O has strengthened the link between the two isotopes. Further, fires have significantly enhanced the 13C-derived intrinsic water-use efficiency (iWUE) and largely altered the relationships between δ13C, δ18O, iWUE and climate (temperature and precipitation). A dual-isotope conceptual model revealed that an initial co-increase in δ13C and δ18O in the fire year can be mainly attributed to a reduction in stomatal conductance with a constant photosynthetic rate. However, this physiological response would shift to different patterns over post-fire time between sites, which might be partly related to spring temperature. This study is beneficial to better understand, from a physiological perspective, how fire-tolerant tree species adapt to a fire-prone environment. It should also be remembered that the limitation of model assumptions and constraints may challenge model applicability and further inferred physiological response.

[Bark characteristics of Betula platyphylla at different heights and their environmental interpretation under fire disturbance]. / ç«å¹²æ‰°ä¸‹ç™½æ¡¦æ ‘å¹²ä¸åŒé«˜åº¦çš„æ ‘çš®æ€§çŠ¶åŠå ¶çŽ¯å¢ƒè§£é‡Š.

Betula platyphylla is a pioneer tree species after fire disturbance in forest communities in the Daxing'an Mountains of China. Bark, as an external structure of vascular cambium, plays an important role in protection and transport. To understand the survival strategy of B. platyphylla under fire disturbance, we analyzed the functional traits of inner and outer bark of B. platyphylla at different heights (0.3, 0.8 and 1.3 m) in natural secondary forest of the Daxing'an Mountains. We further quantified the explanation of three environmental factors (stand, topography and soil) and identified the key factors driving the changes in those traits. The results showed that the relative inner bark thickness of B. platyphylla in burned plot followed an order of 0.3 m (4.7%) > 0.8 m (3.8%) > 1.3 m (3.3%), which was 28.6%, 14.4% and 3.1% higher than that in the unburned plot (30-35 years without fire disturbance), respectively. The relative outer bark thickness and the relative total bark thickness showed similar pattern with tree height. Fire had different effects on other bark functional traits of B. platyphylla. The inner bark density of B. platyphylla in burned plot was significantly decreased by 3.8%-5.6% and water content was significantly increased by 11.0%-12.2%, compared with that in unburned plot across the three heights. However, the contents of carbon, nitrogen, and phosphorus in inner (or outer) bark were not significantly affected by fire. Further, the mean inner bark nitrogen content at 0.3 m in burned plot (5.24 g·kg-1) was significantly higher than that at the other two heights (4.56-4.76 g·kg-1). Environmental factors explained 49.6% and 28.1% of the total variation in inner and outer bark functional traits, respectively, with the highest single explanation (18.9% or 9.9%) of soil factors. Diameter at breast height was the most important factor affecting the growth of inner and outer barks. In summary, fire affected survival strategies of B. platyphylla (e.g., increased the resource allocation to the base bark) via changing the environment factors, which would help improve their defense ability under fire disturbance.

A strong and tough gelatin/polyvinyl alcohol double network hydrogel actuator with superior actuation strength and fast actuation speed.

Hydrogels are widely used in actuators that are applied in numerous fields such as multifunctional sensors, soft robots, artificial muscles, manipulators and microfluidic valves, and yet their applications in soft robots and artificial muscles are often limited by low actuation strength and slow actuation speed. Here, we develop a hydrogel actuator with high actuation strength (contraction strength of 850 kPa), fast actuation speed (response time of 90 s) and high energy density (output working density of 72 kJ m-3) by introducing a storing-releasing elastic potential energy method into a double network hydrogel. The high actuation strength is owing to the double network hydrogel, which possesses a high elastic modulus of 1.3 MPa, fracture strength of 1.8 MPa, and fracture energy of 16 kJ m-2. The fast actuation speed is due to the storing-releasing elastic potential energy method, which stretches the hydrogel and locks the hydrogel at deformed shape under external stimuli to store the elastic potential energy and then makes the hydrogel contract rapidly under new stimuli to release the pre-stored energy. A capture actuator and a hand muscle actuator are fabricated to achieve strong and fast actuation. The hydrogel actuator has shown potential applications in soft robots and artificial muscles.

Polyvinyl alcohol/gelatin hydrogels regulate cell adhesion and chromatin accessibility.

Cell adhesion has a critical influence on various processes such as cancer metastasis and wound healing. Many substrates have been used for studying cell adhesion and its related biological processes, it is still highly desirable to have a simply prepared and low-cost substrate suitable for regulating cell adhesion. In this study, we produced a series of polyvinyl alcohol/gelatin hydrogels with different gelatin concentrations via dry-annealing method. Our data showed that the protein adsorbing capability was enhanced and cell adhesion area and the ratio of non-spherical cells were increased with the increment of gelatin concentration. We also observed that varying cell adhesion conditions induced by polyvinyl alcohol /gelatin hydrogels resulted in expression level changes of genes involved in mechanotransduction from extracellular matrices (ECM) to the nucleus. In particular, we detected a widespread increase in chromatin accessibility under poor cell adhesion condition. This work provides a useful hydrogel system for regulating cell adhesion and opens up new possibilities for the design of biomaterials for cell adhesion study.

Stiff and strong hydrogel tube with great mechanical properties and high stability in various solutions.

Hydrogel tubes are widely used in fields such as artificial blood vessels, drug delivery, biomedical scaffolds and cell adhesion, yet their application is often limited by unsatisfactory mechanical properties and poor stability in various solutions. Herein, a novel hydrogel tube exhibiting a remarkable mechanical performance and stability in various solutions is prepared by introducing a dual physically cross-linked double network (DN) hydrogel matrix. The obtained hydrogel tube can withstand ∼60 N load without fracture and be stretched to over twice its original length before and after immersing in various solutions. The great mechanical properties and stability in various solutions of hydrogel tubes are due to the introduction of a dual physically cross-linked poly(acrylamide-co-acrylic acid)/carboxymethylcellulose sodium/Fe3+ DN hydrogel, which possesses high elastic modulus (3.71 MPa), fracture energy (15.4 kJ m-2), and great stability in various solutions. In addition, the hydrogel tubes with different thickness, diameters, shapes and the multiple branched hydrogel tubes can also be fabricated to enable further functionalization for application requirements. Therefore, this new type of hydrogel tube presents tremendous potential for applications in biomedical and engineering fields.

Atmospheric Pb induced hormesis in the accumulator plant Tillandsia usneoides.

While numerous studies reported hormesis in plants exposed to heavy metals, metals were commonly added in the growth substrate (e.g. soil or solution). The potential of heavy metals in the atmosphere to induce hormesis in plants, however, remains unknown. In this study, we exposed the widely-used accumulator plant Tillandsia usneoides to 10 atmospheric Pb concentrations (0-25.6 µg·m-3) for 6 or 12 h. Three types of dose-response relationships between different response endpoints (biomarkers) and Pb concentrations were found for T. usneoides. The first was a monophasic dose response, in which the response increased linearly with increasing Pb concentrations, as seen for metallothionein (MT) content after a 6-h exposure. The second and dominating type was a biphasic-hormetic dose response, exhibited by malondialdehyde (MDA), superoxide anion radical (O2-), and superoxide dismutase (SOD) after 6 or 12 h of exposure and by glutathione (GSH) and MT content after 12 h of treatment. The third type was a triphasic dose response, as seen for leaf electric conductivity after 6 or 12 h of exposure and GSH after 6 h of exposure. This finding suggests that Pb inhibited the response of T. usneoides at very low concentrations, stimulated it at low-to-moderate concentrations, and inhibited it at higher concentrations. Our results demonstrate diverse adaptation mechanisms of plants to stress, in the framework of which alternating between up- and down-regulation of biomarkers is at play when responding to different levels of toxicants. The emergence of the triphasic dose response will further enhance the understanding of time-dependent hormesis.

Extremely stretchable and tough hybrid hydrogels based on gelatin, κ-carrageenan and polyacrylamide.

Nowadays, several approaches are being suggested to endow hydrogels with improved mechanical properties for practical applications as cartilage and skin replacements, soft electronics, and actuators. However, it remains a challenge to develop DN gels with both high fracture toughness and fracture stretch. Here, we introduce (bio)polyelectrolyte complexes (PECs) consisting of gelatin and κ-carrageenan as the first brittle network and covalently crosslinked polyacrylamide (PAAm) as the second stretchable network to fabricate a highly stretchable and notch-insensitive gelatin/κ-carrageenan/PAAm hydrogel. The unprecedented high stretchability (∼51.7) is ascribed to the reduction of stress concentration and defects in the network structure through the fracture of the PEC gel. In addition, a high fracture toughness (∼16053.34 J m-2) is achieved by effective energy transfer between the PECs and PAAm gel due to their covalent crosslinking, and efficient energy dissipation through destroying inter- and intramolecular interactions in the PEC gel.

Preparation of stretchable and self-healable dual ionically cross-linked hydrogel based on chitosan/polyacrylic acid with anti-freezing property for multi-model flexible sensing and detection.

As a kind of promising material for flexible wearable electronics, conductive hydrogels have attracted extensive interests of researchers for their inherent merits such as superior mechanical properties, biocompatibility, and permeability. Herein, we constructed a new type of highly stretchable, anti-freezing, self-healable, and conductive hydrogel based on chitosan/polyacrylic acid. The large amount of ions inside the network had five functions for the proposed hydrogel, including excellent mechanical behaviors, high conductivity, self-recovery, self-healing and anti-freezing capability. Consequently, the proposed hydrogel possessed tunable stretchability (1190-1550%), tensile strength (0.96-2.56 MPa), toughness (5.7-14.7 MJ/m3), superior self-healing property (self-healing efficiency up to 83.7%), high conductivity (4.58-5.76 S/m), and excellent anti-freezing capability. To our knowledge, the self-healable hydrogel with balanced tensile strength, toughness, conductivity, and low-temperature tolerance can hardly be achieved till now. Furthermore, the conductive hydrogels exhibited high sensitivity (gauge factor up to 10.8) in a broad strain window (0-1000%) and could detect the conventional motion signals of human body such as bending of a knuckle, swallowing, and pressure signal at both room temperature and -20 °C. Moreover, the hydrogels could also be fabricated as flexible detectors to identify different temperatures, different kinds of solutions, and different concentrations of the solution.

An extremely tough and ionic conductive natural-polymer-based double network hydrogel.

Hydrogels are widely used in fields such as drug delivery, tissue regeneration, soft robotics and flexible smart electronic devices, yet their application is often limited by unsatisfactory mechanical behaviors. Among the various improvement strategies, double network (DN) hydrogels from synthetic polymers demonstrated impressive mechanical properties, while those from natural polymers were usually inferior. Here, a novel DN hydrogel composed fully of natural polymers exhibiting remarkable mechanical properties and conductivity is prepared by simply soaking a virgin gellan gum/gelatin composite hydrogel in a mixed solution of Na2SO4 and (NH4)2SO4. This hydrogel exhibits a tunable Young's modulus (0.08 to 42.6 MPa), good fracture stress (0.05 to 7.5 MPa), good fracture stretch (1.4 to 7.1), high fracture toughness (up to 27.7 kJ m-2), and high ionic conductivity (up to 11.4 S m-1 at f = 1 kHz). The improvement in the mechanical properties of the DN gel is attributed to the chain-entanglement crosslinking points introduced by SO42- in the gelatin network and the electrostatic interaction crosslinking points introduced by Na+ in the gellan gum network. The high ionic conductivity of the DN gel is attributed to the infiltration of the DN gel in a salt solution of high concentration. The developed gellan gum/gelatin DN hydrogel has shown a new pathway towards strengthening natural-polymer-based DN hydrogels and towards potential applications in biomedical engineering and flexible electronic devices.

Cellular and subcellular distribution and factors influencing the accumulation of atmospheric Hg in Tillandsia usneoides leaves.

Atmospheric Hg is a highly toxic heavy metal with bioaccumulative properties. However, relatively few studies have focused on the distribution of Hg in cellular and subcellular structures of plants and factors influencing its accumulation. In this study, we selected Tillandsia usneoides, which is a widely used bioindicator for Hg, to analyze the concentration of Hg in different cells (foliar trichomes, epidermal cells, mesophyll cells, and vascular bundle cells), different subcellular structures (cell wall, cell membrane, vacuoles, and organelles) and different cell wall components (pectin, hemicellulose 1, and hemicellulose 2). It was determined that Hg was present in different types of cells, but there was no significant difference, suggesting that atmospheric Hg circulates dynamically in the surface and internal structural cells of T. usneoides leaves. Subcellular analysis showed that as Hg concentration increased, more Hg accumulated in the vacuoles and cell wall through the compartmentalization mechanism. Hemicellulose had the highest content of Hg, indicating that it is the primary Hg-binding component of the cell wall. The FTIR analysis results showed that after the Hg treatment, the cell wall -OH and COO- absorption peaks changed most significantly, indicating that these functional groups play a vital role in the Hg accumulation process.

Biomarker Responses of Spanish Moss Tillandsia usneoides to Atmospheric Hg and Hormesis in This Species.

Hg is an environmental pollutant with severe biotoxicity. Epiphytic Tillandsia species, especially Spanish moss T. usneoides, are widely used as the bioindicator of Hg pollution. However, the effects of different Hg concentrations on Tillandsia have been rarely studied and the occurrence of hormesis in Tillandsia species has not been determined. In this study, T. usneoides was subjected to stress induced by 15 concentrations of gaseous Hg ranging from 0 to 1.8 µg m-3 through a misting system and then Hg content and eight common biomarkers in leaves were measured. The results showed that leaf Hg content significantly increased with Hg concentration, showing a linear relationship. However, there were no obvious mortality symptoms, indicating that T. usneoides showed strong resistance to Hg. Conversely, there were no simple linear relationships between changes in various biomarkers following Hg treatment of T. usneoides and Hg concentration. With increasing Hg concentration, malondialdehyde (MDA) content did not change significantly, superoxide anion radical content decreased gradually, superoxide dismutase (SOD) content decreased to the bottom and then bounced back, electrical conductivity increased, and glutathione (GSH) and metallothionein (MT) content increased to the peak and then dropped. The coefficient of determination of the dose-effect curves between SOD, GSH, and MT contents and Hg concentration was high, and the dose-effect relationship varied with hormesis. The present study is first to confirm hormesis induced by heavy metal pollution in Tillandsia species.

Fully physically cross-linked double network hydrogels with strong mechanical properties, good recovery and self-healing properties.

Combining a hydrophobic interaction crosslinked curdlan as the first network and hydrophobic interaction crosslinked polyacrylamide as the second network, we have fabricated a curdlan/HPAAm double network (DN) hydrogel using a one-pot method. The resulting DN hydrogel exhibited good mechanical properties, i.e. an elastic modulus of 103 kPa, a tensile fracture strength of 0.81 MPa, a tensile stretch of 25.3 and a compressive stress of 62.5 MPa when the compressive strain increased up to 99%. The DN gel could withstand ten compression tests under 90% compressive strain without observable damage. The DN gel demonstrated 84% stiffness recovery and 97% toughness recovery after the deformed samples were relaxed and stored at 95 °C for 4 h. The stiffness and fracture stress of the DN gel were enhanced after sterilization treatment at 120 °C. Furthermore, the gels exhibited 52% self-healing of fracture stretch after the samples were cut and brought into contact at 95 °C for 4 h. The self-recovery and self-healing properties of the DN gel both originated from the first curdlan network via the reformation of hydrophobic interactions and the second HPAAm network via reformation of the broken hydrophobic associations.

Tough and Stretchable Dual Ionically Cross-Linked Hydrogel with High Conductivity and Fast Recovery Property for High-Performance Flexible Sensors.

As a kind of typical soft and wet material, hydrogel has been increasingly investigated as another way to develop flexible electronics. However, the traditional hydrogel with poor strain and strength performance cannot meet the requirements for stretchable electronics; fabricating a stretchable hydrogel with balanced tensile strength, toughness, and conductivity is still a big challenge. Herein, a new type of physically cross-linked hydrogel with poly(acrylamide-co-acrylic acid)-Fe3+ and chitosan-SO42- dual ionic networks via facile free radical polymerization and soaking processes is developed to fabricate excellent high-performance flexible sensors. The abundant Fe3+ and SO42- ions in the hydrogel can not only construct tough and strong dual ionic networks but also give the hydrogel high conductivity. Consequently, the optimal hydrogel possesses high tensile strength (∼5.1 MPa), large strain capacity (∼1225%), elasticity (∼1.13 MPa), high toughness (∼32.1 MJ/m3), and high conductivity (3.04 S/m at f = 0.1M), as well as rapid self-recovery property. Furthermore, the hydrogel conductor has high stretching sensitivity with a gauge factor of 6.0 at strain of 700% and was able to detect conventional motions of the human body such as the motions of the knuckle, speaking, and swallowing, which indicates that our ionic conductive hydrogels can be used to fabricate excellent high-performance flexible sensors.