# Electrically Conductive Nanocomposite Fibers for Flexible and Structural Electronics
## 5 Conclusions  
The research study has investigated the possibility of manufacturing composite conductive fibers with a small diameter to be used in textronic systems. During the research, we used a flexible SBS polymer as the composite matrix and investigated three types of functional phases--CNTs, graphite, and a mixture of these carbon structures.  
The main objective of the research was to develop composite fibers with diameters below 0.5 mm, electrical conductivity above 0.2 \(\Omega\)m, together with high elasticity and stretchability.  
The lowest resistivity characterized the composites containing CNTs, but the high content of the functional phase made it impossible to extrude fibers with diameters below 0.5 mm in a continuous manner. Additionally, such composites were characterized by a non-linear current-voltage characteristic, which is typical for the semiconducting character of carbon nanotube conductivity.  
Composites containing graphite as a functional phase were characterized by worse electrical conductivity and only the content of 50 vol. % allowed us to obtain resistivity below the assumed value of 0.2 \(\Omega\)m. However, fibers with such a high content of graphite content presented unsatisfactory mechanical properties and broke below 40% of the relative elongation.  
Using a functional phase in the form of a mixture of CNTs and graphite made it possible to obtain fibers with optimal electrical and mechanical properties. Composite fiber  
Figure 6: Cotton glove with sewed flex sensor; (a) during flexing, (b) un-flexed, and (c) the resistance response.  
containing 5 vol. % of CNT and 10 vol. % of graphite showed an average resistivity below 0.2 \(\Omega\)m and an elongation on the break above 100%. In addition, the use of a two-component conducting phase allowed for a significant increase in the linearity of the current-voltage characteristics in the measurement range from 0 to 30 V. Thanks to the constant resistivity of the composite, it was possible to use it more widely, independent of the value of the supply voltage. The present research also presented methods for the practical application of the developed composite fibers. Tests have shown that the composite fibers can be integrated into standard fabrics in two ways. Thanks to the small diameter of the composite fibers, it was possible to sew them directly into the fabrics, similar to sewing-in textile fibers. Another way of creating conductive paths on the textile substrates was to press the fiber into the fabric structure, thermally. Both methods made it possible to produce conductive paths on the textile substrates and thus use the developed fibers in textronics.  
The piezoresistive properties of the developed fibers were also observed during testing. As the stretching of the fiber increased, its resistance increased. An example of a bending sensor sewed directly into a cotton glove was presented using this phenomenon. This example presented the possibility of using the developed materials not only in engineering solutions, but also in bioengineering, sports, and medicine, including telemedicine, which plays an increasingly important role in health care.  
Conceptualization, B.P. and A.S.; methodology, B.P., P.W., M.K., A.S. and M.S.; validation, M.K. and A.S.; formal analysis, B.P., A.S. and M.S.; investigation, P.W., M.K. and A.S.; resources, A.S. and M.S.; writing--original draft preparation, B.P.; writing--review and editing, M.S.; visualization, B.P.; supervision, A.S. and M.S.; project administration, M.S.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the Foundation for Polish Science within the FirstTEAM/2016-1/7 project.  
The data presented in this study are available on request from the corresponding author.  
This work was supported by the Institute of Metrology and Biomedical Engineering, Warsaw University of Technology, Faculty of Mechatronics.  
The authors declare no conflict of interest.