Progress in the Preparation and Application of Inulin-Based Hydrogels.

Inulin, a natural polysaccharide, has emerged as a promising precursor for the preparation of hydrogels due to its biocompatibility, biodegradability, and structural versatility. This review provides a comprehensive overview of the recent progress in the preparation, characterization, and diverse applications of inulin-based hydrogels. Different synthesis strategies, including physical methods (thermal induction and non-thermal induction), chemical methods (free-radical polymerization and chemical crosslinking), and enzymatic approaches, are discussed in detail. The unique properties of inulin-based hydrogels, such as stimuli-responsiveness, antibacterial activity, and their potential as fat replacers, are highlighted. Special emphasis is given to their promising applications in drug delivery systems, especially for colon-targeted delivery, due to the selective degradation of inulin via colonic microflora. The ability to incorporate both hydrophilic and hydrophobic drugs further expands their therapeutic potential. In addition, the applications of inulin-based hydrogels in responsive materials, the food industry, wound dressings, and tissue engineering are discussed. While significant progress has been achieved, challenges and prospects in optimizing synthesis, improving mechanical properties, and exploring new functionalities are discussed. Overall, this review highlights the remarkable properties of inulin-based hydrogels as a promising class of biomaterials with immense potential in the biomedical, pharmaceutical, and materials science fields.

Ultra-stretchable and conductive polyacrylamide/carboxymethyl chitosan composite hydrogels with low modulus and fast self-recoverability as flexible strain sensors.

There is a great demand for the fabrication of soft electronics using hydrogels due to their biomimetic structures and good flexibility. However, conventional hydrogels have poor mechanical properties, which restricts their applications as stretchable sensors. Herein, a facile one-step strategy is proposed to fabricate tough and conductive hydrogels by making use of the graftability of carboxymethyl chitosan without extra conductive matter and crosslinking agent. The obtained polyacrylamide/carboxymethyl chitosan composite hydrogels possess outstanding transmittance and excellent mechanical performances, with tensile breaking stress of 630 kPa, breaking strain of 4560 %, toughness of 8490 kJ/m3. These hydrogels have low modulus of 5-20 kPa, fast recoverability after unloading, high conductivity of ∼0.85 S/m without the addition of other conductive substances and good biocompatibility. The ionic conductivity of the gels originates from the counterions of carboxymethyl chitosan, affording the hydrogels as resistive-type sensors. The resultant hydrogel sensors demonstrate a broad strain window (0.12-1500 %), excellent linear response, high sensitivity with the gauge factor reaching 11.72, and great durability, capable of monitoring diverse human motions. This work provides a new strategy to develop stretchable conductive hydrogels with promising applications in the fields of artificial intelligence and flexible electronics.

Advances in the Preparation of Tough Conductive Hydrogels for Flexible Sensors.

The rapid development of tough conductive hydrogels has led to considerable progress in the fields of tissue engineering, soft robots, flexible electronics, etc. Compared to other kinds of traditional sensing materials, tough conductive hydrogels have advantages in flexibility, stretchability and biocompatibility due to their biological structures. Numerous hydrogel flexible sensors have been developed based on specific demands for practical applications. This review focuses on tough conductive hydrogels for flexible sensors. Representative tactics to construct tough hydrogels and strategies to fulfill conductivity, which are of significance to fabricating tough conductive hydrogels, are briefly reviewed. Then, diverse tough conductive hydrogels are presented and discussed. Additionally, recent advancements in flexible sensors assembled with different tough conductive hydrogels as well as various designed structures and their sensing performances are demonstrated in detail. Applications, including the wearable skins, bionic muscles and robotic systems of these hydrogel-based flexible sensors with resistive and capacitive modes are discussed. Some perspectives on tough conductive hydrogels for flexible sensors are also stated at the end. This review will provide a comprehensive understanding of tough conductive hydrogels and will offer clues to researchers who have interests in pursuing flexible sensors.

Conductive silk fibroin hydrogel with semi-interpenetrating network with high toughness and fast self-recovery for strain sensors.

Regenerated silk fibroin (RSF) hydrogels have been extensively studied in the fields of biomedicine and wearable devices in recent years due to their outstanding biocompatibility. However, the pure RSF hydrogels usually exhibited frangibility and low ductility, limiting their application in many aspects severely. Herein, we demonstrate a tough RSF/poly (N, N-dimethylallylamine) hydrogel with semi-interpenetrating network, which possesses good mechanical properties with high stretchability (εb = 900%), tensile strength (σb = 101.7 kPa), toughness (Wf = 516.7 kJ/m3) and tearing fracture energy (T = 407.3 J/m2). Besides, the gels show low residual strain in the cyclic tests and rapid self-recovery (80% toughness recovery within 5 min with the maximum strain of 400%). Moreover, the gels also show high ionic conductivity due to the incorporation of the NaCl and the hydrogel can act as an ideal candidate for strain sensor with high sensitivity (GF = 1.84), admirable linearity, and good durability (1000 cycles with the strain of 100%). When used as a wearable strain sensor for monitoring human movements, it also can detect small and large deformations with high sensitivity. It is expected that this work can provide a new strategy for the fabrication of smart RSF-based hydrogels and expand their application in multiple scenarios.

Multi-level encryption of information in morphing hydrogels with patterned fluorescence.

Fluorescent hydrogels have attracted tremendous attention recently in the field of information security due to the booming development of information technology. Along this line, it is highly desired to improve the security level of concealed information by the advancements of materials and encryption technologies. Here we report multi-level encryption of information in a bilayer hydrogel with shape-morphing ability and patterned fluorescence. This hydrogel is composed of a fluorescence layer containing chromophore units in the poly(acrylic acid) network and an active layer with UV-absorption agents in the poly(N-isopropylacrylamide-co-acrylic acid) network. The former layer exhibits tunable fluorescence tailored by UV light irradiation to induce unimer-to-dimer transformation of the chromophores, facilitating the write-in of information through photolithography. The latter layer is responsive to temperature, enabling morphing of the bilayer hydrogel. Therefore, the bilayer hydrogel encoded with patterned fluorescent patterns can deform into three-dimensional configurations at room temperature to conceal the information, which is readable only after successive procedures of shape recovery at an appropriate temperature and under UV light irradiation from the right direction. The combination of morphing materials and patterned fluorescence as a new avenue to improve the encryption level of information should merit the design of other smart materials with integrated functions for specific applications.

Hydrolyzed Hydrogels with Super Stretchability, High Strength, and Fast Self-Recovery for Flexible Sensors.

Polyacrylamide is widely employed in constructing functional hydrogels. However, the volume expansion of this hydrogel in water weakens its mechanical properties and restricts its application. Herein, we report a strategy to convert the swollen and weak polyacrylamide/carboxymethyl chitosan hydrogel into a strong and tough one by hydrolysis in acid solution with an elevated temperature. The obtained hydrolyzed hydrogels possess a high strength, toughness, and tearing fracture energy of 5.9 MPa, 22 MJ/m3 and 7517 J/m2, which are 254, 535 and 186 times higher than those of the original swollen one, respectively. In addition, the gels demonstrate low residual strain and rapid self-recovery abilities. Moreover, the gels have good shape memory behavior controlled by temperature. Furthermore, the gels can be worked as strain sensors with a broad strain window, high sensitivity, excellent linear response, and great durability in monitoring human motions after immersing treatment in a normal saline solution. This work provides a new method for preparing the stretchable and tough polyacrylamide-based hydrogels used in the areas of soft actuators and flexible electronics.

Villi-like poly(acrylic acid) based hydrogel adsorbent with fast and highly efficient methylene blue removing ability.

Organic dye-containing wastewater has become an increasingly serious environmental problem due to the rapid development of the printing and dyeing industry. Hydrogel is a promising adsorbent for organic dyes because of its unique three-dimension network structure and versatile functional groups. Though many efforts have been made in hydrogel adsorbents recently, there is still a critical challenge to fabricate hydrogel adsorbent with high adsorption capacity and high efficiency at the same time. To address this concern, we developed a calcium hydroxide nano-spherulites/poly(acrylic acid -[2-(Methacryloxy)ethyl]trimethyl ammonium chloride) hydrogel adsorbent with novel villi-like structure. The hydrogels were prepared through a simple free radical copolymerization method using calcium hydroxide nano-spherulites as crosslinker. The resultant hydrogel adsorbents showed a maximum adsorption capacity of 2249 mg/g in a 400 mg/L methylene blue solution and a high removal ratio of 98% in 1 h for a 50 mg/L methylene blue solution. In addition, the adsorption behaviors of our hydrogel adsorbents could be well described by pseudo-second-order kinetic model and Langmuir adsorption isotherm model. Furthermore, this kind of hydrogel adsorbent showed selective adsorption behavior for methylene blue. Altogether, the hydrogel adsorbent developed in this work has a high capacity and high efficiency in organic dye removing and promised a great potential in wastewater treatment application.

Printable and Recyclable Conductive Ink Based on a Liquid Metal with Excellent Surface Wettability for Flexible Electronics.

Flexible electronics greatly facilitate human life due to their convenience and comfortable utilization. Liquid metals are an ideal candidate for flexible devices; however, the high surface tension and poor surface wettability restrict their application on diverse substrates. Herein, a printable and recyclable ink composed of poly(vinyl alcohol) and a liquid metal (PVA-LM) was developed to resolve these problems. The materials were designed considering the compatibility between PVA and the liquid metal, and the composite theory was applied to determine the component proportion. The developed composites improved the surface wettability of the liquid metal on diverse substrates, and three-dimensional (3D) printing technology was chosen to maximize the use of this material. Moreover, the PVA-LM ink showed excellent conductivity of about 1.3 × 105 S/m after being turned on, which favored the designing of alarm systems and object locators. The flexible sensors produced with this ink have broad application, high sensitivity, and superstable signal generation even after 200 cycles. When acting as strain sensors, the constructed composites had high sensitivity for monitoring the human movements. Furthermore, liquid metals in printed products can be recycled under alkaline conditions. This study opens a new direction for the next generation of environmentally friendly flexible devices.

A semi-interpenetrating network ionic composite hydrogel with low modulus, fast self-recoverability and high conductivity as flexible sensor.

There is a growing demand for hydrogel-based sensors due to their biomimetic structures and properties, as well as biocompatibility. However, it is still a challenge to fabricate hydrogel sensor with integration of good mechanical properties and high conductivity. Herein, a tough and conductive hydrogel is developed with semi-interpenetrating network formed by incorporating carboxymethyl chitosan and sodium chloride into polyacrylamide network. The hydrogels have high tensile strength, elongation and toughness, but low modulus comparable to human skin. In addition, the hydrogels exhibit fast self-recovery and satisfactory self-healing capabilities. Owing to the existence of sodium chloride, the hydrogel also has high conductivity, good water retention property and anti-freezing ability. When used as a strain sensor, it demonstrates a broad strain window and shows a high sensitivity in monitoring human motions. This work provides a facile method in fabricating multifunctional ionic conductive hydrogel for applications in wearable electronics and soft robotics.