**1. Introduction**

A sensor is a device that responds to a chemical or physical stimulus by generating a signal that can be analyzed electronically. In general, to be suitable, sensors and devices incorporating them must have a fast response to external stimulus, be able to detect fine variations in the analyte, have a low recovery time, identify the desired analyte among other stimuli, and be a reliable tool, easy to operate, preferable also in *in situ* measurements [1]. In addition, it is desirable to have a low cost and environmental compatibility. Although there are a large number of sensors proposed in the literature, continuous concerns for the improvement of sensors are justified by the need to improve existing systems as well as to find new promising alternatives that

allow the change of old analysis and measurement technologies. A widely investigated possibility in the last decades is the use of nanomaterials, a category that seems to offer an infinite practical field of solutions for the most diverse problems. Among the various nanomaterials, nanostructured carbonaceous materials show numerous advantages compared to other materials due to their specific properties as well as convenient costs, which make them suitable for use as technological sensors. Therefore, special attention will be paid in this chapter to recent advances in the field of resistive temperature sensors using such materials.

Temperature sensors are indispensable in various fields of engineering (electrical, automotive, aerospace, communications, civil engineering [2], health care, human machine interface, robotics, etc. [3]). Therefore, among the sensing technologies, the temperature sensor is probably the most widely used. The signal given by resistive temperature sensors consists of the increase or decrease of resistivity or capacitance with temperature [2]. The continuous improvement of the performance and manufacturing processes of sensors in general and of temperature in the present case, as well as the expansion of application fields, are current concerns of materials science and related disciplines.

Among the directions of perspective or current interest for thermal or multifunctional sensors that include temperature detection are flexible electronics [2] for intelligent house applications (e.g., electronic wallpaper, an interactive system that incorporates a network of sensor temperature and an air conditioning control system [4]), robotics (humanoid artificial skin [5, 6]), control of working conditions in mechanical or electronic systems, instruments wearables for physiological monitoring, smart packaging that indicates the state of freshness of food, etc. [6, 7]. In such applications, portable temperature sensors, flexible themselves, capable of detecting several signals simultaneously (temperature, pressure, and strain), the so-called multifunctional sensors (or physical sensors) are often required, or, on the contrary, the temperature signal should not be affected by the action of other factors, such as mechanical deformation (monofunctional sensors, in our case, temperature sensors) [7]. Some of the portable, flexible sensors proposed in recent years are PTC resistive sensors [6, 7].

A major obstacle to the practical realization of stretchable resistive temperature sensors from composite materials is the limited ability of the conductive phase to be stretchable during use, that is, to retain its sensitivity unchanged to repeated, largeamplitude deformations. Obviously, work must be done on the conductive phase, by finding conductive nanoparticles, capable of favorable interactions with the macromolecular chains of an optimally chosen elastomeric matrix. For this purpose, metal nanoparticles, carbon nanotubes (CNTs), graphene, as well as different preparation techniques, have been tested. The polymer substrates used for flexible printed sensors are mainly polymers containing aromatic rings in their structure, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI), which are available as films with large surface areas, at reasonable prices, allowing the obtaining of reasonable production costs [8].

Stretchable (elastic) temperature sensors are usable in a wide range of applications, such as wearable real-time health care devices, monitoring working conditions in mechanical or electronic devices, wearable instruments for physiological monitoring, and smart packaging. Their realization involves the use of matrices or elastic supports. Temperature sensors often need to accompany stretchable strain sensors in applications such as wearables for recording human physical movements, therapy devices, and health monitoring such as heart rate monitoring [2]. Extensible strain and temperature sensors are also required for measuring deformations on extensible and curved surfaces (deformation of the skin on the wrist, e.g.) or for measuring the stress developed on the curved surfaces of pressure vessels, often requiring that local temperature information to accompany the strain information.

The properties of conductive polymer composites (CPCs) as temperature sensors have been reviewed in various previous works [9–12].

This chapter is dedicated to the recent progress made in the field of temperature sensors with PTC properties and their applications, which tries to develop and complement the previous information, aiming also to give an overview of the subject. Although not the subject of this chapter, some notable examples of NTC sensors are also included.
