High strength, anti‐freezing, and conductive poly(vinyl alcohol)/urea ionic hydrogels as soft sensor

Abstract

Hydrogel‐based sensors with excellent flexibility and stretchability have received extensive attention. However, fabricating anti‐freezing, high‐strength, and conductive hydrogel‐based sensors remains a significant challenge. A stretchable sensor with high strength and anti‐freezing herein is constructed using poly(vinyl alcohol)/urea (PVA/urea) ionic hydrogels. The PVA/urea ionic hydrogels were prepared by a simple strategy of soaking the freeze–thawed PVA hydrogels in sodium citrate (Na3Cit) water/urea aqueous solutions. The presence of Na3Cit in the hydrogel produces hydrophobic aggregation, endowing the PVA/urea ionic hydrogel with high mechanical properties (~1.51 MPa) and makes the hydrogel have high strain gauge factors (GF = 1.94). The synergistic effect of Na3Cit and urea endows the PVA/urea ionic hydrogel with anti‐freezing properties (~−20°C). The obtained sensors have excellent stability (100 cycles at ε = 85%) and could detect various human activities even at −20°C. More importantly, joint motion can be monitored wirelessly via Bluetooth at −20°C. This work provides a feasible method to construct anti‐freezing, high‐strength, and conductive hydrogel. It paves the way for versatile applications in human‐motion detection, intelligence device, and biomimetic skin.

Unimpaired highly extensible tough chemically crosslinked hydrogel after experiencing freeze/thaw and boiling processes

“…[11] Due to the potential detrimental effect of freezing water on the physical and mechanical behaviors of tough hydrogels, attempts have been made to make antifreeze physically crosslinked hydrogels by taking advantage of supramolecular interactions [12] or reducing the freezing point of a hydrogel by adding glycerol and/or NaCl to the hydrogel. [13][14][15][16][17] Other studies in this area include fabrication of ionic anti-freezing stretchable hydrogel based on poly(vinyl alcohol)/urea, [18] stretchable and anti-freezing hybrid DN organohydrogels, [19] biomimetic organohydrogel electrolytes, [20] and design of super-elastic mineral hydrogel based on dynamic physical crosslinking between hydrated calcium ion clusters and amide groups of polyacrylamide network. [21] It should be noted that some of the strategies that were mentioned above to improve the behavior of hydrogels for experiencing high temperature variations, such as changing the freezing point of hydrogels, may affect their other functionalities.…”

A novel PVA/MWCNTs hydrogel with high toughness and electromagnetic shielding properties

“…The morphology of the PVA/MWCNTs hydrogel was observed by a scanning electron microscopy (SEM, Quanta FEG 250, American FEI Company) at an accelerating voltage of 20 kV (the samples were freeze‐dried before testing). Apparent density ( ρ ) and water content ( Φ ) can be obtained by simple measurement and formula (1–2) 37,38 : where Ws was the weight of the original hydrogel sample, and Wd was the weight of the dried hydrogel sample after 24 h of drying at 70°C in a vacuum drying chamber. where W is the weight of the hydrogel, D is the diameter, and H is the thickness of the hydrogel.…”

“…The morphology of the PVA/MWCNTs hydrogel was observed by a scanning electron microscopy (SEM, Quanta FEG 250, American FEI Company) at an accelerating voltage of 20 kV (the samples were freeze-dried before testing). Apparent density (ρ) and water content (Φ) can be obtained by simple measurement and formula (1-2) 37,38 :…”

Preparation of hydrogen‐bonded supramolecular dual‐network hydrogels with tunable mechanical properties

“…1 Hydrogels with excellent mechanical properties also have many promising practical applications such as biosensors, wound dressings, tissue engineering, drugcontrolled release, supercapacitors, and other fields. [2][3][4][5][6] In biological soft tissues such as cartilage, in the gel state, the water content can be as high as 75%, with excellent mechanical properties, which inspired many researchers to create hydrogels with similar properties. However, traditional hydrogels have poor mechanical properties and weak energy dissipation during deformation, which greatly limits their applications.…”

“…Hydrogels are three‐dimensional polymer networks containing large amounts of water, and most of them have high water content, biocompatibility, and functionality 1 . Hydrogels with excellent mechanical properties also have many promising practical applications such as biosensors, wound dressings, tissue engineering, drug‐controlled release, supercapacitors, and other fields 2–6 . In biological soft tissues such as cartilage, in the gel state, the water content can be as high as 75%, with excellent mechanical properties, which inspired many researchers to create hydrogels with similar properties.…”