Recent advances on the fabrication methods of nanocomposite yarn-based strain sensor
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Nanotechnology Reviews 2021; 10: 221–236
Review Article
Xiaoning Tang#, Deshan Cheng#, Jianhua Ran#, Daiqi Li, Chengen He, Shuguang Bi*,
Guangming Cai*, and Xin Wang*
Recent advances on the fabrication methods of
nanocomposite yarn-based strain sensor
https://doi.org/10.1515/ntrev-2021-0021 of core–sheath/wrapping yarn strain sensor as-fabricated
received December 21, 2020; accepted March 26, 2021 by traditional spinning technique were well summarized.
Abstract: Yarn-based strain sensor is an emerging candi- Finally, promising perspectives and challenges together
date for the fabrication of wearable electronic devices. with key points in the development of yarn strain sensors
The intrinsic properties of yarn, such as excellent light- were presented for future endeavor.
weight, flexibility, stitchability, and especially its highly Keywords: strain sensor, conductive, coating, core–sheath
stretchable performance, stand out the yarn-based strain
sensor from conventional rigid sensors in detection of
human body motions. Recent advances in conductive
materials and fabrication methods of yarn-based strain 1 Introduction
sensors are well reviewed and discussed in this work.
Coating techniques including dip-coating, layer by layer Smart textiles have gradually been a hot topic in both
assemble, and chemical deposition for deposition of con- industry and academia because of the growing demands
ductive layer on elastic filament were first introduced, and of performance and functions from textiles [1]. With elec-
fabrication technology to incorporate conductive compo- tronic components effectively integrated into fibrous
nents into elastic matrix via melt extrusion or wet spinning substrate, electronically smart textiles represent as an
was reviewed afterwards. Especially, the recent advances attractive platform for wearable device integration, such
as wearable sensor, wearable heater, and wearable color-
changing display [2,3]. Because of their robust sensi-
tivity, smart textiles can sense external stimuli including
# These authors contributed equally to this work.
thermal, mechanical, chemical, electrical, magnetic, and
optical [4]. In practical applications, smart textiles may
* Corresponding author: Shuguang Bi, State Key Laboratory of New be capable of sensing, actuation processing, and energy
Textile Materials and Advanced Processing Technologies, Wuhan harvesting through their response to stimulation beha-
Textile University, Wuhan 430200, China; Hubei Key Laboratory of vior in an intelligent way [5]. Among various functions,
Biomass Fibers and Eco-Dyeing & Finishing, School of Chemistry the accurate sense of strain plays an important role for
and Chemical Engineering, Wuhan Textile University,
wearable devices [6,7]. Strain sensing component has
Wuhan 430200, China, e-mail: sgbi@wtu.edu.cn
* Corresponding author: Guangming Cai, State Key Laboratory of
been considered as the foundation to measure the shape,
New Textile Materials and Advanced Processing Technologies, and detect the posture and movement of human body [8].
School of Textile Science and Engineering, Wuhan Textile University, The strain measurement on the surface can directly pro-
Wuhan 430200, China, e-mail: guangmingcai2006@163.com vide the detailed physical information. Traditional resis-
* Corresponding author: Xin Wang, Centre for Materials Innovation tance strain gages can monitor strain at small length
and Future Fashion, School of Fashion and Textiles, RMIT University,
scale along specific directions, where the deformation
Brunswick 3056, Australia, e-mail: xin.wang@rmit.edu.au
Xiaoning Tang, Deshan Cheng, Daiqi Li, Chengen He: State Key strain range is generally less than 5% [9,10]. Textile strain
Laboratory of New Textile Materials and Advanced Processing sensor is promising for monitoring large range sensing in
Technologies, School of Textile Science and Engineering, smart garment because of the good flexibility of fibrous
Wuhan Textile University, Wuhan 430200, China materials [11]. Recently, textile strain sensors with spe-
Jianhua Ran: State Key Laboratory of New Textile Materials and
cific functionality emphasizing limbs and trunk motion
Advanced Processing Technologies, Wuhan Textile University,
Wuhan 430200, China; Hubei Key Laboratory of Biomass Fibers and
detection have attracted growing attentions for potential
Eco-Dyeing & Finishing, School of Chemistry and Chemical applications [12]. The development of textile strain sensor
Engineering, Wuhan Textile University, Wuhan 430200, China is essential for the fabrication of wearable electronics.
Open Access. © 2021 Xiaoning Tang et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0
International License.
222 Xiaoning Tang et al.
Textiles have been considered as the ideal platform to applications as wearable electronics. Wang et al. [34] have
integrate diverse flexible electronic devices for development reported the progress in textile-based strain sensor for
of wearable systems [13]. Textile strain sensors with excel- human movement detection. These literatures have pro-
lent flexibility are promising for potential smart clothing vided significant insights and contributed greatly to the
applications, as downstream development of wearable development of yarn-based sensor with resistive, capacitive,
devices [14,15]. Generally, textile strain sensors can be suc- and piezoelectrical properties as sensing systems. However,
cessfully fabricated by coating conductive materials onto a compressive review focusing on nanocomposite yarn-
fabric substrate [16]. However, the coating process results based stretchable strain sensor for wearable electronics
in the damage in the strenuous mechanical deformations, would enrich the knowledge of textile-based electronics
and thus it is an interesting topic to enhance the fastness of and complement to the existing reviews in smart textile
the coated conductive layer [17]. As the intermediate com- areas. Thus, this work focused on yarn strain sensors with
ponent in textile processing line, elastomeric conductive good weaving capacity with enhanced stability, which is
filament and/or yarn can be directly woven or knitted into one of the most explored areas in textile sensor. This work
textile substrate [18]. These filaments and yarns are realized reviewed different conductive materials and fabrication
by the widely used scalable fiber spinning technology, such methods of yarn-based strain sensors, with the scope cov-
as wet spinning and melt-extrusion spinning [19]. In addi- ering the following three sections: (1) coated filament and
tion, the structural geometry manipulation of yarn by heli- staple fiber yarn; (2) melt extrusion and wet spinning; and
cally winding conductive fibers onto core substrate was used (3) twisting structure design of yarn sensor. An attempt was
to develop nanocomposite yarn with multiple layers [20]. also made to review and critically comment on numerous
The wrapped compression spring structure of the yarn is a literatures and current limitations together with insights
powerful tool to achieve long-range elasticity, which is ideal with respect to yarn-based strain sensor were presented.
in developing wearable sensors. This work will directly benefit the development of yarn-based
Fibrous materials are typically divided into three wearable sensors for better wearable electronic devices.
types at different processing levels: fibers, yarns, and
fabrics [21]. Yarn and fabric are defined as hierarchically
structured fibrous materials assembling fibers at one-
dimensional and two-dimensional levels, respectively [22]. 2 Coated filament and/or
Electrically conducting fibers and yarns are essential candi-
dates for wearable electronic devices including sensor,
staple yarn
antennae, signal processor, and energy harvester [23]. The
Coating of electrically conductive layer onto the surface
electrical resistance of the yarn varies with the applying
of fibers is a simple measure to impart electronic capabili-
strain, developing a relationship between mechanical and
ties to fibrous materials for strain sensor applications. Such
electrical signals [24]. Among textile-based sensors at dif-
a coating process can be successfully done at different
ferent hierarchical levels, yarn sensors are flexible, light-
hierarchical levels of textile structure, such as fiber, yarn,
weighted, and comfortable [25]. One important advantage
or fabric [35–38]. In general, electrical conductivity can be
of yarn-based sensor is that it has great processing potential
achieved by coating conductive materials including intrin-
for developing different products. Yarn sensor can be easily
sically conductive polymers [39,40], conducting polymer
woven or knitted into different textures for the integration of
composites [41], metals [42], carbon nanotubes [43–45],
sensors within fabric structure [26,27]. In principle, yarn
carbon nano-powders [46,47], and graphene [48]. Accord-
sensor can be designed and fabricated to detect various
ing to reported studies, electrical deposition [49], dip-
stimuli such as pressure, strain, proximity, and temperature.
coating [50], and chemical vapor deposition [51] were suc-
Yarn sensor has been demonstrated for many applications
cessfully used to obtain conductive coating. Especially,
including biomedical monitoring [28], security [29], sports
direct dip-coating and chemical deposition approaches
[30], and display [31]. Several reviews have been published
were widely used to fabricate yarn sensor.
to discuss the progress of textile strain sensor. For instance,
Heo et al. [32] summarized the emerging trends of wearable
textile electronics by incorporating e-textiles into fiber-based
electronic apparel as a smart platform for displays, sensors, 2.1 Dip-coating
and batteries. Seyedin et al. [33] have gathered the most
recent advances in textile strain sensor including fabri- Dip-coating is a widely used facile technique to deposit
cation technology, performance evaluation, and various functional layer onto various substrates including metallic,
Recent advances on the fabrication methods of yarn strain sensor 223
ceramic, polymer films, and fibrous materials [52,53]. Con-
sidering its easy-operation and low cost, dip-coating has
gradually been an increasingly hotspot [54]. The dip-
coating treatment of thin conductive layer onto the surface
of yarn to prepare strain sensor has also exhibited the
advantage of high efficiency.
Silver nanowires (AgNWs) were interpenetrated into
polyolefin elastomer nanofibrous yarn through dipping
treatment [55], and three-dimensional interpenetrating
AgNWs were uniformly distributed in the polyolefin elas- Figure 1: (a and b) Steps in the fabrication of buckling-structured
tomer nanofiber spacing to generate electrical conduc- elastic conducting sheath–core fiber [59]. Copyright 2016, Wiley.
tivity. Li et al. [56] prepared multi-scale nanocomposites
consisting of 0D silver nanoparticles, 1D AgNWs, and 2D
gauge factor and excellent repeatability for the as-prepared
nanosheet structured MXene, and these nanocomposites
yarn strain sensor. The surface morphology of strain sensor
exhibited good electrical conductivity. Polydopamine was
under releasing and stretching is shown in Figure 2(a–c).
also used to enhance the loading of conductive elements,
A simple system was also designed to precisely regulate the
such as delaminated MXene dispersions and silver nano-
robot hand movement, in which yarn strain sensor was
particles, on elastic yarns via dipping or self-growth. The
used as a controlling device. In addition, Chinese brush
as-prepared composite yarn strain sensor showed remark-
pen was used to prepare sliver/waterborne polyurethane
able high strain and sensitivity, and it was able to detect
coating [61], so as to build multi-scale wrinkled microstruc-
both large and small deformation of human body effec-
tures on the surface of polyurethane fibers, as shown in
tively. Furthermore, highly conductive and machine-wash-
Figure 2(d), The core–shell yarn sensor with wrinkled micro-
able silk yarn sensor was developed by dip-coating of Ag
structures can be used to develop flexible piezoresistive
nanowire and PEDOT:PSS composite layer [57]. The silk
devices for the detection of pressure and bending deforma-
yarn strain sensor exhibited high conductivity with excel-
tions. The proposed sensor exhibited high sensitivity, low
lent washability, which largely enhanced its potential in
detection limit, and excellent stability with faster response
wearable applications. Niu et al. [58] used elastic polyur-
because of the dramatically reduced viscoelastic effect.
ethane yarn as the substrate to prepare graphene-coated
yarn strain sensor. A facile roll-to-roll process is feasible
to achieve the large-scale production of nanofibrous com-
posite yarn strain sensor. The yarn was alternately dipped 2.2 Layer-by-layer and ultrasonic-assisted
into polyurethane yarn (PUY) and graphene solution dip-coating
repeatedly followed by polydopamine coating around
the reduced graphene oxide (rGO) layer in the reaction Multiple dipping process was also developed to strengthen
process. The repeated process is necessary to fabricate the deposited layer. Zheng et al. [62] deposited graphene
the polydopamine (PDA) layer on polyurethane core yarn nanosheets onto cotton fabrics followed by the encapsula-
strain sensor. The obtained sensor could be easily incorpo- tion of polydimethylsiloxane. Wu et al. [63] proposed a
rated into textile structure with good comfort and aesthetic facile and cost efficient layer-by-layer dip-coating method
appearance as wearable devices. for the fabrication of highly sensitive strain sensor, as
Some novel coated structures can also be fabricated shown in Figure 3(a). Polyurethane yarn was first coated
via dip-coating. According to Wang et al. [59], cylindrical with conductive natural rubber layer modified by carbon
wood rod was dipped into molten rubber vertically fol- black fillers. Then, the as-treated yarn was dipped into
lowed by fast withdrawn, and then the attached liquid positively and negatively charged solutions alternately.
was solidified in air to produce rubber fiber. The as-fabri- Therefore, the electrostatic layer was deposited onto the
cated rubber fiber was highly stretched to large deforma- surface of polyurethane yarn. The composite yarn with
tion when the fiber was wrapping with carbon nanotube excellent electrical conductivity can be used as strain
sheet, resulting in a hierarchical buckling structure when the sensor. Using polyurethane yarn as elastic core, graphene
forced strain was released, as shown in Figure 1(a and b). Wu and poly(vinyl alcohol) composites were also coated on the
et al. [60] used dip-coating and roll-to-roll technique to surface as conductive sheath by layer-by-layer assembly
prepare sheath–core yarn strain sensor with large scale. method [64]. Recently, the composite of conductive Ag-
The synergistic crack and elastic effects resulted in a high nanoparticles and graphene micro-sheet was used as a
224 Xiaoning Tang et al. Figure 2: (a–c) Diagrammatic sketch of the crack and elastic effects in the breathing sheath–core fiber strain sensor [60]; (d) fabrication process of core–shell conductive fiber with wrinkled microstructure [61]. (a–c) Copyright 2019, American Chemical Society; (d) Copyright 2016, Wiley. Figure 3: (a) Schematic process for the fabrication of CPC@PU yarn by LBL assembly [63]; (b) sheath–core structured graphite/silk strain sensors by dry-Meyer-rod-coating [66]. (a and b) Copyright 2019, American Chemical Society.
Recent advances on the fabrication methods of yarn strain sensor 225
sheath, and silicone encapsulation layer was fabricated on on polyurethane filaments for deposition of silver particles,
the surface of core polyurethane yarn via layer-by-layer as shown in Figure 4(a). It was found that the silver com-
assembly coating [65]. Furthermore, facile dry-Meyer-rod- ponents were well deposited because of the catechol
coating process was reported to prepare conductive sheath/ groups of polydopamine. The developed flexible yarn strain
core-structured graphite/silk yarn strain sensor, as shown sensor with high elasticity and linearity can be applied as
in Figure 3(b) [66]. wearable strain sensing devices. Liu et al. [72] prepared
Ultrasonic processing is an effective method to imple- poly-pyrrole nanostructure layer-coated electro-spun poly-
ment modification of fibrous substrate by nanomaterials, acrylonitrile nanofiber yarn through an in situ chemical poly-
and it can achieve improved properties in comparison merization treatment. The yarn strain sensor exhibited high
with facile dip-coating approach [67]. Li et al. [68] pre- sensitivity and fast response time even in ammonia atmo-
pared highly conductive and stretchable electro-spun sphere. Hong et al. [73] developed a continuous conductive
thermoplastic polyurethane yarn sensor. The yarn was treatment of yarn by in situ polymerization. Conductive silk
first decorated with both multi-walled and single-walled fibroin yarn was coated with polyaniline by a modified
carbon nanotubes under the ultrasonication treatment, method with reduced consumption of reaction solution
and the synergistic effect resulted in an increase in elec- and improved efficiency. Both silk fibroin surface and the
trical conductivity for better strain sensing applications. gap between yarns were covered and filled with polyaniline.
Souri et al. [69] reported the systematic effects on the fab- The as-treated yarn showed the potential to be used as strain
rication of electrically conductive natural fiber yarn via sensors in smart textiles. In conclusion, in situ polymeriza-
coating with graphene nanoplatelets and carbon black in tion is an effective method to deposit conductive compo-
a ultrasonication bath. The results indicated that flax yarn nent on fibrous materials. The chemical bonding effects
as abundant, cost-effective, and lightweight natural mate- can improve the adhesivity between strain sensing layer
rials could be used for stretchable strain sensing devices. and yarn surface.
Recently, superhydrophobic strain sensor based on conduc-
tive polyurethane/carbon nanotubes with polydimethyl-
siloxane matrix was loaded onto electrospun nanofiber
surface by ultrasonication induced decoration. Because of 3 Melt extrusion and wet spinning
the ultrasonic effects, carbon nanotubes were uniformly dis-
persed on the nanofiber surface, and a hierarchical conductive Electrically conductive yarns for wearable devices are
network was successfully built [70]. According to reported generally composed of metallic fibers, carbon nanotubes,
studies, ultrasonication is effective in improving the decentra- graphene, and conductive polymers [74–76]. Polymer
lization of conductive nanofillers. Multiple dipping process and conductive component can be integrated via either
allows regulation of electrical conductivity of yarn strain melt extrusion or wet spinning method. The improvement
sensor via adjusting the dipping times. Recently, layer-by- of the strength and durability has been an important
layer dip-coating assembly has been widely used in the topic [77]. Lin et al. [78] prepared polyester yarns coated
fabrication of yarn strain sensor. Staple fiber yarn or filament with polypropylene and carbon nanotubes layer using
was alternately dipped into different aqueous solutions with melt extrusion method. Polyester yarn as the core com-
opposite charges. The developed yarn strain sensor exhibited ponent has an easy-processing feature, and it provides
good flexibility and stretchability, which can be easily incor- robust mechanical properties for the final conductive
porated into textile structures through weaving, knitting, and yarns. However, the as-prepared yarn has been limited
braiding for wearable sensing applications. to detect tiny body movement because of its poor elasti-
city in strain sensing applications [79]. Recently, Liao
et al. [80] proposed a cluster-type microstructure strategy
2.3 Chemical deposition coating for fabrication of yarn strain sensor using nozzle jet
printing method, as shown in Figure 4(b). This work
In situ chemical deposition to fabricate layers on fibrous has indicated that the intrinsic elasticity of textiles can
materials exhibits several advantages including environ- be used to realize unique microstructure design when
mental friendliness, robust bonding strength, and cost- nozzle jet printing the conductive layer.
effectiveness. It is an effective technique to achieve a homo- Wet spinning technique of yarn strain sensor fabrica-
genous integration between inorganic components and tion has been widely used. Generally, the process mainly
polymer matrix. Liu et al. [71] developed an in situ poly- includes the following steps: (1) the preparation of spin-
merization method to prepare adherent polydopamine film ning solution; (2) the formation of fine stream by pressing
226 Xiaoning Tang et al. Figure 4: (a) Sketch map of conductive polyurethane filaments by in situ reduction and electroless silver plating [18]; (b) fabrication of strain sensor with cluster-type microstructures [80]; (c) AgNWs and AgNPs in the composite fiber under different conditions [81]. (a) Copyright 2017, Elsevier; (b) Copyright 2019, American Chemical Society; (c) Copyright 2015, Wiley. the solution out of the spinneret; (3) the production of the sensing behavior of yarn strain sensor can be well primary fiber because of the coagulation of the solution; regulated by facile alteration of loop configuration and and (4) coiling and direct post-treatment [82–84]. Wet stitch insertion, resulting in five different knit prototypes. spinning has been widely used to fabricate yarn strain Recently, He et al. [86] reported a novel highly sensitive sensor. For instance, Seyedin et al. [28] developed a wet- strain sensor based on multi-walled carbon nanotube spinning method to fabricate electrically conductive and and thermoplastic polyurethane through wet spinning highly stretchable yarn. It has been found that the poly- process. The composite filament exhibited strong tensile urethane/PEDOT:PSS yarn exhibited robust mechanical strength and ultra-high sensitivity with an approximate properties to meet the requirement of knitting technique. gauge factor of 2,800, which fulfills the requirement of Highly stable sensor was successfully fabricated by the accurate detection of wearable electronics. Melt extrusion co-knit processing with commercial Spandex yarn. The method can effectively avoid the breakage phenomenon production of highly stretchable conductive yarn con- of spinning with high loading fillers. The strain sensor sisting of AgNWs and nanoparticles in styrene–butadiene– exhibited a considerably high sensitivity and high stretch- styrene elastomeric matrix was also reported [81]. Sliver ability because of the unique design of its geometric struc- nanowires decorated styrene–butadiene–styrene fila- ture and the effectiveness of the conductive sensing materials. ment was fabricated by facile wet spinning method, in The structure evolvement has increased the permeation which sliver nanoparticles were deposited on the surface of conductive components into the inner region of yarn in and inner region of composite filament via repeated the filament formation process. cycles of silver precursor absorption and reduction treat- Coaxial wet spinning was used to achieve diversified ment, as shown in Figure 4(c). Seyedin et al. [85] also design of filament. It is a powerful tool to integrate dif- achieved large scale production of conductive elasto- ferent components into a sheath–core structure [88–90]. meric filament, which can be used as strain sensor For instance, Zhou et al. [87] prepared coaxial filament to detect large strains with high stability. Furthermore, made of thermoplastic matrix via coaxial wet spinning
Recent advances on the fabrication methods of yarn strain sensor 227
assembly technique, as shown in Figure 5(a and b). Solu-
tion stretching–drying–buckling approach was also devel-
oped to obtain desired hierarchical structures, and the
prepared yarn strain sensor exhibited self-buckled conduc-
tive core. The unique buckled structure is beneficial in
improving the stability of electrical conductivity. The yarn
strain sensor can be repeatedly stretched up to 680%
with less than 4% resistance change, and thus it can be
used in high-conductivity applications. Guo et al. [91]
reported the fabrication of stretchable optical strain sensor
with wide sensing range and high sensitivity. The yarn
strain sensor was fabricated with highly stretchable poly-
dimethylsiloxane matrix through core/cladding step-index
configuration design, where the inner space was embedded
with conductive gold nanoparticles. The as-prepared strain
sensor can be used in the integration of wearable electro-
nics, including human–machine operation, personal health-
care, and intelligent robotic. Figure 6: (a and b) Schematic illustration of the modeling structure
and the schematic drawing of doubled covered structure [92];
(c) fabrication process of the PU/Cotton/CNT yarn [94] (a and b)
Copyright 2015, Wiley; (c) Copyright 2016, American Chemical
Society.
4 Twisting structure design of
yarn sensor helically winded polyester fibers, because of which the
yarn exhibited excellent bending and torsion-sensitive
4.1 Sheath–core spun yarn efficiency after a dip-coating and reduction treatment
to obtain electrical conductivity. In another work [93],
The sensing of multiple mechanical deformations has commercial composite yarn consisting of central elastic
posed an urgent challenge to both industrial and aca- rubber latex thread and winding polyurethane fibers was
demic researchers. Cheng et al. [92] developed a facile used as scaffold. The stretched yarn was deposited with P
and low-cost strategy to fabricate graphene-based com- (VDF-TrFE) nanofibers followed by the deposition of
posite yarn with compressive spring architecture, as sliver as conductive layer. Wang et al. [94] designed a
shown in Figure 6(a and b). The sheath–core spun yarn wrapping and coating device to achieve the fabrication
consisted of a stretchable core polyurethane filament and of cotton/polyurethane core-spun yarn, as shown in
Figure 5: (a and b) Coaxial wet‐spinning process for encapsulating the conductive dispersion in an elastic thermoplastic elastomer (TPE)
channel [87]. Copyright 2019, Wiley.
228 Xiaoning Tang et al.
Figure 7: (a–c) Schematic illustrations concerning the preparation of CNT/cotton roving and composite fabric [100]; (d–f) SEM of braided
composite yarns by in situ polymerization of poly-pyrrole at different magnifications [101]. (a–c) Copyright 2018, American Chemical
Society; (d–f) Copyright 2019, American Chemical Society.
Figure 6(c). During the winding process of cotton fibers stretching process, sheath–core yarn with wrinkle-assisted
on polyurethane filament surface, conductive single-wall crack microstructure can be fabricated. Cracked conduc-
carbon nanotube was incorporated into the core-spun tive network was integrated with structural wrinkles to
yarn through coating treatment. The self-designed equip- obtain wrinkle-assisted crack microstructure yarn strain
ment with simplicity can well achieve the uniform sensors [98]. The yarn strain sensor exhibited high
covering of twisted fibers and scalable production of sensitivity with the gauge factor of 1344.1, ultra-low
sheath–core yarn [95]. Considering the brittleness of carbon detection limit of less than 0.1% sensing range, excellent
nanofiber yarn, the generated subtle cracks can increase the durability, large workable deformation, and sensitive
sensitivity during stretching process. Yan et al. [96] reported response to bending stimuli. Apart from polyurethane
the fabrication of sheath–core helical yarn through carbo- fiber, latex filament was also used to fabricate sheath–
nization of core cotton yarn while electrospun polyacrylo- core spun yarn strain sensor. Flexible latex was wrapped
nitrile nanofiber was used as a wrapping sheath. The yarn with polyester filaments followed by the deposition of
was effective to monitor subtle strains as lower as 0.1% conductive poly-pyrrole [99].
with good sensitivity. Recently, natural silk fiber was also Traditional yarn manufacturing technique was also
functionalized by tailor-made carbon nanotube paint to used to fabricate yarn strain sensor. Conductive compo-
fabricate strain sensor, thus to detect the physical stimuli site yarn consisting of spandex filament as the stretchable
of human body [97]. core and carbon nanotube as the sheath was successfully
Considering the mismatch of modulus and elasticity fabricated [100]. Carbon nanotubes were incorporated into
between carbon nanotubes and polyurethane yarn in the cotton rovings by dipping and drying treatment before the
Recent advances on the fabrication methods of yarn strain sensor 229 Figure 8: (a) Illustration of the spinning process involving the fabrication of helical yarn from carbon nanotube film [108]; (b–g) fabrication and characterization of double-helix carbon nanotube yarn [109]. (a) Copyright 2012, Wiley; (b–g) Copyright 2013, American Chemical Society. traditional sirofil spinning, as shown in Figure 7(a–c). fabricate highly stretchable cotton/carbon nanotube sheath– Cotton roving was first dipped into the CNT solution, core yarn. The yarn exhibited excellent stretchable capacity thus to prepare CNT-cotton roving. Then sirofil spinning because of the unique spring-like structure, which can be technique was used to achieve the winding process of used as wearable strain sensors with ultrahigh strain sensing Spandex yarn. The as-prepared CNT/cotton/spandex com- range (0–350%), thus for both subtle and large human posite yarn exhibited excellent electrical conductivity and motion monitoring [102]. improved mechanical properties with super-stretchability. Apart from wrapped fibers, nanomaterials can also Pan et al. [101] reported the fabrication of yarn strain be used as a sheath layer to prepare sheath–core yarn sensor by in situ polymerization of poly-pyrrole on poly- strain sensor. Spandex filament was continuously wrapped dopamine templated core-spun yarn surface, as shown in with carbon nanotube aerogel sheets to prepare conducting Figure 7(d–f). The composite yarn has unique braid struc- stretch fabric via interlocking circular knitting technique ture with a cauliflower-like poly-pyrrole layer formed on [103]. Wang et al. [104] performed thermal annealing treat- the surface. The result indicated that the braid structure ment of graphene-based stretchable conductive yarn to together with the elastic polymer exhibited a steady-going increase electrical conductivity, and the yarn was used in and reversible strain sensing performance. Furthermore, the fabrication of twist-spinning graphene film. The core– multiple spinning optimization process was also used to sheath structure can also be obtained by dipping pure
230 Xiaoning Tang et al. Figure 9: Preparation and morphology of CNTs/PU helical yarn. (a) The fabrication scheme of the helical CNTs/PU yarn. The optical images of (b) PU nanofibers strip, (c) intermediate state of the twisting process, (d) the twisted CNTs/PU yarn, and (e) the overtwisted CNTs/PU helical yarn. SEM images of (f) aligned PU nanofibers, (g) torsion angle during twisting process, (h) the uniform straight CNTs/PU yarn, (i) the helical CNTs/PU yarn with spring-like coils, (j) CNTs network on PU nanofibers surface, and (k) high magnification of CNTs network. The cross section SEM of (l) CNTs/PU twisted yarn and (m) amplification image of section [112]. Copyright 2019, American Chemical Society. carbon nanotube yarn into polyvinyl alcohol solutions 4.2 Helical yarn [105]. This method prevents the slippage of carbon nano- tube bundles, allowing the yarn sensor to exhibit stable The unique compression spring structure because of the resistance with cyclic loaded strain for promising switch- helically twisted filament is an effective approach to obtain type strain sensor applications. Recently, super-elastic poly- good elasticity. Different from the above-mentioned urethane yarn was wrapped by graphene nanosheets/gold/ sheath–core structure, helical structure can be fabricated graphene nanosheet film and thin polydimethylsiloxane by a single yarn. Zhao et al. [107] developed a prototype layer [106]. The composite structure provided the yarn strain carbon nanotube yarn strain sensor with excellent repeat- sensor with high flexibility and sensitivity, wide strain-sen- ability and stability for in situ structural health detection. sing range, and excellent waterproof efficiency. Compared The yarn was directly spun from the as-grown carbon with conventional strain sensors based on metal and semi- nanotube arrays, and the twisting process resulted in an conductors, the sensing range of general yarn strain sensor electrically conductive pathway in the longitudinal direc- has increased. However, typical yarn-based sensor is still tion. It is a promising strain sensor as the electrical resis- limited to detect the strenuous body movements involving tance increases linearly with tensile strain. Shang et al. tensile strain, such as bending and twisting. The elasticity [108] fabricated yarn-derived spring-like carbon nanotube of sheath–core yarn strain sensor can be dramatically rope consisting of uniformly arranged loops, as shown in enhanced using polyurethane surrounded by conventional Figure 8(a). The spring-like rope was obtained by over- yarn. The excellent flexibility of core filament provided twisting the randomly oriented carbon nanotube film using robust resilience after the stretching process, resulting in a modified spinning technique. Furthermore, Shang et al. a highly stretchable yarn strain sensor. [109] prepared yarn-derived two-level hierarchical composite
Recent advances on the fabrication methods of yarn strain sensor 231
nanotube yarn to convert mechanical movement into
electrical energy because of the stretch-induced capaci-
tance variability. The yarn can be used to generate elec-
trical power by periodic deformations, e.g., the movement
of stomach can be measured during both food mixing and
emptying into the intestine to produce the electrical
pulses signals. In comparison with the mostly studied
straight carbon nanotube yarn, the hierarchical yarn
exhibited both good elasticity and high strength for the
fabrication of textile strain sensor. The unique structure
exhibited separated two-stage fracture behavior, which is
important to prevent catastrophic failure in potential
strain sensor applications. The helical nanotube yarn
can also be over twisted into highly entangled structure
consisting of tangled double-helix segments, thus for
wearable electronic devices.
4.3 Fancy twisting
Based on conventional sheath–core and helical struc-
tures, novel fancy twisted structure was fabricated to
improve both mechanical and strain sensing properties.
Figure 10: (a) Schematic diagram of stretchable self-powered yarn
It has exhibited the advantage of complex structure
strain sensor [115]; (b–e) schematic illustration and morphology of
design, which can meet the requirements of different
the yarn sensor under stretching and releasing conditions [117].
(a) Copyright 2015, Wiley; (b–e) Copyright 2019, American Chemical strain sensing applications. Shang et al. [114] fabricated
Society. a straight-helical-straight hybrid structure yarn strain
sensor with enhanced electromechanical properties. Com-
pared with long all-helical yarn, short segment with finite
structure consisting of twisted double-helical yarns, as loops was incorporated into a straight yarn to produce partial-
shown in Figure 8(b–g). The yarn end was adaptive to helical structure. The yarn exhibited good elasticity and
the recoverable drag, resulting in a large linear change resilience under suitable strains and linear resistance–
of tensile strain against electrical resistance. The exten- strain relationship. Zhong et al. [115] fabricated an active
sively twisted effects of entanglement enable the yarn to fiber-based strain sensor consisting of different threads mod-
be a stretchable strain sensor [110]. ified by carbon nanotube, as shown in Figure 10(a). These
Ultra-high stretchable sensor was also fabricated by threads were entangled to produce double-helical structured
dry-spun method, in which carbon nanotube fibers were yarn, and then the threads were wrapped around silicone
grown on flexible substrates [111]. This method is bene- filament to form core–sheath yarn strain sensor. For stretch-
ficial in reducing the conductive path, enabling carbon able wearable device, the wrapped threads can be used as
nanotube fibers to be used as highly sensitive strain the elastic components because of the unique spring defor-
sensor. Gao et al. [112] fabricated carbon nanotubes and mation. Bi-sheath buckled structure was also designed to
polyurethane nanofiber composite helical yarn, in which increase the contact between adjacent buckles, in which
the synergy between mechanical properties and spring- two layers of buckled carbon nanotube sheets and buckled
like microgeometry resulted in the high elasticity, as rubber interlayer coaxially existed on the elastic filament [116].
shown in Figure 9. Carbon nanotubes were effectively These sheets were totally contacted with the rotating rubber
winding-locked into the helical yarn through twisting filament to obtain a well-linear resistance–strain dependence.
approach for high conductivity. The interlaced conduc- Lu et al. [117] developed carbon nanotube/rubber core-
tive helical network at both microlevel and macrolevel sheath yarn with double-leveled helical gaps, in which
can achieve robust recoverability and tensile elongation. electrically conducting fibers were wrapped around a
Recently, Jang et al. [113] fabricated coiled helical carbon highly stretchable core filament, as shown in Figure 10(b–e).232 Xiaoning Tang et al. The yarn was used to detect ultralow strain of 0.01% with a Author contributions: All authors have accepted respon- wide sensing range (>200%), rapid response (
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