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Polydopamine (PDA)-based self-adhesive hydrogel sensors are extensively explored but it is still a challenge to construct PDA-based hydrogels by free radical polymerization. Herein, a new approach to construct self-adhesive hydrogels by conducting free radical polymerization in both aqueous phase and micelle phase is developed. The following two-phase polymerization processes account for the formation of the self-adhesive hydrogels. The first one is the polymerization of acrylamide (AM) and dopamine (DA) in aqueous phase to form adhesive component PAM-PDA (PAM, polyacrylamide; PDA, polydopamine). The second one is the polymerization of hydrophobic monomer 2-methoxyethyl acrylate (MEA) in micelles of an amphiphilic block copolymer Pluronic F127 diacrylate (F127DA). The poly(2-methoxyethyl acrylate) (PMEA) networks help to maintain the high robustness of the hydrogel. Because PMEA and PDA form in relatively separated phases, the inhibition effect of PDA on the free radical polymerization process of PMEA is weakened. Based on this mechanism, mechanically strong and adhesive hydrogels are achieved. The introduced ions during preparation process, such as Na+, OH– and K+, endow the resulting hydrogels ionic conductivity. Resistive strain sensor of the hydrogel achieves a high gauge factor (GF) of 5.26, a response time of 0.25 s and high sensing stability. Because of the adhesiveness, such hydrogel sensor can be applied as wearable sensors in monitoring various human motions. To further address the freezing and drying problems of the hydrogels, organohydrogels are constructed in glycerol-water mixed solvent. The organohydrogels exhibit outstanding anti-freezing property and moisture retention ability, and their adhesiveness is well maintained in subzero conditions. Capacitive pressure sensors of the organohydrogels possessing a GF of 2.05 kPa−1, high sensing stability and reversibility, are demonstrated and explored in monitoring diverse human motions.

Self-recoverability and stretchability are critical for high-performance hydrogel sensors, but developing hydrogels with combined self-recoverability and stretchability remains a challenge. Here, s...

Highly sensitive capacitive-type pressure sensor has been achieved by fabricating reliefs on solution-processable hydrogel electrodes. Hybrid PVA/PANI hydrogels (PVA, poly(vinyl alcohol); PANI, polyaniline) with a fully physically cross-linked binary network are selected as the electrodes of the pressure sensors. On the basis of the solution processability, reliefs are fabricated on the surface of PVA/PANI hydrogel electrodes by a template method. The gauge factor (GF) is enhanced by introducing reliefs and regulated by controlling the composition and relief dimension of hydrogel electrodes. The optimized pressure sensor containing reliefs achieves the highest GF of 7.70 kPa

The problem of global water pollution is becoming more and more severe, among which organic dyes and heavy metal ions are two typical types of the most common pollutants. The adsorption method for water purification and wastewater treatment is widely studied and applied. Hydrogel has unique advantages in the field of adsorption due to its three-dimensional porous structure. In this paper, a new type of self-healing hydrogels based on reversible covalent bond were prepared by mixing poly(vinyl alcohol) (PVA) and 2-aminophenylboronic acid modified polyacrylic acid (PAA-2APBA). In addition, the introduction of laponite nanoparticles into the hydrogel can increase both the mechanical strength and adsorption efficiency. This low-cost PAA-2APBA/PVA/laponite nano-composite hydrogel could efficiently remove the organic dyes and heavy metal ions in model waste water through simple immersion, which makes it have application prospects in the fields of both biomedical and environmental engineering.

Ionogels are ideal candidate materials for flexible sensors, but their stretchability and fatigue resistance are limited. Herein, highly stretchable, fatigue-resistant, electrically conductive, and temperature-tolerant ionogels are investigated and further applied in fabricating high-performance flexible sensors. The ionogels consist of a poly(acrylic acid) (PAA) network and a commonly used room-temperature ionic liquid (RTIL) named 1-ethyl-3-methylimidazolium dicyanamide ([EMIm][DCA]). Dually acrylated Pluronic F127 (F127DA) was utilized to cross-link the PAA network, and [EMIm][DCA] was physically confined in the PAA network. Because of their special cross-linking structure, the PAA ionogels are highly stretchable (>850%), tough, and fatigue-resistant, and they are also conductive, transparent, and temperature-tolerant because of the existence of [EMIm][DCA]. On the basis of their integrated performances, the PAA ionogels were further utilized to fabricate strain sensors and pressure sensors. The ionogel-based strain sensors have high sensitivity, low response time (200 ms), wide strain-sensing range (0-750%), excellent durability (>1400 cycles), and good temperature tolerance and can be applied to detect various human motions. The pressure sensors also have a high response speed (256 ms) and excellent sensitivity (GF = 0.73 kPa-1), which offers an opportunity to detect force generated by finger touching and water droplets.

Click chemistry is an effective tool to fabricate three-dimensional gel network in an accurate way. In this work, Pluronic F127 gels are prepared by thiol–ene click chemistry, utilizing double-bond end-capped Pluronic F127 (F127DA) and pentaerythritol tetras-mercaptopropionate (PETMP) as the starting materials and triethylamine as the catalyst. Both the mass fraction of F127DA and the initial concentration of PETMP have effects on the properties of the resulting F127 gels. The optimized F127 gel prepared under a F127DA mass fraction of 15 wt% and a PETMP concentration of 17.62 mmol/L exhibits the highest swelling ratio (10) and has the largest elongation at break (194%). When the F127DA mass fraction and PETMP concentration are fixed at 15 wt% and 48.26 mmol/L, respectively, the resulting F127 gel has a high stress of 0.25 MPa. The present example has presented a method to fabricate three-dimensional gel network in a controlled way.

This work describes the preparation, characterization and removal capability of a novel biomass derived carbonaceous sponges (CS) and their nanocomposites. The CS has hierarchically porous structure which is composed of lamellar structures and secondary porous structures. The pore size is on a scale from 1 nm to 200 μm. Utilizing the CS as adsorbents, rapid removal of model organic molecules, including methylene blue (MB), methyl orange (MO) and crystal violet (CV), from their aqueous solutions can be completed within 1 min with the assistance of pressure and the removal efficiency reaches up to 100%, 81% and 98%, respectively. The removal capabilities for CS towards MB, MO and CV are 0.0769 g/g, 0.2218 g/g and 1.0384 g/g, respectively and 0.0635 g/g, 0.0977 g/g and 0.8634 g/g, respectively for CS nanocomposites.

Flexible strain sensors are highly used in soft robotics, human–machine interfaces and health monitoring devices. However, it is still a big challenge to construct strain sensors with excellent mechanical properties and broad sensing ranges. In this study, a class of extremely stretchable and electrically conductive hydrogels with dually synergistic networks are fabricated for wearable resistive-type strain sensors. Dually synergistic networks are composed of a soft poly(acrylic acid) (PAA) network and a rigid conductive polyaniline (PANI) network. The PAA network is crosslinked by amphiphilic block copolymers, and the PANI network is chemically doped and ionically crosslinked by phytic acid and these two networks are further interlocked by physical entanglements, hydrogen bonds and ionic interactions. The resulting hydrogels have high tensile strength, controllable conductivity and large tensile deformation (1160%). Moreover, these hydrogels are utilized for fabricating strain sensors with good sensitivity and a wide sensing range (0–1130%). The high performances of hydrogels make such strain sensors suitable for wearable devices monitoring both subtle and large strains induced by human motions, including moving of two hands, bending of joints, conducting of gestures and swallowing.

Stretchable and elastic double network (DN) hydrogels are increasingly investigated for flexible electronics but their recyclability is rarely considered. Herein, recyclable, stretchable and conductive DN-hydrogels are explored and utilized in flexible resistive-type strain sensors. Such DN-hydrogels are fully physically cross-linked and composed of a crystalline domain cross-linked poly(vinyl alcohol) (PVA) network and a Tb3+-cross-linked poly(acrylic acid sodium) (PAANa–Tb3+) network. Due to their unique structure, DN-hydrogels have excellent recyclability and flexibility, and they are also conductive and photoluminescent due to the presence of Tb3+. Utilizing DN-hydrogels, flexible strain sensors are assembled and further applied to monitor large human motions, such as moving of two hands and bending of joints, as well as subtle physiological activities including moving of the bicipital muscle, swallowing and breathing. Overall, the present example not only helps to understand the toughening and strengthening mechanism of fully physically cross-linked DN-hydrogels, but also offers a clue to develop multifunctional hydrogels for recyclable flexible electronics.

Hydrogel-based flexible sensors are of promising applications in various fields, but fabrication of such sensors with integrated high performances remains a challenge. In this work, flexible sensors (both strain sensors and pressure sensors) with integrated high performances are fabricated utilizing double network (DN) organohydrogels. Because of the unique structure of DN organohydrogels, the flexible sensors exhibit intrinsic adhesion without introducing components that are often used to obtain adhesive hydrogels, such as polydopamine, nucleobases or proteins. In addition, outstanding temperature tolerance (−18 to 80 °C), high stretchability (>2000%), tensile strength (>300 kPa), self-healing ability (96.5%) and transparency (90%) are also achieved. Resistive-type strain sensors of DN organohydrogels achieve high gauge factor (GF = 2.58), low response time (0.18 s), large sensing range (0–1000%) and reversible sensing ability (>1000 cycles). Sandwich-shaped capacitive-type pressure sensors comprising DN organohydrogel electrodes with reliefs exhibit a high sensitivity of 2.14 kPa−1. Such flexible sensors can be applied in monitoring various human motions and subtle physiological activities and further promoted as wireless sensors on the basis of a Bluetooth protocol.