**3.2 Sensors containing a conductive composite with thermoset matrix**

Han et al. [2] reported the obtaining of a flexible, high mechanical strength GnPs/ epoxy resin composite film exhibiting an electrical conduction percolation threshold of 1.08% (vol) GnPs. Compared to the neat polymer, the composite shows improved mechanical properties (Young's modulus +1344%, tensile strength +66.7%, and good electrical response to bending or twisting up to 180<sup>∘</sup> ). The percolation threshold value calculated for the film of this composite (1.08% by vol.) corresponds to the formation of a global network of connected GnPs inside the epoxy matrix, which ensures the mobility of electrons in the matrix and, implicitly, the insulator/(semi)conductor transformation of the material. After overcoming the percolation threshold, the electrical conductivity of the material increases linearly and steadily with increasing GnPs content due to the creation of numerous conductive paths through the composite. Thus, at a content of 10% (vol.) GnPs, the composite exhibits an electrical conductivity of 0.01 S/cm, which corresponds to an increase of 11 orders of magnitude compared to the initial resin. The film acts exclusively as a temperature sensor at T > 20 <sup>∘</sup> C, having a linear and stable resistive response in the range of 20–110 <sup>∘</sup> C; the temperature coefficient of resistivity is 0.0063 <sup>∘</sup> C<sup>1</sup> , higher than the standard Pt-based temperature sensor (0.0039 <sup>∘</sup> C<sup>1</sup> [7]).

#### **3.3 Sensors obtained through additive manufacturing technologies**

Additive manufacturing (AM) technologies have completely changed the approach to R&D and production problems in various fields, such as electronics, aerospace technologies, biomedical applications, wearable technologies, and automotive industry. AM enables the faster translation of projects into marketable industrial

products starting from the conceptualization of a three-dimensional (3D) model and reaching the manufacture of printed landmarks. Since AM technologies do not require tools and molds for making landmarks, as is the case for classical technologies, it follows that the application of these technologies can lead to significant savings in materials, time and labor, as well as an increase in the quality and reproducibility of production [16]. Different additive manufacturing techniques such as DIW (direct ink writing), DLP (direct light projection), FFF (fused filament fabrication) or FDM (filament deposition modeling), SLA (stereolithography), and SLS (select laser sintering) are discussed briefly by [16].

A stretchable temperature sensor based on GnPs/PDMS composite, insensitive to mechanical deformation, made by an additive manufacturing technology (3D printing) was reported in the reference [7]. The sensor exhibits a high sensitivity in temperature detection, being characterized by a temperature coefficient of resistance value of 0.0080 <sup>∘</sup> C<sup>1</sup> , practically more than twice the TCR value of the standard Pt sensor.

#### **3.4 Sensors based on sandwich structures**

Sandwich structures also fall into the category of composites; in this case, the functionality of the material is given by the layers of overlapping materials. Chen et al. [3] recently reported the realization of a flexible sandwich temperature sensor from laser-reduced graphene oxide (LrGO) deposited on a PET support (**Figure 6**). The critical parameters of the sensor fabrication process are the concentration of the aqueous GO solution and the distance between the laser scan lines. For GO reduction, the minimum laser power density must exceed the GO reduction threshold but not reach the level at which ablation of the LrGO layer and PET substrate occurs. A power of 6.5 W and a scanning speed of 2000 m/s were chosen for this purpose for the UV laser with λ = 355 nm. The flexible sensor can be bent on curved surfaces, thus enabling *in situ* temperature measurement, is applicable for monitoring human breathing and space-temporal temperature variation on curved surfaces, and has great potential for realizing noncontact human-machine interfaces.

#### **3.5 Other examples**

#### *3.5.1 Wearable sensors*

Many of the applications of CNT/polymer composites are as strain sensors [2], but the association between polymer materials and CNTs can produce other interesting

#### **Figure 6.**

*Structure of a sandwiched temperature sensor based on laser-reduced graphene oxide (adaptation after R. Chen et al. 2022): 1—Support polymer film (PET, 0.125 nm thickness); 2—T-shaped gold electrodes (Au/Ti 30/ 20 nm, deposited by sputtering); 3—Laser-reduced (*in situ*) graphene oxide layer (deposited from aqueous solution); 4—Conductive silver paint (for wire/electrode soldering); 5—Wire; 6 – PI tape (for sensor packaging): a—Side view; b—Top view.*

## *Novel PTC Composites for Temperature Sensors (and Related Applications) DOI: http://dx.doi.org/10.5772/intechopen.110358*

effects for emerging applications. Textiles, as assemblies of fibrous materials, single or multiple, have properties that depend on the nature of the fibers, the treatments applied to them, as well as the method of obtaining—weaving, knitting, or felting (nonwoven textiles). The integration of CNTs in textiles enables the development of wearable technologies and smart textiles, with customizable properties and functionality, through recent approaches to the synthesis of CNT-textile fiber hybrid materials. Such textile materials possess a number of important advantages over ordinary textile materials, such as low weight, integrated nonelectronic regulation of body temperature, selfcleaning without water, and appearance adapted to the requirements of clothing production [66]. Kubley et al. [66] presented a way to synthesize a sheet of carbon nanotube hybrid (CNTH) and tested ways to integrate it into fabrics for various applications, as well as their potential applications in technical and smart textiles.

Continuous monitoring of body temperature is developing rapidly, based on numerous innovations [67]. The use of additive manufacturing techniques enables the large-scale production of flexible temperature sensors, which is now a well-defined direction of development [16, 67]. Another direction is represented by bio-inspired materials and structures, such as octopus legs that have a high ability to attach to skin or other supports [19]. The sensor is resistive type and consists of a hydrogel composite formed by a poly(N-isopropylacrylamide) matrix in which PEDOT:PSS and CNTs are as conducting phases. The NTC sensor has high sensitivity between 25 and 40<sup>∘</sup> C, allowing accurate detection of temperature differences of 0.5 <sup>∘</sup> C and can be used in skin attachment, wearable medical, and health care applications.

## *3.5.2 Robotic elements with resistive sensors*

In robotics, an important concern is the development and production of complex materials that reproduce the functionality of human skin (the so-called humanoid artificial skin), for the development of robots with close human characteristics, as well as for reparative surgery applications. In order to imitate the functionality of human skin, in robotics or prosthetics applications, two complex functions must be ensured, namely (i) detecting the characteristics of the surrounding environment (temperature, hardness, and slip), and the second, (ii) is to grasps various objects to move/ condition them. Both of these functions require the existence of a matrix of tactile, force, and temperature sensors connected to a central acquisition system, where the image (map) of temperature and roughness of the environment with which the skin comes into contact will be obtained. Corresponding to the function (i) and based on the environmental information, the force necessary to grasp the target object will be dosed or, if the temperature is outside the established safety range (function II), it will be decided to avoid the contact with it. In addition, the sensor array (just like the skin) must have high elasticity, allowing for bending-rotating movements similar to those performed by the human hand. Obviously, practical realization involves the use of advanced manufacturing technologies such as additive manufacturing and the use of micromechanical systems (MEMS) [40].

Harada et al. [68] made a tactile force sensor (on three axes) in the form of a double 3x3 matrix, which contains a network of temperature sensors and a network of strain sensors. The device obtained exclusively through printing technologies allows the detection of sliding/frictional forces, the sense of touch (pressure), the detection of temperature, as well as the grasping/holding of objects. The temperature sensor (NTC type) consists of a printed CNT/PEDOT:PSS composite, with silver electrodes, and presents a temperature coefficient of 0.25%C<sup>1</sup> , in the range of 20–80 <sup>∘</sup> C.

Many of the temperature sensors used to obtain artificial skin exhibit NTC effect [10, 18, 68]. Nuthalapati et al. [69] reported a temperature sensor made of a highly sensitive rGo-Pd/kapton composite, which exhibits NTC effect at Pd contents lower than 1:4 (rGo:Pd) and PTC effect at ratios of 1:6 and 1:8. However, the sensitivity to temperature detection decreases as the Pd content increases.
