# Electrically Conductive Nanocomposite Fibers for Flexible and Structural Electronics
## 4 Applications  
The developed composite fibers can be used both as electrically conductive paths in textronics and as tensile sensors due to the piezoresistive phenomenon. According to the literature, there are many methods to integrate conductive fibers into the fabric, but two main ways can be determined: sewing [36; 37; 38] or deposition directly into the fabric [39; 40; 41; 42; 43]. Both methods of producing conductive paths on the fabrics were tested during our research. The 5 mm-diameter fibers were hand-sewn into the cotton fabric. This example demonstrates the possibility of efficiently fabricating a conductive path by directly sewing a single composite fiber into the fabric without necessarily manufacturing the entire fabric with composite material. The LED was glued to the sewn-in fibers with conductive glue. Applying a voltage caused the LED to light up, which meant that the process of sewing-in the fibers did not negatively affect their electrical conductivity (Figure 4). Another way to create a conductive path directly on the fabric is to transfer the composite fiber by a thermal process, in which the composite fiber is plasticized and bonds to the fabric. Such a \"print\" on the cotton fabric was made using an ordinary iron by plasticizing the composite fiber and exerting pressure causing a good mechanical bonding. The manufactured path presented high adhesion to the fabric and could not be removed by hand. In addition, the composite did not lose its elastic properties. It was possible to bend and fold the fabric with the \"printed\" path without significant deterioration of the electrical conductivity (Figure 5). Figure 4: The sewing-in of the composite M5 fiber into a cotton glove; (**a,c**) unpowered and (**b,d**) powered. The developed composite fibers also show piezoresistive properties, which allows them to produce stress, displacement, and bend sensors. Similar piezoresistive composites have been successfully used to monitor human movements [44]. In order to examine if developed conductive fibers could be used for a similar purpose, a single 0.5 mm diameter fiber was sewed into the cotton glove. The ends of the fiber were connected to a multimeter to detect any resistance change upon movement of the finger. After the authors put on the glove and flexed the finger, a change in the path resistance was observed (Figure 6). This effect could be repeated for each finger of the glove. Each finger was bent multiple times and the resistance increased with bending. The magnitude of the response is significant at around 10%. The sensitivity of such a fabricated sensor can be modified by using a composite fiber with a smaller diameter, which results in a smaller sensor response when bent, or by making several parallel stitches, which will increase the sensor response when bent. It is worth noting that the fabricated sensor does not require additional circuitry to amplify the received signal. Such printed devices could be used in the field of biomechanics to print customized sensors for patients to assist in their rehabilitation after accidents and injuries. Figure 5: Conductive path fabricated by the thermal process on the cotton fabric; (**a**) unpowered, (**b**) powered, and (**c**) powered and bent. The conductive fibers in the example applications were in direct contact with human skin. Therefore, it is essential to be aware of the potential dangers of using carbon nanoparticles, especially CNTs, which are recognized as potentially hazardous to human health [45,46]. However, it must be taken into account that such a risk to human health is posed only by nanoparticles in their free, unbanded state and not in the form of the polymer nanocomposites from which the fibers are made. The literature that CNTs cannot be released from the polymer composites, despite the visible mechanical damage and wear of the composite. Especially safe are the composites with a flexible polymeric matrix, which is the SBS polymer used in this study [47, 48, 49].