Table 2
Summary of characteristics of flexible flow sensors a
| Materials | Principles | Applications | Performance | Ref. |
| Ni resistors/PI film | Thermal anemometer | Hydrodynamic experiments | Temperature coefficient of resistance: 4250 ppm/°C | [26] |
| WTi/Silicon substrate |
Thermal calorimeter | Airflow separation and reattachment detection | Sensitivity: 30.4 mV/(m/s) Response time: 4.1 ms Threshold: 0.26 mm3/s |
[47] |
| Cr-Au film/PI film | Thermal calorimeter | Assessment of both macrovascular and microvascular flow | ‒ | [34,49] |
| Cr-Au electrodes/PI film | Thermal calorimeter | ‒ | Detection range: 30–170 m/s | [48] |
| VOx thermistors/PI film | Thermal calorimeter | Relative airflow velocity estimation and wing vibration perception | Resolution: 0.11 mm/s Angular resolution: 0.1° Temperature coefficient of resistance: −2%/K |
[82] |
| Ag electrodes, SnO2-carbon/PET film | Thermal calorimeter | Accurate wearable skin temperature monitoring | Sensitivity: 0.08 °C s/m 50-fold enhancement in the thermal sensing accuracy |
[16] |
| Laser-induced-graphene electrodes/PI film | Combined thermal anemometer and calorimeter | Respiration detection and monitoring of the movement of a robotic arm | Sensitivity: 162 K s/m Detection limit: ~25 m/s Response time: 6 s Temperature coefficient of resistance: −0.035%/K |
[50] |
| Cr-Au electrodes/PI film | Combined thermal anemometer and calorimeter | Abnormal breath conditions measurement | Detection limit: 4 m/s Response time: 193 ms |
[89] |
| Vertical graphene nanosheets/PDMS film | Piezoresistive effect | Detection of oscillatory flow inside the semicircular canal | Sensitivity: 103.91 mV s/mm Threshold: 1.127 mm/s Response time: 3 s |
[57] |
| Laser-induced graphene fiber/PI film | Piezoresistive effect | Weather forecasting, communication for quadriplegic aphasia | Sensitivity: 0.11 s/m Threshold: 0.0023 m/s Response time: 0.5 s |
[59] |
| Carbon nanotubes/silk fabric | Piezoresistive effect | Transmitting Morse code, warning devices and buzzer alarm for the blind | Threshold: 0.05 m/s Detection limit: 7.0 m/s Response time: 1.3 s |
[30] |
| P(VDF-TrFE)/BTO nanofiber/PI film | Piezoelectric effect | ‒ | Sensitivity: 0.08 pC s/m Threshold: 0.23 mm/s Detection limit: 5 mm/s |
[60] |
| Cr-Au layer/ZnO-Al film | Doppler effect | ‒ | Temperature coefficient of frequency: 280 ppm/K Relative sensitivity: 2176.8 ppm Response time: 2 s |
[61] |
| Piezoelectric composite, Cu/PI electrode | Doppler effect | Blood flow velocity monitoring and cuff experiments | Error: < 8.69% | [62] |
| ‒ | Acoustic time-of-flight | ‒ | Resolution: 0.03 L/s Linearity: > 95% |
[66] |
| 304 steel alloy, PVDF piezoelectric and silk fibroin electrode | Triboelectric and piezoelectric effects | Stall sensing | [81] | |
| SYLGARD 184 silicone elastomer/A-PET/CPP film | Triboelectric effects | Target motion status monitoring and tracking for marine intelligent equipment | Natural resonant frequency: 2 Hz | [94] |
a: Ni, nickel; PI, polyimide; WTi, tungsten titanium; Cr, chromium; Au, gold; VOx, vanadium oxide; Ag, silver; SnO2, tin(IV) oxide; PET, polyethylene terephthalate; PDMS, polydimethylsiloxane; P(VDF-TrFE)/BTO, poly(vinylidene fluoride-trifluoroethylene)/barium titanate; ZnO, zinc oxide; Al, aluminum; PVDF, poly(vinylidene fluoride); A-PET/CPP, aluminum-coated polyethylene glycol terephthalate/cast polypropylene.
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