Electrical properties of conductive fabrics for operating capacitive touch screen displays

Abstract

Electrically conductive textiles have many potential applications, such as sensors, static charge dissipation, and electro-magnetic interference shields. In this study, two different types of core spun yarns were produced with silver-plated nylon filaments as the conductive material. The electrical characteristics of the core spun yarns and the fabric samples knitted with these yarns were investigated. It was clear that the surface resistance of each type of knitted fabric depends on the surface exposure of the conductive materials. However, both knit types exhibited reasonable features for application as a touching operator for capacitive touch screen panels.

Keywords

conductive fabrics core spun yarns smart fabrics capacitive touch screens

Since the early 21st century, the consumer electronic device industry, including smartphones, E-books, personal digital assistants (PDAs), MP3 players, and so on, has grown continuously and rapidly. In particular, these days, smartphone sales are projected to surpass that of desktop PCs.¹ Smartphones have become integrated into the daily lives of many people, not just one group or another, for example younger generations or business people. Smartphones are used throughout the day for sharing videos with friends or checking emails, as well as communicating via text messages and social networking platforms. A recent Ericsson Consumer Lab study showed that smartphones allow people to go online the very instant they have the impulse to do so. Thus, Internet access is becoming more spontaneous, unplanned, and ubiquitous.²

Currently, touch screen displays are the primary input device of smartphones, tablet computers, and other mobile devices.³ While there are many touch technologies in existence, the resistive and capacitive touch technologies dominate and lead the touch screen panel industry. A resistive touch screen panel is coated with a thin, metallic, electrically conductive resistive layer that causes a change in the electrical current when touched; this is registered as a touch event and is sent to the controller for processing.⁴ However, the capacitive touch screen panel that is used widely in smartphones is coated with a material that stores electrical charges.⁵ When the panel is touched, a small amount of charge is drawn to the point of contact; there are circuits located at each corner of the panel to measure the charge and send the information to the controller for processing. Due to its reliance on capacitive materials, the capacitive panel must be touched using a human finger or something that is composed of a conductive material.

Electrically conductive textiles have been developed for various applications, such as textile-based wearable sensors,⁶ static charge dissipation,^7,8 electro-magnetic interference shields,⁹ and so on. For the purpose of textile interconnections among electronic devices and components, different types of conductive yarns have been developed. Firstly, thin conductive wires with a diameter of several micrometers have been combined to form a multifilament, mechanically more stable form. Secondly, polymer filaments have been coated or embedded with conductive substances, such as silver, carbon, copper, or titanium. Thirdly, thin conductive wires have been combined with textile fibers. Furthermore, core-sheath structured yarns, in which copper or stainless steel wires are used as the core material, have been reported for electro-magnetic shielding applications.^10,11 According to a previous report,¹² each yarn exhibits different electrical performances and their applications differ depending on their electrical capabilities.

In this study, the textile products produced with electro-conducting yarn were used as glove materials. The gloves could not only be used to operate a capacitive touch panel screen in cold climates during winter, but also would feel good to the wearer and have enhanced durability. The electro-conducting yarn was prepared using conventional core spun yarn technology, and cotton/wool (50/50%) fibers were used as the sheath with silver-plated nylon filaments as the core component. Two different types of core spun yarns were manufactured and their electrical properties were investigated. Furthermore, the surface resistances of the sample fabrics that were knitted using the two types of core spun yarns were measured and the functionality of the knit as a touch operator for a capacitive touch screen panel display was evaluated.

Experimental details

Materials

Silver-plated nylon filaments were used as the conductive material (Qingdao Hengtong X-Silver Specialty Textile Co., Ltd, Shandong, China); the yarn count was 11.1 tex. In order to develop the core structured spun yarn, a roving composed of a 50/50% cotton/wool blend (590.5 tex) was used to cover the conductive filament, because wool–cotton mixed textiles are commonly used for manufacturing gloves for cold climates and winter.

Sample preparation

Yarns

The conductive yarns developed in this study consist of two parts – a core and a sheath – as shown in Figure 1. A conventional ring spinning frame (Spintester for cotton spinning, CREATIVE BA 2040, CAIPO, Italy) was used to manufacture the conductive spun yarn. The spinning parameters of the ring spinning system are given in Table 1. The core material was positioned differently in both yarns, as shown in Figure 1. The Type A yarn has the conductive material at the center, and the Type B yarn has the conductive material on either side.

Figure 1.

Schematic cross-sectional views of the conductive core ring spun yarns.

Table 1.

Spinning parameters

Roving &filament properties	Fiber composition	Cotton 50% + wool 50%
	Linear density (tex) of roving	590.5
	Filament composition	Silver-plated nylon
	Linear density (tex) of filament	11.1
Spinning condition	Total draft ratio	12.32
	Break draft	1.3
	Linear density (tex)	47.9
	Twist (TPI)	12.02
	Speed of spindle (rpm)	10,000

Knit fabrics

The Type A and Type B yarns were used to produce conductive fabrics. By plying the Type A and B yarns, three combinations of AA, BB, and AB yarns were obtained. These blended yarns were knitted into a single jersey (weft knit) using a Fiber Analysis Knitter Sampler circular knitting machine (LH122, Lawson-Hemphill, USA; cylinder: 8.89 cm, 160 needles; Tension: 6 g).

Dyeing and washing treatment

The knit was dyed by direct dye (Blue color, Prodye, FINE, Korea) through a small Package Dyeing Machine (Colortec, Roaches International Ltd, UK). Table 2 displays the components of the dyeing bath. Washing samples were prepared according to AATCC Test Method 61-09 at 20 ± 5°C for 45 min.

Table 2.

Dyeing method

Dye bath	Reactive dye	Na₂SO₄	Na₂CO₃	Penetrant
Conc.	3% (owf)	80% (owb)	20 g/l	2 g/l

Characterization

The surface morphologies of the fabric samples were examined using a scanning electron microscope (SEM; S-3000 N, Hitachi, Japan) and video microscope (Video Analyzer 2000, Mesdan, Italy). The silver contents were investigated using an inductively coupled plasma-optical emission spectrometer (ICP-OES; Perkin Elmer, USA). The electrical resistance of the conductive spun yarn and the knit fabrics made from the conductive yarns were measured using a Fluke 8846 A Multimeter (Norwich, UK), as shown in Figure 2. In addition, the objective surface exposure of silver-plated filament on the knit fabric was determined using an image analysis process. Through the image analysis process, each image was first converted to grayscale, and the contrast threshold was adjusted to be in the range of 200–255; finally, the pixel histogram was analyzed using the ImageJ image processing program (National Institutes of Health, USA).

Figure 2.

Resistance measurements for the (a) conductive yarn and (b) fabric.

For the measurement of the fabric conductivity, two parallel electrodes with a length of 20 mm were in contact with the fabric based on a modified version of the AATCC-76 method.¹³ The size of the sample specimen was 20 mm × 80 mm, and it could accommodate the width of the electrodes. The sample specimen was positioned on the testing apparatus. Two electrodes were placed on both sides and a clear plastic panel covered the top. Then, a mass of 98 N was applied to the sample and the electrical resistance of the sample fabric for the given area was measured using a multimeter. The surface resistance (R_s) is given by

R_{s} (Ω / square) = \frac{W}{D} R^{'},

where R′ is the resistance measured by the multimeter, and W and D are the width of the sample and the distance between the two electrodes, respectively.

The subjective sensitivity of the conductive knit fabric was investigated as a tool to operate a capacitive touch panel screen. One hundred persons participated in this study; they were aged in their 20 s and 30 s, and were selected randomly in Seoul, Korea. The subjective evaluation progressed as follows. The participant wore a finger cover made from the conductive knit fabric and touched some specific point on a capacitive touch panel screen (iPad 2, Apple Inc., USA) in accordance with the directions in the accompanying questionnaire. If the touch worked correctly, the result was marked as such on the questionnaire. Every subject attempted to use the iPad 2 ten times for each fabric sample (AA, BB, and AB).

Results and discussion

Surface appearance characterization

The surface images of each type of core spun yarn were captured with a video microscope system, as shown in Figure 3. It is clear that the Type A yarn with the conductive material in the middle had better coverage than that of the Type B yarn. In addition, each type of yarn (single yarn; A and B) and the blended yarns (plied yarn; AA, BB, and AB combinations) were knitted and their surface images were captured, as shown in Figure 4. The structural density of the knit fabric made from the single yarns exhibited approximately 34% higher conductivity than those of the knit fabric made from the plied yarns. Figure 5 shows the surface exposure index of the conductive materials for each fabric sample, which was determined using the number of pixels on the digitally processed image. Overall, as the exposure index decreased, the conductive material (silver-plated nylon filament) was less open. Hence, the surface exposure index of the Type AB, BB, and B fabrics appeared to exhibit a similar distribution, and those of the Type AA and A fabrics appeared to exhibit a similar distribution. Furthermore, the exposure indexes of the knit fabric, including the Type B yarn, were significantly greater than those of the knit fabric including only Type A yarns. However, the Type AB combination was expected to exhibit not only superior electrical conductivity compared to the Type BB and B combinations, but to also demonstrate reasonable exposure effects similar to those in the Type AA and A combinations.

Figure 3.

Surface images of the core spun yarn captured with a video microscope system.

Figure 4.

Surface images of each type of fabric captured with a video microscope system: knitted with (a) type A, (b) type B, (c) type AA, (d) type BB, and (e) type AB. Scale bar: 36 µm.

Figure 5.

Surface exposure index (number of pixels on the digitally processed image).

Surface morphology of the conductive yarns

The SEM images in Figures 6(a)–(c) present the morphology of the conductive yarn from the untreated fabric, dyed fabric, and washed fabric. It appears that the silver coatings on the yarns were cracked after the dyeing and washing process. This result is connected because the silver concentrations of Figures 6(a)–(c) were 19.94, 13.86, and 3.8 g/kg; these decreases in silver concentrations are a result of the silver coatings peeling off due to the physical force during the dyeing and washing processes. Also, the mechanical damage instantly affects the surface conductivity of the conductive core yarn, as shown in Table 3. However, even though the silver concentration of the fabric significantly decreased, the electrical conductivity did not significantly decrease after dyeing and washing.

Figure 6.

Scanning electron microscope images of (a) pristine, (b) dyed, and (c) washed conductive yarn.

Table 3.

Resistance of each conductive yarn and silver concentration (%)

	Silver coating (conductive material)	Conductive core spun yarn (cotton/wool)
	Original	After knitting	After dyeing	After dyeing and washing
Resistance (Ω/cm)	5.52	4.49	12.23	11.84
Silver conc.	100%	100%	70%	19%

Electrical conductivity

Electrical properties of the core spun yarns

The linearity of electrical resistance of the conductive core spun yarn (Type A) was plotted as a function of the yarn length, as shown in Figure 7. As expected, the surface resistance increased linearly with increases in the length of the core spun yarn. This indicates that the conductive core spun yarn complied with the general phenomenon of a conductor: for a given material, the cross-sectional area is inversely proportional to the resistance and the resistance is proportional to the length. However, the electrical resistance of the raw conductive material (silver-plated nylon multifilament) and core spun yarn (Type A; before and after the dyeing/washing process) were investigated, and the resulting values are presented in Table 3. The electrical resistance is represented in Ω/cm, which is reflective of the one-dimensional nature of the electrical threads. The resistance calculation in a form similar to the metal wires is impossible for the coated fibers due to the uncertainty with the total volume and cross-sectional area of the conductive layers.¹⁴

Figure 7.

Resistance linearity of core spun yarn (Type A).

Electrical properties of the knit fabric

The electrical resistance of the knit fabrics made from the Type A, B, AA, AB, and BB yarns and their touch abilities were investigated as shown in Figure 8. As expected, as the content of the Type B yarn increased in the knit fabric, the surface resistance decreased. However, it was observed that the touch ability was not directly affected by the electical conductivity of the matrix, because the touch operation was generated by a distortion of the screen’s electric field, which is measurable as a change in capacitance. The success rate of the operating touches of the Type AA, AB, and BB fabrics were 74.7%, 99.8%, and 99.9%, respectively. This implies that the conductive knit fabric operates the capacitive touch screen panel well for a small portion of the open conductive material (AB). In addition, even though the open conductive part was insufficient (AA), the conductive knit fabric worked reasonably on the panel. It appears that the electrically charged particles in the Type A yarn are able to interact with the capacitive touch panel through touch pressure, which forms an electrical field between the conductor (silver) and the capacitive touch panel.¹⁵

Figure 8.

Success rate of operating touches as a function of electrical resistances: knit fabrics made of type A, B, AB, BB, and AA yarns.

Conclusion

In this study, the electrical properties of conductive core spun yarns and the fabrics knitted from those yarns were investigated. Using the conventional core/sheath spinning process, the conductive core spun yarn was manufactured. The surface exposure of the conductive materials was adjusted by placing the core material in different positions. It is clear that the surface resistance of each type of knitted fabric was dependent on the surface exposure of the conductive materials. Furthermore, with increases in the exposure of the conductive material, the sensitivity of the conductive knit fabric as a tool to operate capacitive touch panel screens increased. However, it was also observed that the touch ability was not directly affected by the electrical conductivity of the matrix. Therefore, it was found that the knit fabrics could be good materials for manufacturing gloves used in operating capacitive touch screens in cold climates and winter. In future studies, the optimization of the yarn and fabric structure will be investigated for better operation of capacitive touch pad screens. Also, economic aspects, such as the use of conductive materials and durability of the knitted fabric, will also be evaluated.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

References

Snol L. More smartphones than desktop PCs by 2011, http://www.pcworld.com/article/171380/more_smartphones_than_desktop_pcs_by_2011.html (2009, accessed 10 October 2011).

Ericsson Consumer Lab. From apps to everyday situations, http://www.ericsson.com/res/docs/2011/silicon_valley_brochure_letter.pdf (2011, accessed 18 December 2011).

Vaughan-Nichols

. New interfaces at the touch of a fingertip. IEEE Comput 2007; 40: 12–15.

Downs

. Using resistive touch screens for human/machine interface. Analog Appl J 2005; 3Q: 5–10.

Barrett

Omote

. Projected-capacitive touch technology. Inf Disp 2010; 26: 16–21.

Huang

Shen

Tang

. A wearable yarn-based piezo-resistive sensor. Sens Actuators A 2008; 141: 396–403.

Maclaga

Fisher

. Static dissipation mechanism in carpets containing conductive fibers. Textil Res J 2001; 71: 281–286.

Hurst NJA and Barker S. Dissipation of static electricity in workwear. Patent 6767603, USA, 2004.

Aniołczyk

Koprowska

Mamrot

. Application of electrically conductive textiles as electromagnetic shields in physiotherapy. Fibers Textil East Eur 2004; 12: 47–50.

10.

Lou

. Process of complex core spun yarn containing a metal wire. Textil Res J 2005; 75: 466–473.

11.

Ramachandran

Vigneswaran

. Design and development of copper core conductive fabrics for smart textiles. J Ind Textil 2009; 39: 81–93.

12.

Teo

Ramakrishna

. Electrospun nanofibers as a platform for multifunctional, hierarchically organized nanocomposite. Compos Sci Technol 2009; 69: 1804–1817.

13.

Wu J. Synthesis. PhD Thesis, University of Wollongong, Australia, 2004.

14.

Shim

Chen

Doty

. Smart electronic yarns and wearable fabrics for human biomonitoring made by carbon nanotube coating with polyelectrolytes. Nano Lett 2008; 8: 4151–4158.

15.

Wangsness

. Electromagnetic fields, 2nd edn, New York: John Wiley & Sons, 1986.