Natural polymers-enhanced double-network hydrogel as wearable flexible sensor with high mechanical strength and strain sensitivity

Conductive hydrogels have shown great prospects as wearable flexible sensors. Nevertheless, it is still a challenge to construct hydrogel-based sensor with great mechanical strength and high strain sensitivity. Herein, an ion-conducting hydrogel was fabricated by introducing gelatin-dialdehyde β-cyclodextrin (Gel-DACD) into polyvinyl alcohol-borax (PVA-borax) hydrogel network. Natural Gel-DACD network acted as mechanical deformation force through non-covalent cross-linking to endow the polyvinyl alcohol-borax/gelatin-dialdehyde β-cyclodextrin hydrogel (PGBCDH) with excellent mechanical stress (1.35 MPa), stretchability (400%), toughness (1.84 MJ/m³) and great fatigue resistance (200% strain for 100 cycles). Surprisingly, PGBCDH displayed good conductivity of 0.31 S/m after adding DACD to hydrogel network. As sensor, it showed rapid response (168 ms), high strain sensitivity (gage factor (GF) = 8.57 in the strain range of 200%-250%) and reliable sensing stability (100% strain for 200 cycles). Importantly, PGBCDH-based sensor can accurately monitor complex body movements (knee, elbow, wrist and finger joints) and large-scale subtle movements (speech, swallow, breath and facial expressions). Thus, PGBCDH shows great potential for human monitoring with high precision.     

Hydrophobic association and ionic coordination dual crossed-linked conductive hydrogels with self-adhesive and self-healing virtues for conformal strain sensors

As a promising functional material, conductive hydrogel has attracted extensive attention, especially in flexible sensor field. Despite the recent developments, current hydrogels still experience several issues, such as limited stretchability, lack of self-recovery and self-healing capability, and insufficient self-adhesion. Herein, dual cross-linked (DC) poly (AA-co-LMA)_SDS/Fe³⁺ hydrogels are fabricated subtly on the basis of ionic coordination interactions and the poly (AA-co-LMA)_SDS hydrophobic association networks, which may provide one plausible routine to compensate the mentioned drawback of hydrogels. The hydrophobic association and ionic coordination networks work synergistically to endow the hydrogels remarkable stretchability (>1200%), high-fracture strength (≈ 820 kPa), and excellent self-healing capability. In addition, the DC hydrogel-based strain sensors displayed a broad sensing range (0 ∼ 900%), conspicuous sensitivity (strain 0% ∼ 200%, gauge factor = 0.53; strain 200% ∼ 500%, gauge factor = 1.23; strain 500% ∼ 900%, gauge factor = 2.09), and pronounced durability. What's more, the self-adhesive feature ensures the strain sensor always forming a good conformal contact with the skin during human movements and displaying remarkable bidirectional detection capability.     

MXene-composited highly stretchable,sensitive and durable hydrogel for flexible strain sensors

The flourishing development in flexible electronics has provoked intensive research in flexible strain sensors to realize accurate perception acquisition under different external stimuli.However,building hydrogel-based strain sensors with high stretchability and sensitivity remains a great challenge.Herein,MXene nanosheets were composited into polyacrylamide-sodium alginate matrix to construct mechanical robust and sensitive double networked hydrogel strain sensor.The hydrophilic MXene nanosheets formed strong interactions with the polymer matrix and endowed the hydrogel with excellent tensile properties(3150%),compliant mechanical strength(2.03 kPa~(-1) in Young's Module) and long-lasting stability and fatigue resistance(1000 dynamic cycles under 1,600% strain).Due to the highly oriented MXene-based three dimensional conductive networks,the hydrogel sensor achieved extremely high tensile sensitivity(18.15 in gauge factor) and compression sensitivity(0.38 kPa~(-1) below 3 kPa).MXene hydrogel-based strain sensors also displayed negligible hysteresis in electromechanical performance,typical frequent-independent feature and rapid response time to external stimuli.Moreover,the sensor exhibited accurate response to different scales of human movements,providing potential application in speech recognition,expression recognition and handwriting verification.     

Strong and Ultra-tough Ionic Hydrogel Based on Hyperbranched Macro-Cross-linker: Influence of Topological Structure on Properties

The application of hydrogels often suffers from their inherent limitation of poor mechanical properties. Here, a carboxyl-functionalized and acryloyl-terminated hyperbranched polycaprolactone (PCL) was synthesized and used as a macro-cross-linker to fabricate a super strong and ultra-tough ionic hydrogel. The terminal acryloyl groups of hyperbranched PCL are chemically incorporated into the network to form covalent cross-links, which contribute to robust networks. Meanwhile, the hydrophobic domains formed by the spontaneous aggregation of PCL chains and coordination bonds between Fe³⁺ and COO⁻ groups serve as dynamic non-covalent cross-links, which enhance the energy dissipation ability. Especially, the influence of the hyperbranched topological structure of PCL on hydrogel properties has been well investigated, exhibiting superior strengthening and toughening effects compared to the linear one. Moreover, the hyperbranched PCL cross-linker also endowed the ionic hydrogel with higher sensitivity than the linear one when used as a strain sensor. As a result, this well-designed ionic hydrogel possesses high mechanical strength, superior toughness, and well ionic conductivity, exhibiting potential applications in the field of flexible strain sensors.     

A novel polyacrylamide nanocomposite hydrogel reinforced with natural chitosan nanofibers

Polyacrylamide (PAM) was used as a matrix material for fabricating novel nanocomposite hydrogels reinforced with natural chitosan nanofibers (CNFs) via in situ free-radical polymerization. The nanocomposite's structure, strength, morphology and rheological properties were investigated. The results showed that the CNFs had a strong interaction with PAM through hydrogen and covalent bondings. The CNFs acted as a multifunctional cross-linker and a reinforcing agent in the hydrogel system. The compression strength and storage modulus of the nanocomposite hydrogels were significantly higher than those of the pure PAM hydrogels and the corresponding PAM/chitosan semi-interpenetrating polymer network (PAM-SIPN) hydrogels. The swelling ratio (SR) of the nanocomposite hydrogels was lower than that of the PAM hydrogel, but was similar to that of the PAM-SIPN hydrogel. Among the CNF contents used, the 1.5 wt% CNF loading level showed the best combined swelling and mechanical properties for the hydrogels.     

Electroresponsive Behavior of Sulfonated Benzal Poly(vinyl alcohol) Hydrogel Under Direct-Current Electric Field

A novel sulfonated benzal poly(vinyl alcohol) (S-B-PVA) hydrogel was prepared by sulfonating benzal poly(vinyl alcohol) hydrogel with concentrated sulfuric acid, and its swelling properties, mechanical properties, and electroresponsive behavior in Na₂SO₄ solutions were studied. The results indicated that the water take-up ability of the hydrogel decreased with the increasing ionic strength of Na₂SO₄ solution. The Young's modulus, elongation at break and tensile strength of the hydrogel swollen in deionized water is 8.38 MPa, 22.2% and 3.14 MPa, respectively. The hydrogel swollen in Na₂SO₄ solution bent toward the cathode under non-contact dc electric fields, and its bending speed and equilibrium strain increased with the increasing of applied voltage. The electroresponsive behavior of the hydrogel was also affected by the electrolyte concentration of external Na₂SO₄ solution, and there is a critical ionic strength of 0.1 at which the maximum equilibrium strain of the hydrogel occurs. Under a cyclically varying electric field, the hydrogel exhibited a good reversible bending behavior.     

A Highly Elastic and Reversibly Stretchable All‐Polymer Supercapacitor

Multiple stretchability has never been demonstrated as supercapacitors because the hydrogel used cannot fully recover after being heavily deformed. Now, a highly reversibly stretchable all‐polymer supercapacitor was fabricated using a developed double network hydrogel (DN hydrogel) as electrolyte and pure polypyrrole (PPy) as electrode. The DN hydrogel provides excellent mechanical properties, which can be stretched up to 500 % many times and then restore almost 100 % of the original length. To fabricate the fully recoverable stretchable supercapacitor, we annealed a free‐standing pure conducting polymer film as electrode so that the electrodes induced retardance is minimized. The as‐fabricated DN hydrogel/pure conducting polymer supercapacitors can be perfectly recovered from 100 % strain with almost no residual deformation left and the electrochemical performance can be maintained even after 1000 stretches (but not bending).     

Robust Hydrogel Adhesive with Dual Hydrogen Bond Networks

Hydrogel adhesives are attractive for applications in intelligent soft materials and tissue engineering, but conventional hydrogels usually have poor adhesion. In this study, we designed a strategy to synthesize a novel adhesive with a thin hydrogel adhesive layer integrated on a tough substrate hydrogel. The adhesive layer with positive charges of ammonium groups on the polymer backbones strongly bonds to a wide range of nonporous materials’ surfaces. The substrate layer with a dual hydrogen bond system consists of (i) weak hydrogen bonds between N,N-dimethyl acrylamide (DMAA) and acrylic acid (AAc) units and (ii) strong multiple hydrogen bonds between 2-ureido-4[1H]-pyrimidinone (UPy) units. The dual hydrogen-bond network endowed the hydrogel adhesives with unique mechanical properties, e.g., toughness, highly stretchability, and insensitivity to notches. The hydrogel adhesion to four types of materials like glass, 316L stainless steel, aluminum, Al₂O₃ ceramic, and two biological tissues including pig skin and pig kidney was investigated. The hydrogel bonds strongly to dry solid surfaces and wet tissue, which is promising for biomedical applications.     

High elasticity,strength, and biocompatible amphiphilic hydrogel via click chemistry and ferric ion coordination

An amphiphilic interpenetrating polymer network hydrogel was designed and synthesized using click chemistry and ferric ion coordination. The first polymer network was formed through the reaction of azide‐modified PEG (N₃‐PEG_n‐N₃) and alkynyl‐pendant linear PPG derivatives ((PPG_m(C≡CH))_n) through click chemistry and mixed with poly(ethylene glycol‐dopamine) macromolecules. The second polymer network was formed through ferric ion coordination with poly(ethylene glycol‐dopamine). Interpenetrating polymer networks give the hydrogel unique amphiphilic properties and higher mechanical strength and thermal stability. Swelling ratio and degradation rate could be adjusted by controlling the ratio of poly(ethylene glycol‐dopamine) in the hydrogel network. Given that in vivo subcutaneous implantation revealed no infection and no obvious abnormalities, the hydrogel exhibits high biocompatibility. The feature indicates that these hydrogels have a promising application in the field of biomaterials and tissue engineering. Copyright © 2016 John Wiley & Sons, Ltd.     

Flexible,adhesive, strain-sensitive,and skin-matchable hydrogel strain sensors for human motion and handwritten signal monitoring

Flexible hydrogel strain sensors have made great progress in medical applications, human motion detection, and human-machine interactions. However, the design of hydrogels to realize the synergistic responses of excellent mechanical properties, robust adhesion, and stable sensing is still a highly challenging task. Herein, we report a multifunctional hydrogel (PAMT hydrogel) by crosslinking acrylic acid (AA), 2-methacryloyloxyethyl phosphorylcholine (MPC) and tannic acid (TA) to form a polymer network via a simple one-pot free radical polymerization. Among them, the dynamic bonding with hydrogen bonding between PAA chains and TA significantly improved the stretch ability of the hydrogels (700%), and the abundant catechol groups on TA endowed the hydrogels with strong and stable adhesion properties (the adhesion strength to glass reached 248 N m⁻¹). When applied to human skin, the hydrogel can be easily peeled off without leaving any residue. Furthermore, the strain sensor assembled using PAMT hydrogel could not only effectively detect the movement in different parts of the human body, but also be used for precise handwritten recognition and electric skin of silicone prosthetic hand. Due to the addition of MPC and TA, the conductive hydrogel has good biocompatibility and no harm to human body. Therefore, PAMT hydrogel has opened up a new vision for the development of intelligent detection and bionic intelligent robots.     

Water-Resistant and Stretchable Conductive Ionic Hydrogel Fibers Reinforced by Carboxymethyl Cellulose

Conductive ionic hydrogels (CIH) have been widely studied for the development of stretchable electronic devices, such as sensors, electrodes, and actuators. Most of these CIH are made into 3D or 2D shape, while 1D CIH (hydrogel fibers) is often difficult to make because of the low mechanical robustness of common CIH. Herein, we use gel spinning method to prepare a robust CIH fiber with high strength, large stretchability, and good conductivity. The robust CIH fiber is drawn from the composite gel of sodium polyacrylate (PAAS) and sodium carboxymethyl cellulose (CMC). In the composite CIH fiber, the soft PAAS presents good conductivity and stretchability, while the rigid CMC significantly enhances the strength and toughness of the PAAS/CMC fiber. To protect the conductive PAAS/CMC fiber from damage by water, a thin layer of hydrophobic polymethyl acrylate (PMA) or polybutyl acrylate (PBA) is coated on the PAAS/CMC fiber as a water-resistant and insulating cover. The obtained PAAS/CMC-PMA and PAAS/CMC-PBA CIH fibers present high tensile strength (up to 28 MPa), high tensile toughness (up to 43 MJ/m³), and good electrical conductivity (up to 0.35 S/m), which are useful for textile-based stretchable electronic devices.     

Fracture Behavior of Two Highly Stretchable Double Network Hydrogels

The static and dynamic mechanical behavior of two double network (DN) hydrogels, alginate/polyacrylamide (PAAm) hybrid hydrogel and sodium poly(2-acrylamido-2-methylpropanesulfonic acid) PNaAMPS/PAAm, is presented to understand the role played by different cross-linked networks on fracture and recovery properties. Although with a smaller modulus, alginate/PAAm hybrid hydrogel had a much higher stretchability, whether with or without notches, in the tensile tests. Continuous step strain measurement by using a strain-controlled parallel-plate rheometer showed that alginate/PAAm can immediately recover its mechanical properties after breakdown, while PNaAMPS/PAAm didn't show mechanical recovery at all.     

Strong,Supertough and Self-Healing Biomimetic Layered Nanocomposites Enabled by Reversible Interfacial Polymer Chain Sliding

Despite the remarkable progress in ultrastrong mechanical laminate materials, the simultaneous achievement of toughness, stretchability and self-healing properties in biomimetic layered nanocomposites remains a great challenge due to the intrinsic limitations of their hard essences and lack of effective stress transfer at the organic-inorganic fragile boundary. Here, an ultratough nanocomposite laminate is prepared by constructing chain-sliding cross-linking at the interface between sulfonated graphene nanosheets and polyurethane layers based on the ring molecules sliding on the linear polymer chains to release stresses. Unlike traditional supramolecular bonding toughening with limited sliding spacing, our strategy enables interfacial molecular chains reversible slippage when the inorganic nanosheets bear stretching force, providing sufficient interlayer spatial distance for relative sliding to dissipate more energy. The resulting laminates exhibit strong strength (22.33 MPa), supertoughness (219.08 MJ m⁻³), ultrahigh stretchability (>1900 %) and self-healing ability (99.7 %), which far surpass most of reported synthetic and natural laminate materials. Moreover, the fabricated proof-of-concept electronic skin shows excellent flexibility, sensitivity and healability for human physiological signals monitoring. This strategy breaks through the challenge that traditional layered nanocomposites are intrinsically stiff and opens up the functional application of layered nanocomposites in flexible devices.     

MnO<Subscript>2</Subscript> Nanoparticles and Carbon Nanofibers Nanocomposites with High Sensing Performance Toward Glucose

By combining the advantages of manganese dioxide nanoparticles (MnO₂ NPs) and carbon nanofibers (CNFs), a biosensing electrode surface as a high-performance enzyme biosensor is designed in this work. MnO₂ NPs and CNFs nanocomposites (MnO₂–CNFs) were prepared by using a simple hydrothermal method and then were characterized by scanning electron microscopy, powder X-ray diffraction, fourier transform infrared spectroscopy, energy dispersive spectrometry and electrochemisty. The results showed that MnO₂ NPs are uniformly attached to the surface of CNFs. Meanwhile, the MnO₂–CNFs nanocomposites as a supporting matrix can provide an efficient and advantageous platform for electrochemical sensing applications. On the basis of the improved sensitivity of MnO₂–CNFs modified electrode toward H₂O₂ at low overpotential, a MnO₂–CNFs based glucose biosensor was fabricated by monitoring H₂O₂ produced by an enzymatic reaction between glucose oxidase and glucose. The constructed biosensor exhibited a linear calibration graph for glucose in a concentration range of 0.08–4.6 mM and a low detection limit of 0.015 mM. In addition, the biosensor showed other excellent characteristics, such as high sensitivity and selectivity, short response time, and the relative low apparent Michaelis–Menten constant. Analysis of human urine spiked with glucose at different concentration levels yielded recoveries between 101.0 and 104.8%.     

Highly stretchable,self-adhesive,and self-healable double network hydrogel based on alginate/polyacrylamide with tunable mechanical properties

A number of synthetic hydrogels suffer from low mechanical strength. Despite of the recent advances in the fabrication of tough hydrogels, it is still a great challenge to simultaneously construct high stretchability, and self-adhesive and self-healing capability in a hydrogel. Herein, a new type of double network hydrogel was prepared based on irreversible cross-linking of polyacrylamide chains and Schiff-base reversible cross-linking between glycidyl methacrylate-grafted ethylenediamine and oxidized sodium alginate (OSA). The combination of both cross-linkings and their synergistic effect provided a novel hydrogel with high strength, stretchable, rapid self-healing, and self-adhesiveness to different material. Besides, the hydrogels with diverse OSA content could maintain their original shapes after loading–unloading tensile test. The resulting hydrogel has a great potential in various fields for supporting and load-bearing substance.     

Effect of agar hydrogel and magnesium ions on the biomimetic mineralization of calcium carbonate

Here, agar hydrogel was selected as diffusion medium and template to control the biomimetic mineralization of calcium carbonate (CaCO₃). Due to three dimensional network structures and abundant functional groups (such as, hydroxyl groups), Ca²⁺ ions were uniformly distributed in the network and electrostatically attracted. The diffusion speed and range of CO₃^2? ions were mediated by the concentration of hydrogel medium. Under the synergistic effect of Mg²⁺ ions, the crystal CaCO₃ was induced by gas phase diffusion method in the hydrogel system. The results showed that the concentrations of Mg²⁺ ions and agar hydrogel had no obvious effect on the calcite phase of CaCO₃, but the morphologies and sizes changed with concentrations of medium and Mg²⁺ ions. Attribute to template effect, the crystallization behavior and growth rate of CaCO₃ crystals were regulated. Since Mg²⁺ ions were easily adsorbed on the surfaces of unit cell, the unique structure of CaCO₃ was precisely controlled. This study provides a useful reference and inspiration for the understandings of the contributions of ion supply rate in bio-mineralization and hydrogel medium in biomimetic mineralization.     

Application of sensitive hydrogels in chemical and pH sensors

In the present work, pH-sensitive poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) blends as well as hydrogels based on poly(N-isopropylacrylamide) (PNIPAAm), which are sensitive to organic solvent concentration in aqueous solutions, were used in silicon micromachined sensors. A sensitivity of approximately 15 mV/pH was obtained for a pH sensor with a 50 μm thick PVA/PAA hydrogel layer in a pH range above the acid exponent of acrylic acid (pK_a=4.7). The output voltage versus pH-value characteristics and the long-term signal stability of hydrogel-based sensors were investigated and the measurement conditions necessary for high signal reproducibility were determined. The influence of the preparation conditions of the hydrogel films on the sensitivity and response time of the chemical and pH sensors is discussed.     

Sponge-inspired multisensory hydrogel

The preparation of smart sensors with multiple sensory systems, reusable property, and high sensitivity remains a challenge. Here, inspired by a sponge, we develop a multisensory PCA–poly(vinyl alcohol) (PVA)/borax–LiCl hydrogel (named PPL hydrogel) with low cost and simple fabrication process using sodium polyacrylate (PAAS), PVA, and lithium chloride (LiCl) to address this challenge. PPL hydrogel has a sponge-like porous structure and can repeatedly “absorb” and “drain” water, while maintaining good tensile (strain up to 1442%) and electrical conductivity. PAAS builds the main skeleton of PPL hydrogels and provides the basis for building the pore structure, PVA enhances the mechanical properties of the crosslinked network, while LiCl ionic solution further improves the conductivity, which can reach 8.8 s m⁻¹ and be increased nearly 42 times better than without LiCl. Therefore, this makes PPL hydrogel a very sensitive sensing system for wearable devices that can be used to detect signals from the human body. Additionally, PPL hydrogel is also capable of detecting temperature due to its temperature-sensitive properties. Moreover, PPL hydrogel can also roughly identify the basic properties of different solvents. Our simple, low-cost and multisensory PPL hydrogel offers promising opportunities for multisensory sensors, even functional/smart materials, flexible/wearable devices, and medical care.     

Direct current electrodeposition of carbon nanofibers in DMF

Carbon nanofibers (CNFs) were electrodeposited on indium tin oxide (ITO) electrodes by using a DC electric field from N,N′-dimethylformamide (DMF). An improved dispersion of CNFs has been found in DMF solution compared to ethanol and acetonenitrile. After treated by concentrated H₂SO₄/HNO₃, CNFs were dispersed uniformly and stably in DMF. During the electrodeposition process, CNFs moved towards anode indicating the negative charge of the nanofibers. Effects of electric field strength, CNF concentration in the suspension, and the solvents used for CNF dispersion were examined on the deposition nature of CNFs.     

Facile preparation of pH-sensitive poly(acrylic acid-co-acrylamide)/SiO2 hybrid hydrogels with high strength by in situ frontal polymerization

A series of poly(acrylic acid-co-acrylamide) (PAA)/SiO₂ hybrid hydrogels were prepared by in situ frontal polymerization. It was found that the increase in the concentration of SiO₂ nanoparticles could lead to the increase in front velocity (V _f) and the highest front temperature (T _max). This may be attributed to the fact that SiO₂ nanoparticles could increase the liquid viscosity of reaction mixture. The obtained PAA/SiO₂ hybrid hydrogels were characterized by SEM and Fourier transform infrared spectroscopy spectrum and swelling measurements. The pH-sensitive swelling behaviors showed that the prepared PAA/SiO₂ hybrid hydrogel had high pH sensitivity in different pH buffer solutions. Mechanical property test indicated that the PAA/SiO₂ hybrid hydrogels exhibited a high compressive strength while remaining a high swelling radio (SR). The maximum of compressive strength and SR of the hybrid hydrogel may reach 42.6 kPa and 17.8, respectively, which was much higher than that of pure PAA hydrogel.