High-strain sensitive zwitterionic hydrogels with swelling-resistant and controllable rehydration for sustainable wearable sensor

A gelatin-based ionogel with anti-swelling properties for underwater human physiological signal detection

A hydrogel is an ideal matrix material for flexible electronic devices, electronic skin and health detection devices due to its outstanding flexibility and stretchability. However, hydrogel-based flexible electronic devices swell once they are placed in a high humidity or underwater environment. The swelling behavior could damage the internal structure of hydrogels, ultimately leading to the reduction or complete loss of mechanical properties, electrical conductivity and sensing function. In order to resolve the above problems, a double network ionogel with remarkable anti-swelling behavior, stretchability and conductive properties was prepared. The ionogel consisted of gelatin (G) and copolymerization of acrylic acid (AA), 2-hydroxyethyl methacrylate (HEMA), butyl acrylate (BA), dimethylaminoethyl methacrylate maleate (D) and N,N'-methylene-bis-acrylamide (MBAA). Due to the dense crosslinking network and hydrophobic interaction, the ionogel exhibited remarkable anti-swelling properties (7.64% of the 30-day equilibrium swelling ratio in deionized water). D and MBAA were simultaneously introduced into the ionogel system as cross-linking agents to provide a large number of cross-linking points, improving the cross-linking density of the ionogel. Importantly, the introduction of D avoided ionic leakage by free radical copolymerization. Furthermore, the ionogel maintained stable mechanical properties and conductivity after being submerged in deionized water owing to remarkable anti-swelling performance. The mechanical properties of the ionogel retained 89.75% of the initial mechanical properties after a 5-day immersion in deionized water. Therefore, this ionogel could be employed as an underwater flexible wearable sensor for high humidity or underwater motion monitoring.

Design of smart temperature-sensitive terpolymeric hydrogel for multi-applications in liquid chromatography

Hydrogels with a unique three-dimensional network structure have been widely used in a variety of fields. However, hydrogels are prone to swelling under water-rich conditions, which severely limits their application in liquid chromatography. Therefore, producing a hydrogel with reliable performance and good mechanical property is essential. Smart temperature-sensitive chromatographic packings have attracted extensive attentions in recent years. In this work, sodium 4-styrenesulfonate and 1-octadecene were introduced into the poly(N-isopropylacrylamide) hydrogel to improve mechanical property and separation performance. As a consequence, a smart temperature-sensitive terpolymeric hydrogel modified silica stationary phase (ION-hydrogel@SiO2) was synthesized for multimode liquid chromatographic separation. It was found that this new ION-hydrogel@SiO2 column exhibited excellent chromatographic separation ability for a wide range of analytes. To a certain extent, this new column has a higher chromatographic separation efficiency compared to the commercial C18 column and XAmide column. Moreover, the use of low proportion of organic phase in chromatographic separation is conducive to the realization of green chromatography. By investigating the chromatographic separation mechanism, it has been demonstrated that the hydrogen bonding interaction is primarily responsible for the temperature-sensitive behavior of the hydrogel. Finally, the ION-hydrogel@SiO2 column was used for the determination of pyridoxine in the commercially available tablet samples. In conclusion, this study presents a feasible idea for the development of novel copolymer hydrogels as liquid chromatographic stationary phases.

Recent Progress of Anti-Freezing, Anti-Drying, and Anti-Swelling Conductive Hydrogels and Their Applications

Hydrogels are soft-wet materials with a hydrophilic three-dimensional network structure offering controllable stretchability, conductivity, and biocompatibility. However, traditional conductive hydrogels only operate in mild environments and exhibit poor environmental tolerance due to their high water content and hydrophilic network, which result in undesirable swelling, susceptibility to freezing at sub-zero temperatures, and structural dehydration through evaporation. The application range of conductive hydrogels is significantly restricted by these limitations. Therefore, developing environmentally tolerant conductive hydrogels (ETCHs) is crucial to increasing the application scope of these materials. In this review, we summarize recent strategies for designing multifunctional conductive hydrogels that possess anti-freezing, anti-drying, and anti-swelling properties. Furthermore, we briefly introduce some of the applications of ETCHs, including wearable sensors, bioelectrodes, soft robots, and wound dressings. The current development status of different types of ETCHs and their limitations are analyzed to further discuss future research directions and development prospects.

An Intrinsic Self-Healable, Anti-Freezable and Ionically Conductive Hydrogel for Soft Ionotronics Induced by Imidazolyl Cross-Linker Molecules Anchored with Dynamic Disulfide Bonds

Hydrogels are ideal materials for flexible electronic devices based on their smooth ion channels and considerable mechanical flexibility. A substantial volume of aqueous solution is required to enable the smooth flow of ions, resulting in the agony of low-temperature freezing; besides, long-term exposure to bending/tensile tress triggers fatigue issues. Therefore, it is a great challenge to prepare hydrogels with both freeze-resistance and long-term durability. Herein, a polyacrylic acid-based hydrogel with both hydrophobic interaction and dynamic reversible covalent bonding cross-linking networks is preparing (DC-hydrogel) by polymerizing a bi-functional imidazole-type ionic liquid monomer with integrated disulfide and alkene bonds (DS/DB-IL) and an octadecyl methacrylate, achieving self-healing. The DS/DB-IL anchored into the polymer backbone has a high affinity with water, reducing the freezing point of water, while the DS/DB-IL with free ions provides superior ionic conductivity to the DC-hydrogel. The polyacrylic acid with abundant carboxyl gives hydrogel good self-adhesiveness to different substrates. Ionotronics with resistance-type sensors with stable output performance are fabricated and explored its application to joint motion and health information. Moreover, hydrogel-based sensing arrays with high resolution and accuracy are fabricated to identify 2D distribution of stress. The hydrogels have great promise for various ionotronics in many fields.

Mussel-inspired adhesive and anti-swelling hydrogels for underwater strain sensing

The application of hydrogels in an underwater environment is limited due to their swelling behavior and the existence of a hydration layer. In this study, a hydrogel based on poly(sulfobetaine methacrylate) (PSBMA), tannic acid (TA) and montmorillonite (MMT) was prepared with excellent anti-swelling properties and underwater self-adhesion properties. The PSBMA hydrogel has excellent anti-swelling properties due to the strong electrostatic interaction between charged groups of PSBMA chains. Inspired by marine mussels, tannic acid modified montmorillonite (TA@MMT) was introduced. Natural polyphenol tannic acid, as a catechol donor, provides a large number of catechol groups for hydrogels. Montmorillonite acts as the physical cross-linking point of PSBMA chains through electrostatic interaction to improve the cohesion of the hydrogel. By combining the adhesion mechanism of zwitterions and catechol, the hydrogel maintains adhesion in air and underwater environments. In addition, a strain sensor was prepared based on the PSBMA/TA@MMT hydrogel, which can closely fit the human skin and stably monitor different movements in air and in underwater environments. Through a Bluetooth communication system, long-distance information transmission can be achieved. Therefore, the PSBMA/TA@MMT hydrogel broadens the application prospect of wearable devices in the underwater environment.

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.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

Hydrogels with tailor-made swelling-shrinkable properties have aroused considerable interest in numerous biomedical domains. For example, as swelling is a key issue for blood and wound extrudates absorption, the transference of nutrients and metabolites, as well as drug diffusion and release, hydrogels with high swelling capacity have been widely applicated in full-thickness skin wound healing and tissue regeneration, and drug delivery. Nevertheless, in the fields of tissue adhesives and internal soft-tissue wound healing, and bioelectronics, non-swelling hydrogels play very important functions owing to their stable macroscopic dimension and physical performance in physiological environment. Moreover, the negative swelling behavior (i.e., shrinkage) of hydrogels can be exploited to drive noninvasive wound closure, and achieve resolution enhancement of hydrogel scaffolds. In addition, it can help push out the entrapped drugs, thus promote drug release. However, there still has not been a general review of the constructions and biomedical applications of hydrogels from the viewpoint of swelling-shrinkable properties. Therefore, this review summarizes the tactics employed so far in tailoring the swelling-shrinkable properties of hydrogels and their biomedical applications. And a relatively comprehensive understanding of the current progress and future challenge of the hydrogels with different swelling-shrinkable features is provided for potential clinical translations.

Programmable nanocomposites of cellulose nanocrystals and zwitterionic hydrogels for soft robotics

Stimuli-responsive hydrogels have garnered significant attention as a versatile class of soft actuators. Introducing anisotropic properties, and shape-change programmability to responsive hydrogels promises a host of opportunities in the development of soft robots. Herein we report the synthesis of pH-responsive hydrogel nanocomposites with predetermined microstructural anisotropy, shape-transformation, and self-healing. Our hydrogel nanocomposites are largely composed of zwitterionic monomers and asymmetric cellulose nanocrystals. While the zwitterionic nature of the network imparts both self-healing and cytocompatibility to our hydrogel nanocomposites, the shear-induced alignment of cellulose nanocrystals renders their anisotropic swelling and mechanical properties. Thanks to the self-healing properties, we utilized a cut-and-paste approach to program reversible, and complex deformation into our hydrogels. As a proof-of-concept, we demonstrated the transport of light cargo using tethered and untethered soft robots made from our hydrogels. We believe the proposed material system introduce a powerful toolbox for the development of future generations of biomedical soft robots.

A Rapidly and Highly Self-Healing Poly(Sulfobetaine Methacrylate) Hydrogel with Stretching Properties, Adhesive Properties, and Biocompatibility

This study focuses on the preparation of stretchable zwitterionic poly(sulfobetaine methacrylate) (PSBMA) hydrogels. To address the weak mechanical properties of chemically crosslinked PSBMA hydrogels, a physical crosslinking method utilizing hydrophobic interactions to crosslink hydrogels to approach tough properties is developed. Here, sodium dodecyl sulfate (SDS)-based micelle is used as a physical crosslinker to prepare physically crosslinked PSBMA (PSBMA_phy ) hydrogels, and ethylene glycol dimethylacrylate (EGDMA) is used to prepare a control group of chemically crosslinked PSBMA (PSBMA_chem ) hydrogels. The mechanical properties of the two hydrogels are compared, and PSBMA_phy hydrogels exhibit greater flexibility than the PSBMA_chem hydrogels. When the PSBMA_phy hydrogels are subjected to external forces, the micelles act as dynamic crosslinking sites, allowing the stress to disperse and prevent the hydrogel from breaking. In addition, the PSBMA_phy hydrogels have nearly 100% self-healing properties within 2.5 min. The PSBMA_phy hydrogels exhibit usable adhesive properties to porcine skin and subcutis. MTT and hemolysis tests show that the PSBMA_phy hydrogels have excellent biocompatibility and hemocompatibility. This study proposes that the multifunctional PSBMA_phy hydrogels with micelles will be potential to carry drugs for use in drug delivery systems in the future.