Stretchable and Highly Sensitive Braided Composite Yarn

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Applications of Polymer, Composite, and Coating Materials

Stretchable and Highly Sensitive Braided Composite Yarn@Polydopamine@Polypyrrole for Wearable Applications Junjie Pan, Mengyun Yang, Lei Luo, Anchang Xu, Bin Tang, Deshan Cheng, Guangming Cai, and Xin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18823 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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ACS Applied Materials & Interfaces

Stretchable and Highly Sensitive Braided Composite Yarn@Polydopamine@Polypyrrole for Wearable Applications Junjie Pana, Mengyun Yanga, Lei Luoa, Anchang Xua, Bin Tang a,b, Deshan Chenga*, Guangming Caia*, Xin Wangc* aState

Key Laboratory of New Textile Materials and Advanced Processing Technologies, Wuhan Textile University, Wuhan 430073, China bInstitute for Frontier Materials, Deakin University, Geelong 3216, Australia cSchool of Fashion and Textiles, RMIT University, Melbourne 3056, Australia Abstract Flexible wearable devices for various applications have attracted research attention in recent years. To date, it is still a big challenge to fabricate strain sensors with a large workable strain range while maintaining their high sensitivity. Herein, we report the fabrication of highly sensitive wearable strain sensors from braided composite yarns (BYs) by in-situ polymerization of polypyrrole (PPy) on the surface of yarns after polydopamine templating (BYs-PDA). The electromechanical performance and strain sensing properties of the fabricated braided composite yarn@polydopamine@polypyrrole (BYs-PDA-PPy) were investigated. Due to the unique braided structure of BYs, the BYs-PDA-PPy strain sensors exhibit fascinating performance, including a large workable strain range (up to 105 % strain), high sensitivity (gauge factor of 51.2 in strain of 0%-40% and of 27.6 in strain of 40%-105%), long-term stability and great electrical heating performance. Furthermore, the BYs-PDA-PPy sensors can be used in real-time monitoring subtle and large human motions. The BYs-PDA-PPy strain sensors can also be woven into fabrics for large area electric heating. These results demonstrate the potential of BYs-PDA-PPy in wearable electronics. Keywords: Braided composite yarns; Polydopamine; PPy; Strain sensor; Electromechanical performance Introduction Flexible and stretchable wearable devices have attracted more and more attention due to their great application potential in areas such as wearable displays,1, 2 smart clothing,3,4 human motion5,6 and health monitors.7, 8 Flexible strain sensors, as a component of wearable electronics, are mainly used in developing electronic skins9-10 and wearable monitoring systems.11-15 The traditional metal/semiconductor-based strain sensors exhibited low stretchability or narrow sensing ranges.16-19 Recently, flexible strain sensors based on conductive sensing materials with elastic polymer matrix have been widely reported.20-23 The most typical used strain sensing materials include metal nanowires, conducting polymers, and carbon nanomaterials like carbon nanotubes and graphene.24-30 Easy preparation, low cost, continuous production are important factors that determine the application potential of polymer based composite sensors,31 thus these nanomaterials were incorporated either by depositing them directly on the top surface or by embedding them into the matrix of the stretchable polymer stem.32-35 Wearable strain sensors with a large workable strain

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range and high sensitivity simultaneously require a high stretchability and the reliable mechanism of sensing. Besides, direct contact of these polymers with body causes the mismatch between sensor and skin, leading to failure in response. Textiles have flexibility, high tensile strength, high recoverable deformation and friendly contact with human body, so that they have been widely used as substrates for strain sensors.36-39 For example, elastic fiber and fabrics were used to fabricate strain sensors, but the gauge factor was low.40, 41 Carbonized textiles were also used to fabricate sensors with sensitivity and large strain sensing range, but the carbonization process destroys the structure of fabrics with mechanical properties deteriorated and the complicated fabrication process hinders their practical application potential.42-44 Graphene functionalized double-covered yarns were used to fabricate strain sensors with large sensing ranges, but the gauge factor was low.45 Our group reported the scalable preparing of cotton/carbon nanotubes/spandex yarn for wearable application,46 but still the gauge factor of the elastic yarn is low. Therefore, it is still a big challenge to develop a flexible fiber-base strain sensor with both high sensitivity and large strain sensing range. Furthermore, graphene and carbon nanotube as conductive materials incorporated into fibrous system are not very conductive due to the lack of continuous contact conduction mechanism, thus their applications are highly limited.36,40,41 Polypyrrole (PPy) is highly electrically conductive, and it can be easily deposited through in situ polymerizing on the surface of fiber for wearable devices.47 Considering the frequent strains and deformation of the wearable sensors in real use, application of PPy as flexible sensor would need a combining mechanism between the substrate materials and the PPy. Dopamine is a critical functional element of adhesive proteins excreted by mussels and other marine organisms, and it can polymerize to form a polydopamine (PDA) layer on almost any surface towards versatile functional coating.48, 49 PDA is rich in catechol and amine functional groups to ensure strong adhesion to any substrates including PET,50 resulting in durable and strong coated surface on the substrates.51 Durable and highly flexible wearable sensor from fibrous materials with a large workable range is highly demanded, but it is of great challenge as the special textile structure together with bonding mechanism between fibers and conductive components have to be developed synergistically. In this work, we developed a simple, low-cost and scalable fabricating strategy for preparing highly sensitive and stretchable strain sensor from braided composite yarns, and the yarns were demonstrated to be sensitive wearable sensors with large workable range and wearable electronics with heating performance. 2 Experimental 2.1 Materials PET fibers (polyethylene terephthalate, ≈20 µm in diameter) and the elastic polymers (a rubber core fiber, 0.5 mm in diameter, 0.0123 g/5 cm) were scoured with deionized water followed by a cleaning in acetone before use. 3-hydroxytyramine hydrochloride (dopamine hydrochloride) was purchased from Aldrich Chemical Co. (Milwaukee, USA). Tris(hydroxymethyl) aminomethane (Tris) pyrrole, ethanol, anhydrous ferric chloride, triton X-100, sodium dodecyl sulfate (SDS), dodecyl benzene sodium sulfonate (SDBS), and camphorsulfonic acid (CSA) were purchased from Aladdin Chemical Co. (Shanghai, China). All the chemicals were in analytic grade and were used without further purification. 2.2 Fabrication


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The fabrication process of the BYs-PDA is shown in Figure 1a. Firstly, the PET fibers were cleaned ultrasonically in deionized water for 30 min. Subsequently, a dopamine solution with a concentration of 10 mM was dissolved in the Tris buffer solution with the pH value adjusted to 8.5 by adding HCl. Then, the clean PET fibers were directly dipped into the freshly prepared dopamine solution at room temperature. After stirring for 24 h, the PET fibers were washed thoroughly with deionized water followed by hanging dry. The resulted PET fibers were wounded on bobbins for spinning. Secondly, eight PET fiber bobbins together with the elastic polymer were fed into the braiding machine to fabricate braided composite yarns (denoted as BYs-PDA).

Figure 1. Schematics of the fabrication process of BYs-PDA (a), BYs-PDA-PPy (b) and functional groups interactions between PPy, PDA and PET (c). The fabrication process of the BYs-PDA-PPy is shown in Figure 1b. The as-fabricated BYs-PDA yarns were dipped into a pyrrole aqueous solution (with different concentrations) with sodium dodecyl benzenesulfonate (SDBS) and cetyltrimethylammonium bromide (CTAB) as surfactants for 1 h. An aqueous solution of iron (III) chloride hexahydrate (FeCl36H2O, 0.5 M) was slowly dropped into the solution to start the polymerization process, and the polymerization was carried out at 0 oC for 2 h. There was an immediate color change from brown to black when PPy was polymerized (Figure 1b). The fabricated BYs-PDA-PPy was then used as the wearable sensor. In addition, the BYs-PDA-PPy was woven into a BYs-PDA-PPy fabric for studying wearable heating performance



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(Figure 1b). The functional groups interactions between PPy, PDA and PET are shown in Figure 1c. The obtained BYs-PDA-PPy was thoroughly rinsed with water and dried in a vacuum oven at 50 oC. Copper wires were attached onto both ends of BYs-PDA-PPy as electrodes, and silver paste was used to glue the electrodes to the yarn so as to ensure a good contact. 2.3 Measurements and characterizations The surface morphology and elemental composition were observed on a scanning electron microscope (SEM, JSM-5600LV, JEOL, Japan) with an energy dispersive X-ray spectrum (EDX, Oxford Instruments, Oxford, UK). Optical photographs were taken by an optical camera (OLYMPUS DSX510) under ordinary white light. Fourier transform infrared (FTIR) spectra were recorded by a Tensor 27 spectrometer (Bruker, Germany) equipped with attenuated total reflectance (ATR) cell. Mechanical properties were measured using an Instron 5566 Materials Testing System. Tensile strength of a single yarns was measured at a gauge length of 250 mm with the drawing rate of 80 mm/min and each specimen was tested 10 times. The resistance of BYs-PDA-PPy was recorded by a digital multi-meter (Keysight Truevolt 34465A). The Origin 8.0 was used as data plotting software. The yarn sample (length 3 cm) was fixed between two insulators and the gauge length was set as 3 cm (see test details in Figure S1). The resistance of the test leads was 6.2  and the same leads were used in every test. The two insulators were controlled by a motor to perform traverse movements, creating a cycling movement with the speed adjustable (typical speed 50 mm/min). After each experiment, the yarn was recovered for at least 10 minutes. The relative change of the resistance was calculated by the formula: ∆R/R0= (R- R0)/R0 (1) where R0 represents the original resistance and R is the resistance of the sample under applied strain. The gauge factor (GF) of the BYs-PDA-PPy strain sensor was defined as: δ(∆R/R0)/δε (2) where ε denotes the applied strain and ∆R represents the relative change of the resistance as calculated by the Formula (1). The human body motions were monitored using human models. The 2 cm length BYs-PDA-PPy sensors were attached onto human body using insulating adhesive tape. Different locations of body, such as knuckle, finger, wrist, biceps, brachii, and elbow, were selected to do the test. The temperature change and thermal imaging feature were measured by an Infrared thermal camera (FLIR ONE Pro). In the test, the BYs-PDA-PPy fabrics were heated for 150 s and then the surface temperature together with the thermal images were recorded. 3. Results and discussion Conductive PPy was deposited onto the surface of braided composite yarns through PDA templating. The high elasticity of composite yarns provides the strain sensor with large stretchability and thus a broad sensing range (up to 105%). Braiding structure of the composite yarns ensures the high sensitivity of the strain sensor. The PDA ensures the strong adhesion between PPy and fibers (Figure 1c), endowing durability and repeatability of the sensor. More significantly, the strain sensor can be woven into fabrics for wearable applications as motion monitors and electrical heaters.


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3.1. Structure Owing to the fabrication method, the BYs-PDA shows a typical braided structure as shown in Figure 2a. The PET fibers bundles intertwined from different directions, forming a compact and interlaced structure on the surface of the elastomeric polymer. The cross-sectional view clearly reveals that the PET fibers are wrapped around the polymer core (Figure 2a). The PET fibers are fully covered with polydopamine films (Figure 2b-e) as a result of dip coating of PDA. The surface of fiber is not smooth (Figure 2e) with grooves and uneven morphology. The corresponding cross-sectional view of BYs-PDA further confirms the skin-core structure of PET fiber winding round the elastomeric polymer.



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Figure 2. Optical microscopic images of BYs-PDA (a); SEM of BYs-PDA at different magnifications (b-e); EDS spectra of PET fibers and BYs-PDA (f-g); FTIR spectra of PET fiber and BYs-PDA (h) and SEM photos of BYs-PDA-PPy with different magnifications (i-l). The chemical composition analysis from EDS spectra (Figure 2f-g) reveals that the signal of nitrogen can be detected on BYs-PDA as compared to that of PET fiber, which further proves the successful polymerization of PDA on the surface of PET fibers. The FTIR spectrum of BYs-PDA (Figure 2h) shows a broad peak around 340 cm-1 corresponding to the O-H stretching vibrations of catechol groups and a peak at 1610 cm-1 attributing to the ring structure vibrations and N-H bending vibrations.52 The existence of new peaks as compared to the FTIR spectrum of PET fiber is due to the coating of PDA. The PDA templates on the surface of PET fibers has enhanced the in situ polymerization of PPy, resulting in a thick polymeric layer of PPy on the surface of BYs-PDA-PPy (Figure 2i-j). In the magnified SEM images (Figure 2k-l), cauliflower-like PPy polymeric layer can be seen from the surface of fibers. In the fabrication process of BYs-PDA-PPy as shown in Figure 1b, the polymerization of pyrrole was happening in the solution while the resulted PPy was attracted onto PDA layer, forming functional group between the PPy polymeric layer and PDA (Figure 1c). The PPy was thus anchored onto the surface of PDA layer to initiate an even coating without accumulation. The uneven surface with some particles of the PDA layer contributed to the forming of accumulated cauliflower like structure of PPy. 3.2. Properties The mechanical performance is important for strain sensors. With the special braided structure, BYs-PDA-PPy represents mechanical properties with large strains, as shown the representative force-elongation curves in Figure 3a. Owing to the good mechanical properties of BYs, the BYs-PDA and BYs-PDA-PPy show an elongation of >200% and a force of 25 N at breaking (Figure 3b). The elastic polymer as the core provides deformation capacity to the BYs, so that the yarn is highly stretchable. However, after coating with PDA, the force at breaking of the BYs-PDA decreased by 20%. The existence of PDA within the BYs has affected the inter-fiber structure, as a result the strength was deteriorated. At the same time, the stretchability of BYs was lower after PDA coating. Due to the PDA layer on the surface of and in between fibers bridging the fibers and polymer together, the deformation of PET fibers became harder. It was also noted that the force at breaking increased slightly with PPy polymerization, and this might be attributed to the formation of the compact layer of PPy on the yarn surface. The >200% elongation of BYs-PDA-PPy meets the requirement of strain sensor in wearable applications.53, 54


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It is expected that the BYs-PDA-PPy will have a strong durability against abrasion due to the formed group bonding between BY, PDA and PPy (Figure 1c). To prove the strong adhesion of the PPy to the BYs-PDA, a simple scotch tape adhesion test was performed. It can be seen clearly from Figure S2 that no peeling can be observed for the BYs-PDA-PPy after 30 cycles of scotch tape adhesion test. The resistance does not change much after the repeated scotch tape adhesion test, indicating the strong adhesion of the PPy to the BYs-PDA. The BYs-PDA-PPy shows electrical conductivity due to the coating of PPy on its surface and each specimen was tested at 5 different positions. The electrical resistance of BYs-PDA-PPy is shown in Figure 4. As the concentration of pyrrole aqueous solution increases, the resistance of BYs-PDA-PPy decreases quickly due to the increase of the content of PPy. However, when the concentration of pyrrole is higher than 0.8 mol/L, the resistance of BYs-PDA-PPy tends to be stabled with the lowest resistance of 0.75 k/cm at 0.8 mol/L.

Figure 3. Force-elongation curves (a) and values of force and elongation (b) for BYs, BYs-PDA and BYs-PDA-PPy.

Figure 4. Electrical resistance and PPy content of BYs-PDA-PPy under different concentrations of pyrrole aqueous solution. 3.3. Performance The relative resistance variation (∆R/R0) of BYs-PDA-PPy under various strain loading conditions is shown in Figure 5a. There is an evident increase of ∆R/R0 with the increase of strain from 0 to 105%, suggesting that the BYs-PDA-PPy has a very wide strain sensing range. The curve is divided into two parts with the strain range of 0%-40% and 40%-105%, corresponding to the gauge factor (GF) of 51.2 and 27.6, respectively. The change of resistance is attributed to the deformation of the PET fibers as a result of the strain. The stretching of BYs-PDA-PPy forced the PET fibers to incline,



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resulting in the increase of the winding angle. The contact within PPy layer were then affected due to the movement of PET fibers, so that the change of resistance increased gradually. Compared with the reported flexible fiber-based strain sensors in literatures (Table 1), the sensor reported in this work has the highest GF in the 0%-105% strain range with a reasonably large sensing range. The sensitivity and workable strain range are the determining factors for wearable sensors to detect human motions. Due to the imperfection of the structure of materials and the sensing mechanism, the current fiber-based strains sensors could not balance between the sensitivity and the workable strain range. Figure 5b-c shows the relative resistance variation of BYs-PDA-PPy under cyclic stretching-releasing strain of 1%, 2%, 3%, 4%, 5%, 10%, 20%, 50%, 100% and 120%. The ∆R/R0 increases linearly with the increase of the strain in the range of 0-100% (see Figure S3a), which is consistent with the results shown in Figure 5a. Meanwhile, the ∆R/R0 returns back to its original value in the releasing process. It can be seen that the value of ∆R/R0 experiences the same change in each cycle with the almost same value of each peak as shown in Figure 5b-c and the error bars in Figure S3a, and this is due to the similar deformation and recovery of the conductive network under the same strain. It is noticed that the ∆R/R0 value decreases when the strain exceeds 105%, indicating the limitation of the strain range of the BYs-PDA-PPY. Some shoulder peaks can be found from Figure 5c, and this is probably due to the hysteresis effect of composites under larger strains.48 The results demonstrate that the BYs-PDA-PPy sensor is very sensitive with cyclic stability and repeatability under different strains.

Figure 5. Relative resistance change (∆R/R0) as a function of strain with the loading speed 50 mm/min of BYs-PDA-PPy (a) (Inset: photo of the testing); ∆R/R0 versus cyclic strain of 1%, 2%, 3%, 4%, 5%, 10%, 20%, 50%, 100% and 120% (b-c); ∆R/R0 under cyclic strain of 10% at different loading speeds (d); Effect of PPy concentration on the ∆R/R0 (10% strain) (e); Durability of BYs-PDA-PPy under cyclic strain of 10% (f). The BYs-PDA-PPy sensor has a high adaptivity to the stimuli. Figure 5d shows the ∆R/R0 with a


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cyclic strain of 10% under different drawing speeds. When the tensile speed increases gradually from 10 mm/min to 100 mm/min, the ∆R/R0 of the sensor exhibits almost constant values under each cyclic stretching/releasing. Statistical analysis of the peak values of ∆R/R0 (Figure S3b) shows the peak values are not significantly different when the speed is lower than 100 mm/min. In addition, the ∆R/R0 of the sensor exhibits almost constant values at 20% cyclic strain with different loading speeds (see Figure S4). The adaptivity of the sensor is crucial in wearable applications as a reliable response under different external stimuli ensures the reliability of the sensor. In order to investigate the effect of PPy concentration on stretchability and sensitivity of the BYs-PDA-PPy, the ∆R/R0 versus strain under different PPy concentrations were investigated. It can be seen from Figure 5e that PPy concentration has limited affection on the sensing performance of BYs-PDA-PPy as the curves for ∆R/R0 are almost the same with a bit variation in the peak value (Figure 3Sc). Besides, the periodic spectrum of the ∆R/R0 suggests that the stretchability of the BYs-PDA-PPy has not been affected by the change of PPy concentration. Therefore, in a certain range (such as 10% strain) the pyrrole concentration has little effect on the sensibility of BYs-PDA-PPy. Furthermore, the BYs-PDA-PPy sensor has cyclic stability, as shown in Figure 5f the response of the sensor under ≈400 times cyclic strain of 10% at a tensile speed of 10 mm/min. The ∆R/R0 value experiences almost identical rising and falling in each cycle (inset of Figure 5f), which proves the reproducibility and stability of the sensor. Table 1 A comparison of the gauge factor (GF) of different fiber-based sensors. Strain sensor Graphene-based fiber45 CNTs coating fabric55 Carbonized cotton fabric44 Carbonized silk fabric42 Carbon nanotubes/elastic bands48 Graphene coating fabric40 CNT/cotton/Spandex composite yarn46 SWNT/MWNT polyurethane yarns30 Conductive cotton and wool fabrics37 MCNT/thermoplastic elastomer buckled sheath-core fiber26 Graphene textile56 BYs-PDA-PPy

Strain range 0-1% 1-50% 0-6% 8-50% 55-76% 0-80% 80%-140% 0-250% 250–500% 0-250% 780%-920% 0-10% 10%-18% 0-40% 40-400% 0-20% 20-120% 0-120% 120-150% 0-150% 200-1500% 0-8% 0-40%


Gauge factor 10 3.7 15.6 8.1 5.4 25 64 9.6 37.5 5.06 129.2 18.5 12.1 3 0.1 1.67 1.24 1.67 6.05 21.3 34.22 26 51.2


40-105%

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27.6

The sensitivity, adaptivity, stability, durability and large workable sensing range of the BYs-PDA-PPy are due to the extraordinary structure of the yarn. The sensing mechanism of BYs-PDA-PPy is depicted in Figure 6. The change of structure upon a strain incurs the contacts between conductive PPys, so that the change of resistance is presented to sense the strain. When the BYs-PDA-PPy was drawn, the PET fibers on its surface inclined and the winding angle increased (Figure 6a-c). For example, the winding angle increased from 13o to 22o and then 35o when the elongation was 0%, 30% and 50%, respectively. As shown in Figure 6d, the contact area between neighboring fiber strands (overlapping point) decreases as a result of the increase of the winding angle. Besides, the increase of winding angle also leads to the increase of the distance between PET fibers with gaps generated in between them. It is estimated that the resistance of each overlapping point will be increased due to the less conductive capability as a result of the decrease of contacting area. As a whole, all the overlapping points working together makes the resistance of BYs-PDA-PPy increase tremendously. Considering the fabrication of the BYs as shown in Figure 1a, the BYs-PDA-PPy can be regarded as a combination of eight PET fibers. The sensor can thus be viewed as eight separate resistances connected in parallel, as proposed the model in Figure 6e. The overall resistance of the model is decided by the resistance of each resistance and increase in resistance of each paralleled resistance will contribute to the increase of the overall resistance. This model can be validated by experimental work, as the increase of resistance in each PET fiber made the resistance of the BYs-PDA-PPy increased greatly as a whole.

Figure 6 The sensing mechanism: optical images of BYs at the strain of 0% (a), 30% (b) and 50% (c); the schematics of the stretching of BYs-PDA-PPy (d); and the resistance model (e). 3.4. Wearable applications The high sensitivity, the large workable strain range and stability enable the strain sensors to monitor full-range human body motions. Due to the large workable strain range, large deformation of the human body, such as motions of joints can be recognized by the strain sensor. As shown in Figure


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7a, the BYs-PDA-PPy strain sensors were fixed onto a knuckle (insets of Figure 7a) to test their response to the bending of the finger. When the finger was kept straight, the ∆R/R0 of the strain sensor was zero. The ∆R/R0 value of strain sensor showed a real-time increase with the rapidly bending the finger. After the finger returning back to be straight, the sensor relaxed to its original situation and the ∆R/R0 dropped back to its original value. The cyclic bending and straightening of the finger resulted in a periodical increasing and dropping of the ∆R/R0, and a spectrum was generated as shown in Figure7a. The movement range can be reflected by the magnitude of the spectrum as shown in Figure 7b. With the increase of the bending angle, the magnitude of ∆R/R0 increased accordingly, indicating the fast response and high sensitivity of the BYs-PDA-PPy sensor. Similarly, the stretchable strain sensor was used to monitor various movements of a human wrist (Figure 7c). The ∆R/R0 of the strain sensor increased and decreased periodically when the wrist was repeatedly bending. The sensor could even sense the wrist for detecting hand-writing, including writing English alphabets and words. Figure7d presents the spectrum of ∆R/R0 corresponding to the writing of different English letters, such as “A”, “Sensor”, and “Sensitivity”. English characters from one to ten were also presented by the spectrum of the ∆R/R0 (see Figure S5, Supporting Information). It could be found from the spectrum that the difference between writing different words was very obvious, and the spectrum was reproducible when writing the same word.



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Figure 7. Detection of various human motions using BYs-PDA-PPy sensor, such as finger bending (a-b), wrist activity (c), writing (d), biceps brachii movement (e), elbow bending (f), clenching fist (g), bending leg (h), facial expression (i), expressions (j), breathing (k), speaking (l). Figure 7e shows the ∆R/R0 of BYs-PDA-PPy during the biceps brachii movement. the ∆R/R0 of BYs-PDA-PPy increased and then returned to its initial value in a highly repeatable manner when bending arm. The sensor was also fixed on the elbow joint to monitor the bending of elbow. Figure 7f demonstrates that the peak value of the ∆R/R0 spectrum is quite different under a small bending and a deep bending. A slight movement of muscle can also be detected as shown in Figure 7g, and the spectrum of ∆R/R0 clearly reflects the movement of clenching fist. Similarly, the extending and bending of knee can be monitored by fixing the sensor on the knee (Figure 7h). The ∆R/R0 value


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changed responsively when the leg was extending and bending, and a periodically changed spectrum was obtained as shown in Figure 7h. Based on the sensitivity of the BYs-PDA-PPy sensor to respond to a slight strain, subtle strain deformation of human body was detected as shown in Figure7i-n. Facial expression such as smiling could be detected and recognized by the strain sensor (Figure 7i). The ∆R/R0 value increased immediately to its peak when the trial person smiled, and it dropped back to its original value when the face was relaxing. A recognizable spectrum of the ∆R/R0 value was then obtained as shown in Figure7i. The signal shows quite good repetitive patterns, which verifies the good stability and repeatability of the sensor. Meanwhile, the BYs-PDA-PPy strain sensor was attached onto the corner of an eye, and the movement of blinking generated another spectrum of the ∆R/R0 (Figure S6). In addition, the strain sensor was fixed on the eyebrow to detect and recognize the expression of surprising and frowning (Figure 7j), When the subject was surprised, the forehead rose and resulted in the increase of the resistance of sensor. Conversely, when the subject frowned, a shrinkage of strain sensor and the resistance decreased accordingly. Respiration rate could be detected by the BYs-PDA-PPy strain sensor by attaching it on the skin near the abdomen, as shown in Figure 7k. The spectrum of ∆R/R0 value shows different patterns corresponding to shallow and deep breath, respectively. Figure 7l shows the performance of the BYs-PDA-PPy strain sensors in phonetic recognition. The spectrum shows distinctive and repeatable signal patterns for each word. The sensor was also used to monitor real-time motions such as drinking water (Figure S7a). Drinking with different volumes could also be detected (see Figure S7b). The above results indicate the BYs-PDA-PPy strain sensors are promising in detecting human motions.

Figure 8. Evolution of temperature of the BYs-PDA-PPy heater at different voltages (a); cyclic electrical thermal test for the heaters under 6V voltage (b); and the BYs-PDA-PPy heater fixed on a finger (c-d). The BYs-PDA-PPy can be used as a wearable heating device. For the purpose of manufacturing a heater, the BYs-PDA-PPy was woven into a fabric (4×2 cm2) (see Figure S8a). Figure 8a indicates the typical time-dependent surface temperature evolvement of the BYs-PDA-PPy heater under different voltages of 2, 4, 6, and 8 V. The heater generated heat immediately and reached to its corresponding temperature in a short period of time, and the higher the applied voltage, the higher the surface temperature of the fabric. The surface temperature dropped immediately when the power was turned off, and then the surface temperature returned back gradually after 250 s. The cyclic electrical heating performance for the heater under an applied voltage of 6V is shown in Figure 8b. Upon a periodical on-off of the applied voltage, the heater reached to its maximum temperature of



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about 75 oC and then dropped back. The similar pattern of each peak indicates the superior stability and repeatability of the BYs-PDA-PPy heaters. At the same time, it can be seen that the heater has cyclic stability and repeatability at different voltages of 2, 4, 6, and 8 V (see Figure S8b). For wearable application, the BYs-PDA-PPy fabric was fixed to a finger for thermal therapy (Figure 8c-d). The thermal images reveal the activity of the electrical heater on the specific region. The temperature on hand surface was about 36 °C, and the distribution of temperature was homogeneous (Figure 8c). When an applied voltage of 4 V was applied to the heater, the heater reached to 43 oC while the other areas of the hand remained in the previous temperature (Figure 8d). This trial experiment demonstrates the simplicity of incorporating the fabric heater for some important applications as wearable devices with controlled variation of temperature in specific regions of body. The electrothermal performance can be investigated when the yarn is subject to different tensile/torsional deformations or under different environmental conditions. It is also expected that the application of the as-fabricated fabric heater can be further explored in medical therapy, protective clothing, electro-thermochromic fabrics with color changing and pattern design, and displaying textiles. 4. Conclusions In summary, a highly stretchable and sensitive wearable strain sensor was fabricated from braided composite yarns. The in-situ polymerization of PPy was generated on the PDA templated surface of the braided PET fibers to provide electrical conduction, and the special structure of the braided fibers together with the elastic polymer provided a stable, reversible and repeatable strain sensing mechanism. The BYs showed unique braid structure and the cauliflower-like PPy layer were formed on the surface of BYs after coating with PDA. The tensile data suggested that the BYs-PDA-PPy has great mechanical properties. The BYs-PDA-PPy strain sensors exhibited a large workable strain range, with high sensitivity and responsivity (gauge factor of 50.8 in strain of 0%-40% and that of 29.7 in strain of 40%-105%), and long-term stability and durability. The BYs-PDA-PPy sensor accurately monitored real-time human motions including large and subtle movements. Moreover, the BYs-PDA-PPy strain sensors were woven into a fabric as a wearable electric heater. The BYs-PDA-PPy showed a big potential as a flexible strain sensor and a wearable electric heater, demonstrating its great potential as wearable electronics. Supporting Information Schematics and photos of the measurement unit to test the yarn resistance; Photo and results of scotch tape adhesion test of BYs-PDA-PPy; the ∆R/R0 of BYs-PDA-PPy under cyclic strain of 20% at different loading speeds; the spectra of ∆R/R0 of BYs-PDA-PPy sensor to detect human motions of writing English letters, closing and opening eyes, and drinking water; Photo of the fabrics from BYs-PDA-PPy and the cyclic thermal test under different applied voltages. Corresponding Authors *(D.C.) E-mail: [email protected] *(G.C.) E-mail: [email protected] *(X.W.) E-mail: [email protected] Acknowledgements


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Stretchable and Highly Sensitive Braided Composite Yarn

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