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SCIENCE CHINA Materials, Volume 60, Issue 11: 1026-1062(2017) https://doi.org/10.1007/s40843-017-9077-x

Advanced carbon materials for flexible and wearable sensors

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  • ReceivedJun 2, 2017
  • AcceptedJul 12, 2017
  • PublishedSep 1, 2017

Abstract

Flexible and wearable sensors have drawn extensive concern due to their wide potential applications in wearable electronics and intelligent robots. Flexible sensors with high sensitivity, good flexibility, and excellent stability are highly desirable for monitoring human biomedical signals, movements and the environment. The active materials and the device structures are the keys to achieve high performance. Carbon nanomaterials, including carbon nanotubes (CNTs), graphene, carbon black and carbon nanofibers, are one of the most commonly used active materials for the fabrication of high-performance flexible sensors due to their superior properties. Especially, CNTs and graphene can be assembled into various multi-scaled macroscopic structures, including one dimensional fibers, two dimensional films and three dimensional architectures, endowing the facile design of flexible sensors for wide practical applications. In addition, the hybrid structured carbon materials derived from natural bio-materials also showed a bright prospect for applications in flexible sensors. This review provides a comprehensive presentation of flexible and wearable sensors based on the above various carbon materials. Following a brief introduction of flexible sensors and carbon materials, the fundamentals of typical flexible sensors, such as strain sensors, pressure sensors, temperature sensors and humidity sensors, are presented. Then, the latest progress of flexible sensors based on carbon materials, including the fabrication processes, performance and applications, are summarized. Finally, the remaining major challenges of carbon-based flexible electronics are discussed and the future research directions are proposed.


Funded by

National Natural Science Foundation of China(51672153,51422204,51372132)

National Key Basic Research and Development Program(2016YFA0200103,2013CB228506)


Acknowledgment

This work was supported by the National Natural Science Foundation of China (51672153, 51422204 and 51372132) and the National Key Basic Research and Development Program (2016YFA0200103 and 2013CB228506).


Interest statement

The authors declare that they have no conflict of interest.


Contributions statement

Zhang Y and Jian M proposed the concept and outline of the manuscript. Jian M drafted the manuscript and all authors discussed and revised it.


Author information

Muqiang Jian received his BSc degree in chemical engineering and technology from Northwestern Polytechnical University in 2013. Now he is a PhD candidate in Prof. Yingying Zhang’s group at the Department of Chemistry and Center for Nano and Micro Mechanics of Tsinghua University. His current research is the synthesis of CNTs and their applications in flexible sensors.


Yingying Zhang received her PhD degree from Peking University in 2007. She then worked as a postdoctoral fellow at Los Alamos National Laboratory, USA (2008–2011). Currently, she is an associate professor at the Department of Chemistry and Center for Nano and Micro Mechanics of Tsinghua University. Her research interest is the synthesis of carbon materials and their applications in flexible sensors and wearable electronics.


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  • Figure 1

    Macroscopic assemblies of carbon materials for flexible sensors and their applications. The insets are reprinted from the following sources. 1D fibers: reprinted with permission from Ref. [74], Copyright 2004, the American Association for the Advancement of Science; reprinted with permission from Ref. [75], Copyright 2016, Wiley-VCH. 2D films: reprinted with permission from Ref. [76], Copyright 2014, Wiley-VCH; reprinted with permission from Ref. [68], Copyright 2016, Wiley-VCH. 3D monoliths: reprinted with permission from Ref. [77], Copyright 2010, Wiley-VCH; reprinted with permission from Ref. [78], Copyright 2013, Wiley-VCH. Strain sensors: reprinted with permission from Ref. [11], Copyright 2011, Nature Publishing Group; reprinted with permission from Ref. [73], Copyright 2015, Wiley-VCH. Pressure sensors: reprinted with permission from Ref. [79], Copyright 2014, Wiley-VCH; reprinted with permission from Ref. [18], Copyright 2016, Wiley-VCH. Temperature sensors: reprinted with permission from Ref. [80], Copyright 2016, Wiley-VCH; reprinted with permission from Ref. [81], Copyright 2015, the American Chemical Society. Humidity sensors: reprinted with permission from Ref. [82], Copyright 2015, Wiley-VCH; reprinted with permission from Ref. [23], Copyright 2015, Wiley-VCH.

  • Figure 2

    Fabrication processes and characterizations of CNT fibers. (a) Schematic illustration showing the wet spinning of CNT fibers. (b) CNT fibers with high flexibility. Reprinted with permission from Ref. [169]. Copyright 2000, the American Association for the Advancement of Science. (c) Photograph of a CNT solution. (d) Scanning electron microscope (SEM) image of CNT fibers with highly aligned CNTs. (e) Continuous CNT fibers collected by a winding drum. Reprinted with permission from Ref. [171]. Copyright 2013, the American Association for the Advancement of Science. (f) A CNT fiber pulled out from an array. Reprinted with permission from Ref. [172]. Copyright 2002, Nature Publishing Group. (g) SEM images of CNT fibers drawn from a CNT array with twisting. Reprinted with permission from Ref. [173]. Copyright 2004, the American Association for the Advancement of Science. (h) Schematic of direct spinning CNT fibers from CVD furnaces. Reprinted with permission from Ref [74]. Copyright 2004, the American Association for the Advancement of Science. (i) A continuous CNT fibers obtained by direct spinning from CNT aerogels. Reprinted with permission from Ref. [174]. Copyright 2010, Wiley-VCH. (j) Post-treatment of CNT fibers to obtain high-performance fibers. Reprinted with permission from Ref. [175]. Copyright 2014, Nature Publishing Group.

  • Figure 3

    Fabrication and morphology of graphene fibers. (a) Schematic diagram showing the process to prepare GO fibers by wet-spinning methods and an image showing the continuous GO fiber wound around a ceramic reel. Reprinted with permission from Ref. [179]. Copyright 2013, Wiley-VCH. (b) SEM images of graphene fibers and fibers woven with cotton threads. Reprinted with permission from Ref. [180]. Copyright 2011, Nature Publishing Group. (c) Photograph and SEM images of graphene microtubings obtained by a confined hydrothermal technique. Reprinted with permission from Ref. [181]. Copyright 2012, the American Chemical Society. (d) Photograph of a free-standing GO film and SEM images of a GO film twisting for fibers. Reprinted with permission from Ref. [182]. Copyright 2014, the American Chemical Society. (e) Photograph and SEM images of a graphene fiber produced by CVD. Reprinted with permission from Ref. [184]. Copyright 2015, Wiley-VCH.

  • Figure 4

    CNT fibers for flexible sensors. (a) Fabrication process of strain sensors by putting CNT yarns drawn from an array on pre-stretched substrates. (b) Relative change of resistance as a function of strains for three types of strain sensors based on CNT fibers. The sensor with a pre-stretched substrate shows an extreme stretchability. (c) The strain sensor integrated into a glove used for monitoring finger motions. (a–c) Reprinted with permission from Ref. [96]. Copyright 2015, the American Chemical Society. (d) Schematic illustration showing the fabrication process of a sheath-core fiber and the hierarchical buckling shown in the SEM image. (e) Sheath-core CNT fibers for strain sensors. (d, e) Reprinted with permission from Ref. [108]. Copyright 2015, the American Association for the Advancement of Science. (f) The composite fibers composed of CNT sheets and elastic fibers woven into a grove. Reprinted with permission from Ref. [97]. Copyright 2016, the American Chemical Society. (g) CNT fibers for fabrication of fabrics to detect pressure (h) and humidity (i). (g–i) Reprinted with permission from Ref. [155]. Copyright 2015, Wiley-VCH. (j) Preparation of a hydrophilic CNT fiber for detecting humidity. Reprinted with permission from Ref. [168]. Copyright 2015, Wiley-VCH.

  • Figure 5

    Graphene fibers for flexible sensors. (a) Graphene fibers prepared by CVD combined with PVA for strain sensors. Reprinted with permission from Ref. [38]. Copyright 2015, American Chemical Society. (b) SEM images of a helical graphene fiber derived from GO films. (c) Performance of a graphene fiber-based temperature sensor. Adapted with permission from Ref. [189]. Copyright 2016, Royal Society of Chemistry. (d) An asymmetric graphene/GO fiber prepared by laser reduction of a GO fiber for humidity sensors. Reprinted with permission from Ref. [167]. Copyright 2013, Wiley-VCH. (e) Schematic and characterizations of a double-helix graphene fiber made of a graphene fiber and a graphene fiber coated a layer of carbon nitride. (f) A double-helix graphene fiber for sensing temperature, pressure and humidity. Reprinted with permission from Ref. [190]. Copyright 2015, Wiley-VCH.

  • Figure 6

    Flexible fibers containing carbon materials for flexible sensors. (a) Schematic illustration showing the fabrication process of a self-powered strain sensor made of an elastic fiber and two cotton threads coated with CNTs and PTFE/CNTs, respectively. (b) Output currents as a function of strains. Reprinted with permission from Ref. [191]. Copyright 2015, Wiley-VCH. (c) Schematic diagram showing the preparation of graphene-based sensors using the polymer fibers as the templates. (d) Graphene-based sensors for sensing low strains. (e) Applications in body motions and phonation. Reprinted with permission from Ref. [192]. Copyright 2015, Wiley-VCH. (f) Fabrication process of a strain sensor with core-sheath structured silk/graphite fibers. (g) Strain sensor with a GF of 14.5 within 15%. (h) Detection of finger motions. Reprinted with permission from Ref. [194]. Copyright 2016, the American Chemical Society.

  • Figure 7

    Fabrication and morphology of CNT films. (a) Transparent CNT films prepared by a vacuum filtration process. Reprinted with permission from Ref. [197]. Copyright 2004, the American Association for the Advancement of Science. (b, c) Schematic illustration (b) and SEM image (c) of CNT films prepared by a rod coating method. Reprinted with permission from Ref. [198]. Copyright 2009, the American Chemical Society. (d) CNT films prepared through a spin-coating process and the atomic force microscope (AFM) image. Reprinted with permission from Ref. [199]. Copyright 2009, the American Chemical Society. (e) CNT film prepared by a spray-coating method. Reprinted with permission from Ref. [200]. Copyright 2009, Wiley-VCH. (f) A free-standing CNT film directly prepared through CVD and its SEM image showing the alignment of CNT bundles. Reprinted with permission from Ref. [206]. Copyright 2007, American Chemical Society. (g) Photograph of the continuous CNT film drawn out from a CNT array. Reprinted with permission from Ref. [207]. Copyright 2005, the American Association for the Advancement of Science.

  • Figure 8

    Fabrication and morphology of graphene (or GO) and hybrid films. (a) Morphology and structure of GO films prepared by a vacuum filtration process. Reprinted with permission from Ref. [212]. Copyright 2007, Nature Publishing Group. (b) Schematic illustration of a dip-coating process of a GO dispersion for films. Reprinted with permission from Ref. [213]. Copyright 2014, the Royal Society of Chemistry. (c) SEM image and photograph of graphene films prepared by a casting process. Reprinted with permission from Ref. [76]. Copyright 2014, Wiley-VCH. (d) Schematic illustration showing the preparation of graphene films by a spray-coating method. Reprinted with permission from Ref. [214]. Copyright 2012, the American Chemical Society. (e) Schematic diagram to assemble graphene films at the liquid/air interface and the optical and SEM image of the obtained graphene film. Reprinted with permission from Ref. [215]. Copyright 2015, Wiley-VCH. (f) Photograph of a graphene woven fabric prepared by CVD. Reprinted with permission from Ref. [216]. Copyright 2012, Nature Publishing Group. (g) Schematic and SEM image of graphene/CNT hybrid films. Reprinted with permission from Ref. [217]. Copyright 2015, Wiley-VCH.

  • Figure 9

    Flexible sensors based on CNT films. (a‒d) Fabrication process of SWNT film-based strain sensors (a) and SEM image of fractural morphology at 100% strain (b). Flexible strain sensor attached to human knee joint (c) for detecting the motions (d). Reprinted with permission from Ref. [11]. Copyright 2011, Nature Publishing Group. (e) Schematic illustration of the flexible strain sensors made of hybrid films containing poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and SWNT. (f) Schematic diagram of the multifunctional sensors based on porous PDMS and SWNT films and the sensors for distinguishing pressure and bending. Reprinted with permission from Ref. [106]. Copyright 2014, Wiley-VCH. (g) Flexible sensor mounted on human body for monitoring emotional expressions. (e, g) Reprinted with permission from Ref. [109]. Copyright 2015, the American Chemical Society.

  • Figure 10

    Graphene and CNT/graphene films for flexible sensors. (a) Relative change of resistance of graphene films as a function of strains. Reprinted with permission from Ref. [112]. Copyright 2013, Elsevier. (b) Schematic illustration showing the fabrication of flexible strain sensor using a graphene woven fabric. (c) Flexible sensors made of graphene fabrics for detecting hand motions and emotional expressions. Reprinted with permission from Ref. [13]. Copyright 2014, Wiley-VCH. (d) Schematic illustration of the fabrication of high-performance strain sensor using a fish-scale-like graphene and the SEM image of graphene films. (e) Relative resistance change within 80% strain. Reprinted with permission from Ref. [236]. Copyright 2016, the American Chemical Society. (f) SEM image of the hierarchically structured graphene films. (g) Schematic illustration of the highly sensitive pressure sensor made of the microstructured graphene. Reprinted with permission from Ref. [18]. Copyright 2016, Wiley-VCH. (h) Schematic illustration of the flexible pressure sensors made of CNT/graphene hybrid films and microstructured PDMS substrates. (i) SEM image of the CNT/graphene hybrid film on the microstructured PDMS substrate. (j) Relative current change within 6 kPa. (k) Sensors used to detect the small water drops. Reprinted with permission from Ref. [130]. Copyright 2017, Wiley-VCH.

  • Figure 11

    Carbon materials hybridized with polymers or fabrics for flexible sensors. (a) Graphene coated non-woven fabrics for flexible sensors. Reprinted with permission from Ref. [113]. Copyright 2016, the Royal Society of Chemistry. (b) Strain sensors made of carbon black-PDMS for sensing deformations of human skin. Reprinted with permission from Ref. [240]. Copyright 2012, Wiley-VCH. (c) Carbon black-decorated fabrics for measurement of blood pressure. Reprinted with permission from Ref. [67]. Copyright 2016, Wiley-VCH.

  • Figure 12

    Carbon materials derived from bio-materials for flexible sensors. (a) Schematic diagram for the preparation of strain sensors based on carbonized silk fabrics. (b) Strain sensor attached on human wrist for monitoring the bending and rotation of a wrist. Reprinted with permission from Ref. [68]. Copyright 2016, Wiley-VCH. (c) Highly flexible and sensitive strain sensors made of carbonized cotton fabrics. (d) Relative changes of resistance as a function of joint motion and finger bending. Reprinted with permission from Ref. [69]. Copyright 2016, Wiley-VCH. (e) Schematic illustration to fabricate pressure sensor made of carbonized silk nanofibers and flexible substrate. (f) Current changes as a function of finger motions when picking grapes. (g) A pressure sensor matrix for pressure distribution mapping. Reprinted with permission from Ref. [241]. Copyright 2017, Wiley-VCH.

  • Figure 13

    The application of carbon film-based flexible sensors in monitoring physiological and environmental signals. (a‒c) Schematic illustration showing the structure of flexible pressure sensor made of thin SWNT films and microstructured substrates (a), measurement of phonation (b) and pulse (c) using the pressure sensors. Reprinted with permission from Ref. [79]. Copyright 2014, Wiley-VCH. (d) Fabrication process of the conductive dry adhesive based on composites containing PDMS, CNTs and graphene. (e) Comparison of the ECG signals obtained by the conductive dry adhesives and commercial adhesives. (f) Good stability of ECG signals during movement states. Reprinted with permission from Ref. [72]. Copyright 2016, the American Chemical Society. (g) Highly sensitive strain sensors made of carbonized silk fabrics for measuring respiration in relaxation and after exercise. Reprinted with permission from Ref. [68]. Copyright 2016, Wiley-VCH. (h) Self-powered respiratory monitor based on GO films. Reprinted with permission from Ref. [23]. Copyright 2015, Wiley-VCH. (i) Stretchable temperature sensors made of crumpled graphene films. Reprinted with permission from Ref. [81]. Copyright 2015, the American Chemical Society. (j) Schematic illustration showing the integration of temperature sensors, humidity sensors and pressure sensors based on graphene and GO films. (k) Sensor arrays for monitoring humidity, temperature and pressure. Reprinted with permission from Ref. [94]. Copyright 2016, Wiley-VCH.

  • Figure 14

    Fabrication and morphology of CNT monoliths. (a) Photograph and SEM image of CNT/PVA aerogels prepared by a critical-point-drying process. Reprinted with permission from Ref. [244]. Copyright 2007, Wiley-VCH. (b) Morphology of CNT foams using ice as the template. Reprinted with permission from Ref. [245]. Copyright 2007, the American Chemical Society. (c) Photograph of a PU sponge before and after coated with CNTs. (d) SEM image of a sponge coated with CNTs. Reprinted with permission from Ref. [246]. Copyright 2012, the Royal Society of Chemistry. (e) Optical and SEM images of a SWNT array with a height of 2.5 mm. Reprinted with permission from Ref. [247]. Copyright 2004, the American Association for the Advancement of Science. (f) Schematic illustration and SEM image of a high CNT array. Reprinted with permission from Ref. [248]. Copyright 2016, the American Chemical Society. (g) Photographs and SEM image of CNT sponges synthesized by CVD. Reprinted with permission from Ref. [77]. Copyright 2010, Wiley-VCH.

  • Figure 15

    Fabrication and structure characterizations of graphene monoliths. (a) Optical and SEM images of graphene hydrogels through a hydrothermal process. Reprinted with permission from Ref. [251]. Copyright 2010, the American Chemical Society. (b) Schematic illustration to prepare graphene aerogels. Reprinted with permission from Ref. [252]. Copyright 2013, Wiley-VCH. (c) Illustration and SEM image showing the formation mechanism and the structure of graphene monoliths through a freeze-drying method. Reprinted with permission from Ref. [254]. Copyright 2012, Nature Publishing Group. (d, e) Schematic diagram (d) and SEM images (e) of graphene/sponge composites using a sponge as the template. Reprinted with permission from Ref. [255]. Copyright 2014, Wiley-VCH. (f) Preparation process and photograph of graphene foams using Ni foams as the template. Reprinted with permission from Ref. [256]. Copyright 2011, Nature Publishing Group. (g) Schematic representation for preparation of graphene aerogels by unzipping CNT sponges. Reprinted with permission from Ref. [257]. Copyright 2014, Wiley-VCH.

  • Figure 16

    CNT and graphene monoliths for flexible sensors. (a) Flexible sensors made of two intersecting CNT-PU sponges for omnidirectionally measuring bending. (b) Detection of pressure based on the triboelectric effect. Reprinted with permission from Ref. [265]. Copyright 2017, Wiley-VCH. (c) SEM image of graphene monoliths through a freeze-drying method. (d) High-frequency response of graphene monoliths under the frequency of 500 Hz. Reprinted with permission from Ref. [266]. Copyright 2016, Wiley-VCH. (e) Photograph of graphene/polyimide (PI) composites with a high flexibility and the composites based pressure sensors with a good performance. Reprinted with permission from Ref. [102]. Copyright 2015, the American Chemical Society. (f) Optical image of the graphene/PU sponge and current-voltage curves of the sponge under different pressures. Reprinted with permission from Ref. [267]. Copyright 2013, Wiley-VCH. (g) Optical and SEM image of the composites containing graphene sheets and the lightly cross-linked polymer. (h) Relative change of resistance as a function of strain. (i, j) Applications of the flexible sensor for measuring breathing and pulse (i) and the footsteps of a walking spider (j). Reprinted with permission from Ref. [264]. Copyright 2016, the American Association for the Advancement of Science.

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