Ionogel-based flexible stress and strain sensors

Figures & data

Table 1. The ions of common used ILs in IFSS

Cation	Anion	Poly ionic liquid
1-ethyl-3-methyl imidazolium (EMIM)	Dicyanamide (DCA)	Poly(1-butyl-3-vinyl imidazolium fluoborate)
1-vinyl-3-ethyl imidazolium (VEIM)	Tricyanomethanide (TCM)
1-Butyl-3-methyl imidazolium (BMIM)	Bis(trifluoromethylsulfonyl)imide (TFSI/NTf2)
1,2-dimethyl-3-ethoxyethyl-imidazolium (DEIM)	Ethylsulfate (EtSO4)
1-ethy-l-3-methyl imidazolium (C2mm)	Trifluoromethanesulfonate (OTF)
1-(2-hydroxyethyl)-3-methylimidazolium (HoEMIM)	BF4^−, Cl^−

Figure 1. Free radical polymerization of ionogel. (a) The one-pot photopolymerization of ionogel based on BA, HDDA and [BMIM]TFSI through 1173 photoinitiator [40]. (b) The photopolymerization of ionogel based on AA, PEGDA and [C₂mim][EtSO₄] [42]. (c) The thermal polymerization of NNMBA [44]. (d) The one-pot thermal polymerization of DN ionogel [47]. (e) Integrating metal coordination bonds in a loosely cross-linked network of ionogel during thermal polymerization [48]

Table 2. Synthesis and materials of ionogel for flexible stress and strain sensors

ILs	Monomer, cross-linker or precursor	Synthesis	Initiator /catalyst	Features	REF.
[BMIM]BF4	Vinyl [BMIM]BF4; acrylate-terminated hyperbranched polymer	Photopolymerization	1-hydroxycyclohexyl phenyl ketone	Wide temperature window (−60 ~ 250 °C)	[49]
	Pluronic F127 ended with isocyanatoethyl methacrylate		2-hydroxy-2-methylpropiophenone (HOMPP)	Shear-thinning and yielding characteristics	[50]
	N,N-Dimethylacrylamide (DMAA) and N,N-methylenebisacrylamide (NNMBA)		2,2-diethoxyacetophenone	-	[51]
	Tetraethyl orthosilioate (TEOS); DMAA; NNMBA		2-oxoglutaric acid (2-OA)	Conductive ink	[52]
	Graphene oxide nanoplatelets	π–π interactions	Hydrazine	Monolithic structure	[53]
	Poly(amic acid) (PAA), 4,40-diaminodiphenyl ether (ODA),	Chemical imidization; solvent displacement and hydrogenbond interaction	Pyridine and acetic anhydride (imidizationagents)	Wide thermal range (−60 to 180°C)	[54]
[EMIM][DCA]	AA; Dually acrylated Pluronic F127 (F127DA)	Photopolymerization and solvent exchange	α-ketoglutaric acid	Fatigue-resistant for device	[55]
	3-dimethyl (methacryloyloxyethyl) ammonium propane sulfonate (DMAPS); AA	Photopolymerization	Ammonium persulfate (APS)	Wide temperature window (−20 ~ 100°C)	[43]
	Poly(vinyl alcohol) (PVA)−poly(vinylpyrrolidone) (PVP)	Hydrogen bonds	-	Self-healable	[56]
	AMPS and NNMBA	Photopolymerization and solvent exchange; electrostatic interactions	α-ketoglutaric acid	High Conductivity	[57,58]
[VEIM][DCA]	NNMBA	Thermal polymerization	APS	Wide detection range	[44]
	SiO2 solution			Microporous structure; Self-healable	[45,46]
[BMIM]TFSI	Butyl acrylate (BA); 1,6-Bis(acryloyloxy)hexane (HDDA)	Photopolymerization	HOMPP	Self-adhesion	[40]
	Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP); Methyl methacrylate (MMA), butylmethacrylate (BMA); poly(ethylene glycol) dimethacrylate (PEGDMA)	Thermal polymerization	2,2ʹ -azobis(2-methyl propionitrile) (AIBN)	Dual-Network; Temperature tolerance (−40 to 80°C)	[47]
[EMIM]Cl	Acrylic acid(AA); poly(ethylene glycol) diacrylate (PEDGA)	Photopolymerization	HOMPP	Temperature detection function	[41]
	Hydroxyethyl acrylate (HEA) and agarose; PEDGA				[59]
[C2mim][EtSO4]	AA; PEDGA	Photopolymerization	α-ketoglutaric acid	Nonvolatile, compliant and ionic conductor	[42]
	Fe3O4 nanoparticles coated with poly (acrylic acid); AA; PEDGA	Thermal polymerization; coordination bonds, hydrogen bonds and the ionic bonds	APS	High stretchability and Self-healable	[48]
[DEIM][TFSI]	Poly(ε-caprolactone) (PCL) diol, poly(ethylene glycol) (PEG), N,N’-di-tert-butylethane-1,2-diamine, and 1,6-diisocyanatohexane	Condensation reaction; hydrogen bonds and hindered urea bonds	Hexamethylene diisocyanate	Long-term durability and self-healable	[60]
[C2mim][NTf2]	Ethyl acrylate (EA), ethylene glycol dimethacrylate (EGDMA)	Photopolymerization; hydrogen bonding	Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (PBPO)	Dual-Network; mechanically robust, high stability and ultrahigh stretchability; temperature tolerance (−70 to 100°C)	[61]
HoEMIMBF4	Polyvinylpyrolidone (PVP), (hydroxyethyl)methacrylate (HEMA) and acrylonitrile (AN), thylene glycol dimethacrylate (EGDMA) 3,4-dihydroxyhydrocinnamic (HCA) coated BaTiO3;	Photopolymerization	Dimethoxy-phenylacetophenone, DMPA	Soft piezoionic/piezoelectric nanocomposites	[62]
Poly([BMIM]BF4)	Pentaerythritol tetraacrylate (PETA); PEDGA; benzene tetracarboxylic acid (BTCA); 1,2-ethanedithiol (ED)	Thiol-ene click chemistry and solvent exchange	Triethylamine (TEA)	Wide temperature window (−75 ~ 340 °C), fatigue-resistant	[63]
[EMIM][TFSI]	P(VDF-HFP)	Electrostatic interaction	-	Introionic, pressure sensing	[64,65,66,67]
	Thermoplastic Polyurethane (TPU)				[68,69,70]
[EMIM][TCM]	PEGDA	Photo polymerization	HOMPP		[71]
	PEDGA, cellulose paper			Introionic, paper-based	[72]
[EMIM][OTF]	PVA, cellulose pulp	Electrostatic interaction	-	Introionic, paper-based	[73]
	P(HEMA), cellulose fiber	Photo polymerization	HOMPP		[74]

Figure 2. Other synthesis methods of ionogel. (a) Locking ILs in DN network through electrostatic interactions [57]. (b) Forming polyimide ionogels by solution displacement attributed to hydrogen bonding [54]. (c) Locking ILs into the elastomer network through hydrogen bonding [61]. (d) The thiol-ene click reaction for preparing DN click ionogel [63]

Figure 3. Solution casting and 3D printing molding process of IFSS. (a) Solution casting during synthesizing ionogel [41]. (b) 3D printing manufactured complex architectures [49]. (c) 3D printed hourglass auxetic structure [50]. (d) Volume production of IFSS by 3D printing [53]

Table 3. The principle, forming process of ionogel and device structures of IFSS

Principle	Sensing function	Required characteristics	Forming of ionogel	Device structure	REF.
Resistive	Tensile strain	Stretchable and mechanical robust	3D printing	Ionogel film or tube	[49]
				Ionogel crosslinked network	[50]
				Integrally-formed and encapsulated with elastomer	[53]
			Solution cast	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		Ionogel film	[41,59]
	Pressure	Pressure sensitive			[44,46]
Resistive and capacitive	Tensile strain and pressure	Stretchable, mechanical robust and pressure sensitive		Ionogel film or ionogel electrodes sandwiched with separator	[55]
Capacitive	Pressure	Pressure sensitive		Sandwiched with flexible electrodes and sensor array	[51]
Triboelectric and resistive	Pressure	Pressure sensitive	Writing	All-paper based and sandwiched with separator or dielectric layer	[52]
Triboelectric	Pressure	Self-powering and pressure sensitive	Solution cast	Sandwiched with dielectric layer	[63]
	Bending strain			Sandwiched with flexible separator or dielectric layer	[43]
	Pressure			Sandwiched with patterned flexible dielectric layer	[58]
Piezoelectric	Directional discriminative pressure sensing	Piezoelectric		Depositing metal electrodes on opposite sides of ionogel bulk	[62]
Interfacial iontronic	Pressure	High unit area capacitance (UAC) and signal-to-noise ratio (SNR)	Spin coating	Patterned ionogel film sandwiched with flexible electrodes and shell	[64]
				Irregular bump structured conductive elastomer sandwiched with ionogel film and PDMS shell	[66]
				Micropatterned ionogel film sandwiched with ITO/PET films	[75]
				Leaf surface patterned ionogel from sandwiched conductive elastomer	[65]
	Tactile controller			Sandwiched electric skin and integrated with circuit; sensor array	[69]
	Pressure		Solution cast	Sandwiched membrane with chamber and sensor array	[71]
	Tactile and pressure		Printing or writhing	All-paper based sensor array	[73]
	Pressure			Paper-based and packaged with complex shaped shell	[72]
				All-paper based, sandwiched with printing electrodes and has different origami structures	[74]
			Electro-spinning	All-fabrics based sensor sandwiched with conductive fabrics	[67]
				Sandwiched with Al foil and insulator shell	[76]

Figure 4. IFSS with sandwich-like structures. (a) IFSS with EcoFlex shell [45]. (b) Ionogel as the electrode of capacitive type sensor [55]. (c) Single-electrode mode TENG IFSS prepared by stacking ionogel and PDMS dielectric layer [43]. (d) Contact-separation mode TENG IFSS working by contacting ionogel with patterned PDMS dielectric layer [58]. (e) Paper-based IFSS prepared by direct-writing ionogel precursor solution on paper [52]

Figure 5. Working principles of IFSS. (a) Piezoresistive-type IFSS [44]. (b) IFSS based on piezoionic and piezoelectric mechanism [62]. (c) The interfacial iontronic pressure sensor based EDL mechanism [65]. (d) Single-electrode TENG principle [63]. (e) Contact-separation TENG principle [58]

Table 4. The major characteristic parameters of ionogel based tensile strain sensors

Composition	MAX. Strain (%)	σ (mS/cm)	GF (at ε)	Response time (ms)	DH	Durability (Cycles)	Temperature tolerance(°C)	REF.
[BMIM]BF4 PIL	>1000	5.8	1.98–2.77	200	-	2000	−60 ~ 250	[49]
[BMIM]BF4 Modified pluronic F127	310	-	-	-	-	-	-	[50]
[EMIM]Cl PAA	2200	0.49	1.6 (200%)	100 (50%)	0.54% (200%)	-	−30 ~ 70	[41]
[BMIM]TFSI Poly(butylacrylate)	700	0.75 (RT)	1.7 (100%), 3.5 (600%)	180 (100%)	2.56% (600%)	-	−30 ~ 100	[40]
[EMIM][DCA] PAA	>850	-	1.29 (0 − 400%) 2.08 (400–750%)	200	-	>1400	-	[55]
[DEIM][TFSI] PU	327	1.2	1.23 (0–50%) 1.54 (50–300%)	-	-	10,000	-	[60]
[BMIM]BF4 GO	350	2.77 ~ 2.92	0.54–2.41	-	<3.5% (300%)	5000	-	[53]
[EMIm][DCA] PVA-PVP	821	19.7	1.01 (0–200%) 1.85 (>200%)	-	-	-	−56.8 ~ 246.1	[56]
[EMIM][DCA] PAMPS DN	158	19	-	-	-	-	−70 ~ 100	[57]
[EMIM]Cl Agarose/PHEA DN	700	0.12 (25°C)	1.1(100%)	80	0.75% (100%)	>500	−30 ~ 60	[59]
[C2mim] EtSO4 Fe3O4@PAA/PAA	2000	1	20 (800 ~ 1400%)	-	-	>500	−20 ~ 60	[48]
[Bmim][BF4] PI	320	1.9–5.2	0.845	-	-	>100	−60 ~ 180	[54]
[BMIM]TFSI P(VDF-HFP) P(MMA-co-BMA) DN	307	>1	1.62	240	-	-	−40 ~ 80	[47]
[C2mim][NTf2] PEA DN	5000	1.2 (30°C)	1.24(1%), 1.83(100%)	-	-	>10,000 (100%)	−70 ~ 100	[61]
[VEIm][DCA] SiO2 solution	3000	-	1 ~ 20	183/319	-	500	-	[45]

Table 5. The major characteristic parameters of ionogel based pressure sensors

Composition	Structure Characteristics	Sensitivity	Response time (ms)	Operating voltage	Working range	REF.
[BMIM]BF4 PDMAAm	Parallel capacitor	0.094 nF/Pa	-	-	48.2 ~ 457 Pa	[51]
[VEIm][DCA] PIL	Piezoresistive	15.4 kPa^−1	72/753	10 V	5 Pa~5kPa	[44]
[VEIm][DCA] SiO2 gel	Piezoresistive microstructured ionogels	69.61 kPa^−1	100	-	20 Pa ~ 4.8 kPa	[46]
[Bmim][BF4] SiO2/PDMAAm DN	All-paper based Two modes	0.304 N^−1; 20.6 mV/N; 0.182 nA/N	-	Self-powering at TENG mode	0.3 ~ 0.9 N; 0.45 ~ 6.5 N	[52]
[EMIM][DCA] PAMPS DN	Transparent TENG	1.76 V/N at 50% ε	514	Self-powering	0.1 ~ 1 N	[58]
[EMIM][TCM] PEGDA	Micro sensing chamber	3.1 nF /kPa	<1	-	1 ~ 750 kPa	[71]
[EMIM][TFSI] P(VDF-HFP)	Sandpaper molded ionogel	131.5 kPa^−1	43	-	1.12 Pa ~ 2 kPa	[64]
[EMIM][TFSI] P(VDF-HFP)	Sandpaper molded ionogel	9.55 kPa^−1	<52	0.1 V	0 ~ 8kPa	[66]
[EMIM][TFSA] P(VDF-HFP)	Micropatterned Pyramidal Ionogel	41 kPa^−1	<20	0.25 V	1 Pa ~ 50 kPa	[75]
[EMIM][TFSI] P(VDF-HFP)	Leaf molded ionogel	54.31 kPa^−1	29	-	0.1 Pa ~ 115 kPa	[65]
[EMIM][TFSI] TPU	Pyramid-Plug Structure	1.93 kPa^−1	47		10.2 Pa ~	[68]
[EMIM][TFSI] TPU, silica	Biomimic ion pump	48.1 kPa^−1	-	1 mV	0 ~ 135 kPa	[69]
[EMIM][OTF] PVA, cellular pulp	Sensing paper	10 nF kPa^−1 cm^−2	5 /4	-	6.25 Pa ~ 25 kPa	[73]
[EMIM][TFSI] P(VDF-HFP)	Sensing fabrics	114 nF/ kPa	4.2	-	2.4 Pa ~ 10.7 kPa	[67]
[EMIM][TCM] PEDGA, cellular paper	Paper based sensing component	20 nF/ N	-	-	-	[72]
[EMIM][OTF] P(HEMA), cellulose fiber	Paper based sensing origami	1 nF kPa^−1 cm^−2	6 /4	-	5.12 Pa ~ 200 kPa	[74]
[EMIM] [TFSI] TPU	Biomimic pillar patterns	25.8 nF/ kPa	47 /63	1 mV	10.2 Pa ~ 50 kPa	[70]

Figure 6. The characteristics and properties of IFSS. (a) The transmittance of ionogel in visible light range [61]. (b) The maximum elongation of ionogel [61]. (c) The temperature sensibility of IFSS [59]. (d) IFSS may have different GF in different elongation ranges [48]. (e) The GF of capacitive IFSS [55]. (f) The GF of triboelectric-type IFSS [52]. (g) The resistance change of IFSS has a good linear relationship with strain [41]. (h) An resistance IFSS with 80 ms responsive time [59]. (i) High stability with 10,000 cycles of duration [60]. (j) Self-healable ionogel [56]. (k) An IFSS that can work in the temperature range of −60 ~ 250 °C [49]

Figure 7. The development progress of (a) maximum strain, sensitivity (sensitivity is not provided in Ref. [50,57]) and (b) temperature tolerance on ionogel based tensile strain sensors; and the development progress of (c) sensitivity, response time (response time is not provided in Ref. [51,69]) and (d) working range on ionogel based pressure sensors in recent years

Figure 8. The IFSS can monitoring human limb movements, such as (a) finger bending, (b) wrist flexing, (c) elbow bending [49] and (d) subtle wrist bending [45]

Figure 9. (a) Controlling an UVA by eyes blinking through an IFSS [45]; (b) A smart wristband integrated IFSS array with circuit components consolidated on a flexible printed circuit board (PCB) can control the drone flight [69]; (c) A smart glove can sensing distribution of pressure [67]

Figure 10. IFSS can real-time sensing physiological characteristics, such as (a) breathing [55], (b) swallowing [47], (c) speaking [45], (d) pulse beating [52] and (e) eye movements [64], for monitoring human health situation

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