An adhesive, anti-freezing, and environment stable zwitterionic organohydrogel for flexible all-solid-state supercapacitor

Silk fibroin-based hydrogels with low hysteresis, self-adhesion, and tunable ionic conductivity for wearable devices

Silk Fibroin (SF) hydrogels are easy to functionalize and possess biocompatibility, making them highly promising for the development of flexible electronic devices and wearable equipment. However, fabricating SF-based hydrogels with multiple functions such as low hysteresis, self-adhesion, and high elasticity, while constructing flexible wearable electronic devices with high sensitivity and fidelity, remains a challenge to date. To address these issues, this work reports a one-step preparation of a fully polymer-based triple-network hydrogel through precursor solution pH pre-regulation, with polyacrylamide (PAM) as a brittle network, methyl cellulose (MC) as a tough network, and SF as a zwitterionic macromolecule. The introduction of MC effectively regulate the network aperture of the hydrogel, so as to improve the ion transport capacity and realize the high conductivity of the hydrogel. Through the regulation of the precursor solution pH, the cross-linking degree of the PAM network, the hydrogen bonding interactions between the triple networks, and the interfacial properties were simultaneously modulated, resulting in a reduction in hysteresis of the hydrogel from 21.4 % to 7.2 %, an increase in conductivity from 0.34 S·m-1 to 0.57 S·m-1, an increase in elastic modulus from 18.6 kPa to 58.9 kPa, and an improvement in interfacial adhesion from 4.5 kPa to 15.48 kPa. The prepared SF-based hydrogel was assembled into flexible electronic patches and adhered to different parts of the human body, enabling self-adhesive, multi-channel, wireless detection of human multi-scale movements. The hydrogel prepared in this work also demonstrates exceptional potential in fields such as electrocardiogram monitoring, electromyogram detection, information encryption, and self-powered devices. The method reported in this paper provides new insights for the synergistic enhancement of mechanics, electricity, and adhesion in natural polymer-based hydrogels.

High Output Voltage Aqueous Supercapacitors by Water Deactivated Electrolyte over Wide Temperature Range

Confined by factors such as low operating voltage, poor temperature resistance, and instability at high voltage, the energy density of conventional symmetric aqueous supercapacitors is undesirable over a wide temperature range. It is still challenging to develop aqueous flexible supercapacitors (AFSCs) that can provide stable and high voltage output (>2.0 V) at extreme ambient temperatures. Here, a strategy for constructing AFSC with ultrahigh output voltages over a wide temperature range is proposed through the development of organohydrogel electrolytes (OHEs) with excellent water deactivation, which achieve a notable output voltage of 3.0 V, and unprecedented energy densities of 23.16 µWh cm-2 at -40 °C (beyond 25 °C), surpassing the performance of all previously reported symmetric supercapacitors with aqueous electrolytes. Theoretical calculations and experimental analyses show that OHEs can deactivate water to increase the output voltage limit of AFSCs by enhancing intermolecular interactions and regulating inter Helmholtz plane. Meanwhile, it also shows excellent flexibility and cycling stability (80.5% after 20 000 cycles at 25 °C and 97.0% after 50 000 cycles at -40 °C). More importantly, OHEs enable AFSCs switchable output voltages (from 2.5 to 3.0 V), making it possible to operate supercapacitors with high energy density and stability at low temperatures.

Calcium alginate reinforced zwitterionic double network hydrogel with mechanical robustness and antimicrobial activity for freshwater shrimp spoilage detection

Hydrogel indicators promise to monitor food spoilage, but their poor mechanics can cause defects in transport. Herein, a novel zwitterionic double network (DN) hydrogel was developed by polymerizing arylamide and sulfobetaine methacrylate in an alginate-Ca2+ system. This hydrogel exhibited enhanced mechanical properties, including a maximum 2087 % breaking elongation and 135 ± 12 kJ/m3 toughness, significantly outperforming the current zwitterionic DN hydrogels, which typically exhibit less than 1800 % breaking elongation, capable of supporting 150 g-136 times its own weight. Importantly, it also demonstrated excellent anti-fatigue and tear resistance, with a tearing energy of 2200 ± 180 J/m2. The abundant zwitterionic polymers, hydroxyl groups, and amino groups within hydrogel contribute to its strong adhesiveness to various substrates. Additionally, the hydrogel exhibited antimicrobial activity against E. coli and S. aureus. Furthermore, a pH/NH3 dual-sensitive hydrogel indicator, using phenol red, bromocresol purple, and their mixtures in ratios of 1:1, 1:2, and 2:1, was developed for monitoring shrimp freshness. The results showed that the hydrogel indicator with phenol red, effective at 4 °C and 25 °C, showed clear color changes (ΔE > 80) in response to spoilage and strong correlations between ΔE and TVB-N (R2 > 0.98) and pH (R2 > 0.97), surpassing the existing hydrogel indicators used for shrimp freshness monitoring, which generally have a correlation coefficient of only 0.86. This novel dual-responsive hydrogel, with excellent mechanical performance and TVB-N sensitivity, is suitable for real-time freshness monitoring in intelligent packaging.

Aqueous AlCl3/ZnCl2 solution room-induced the self-growing strategy of expanded topological network for cellulose/polyacrylamide-based solid-state electrolytes

The green synthesis strategy for cellulose-containing hydrogel electrolytes is significant for effectively managing resources, energy, and environmental concerns in the contemporary world. Herein, we propose an all-green strategy using AlCl3/ZnCl2/H2O solvent to create cellulose/polyacrylamide-based hydrogel (AZ-Cel/PAM) with expanded hierarchical topologies. The aqueous AlCl3/ZnCl2 facilitates the efficient dissolution of cellulose at room temperature, and the dispersed Al3+-Zn2+ ions autocatalytic system catalyzes in-situ polymerization of acrylamide (AM) monomer. This expands the AM network within the cellulose framework, forming multiple bonding interactions and stable ion channels. The resulting hybrid hydrogel exhibits improved mechanical properties (tensile strength of 56.54 kPa and compressive strength of 359.43 kPa) and enhanced ionic conductivity (1.99 S/m). Furthermore, it also demonstrates excellent adhesion, freeze resistance (-45 °C), and water retention capabilities. Quantum simulations further clarify the mechanical composition and ion transport mechanism of AZ-Cel/PAM hydrogels. The assembled supercapacitor with the hydrogel electrolyte, demonstrates an ideal area-specific capacitance of 203.80 mF/cm2. This all-green strategy presents a novel approach to developing sustainable energy storage devices.

Mechanically Robust and Highly Conductive Poly(ionic liquid)/Polyacrylamide Double-Network Hydrogel Electrolytes for Flexible Symmetric Supercapacitors with a Wide Operating Voltage Range

Flexible electronic devices, such as supercapacitors (SCs), place high demands on the mechanical properties, ionic conductivity, and electrochemical stability of electrolytes. Hydrogels, which combine flexibility and the advantages of both solid and liquid electrolytes, will meet the demand. Here, we report the synthesis of novel poly(ionic liquid)/polyacrylamide double-network (DN) (PIL/PAM DN) hydrogel electrolytes containing different metal salts via a two-step γ-radiation method. The resultant Li2SO4-1.0/PIL/PAM DN hydrogel electrolyte possesses excellent mechanical properties (tensile strength of 3.64 MPa, elongation at break of 446%) and high ionic conductivity (24.1 mS·cm-1). The corresponding flexible SC based on the Li2SO4-1.0/PIL/PAM DN hydrogel electrolyte (SC-Li2SO4) presents improved ion diffusion, ideal electrochemical double-layer capacitor behavior, good rate capability, and excellent cyclic stability. Moreover, symmetric SC-Li2SO4 achieves a wide operating voltage range of up to 1.5 V, with a maximum energy density of 26.0 W h·kg-1 and a capacitance retention of 94.1% after 10,000 galvanostatic charge-discharge cycles, owing to the deactivation of free water molecules by the synergistic effect of PIL, PAM, and SO42-. Above all, the capacitance of SC-Li2SO4 is well-maintained after overcharge, overdischarge, short circuit, extreme temperature, compression, and bending tests, indicating its high security and flexibility. This work reveals the enormous application potential of PIL-based conductive hydrogel electrolytes for flexible electronic devices.

Cellulose nanofiber hydrogel with high conductivity electrolytes for high voltage flexible supercapacitors

Although flexible double layer capacitors based on hydrogels overcome the drawbacks of commercial double layer capacitors such as low safety and non-deformability, it is still considered as attractive challenges to achieve high conductivity for hydrogel electrolytes as well as high operating voltages for hydrogel flexible supercapacitors. In this paper, ion migration channels were engineered by immobilizing positive and negative charges on polymer skeleton and dispersing cellulose nanofibers in the polymerized polyelectrolyte network, providing ultra-high ionic conductivity (103 mS cm-1). In addition, K3[Fe(CN)6] was introduced through a soaking method, leading to redox reactions on the surface of carbon electrode during charging and discharging, supporting a relatively wide voltage window (1.8 V). Moreover, the specific capacitance at high current remained 55 % of the specific capacitance at low current, indicating excellent rate performance. In addition, the device displayed high cycling stability (80.05 % after 10,000 cycles). Notably, we successfully light up the red LED with only one device. Accordingly, this work provides a feasible design concept for the development of cellulose nanofibers (CNF) hydrogel-based solid-state electrolyte with high conductivity for flexible supercapacitors with wide potential window and high energy density.

Preparation of Tough and Adhesive PVA/P(AM-AMPS)/Glycerol/Laponite/Na2SO4 Organohydrogels for All-Solid-State Supercapacitors and Self-Powered Wearable Strain Sensors

Hydrogel electrolytes are ideal for flexible wearable electronic devices because of their high ionic conductivity, flexibility, and biocompatibility. However, some problems, such as poor mechanical properties, low conductivity, and lack of adhesivity, are encountered in the process of hydrogel preparation and application, which restrict the further development of hydrogel electrolytes. In this study, PVA was used as the first network, and P(AM-co-AMPS) as the second network to prepare a double-network hydrogel electrolyte. Laponite and Na2SO4 were introduced into the hydrogel during hydrogel formation as the nanofiller and salt with the salting-out effect to enhance its mechanical properties. The hydrogel electrolyte with high toughness (1663 kJ·m-3), adhesivity (77 kPa), and ionic conductivity (1.7 S·m-1) was obtained. In addition, the hydrogel electrolyte also has excellent antifatigue performance. In the 10 consecutive tensile cycles, the tensile strength does not decay. Due to the high adhesivity of the hydrogel electrolyte, a symmetrical all-solid-state supercapacitor was assembled with a tight interface between the hydrogel electrolyte and the AC/CNT composite electrode. The supercapacitor has a high specific capacitance of 186.1 mF·cm-2 at the current density of 1 mA·cm-2. In addition, the capacitor has good flexibility and can withstand bending at various angles. The hydrogel electrolyte also has excellent strain sensing performance, with an ultrafast tensile response time (0.17 s) and high sensitivity factor (GF = 10.01). Finally, the self-powered sensor system composed of a supercapacitor as the power supply device and hydrogel electrolyte as the sensing part was obtained and applied to human motion monitoring, which provides a potential application in the integrated flexible electronic system.

Lignosulfonate sodium and ionic liquid synergistically promote tough hydrogels for intelligent wearable human-machine interaction

Flexible wearable devices are garnering significant interest, with conductive hydrogels emerging as a particularly notable category. While many of these hydrogels offer impressive conductivity, they often lack the innate ability to adhere autonomously to human skin. The ideal hydrogel should possess both superior adhesion properties and a wide responsive range. This study introduces a novel double-network conductive hydrogel, synthesized from lignosulfonate sodium and ionic liquid using a one-pot method. The gel's mechanical robustness (fracture elongation of ∼3500 % and tensile strength of ∼130 kPa) and exceptional conductivity sensing performance arise from the synergistic effects of electrostatic interactions, dynamic hydrogen bonding, and a three-dimensional network structure. Additionally, the phenolic hydroxyl and sulfonic groups from lignosulfonate sodium imbue the hydrogel with adhesive qualities, allowing it to easily bond with varied material surfaces. This hydrogel excels in human physiological signal detection and wireless monitoring, demonstrating a rapid response time (149 ms) and high sensitivity (a maximum gauge factor of 10.9 for strains between 400 and 600 %). Given these properties, the flexible, self-adhesive, and conductive hydrogel showcases immense promise for future applications in wearable devices and wireless transmission sensing.

Ultrastretchable and highly conductive hydrogels based on Fe3+- lignin nanoparticles for subzero wearable strain sensor

Conductive hydrogels have attracted considerable interest for potential applications in soft robotics, electronic skin and human monitoring. However, insufficient mechanical characteristics, low adhesion and unsatisfactory electrical conductivity severely restrict future application possibilities of hydrogels. Herein, lignin nanoparticles (LNPs)-Fe3+-ammonium persulfate (APS) catalytic system was introduced to assemble Poly(2-hydroxyethyl methacrylate)/LNPs/Ca2+ (PHEMA/LNPs/Ca) hydrogels. Due to the abundant metal coordination and hydrogen bonds, the composite hydrogel displayed ultrahigh stretchable capacity (3769 %), adhesion properties (248 kPa for skin) and self-healing performance. Importantly, hydrogel sensors possess with high durability, strain sensitivity (GF = 8.75), fast response time and freeze resistance (-20 °C) that could be employed to monitor motion signals in low-temperature regime. Therefore, the LNPs-Fe3+ catalytic system has great potential in preparing hydrogel for various applications such as human-computer interaction, artificial intelligence, personal healthcare and subzero wearable devices. At the same time, incorporation of natural macromolecules into polymer hydrogels is tremendous research significance for investigating high-value utilization of lignin.

Establishing a Corrugated Carbon Network with a Crack Structure in a Hydrogel for Improving Sensing Performance

Electronic conductive hydrogels have prompted immense research interest as flexible sensing materials. However, establishing a continuous electronic conductive network within a hydrogel is still highly challenging. Herein, we develop a new strategy to establish a continuous corrugated carbon network within a hydrogel by embedding carbonized crepe paper into the hydrogel with its corrugations perpendicular to the stretching direction using a casting technique. The corrugated carbon network within the as-prepared composite hydrogel serves as a rigid conductive network to simultaneously improve the tensile strength and conductivity of the composite hydrogel. The composite hydrogel also generates a crack structure when it is stretched, enabling the composite hydrogel to show ultrahigh sensitivity (gauge factor = 59.7 and 114 at strain ranges of 0-60 and 60-100%, respectively). The composite hydrogel also shows an ultralow detection limit of 0.1%, an ultrafast response/recovery time of 75/95 ms, and good stability and durability (5000 cycles at 10% strain) when used as a resistive strain sensing material. Moreover, the good stretchability, adhesiveness, and self-healing ability of the hydrogel were also effectively retained after the corrugated carbon network was introduced into the hydrogel. Because of its outstanding sensing performance, the composite hydrogel has potential applications in sensing various human activities, including accurately recording subtle variations in wrist pulse waves and small-/large-scale complex human activities. Our work provides a new approach to develop economical, environmentally friendly, and reliable electronic conductive hydrogels with ultrahigh sensing performance for the future development of electronic skin and wearable devices.

Cartilage structure-inspired elastic silk nanofiber network hydrogel for stretchable and high-performance supercapacitors

Flexible supercapacitors are an important portable energy storage but suffer from low capacitance, inability to stretch, etc. Therefore, flexible supercapacitors must achieve higher capacitance, energy density, and mechanical robustness to expand the applications. Herein, a hydrogel electrode with excellent mechanical strength was created by simulating the collagen fiber network and proteoglycan in cartilage using silk nanofiber (SNF) network and polyvinyl alcohol (PVA). The Young's modulus and breaking strength of the hydrogel electrode increased by 205 % and 91 % compared with PVA hydrogel owing to the enhanced effect of the bionic structure, respectively, which are 1.22 MPa and 1.3 MPa. The fracture energy and fatigue threshold reached 1813.5 J/m² and 1585.2 J/m², respectively. The SNF network effectively connected carbon nanotubes (CNTs) and polypyrrole (PPy) in series, affording a capacitance of 13.62 F/cm² and energy density of 1.2098 mWh/cm². This capacitance is the highest among currently reported PVA hydrogel capacitors, which can maintain >95.2 % after 3000 charge-discharge cycles. This capacitance Notably, the cartilage-like structure endowed the supercapacitor with high resilience; thus, the capacitance remained >92.1 % under 150 % deformation and >93.35 % after repeated stretching (3000 times), which was far superior to that of other PVA-based supercapacitors. Overall, this effective bionic strategy can endow supercapacitors with ultrahigh capacitance and effectively ensure the mechanical reliability of flexible supercapacitors, which will help expand the applications of supercapacitors.

Salt Transport in Crosslinked Hydrogel Membranes Containing Zwitterionic Sulfobetaine Methacrylate and Hydrophobic Phenyl Acrylate

Produced water is a by-product of industrial operations, such as hydraulic fracturing for increased oil recovery, that causes environmental issues since it includes different metal ions (e.g., Li+, K+, Ni2+, Mg2+, etc.) that need to be extracted or collected before disposal. To remove these substances using either selective transport behavior or absorption-swing processes employing membrane-bound ligands, membrane separation procedures are promising unit operations. This study investigates the transport of a series of salts in crosslinked polymer membranes synthesized using a hydrophobic monomer (phenyl acrylate, PA), a zwitterionic hydrophilic monomer (sulfobetaine methacrylate, SBMA), and a crosslinker (methylenebisacrylamide, MBAA). Membranes are characterized according to their thermomechanical properties, where an increased SBMA content leads to decreased water uptake due to structural differences within the films and to more ionic interactions between the ammonium and sulfonate moieties, resulting in a decreased water volume fraction, and Young’s modulus increases with increasing MBAA or PA content. Permeabilities, solubilities, and diffusivities of membranes to LiCl, NaCl, KCl, CaCl2, MgCl2, and NiCl2 are determined by diffusion cell experiments, sorption-desorption experiments, and the solution-diffusion relationship, respectively. Permeability to these metal ions generally decreases with an increasing SBMA content or MBAA content due to the corresponding decreasing water volume fraction, and the permeabilities are in the order of K+ > Na+ > Li+ > Ni2+ > Ca2+ > Mg2+ presumably due to the differences in the hydration diameter.