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. 2020 Aug 11;20(16):4484. doi: 10.3390/s20164484

Table 4.

Summary of active materials, sensing principles used in wearable sensors along with their sensing properties.

SN Active Material
(Structure)		 Sensing
Principles		 Limit of
Detection
(kPa) 1 		Maximum
Detection
(kPa) 2 		Sensitivity
(kPa−1) 3 		Reference Notes
1.0 CNT/PDMS
(Porous Structure)		 Piezoresistive 0.25 100.0 0.588 4 [257]

  • No blood pressure measurement application.

  • Not all properties were reported (e.g., hysteresis, stability test).

  • High limit of detection.
2.0 CNT@EcoFlex
(Buckled Structure)		 Piezocapacitive 50.0 1000 230 [244]

  • No blood pressure measurement application.

  • Not all properties were reported (e.g., hysteresis, stability test).

  • High limit of detection.
3.0 ACNT-Graphene
/PDMS
(CVD)		 Piezoresistive 0.0006 0.3 19.8 [259]

  • No blood pressure measurement application.

  • Response time is <16.7 ms. 5

  • Stability for more than 35,000 loading/unloading cycles. 6

  • Operating voltage is 0.03 V. 7

  • Low maximum detection.
4.0 CNT/PDMS
(Patterned Microstructure)		 Piezoresistive 0.0002 59.0 15.1 [258]

  • No blood pressure measurement application.

  • Performance changed under cycle-test with constant pressure.

  • Not all properties were reported.

  • The operating voltage is 10.0 V. 7
5.0 VACNT/PDMS
(T-CVD)		 Piezoresistive 0.002 10.0 0.3 and
up to 0.7		 [260]

  • Blood pressure measurement application without calibration, need to assess accuracy.

  • Response time is around 162 ms. 5 Acceptable reproducibility over 5000 cycles of pressure loading/ unloading. 6
6.0 SWCNTs/PDMS
(Silk Molded Microstructure)		 Piezoresistive 0.0006 1.2 1.8 [272]

  • No blood pressure measurement application but heart pulse.

  • Response time is 10 ms. 5

  • Stability for around 67,500 loading/unloading cycles. 6

  • Operating voltage is 2.0 V. 7
7.0 CB@PU Sponge Piezoresistive 0.091 16.4 0.068 [240]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 50,000 loading/unloading cycles. 6

  • Response time is 20 ms. 5
8.0 MWCNT-rGO@PU Foam Piezoresistive 0.0035 2.7 0.022 [256]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 5000 loading/unloading cycles. 6

  • Response time is < 30 ms. 5
9.0 SWCNTs/PDMS Optical 1.0 - 0.2 [201]

  • No blood pressure measurement application.

  • Response time is 300 ms. 5

  • No stability test information was reported.

  • Transparent optical application.
10.0 CNT/3D Microporous Elastomeric Dielectric Layer Piezocapacitive 0.0001 130 0.601 [194]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 1000 loading/unloading cycles. 6

  • Operating voltage can be 1.0 V. 7

11.0 Graphene@PU Piezoresistive 0.009 10.0 0.26 [273]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 10,000 loading/unloading cycles. 6

  • Not all properties were reported.
12.0 Graphene
(MEMS)		 Piezoresistive 0.1 - 3.4 × 10−6 [274]

  • No blood pressure measurement application.

  • Response Time is 0.9 s. 5

  • Operating voltage is 2.5 V. 7

  • Not all properties were reported.
13.0 Graphene Paper Piezocapacitive 2.0 20.0 17.2 [275]

  • No blood pressure measurement application, but heart pulse.

  • Response time is120 ms. 5

  • Stability for around 300 loading/unloading cycles. 6

  • Fabricated by Dip Coating.
14.0 Graphene Electrode
(T-CVD)		 Piezocapacitive 4.4 × 10−5
(1 mg)		 - 3.19 [247]

  • No blood pressure measurement application, but it sounds to be suitable for this application.

  • The applied voltage is 3.3 V. 7

  • Stability for around 500 loading/unloading cycles. 6

  • Response time is 30 ms. 5
15.0 Graphene
(Porous GS)		 Piezoresistive 0.3 10.0 0.046 [276]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 200 loading/unloading cycles. 6
16.0 Graphene Electrode Piezocapacitive 8.0 - 6.55 [268]

  • No blood pressure measurement application.

  • Stability for around 500 loading/unloading cycles. 6

  • Response time is 70ms. 5 =
17.0 Suspended
Graphene /Polymer
(Heterostructure
Membranes)		 Piezocapacitive 80.0 - 123ZF [277]

  • No blood pressure measurement application.

  • Not all properties were reported.
18.0 Graphene Tribotronics FET 1.0 - 0.02 [278]

  • No blood pressure measurement application.

  • Stability for around 1700 loading/unloading cycles. 6 =

  • Response time is 30 ms. 5 =

  • Operating voltage can be 0.5 V. 7 =

  • Suitable for electronic skin and touch screen applications.
19.0 rGO/PANI
Wrapped Sponge		 Piezoresistive 0.1 27.0 0.152 [279]

  • No blood pressure measurement application.

  • Response time is around 96 ms. 5 =

  • Stability for around 9000 loading/unloading cycles. 6 =

  • High current output (i.e., ∼300 μA at 1.0 V. 7 =
20.0 PNIPAm/CMC/ rGO DN Hydrogel Thermo-resisitive - 800.0 - [280]

  • No blood pressure measurement application.

  • It is also sensitive to temperature (30 °C–45 °C).
21.0 rGO/PU
Sponge		 Piezoresistive 4.84 × 10−5 (1.1 mg) - 0.21 [281]

  • No blood pressure measurement application.

  • Stability for around 10,000 loading/unloading cycles. 6 =

  • Response time is about 100 ms. 5 =

  • Not all properties were reported.
22.0 rGO/PDMS Film
(Pattered Micropyramid)		 Piezoresistive 0.0015 1.4 5.5 [243]

  • No blood pressure measurement application.

  • Response time is 0.2 ms. 5 =

  • Stability for around 5000 loading/unloading cycles at 100 Pa. 6 =

  • The applied voltage is 1.0 V. 7 =
23.0 rGO Films with Continuous
Gradient
Wrinkles		 Piezoresistive 0.0042 3.0 178.0 [282]

  • No blood pressure measurement application.

  • Stability for around 1200 loading/unloading cycles. 6 =

  • Response time is 131 ms. 5 =

  • Operating Voltage can be 1.0 V. 7 =
24.0 Large-Scale
Polystyrene
Ball@rGO Core Shell NPs		 Piezoresistive 0.003 3.0 50.9 [283]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 20,000 loading/unloading cycles. 6

  • Response time is 50 ms. 5

  • Operating Voltage can be 1.0 V. 7
25.0 Graphene
(Electrode Microconformal)		 Piezocapacitive 4.4 × 10−5
(1 mg)		 - 7.68 [190]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 5000 loading/unloading cycles. 6

  • The applied voltage is 1.0 mV. 7

  • Response time is 30ms. 5
26.0 Integrated
Arrays of Air-Dielectric Graphene Transistors		 FET 0.25 3000 2.05 × 10−4 [270]

  • No blood pressure measurement application.

  • Response Time is 30 ms. 5

  • Stability for around 1000 loading/unloading cycles. 6

  • The gate voltage is 25.0 V. 7
27.0 Graphene Transistor
Array
(Direct-Contact Tribotronic Planar)		 FET 0.16 mm−1 - - [269]

  • No blood pressure measurement application.

  • Stability for around 1000 loading/unloading cycles. 6

  • Response time is 15 ms. 5

  • Gate voltage can be 2.0 V. 7
28.0 Graphene
(Direct Laser Scribing PDMS)		 Piezoresistive 0.028 - 480.0 [271]

  • No blood pressure measurement application.

  • Stability for around 4000 loading/unloading cycles. 6

  • Response time is 0.002 ms. 5

  • Operating voltage can be 5.0 V. 7
29.0 3D Graphene
Film
(Fingerprint Like Patterned) 		Piezoresistive 0.0002 75.0 110.0 [242]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 10,000 loading/unloading cycles. 6

  • Response time is 30 ms. 5

  • Operating voltage can be 3.0 V. 7
30.0 GO (Spray Coating through a Stencil Mask) Piezocapacitive 0.24 × 10−3 - 0.8 [192]

  • No blood pressure measurement application.

  • Response time is around 100 ms. 5

  • Maximum hysteresis is 5.0%@ 1.4 kPa.

  • Stability for around 100 loading /unloading cycles. 6
31.0 MoS2/GPN /Ecoflex
(T-CVD)		 Piezoresistive 0.6 25.4 6.06 [284]

  • Suitable for blood pressure measurement application.

  • Stability for around 4000 loading/unloading cycles. 6

  • Response Time is 200 ms. 5
32.0 PVA NWs/ Wrinkled
Graphene Film		 Piezoresistive 0.00224 - 28.34 [285]

  • No blood pressure measurement application.

  • Stability for around 6000 loading/unloading cycles. 6

  • The applied voltage is 0.1V. 7

  • Not all properties were reported.
33.0 rGO Film/
PDMS Arrays		 Piezoresistive 0.0013 225.0 1.71 [238]

  • No blood pressure measurement application.

  • Stability for around 5000 loading/unloading cycles. 6

  • Response time is 6 ms. 5

  • Operating Voltage can be 1.0 V. 7
34.0 P(VDF-TrFe)
/rGO		 Piezoresistive 0.0012 - 15.6 [251]

  • No blood pressure measurement application.

  • Stability for around 100,000 loading/unloading cycles. 6

  • Response time is 5 ms. 5

  • The applied voltage is 1.0 mV. 7
35.0 PEDOT:PSS
/PUD
(Pattered Micropyramid)		 Piezoresistive 0.023 8.0 10.3 [176]

  • No blood pressure measurement application but heart pulse.

  • Response time 0.2 s. 5

  • Stability 800 loading/unloading cycles. 6

  • Applied Voltage is 0.2 V. 7
36.0 PVDF-HFP/PEDOT
(3D Electrospun Nanofibers)		 Piezoresistive 0.001 30.0 13.5 [286]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 10,000 loading/unloading cycles. 6

  • Response time is around 0.4s. 5

  • The applied voltage is 1.0 V. 7
37.0 P(VDF-TrFe) (Electrospun Nanofiber) Piezoresistive 0.0001 0.012 0.00041 [287]

  • No blood pressure measurement application.

  • Stability for around 10,000 loading / unloading cycles. 6
38.0 [PPy@PVA-co-PE] and POE Nanofibers Piezoresistive 0.0013 7.0 1.24 [288]

  • Suitable for blood pressure measurement application.

  • Stability for around 250 loading/unloading cycles. 6

  • Operating voltage can be 2.0 V. 7
39.0 Au NWs/Tissue Paper
(Dip-Coating)		 Piezoresistive 0.013 - 1.14 [289]

  • No blood pressure measurement application but heart pulse.

  • The operating voltage is 1.5 V 7 with low energy consumption that is <30 µW.

  • Response time is <17 ms. 5

  • High Stability for around 50,000 loading/unloading cycles.
40.0 Au NP Densely Packed µNW based Pressure Piezoresistive 0.025 0.0801 [290]

  • No blood pressure measurement application.

  • Stability for around 6000 loading/unloading cycles. 6

  • Response time is <270 ms. 5

  • Not all properties were reported.
41.0 Ag IDEs and PdOx NP
(Percolative Metal NP Arrays) 		Piezoresistive 0.0005 1.0 0.13 [248]

  • No blood pressure measurement application.

  • Stability for around 500 loading/unloading cycles. 6

  • Hysteresis induced by a 1.0 kPa applied pressure showed a shift of 0.012% from the initial value.
42.0 AG NWs
(Ag NW Flower) 		Piezocapacitive 0.0006 115.0 1.54 [195]

  • No blood pressure measurement application.

  • Stability for around 5000 loading/unloading cycles. 6

  • Not all properties were reported.
43.0 Ge/Si Core/shell NW PSR
(OLED)		 FET 1.0 - 42.7 [210]

  • No blood pressure measurement application.

  • Response time is 1 ms.5

  • Operating voltage is 10 V. 7

  • Not all properties were reported.
44.0 MOF
(CuTCNQ)		 Piezoresistive 0.00073 3.0 6.25 [249]

  • Suitable for blood pressure measurement application.

  • Stability for around 10,000 loading/unloading cycles. 6

  • Response time is 10 ms. 5

  • Operating voltage can be 1.0 V. 7

  • MOF is relatively expensive.
45.0 C-MOF/PANIF @PU Sponge Piezoresistive 0.001 60.0 158.26 [291]

  • No blood pressure measurement application, but heart pulse.

  • Stability for around 15,000 loading/unloading cycles. 6

  • Response time is < 22 ms. 5
46.0 C-MOF-5
Derived Porous Carbon		 Piezoresistive 0.02 1.0 15.63 [292]

  • No blood pressure measurement application.

  • Stability for around 2000 loading/unloading cycles. 6

  • Response time is < 65 ms. 5

  • Operating voltage is 3.0 V. 7
47.0 MXenes
Nanosheets (Ti3C2Tx)		 Piezoresistive 0.0102 30.0 0.55 [200]

  • No blood pressure measurement application.

  • Stability for around 10,000 loading/unloading cycles.6

  • Response time is less than 11 ms.5

  • Operating voltage is 0.01 V.7
48.0 MXenes
Nanosheets (Ti3C2)		 Piezoresistive - 0.351 7.5 [250]

  • No blood pressure measurement application.

  • Stability for around 4000 loading/unloading cycles. 6

  • Response time is < 30 ms. 5

  • Operating voltage is 1.0 V. 7
49.0 Pt-coated
Polymeric Nanofibers
(Nanohair)		 Piezoresistive 0.005 - 11.35 [293]

  • No blood pressure measurement application.

  • Response time is 50 ms. 5

  • Stability for around 10,000 loading/unloading cycles. 6
50.0 ITO (3D Printed Mold) Piezocapacitive 0.003 4.0 1.62 [294]

  • No blood pressure measurement application but heart pulse.

  • Stability for around 1000 loading/unloading cycles. 6

  • Operating voltage can be 3.0 V. 7

  • Response time is < 10 ms. 5

51.0 Ag Flexible Piezoelectret-Based Pressure Sensor Piezoelectric - 2.5 15.0 [295]

  • No blood pressure measurement application.

  • Stability for around 36,000 loading/unloading cycles with constant pressure. 6

  • Perfluoro (alkoxy alkane) electret.

  • Not all properties were reported.
52.0 ITO/PDMS
(Pattered micro-pyramid)		 Piezocapacitive 0.003 20.0 0.55 [296]

  • No blood pressure measurement application.

  • Response time is 1.0 s. 5

  • Stability for around 10,000 loading/unloading cycles. 6

  • The gate voltage is higher than 5.0 V. 7
53.0 Ag NWs (Embedded PDMS Electrode with Microarray Structure) Piezocapacitive 0.003 5.0 2.94 [297]

  • No blood pressure measurement application.

  • Stability for around 1000 loading/unloading cycles. 6

  • Response time is less than 50 ms. 5

  • Not all properties were reported.

1 The smallest change that can be detected by a sensor. 2 The largest change that can be detected by a sensor. 3 The rate of change in output signal when responding to a change in stimulus. 4 Calculated Sensitivity. 5 The time interval within which a sensor can detect the smallest change in stimulus. 6 The rate of change in sensitivity and/or other sensor properties over continuous cycles of loading/unloading. 7 The sensor operating voltage should be as small as fractions of an mV to reduce power consumption, improve the battery life of the wearable system and/or support the integration with other energy harvesting applications.