High-Performing Conductive Hydrogels for Wearable Applications.

A ionic liquid enhanced conductive hydrogel for strain sensing applications

Degradable conductive self-healing hydrogels based on dextran-graft-tetraaniline and N-carboxyethyl chitosan as injectable carriers for myoblast cell therapy and muscle regeneration

Flexible and wearable devices play a pivotal role in the realm of smart portable electronics due to their diverse applications in healthcare monitoring, soft robotics, human-machine interfaces, and artificial intelligence. Nonetheless, the extensive integration of intelligent wearable sensors into mass production faces challenges within a resource-limited environment, necessitating low-cost manufacturing, high reliability, stability, and multi-functionality. In this study, a cost-effective fiber laser direct writing method (fLDW) was illustrated to create highly responsive and robust flexible sensors. These sensors integrate laser-induced graphene (LiG) with mixed metal oxides on a flexible polyimide film. fLDW simplifies the synthesis of graphene, functionalization of carbon structures into graphene oxides and reduced graphene oxides, and deposition of metal-oxide nanoparticles within a single experimental laser writing setup. The preparation and surface modification of dense oxygenated graphene networks and semiconducting metal oxide nanoparticles (CuO x , ZnO x , FeO x ) enables rapid fabrication of LiG/MO x composite sensors with the ability to detect and differentiate various stimuli, including visible light, UV light, temperature, humidity, and magnetic fluxes. Further, this in situ customizability of fLDW-produced sensors allows for tunable sensitivity, response time, recovery time, and selectivity. The normalized current gain of resistive LiG/MO x sensors can be controlled between -2.7 to 3.5, with response times ranging from 0.02 to 15 s, and recovery times from 0.04 to 6 s. Furthermore, the programmable properties showed great endurance after 200 days in air and extended bend cycles. Collectively, these LiG/MO x sensors stand as a testament to the effectiveness of fLDW in economically mass-producing flexible and wearable electronic devices to meet the explicit demands of the Internet of Things.

Flexible piezo-resistive pressure sensor based on conducting PANI on paper substrate

Superelastic and highly sensitive conductive hydrogel sensor enabled by spatially confined assembly of MXene within bacterial cellulose network.

Recently, there has been a great development in the field of smart materials and energy devices. Among many energy devices, nanogenerators that transform waste mechanical energy into electrical energy have attracted considerable research interest in the last 10 years. Polymers with unique characteristics, such as ferroelectric, pyroelectric, piezoelectric, along with flexibility, easy shaping, lightness, and ductility, are frequently used in nanogenerators. The chapter covers recent development in textile-based nanogenerators based on triboelectric, pyroelectric, and piezoelectric mechanisms. Textile-based nanogenerators have previously been proposed for biomechanical energy harvesting for self-powered and low-power electronics. However, with the advancement in nanogenerators, textile-based, human, and environmentally friendly flexible nanogenerators for applications in healthcare monitoring and wearable sensors are drawing attention. This chapter first provides the current state and developments in textile-based nanogenerators and then provides the obstacles and suggestions for the future.

Conductive hydrogels are promising multifunctional materials for wearable sensors, but their practical applications require combined properties that are difficult to achieve. Herein, we developed a flexible wearable sensor with double-layer structure based on conductive composite hydrogel, which included the outer layer of silicone elastomer (Ecoflex)/silica microparticle composite film and the inner layer of P(AAm-co-HEMA)-MXene-AgNPs hydrogel. Through covalently cross-linking silicone elastomer on the surface of the hydrogel polymer, we bonded a thin Ecoflex film (100 μm) on the P(AAm-co-HEMA)-MXene-AgNPs hydrogel with robust interface, which can easily adhere to the Ecoflex/SiO2 microparticle composite film by silicone glue. The Ecoflex/SiO2 microparticle composite film endows the strain wearable sensor with superhydrophobic function that could maintain the stability under stretching or bending. Moreover, it can effectively resist the interference of water droplets and water flow. The P(AAm-co-HEMA)-MXene-AgNPs hydrogel exhibits outstanding antibacterial activity to inhibit Staphylococcus aureus, Escherichia coli, and even drug-resistant Escherichia coli. In addition, the flexible wearable sensor exhibited good self-adhesive performance by changing the reaction temperature of hydrogel and can adhere strongly onto various materials. The conductive composite hydrogel reported in this work contributes an innovative strategy for the preparation of multifunctional flexible wearable sensor.

A green MXene-based organohydrogel with tunable mechanics and freezing tolerance for wearable strain sensors

Conductive hydrogels integrate the conductive performance and soft nature, which is like that of human skin. Thus, they are more suitable for the preparation of wearable human-motion sensors. Nevertheless, the integration of outstanding multiple functionalities, such as stretchability, toughness, biocompatibility, self-healing, adhesion, strain sensitivity, and durability, by a simple way is still a huge challenge. Herein, we have developed a multifunctional chitosan/oxidized hyaluronic acid/hydroxypropyl methylcellulose/poly(acrylic acid)/tannic acid/Al3+ hydrogel (CS/OHA/HPMC/PAA/TA/Al3+) by using a two-step method with hydroxypropyl methylcellulose (HPMC), acrylic acid (AA), tannic acid (TA), chitosan (CS), oxidized hyaluronic acid (OHA), and aluminum chloride hexahydrate (AlCl3·6H2O). Due to the synergistic effect of dynamic imine bonds between CS and OHA, dynamic metal coordination bonds between Al3+ and -COOH and/or TA as well as reversible hydrogen, the hydrogel showed excellent tensile property (elongation at break of 3168%) and desirable toughness (0.79 MJ/m3). The mechanical self-healing efficiency can reach 95.5% at 30 min, and the conductivity can recover in 5.2 s at room temperature without stimulation. The favorable attribute of high transparency (98.5% transmittance) facilitates the transmission of the optical signal and enables visualization of the sensor. It also shows good adhesiveness to various materials and is easy to peel off without residue. The resistance of the hydrogel-based sensors shows good electrical conductivity (2.33 S m-1), good durability, high sensing sensitivity (GF value of 4.12 under 1600% strain), low detection limit (less than 1%), and short response/recovery time (0.54/0.31 s). It adhered to human skin and monitored human movements such as the bending movements of joints, swallowing, and speaking successfully. Therefore, the obtained multifunctional conductive hydrogel has great potential applications in wearable strain sensors.

Solvent-induced in-situ self-assembly lignin nanoparticles to reinforce conductive nanocomposite organogels as anti-freezing and anti-dehydration flexible strain sensors

Self-healing and adhesive MXene-polypyrrole/silk fibroin/polyvinyl alcohol conductive hydrogels as wearable sensor

Low temperature tolerant, ultrasensitive strain sensors based on self-healing hydrogel for self-monitor of human motion

Flexible wearable sensors are emerging as next-generation tools to collect information from the human body and surroundings in a smart, friendly, and real-time manner. A new class of such sensors with various functionality and amenability for the human body is essential for this goal. Unfortunately, the majority of the wearable sensors reported so far in the literature were of a single function (mostly strain sensors) and just a prototype without thinking of continuous mass production. In this paper, we report a series of multifunctional conductive hydrogel/ thermochromic elastomer hybrid fibers with core-shell segmental configuration and their application as flexible wearable strain and temperature sensors to monitor human motion and body/surrounding temperatures. Specifically, a conductive reduced-graphene-oxide-doped poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide (rGO-poly(AMPS-co-AAm)) hydrogel and a thermochromic elastomer containing silicon rubber and thermochromic microcapsules are chosen as strain-sensitive and thermosensitive materials, respectively. A core-shell segmental structure is realized by programming the extrusion of either conductive hydrogel precursor solution or a thermochromic elastomer prepolymer as a core layer via dual-core coaxial wet spinning. Depending on the assembly order and length of the conductive hydrogel and the thermochromic elastomer, the as-prepared hybrid fibers can be used for different purposes, i.e., human-motion monitoring, body or room temperature detection, and color decoration. The strategy described above, i.e., fabrication of core-shell segmental fibers via the wet-spinning method, is especially suitable for mass production in industry and can be further extended to fabricate flexible wearable devices with more components and more functions such as transistors, sensors, displays, and batteries.

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

Wearable flexible sensors based on integrated microfluidic networks with multiplex analysis capability are emerging as a new paradigm to assess human health status and show great potential in application fields such as clinical medicine and athletic monitoring. Well-designed microfluidic sensors can be attached to the skin surface to acquire various pieces of physiological information with high precision, such as sweat loss, information regarding metabolites, and electrolyte balance. Herein, the recent progress of wearable microfluidic sensors for applications in healthcare monitoring is summarized, including analysis principles and microfabrication methods. Finally, the challenges and opportunities for wearable microfluidic sensors in practical applications are discussed.