Flexible wearable sensors - an update in view of touch-sensing

Figures & data

Figure 1. Flexible touch sensors, consisting of Principles, Materials, Technologies, and Applications in e-skins. Reproduced with permission from Ref [6]. copyright 2019 Elsevier B.V., e-textiles. Adapted with permission from Ref [11]. copyright 2018 Springer Nature, e-healthcare. Adapted with permission from Ref [7]. copyright 2017 American Chemical Society, e-control. Adapted with permission from Ref [5]. copyright 2020 IEEE

Table 1. Summary the electrical and mechanical performance of the different sensors and sensing technologies, where PDMS is polydimethylsiloxane, CNTs are carbon nanotubes, SWNTs are single-walled carbon nanotubes, ITO is indium tin oxide, CNFs are carbon nanofibers, PPy is polypyrrole, BCZT is (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃, AgNWs are silver nanowires

Ref.	Material	Principle	Sensitivity	Response time	Working range	Thickness
[7]	PDMS, SWNTs, Si, PET	Resistive	1 kPa^−1	20 ms	0.1–100 kPa	~ 900 µm
[13]	Single-layer ion gel, Copper electrodes	Capacitive	2.266 kPa^−1	< 1s	1–25 kPa	> 200 µm
[14]	Au/PET, PDMS	Capacitive	0.42 kPa^−1	70 ms	1–9 kPa	100 µm
[18]	Paper, Silver paste, CNTs	Resistive	2.56–5.67 kPa^−1	30 ms	0–20 kPa	> 1 mm
[20]	Hemispheric microstructures, PDMS, Au, Ag Paste	Resistive	196 kPa^−1	26 ms	0–100 kPa	-
[21]	Microstructures, PDMS, ITO‐PET, CNFs	Resistive	3.6 kPa^−1	20–50 ms	0–2 kPa	> 1.5 mm
[22]	Pyramid microstructures, PDMS, PPy/PDMS, Au/Cr	Resistive	> 200 kPa^−1	50 us	0.1–1,000 Pa	-
[23]	PDMS, BCZT particles	Piezoelectric	0.55 kPa^−1	-	28.8 V output, 50–1,000 kPa	0.5 mm
[24]	Graphene field-effect transistor (GFET), Metal-insulator-metal (MIM)	Piezoelectric	0.00455 kPa^−1	-	100 mV operation, 23.54–94.18 kPa	-
[25]	Silk fibroin, AgNWs, PDMS	Triboelectric	-	7 ms	~ 90 V output, 8–22 kPa	0.12 mm
[27]	PDMS, Paper, Graphite (Pencil)	Multi-Touch/Capacitive	0.62 kPa^−1	> 200 ms	0.5–10 kPa	> 250 µm
[28]	Ni-coated textile, CNT, Acrylate polymers	Multi-Touch/Resistive	14.4 kPa^−1	24 ms	0–15 kPa	-

Figure 2. Schematic illustrations: (a) The flexible electrode using spin coating with electron-induced perpendicular graphene (EIPG), anodic aluminum oxide (AAO), and polydimethylsiloxane (PDMS). Reproduced with permission from Ref [4]. copyright 2020 Springer Nature. (b) The inkjet-printing controllable process for the evaporation-driven convective particle self-assembly at the contact line. Adapted with permission from Ref [73]. copyright 2020 Wiley-VCH. (c) The electrospinning and electrospraying processes of the AgNF-AgNW hybrid electrode. Adapted with permission from Ref [53]. copyright 2018 Springer Nature

Figure 3. (a) Three-dimensional fingertip-shaped artificial skin device by 3D printing, which can sense exact touch location and heal mechanical damage spontaneously. Reproduced with permission from Ref [80]. copyright 2019 American Chemical Society. (b) Schematic illustration of the e‐skin consisting of two electrodes and the natural material (flower and leaf) as the dielectric layer in between. Besides, the graphs show the capacitance change and the sensitivity when applied pressure on three types of e-skins based critical point dried rose petal, rose leaf, and acacia leaf. Adapted with permission from Ref [81]. copyright 2018 Wiley-VCH. (c) Soft-touch textile sensors in high-voltage TASER test to short out the electrical shock. Adapted with permission from Ref [86]. copyright 2020 American Chemical Society. (d) Omniphobic triboelectric nanogenerators (RF‐TENGs) from e-textiles with the shape of a cat powering two LEDs embroidered as eyes (when touch). Adapted with permission from Ref [88]. copyright 2019 Wiley-VCH

Figure 4. a) The photoplethysmogram (PPG) sensor hybrids an organic phototransistor (OPT) with an inorganic light‐emitting diode for cardiovascular monitoring, consisting of the fabrication process, the real picture of the sensor, and the OPT-LED structure are directly laminated onto the finger. Reproduced with permission from Ref [93]. copyright 2017 Wiley-VCH. b) The fingers of the hand are attached to the nanomesh film devices for tissue-temperature sensing and schematic illustration of remote monitoring of the sensing performance of the assembled film devices by wireless communication with a mobile phone. Adapted with permission from Ref [94]. copyright 2019 Wiley-VCH

Figure 5. Application of the programmable touch operation platform based on the user-interactive e-skin (SUE-skin) for tracks recognition: (a) Optical photograph and four-electrodes schematic of the SUE-skin. (b) Light intensity and output voltage of the SUE-skin under different pressures and different prestress. (c) Touch operation demonstration for the audio controlling and the characters displaying. Adapted with permission from Ref [95]. copyright 2018 Wiley-VCH

Lee Y, Kim J, Jang B, et al. Graphene-based stretchable/wearable self-powered touch sensor. Nano Energy. 2019;62:259–267.

 Web of Science ®Google Scholar

Xiong J, Cui P, Chen X, et al. Skin-touch-actuated textile-based triboelectric nanogenerator with black phosphorus for durable biomechanical energy harvesting. Nat Commun. 2018;9(1):4280.

 PubMed Web of Science ®Google Scholar

Chang H, Kim S, Jin S, et al. Ultrasensitive and highly stable resistive pressure sensors with biomaterial-incorporated interfacial layers for wearable health-monitoring and human–machine interfaces. ACS Appl Mater Interfaces. 2018;10(1):1067–1076.

 PubMed Web of Science ®Google Scholar

Ozioko O, Karipoth P, Hersh M, et al. Wearable assistive tactile communication interface based on integrated touch sensors and actuators. IEEE Trans Neural Syst Rehabil Eng. 2020;28(6):1344–1352.

 PubMed Web of Science ®Google Scholar

B J P, Oh S, Kim FS, et al. Pixel-free capacitive touch sensor using a single-layer ion gel. J Mater Chem C. 2019;7:10264–10272.

 Web of Science ®Google Scholar

Luo Y, Shao J, Chen S, et al. Flexible capacitive pressure sensor enhanced by tilted micropillar arrays. ACS Appl Mater Interfaces. 2019;11(19):17796–17803.

 PubMed Web of Science ®Google Scholar

Chen S, Song Y, Flexible XF. Highly sensitive resistive pressure sensor based on carbonized crepe paper with corrugated structure. ACS Appl Mater Interfaces. 2018;10(40):34646–34654.

 PubMed Web of Science ®Google Scholar

Wang Z, Zhang L, Liu J, et al. Flexible hemispheric microarrays of highly pressure-sensitive sensors based on breath figure method. Nanoscale. 2018;10:10691–10698.

 PubMed Web of Science ®Google Scholar

Peng S, Blanloeuil P, Wu S, et al. Rational design of ultrasensitive pressure sensors by tailoring microscopic features. Adv Mater Interfaces. 2018;5(18):1800403.

 Web of Science ®Google Scholar

Li H, Wu K, Xu Z, et al. Ultrahigh-sensitivity piezoresistive pressure sensors for detection of tiny pressure. ACS Appl Mater Interfaces. 2018;10(24):20826–20834.

 PubMed Web of Science ®Google Scholar

Gao X, Zheng M, Yan X, et al. The alignment of BCZT particles in PDMS boosts the sensitivity and cycling reliability of a flexible piezoelectric touch sensor. J Mater Chem C. 2019;7:961–967.

 Web of Science ®Google Scholar

Yogeswaran N, Navaraj WT, Gupta S, et al. Piezoelectric graphene field effect transistor pressure sensors for tactile sensing. Appl Phys Lett. 2018;113:014102.

 Web of Science ®Google Scholar

Gogurla N, Roy B, Park JY, et al. Skin-contact actuated single-electrode protein triboelectric nanogenerator and strain sensor for biomechanical energy harvesting and motion sensing. Nano Energy. 2019;62:674–681.

 Web of Science ®Google Scholar

Lee K, Lee J, Kim G, et al. Rough-surface-enabled capacitive pressure sensors with 3D touch capability. Small. 2017;13(43):1700368.

 Web of Science ®Google Scholar

Liu M, Pu X, Jiang C, et al. Large-area all-textile pressure sensors for monitoring human motion and physiological signals. Adv Mater. 2017;29(41):1703700.

 Web of Science ®Google Scholar

Chen S, Wang Y, Yang L, et al. Electron-induced perpendicular graphene sheets embedded porous carbon film for flexible touch sensors. Nano-Micro Lett. 2020;12(1):136.

 PubMed Web of Science ®Google Scholar

Liu L, Pei Y, Ma S, et al. Inkjet printing controllable polydopamine nanoparticle line array for transparent and flexible touch-sensing application. Adv Eng Mater. 2020;22(4):1901351.

 Web of Science ®Google Scholar

An BW, Heo S, Ji S, et al. Transparent and flexible fingerprint sensor array with multiplexed detection of tactile pressure and skin temperature. Nat Commun. 2018;9(1):2458.

 PubMed Web of Science ®Google Scholar

Park S, Shin B-G, Jang S, et al. Three-dimensional self-healable touch sensing artificial skin device. ACS Appl Mater Interfaces. 2020;12(3):3953–3960.

 PubMed Web of Science ®Google Scholar

Wan Y, Qiu Z, Huang J, et al. Natural plant materials as dielectric layer for highly sensitive flexible electronic skin. Small. 2018;14(35):e1801657.

 PubMed Web of Science ®Google Scholar

Ghosh S, Nitin B, Remanan S, et al. A multifunctional smart textile derived from merino wool/nylon polymer nanocomposites as next generation microwave absorber and soft touch sensor. ACS Appl Mater Interfaces. 2020;12(15):17988–18001.

 PubMed Web of Science ®Google Scholar

Medeiros MSD, Chanci D, Moreno C, et al. Waterproof, breathable, and antibacterial self-powered e-textiles based on omniphobic triboelectric nanogenerators. Adv Funct Mater. 2019;29(42):1904350.

 Web of Science ®Google Scholar

Xu H, Liu J, Zhang J, et al. Flexible organic/inorganic hybrid near-infrared photoplethysmogram sensor for cardiovascular monitoring. Adv Mater. 2017;29(31):1700975.

 Web of Science ®Google Scholar

Gong M, Wan P, Ma D, et al. Flexible breathable nanomesh electronic devices for on-demand therapy. Adv Funct Mater. 2019;29(26):1902127.

 Web of Science ®Google Scholar

Zhao X, Zhang Z, Liao Q, et al. Self-powered user-interactive electronic skin for programmable touch operation platform. Sci Adv. 2020;6(28):eaba4294.

 PubMed Web of Science ®Google Scholar