Highly Stretchable, Compressible, Adhesive, Conductive Self-healing Composite Hydrogels with Sensor Capacity

Boron nitride microfiber reinforced polyacrylic acid hydrogels with excellent self-adhesion, fast pH response, and strain sensitivity

Hydrogels are widely utilized in the sensor field, but their inadequate adhesion presents a significant obstacle. Herein, a new multifunctional BNMFs/PAA composite hydrogel was prepared via the incorporation of one-dimensional porous boron nitride microfibers (BNMFs) and polyacrylic acid (PAA) hydrogels. BNMFs, as a reinforcing filler, play a very important role in enhancing the properties of the composite hydrogels. In particular, the porous micrometer structure plays a unique role in improving the adhesion properties of PAA hydrogels. The steric hindrance and the rich hydroxyl functional groups coming from BNMFs are key factors for the excellent adhesion of the composite hydrogels. The composite hydrogels show strong adhesion to various substrate materials. For iron plates and biological tissues, the adhesion energy can reach 1377 J m-2 and 317 J m-2, respectively. In addition, the developed BNMFs/PAA composite hydrogels exhibit excellent mechanical properties. The fracture strain of the composite hydrogels is increased by 2.4 times compared to pure PAA hydrogels. The hydrogen bonds formed between BNMFs and PAA are conducive to the mechanical properties of the BNMFs/PAA composite hydrogels. Meanwhile, BNMFs as fillers play a role in carrying and dissipating force. Furthermore, the BNMFs/PAA composite hydrogels have excellent strain and pH response characteristics. This is because the crosslinking network of the composite hydrogels becomes loose after the addition of BNMFs, resulting in rapid ion transport pathways. Therefore, the developed BNMFs/PAA composite hydrogels will have broad application prospects in the fields of motion monitoring, intelligent skin and biological adhesives.

Seamless Integration of Conducting Hydrogels in Daily Life: From Preparation to Wearable Application

Conductive hydrogels (CHs) have received significant attention for use in wearable devices because they retain their softness and flexibility while maintaining high conductivity. CHs are well suited for applications in skin-contact electronics and biomedical devices owing to their high biocompatibility and conformality. Although highly conductive hydrogels for smart wearable devices are extensively researched, a detailed summary of the outstanding results of CHs is required for a comprehensive understanding. In this review, the recent progress in the preparation and fabrication of CHs is summarized for smart wearable devices. Improvements in the mechanical, electrical, and functional properties of high-performance wearable devices are also discussed. Furthermore, recent examples of innovative and highly functional devices based on CHs that can be seamlessly integrated into daily lives are reviewed.

Weft-Knitted Spacer Fabric for Highly Stretchable-Compressible Strain Sensor, Supercapacitor, and Joule Heater

The development of wearable electronic devices has greatly stimulated the research interest of textile-based strain sensors, which can effectively combine functionality with wearability. In this work, the fabrication of highly stretchable and compressible strain sensors from weft-knitted spacer fabric was reported. Carbon nanotubes and polypyrrole were deposited on the surface of fabric via an in situ polymerization approach to reduce the electrical resistance. The as-fabricated WSP-CNT-PPy strain sensor exhibits high electrical conductivity and stable strain-sensing performance under different stretching deformations. The WSP-CNT-PPy strain sensor can be stretched up to 450% and compressed to 60% with a pressure of less than 50 KPa, which can be attributed to the unique loop and interval filament structures. The distinguishing response efficiency of WSP-CNT-PPy can effectively detect faint and strenuous body movements. In addition, the electrochemical behavior of WSP-CNT-PPy was also characterized to study the comprehensive properties. The electro-heating performance was also evaluated for feasible Joule heater applications. This work demonstrates the practicability of WSP-CNT-PPy strain sensor fabric for real-time monitoring in promising wearable garments.

Stretchable, compressible, and conductive hydrogel for sensitive wearable soft sensors

Conductive hydrogels hold great promises in wearable soft electronics. However, the weak mechanical properties, low sensitivity and the absence of multifunctionalities (e.g., self-healing, self-adhesive, etc.) of the conventional conductive hydrogels limit their applications. Thus, developing multifunctional hydrogels may address some of these technical issues. In this work, a multifunctional conductive hydrogel strain sensor is fabricated by incorporating a conductive polymer Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) into a mechanically robust poly (vinyl alcohol) (PVA)/ poly (acrylic acid) (PAA) double network (DN) hydrogel. The as-prepared hydrogel sensor could span a wide spectrum of mechanical properties by simply tuning the polymer composition and the number of freezing-thawing cycles. In addition, the dynamic hydrogen bonding interactions endow the hydrogel sensor with self-healing property and reversible adhesiveness on diverse substrates. Moreover, the hydrogel sensor shows high sensitivity (Gauge Factor from 2.21 to 3.82) and can precisely detect some subtle human motions (e.g., pulse and vocal cord vibration). This work provides useful insights into the development of conductive hydrogel-based wearable soft electronics.

Soft Stretchable Conductive Carboxymethylcellulose Hydrogels for Wearable Sensors

Hydrogels that have a capability to provide mechanical modulus matching between time-dynamic curvilinear tissues and bioelectronic devices have been considered tissue-interfacing ionic materials for stably sensing physiological signals and delivering feedback actuation in skin-inspired healthcare systems. These functionalities are totally different from those of elastomers with low ionic conductivity and higher stiffness. Despite such remarkable progress, their low conductivity remains limited in transporting electrical charges to internal or external terminals without undesired information loss, potentially leading to an unstable biotic–abiotic interfaces in the wearable electronics. Here, we report a soft stretchable conductive hydrogel composite consisting of alginate, carboxymethyl cellulose, polyacrylamide, and silver flakes. This composite was fabricated via sol–gel transition. In particular, the phase stability and low dynamic modulus rates of the conductive hydrogel were confirmed through an oscillatory rheological characterization. In addition, our conductive hydrogel showed maximal tensile strain (≈400%), a low deformations of cyclic loading (over 100 times), low resistance (≈8.4 Ω), and a high gauge factor (≈241). These stable electrical and mechanical properties allowed our composite hydrogel to fully support the operation of a light-emitting diode demonstration under mechanical deformation. Based on such durable performance, we successfully measured the electromyogram signals without electrical malfunction even in various motions.

Self-Healing Mechanism and Conductivity of the Hydrogel Flexible Sensors: A Review

Sensors are devices that can capture changes in environmental parameters and convert them into electrical signals to output, which are widely used in all aspects of life. Flexible sensors, sensors made of flexible materials, not only overcome the limitations of the environment on detection devices but also expand the application of sensors in human health and biomedicine. Conductivity and flexibility are the most important parameters for flexible sensors, and hydrogels are currently considered to be an ideal matrix material due to their excellent flexibility and biocompatibility. In particular, compared with flexible sensors based on elastomers with a high modulus, the hydrogel sensor has better stretchability and can be tightly attached to the surface of objects. However, for hydrogel sensors, a poor mechanical lifetime is always an issue. To address this challenge, a self-healing hydrogel has been proposed. Currently, a large number of studies on the self-healing property have been performed, and numerous exciting results have been obtained, but there are few detailed reviews focusing on the self-healing mechanism and conductivity of hydrogel flexible sensors. This paper presents an overview of self-healing hydrogel flexible sensors, focusing on their self-healing mechanism and conductivity. Moreover, the advantages and disadvantages of different types of sensors have been summarized and discussed. Finally, the key issues and challenges for self-healing flexible sensors are also identified and discussed along with recommendations for the future.

Mussel-inspired hydrogels as tough, self-adhesive and conductive bioelectronics: a review

To overcome the wearable sensor's defects and achieve the goal of robust mechanical properties, long-term adhesion, sensitive electrical conductivity, the multifunctional hydrogels were inspired by various mussels on the base of catechol and its analogues. In this review, we review the strategies for improving the mechanical strength, adhesion, conductivity and antibacterial properties of mussel-inspired hydrogels as bioelectronics. Double network structures, nanocomposites, supramolecular block polymers and other strategies were utilized for achieving tough hydrogels to prevent tensile fractures under high deformation. Many mussel-inspired chemistries were incorporated for constructing skin-attachable hydrogel strain sensors and some strategies for controlling the oxidation of catechol were employed to achieve long-term adhesion. In addition, electrolytes, conductive fillers, conductive polymers and their relevant hydrophilic modifications were introduced for fabricating the conductive hydrogel bioelectronics to enhance the conductivity properties. Finally, the challenges and outlooks in this promising field are featured from the perspective of materials chemistry.

Surface Modification of Nanofibers by Physical Adsorption of Fiber-Homologous Amphiphilic Copolymers and Nanofiber-Reinforced Hydrogels with Excellent Tissue Adhesion

Herein, we report a simple approach to modify hydrophobic PCL nanofibers by adsorption of a fiber-homologous amphiphilic triblock copolymer (PCL-b-PEG-b-PCL, PCEC). The modified PCL nanofibers were then utilized to reinforce a physical hydrogel, which was formed by micellar crosslinking of the same PCEC triblock copolymer. Therefore, the copolymer played a dual role in not only dispersing and stabilizing nanofibers but also additionally providing a framework for the hydrogel matrix. The mechanical strength of the hydrogel was significantly enhanced by addition of the modified PCL nanofibers, and the gel modulus can be tuned by varying the concentration of the copolymer and nanofibers. The effect of nanofiber size and content on the mechanical properties of the hydrogel matrices was studied. Different from hydrogel composites that were reinforced by 2D fiber meshes or 3D woven fiber networks, this free fiber-reinforced hydrogel can be readily injected to adapt to the environmental shape and self-heal. The hydrogel composites showed superior tissue adhesion properties compared to the commercially available fibrin glue, especially in muscle adhesion. Due to its injectable and self-healing properties, this nanofiber-reinforced hydrogel may have great potential as a new type of tissue sealant.

Multifunctional conductive hydrogels and their applications as smart wearable devices

Recently, hydrogel-based conductive materials and their applications as smart wearable devices have been paid tremendous attention due to their high stretchability, flexibility, and excellent biocompatibility. Compared with single functional conductive hydrogels, multifunctional conductive hydrogels are more advantageous to match various demands for practical applications. This review focuses on multifunctional conductive hydrogels applied for smart wearable devices. Representative strategies for conduction of hydrogels are discussed firstly: (1) electronic conduction based on the conductive fillers and (2) ionic conduction based on charged ions. Then, the common and intensive research on multiple functionalities of conductive hydrogels, such as mechanical properties, conductive and sensory properties, anti-freezing and moisturizing properties, and adhesion and self-healing properties is presented. The applications of multifunctional conductive hydrogels such as in human motion sensors, sensory skins, and personal healthcare diagnosis are provided in the third part. Finally, we offer our perspective on open challenges and future areas of interest for multifunctional conductive hydrogels used as smart wearable devices.

Combination of Self-Healing Butyl Rubber and Natural Rubber Composites for Improving the Stability

The self-healing composites were prepared from the combination of bromobutyl rubber (BIIR) and natural rubber (NR) blends filled with carbon nanotubes (CNT) and carbon black (CB). To reach the optimized self-healing propagation, the BIIR was modified with ionic liquid (IL) and butylimidazole (IM), and blended with NR using the ratios of 70:30 and 80:20 BIIR:NR. Physical and chemical modifications were confirmed from the mixing torque and attenuated total reflection-fourier transform infrared spectroscopy (ATR-FTIR). It was found that the BIIR/NR-CNT_CB with IL and IM effectively improved the cure properties with enhanced tensile properties relative to pure BIIR/NR blends. For the healed composites, BIIR/NR-CNT_CB-IM exhibited superior mechanical and electrical properties due to the existing ionic linkages in rubber matrix. For the abrasion resistances, puncture stress and electrical recyclability were examined to know the possibility of inner liner applications and Taber abrasion with dynamic mechanical properties were elucidated for tire tread applications. Based on the obtained T_g and Tan δ values, the composites are proposed for tire applications in the future with a simplified preparation procedure.