Ultrastiff, Tough, and Healable Ionic-Hydrogen Bond Cross-Linked Hydrogels and Their Uses as Building Blocks To Construct Complex Hydrogel Structures

We report the ultrastiff and tough poly(acrylamide- co-acrylic acid)/Na-alginate/Fe3+ (P(AM- co-AA)/Na-alginate/Fe3+) hydrogel via the formation of hybrid ionic-hydrogen bond cross-linking networks. The optimal P(AM- co-AA)/Na-alginate/Fe3+ hydrogel possessed super high elastic modulus (∼24.6 MPa), tensile strength (∼10.4 MPa), compression strength (∼44 MPa), and toughness (∼4800 J/m2). The P(AM- co-AA)/Na-alginate/Fe3+ hydrogel was highly stable and maintained its superior mechanical properties in 0.5-2 M NaCl solution, aqueous solution with pH ranging from 4 to 10. The ionic cross-linking networks of the P(AM- co-AA)/Na-alginate/Fe3+ hydrogels can be locally and selectively dissociated by treating with aqueous NaOH solution with pH of 13 for 1 min and reformed by locally adding the additional Fe3+ solutions, making the hydrogels healable and cohesive. The healed hydrogels from the cutting surfaces can bear a tensile strength of up to 7.1 MPa. Various complex hydrogel structures can be constructed by using the P(AM- co-AA)/Na-alginate/Fe3+ hydrogels as building blocks via the adhesion of as-prepared hydrogels.

Tough and Stretchable Dual Ionically Cross-Linked Hydrogel with High Conductivity and Fast Recovery Property for High-Performance Flexible Sensors

As a kind of typical soft and wet material, hydrogel has been increasingly investigated as another way to develop flexible electronics. However, the traditional hydrogel with poor strain and strength performance cannot meet the requirements for stretchable electronics; fabricating a stretchable hydrogel with balanced tensile strength, toughness, and conductivity is still a big challenge. Herein, a new type of physically cross-linked hydrogel with poly(acrylamide-co-acrylic acid)-Fe3+ and chitosan-SO42- dual ionic networks via facile free radical polymerization and soaking processes is developed to fabricate excellent high-performance flexible sensors. The abundant Fe3+ and SO42- ions in the hydrogel can not only construct tough and strong dual ionic networks but also give the hydrogel high conductivity. Consequently, the optimal hydrogel possesses high tensile strength (∼5.1 MPa), large strain capacity (∼1225%), elasticity (∼1.13 MPa), high toughness (∼32.1 MJ/m3), and high conductivity (3.04 S/m at f = 0.1M), as well as rapid self-recovery property. Furthermore, the hydrogel conductor has high stretching sensitivity with a gauge factor of 6.0 at strain of 700% and was able to detect conventional motions of the human body such as the motions of the knuckle, speaking, and swallowing, which indicates that our ionic conductive hydrogels can be used to fabricate excellent high-performance flexible sensors.

Preparation of stretchable and self-healable dual ionically cross-linked hydrogel based on chitosan/polyacrylic acid with anti-freezing property for multi-model flexible sensing and detection

As a kind of promising material for flexible wearable electronics, conductive hydrogels have attracted extensive interests of researchers for their inherent merits such as superior mechanical properties, biocompatibility, and permeability. Herein, we constructed a new type of highly stretchable, anti-freezing, self-healable, and conductive hydrogel based on chitosan/polyacrylic acid. The large amount of ions inside the network had five functions for the proposed hydrogel, including excellent mechanical behaviors, high conductivity, self-recovery, self-healing and anti-freezing capability. Consequently, the proposed hydrogel possessed tunable stretchability (1190-1550%), tensile strength (0.96-2.56 MPa), toughness (5.7-14.7 MJ/m3), superior self-healing property (self-healing efficiency up to 83.7%), high conductivity (4.58-5.76 S/m), and excellent anti-freezing capability. To our knowledge, the self-healable hydrogel with balanced tensile strength, toughness, conductivity, and low-temperature tolerance can hardly be achieved till now. Furthermore, the conductive hydrogels exhibited high sensitivity (gauge factor up to 10.8) in a broad strain window (0-1000%) and could detect the conventional motion signals of human body such as bending of a knuckle, swallowing, and pressure signal at both room temperature and -20 °C. Moreover, the hydrogels could also be fabricated as flexible detectors to identify different temperatures, different kinds of solutions, and different concentrations of the solution.

Stretchable and Self‐Healable Organohydrogel as Electronic Skin with Low‐Temperature Tolerance and Multiple Stimuli Responsiveness

The design and fabrication of electronic skin that mimics the human somatosensory system has attracted a great deal of attention. However, existing materials could hardly achieve the unique characteristics that natural skin possesses, including excellent mechanical flexibility, self‐healing ability, the sensory ability of tension, pressure, temperature, humidity, and the ability to secrete sweat through various sensory receptors and nervous pathways. In this paper, a new type of stretchable (over 2150%), tough (over 4 MJ m−3), conductive (up to 1.68 S m−1), and self‐healable (self‐healing efficiency up to 100%) organohydrogel with low‐temperature tolerance (stretchable below −50 °C) and multiple stimuli responsiveness is prepared by a simple “one‐pot” strategy at room temperature. The synthesized electronic skin with this organohydrogel can detect slight changes in tension, pressure, temperature, and humidity, and even distinguish between saline and alkaline solutions, showing high sensitivity in the broad strain window. This organohydrogel also has application prospects in humanoid robotics, flexible energy storage technologies, and health‐monitoring devices.