Transparent Stretchable Dual-Network Ionogel with Temperature Tolerance for High-Performance Flexible Strain Sensors

Abstract

A stretchable transparent double network ionogel composed of physically cross-linked poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDFco-HFP)) and chemically cross-linked poly(methyl methacrylate-co-butylmethacrylate) (P(MMA-co-BMA)) elastomer networks within [EMIM][TFSI] ionic liquid was fabricated through a facile one-pot thermal polymerization. The dualnetwork (DN) ionogel presents good mechanical performance (failure tensile stress 2.31 MPa, strain 307%) with a high loading of ionic liquid (70 wt %) for achieving required ionic conductivity (>0.1 S/m at room temperature). The transparent chemical cross-linked P(MMA-co-BMA) elastomer network endows high transparency (>93%) and high stretchability to the DN ionogel. The DN ionogel maintains good toughness, elasticity, and transparency in a wide temperature range (−40 to 80 °C) for the application in a harsh environment. In addition, the sensitivity of the DN ionogel to the change of environment temperature and deformation was detected and described. The practical potential of the DN ionogel in flexible electronic devices is further revealed by fabricating DN ionogel strain sensors to detect the movement of different human limbs including the bending of the finger, wrist, and elbow as well as the slight throat jitter during the swallowing and vocalization, showing fast response, high sensitivity, and good repeatability.

Materials, Electrical Performance, Mechanisms, Applications, and Manufacturing Approaches for Flexible Strain Sensors

“…The synchronized delivery and control of the signal from human body to detector or actuator are convenient, expeditious, effective, and accurate compared with traditional rigid conducting and semiconducting materials based on smart devices [ 19 , 20 ]. Besides, the excellent stretchability [ 21 , 22 ], transparency [ 23 , 24 , 25 ], wearability [ 7 , 26 , 27 ], and biocompatibility [ 28 , 29 , 30 ] endows flexible and wearable electronics with alluring prospect, which the functions infinitely tend to real human skins or beyond [ 31 , 32 , 33 ].…”

Recent Progress in Essential Functions of Soft Electronic Skin

“…Compared to hydrogel-based strain sensor, ionogel-based strain sensor normally exhibits better anti-freezing and antidrying properties because of the nonvolatility and thermal stability of ILs. For instance, Lan et al [106] constructed the double network ionogels composed of cross-linked poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) and poly(methyl methacrylate-co-butylmethacrylate) (P(MMA-co-BMA)) networks combined with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]) IL, which presented good mechanical properties and flexibility in a wide temperature range from −40 to 80 °C. Sun et al [107] proposed a strategy of one-pot in situ photopolymerization to fabricate the PAA/1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) ionogels.…”

Ionogel-based flexible stress and strain sensors

“…With the advent of the Internet of Things, considering more concern on human health in daily life, real-time health monitoring is becoming a great demand. IFSS can meet such requirements by virtue of their sensitive detection of breathing [55], swallowing [47], voice [45], human pulse [52,59], and eye movements [45,64](Figure 10). Compared with traditional sensors, IFSS have high sensitivity and fit the human skin better, and much more convenient to use than traditional non-flexible sensing device.…”

“…Besides, compared with complicated circuit design and components integration of traditional sensors, manufacturing process of IFSS is relatively simple. [55], (b) swallowing [47], (c) speaking [45], (d) pulse beating [52] and (e) eye movements [64], for monitoring human health situation.…”

“…Ionogel film [40,47,48,54,56,57,60,61] Sandwiched with flexible shell and copper foil [45] Tensile strain and temperature Stretchable, mechanical robust and thermosensitive…”