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

Abstract

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)-Fe 3+ and chitosan-SO 4 2− dual ionic networks via facile free radical polymerization and soaking processes is developed to fabricate excellent highperformance flexible sensors. The abundant Fe 3+ and SO 4 2− 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/m 3 ), 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.

Cellulose Nanofibrils Enhanced, Strong, Stretchable, Freezing‐Tolerant Ionic Conductive Organohydrogel for Multi‐Functional Sensors

“…b–g) Mechanical performance of PVA‐CNF organoydrogels. b) The tensile stress–strain curves of PVA‐CNF organohydrogels with varying CNF contents, c) Stress and Young's modulus values of PVA‐CNF organohydrogels with varying CNF contents, d) Strain and toughness values of PVA‐CNF organohydrogels with varying CNF contents, e) Ashby plots of stress and strain values of reported ionic conducting (organo)hydrogels, including 1‐ethyl‐3‐methylimidazolium dicyanamide/poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid) ([EMIm][DCA]/PAMPS) organohydrogel, [ 29 ] polyethylene glycol (PEG)/poly(acrylamide‐co‐acrylic acid) (PAMAA) hydrogel, [ 30 ] hydroxylpropyl cellulose/PVA hydrogel, [ 7b ] poly(styrene‐b‐ethylene oxide‐b‐styrene) (SOS) with 1‐ethyl‐3‐methylimidazolium bis(trifluoromethyl)sulfonyl amide ([EMI][TFSA]) organohydrogel, [ 31 ] PVA/poly[2‐(methacryloyloxy)ethyl]dimethyl‐(3‐sulfopropyl)]PSBMA hydrogel, [ 32 ] and PAM‐PEGDMA hydrogel, [ 5b ] f) Compressive stress–strain curves of PVA‐CNF organohydrogels with varying CNF contents. g) Cyclic compressive stress–strain curves of PVA‐1% CNF organohydrogel.…”

“…Characterization of ionic conduction of PVC‐CNF ionic conductive organohydrogels. EIS Nyquist plot of ionic conductive PVA‐1% CNF organohydrogel after soaking in a) 0.05 m and b) 1 m NaCl solution; c) ionic conductivity of PVA‐CNF organohydrogels with varying NaCl concentrations; d) Ashby plot of ionic conductivity and tensile stress with other reported ionic conductive (organo)hydrogels; [ 7c,11,13,19,21,29,38 ] e) Comparisons of luminance of LEDs (working voltage of 2.4 V) using PVA‐1% CNF as the conductor (top row) before and (bottom row) after (left column) stretching and (right column) pressing.…”

“…The Ashby plot of ionic conductivity and tensile stress of the ionic conductive (organo)hydrogel in Figure 3d showed that our PVA‐CNF organohydrogel is superior to the reported samples in term of balanced performance. Although high ionic conductivities of 6.2 and 5.5 S m −1 have been reported for poly (ionic liquid) (PIL) hydrogel [ 38a ] and polyacrylamide (PAM)‐alginate double network hydrogel, [ 7c ] respectively, their respective tensile strength are as low as 0.035 and 0.2 MPa. On the other hand, the strong gelatin organohydrogel (1.91 MPa) [ 18b ] and ionic liquid (PIL‐PEGDA) based ionogel (2.28 MPa) only presented unsatisfied low ionic conductivities of 0.47 and 0.83 S m −1 , respectively.…”

“…Hydrogel, consisting of a polymeric network structure with over 90% of water, can serve as an ideal ionic conductor when incorporating with electrolyte salts. [ 6 ] Due to the high mobility of electrolyte ions in water, the ionic conductive hydrogel (ICH) can show exceptional ionic conductivity of as high as 3.4–5.5 S m −1 , [ 7 ] which is in the same order as conductive polymer based hydrogels. [ 8 ] Despite the high ionic conductivity and transparency, there exist several intrinsic drawbacks that limit its wide application.…”

“…In addition, conventional ICHs use water as the ion conducting medium, which can freeze at sub‐zero temperatures and evaporate even under ambient condition, hindering their durability and practical application in real life. [ 7c,16 ] Inspired from nature where freezing tolerance in plants was introduced by the lipid species in cell membrane to suppress water freezing, [ 16,17 ] binary solvent systems, including ethylene glycol or glycerol/H 2 O, [ 11,13,18 ] benzyltrimethyl ammonium hydroxide/H 2 O, [ 19 ] betaine or proline/H 2 O, [ 20 ] has been introduced to fabricate freezing‐tolerant ICHs. Incorporating cryoprotectant into hydrogel can suppress ice crystal formation as well as increase the crosslink density, leading to a stronger organohydrogel.…”