**4. Aspects of Manufacturing**

### *4.1. Integration of Electrically Conductive Elements*

The central element to integrate sensors in textiles is to achieve stable connections. Figure 6 shows an output of a literature search on manufacturing techniques in textiles. The concepts are highlighted with green for "conductive printing on textiles", blue for "conductive deposition on textiles", and red for "conductive coating on textiles". The research dynamics of the three approaches increased over the period of 29 years, which indicates the growing interest in techniques to miniaturize textile sensors.

**Figure 6.** Research activity in coating, deposition, and printing processes to manufacture conductive structure in textiles.

In general, the coating or deposition of conductive materials has more advantages than the soldering, welding, and weaving of metallic wires or sensors in textiles. The classification of products was related to the aspect by which the coating or deposition of materials can be conducted on large textile areas and spatially defined structures such as yarns or fibers. The coating or deposition of conductive materials leads to a conductive thin layer formation on substrates compared to soldering. Thus, coated conductive textiles are flexible and have a higher motion of freedom compared to soldered textiles. The scientist makes the choice of textile materials because it is more likely that different materials and textile structures will be used in each scientific field. The properties of conductive materials are determined by the firm or loose textile structures, their swelling properties, and the amount of reactive groups.

The advantages of coated or deposited materials are the low weight and the thin coating thickness. Thus, conductive coatings better retain the flexibility, bending, and stretch properties of textiles compared to rigid metallic wires or sensors. Consequently, the combination of conductive coatings and deposits on textiles also contributes to the construction of truly miniaturized textile thermocouples. A truly textile thermocouple was described in the Introduction.

The manufacturing of electrically conductive substrates on textiles can be performed by different techniques e.g., soldering, stapling, and bonding components through conductive adhesives. Soldered substrates do not withstand the bending of textiles. Stapled substrates increase the wear and tear of textiles due to the rigid structure and reduce the freedom of movement. The connection between textiles and conductive substrates can be made by a flexible conductive material [49].

Representative examples for such materials are bicomponent fibers that include poly(vinylidene fluoride) as the sheath material, carbon black, and high-density polyethylene as the core material. Bicomponent yarns were made during the melt spinning process with two screw extruders consisting of the core and sheath material [50]. Conductive core–sheath yarns of copper core filament and cotton sheath were manufactured through the Dref-3 friction spinning method. The core–sheath yarns were made of copper filaments as a core and cotton fibers as sheath. These yarns showed a resistance of 3 to 28 M Ω and a shielding e ffect of 760 to 860 MHz at a core sheath ratio of Cu 0.26 gram per meter and cotton 0.13 grams per meter [51]. Elastic conducting inks were made of Ag flakes, fluorine rubber, and fluorine surfactant, which showed a conductivity of 182 Scm−<sup>1</sup> during stretching. These materials were used as wearable electromyogram sensors to detect the signal activity of the muscles of the forearm. Elastic conductor inks were printed on polyimide stencil masks, which formed flexible conductor wires on the upper side of the textile and an elastic conductor vital electrode on the lower side of the textile [52]. Wearable electronic textiles were created by a lockstitching method and were used in apparel textiles. Conductive assemblies were made by stitching conductive threads (such as silver, aluminum, stainless steel, copper, and carbon) on the surface of a cellulose and stitching thermofusible threads (polyamide, polyolefin, and polyvinyl) on the polyester/elastane [53].

The surface of cotton textiles was rendered conductive by impregnating with carboxylated multiwall carbon nanotubes by dispersion. The cotton fabrics were treated with aqueous NaOH/urea mixture at −10 ◦C for 1 h and showed a low electrical resistivity of 281 Ω cm [54]. The formation of conductive textiles was manufactured through screen printing of the FeCl3 and by applying high voltage from 5 to 30 kV during the coating of pyrrole by vapor deposition. The high voltage along the polypropylene-coated fabric stabilized pyrrole monomers during vapor deposition [55]. Non-conductive epoxy surfaces were laminated with copper sheets by the pressing method. Afterwards, these materials were activated with stannous/palladium chloride particles. The epoxy substrates were made conductive after 20 h of electroless copper plating [56]. The formation of conductive tracks of 1.5 and 4.0 mm was achieved on cotton textiles by the reduction of silver nitrate from sodium borohydride during the spray deposition. Subsequently, the silver seeded tracks were plated selectively with copper during the electroless process from aqueous solution [57]. Copper foils were used to form circuits in cloths, which consisted of silk organza fibers. The electrical circuits structure was manufactured by embroidery and by an industrial sewing machine [58].

Additionally, conductive coatings on cotton fabrics were manufactured by the surface activation in NaOH and poly(diallyldimethylammoniumchloride) solution. The activated cotton fabrics were impregnated with NaBH4 in aqueous solution, and afterwards, a silver nitrate solution was added to the fabric. The cotton fabrics were completely coated with silver nanoparticles after the reduction of silver ions by NaBH4 [59].

Coated textiles powered small consumers without the use of metal wires and impart electromagnetic shielding properties by the examples below. Conductive woven cellulose fabrics power a light-emitting diode (LED) at 20 mA. The copper layer was formed after silver seeding through an electroless deposition in alkaline solution comprising a Cu L-tartrate complex and formaldehyde [60].

Figure 7 describes the electroless deposition method of copper on silver seeded cellulose textiles in alkaline solution. The silver seeded textile is dipped into copper sulfate, formaldehyde, and potassium hydrogen L-tartrate solution (Figure 7a). Formaldehyde is a chemical reducing agent, which reduced copper ions on silver seeds from the copper tartrate complex (TH) to metallic copper (Figure 7a,b). When the deposition proceeds (Figure 7c), copper islands are formed on silver seeds, which then grow to a continuous coating [61].

**Figure 7.** The electroless copper deposition method conducted on cellulose textiles, where the tartrate complex (TH) is a free L-tartrate ligand (**a**). The copper deposition continues on Ag seed (**b**), which leads to the copper layer formation (**c**).

Cotton fabrics imparted conductive properties after the in situ deposition of copper particles and repeated dipping steps in the CuSO4 and Na2S2O4. Copper-coated textiles can be used as flexible and light materials. The copper-coated cotton fabrics showed a shielding property of 6 dB, 10 dB, and 13 dB when the fabric was dipped in the copper sulfate solution 50, 100, and 150 times, respectively [62].

A low electrical resistance of textile material can be achieved also by treatment with conductive polymers after impregnating, vapor deposition, and melt mixing methods. Electro-conductive fabrics can be made from wool, cotton, and silver-coated acrylic yarns. Textiles composed of silver-coated wool yarns and silver-coated cotton/acrylic were used as heating elements in textiles [63]. The incorporation of conductive material during fiber formation also leads to polymer fibers with conductive properties.

A non-woven poly(ethylene oxide) (PEO) matrix was mixed with 3 wt % multiwalled carbon nanotubes (MWNT), which formed conductive polymer composites by an electrospinning process. The maximum electrical resistance of PEO/MWNT composites changed when exposed to methanol, dioxan, and toluene vapors [64]. Conductive monofilaments composites were formed from carbon nanotubes (CNT), polypropylene, poly(ε-caprolactone) (PCL), and polypropylen substrates. The materials manufactured from 50%PP/50%PCL/4%CNT composites showed a resistivity of 1.1 Ωm at 154 ◦C [65].

Coated textiles were used for temperature detection in the range of 15 to 57 ◦C. Conductive polyamide fabrics of 17% Lycra and 83% Tactel (5 cm × 1 cm) were coated from aqueous solution with poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT-PSS). PEDOT-PSS-coated fibers were exposed to environmental temperatures of 15 and 45 ◦C. The electrical resistance of coated fibers decreased with increasing temperature [66]. Conductive polyester yarns were manufactured from copper nanowires and a silicon rubber substrate during a dip-coating. The coated polyester yarns were used as stretchable heating fibers. The composites were woven into a heating fabric and connected to a microcontroller unit to manufacture wearable and smart personal heating systems [67].
