Flexible electronics, which can be distributed on any surface we need, are highly demanded in the development of Internet of Things (IoT), robot technology and electronic skins. Temperature is a fundamental physical parameter, and it is an important indicator in many applications. Therefore, a flexible temperature sensor is required. Here, we report a simple method to fabricate three lightweight, low-cost and flexible temperature sensors, whose sensitive materials are reduced graphene oxide (r-GO), single-walled carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs). By comparing linearity, sensitive and repeatability, we found that the r-GO temperature sensor had the most balanced performance. Furthermore, the r-GO temperature sensor showed good mechanical properties and it could be bent in different angles with negligible resistance change. In addition, the performance of the r-GO temperature sensor remained stable under different kinds of pressure and was unaffected by surrounding environments, like humidity or other gases, because of the insulating layer on its sensitive layer. The easy-fabricated process and economy, together with the remarkable performance of the r-GO temperature sensor, suggest that it is suitable for use as a robot skin or used in the environment of IoT.

We propose a flexible wireless pressure sensor, which uses a graphene/polydimethylsiloxane (GR/PDMS) sponge as the dielectric layer. The sponge is sandwiched between two surfaces of a folded flexible printed circuit with patterned Cu as the antenna and electrode. By adjusting graphene and NH₄HCO₃ concentrations, a composite with 20% concentration of NH₄HCO₃ and 2% concentration of graphene as the dielectric layer is obtained, which exhibits high sensitivity (2.2 MHz/kPa), wide operating range (0–500 kPa), rapid response time (~7 ms), low detection limit (5 Pa), and good stability, recoverability, and repeatability. In addition, the sensor is sensitive to finger bending and facial muscle movements for smile and frown, that are transmitted using wireless electromagnetic coupling; therefore, it has potential for a wide range of applications such as intelligent robots, bionic-electronic skin and wearable electronic devices.

In this paper, we demonstrate a passive wireless surface temperature sensor, based on a high-temperature co-fired ceramic process and a thick-film technology, which is functional up to 1400 °C. In addition, we propose a temperature measurement technique that makes use of the absolute amplitude of the S ₁₁ parameter. The sensor comprises an inductor (created from platinum) deposited on a ceramic substrate (99% alumina), making it cheap and easy to fabricate using a screen-printing technology, due to its simple structure. The use of temperature resistant materials, and a completely passive structure, means that this sensor is robust to harsh environments. The device operates using the principle of $LC$ resonance, illustrated in this paper with a lumped circuit model. The dielectric constant of the substrate increases with increasing temperature, leading to a monotonic variation in the resonant frequency of the sensor, which is retrieved wirelessly via a readout antenna. We set the test distance between the sensor and the antenna to 10 mm and observed a repeatable response between 26 °C and 1400 °C. S ₁₁ amplitude and resonant frequency are used to reflect the change temperature, respectively. And we found that it is more accurate to test the temperature by resonant frequency after comparing the experimental results. The average sensitivity of temperature test by resonance frequency is obtained, which is 14.3 kHz/°C.

This paper presents a passive wireless sensor for simultaneously and remotely measuring pressure and temperature under harsh environments. The sensor consists of a dual LC (inductor and capacitor) resonant circuit, one without a cavity and the other with a cavity capacitor for temperature and pressure sensing, respectively. The low-temperature co-fired ceramic technology is used to fabricate the sensor, making it suitable for high-temperature harsh environment operations. Experimental results show the prototype sensor has temperature sensitivity of 8.15 kHz/°C and pressure sensitivity of 1.96 MHz/Bar up to 400°C.