Strain sensing fabric integrated with carbon nanotube yarn for wearable applications

Abstract

Textiles, as a desired platform for wearable smart technology, can be integrated with smart elements in the hierarchical levels of the fabric structure. In this study, a new way to make strain sensing fabric was developed by embedding a single strain sensing carbon nanotube (CNT)-based yarn into the woven structure. To overcome the abrasion during insertion, the yarn was coated with poly(vinyl alcohol) (PVA) to achieve higher mechanical performance. The gauge factor of the yarn was improved from 0.91 to 1.64. The sensing properties of CNT/PVA yarn integrated strain sensing fabric showed a gauge factor of over 1.1, a degree of linearity of more than 97% and good stability and repeatability during cyclic loading. Fabric with different integrated yarn lengths, patterns and output connections has also been investigated. The results showed that the yarn length and bend over section had a great influence on the strain sensing properties of the fabric. Furthermore, the fabric strain sensor exhibited a quick and precise response to the finger motion detection, demonstrating potential in wearable electronics.

Keywords

wearable electronics strain sensor carbon nanotube yarn electric mechanical properties

Flexible strain sensors are a promising prospect for applications in wearable electronics.^1,2 Compared with rigid and brittle strain sensors, they can withstand larger deformation during stretching, torsion or compressing and provide comfortable contact with human skin. Textiles, as a platform, have been shown to be an effective way to obtain flexible strain sensors, with integrating sensing elements into different levels of the hierarchy structure, including the fiber, yarn and fabric.^3–5 However, to exploit the development of wearable devices, the sensors should be able to support the deformations of the textile simultaneously, without affecting the original textile characteristics, such as softness, breathable and draping properties.^6,7 To respond to these specifications, the sensing element is required to be imperceptible to human beings and compatible to the structure of the textile.

The textile strain sensor can be made by applying conductive coating or introducing conductive fibers or yarns, by which the strain sensing can be induced by contact resistance change.⁸ Conductive materials, such as polypyrrole,⁹ poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PE-DOT:PSS)¹⁰ and reduced graphene oxide^11,12 have been coated on the fabric directly for preparation of flexible strain sensors. However, their electrical durability under abrasion and washing should be considered for practical applications. Knitting or weaving conductive fiber or yarns into strain sensing fabrics is another way that can facilitate the sensor with the preferred structure and properties. For example, Hong et al.¹³ fabricated conductive ultra-high molecular weight polyethylene (UHMWP)/polyaniline (PANI) composite yarn into a cylindrical plain weft-knitted fabric. The gauge factor of the fabric changed from 7.38 to 15.47 with different densities. For knitted fabric, electrical resistance change resulted from a close or distant contact of the adjacent fiber or yarns during the extension, which was caused by the deformation of the loop structure.¹⁴ Similarly, the strain sensor can also be designed into a warp-knitted or woven structure by utilizing the conductive polymer yarns.^15,16 For fabricating the free-standing strain sensing fabric, the conductive fibers or yarns should be scalable and achieve suitable electrical and mechanical properties, which has limited the development of the one-piece all-in-one wearable strain sensing textile.^17,18

To target the positions of measurement, strain sensing fabric can also be obtained by incorporating the yarn strain sensors into the textile structure.¹⁹ The sensing feature of fibers or yarns can be achieved by coating of intrinsically or extrinsically conductive polymers, usually by immersing the yarns into a solution of conductive polymer or non-conductive polymer with conductive fillers.^20–22 There are some other ways to make the strain sensing yarn. For example, carbon nanotube (CNT) yarn (fiber) made by twisting of the axially aligned CNT assembly also exhibits a piezoresistive effect^23,24 and can be further improved by incorporating with polymer.^25,26 However, strain sensing fabrics based on the yarn strain sensors are seldom reported. Recently, Eom et al.²⁷ integrated in situ polymerized PEDOT/PS strain sensing fiber into a woven fabric by using a mechanical sewing machine. The gauge factor of the textile strain sensor was higher than 0.76. Yet, the sensing yarn was attached on the surface of the fabric, which might cause low abrasion resistance.

In this paper, we report a flexible strain sensor by embedding poly(vinyl alcohol) (PVA)-coated CNT yarn into the woven structure. Among many types of CNT yarns, aerogel spun CNT yarn has great advantages due to its good electrical properties and high efficiency during production.²⁸ PVA was carefully chosen as a coating polymer because it has good compatibility with CNT yarns.²⁶ The coated yarn possesses a strain gauge factor of 1.64 and it is quite flexible with a strain at break of 20%. After integration, the textile strain sensor exhibited a strain gauge factor higher than 1 and good stability. Meanwhile, the sensing properties of fabric embedded with different patterns and output connections were investigated under cyclic loading. Furthermore, the strain sensing fabric was attached to a finger to monitor the finger bending motion, and it demonstrated potential application in wearable electronics.

Experimental details

Preparation of strain sensing CNT composite yarn

Aerogel spun CNT yarns with diameter of 40–45 µm were provided by the Suzhou Institute of Nano-Tech and Nano-Bionics (Suzhou China). PVA 1799 with a polymerization degree of 1700 and alcoholysis degree of 99% was bought from Sinopharm Chemical Reagent Co., Ltd. PVA solution (5% concentrated) was prepared by dissolving PVA in a solvent containing equal volumes of deionized water and ethanol. The CNT yarn was immersed in the PVA solution for 30 min and passed through a quilting pin to remove the excess solution. After drying the yarn at room temperature for 24 h, the strain sensing CNT composites yarn can be obtained.

Fabrication and characterization of the strain sensing fabric

As shown in Figure 1, single CNT/PVA yarn of a certain length was integrated into the plain structured cotton fabric in the interlacing warp direction by hand sewing. Two copper wires were connected into the two ends of the integrated yarn by using silver paste. Epoxy glue was further applied on the two ends to ensure a tight and stable connection. To investigate the influence of the embedding shape on the electro-mechanical property of the fabric, yarns with different lengths of 1, 3 and 5 cm and output connections were integrated into the fabric separately. A KEYSIGHT 34461A multimeter was used to test the electrical resistance of the integrated yarn by the two-probe method. The electrical resistance data was output simultaneously during the stretching of a 60 mm × 200 mm substrate fabric with a strain rate of 4.8 mm/min. The micro structural observations of CNT yarns and CNT/PVA yarns were conducted with a field emission scanning electron microscope (FESEM, Hitachi S4800, 5 kV).

Figure 1.

Schematic view of the carbon nanotube (CNT)/poly(vinyl alcohol) (PVA) yarn integrated fabric strain sensor.

Results and discussion

Physical properties of CNT yarn and PVA/CNT yarn

The CNT yarn had a rough surface with whisker CNTs and micro cracks along the twisting angle direction (Figure 2(a)). This morphology of the yarn was formed during the process of spinning CNT yarn via the one-step floating catalyst chemical vapor deposition (CVD) method. To improve the properties of CNT yarn, a layer of PVA coating was applied on the circumference of the yarn. After coating by 5% concentrate PVA solution, the coated CNT yarn shrank with a diameter of 35–42 µm and showed a smooth surface in Figure 2(b), as the yarn was covered by polymer with a densified structure. It also demonstrated an improved mechanical performance over the original CNT yarn. The tensile strength and modulus increased from 228 to 341 MPa and 7.3 to 15 GPa, respectively. Although the failure strain of the PVA-coated CNT yarn was lowered from 30% to slightly over 20%, it could still meet the requirements as a sensor for detecting hand gestures or body posture.^29,30 The high strain of CNT yarn resulted from the rearrangement of CNT alignment during tensile loading. As the PVA combined with CNT in the coated yarn, a dense film structure was obtained, which inhibited CNT rearrangement.

Figure 2.

(a) Scanning electron microscopy (SEM) picture of carbon nanotube (CNT) yarn; (b) SEM picture of CNT/poly(vinyl alcohol) (PVA) yarn; (c) Typical stress–strain curves of CNT yarn and CNT/PVA yarn; (d) Tensile strain-dependent relative resistance change of the CNT yarn and CNT/PVA yarn.

Both CNT yarn and CNT/PVA yarn are electrically conductive with resistivity of 70 ± 10 and 150 ± 10 Ω/cm, respectively. During tensile loading, the electrical resistance of both yarns changed linearly due to the increased intrinsic resistance of individual CNTs.^26,31 As shown in Figure 2(d), the resistance change rate demonstrated a linear relationship with the tensile strain. The resistance change rate was calculated by the ratio of changed resistance ΔR to the original resistance R₀. To evaluate the correlation between the resistance and strain of the yarns, the gauge factor (GF), as a measure of the sensitivity of the strain gauge, was calculated using the following equation

GF = \frac{Δ R / R_{0}}{ɛ}

(1)

where ε is the strain of the yarn. The gauge factor of the original CNT yarn was calculated to be 0.91, while that of the PVA-coated CNT yarn was 1.64, which indicated an improved sensibility with strain. This was because the sensing property of the yarn primarily came from the intrinsic resistance change of the CNT during stretching.^26,32 With PVA coating, the yarn showed an increased mechanical structure, which proved that the load transfer from yarn to CNT was improved. Therefore, when the intrinsic resistance of CNTs increased more, it resulted in higher resistance change. Since the yarn had a large tensile strain of 20%, it could be elongated without breaking with the strain of the fabric lower than 20%. Moreover, the improved tensile strength, strain sensitivity and anti-abrasion property of CNT/PVA yarn made it applicable for integrating into a woven structure as a sensing element.

Electro-mechanical properties of single CNT/PVA yarn inserted in woven fabric

Since the CNT/PVA yarn was much finer compared with normal textile yarns, such as cotton or cotton blend yarns, it could be easily threaded into the woven structure by simply tracing a single warp yarn or weft yarn into the interlacing space of the textile. Figure 3(a) shows a schematic view of a single CNT/PVA yarn inserted into the plain structure in the warp direction. The strain sensor fabricated by using cotton fabric as a platform and CNT/PVA yarn as a sensing element is shown in Figure 3(b). It can be observed that the CNT/PVA yarn exhibited less curvature in the space in between the warp and weft yarns than in the warp yarn and weft yarn of the fabric.

Figure 3.

Strain sensing properties of the strain sensing fabric: (a) schematic view of the structure of strain sensing fabric; (b) strain sensing plain woven fabric with embedding of a single carbon nanotube (CNT)/poly(vinyl alcohol) (PVA) yarn; (c) relative resistance change of CNT/PVA yarn and strain sensing fabric during five cyclic loadings; (d) gauge factors (GFs) of the CNT/PVA yarn and strain sensing fabric during five cyclic loadings.

As shown in Figure 3(c), with five times cyclic loading of 3% strain, the fabric and CNT/PVA yarn both demonstrated cyclic relative resistance change. After the first tensile loading, the electrical resistance of the CNT/PVA-coated yarn and integrated fabric increased approximately by 2.5% and 1.9%, respectively, due to the stress relaxation and realignment of CNT and PVA in the yarn.²⁶ The gauge factor of the fabric increased after the first cycle and became stable at around 1.1. Moreover, the resistance response curve of the fabric showed good linearity of over 97%. The gauge factor and linearity of the yarn and fabric are compared in Table 1. In fact, the resistance change of fabric resulted from the stretching of the embedding yarn, which indicated that the strain sensing properties of the yarn were well maintained in the sensing fabric. This is because the fine and strong PVA-coated CNT yarn has a much stable structure, which make it resistant to abrasion during the stretching and releasing of the fabric.

Table 1.

Gauge factor and linearity of carbon nanotube (CNT)/poly(vinyl alcohol) (PVA) yarn and the fabric sensor during different loading cycles

Loading cycle	Gauge factor		Linearity (%)
Loading cycle	CNT/PVA yarn	Fabric sensor	CNT/PVA yarn	Fabric sensor
1st	1.06	0.64	98.1	97.2
2nd	1.36	0.98	99.0	98.6
3rd	1.33	0.91	98.3	97.8
4th	1.27	1.01	99.1	98.7
5th	1.27	0.93	99.1	98.3

The stability of the sensing fabric was also investigated by up to 30 cyclic tensile loadings with strain of 7%. The electro-mechanical properties of the elastic strain sensing fabric are shown in Figure 4. This fabric can keep a very stable electro-mechanical performance during cyclic loading and would be durable for applications. The strain-dependent relative resistance change in Figure 4(b) shows that the fabric had an electrical response when the strain was higher than 3% at the first loading cycle. After releasing, the relative resistance changes of the fabric increased by 1% and then remained unchanged due to the adjusting of the CNT-coated yarn in the fabric during stretching. For the following cycles, the electrical response started as the strain reached 4.5% and the gauge factor exhibited similar and stable values around 1.3. The excellent electro-mechanical stability of the CNT/PVA yarn embedded fabric during 30 cyclic tensile loadings demonstrates that it is capable of working as a strain sensing fabric for wearable electronics. Compared with other fabric sensors with stiff conductive yarns woven or knitted into the fabric structure, fabric integrated with sensing CNT/PVA yarn has a much lower impact on the fabric, especially its hand feeling and draping properties.

Figure 4.

Electro-mechanical stability of the strain sensing fabric during 30 cyclic tensile loadings: (a) load cycle-dependent relative resistance change; (b) tensile strain-dependent relative resistance change.

Strain sensing fabric with different integrated patterns

To monitor the deformation of the fabric in a specific area, the strain sensing CNT/PVA yarn can be integrated into the fabric with different shapes and different output connections. As shown in Figure 5, CNT/PVA yarns with different lengths of 1, 3 and 5 cm were integrated into the fabric in three different ways. The fabric was applied with a cyclic loading with a tensile strain of 3%. Figure 5(a) shows the strain sensing fabric with straight insertion of yarns with different lengths. By increasing the yarn length from 1 to 3 and 5 cm, the relative resistance change of both fabrics increased and showed smooth curves (Figure 5(d)). In fact, the relative resistance changes of the fabric were calculated based on the resistance change of the inserted yarn. When a longer CNT was inserted into the woven structure, the yarn had more contact with the adjacent warp and weft yarns, making the inserted structure more stable. Therefore, the yarn could take more uniform loading during the tensile stretching of the fabric.

Figure 5.

Strain sensing fabrics integrated with 1, 3 and 5 cm lengths of carbon nanotube/poly(vinyl alcohol) yarn and comparison of their electro-mechanical properties: (a) and (d) for the straight warp insertion pattern; (b) and (e) for rectangle pattern insertion with series output connection; (c) and (f) for rectangle pattern insertion with parallel output connection.

With the same insertion length of the CNT/PVA yarns, they can also be integrated into the fabric with rectangle patterns (Figures 5(b) and (c)). By setting the width of the pattern to 1 cm, the height of the rectangle pattern increased with the increasing of the yarn length. Based on the different output connections, it can be classified as serial integration (Figure 5(b)) and parallel integration (Figure 5(c)). For the former, by increasing of the inserted yarn length, the relative resistance changes obviously decreased. This is because the bend over section of the serial inserted yarn, along the weft direction, was compressed by the adjacent weft yarn during the stretching along the warp direction, and this resulted in the degradation of strain sensitivity. In comparison, the CNT yarns inserted in parallel (Figures 5(c) and (f)) were electrically connected with each other by the conductive paste. These patterns showed less variation of the resistance change rate as there was no bend over section along the weft direction. Therefore, the total length of the inserted yarn had less influence on parallel output resistance.

Monitoring of finger bending motions based on the strain sensing fabric

In order to demonstrate the potential applications of the strain sensing fabric for wearable devices, the fabric was attached to the joint (Figure 6(a)) and segment (Figure 6(b)) part of the index finger to detect the minor strain from bending. When the finger was bent, the resistance change of the fabric was recorded simultaneously by the digital multimeter. As shown in Figure 6, with increasing the bending angel to a certain degree, the electrical resistance of the fabric increased and then remained stable. Compared with the fabric that is fixed on the joint part of the finger, the fabric that is fixed on the segment part of the finger had less increase of resistance due to there being less elongation during the bending. This demonstration clearly showed the feasibility of the fabric sensor applied in wearable electronic structures to detect human motions.

Figure 6.

Monitoring of the finger bending motion with attaching the strain sensing fabric to the joint (a) and segment (b) part of the index finger.

Conclusions

In summary, flexible strain sensing fabrics have been successfully fabricated by integrating strain sensing CNT/PVA composite yarn into woven fabrics. The electro-mechanical performance of the strain sensing fabric showed that the strain sensing properties of the CNT/PVA yarn can be well maintained after insertion into the fabric structure. The fabric sensor exhibited the moderate gauge factors of 1.1 and 1.3 with integration to a plain woven (strain of 3%) and an elastic woven fabric (strain of 7%), respectively. It also showed good repeatability and stability with a linear response of over 97% linearity. Upon integrated different patterns and lengths of yarn into the fabric, the fabric with 5 cm length of single CNT/PVA yarn insertion showed the best strain sensing properties. Furthermore, the fabric sensor showed good sensitivity to detect the finger motion, which indicates that it will facilitate the development of novel wearable electronic devices.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant No. 51503120), the Science and Technology Commission of Shanghai Municipality (Grant No. 14YF1409600) and the Shanghai Education Committee (Grant No. ZZgcd14016).

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