Wearable Temperature Sensors Based on Reduced Graphene Oxide Films

Abstract

With the development of medical technology and increasing demands of healthcare monitoring, wearable temperature sensors have gained widespread attention because of their portability, flexibility, and capability of conducting real-time and continuous signal detection. To achieve excellent thermal sensitivity, high linearity, and a fast response time, the materials of sensors should be chosen carefully. Thus, reduced graphene oxide (rGO) has become one of the most popular materials for temperature sensors due to its exceptional thermal conductivity and sensitive resistance changes in response to different temperatures. Moreover, by using the corresponding preparation methods, rGO can be easily combined with various substrates, which has led to it being extensively applied in the wearable field. This paper reviews the state-of-the-art advances in wearable temperature sensors based on rGO films and summarizes their sensing mechanisms, structure designs, functional material additions, manufacturing processes, and performances. Finally, the possible challenges and prospects of rGO-based wearable temperature sensors are briefly discussed.

Keywords: reduced graphene oxide; temperature sensing; wearable electronics.

Figure 1

(a) Schematic of three main reduction methods of GO. Reproduced with permission from Ref. [34]. Copyright 2020 Institute of Chemistry, Slovak Academy of Sciences. (b) XPS spectra of the initial and HNO₃-treated rGO materials. Reproduced with permission from Ref. [46]. Copyright Royal Society of Chemistry. (c) C1s region of the XPS spectra of the GO and rGO samples. Reproduced with permission from Ref. [44]. Copyright 2017 Elsevier B.V. (d) Raman spectra of GO and rGO including the D, G, 2D and D + D’ bands. Reproduced with permission from Ref. [44]. Copyright 2017 Elsevier B.V.

Figure 6

(a) Transparent and stretchable integrated platform of temperature and strain sensors responding simultaneously to the temperature of human skin and to muscle movement. Reproduced with permission from Ref. [79]. Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Monitoring the temperature of the human body during movement and activity. Reproduced with permission from Ref. [4]. Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Schematic illustration of the sensor array of multifunctional flexible sensors. Reproduced with permission from Ref. [49]. Copyright 2022 Tsinghua University Press. (d) Code lock application of the nine-channel flexible temperature sensor. Reproduced with permission from Ref. [51]. Copyright 2021 Elsevier Ltd. (e) Schematic illustrations of the PrGO temperature sensor array mounted on a human wrist and response of the sensor to the metal and water drop with different temperatures placed on the sensor array. Reproduced with permission from Ref. [19]. Copyright 2023 American Chemical Society. (f) The thumb touch test of the textile-infused array of 6 × 6 temperature sensors. Reproduced with permission from Ref. [80]. Copyright 2018 Elsevier B.V.

Figure 2

Assembly methods of rGO. (a) Schematic illustration of the preparation process of the Cotton/rGO/CNT composite. Reproduced with permission from Ref. [49]. Copyright 2022 Tsinghua University Press. (b) The spin coating method for sensing layer deposition. Reproduced with permission from Ref. [50]. Copyright 2022 by the authors. (c) Drop-coating of GO solution. Reproduced with permission from Ref. [51]. Copyright 2021 Elsevier Ltd. (d) Schematic representation of the fabrication of electrochemical sensors by inkjet printing. Reproduced with permission from Ref. [52]. Copyright 2017 Elsevier B.V. (e) Schematic illustration of electrochemical deposition method for preparing ERGO. Reproduced with permission from Ref. [53]. Copyright Royal Society of Chemistry. (f) Preparation process of rGO/PET and photos corresponding to pure PET, GO/PET, and rGO/PET. Reproduced with permission from Ref. [54]. Copyright 2018 Elsevier B.V.

Figure 3

Temperature sensors based on pure rGO. (a) Schematic demonstration for the fabrication of the rGO shell assembly on a nanofiber surface. Reproduced with permission from Ref. [65]. Copyright 2021 Elsevier Inc. (b) Schematic of the temperature sensor, normalized resistance variation of the temperature sensor during heating and cooling processes, and normalized temperature response of the sensor at different bending angles. Reproduced with permission from Ref. [51]. Copyright 2021 Elsevier Ltd. (c) Schematic diagram of the temperature sensor structure and fabrication process of the skin-attachable temperature sensor. Reproduced with permission from Ref. [66]. Copyright 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Schematic showing the creation of rGOf through the direct laser writing reduction process of the self-assembled GO sheets at the liquid surface. The resultant rGOf is a free-floating fabric sheet, which can be either self-standing or transfer-printed onto any substrate, whether it is rigid or flexible. Reproduced with permission from Ref. [42]. Copyright 2022 WILEY-VCH GmbH. (e) Relative resistance variations as a function of temperature for different laser powers of laser writing for LIG-based interdigital electrode. Reproduced with permission from Ref. [29]. Copyright 2021 Elsevier B.V.

Figure 4

Temperature sensors based on rGO mixed with other materials. (a) The schematic of the fabrication process for PEDOT:PSS/rGO temperature sensor. Reproduced with permission from Ref. [69]. Copyright 2022 the author(s), under exclusive license to Spring Science Business Media, LLC, part of Spring Nature. (b) Schematic illustration of the multifunctional flexible sensor. Reproduced with permission from Ref. [49]. Copyright 2022 Tsinghua University Press. (c) rGO-Pd temperature sensor fabrication and the proposed sensor’s temperature performance. Reproduced with permission from Ref. [20]. Copyright 2021 Elsevier B.V. (d) Schema of the fabrication process for rGO/CNTs@PBT MB temperature sensor and the SEM images. Reproduced with permission from Ref. [70]. Copyright 2022 Elsevier B.V. (e) Schematic illustration of the preparation process and the SEM images of the sensitive layer (sprayed composite of CB and graphene) on the paper substrate. Reproduced with permission from Ref. [16]. Copyright 2019 American Chemical Society.

Figure 5

(a) Sensor application test about breathing rate monitoring and human blowing detection. Reproduced with permission from Ref. [51]. Copyright 2021 Elsevier Ltd. (b) Breathing rate monitoring inside a gas mask. Reproduced with permission from Ref. [66]. Copyright 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) A demonstration system realizing the non-contact, smart, and real-time monitoring of human respiration signals. Reproduced with permission from Ref. [49]. Copyright 2022 Tsinghua University Press. (d) rGO temperature sensor with buffer layer of polyimide film. Reproduced with permission from Ref. [75]. Copyright 2021 Elsevier B.V. (e) Temperature response of the device worn on a human finger and the response current of the device to the temperature of skin touching a hot coffee cup and a cold coffee cup. Reproduced with permission from Ref. [4]. Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (f) Proximity detection experiment. (A color version of this figure can be viewed online. Reproduced with permission from Ref. [51]. Copyright 2021 Elsevier Ltd. (g) Finger touch detection. Reproduced with permission from Ref. [66]. Copyright 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.