Highly Stretchable, Ultra-Sensitive, and Self-Healable Multifunctional Flexible Conductive Hydrogel Sensor for Motion Detection and Information Transmission

Amino Acid-Based Hydrogel with Interpenetrating Gelatin and Cross-Linked by Metal Ions, Providing High Stretchability and Motion Sensitivity

A double network structure with metal ions was created to enhance the mechanical stability of the hydrogels and increase their low conductivity. For this purpose, the P(AM_AcOr_Gelatin) hydrogel was synthesized by combining gelatin, a biocompatible polymer, N-δ-acryloyl-ornithine (AcOr), an amino acid derivative, and acrylamide (AM). Because the amino acid-based monomer added charged groups to the hydrogel network, the hydrogel exhibited improved conductivity and motion sensitivity properties compared with polyacrylamide (PAM) hydrogels. Furthermore, we altered the P(AM_AcOr_Gelatin) hydrogel by introducing the Fe3+ and Cu2+ ions, resulting in the formation of the P(AM_AcOr_Gelatin)-Fe3+ and P(AM_AcOr_Gelatin)-Cu2+ hydrogels. The hydrogels containing metal ions had coordination bonds between the ions, gelatin, and AcOr. Additionally, there were other noncovalent bonds present, resulting in further increased conductivity (approximately 95% improvement) and stretchability (more than double). The conductivity and resistance of the hydrogels changed, depending on the bending position and strain applied to the hydrogel layer. The results demonstrated that the hydrogel layer had good strain sensitivity, with an enhanced gauge factor (GF) of approximately 1.7 (at 250% strain) and a conductivity ranging from 3355 to 4387 μS·cm-1.

PNIPAAm-based temperature responsive ionic conductive hydrogels for flexible strain and temperature sensing

Conductive hydrogels have received much attention in the field of flexible wearable sensors due to their outstanding flexibility, conductivity, sensitivity and excellent compatibility. However, most conductive hydrogels mainly focus on strain sensors to detect human motion and lack other features such as temperature response. Herein, we prepared a strain and temperature dual responsive ionic conductive hydrogel (PPPNV) with an interpenetrating network structure by introducing a covalent crosslinked network of N-isopropylacrylamide (NIPAAm) and 1-vinyl-3-butylimidazolium bromide (VBIMBr) into the skeleton of the hydrogel composed of polyvinylalcohol (PVA) and polyvinylpyrrolidone (PVP). The PPPNV hydrogel exhibited excellent anti-freezing properties (-37.34 °C) and water retention with high stretchability (∼930 %) and excellent adhesion. As a wearable strain sensor, the PPPNV hydrogel has good responsiveness and stability to a wide range of deformations and exhibits high strain sensitivity (GF=2.6) as well as fast response time. It can detect large and subtle body movements with good signal stability. As wearable temperature sensors, PPPNV hydrogels can detect human physiological signals and respond to temperature changes, and the volumetric phase transition temperature (VPTT) can be easily controlled by adjusting the molar ratio of NIPAAm to VBIMBr. In addition, a bilayer temperature-sensitive hydrogel was prepared with the temperature responsive hydrogel by two-step synthesis, which shows great promising applications in temperature actuators.

Highly conductive and stable double network carrageenan organohydrogels for advanced strain sensing and signal recognition arrays

Conductive hydrogels with excellent mechanical properties, a broad detection range, and stability in complex environments have remained a significant challenge for the development of flexible sensors. In this study, a straightforward freeze-thaw cycles strategy was developed to fabricate a polyvinyl alcohol (PVA)/carrageenan (CA)/calcium chloride (CaCl2)/MXene-based double network organohydrogel (PCCME) for highly flexible and responsive strain detection across a broad temperature spectrum. The PCCME organohydrogel features multiple interactive forces including hydrogen bonding, ionic interactions, and microphase crystallization, which contribute to the organohydrogel's exceptional mechanical and electrical performance. The PCCME organohydrogel exhibited excellent performance in a load-unload test repeated 100 times after being maintained at room temperature for 7 days, with a minimal mechanical decay of only 2.6%. Furthermore, the repaired PCCME organohydrogel retained its robust stability after storage at low temperatures followed by placement at room temperature. The organohydrogel sensor not only detects various movement amplitudes of the human body but also recognizes arrays of pressure signals and converts these into digital images, highlighting its significant potential for applications in rehabilitation monitoring, pressure sensing, and human-computer interaction.

Aminated lignin and phytic acid-assisted polyacrylic acid hydrogel sensors with enhanced mechanical properties and strong adhesion

Excellent comprehensive performance of hydrogels can be achieved by synergistically combining multiple interaction mechanisms. In this study, a series of hydrogels with rapid gelation and excellent adhesive, mechanical, self-healing, and conductive properties, driven by covalent bonds and multiple reversible interactions, were constructed by mixing acrylic acid (AA), aminated alkaline lignin (AAL), phytic acid (PA), and Fe3+. The rigid skeletons of polyacrylic acid (PAA) and AAL, as well as the metal coordination bonds formed between them and Fe3+, enhance the mechanical properties of the samples. The samples exhibit excellent tensile strength and compressive strength, reaching 73.7 kPa and 4.6 MPa (under a compressive strain of 80 %), respectively, with a tensile strain of 1142 % under the same condition. Adding PA enhances the compliance and adhesion (148.2 kPa for porcine skin) of the gel and endowed it with good flame retardancy. Additionally, the sample maintained its good mechanical properties and conductivity even after five cutting-healing cycles. Good durability, robust adhesion, and high electrical conductivity of the sample render it a promising strain sensor for electronic devices. This work provides a design strategy for preparing hydrogels with superior adhesion and good comprehensive performance.

Electronic skin based on natural biodegradable polymers for human motion monitoring

The wearability of the flexible electronic skin (e-skin) allows it to attach to the skin for human motion monitoring, which is essential for studying human motion and especially for assessing how well patients are recovering from rehabilitation therapy. However, the use of non-degradable synthetic materials in e-skin may raise skin safety concerns. Natural biodegradable polymers with advantages such as biodegradability, biocompatibility, sustainability, natural abundance, and low cost have the potential to be alternative materials for constructing flexible e-skin and applying them to human motion monitoring. This review summarizes the applications of natural biodegradable polymers in e-skin for human motion monitoring over the past three years, focusing on the discussion of cellulose, chitosan, silk fibroin, gelatin, and sodium alginate. Finally, we summarize the opportunities and challenges of e-skin based on natural biodegradable polymers. It is hoped that this review will provide insights for the future development of flexible e-skin in the field of human motion monitoring.

Dually-crosslinked ionic conductive hydrogels reinforced through biopolymer gellan gum for flexible sensors to monitor human activities

Human-machine interactions, monitoring of health equipment, and gentle robots all depend considerably on flexible strain sensors. However, making strain sensors have better mechanical behavior and an extensive sensing range remains an urgent difficulty. In this study, poly acrylamide-co-butyl acrylate with gellan gum (poly(AAm-co-BA)@GG) hydrophobic association networks and intermolecular hydrogen bonding interactions are used to fabricate dual cross-linked hydrogels for wearable resistive-type strain sensors. This could be an acceptable way to minimize the limitations in hydrogels previously identified. The robust fracture strength (870 kPa) and exceptional stretchability (1297 %) of the hydrogel arise from the collaborative action of intermolecular hydrogen bonding and hydrophobic associations. It also demonstrates exceptional resilience to repeated cycles of uninterrupted stretching and relaxation, retaining its structural integrity. The response and restoration times are 110 and 120 ms respectively. Furthermore, a wide sensing range (0-900 %), notable sensitivity across various strain levels, and an impressive gauge factor (GF) of 31.51 with high durability were observed by the dual cross-linked (DC) hydrogel-based strain sensors. The measured conductivity of the hydrogel was 0.32 S/m which is due to the incorporation of NaCl. Therefore, the hydrogels can be tailored to function as wearable strain sensors that can detect subtle human gestures like speech patterns, distinguish between distinct words, and recognize vibrations of the larynx during drinking, as well as large joint motions like wrist, finger, and elbow. Furthermore, these hydrogels are capable of reliably distinguishing and reproducing various printed text. These findings imply that any electronic device that demands strain-sensing functionality might make use of these developed materials.

Facile Preparation and Study of Self-Healing Water-Absorbing Expanding Elastomers

The absorbent expansion elastomer plays a crucial role in engineering sealing and holds a promising future in this field as infrastructure continues to develop. Traditional materials have their limitations, especially when used in large construction projects where the integrity and reliability of the material are of utmost importance. In this work, a self-healing water-absorbing expansion elastomer was developed for continuous production at a large scale to monitor the sealing conditions of massive infrastructure projects. At room temperature, the material exhibited a repairing efficiency of 57.77 % within 2 h, which increased to 84.02 % after 12 h. This extended reaction time allowed for effective repairs when defects were detected. The material's strength can attain 3 MPa, placing it at the upper echelon among common self-healing materials, thereby granting it a certain level of durability in its application environment. The material's volume expansion rate reached 200-400 % to achieve effective sealing, and the functional filling of the filler endowed the material itself with a favorable external force induction effect and prevented heat accumulation. The conductive detection performance of the absorbent expansion elastomer was improved by utilizing triple self-healing strategies, including dipole-dipole interaction, ion cross-linked network, and externally-aided restoration materials. These strategies were combined with a double packing strategy to enhance the material's properties. This innovative elastomer can be applied in various fields such as tunnel construction, infrastructure development, aerospace sealing, and railway transportation, showcasing significant potential for diverse engineering applications.

Recent Advances in the 3D Printing of Conductive Hydrogels for Sensor Applications: A Review

Conductive hydrogels, known for their flexibility, biocompatibility, and conductivity, have found extensive applications in fields such as healthcare, environmental monitoring, and soft robotics. Recent advancements in 3D printing technologies have transformed the fabrication of conductive hydrogels, creating new opportunities for sensing applications. This review provides a comprehensive overview of the advancements in the fabrication and application of 3D-printed conductive hydrogel sensors. First, the basic principles and fabrication techniques of conductive hydrogels are briefly reviewed. We then explore various 3D printing methods for conductive hydrogels, discussing their respective strengths and limitations. The review also summarizes the applications of 3D-printed conductive hydrogel-based sensors. In addition, perspectives on 3D-printed conductive hydrogel sensors are highlighted. This review aims to equip researchers and engineers with insights into the current landscape of 3D-printed conductive hydrogel sensors and to inspire future innovations in this promising field.

Distinguishing thermoelectric and photoelectric modes enables intelligent real-time detection of indoor electrical safety hazards

Due to the prevalence of electronic devices, intelligent sensors have attracted much interest for the detecting and providing alarms with respect to indoor electrical safety. Nonetheless, how to effectively identify various indoor electrical safety hazards remains a challenge. In this study, we fabricated single-walled carbon nanotube/poly(3-hexylthiophene-2,5-diyl) (SWCNT/P3HT) composites with exceptional bifunctional thermoelectric and photoelectric responses. Through synergy of the thermo-/photoelectric effects, the composites yielded greatly enhanced output voltages compared with the use of thermoelectric effects alone. Interestingly, modes of heat transfer can be effectively distinguished using the nominal Seebeck coefficients. Based on the remarkable output voltages and deviations in the nominal Seebeck coefficients, we developed indoor intelligent sensors capable of effectively identifying and monitoring diverse indoor electrical conditions, including electrical overheating, fire, and air conditioning flow. This pioneering investigation proposes a novel avenue for designing intelligent sensors that can recognize heat transfer modes and hence effectively monitor indoor electrical safety hazards.

Sandwich-Type Self-Healing Sensor with Multilevel for Motion Detection

Real-time detection of various parts of the human body is crucial in medical monitoring and human-machine technology. However, existing self-healing flexible sensing materials are limited in real-life applications due to the weak stability of conductive networks and difficulty in balancing stretchability and self-healing properties. Therefore, the development of wearable flexible sensors with high sensitivity and fast response with self-healing properties is of great interest. In this paper, a novel multilevel self-healing polydimethylsiloxane (PDMS) material is proposed for enhanced sensing capabilities. The PDMS was designed to have multiple bonding mechanisms including hydrogen bonding, coordination bonding, disulfide bonding, and local covalent bonding. To further enhance its sensing properties, modified carbon nanotubes (CNTs) were embedded within the PDMS matrix using a solvent etching technique. This created a sandwich-type sensing material with improved stability and sensitivity. This self-healing flexible sensing material (self-healing efficiency = 70.1% at 80 °C and 6 h) has good mechanical properties (stretchability ≈413%, tensile strength ≈0.69 MPa), thermal conductivity, and electrical conductivity. It has ultrahigh sensitivity, which makes it possible to be manufactured as a multifunctional flexible sensor.

Fabrication of anti-freezing and self-healing nanocomposite hydrogels based on phytic acid and cellulose nanocrystals for high strain sensing applications

For hydrogel-based flexible sensors, it is a challenge to enhance the stability at sub-zero temperatures while maintaining good self-healing properties. Herein, an anti-freezing nanocomposite hydrogel with self-healing properties and conductivity was designed by introducing cellulose nanocrystals (CNCs) and phytic acid (PA). The CNCs were grafted with polypyrrole (PPy) by chemical oxidation, which were used as the nanoparticle reinforcement phase to reinforce the mechanical strength of hydrogels (851.8%). PA as a biomass material could form strong hydrogen bond interactions with H2O molecules, endowing hydrogels with prominent anti-freezing properties. Based on the non-covalent interactions, the self-healing rate of the hydrogels reached 92.9% at -15 °C as the content of PA was 40.0 wt%. Hydrogel-based strain sensors displayed high sensitivity (GF = 0.75), rapid response time (350 ms), good conductivity (3.1 S m-1) and stability at -15 °C. Various human movements could be detected by using them, including small (smile and frown) and large changes (elbow and knee bending). This work provides a promising method for the development of flexible wearable sensors that work stably in frigid environments.