**4. Conclusions**

Regenerated cellulose fibres can be modified through incorporation of CB to implement electrical conductivity. Addition of CB into the viscose dope requires formation of stable dispersions and rather high concentrations of CB to achieve percolation. Through the addition of 20 wt.% CB to the viscose dope, a regenerated cellulose fibre with content of 16.4 wt.% CB could be obtained. The conductivity of a standard viscose fibre of 7.7 × 10−<sup>9</sup> increased to 9.4 × 10−<sup>8</sup> S/m for the modified fibre. A further increase in CB content to 19.8 wt.% and 23 wt.% increased the conductivity substantially, to 8.8 × 10−<sup>6</sup> and 0.044 S/m, respectively. The presence of particulate matter in the fibre structure, however, reduced the tenacity of the fibres to 30–50% of the value of a standard fibre.

In experiments to measure conductivity of rotor rings and yarns, a remarkable influence of relative humidity present in ambient air was observed. Sorption of water into fibre assemblies with high electrical resistance, e.g., 109–1010 Ω, led to a decrease in resistance. In the case of rotor rings manufactured from fibres with 23.1 wt.% CB, however, the initially low volume resistivity of 10 kΩ increased slightly, most probably due to hygral fibre expansion and adsorption of water onto the fibre surface.

Through a combination of the more rigid CB-containing viscose fibres with elastic polyester fibres, a piezo-sensitive nonwoven fabric was manufactured, which demonstrated pressure sensitivity in the range of very low pressure of 400–1000 Pa. Repetitive load/relaxation cycles demonstrated the repeatability and durability of the sensor mat and the stability of the signal.

The results highlight the potential of CB-incorporated viscose fibres as a cheap functional material for pressure sensor production in smart textile applications. The incorporation of carbon black into the viscose fibres led to a black colour, which limits their application in the visible parts of a garment. However, their use for pressure sensing inside a garmen<sup>t</sup> and therapeutic compression textiles, e.g., bandages, in the form of pressure-sensing pads, could be potential applications of the material. The material is of particular interest for sensor design, as the CB-incorporated cellulose fibres are nontoxic, compatible with future recycling, and able to be produced at affordable costs.

The conductive cellulose fibres exhibit high potential for the substitution of nonbiodegradable synthetic material used in other applications and, thus, could become a greener alternative to existing materials used in smart textiles.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/13/22/5150/s1. Figure S1. Rotor ring and measurement of resistance measurement with multimeter; Figure S2. Experimental set-up for the measurement of volume resistance; Figure S3. Experimental set-up for the measurement of yarn resistance; Figure S4. Experimental set-up for the measurement of resistance as function of pressure; Figure S5. Experimental set-up for the cyclic load/release experiments in a tensile testing unit.

**Author Contributions:** Conceptualization, Y.Z., D.M., and T.B.; investigation, M.E., Y.Z., D.M., N.K., J.F.T., A.M.-A., and T.B.; methodology, J.U., Y.Z., and T.B.; project administration, D.M.; writing—original draft, J.U., Y.Z., D.M., and T.B.; writing—review and editing, T.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Austrian research promotion agency (FFG) K-Project tccv (860474) Textile Competence Centre Vorarlberg and the FFG-talente program.

**Acknowledgments:** The authors thank Anna-Lena Moosbrugger for technical support in the experimental work.

**Data Statement:** The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
