Highly Elastic, Sensitive, Stretchable, and Skin-Inspired Conductive Sodium Alginate/Polyacrylamide/Gallium Composite Hydrogel with Toughness as a Flexible Strain Sensor

Abstract

As a classic flexible material, hydrogels show great potential in wearable electronic devices. The application of strain sensors prepared using them in human health monitoring and humanoid robotics is developing rapidly. However, it is still a challenge to fabricate a high-toughness, large-tensile-deformation, strain-sensitive. and human-skin-fit hydrogel with the integration of excellent mechanical properties and high electrical conductivity. In this study, a flexible sensor using a highly strain-sensitive skin-like hydrogel with acrylamide and sodium alginate was designed using liquid metallic gallium as a “reactive” conductive filler. The sensor had a low elastic modulus (30 kPa) similar to that of skin, a high-toughness (2.25 MJ m–3), self-stiffness, a large tensile deformation (1400%), recoverability, and excellent fatigue resistance. Moreover, the addition of gallium might enhance the electrical conductivity (1.9 S m–1) of the hydrogel while maintaining high transparency, and the flexible sensor device constructed from it showed high sensitivity to strain (gauge factor = 4.08) and pressure (gauge factor = 0.455 kPa–1). As a result, the hydrogel sensor could monitor various human motions, including large-scale joint bending and tiny facial expression, breathing, voice recognition, and handwriting. Furthermore, it might even be used for human–computer communication.

Construction of Multifunctional Hydrogels via a Supramolecular Self-Assembled Strategy with Ultrahigh Sensitivity to Strain Responsiveness

“…As shown in Figure C, the dissipated energy decreased from 8.724 kJ/m 3 (1st cycle) to 5.796 kJ/m 3 (2nd cycle) and then remained almost constant in the following cycles, demonstrating the good fatigue resistance ability of the PAA/PEI/Al-TA 4 hydrogel. This is in agreement with the reported work that the dissipated energy was through ionic coordinate bonding . Toughness was also determined to evaluate the mechanical properties of the hydrogels.…”

Facile Synthesis of Highly Stretchable, Tough, and Photodegradable Hydrogels

“…Despite some advantages including high transparency, a certain degree of elasticity, and good stability, these sensors presented high elastic modulus and low flexibility, which hinders the fitting for human motions and limits the detection range. [ 66–68 ]…”

“…Despite some advantages including high transparency, a certain degree of elasticity, and good stability, these sensors presented high elastic modulus and low flexibility, which hinders the fitting for human motions and limits the detection range. [66][67][68] In order to demonstrate possible applications of the developed photodegradable DN hydrogels, in this work we used the H 600 hydrogel as a body-worn sensor for monitoring human health parameters and activity. This is possible due to excellent mechanical properties, that is, high stretchability, and low elastic modulus in the kPa range.…”

Multiple Physical Bonds Cross-Linked Strong and Tough Hydrogel with Antibacterial Ability for Wearable Strain Sensor

“…Alginate, a natural anionic copolymer derived generally from seaweed, is frequently used to fabricate biobased hydrogels, because of its good biocompatibility, controllable biodegradability, and environmental sustainability. ,− The main chain of alginate is configured by (1–4)-linked β-mannuronic acid (M monomer) and α-guluronic acid (G monomer), where the M monomer and G monomer are arranged repeatedly (MM or GG) or alternately (MG). The chelation effect is readily formed between the carboxyl on the G monomer and the divalent (Ca 2+ , Ba 2+ , Cu 2+ ) or trivalent cations (Fe 3+ , Al 3+ ). , Meanwhile, due to the existence of abundant functional groups like −COOH and −OH, alginate hydrogels normally exhibit unique functionality such as pH sensitivity, photochromic ability, and shape memory effect .…”

“…Based on the Lake-Thomas model, the mechanical properties of hydrogels correlate intimately with the number of chains per unit volume, the density of cross-linking points, and the homogeneity of chain length between cross-linking points of the 3D cross-linked scaffold. − ,, To enhance the mechanical properties of hydrogels, various strategies were developed based on either optimizing the individual alginate network or combining the multicomponent system with the alginate network by constructing a double network, − ,, supramolecular interactions, − ionic coordination, and dynamic covalent cross-linking. − For instance, Kim et al built a densely interconnected network in alginate hydrogels employing a reconstructing process including anisotropic densification of pregel and a subsequent ionic cross-linking with rehydration. Depending on the cross-linking ions during rehydration, the alginate hydrogel exhibited a wide range of tensile strength (8–57 MPa) and elastic moduli (94–1290 MPa) .…”