Skin-Inspired Patterned Hydrogel with Strain-Stiffening Capability for Strain Sensors

Abstract

Flexible materials with ionic conductivity and stretchability are indispensable in emerging fields of flexible electronic devices as sensing and protecting layers. However, designing robust sensing materials with skin-like compliance remains challenging because of the contradiction between softness and strength. Herein, inspired by the modulus-contrast hierarchical structure of biological skin, we fabricated a biomimetic hydrogel with strain-stiffening capability by embedding the stiff array of poly(acrylic acid) (PAAc) in the soft polyacrylamide (PAAm) hydrogel. The stress distribution in both stiff and soft domains can be regulated by changing the arrangement of patterns, thus improving the mechanical properties of the patterned hydrogel. As expected, the resulting patterned hydrogel showed its nonlinear mechanical properties, which afforded a high strength of 1.20 MPa while maintaining a low initial Young's modulus of 31.0 kPa. Moreover, the array of PAAc enables the patterned hydrogel to possess protonic conductivity in the absence of additional ionic salts, thus endowing the patterned hydrogel with the ability to serve as a strain sensor for monitoring human motion.

Breathable and Durable Tactile e-Skin Based on S-Shaped Nanofiber Networks by Patterned Additive Manufacturing

“…According to the working conditions, they can be divided into two types of application areas. The first is the field where the tactile e-skin is attached to the real human skin, − and the second is the field where the tactile e-skin is attached to the surface of bionic human skin. − In fact, the tactile e-skin used in both fields has a high demand for breathability. Currently, there are remarkable application cases of tactile e-skin attached to real skin, − which have different sensing principles, have flexible structures, and are made of functional materials.…”

Strain-Stiffening Mechanoresponse in Dynamic-Covalent Cellulose Hydrogels

“…Strain sensors have been designed using strain-stiffening poly(acrylic acid)/poly(acrylamide) hydrogels, which display a deviation from linear viscoelasticity when their stiffness increases upon the application of high strain. 24,25 Strain-stiffening oligo-(ethylene)glycol polyisocyano-peptide hydrogels have also been used as synthetic extracellular matrices; the extent of their strain-stiffening in response to deformations by encapsulated mesenchymal stem cells impacted the differ- entiation of these cells toward osteogenesis or adipogenesis. 3 Additionally, strain-stiffening hydrogels have been engineered to exhibit muscle-like fatigue resistance.…”

“…For example, hydrogels exhibiting shear-thinning and self-healing have been engineered and are widely utilized as injectable vehicles for drug delivery and 3D printing. , Shear-thinning and self-healing behaviors are typically a result of dynamic-covalent bonds or noncovalent interactions, such as hydrogen bonding, metal–ligand coordination, host–guest interactions, and hydrophobic interactions. − These relatively weak interactions are disrupted at high shear rates or high strains but reform upon cessation of the mechanical stimulus, allowing for recovery of the initial hydrogel mechanical properties. Strain sensors have been designed using strain-stiffening poly­(acrylic acid)/poly­(acrylamide) hydrogels, which display a deviation from linear viscoelasticity when their stiffness increases upon the application of high strain. , Strain-stiffening oligo­(ethylene)­glycol polyisocyano-peptide hydrogels have also been used as synthetic extracellular matrices; the extent of their strain-stiffening in response to deformations by encapsulated mesenchymal stem cells impacted the differentiation of these cells toward osteogenesis or adipogenesis . Additionally, strain-stiffening hydrogels have been engineered to exhibit muscle-like fatigue resistance. − …”