Table 1.
The properties of highly stretchable and tough hydrogels.
| Gel | Problems of Traditional Gels that the New Gel Tried to Fix | Design Strategy of the New Gel in the Paper | Gauge Factor (Strain %) | Healing Time and Efficiency | Mechanical Properties | Year | Ref. |
|---|---|---|---|---|---|---|---|
| PVA/SWCNT | No sensing properties over 100% strain, Low Gauge Factor | Introduce SWCNT to increase stretchability, gauge factor, and recovery | 0.24 (100%) 1.51 (1000%) |
Electrical Healing: 3.2 s Appearance: 30–60 s Self-Healing Efficiency: ~98% |
No change in sensor properties after 1000 cycles at 700% strain Excellent Sensing Performance |
2016 | [21] |
| PVA/Graphene | No sensing properties over 100% strain, Low Gauge Factor | Introduce Graphene to increase stretchability, gauge factor, and recovery | 0.92 (1000%) | - | Excellent Sensing Performance | 2016 | [21] |
| PVA/Silver Nanowire | No sensing properties over 100% strain, Low Gauge Factor | Introduce Silver Nanowire to increase stretchability, gauge factor, and recovery | 2.25 (1000%) | Silver nanowire is easily oxidized by air and water | Excellent Sensing Performance | 2016 | [21] |
| Aromatic Polyamic Acid Salt (PAAS) Hydrogel | Poor Mechanical Properties, Preparation is toxic | Prepare in an environmentally friendly way, Adding p-PDA/s-BPDA enhance mechanical properties | - | Self-healed within 1 min at room temperature | Mechanical stress of 500 kPa at 1350% strain, Storage Modulus of 5 × 105 Pa | 2019 | [27] |
| DCh/PPy/PAA | Low Conductivity, Sensitivity, Mechanical Recovery | Create a mechanically/electrically self-healing hydrogel with pressure/extension sensitivity | - | Mechanical Recovery: 2 min 90% Electrical Recovery: 30 s |
Conductivity increases with strength of compression on Hydrogel | 2017 | [28] |
| PVA/PVP/Fe3+ | Low Mechanical Properties, Self-healability, sensitivity | Fabricate a conductive, elastic, self-healing, and strain-sensitive hydrogel | 0.478 (200%) | Self-healing within 5 min, and self-recovery within 30 min | Mechanical Strength of 2.1 MPa of tensile stress | 2017 | [29] |
| PVA/PDA | Low Detection Ranges and sensitivity | A low-modulus PVA hydrogel that is self-healing, PDA makes the hydrogel self-adhesive | - | Self-Healing in 250 ms at ambient temperature | High Sensing Performance in the ranges of Ultralow (0.1%) to High (500%) Strain | 2018 | [24] |
| PEDOT:SL/PAA | Not wearable, Insensitive to pressure/strain Can freeze at subzero temperatures |
PEDOT:SL improves softness and elasticity-promotes strain sensitivity | 7 (100%) | - | Stretched to 7 times original length, recovers with negligible residual strain | 2019 | [30] |
| PAAm/Graphene | Poor mechanical consistence and electrical conductivity | Hydrogel acts as potential scaffold for neuronal growth | 9 (30%) | - | Conductivity:5.4 × 10−5 S/cm | 2018 | [31] |
| PAA/PANI | Self-healing electronics have low durability and stretchability | PANI-based self-healing electronic composite with high stretchability and electrical conductivity | 11.6 (Within 100%) 4.7 (Over 100%) |
Electrical Conductivity Healing Efficiency: 88.2% in 5 min Mechanical Healing Efficiency: 24.3% in 5 min |
Stretchability up to 400% Electrical Conductivity of 0.12 S/cm |
2018 | [22] |
| PAAm/LiCl | Ionogels have lower conductivity than hydrogels | Soft, stretchable electrical devices integrating a conductive hydrogel | 0.84 (40%) | - | Conductivity: 10.39 ± 0.31 S/m | 2017 | [32] |
| PAA/Graphene/Fe3+ | Low stretchability, self-healing, mechanical properties | Covalent bonds -strong, stable network for the hydrogel, Reduced graphene oxide and ions are highly sensitive | 0.31 (100%) 1.32 (500%) |
Recovered nearly 100% initial conductivity | Resistance: 5.8 kΩ Strength: ~300 kPa at 45% strain Tensile Strength: ~400 kPa at 300% strain |
2018 | [33,34] |
| PAA/Al3+ | Poor mechanical properties, Require adhesives | Ions allow high sensitivity to large and subtle motions | 5.5 (100%) 7.8 (2000%) |
Healing efficiency of 88% at 20 min and 92% at 30 min | Ultra-stretchability with a 2952% fracture strain, Compression Performance: 95% strain without fracture Toughness: 5.60 MJ/m3 |
2018 | [25] |
| Dopamine/Talc/PAAm (DTPAM) | Low stretchability and recoverability | Polydopamine-modified talc particles uniformly disperse in PAAm—Enhance mechanical properties/adhesiveness | 0.125 (100%) 0.693 (1000%) |
Appearance healed after 30 min at room temperature | After healing, can still be stretched over 800% Strongly adhesive |
2018 | [34] |
| PAAm/Alginate | Low mechanical robustness and stretchability | PAAm and alginate form a ‘tough’ hydrogel that has a high stretchability and fracture toughness | - | - | Fracture Toughness of ~9000 J/m2 Fatigue Fracture of 53 J/m2 Cycle 1000: Constant resistance to high stretching |
2016 2017 |
[35,36] |
| PAAm/Alginate/CaCl2 | Desired properties lost below freezing point of water | Gel soaked in 30 wt % CaCl2 retains stretchability/toughness/conductivity at below 0 °C | - | - | Fracture Toughness of ~5000 J/m2 | 2018 | [37] |
| PAAm/Alginate Optical Fibers | Fragile against external strain, Low mechanical strength |
Make a tough hydrogel, which has high stretchability and mechanical strength | - | - | Fracture Energy of ~9000 J/m2 Can be elongated to 700% strain |
2016 | [38] |
| PAMPS/PAAm Double Network Gel |
Single network hydrogels showed poor mechanical properties, Fatigue Damage under low cyclic load |
Double Network hydrogels have outstanding mechanical properties | - | - | Average Toughness ~3358 J/m2, Fracture Energy 3779 J/m2, Fatigue Threshold 418 J/m2 | 2018 | [39] |
| PVA/PAAm | Low stretchability and sensitivity | Adhesive Wrinkled microarchitectures and interconnected ridges increase contact area |
- | - | Stretchability up to 500%, Response time of 150 ms, Sensitivity of 0.05 kPa−1 at 0 to 3.27 kPa | 2018 | [40] |
| AAm/2-hydroxyethylacrylate/Liquid Gallium | Low sensitivity, limited stretchability, and poor stability | Use liquid metals as soft fillers in hydrophilic polymer networks to make highly stretchable, force-sensitive hydrogels | - | - | Tensile Strain ~1500%, Compressive Sensitivity of 0.25 kPa | 2019 | [41] |
| PAA/PANI | Limited by fragile and weak properties, like low flexibility | Highly Stretchable PAA/PANI hydrogel | 0.60 (0–800%) 1.05 (800–1130%) |
- | Tensile Deformation: 1160% strain Sensing Range: 0 to 1130% |
2018 | [42] |
| PVA/MXene | Low sensitivity | MXenes have high conductivity and strain sensitivity. MXenes improve mechanical properties |
2, 0 wt % MXene (40%) 25, 4.1 wt % MXene (40%) |
Instantaneous Self-Healing | Stretchability of 3400% Conformability and adhesive to various surfaces, including human skin |
2018 | [23] |
| PAAm/Alginate/Eutectic Gallium | Low Conductivity, Stretchability, High Power Consumption | Eutectic Gallium is highly conductive and used in a known tough hydrogel | - | - | Sensitivity of 100 Pa, can be rehydrated to most of its initial weight (>85%) after 30 drying/soaking cycles | 2018 | [43] |
| PAAm/Agar/LiCl | Low stretchability, Opaque, Poor Mechanical Strength | Conductive, Excellent mechanical properties, stretchability, and sensitivity, Transparent | 1.8 (1100%) | - | Stretchability over 1600%, Tension Strength: 0.22 MPa, Compression Strength: 3.5 MPa | 2019 | [44] |
| PDMS/AAm/NaCl | Capacitance and resistance are affected by stretch, bend, and pressure | Low Cost Materials and methods | - | - | Ionic Resistivity of 0.06 Ω | 2017 | [45] |
| PAAm/LiCl | Low Sheet Resistances and transparency, Brittle | Used as an ionic conductor | - | - | Can operate with over 1000% areal strain Elastic Modulus of 12 kPa |
2016 | [46] |
| PAAm/LiCl/Silicone | LED-based systems are limited by low ultimate strain | Fabricate a hyperelastic light-emitting capacitor (HLEC), using a hydrogel | - | - | Stretches to >480% strain | 2016 | [47] |
| PAAm/Alginate/PDMS | Low mechanical robustness and compatibility | Hydrogel–Elastomer Hybrid that is stretchable, robust, and biocompatible | - | - | - | 2017 | [48] |
| PNAGA-PAMPS/PEDOT-PSSa | Conductive Hydrogels (CHs) are mechanically weak and brittle | PNAGA hydrogels demonstrate high strength, thermoplasticity, and self-healability | - | Self-healed after 3 h in a plastic syringe immersed in a 90 °C water bath | 0.22–0.58 MPa tensile strength, 1.02–7.62 MPa compressive strength, 817–1709% breaking strain | 2017 | [49] |
| PVA/CNF | Low sensitivity, stretchability, self-healability, and transparency | Highly sensitive, stretchable, and autonomously self-healing ionic skin—biocompatible | - | Spontaneously Self-Healed in 15 s | Highly Transparent—Transmittance as high as 90%, Modulus of 11.2 kPa, Elongation Rate of 1900% | 2019 | [50] |
| PVA/Borax | Low stretchability, self-healing, water retention, biocompatibility | PVA and Borax: biocompatible/highly stretchable/easily dissolvable in aqueous solution/have good mechanical performance | - | Self-healed 10 times without affecting electrical conduction of gel | Can be stretched to strains over 5000% | 2019 | [51] |
PVA—Polyvinyl Alcohol; SWCNT—Single-Wall Carbon Nanotube; p-PDA—p-Phenylenediamine; s-BPDA—Biphenyltetracarboxylic dianhydride; DCh—Double-bond Decorated Chitosan; PPy—Polypyrrole; PAA—Polyacrylic Acid; PVP—Polyvinylpyrrolidone; PDA—Polydopamine; PEDOT:SL—Poly (3,4-ethylenedioxythiophene): Sulfonated Lignin; PAAm—Polyacrylamide; PANI—Polyaniline; PAMPS—Poly (1-acrylanmido-2-methylpropanesulfonic acid); AAm—Acrylamide; PDMS—Polydimethylsiloxane; PNAGA-PAMPS—Poly (N-acryloyl glycinamide-co-2-acrylamide-2-methylpropanesulfonic); PEDOT-PSS—Poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate); CNF—Cellulose Nanofibril.