Micelle–Micelle Cross-Linked Highly Stretchable Conductive Hydrogels for Potential Applications of Strain and Electronic Skin Sensors

Abstract

Wearable sensors made of flexible and stretchable hydrogels have garnered significant attention. However, their use has been limited by poor mechanical performance, such as poor toughness, poor self-recovery, and a large response–recovery time. To overcome these limitations, we have developed a novel cross-linking agent-based hydrogel with high stretchability, high toughness, antifatigue properties, and good conductivity. These hydrogels were developed by introducing l-glutamic acid (LGA) into hydrophobically cross-linked polyacrylamide (PAmm) chains. In this system, LGA dynamically cross-linked the micelle–micelle and micelle–polymer chains and greatly regulates the mechanical properties of the hydrogels. The noncovalent synergistic interactions that came with the insertion of LGA enable the hydrogels to achieve high stretchability and high stress values, with fast self-recovery and antifatigue behaviors without the help of foreign stimuli. Additionally, LGA-based hydrogels can function as durable and highly sensitive strain sensors for detecting various mechanical deformations with a fast response–recovery time and high gauge factor value. As a result, the hydrogels have the capability to be designed as wearable strain sensors that are capable of detecting large human joint motions, such as neck twisting, neck bending, and wrist, finger, and elbow movements. Similarly, these hydrogels are capable of monitoring different subtle human motions such as speaking and differentiating between different words, swallowing, and drinking through larynx vibrations. Besides these large and subtle human motions, hydrogels have the ability to differentiate and reproduce different written words with reliability. These LGA-regulated hydrogels have potential applications in electric skins, medical monitoring, soft robotics, and flexible touch panels.

High-Saline-Enabled Hydrophobic Homogeneous Cross-Linking for Extremely Soft, Tough, and Stretchable Conductive Hydrogels as High-Sensitive Strain Sensors

“…The response and recovery times of PAMD-NaCl hydrogel sensors are about 104 and 116 ms, respectively, which was sufficiently accurate for sensing various strains (Figure d). The response time of PAMD-NaCl hydrogel sensors was shorter than those of previously reported double network hydrogel sensors containing hydrophobic associations, − which should be ascribed to the soft feature and high ion conductivity of PAMD-NaCl hydrogels. As shown in Figure e–g, the resistances of PAMD-NaCl hydrogels changed obviously during multiple deformations such as being stretched, compressed, thrusted, bent, and twisted.…”

Multifunctional and self-healable conductive IPN hydrogels functionalized with reduced magnetite graphene oxide for an advanced flexible all in one solid-state supercapacitor

“…Recent developments and progress in the field of hydrogels for their use in versatile applications in daily life make it more promising in general, and particularly in replacing traditional rigid wearable electronic devices. 1 Hydrogels are used in many electrical devices such as smartphones, 2 watches, 3 sensors, 4–6 and in flexible and healable electronic devices. 7 Most of the existing traditional electronic energy storage devices have problems with mechanical wear and damage, and thus, wearing the electronic devices during their routine operation and application is not possible.…”

Micelle-Cross-Linked Hydrogels with Strain Stiffening Properties Regulated by Intramicellar Cross-Linking

“…Furthermore, the mechanical performances of these micelle-cross-linked hydrogels correlated closely to the relaxation dynamics of the micellar cross-linkers. 25,49 Micelles with a high glass transition temperature (T g ) possess a slow relaxation process so that the corresponding micelle-crosslinked hydrogels showed high mechanical strength and fatigue resistance. On the contrary, micelles with a low T g relax rapidly and are easy to be broken by external forces.…”