Recent Advances in Flexible Temperature Sensors: Materials, Mechanism, Fabrication, and Applications

Abstract

Flexible electronics is an emerging and cutting-edge technology which is considered as the building blocks of the next generation micro-nano electronics. Flexible electronics integrate both active and passive functions in devices, driving rapid developments in healthcare, the Internet of Things (IoT), and industrial fields. Among them, flexible temperature sensors, which can be directly attached to human skin or curved surfaces of objects for continuous and stable temperature measurement, have attracted much attention for applications in disease prediction, health monitoring, robotic signal sensing, and curved surface temperature measurement. Preparing flexible temperature sensors with high sensitivity, fast response, wide temperature measurement interval, high flexibility, stretchability, low cost, high reliability, and stability has become a research target. This article reviewed the latest development of flexible temperature sensors and mainly discusses the sensitive materials, working mechanism, preparation process, and the applications of flexible temperature sensors. Finally, conclusions based on the latest developments, and the challenges and prospects for research in this field are presented.

Keywords: application; flexible temperature sensors; mechanical and electrical properties; preparation process; temperature‐sensitive materials; working mechanism.

Figure 1

Brief timeline of the development of the performance and manufacturing methods of flexible temperature sensors. Reproduced with permission.^{[ 35} , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ⁴⁴ , ⁴⁵ , ⁴⁶ , ^{47 ]} Copyright 2007, Elsevier B.V. All rights reserved; Copyright 2012, Tsinghua University Press and Springer; Copyright 2013, Wiley‐VCH; Copyright 2022, American Chemical Society; Copyright 2022, IOP Publishing Ltd; Copyright 2010, Wiley‐VCH; Copyright 2019, Wiley‐VCH; Copyright 2022, Wiley‐VCH.

Figure 2

Structure, electrical properties and stretchability of CNTs modified temperature sensors by a) sericin, Reproduced with permission.^{[ 57 ]} Copyright 2021, Wiley‐VCH, b) carbon black, Reproduced with permission.^{[ 54 ]} Copyright 2020, American Chemical Society, and c) AgNPs. Reproduced with permission.^{[ 69 ]} Copyright 2022, The Royal Society of Chemistry.

Figure 3

Flexible temperature sensor with conductive polymer composite as active material, sensor of left side without substrate and right side with substrate, a) a paper‐based composite sensor composed of carboxylic carbon nanotubes (CCNTs) and poly‐m‐phenyleneisophthalamide (PMIA), Reproduced with permission.^{[ 58 ]} Copyright 2021, American Chemical Society; b) Silver nanoparticles are dispersed in PDMS, Reproduced with permission.^{[ 106 ]} Copyright 2017, American Chemical Society; c) carbon nanotubes (CNTs) and carbon black (CB) are integrated into a poly(vinyl alcohol)/glycerol (PVA/Gly) organohydrogel, Reproduced with permission.^{[ 54 ]} Copyright 2020, American Chemical Society; d) PEDOT:PSS/PANI on the PI substrate, Reproduced with permission.^{[ 93 ]} Copyright 2022, American Chemical Society; e) an R‐GO/P(VDF‐TrFE) nanocomposite channel as a sensing layer, Reproduced with permission.^{[ 89 ]} Copyright 2014, Wiley‐VCH; f) a photoactive silk sericin and PEDOT:PSS on the PI, Reproduced with permission.^{[ 92 ]} Copyright 2020, American Chemical Society.

Figure 4

a,b) the structure and electrical properties of NiO/Ni flexible temperature sensors, Reproduced with permission.^{[ 35} , ^{67 ]} Copyright 2019, Wiley‐VCH, c) Atomic structure and SEM images of the Ni and NiO framework, Reproduced with permission.^{[ 67 ]} Copyright 2021, AIP Publishing LLC.

Figure 5

a) Resistance drift rate and B value of Mn‐Co‐Ni‐O flexible temperature sensors after bending for different times, Reproduced with permission.^{[ 112 ]} Copyright 2023, Wiley‐VCH. b) Stability and reliability testing of Sr‐doped SmMnO₃ flexible temperature sensors, Reproduced with permission.^{[ 36 ]} Copyright 2020, American Chemical Society.

Figure 6

High Performance Metal Sulfide Temperature Sensor Array, a) Ag₂S, Reproduced with permission.^{[ 117 ]} Copyright 2022, Wiley‐VCH, b) MoS₂, Reproduced with permission.^{[ 37 ]} Copyright 2022, American Chemical Society. c) Temperature measurement ranges and sensitivity for various materials.

Figure 7

Classification of flexible substrate materials, Reproduced with permission.^{[ 24 ]} Copyright 2019, Wiley‐VCH.

Figure 8

Schematic illustrations of common polymer substrates, including PET, Reproduced with permission.^{[ 67} , ^{115 ]} Copyright 2021, AIP Publishing, Reproduced with permission. Copyright 2018, American Chemical Society, PI, Reproduced with permission.^{[ 126 ]} Copyright 2021, American Chemical Society, PU, Reproduced with permission.^{[ 131 ]} Copyright 2021, American Chemical Society, PES, Reproduced with permission.^{[ 133 ]} Copyright 2023, MDPI, and PC, Reproduced with permission.^{[ 134 ]} Copyright 2023, Royal Society of Chemistry.

Figure 9

Application of PDMS in flexible/wearable devices. a) The structure of PDMS, fabrication process of the b,c) BTO ceramic array, Reproduced with permission.^{[ 116 ]} Copyright 2019, Wiley‐VCH, and d) CNC‐CNT buckypaper^{[ 56 ]} based on PDMS substrate and encapsulated layer, Reproduced with permission. Copyright 2021, Elsevier B.V.

Figure 10

Properties and applications of hydrogels. a) Optical pattern^{[ 139 ]} and b) scanning electron microscope microstructure^{[ 145 ]} of hydrogels, Reproduced with permission. Copyright 2021, The Royal Society of Chemistry and Copyright 2022, Springer Nature. Hydrogel with excellent c) stretchability^{[ 150 ]} and d) adhesion properties, Reproduced with permission.^{[ 143 ]} Copyright 2021, Wiley‐VCH and Copyright 2021, Elsevier Ltd. e,f) Temperature sensing properties of hydrogels, Reproduced with permission.^{[ 145} , ^{146 ]} Copyright 2022, Springer Nature and Copyright 2022, American Chemical Society.

Figure 11

a) Preparations of cellulose films. Reproduced with permission.^{[ 152 ]} Copyright 2020, American Chemical Society. b) Photographs of cellulose paper‐based flexible devices.^{[ 154 ]} Copyright 2016, American Chemical Society. c) Manufacturing process and properties of CNC/PEDOT:PSS/PVA hydrogels,^{[ 156 ]} Copyright 2023, American Chemical Society.

Figure 12

Four types of temperature sensing mechanisms.

Figure 13

Modification of silver nanocrystals by different ligands (a. inorange ligands, b. orange ligands) leads to a transformation of the transport behavior, and their stability c,d). Reproduced with permission.^{[ 110 ]} Copyright 2017, Wiley‐VCH.

Figure 14

A highly stretchable strain‐insensitive temperature sensor exploits the Seebeck effect, which can detect temperature distribution across the surface of thing. Reproduced with permission.^{[ 161 ]}Copyright 2019, The Royal Society of Chemistry.

Figure 15

Pyroelectric properties of a single Ag/BTO/Ag sensing device. Reproduced with permission.^{[ 116 ]} Copyright 2019, Wiley‐VCH.

Figure 16

a) In situ TEM tests the changes of graphite flakes under different bending radius, Reproduced with permission.^{[ 176 ]} Copyright 2019, Elsevier Ltd. b) In situ KPFM reveals the changes of crack surface potential of a‐C film under different tensile strains.^{[ 175 ]} c) In situ CLSM tests the effect of strain on cracks and wrinkles in a‐C film, Reproduced with permission.^{[ 175 ]} Copyright 2022, Elsevier B.V.

Figure 17

a) Schematic diagram of photolithography, Reproduced with permission.^{[ 186 ]} Copyright 2022, MDPI. b) Flow chart of the photolithography process to fabricate the patterned Ag NFs electrode and to package as the single electrode triboelectric nanogenerator (TENG) sensor array device,^{[ 182 ]} Copyright 2021, MDPI. c) Two working principles of nanoimprinting lithography,^{[ 187 ]} d) 3D structures prepared based on roll‐to‐roll nanoimprinting. Reproduced with permission.^{[ 185 ]} Copyright 2013, The Royal Society of Chemistry. e) Laser direct writing prepared Ni‐NiO‐Ni temperature sensors. Reproduced with permission.^{[ 35 ]} Copyright 2019, Wiley‐VCH.

Figure 18

Four processes for preparing flexible temperature sensors, a) spin‐coating, Reproduced with permission.^{[ 188 ]} Copyright 2023, Elsevier B.V., b) roll‐to‐roll,^{[ 196 ]} Copyright 2019, IEEE., c)screen printing, Reproduced with permission.^{[ 189 ]} Copyright 2021, Elsevier Ltd., and d) inkjet printing, Reproduced with permission.^{[ 190 ]} Copyright 2023, IOP Publishing Ltd.

Figure 19

Spin‐coating metal oxide paste to prepare temperature sensors. a) The process diagram of spin‐coating metal oxide pastes and screen‐printing silver electrodes, b) SEM, and c) the lnρ−1000/T curve of ceramic thick films formed by spin‐coated high estimated content ceramic pastes. Reproduced with permission.^{[ 191 ]} Copyright 2021, Elsevier B.V. d) The glass phase fills in the interstices of the ceramic thick film to form a dense ceramic. Reproduced with permission.^{[ 192 ]} Copyright 2022, Elsevier B.V.

Figure 20

Morphologies of EHD‐printed dots with a–c)the same size (10 µm) and different viscosities, d–f) the same viscosities (10.39 cPs) and different size. Plot depicting the correlation of droplet morphology with solution viscosity and droplet size. Reproduced with permission.^{[ 202 ]} Copyright 2022, American Chemical Society.

Figure 21

3D printed bimodal breathable electronic skin. Reproduced with permission.^{[ 205 ]} Copyright 2022, American Chemical Society.

Figure 22

Chemical vapor deposition schematic and applications. a) Preparing PEDOT/fabric composite by CVD. Reproduced with permission.^{[ 206 ]} Copyright 2022, Elsevier Ltd. b) Structure of molybdenum disulphide flexible temperature sensors. Reproduced with permission.^{[ 37 ]} Copyright 2022, American Chemical Society. c) Transmittance as a function of wavelength for trilayer graphene on glass with an SEM image of graphene‐coated fibers. Reproduced with permission.^{[ 209 ]} Copyright 2020, American Chemical Society.

Figure 23

a) Body‐temperature detection. Reproduced with permission.^{[ 35 ]} Copyright 2019, Wiley‐VCH. b) Breathing rate and blowing detection. Reproduced with permission.^{[ 62 ]} Copyright 2021, Elsevier Ltd. All rights reserved. c) Health monitoring and disease prevention. Reproduced with permission.^{[ 38 ]} Copyright 2022, IOP. d) High throughput wireless body temperature monitoring system. Reproduced with permission.^{[ 61 ]} Copyright 2022, Wiley‐VCH.

Figure 24

a) Wearable device with mechanical dexterous hands for large‐area temperature detection. Reproduced with permission.^{[ 38 ]} Copyright 2022, IOP. b) Robotic fingers for determining the direction of liquid flow. Reproduced with permission.^{[ 35 ]} Copyright 2019, Wiley‐VCH. c) Detecting temperature distribution across the surface of a soft robotic arm,^{[ 161 ]} Copyright 2019, The Royal Society of Chemistry. d) Applications for the flexible temperature sensors in temperature monitoring and warning system. Reproduced with permission.^{[ 210 ]} Copyright 2024, Wiley‐VCH.