Abstract—The emerging area of e-textiles requires electrically conductive threads. We demonstrate that nylon and cotton threads can be made conductive by coating their surfaces with random networks of solution-synthesized silver nanowires. A resistance of 0.8 Ω/cm was achieved and can be engineered by varying the density of the nanowire coating. Because the nanowires are 35 nm in diameter, and the mesh structure does not cover the entire surface like a thin-film, less metal is used compared to conventional silver-coated conductive threads. The nanowire coating is less than 5% of the weight of the typical silver thin-film coatings used in commercial conductive thread. The nanowire-coated thread is also more mechanically flexible, with its resistance increasing only 16% after 200 bending cycles while the resistance of the commercial conductive thread increased by more than 300%.

I. INTRODUCTION

In the growing field of wearable electronics, electrically conductive thread is used for electromagnetic interference (EMI) shielding [1] and wearable antennas [2], as well as functions as interconnects in textile-integrated devices such as health monitoring sensors [3] and solar cells [4]. Commonly used textiles such as cotton, nylon, and polyester are all electrical insulators. Commercially available conductive thread is either a solid metal wire, such as copper or stainless steel fiber, or a non-conductive thread coated with a ~1 μm thick metal film, usually silver. These options are less than ideal as they tend to be stiff and brittle, and they can breakdown after repeated bends [5].

Alternative coatings to achieve flexible conductive threads have recently been proposed, such as conductive polymers [6], graphene flakes, and carbon nanotubes [7]. However, conductive polymers have relatively low conductivity and are unstable in air [8]. Carbon nanotubes and graphene have stability issues in air as well [9], and although the resistance of individual carbon nanotubes or graphene flakes is extremely low, the junction resistance between two overlapping nanotubes or flakes in the film is very high [10-11]. Achieving conductive thread by coating with metallic nanoparticles such as gold, silver, or platinum has also been demonstrated [12-14]. Of these, silver is the most common because it is the most conductive of all metals, is more cost effective than gold or platinum, and is relatively stable in air. However, the conductivity of metallic-nanoparticle coated threads is low because their small size leads to many inter-particle junctions.

*Correspondance: igoldthorpe@uwaterloo.ca

In this work, nylon and cotton threads are instead coated with a thin metallic mesh made up of a random network of silver nanowires. The use of metallic nanowires in textiles has only been briefly mentioned in the literature [15-16] with little accompanying data. Unlike silver nanoparticles, the elongated shape of a nanowire allows for a conductive film to be achieved at a far lower particle density, and therefore there are less junctions. And unlike carbon-based materials, silver nanowires can be sintered to greatly reduce junction resistance. Because the nanowire coating is a mesh rather than a continuous film, and the nanowires used are only 35 nm thick, much less metal is used compared to conventional conductive threads where a metal film coating or a solid wire is used. This leads to less material cost, lower weight, and thinner thread, as well as greater mechanical flexibility. The coating can be simply deposited as a dye with no vacuum or complex processes required.

II. EXPERIMENTAL METHODS

Silver nanowires can be synthesized in solution at low temperatures by the now well-known polyol method [17]. The nanowires are crystalline with five twin planes running along the longitudinal axis [17]. The silver nanowires used in this work were supplied in ethanol from Blue Nano Inc (Charlotte, North Carolina). Their average diameters and lengths were 35 nm and 15 μm, respectively.

Deposition of nanowire films around threads was achieved through dip-coating. The nylon thread was monofilament with a diameter of 700 µm. The cotton thread was multifilament with a diameter of 200 µm. Both types of thread were cleaned in an ultrasonic bath using ethanol, alcohol, and deionized water for 5 min each. Because cotton is hydrophilic, the nanowire solution adheres well to the surface of cotton without any modification However, nylon is hydrophobic. Therefore after cleaning, the nylon threads were submersed in a solution consisting of 91 wt. % ethyl acetate and 9 wt. % resorcinol for 1 minute and then dried in air. Resorcinol creates hydroxyl groups on the surface of nylon, making it more hydrophilic. After this step the nanowire solution adhered well to the nylon. In both the cotton and nylon cases, the density of the nanowire film deposited was varied through the concentration of the nanowires in the coating solution and through the number of dipping steps. After deposition the threads were annealed at 150°C for 60 min to fuse the overlapping nanowire junctions which greatly reduces the resistance.

The resistance was measured using a multimeter, and the nanowire density was calculated from scanning electron

Metal-nanowire Coated Threads for Conductive Textiles

Yahya Atwa and Irene A. Goldthorpe*

The Department of Electrical & Computer Engineering and the Waterloo Institute for Nanotechnology, Waterloo, Canada N2L 3G1

Proceedings of the 14th IEEEInternational Conference on NanotechnologyToronto, Canada, August 18-21, 2014

978-1-4799-5622-7/$31.00 ©2014 IEEE 482

microscopy (SEM) images. Flexibility was measured by bending the thread around a rod with a 6 mm radius and measuring the resistance after successive bends. To investigate washability, the threads were repeatedly immersed in a solution of commercially available detergent dissolved in water for 5 minutes at a time. For comparison purposes, these same tests were performed on a commercially available conductive thread (117/17 2-ply manufactured by Shieldex-U.S., Palmyra, NY). This commercial thread is multifilament nylon, where the surface of each filament is coated with a thin-film of silver.

III. RESULTS AND DISCUSSION

In Fig. 1, nanowire-coated nylon thread is used in a circuit to power an LED to demonstrate that is it conductive. Fig. 2 shows SEM images of nylon and cotton threads coated with silver nanowires. The nanowires form a connected network that extends completely around the diameter and along the length. As the density of nanowires is increased, resistance decreases. The nylon thread in Figs. 2a and 2b was dipped 3 times in a solution containing 1.25 mg/mL of silver nanowires in ethanol and has a resistance of 12 Ω/cm. The cotton thread in Fig. 2c and d was dipped 3 times in a nanowire solution with a concentration of 5 mg/mL and has a resistance of 11 Ω/cm. Table 1 shows the dependence of resistance on density, and the material cost of the nanowires for nylon threads coated with different nanowire densities [18]. A resistance as low as 0.8 Ω/cm was achieved. 0.8 Ω/cm in this case is equivalent to a sheet resistance of 0.18 Ω/square, and the conductivity of the composite (using the cross-sectional area of the nylon thread) is 81 S/cm. For comparison, nylon thread coated with the conductive polymer PEDOT:PSS has been reported to be 40 Ω/cm [19], and nylon coated with carbon nanotubes has been reported to be 49 kΩ/cm [20].

Although silver is an expensive material, so little of it is used so the costs listed in Table 1 are low. Furthermore, in regards to weight, 1 mg/m or less is added to the thread. If the coating were instead a typical 1 µm thick silver film the coating would weigh 23 mg/m, so the nanowire coating is less than 5% the weight of a solid film.

Figure 1. Passing current through nanowire-coated nylon thread (top left of image) to power an LED.

TABLE 1. The dependence of resistance and material cost on the coating density of the nanowires on nylon thread.

Metal density (mg/m) Resistance (Ω/cm) Nanowire cost ($/m) 0.20 12 0.007

0.49 2.5 0.016

1.01 0.8 0.033

Electrically conductive thread must be mechanically flexible and must not degrade significantly after repeated bends, especially when used in applications such as clothing-integrated sensors. The variation of resistance with repeated bending for both the commercial conductive thread and a nanowire-coated nylon thread with the same initial resistance is plotted in Fig. 3. The resistance of the commercial thread increased by more than 4 times, from 2.8 Ω/cm to 12.2 Ω/cm, after being bent 200 times. Silver thin-films are brittle and can crack after repeated bends [21], thus increasing its resistance. Films of silver nanowires are known to be much more flexible [22]. The resistance of the nanowire-coated thread only increased 16% after 200 bends, from 2.8 Ω/cm to 3.2 Ω/cm. Furthermore, when the resistance of nanowire-coated threads was measured when the thread was bent (as opposed to measuring the resistance after returning the thread flat as was done to collect the data in Fig 3), the resistance was actually lower than an unbent thread. For example, when an initially 2.8 Ω/cm nanowire-coated nylon thread was bent around a rod with a radius of 6 mm, its resistance decreased to 2.3 Ω/cm. This may be because bending causes mechanical forces that improve the connections between overlapping nanowires and thus lower their junction resistances. The resistance of the commercial thread, on the other hand, increased from 2.8 Ω/cm to 4.4 Ω/cm when bent.

Figure. 2. SEM images of silver nanowire coated (a),(b) nylon thread and (c),(d) cotton thread.

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Figure 3. The resistance of the nanowire-coated nylon thread and a commercially available conductive thread after repeated bends to a 6 mm radius of curvature

Adhesion of the nanowire coating to the thread is another important parameter. As can be seen in Fig. 4, the resistance of the nanowire-coated nylon thread did not change after five repeated washings in liquid detergent, which indicates that the coating did not come off or degrade. The resistance of the commercial thread also maintained its conductivity after the same washing procedure.

Figure 4. The resistance of the nanowire-coated nylon thread after washing.

IV. CONCLUSION Both cotton and nylon threads were successfully made

electrically conductive by coating their surfaces with meshes of silver nanowires. In comparison to a commercially available conductive thread which has a continuous thin-film coating of silver, the nanowire coating is an open network. This result in the use of less metal, leading to a lower material cost, lower weight, and greatly improved mechanically flexibility. Overall, this work demonstrates a simple, economical, and functional conductive thread for e-textiles.

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