Article

Printable Smart Pattern for MultifunctionalEnergy-Management E-Textile

Mingchao Zhang, Mingyu Zhao,

Muqiang Jian, ..., Xiao Liang,

Junyi Zhai, Yingying Zhang

jyzhai@binn.cas.cn (J.Z.)

yingyingzhang@tsinghua.edu.cn (Y.Z.)

HIGHLIGHTS

A large scalable strategy for

fabrication of fibertronics for

smart E-textile

Printing of sheath-core fibers

using a 3D printer equipped with a

coaxial spinneret

Printing of various aesthetic smart

patterns on textile

Energy-harvesting textile

composed of silk-sheathed

carbon nanotube fibers

A facile one-step fabrication of coaxial fiber-based smart patterns for E-textile

through 3D printing equipped with a coaxial spinneret is reported. Versatile smart

textiles for different purposes can be fabricated by selecting different materials in

construction of the coaxial layers. Examples such as silk energy-harvesting textile

and energy-storage textile with superior performance are demonstrated.

Zhang et al., Matter 1, 1–12

July 10, 2019 ª 2019 Elsevier Inc.

https://doi.org/10.1016/j.matt.2019.02.003

Article

Printable Smart Pattern for MultifunctionalEnergy-Management E-TextileMingchao Zhang,1,2 Mingyu Zhao,1,2 Muqiang Jian,1,2 Chunya Wang,1,2 Aifang Yu,3,4 Zhe Yin,1,2

Xiaoping Liang,1,2 HuiminWang,1,2 Kailun Xia,1,2 Xiao Liang,1,2 Junyi Zhai,3,4,* and Yingying Zhang1,2,5,*

Progress and Potential

E-textiles, inheriting tradition

merits of textiles with

incorporated electric

components, have drawn great

attention in recent years.

However, the attachment of rigid

and bulky electronics on textile

will severely deteriorate the

breathability and flexibility of the

textile. Alternatively, integrating

functional fibers in traditional

fabrics is a versatile and promising

approach while keeping the

intrinsic merits of fabrics.

SUMMARY

Electronic textile (E-textile) has drawn tremendous attention with the develop-

ment of flexible and wearable electronics in recent years. Herein, we report

the direct printing of E-textile composed of core-sheath fibers by employing

a 3D printer equipped with a coaxial spinneret. Customer-designed core-

sheath fiber-based patterns can be printed on textile for various purposes.

For demonstration, we used carbon nanotubes (CNTs) as a conductive core

and silk fibroin (SF) as a dielectric sheath, and fabricated a CNTs@SF core-

sheath fiber-based smart pattern, which was further used as a triboelectricity

nanogenerator textile. The smart textile could harvest biomechanical energy

from human motion and achieve a power density as high as 18 mW/m2.

We also demonstrated the printing of a supercapacitor textile for energy

storage. The direct printing of smart patterns on textile may contribute

to the large-scale production of self-sustainable E-textile with integrated

electronics.

Nevertheless, a practical

approach to integrate smart fibers

with textile is still lacking. Here, we

report a facile one-step

fabricating process for core-

sheath fiber-based smart patterns

on textile for E-textile, which are

realized by a coaxial spinneret-

equipped 3D printer. The printed

smart pattern on textile, which is

demonstrated for energy

management and other

applications, is beyond the

conventionally aesthetic purpose

or trademark identification of

patterns, paving the way for facile

fabrication of E-textile with

various integrated electronics.

INTRODUCTION

Electronic textile (E-textile), which refers to textile or fabric with integrated digital

components,1 has the potential to have characteristics inherited from traditional

fabrics, such as softness, breathability, and stretchability,2 beyond the desired

electronic functions. However, the attachment of rigid electronics on textile will

severely deteriorate its breathability and flexibility.3 Alternatively, incorporation

functional fibers (i.e., fibertronics) in common fabrics is a versatile and promising

way to obtain E-textile while keeping the aforementioned merits of fabrics.4

Thus, developing techniques to fabricate smart fibers with built-in or add-on

electronic functionality attracts significant attention.5 Nevertheless, imparting

desired functions to highly deformable fibers remains a significant technical

challenge.1

On the other hand, practical approaches to integrate smart fibers with textile are still

lacking. The fabrication of fibertronics and their assemblies into fabric are usually

carried out in separate procedures, which is arduous and time consuming.6–9 For

example, a recent work reported the fabrication of an organic light-emitting diode

(LED) fiber through multi-step dip-coating and thermal deposition, followed by be-

ing sewn into fabrics.7 A functional circuitry on cotton yarns was reported by coating

a layer of aluminum using dip-coating and a layer of polymer by chemical vapor

deposition, following by being woven into fabrics.8 Although these approaches

are effective, the processes are still complex and arduous. Therefore, facile or

even one-step fabrication processes of fibertronics, which have potential for mass

production, are highly desirable.

Matter 1, 1–12, July 10, 2019 ª 2019 Elsevier Inc. 1

Recently, 3D printing has rapidly developed into an important state-of-the-art

additive manufacturing technology.10–13 In particular, a 3D printing technique

based on direct ink writing was proved to be an efficient approach to fabricate

fiber-based architectures for soft robotics,10 smart composites,11 and stretchable

electronics.12 Currently a single-axial spinneret is usually used for 3D printing,13

restricting the material choice and structure design of the printed architectures.

We foresee that the introduction of the multi-axial spinneret will greatly widen

the capability of 3D printing techniques, especially for fabrication of multifunc-

tional fibers and smart textile.

Here, we report the one-step fabrication of fiber-based smart patterns for E-textile

through a 3D printer equipped with a coaxial spinneret. The patterns consisted of

core-sheath fibers, which were extruded from a coaxial spinneret and were directly

printed on textile by a 3D printer. The precursor materials for the sheath and the core

fibers could be facilely selected depending on different purposes, enabling the

fabrication of versatile functional E-textiles. As a proof of concept, we used carbon

nanotubes (CNTs) as the core fiber and silk fibroin (SF) as the sheath layer, and

printed core-sheath fiber-based patterns on textile. We further demonstrated the

excellent performance of the obtained smart textile while serving as a triboelectricity

nanogenerator to harvest mechanical energy from human motion. A smart superca-

pacitor textile for energy storage was also demonstrated. The direct printing of

smart patterns on textile enables the facile fabrication of E-textile with various inte-

grated electronics.

1Key Laboratory of Organic Optoelectronics andMolecular Engineering of the Ministry ofEducation, Department of Chemistry, TsinghuaUniversity, Beijing 100084, P. R. China

2Center for Nano andMicroMechanics, School ofAerospace Engineering, Tsinghua University,Beijing 100084, P. R. China

3Beijing Institute of Nanoenergy andNanosystems, Chinese Academy of Science,Beijing 100083, P. R. China

4School of Nanoscience and Technology,University of Chinese Academy of Sciences,Beijing, 100049, P. R. China

5Lead Contact

*Correspondence: jyzhai@binn.cas.cn (J.Z.),yingyingzhang@tsinghua.edu.cn (Y.Z.)

https://doi.org/10.1016/j.matt.2019.02.003

RESULTS AND DISCUSSION

3D Printing of Smart Patterns on Textile

Figure 1A illustrates the printing of core-sheath fiber-based patterns on fabrics us-

ing a 3D printer equipped with a coaxial spinneret. Two injection syringes contain-

ing different inks were connected to a coaxial spinneret, which was fixed on a 3D

printer. Inks containing various materials can be selected depending on the

desired functions of the final power devices. The flexibility, biocompatibility, and

waterproofness of the sheath materials should be considered for practical applica-

tions of E-textile. For demonstration purposes, we used CNT aqueous solution as

the core ink and SF solution as the sheath ink. On this basis, we fabricated an

E-textile that can harvest mechanical energy of human body motion for wearable

energy-management purposes. The CNT ink from the inner spinneret and the SF

ink from the outer spinneret were synchronously injected to form core-sheath

structured fibers. Due to the different cross-sectional areas of the inner and outer

spinneret, different feeding rates of CNT and SF inks were applied to ensure a

similar flow rate of both inks. To ensure the formation of continuous and robust fi-

bers on textile, the moving speed of the coaxial spinneret must match the

extruding rate of the fiber.

Various flexible customer-designed patterns composed of core-sheath functional

fibers can be printed onto fabrics for smart E-textile by controlling the moving

path of the coaxial spinneret using a programmed procedure (Figures 1B and

1C). Figure 1C and its insets show examples of the as-obtained patterns, which

contain Chinese characters meaning PRINTING with hollow structures, English

letters spelling SILK with solid lines, and a pigeon with hollow structures. As shown

in Figure 1D, the obtained smart textile showed good flexibility against twisting

and folding, which was realized based on the good flexibility and robustness of

the printed patterns. The versatility of this approach and the good flexibility of

2 Matter 1, 1–12, July 10, 2019

Figure 1. Direct Printing of Core-Sheath Fiber-Based Patterns on Fabrics for Energy-Management Smart Textile

(A) Schematic illustration showing the 3D printing process using a coaxial spinneret.

(B) Photograph of the 3D printing process.

(C) Photographs of customer-designed patterns on textile, including English letters of SILK, a pattern of a pigeon, and Chinese characters of PRINTING.

(D) A smart textile under twisting and folding, showing its high flexibility.

the printed structures encourage the application of this technique in fabrication of

various smart textiles.

Printing Inks and Their Rheological Properties

To ensure the successful printing of continuous and uniform core-sheath fibers on

textile, the uniformity and rheological properties of the printing inks should be

controlled and adjusted. For example, we studied the process to prepare the SF

ink and the CNT ink, which was essential for the printing of CNTs@SF core-sheath

fibers for the fabrication of a nanogenerator textile. The SF ink was employed to

construct the dielectric sheath of the fiber, and the CNT ink was used to fabricate

the conductive core of the fiber. For the construction of the sheath layer, silkworm

silk was chosen due to its top-level position in the triboelectric series, superior me-

chanical properties, and good biocompatibility when particularly used in wearable

devices.14–16 Figure 2A shows the SF ink, which was prepared by dissolving de-

gummed silk cocoons in a formic acid/CaCl2 solution (for details see Experimental

Procedures). It takes only a fewminutes to obtain a stable and viscous ink when using

the formic acid/CaCl2 solution, whereas it requires arduous and time-consuming

dissolution and dialysis process using other typical dissolution systems such as

CaCl2/ethanol/H2O or LiBr/H2O.17 Moreover, the calcium ions in the resulting fibers

tend to bind with water molecules in air, facilitating achievement of good flexibility

of the printed architectures.18

Matter 1, 1–12, July 10, 2019 3

Figure 2. Printing Inks and Their Rheological Properties

(A) Photographs of silk cocoons and the obtained SF ink.

(B) Optical image showing the SF microfibrils in the SF ink.

(C) Photograph showing the highly injectable SF ink.

(D) Photographs of CNT powder and the CNT ink.

(E) TEM image showing the good dispersion of CNTs.

(F) TEM image of a multiwall carbon nanotube wrapped by polymer on its outer wall; the inset is a zoomed-in image.

(G) Apparent viscosity as a function of shear rate of the CNT and SF inks.

(H) Storage (G0) and loss (G00) modulus as a function of shear stress of the CNT and SF inks.

(I) Photograph of a free-standing CNTs@SF core-sheath fiber after being extruded, showing good spinnability of both inks.

It is noteworthy that this dissolution system could partially dissolve SF fibers into

microfibrils with a diameter of several micrometers (Figures 2B and S1). The confor-

mation of the secondary structure of SF in natural silk fibers could be largely main-

tained, leading to good mechanical properties and water resistance of the printed

structures. This can be evidenced by Raman spectroscopy (Figure S2A). According

to the deconvolution of the amide I region (1,600–1,700 cm�1), the secondary struc-

ture elements of SF with a b-sheet structure content in the regenerated SF fiber ob-

tained from the formic acid/CaCl2 system was estimated to be 53.3%, which is close

to that of the degummed natural silk (57.3%) (Figures S2B and S2C; Table S1).19 In

contrast, the b-sheet structure content obtained from the CaCl2/ethanol/H2O

4 Matter 1, 1–12, July 10, 2019

system was dramatically reduced to 23.3% (Figure S2D). As shown in Figures 2C and

S3, the SF ink extruded from a spinneret under mild extruding forces can solidify

quickly in air and form a self-standing long filament, showing its potential as printing

ink. The mechanical performance of the as-obtained SF fiber (Figure S4) was obvi-

ously better than that of SF fibers obtained from other dissolution systems20 in as-

pects of modulus, breaking stress, strain at breaking, and toughness, indicating its

suitability as robust building blocks for wearable electronics.

Regarding the CNT ink, a good dispersion of the CNTs in the solution is very impor-

tant in enabling a smooth flow when being extruded as well as the formation of a

continuous and conductive core fiber. To this end, CNT powder was dispersed in

an aqueous solution with SDS as the surfactant and polyvinyl acetate (PVA) as a vis-

cosity regulation agent (Figure 2D). The obtained CNT ink showed good uniformity.

Figure 2E shows a transmission electron microscopy (TEM) image of the dispersed

CNTs, exhibiting no observed coagulation. Figure 2F shows a magnified TEM image

of a CNT coated with a polymer layer. Entangled polymer chains can physically

adhere on the outer wall of the CNT, which brings steric hindrance against the strong

p-p stacking and hydrophilic interactions among CNTs, thus enabling good disper-

sion of the CNTs in the ink.

The rheological nature of the SF and CNT inks are also very important factors influ-

encing the quality of the printed fibers on textile. A certain rheological property of

the printing ink is required to ensure a smooth flow to achieve a continuous core-

sheath fiber with good shape fidelity. As shown in Figure 2G, the apparent viscosities

of the SF and CNT inks reduce alongside the increase of shear rates, displaying a

typical shear-thinning behavior. These curves indicate that both SF and CNT inks

have a flow behavior at high shear rates and a solid-like state at rest. Besides, the

SF ink possesses a larger viscosity than the CNT ink, indicating that higher extrusion

stress is required for the SF ink.

Figure 2H displays the storage (G0) and loss (G00) modulus of the SF and CNT inks as a

function of shear stress. The G0 values of both inks exhibit plateaus at low shear

stresses and are larger than the G00, indicating their solid-like behavior at low

stresses.21 After a yield point of �25 Pa for the CNT ink and �150 Pa for the SF

ink, their G0 drops sharply and falls below G00, indicating that both inks behave like

viscous liquids under high shear stresses. The shear stresses during extrusion are

orders of magnitude higher than the yield point stresses of both inks, thus allowing

a smooth flow through the spinneret.21 Once the inks are extruded from the spin-

neret the applied stresses disappear, leading to the formation of solid-like filaments.

The shear-thinning behavior under extrusion and the capability of keeping good

shape fidelity after extrusion of both inks are prerequisites for the formation of

stable, robust, and even free-standing core-sheath structures using the printing

technique (Figure 2I).

Structure of the Core-Sheath Fiber

The morphologies and structures of the CNTs@SF core-sheath fiber were investi-

gated. The core-sheath fibers printed on textile have a diameter of �500 mm

(Figure 3A). The conductive CNT core has a width of �200 mm and is closely encap-

sulated by the SF sheath (Figure 3B). Figure 3C shows the cross-section of the fiber

on a textile, exhibiting distinct areas of the CNT core and the SF sheath. As shown in

Figure 3D, there are entangled CNTs in the core area, enabling an electrical conduc-

tivity of 2.1 3 10�2 S/cm. Moreover, the printed structures on the textile can resist

the peeling force using common adhesive tapes (e.g., Scotch tape, polyimide

Matter 1, 1–12, July 10, 2019 5

Figure 3. Structure and Morphology of the as-obtained CNTs@SF Core-Sheath Fibers on Textile

(A) Scanning electron microscopy (SEM) image of top-view of a core-sheath fiber on textile.

(B) Optical image of top-view of a core-sheath fiber on textile.

(C) SEM image of cross-section of a core-sheath fiber on textile.

(D) SEM image of cross-section of a fiber showing the CNT core.

tape) (Video S1 and Figure S5), indicating the strong interaction between the sub-

strate fabric and the printed core-sheath fibers. In addition the printed fibers are

waterproof (Figure S6), which can be ascribed to the encapsulation of the b-sheet-

rich SF sheath. These results prove the successful construction of CNTs@SF fibers

on textiles, which can be used to fabricate smart patterns beyond traditionally

aesthetic purposes and integrated into fabrics for E-textile.

3D-Printed Energy-Harvesting Textile

For demonstration purposes, a CNTs@SF core-sheath fiber-based pattern printed

on a conventional fabric was used as an E-textile for harvesting biomechanical en-

ergy of human body motion (for details see Experimental Procedures). The basic

working mechanism, which is illustrated in Figure 4A, is based on the coupling effect

of contact electrification and electrostatic induction. The printed SF pattern and a

polyethylene terephthalate (PET) film were chosen as the triboelectric pair. The SF

has a strong ability to lose electrons and the PET tends to gain electrons. Therefore,

the contact/separation of the two parts will generate a variable dipole moment,

leading to the flow of electrons between the two electrodes. We investigated the

performance of a pattern composed of parallel CNTs@SF core-sheath fibers on

textile for energy harvesting (Figure S7). This can generate a short-circuit current

(Isc) peak of 1.4 mA and an open-circuit voltage (Voc) peak of 15 V at a displacement

speed of 10 cm/s, as shown in Figure 4B. In addition, the power density peak can

reach a maximum value of 18 mW/m2 at an external resistance load of 4 MU

(Figure 4C).

Furthermore, the outputs of the different patterns under different contacting/sepa-

rating speeds with PET films were investigated. A pattern of gridline on textile (Fig-

ure 4D) showed that output Voc peaks increased from 30 to 55 V as the displacement

6 Matter 1, 1–12, July 10, 2019

Figure 4. Performance of the 3D-Printed Energy-Harvesting Textile

(A) Schematic illustration of the working mechanism of the smart textile.

(B) Typical output open-circuit voltage (Voc) and short-circuit current (Isc) of a parallel-line pattern on textile (9 3 9 cm, with a spacing of 2 mm).

(C) Output Isc density and power density as a function of resistance of the 3D-printed smart textile.

(D) Schematic illustration of a gridline pattern (9 3 9 cm, with a spacing of 2 mm) on textile.

(E and F) Output Voc (E) and Isc (F) of a gridline pattern on the textile while contacting/separating with a PET film at different displacement speeds (5, 8,

10, 13, 15, and 18 cm/s).

speeds increased from 5 to 18 cm/s (Figure 4E). At the same time, the output Isc peak

also increased from 1.0 to 7.0 mA with the increase of the displacement speeds (Fig-

ure 4F), indicating that a high displacement speed leads to a high output of current.

For comparison, the pattern of parallel core-sheath fibers on textile (Figure S7) only

generated Voc peaks from 14 to 17 V and Isc peaks from 0.4 to 2.7 mA as the displace-

ment speeds increased from 5 to 18 cm/s (Figure S8). These results can be inter-

preted by considering the fact that a pattern with a larger effective area brings about

greater electron transfer to achieve a higher output voltage and current. In addition,

the long-term performance of the printed textile was explored, which showed stable

results during 15,000 cycles of loading/unloading (Figure S9), indicating its good

mechanical durability and stability. Compared with recently reported silk-based

triboelectric nanogenerators,22,23 the printed textile in this work showed obviously

higher output power.

3D-Printed Energy-Storage Textile

We also demonstrated the direct printing of supercapacitors on fabrics for energy-

storage textile using the 3D printing technique with a coaxial spinneret. The

Matter 1, 1–12, July 10, 2019 7

designed smart pattern for supercapacitor was composed of two core-sheath fiber

electrodes on textile (for details see Experimental Procedures and schematic

illustration in Figure S10). Its electrochemical performance was measured through

a cyclic voltammetry (CV) method at scanning rates from 5 to 100 mV/s (Fig-

ure S11A). The quasi-rectangular curves of these CV curves indicate the good

electrochemical double-layer capacitance properties of the smart pattern.24 The

galvanostatic charge/discharge (GCD) curves (Figure S11B) at different current

density are close to triangular shapes, indicating good charge transportation

across the two fiber electrodes. According to the GCD curves, the areal capaci-

tance of the smart pattern was calculated to be 26.42 mF/cm2 at a current density

of 0.42 mA/cm2, and the corresponding areal energy density and areal power

density were 0.33 mWh/cm2 and 31.85 mW/cm2, respectively. The stability of the

supercapacitor under mechanical deformation was also investigated. The GCD

curves measured under different bending degrees up to 150� are similar to each

other (Figure S12), indicating the good flexibility and superior stability of the

energy-storage textile. In addition, apart from the aforementioned CNTs@SF

and CNTs@SCC structures, other versatile materials-based structures, such as gra-

phene oxide@SF and reduced graphene oxide@SF structures, can also be printed

(Figure S13).

3D-Printed Energy-Management Textile for Smart Clothes

The smart patterns can be directly printed or integrated into clothes/garments for

wearable energy-management systems. As a proof of concept, smart patterns

were printed on different parts of clothes to harvest biomechanical energy

from the movement of the human body, such as walking and running (Figure 5A;

Videos S2 and S3). The touching/separating of the smart pattern with an opposite

PET film on the underarm sleeve induced by moving the arms can generate

an alternating current. Figure 5Ai shows the typical alternating current with a

maximal Isc peak of �1.8 mA/cm2. For practical applications, a bridge rectifier

was usually employed to convert the alternating current to direct current for further

applications (see the rectifying circuit diagram shown in Figure 5Aii). Figure 5Aiii

shows the rectified Isc. The collected energy can be stored in capacitors for

later uses.

Next, the performance of the energy-management textile at different displacement

speeds to charge capacitors was investigated. As the displacement speeds

increased from 2 to 18 cm/s, the required time to charge a 3.3-mF capacitor to a

voltage of 3 V shortened from �130 s to �10 s (Figure 5B). Capacitors with different

capacitances were charged at a fixed displacement speed of 13 cm/s. As shown in

Figure 5C, a capacitor with larger capacitance required a longer time to reach 5 V.

The energy stored in the capacitors could be used to drive small electronic devices.

For examples, 14 LED bulbs could be lit up using the smart pattern power system

(Figure 5Di). Besides, the printed power textile could also drive an electrical watch

(Figure 5Dii) and an electrical timer (Figure S14). These results not only show the

potential applications of the printed E-textile for wearable energy-management

systems, but also indicate the feasibility of the directly printed E-textile for smart

clothes.

Conclusion

In summary, we reported the direct printing of core-sheath fiber-based smart pat-

terns for energy-management E-textile. By selecting different inks for the sheath

and core layer of the coaxial spinneret on the 3D printer, versatile energy-manage-

ment smart textile can be fabricated. For our demonstration, we used CNT ink and

8 Matter 1, 1–12, July 10, 2019

Figure 5. Application of the 3D-Printed E-Textile for Energy Management

(A) Schematic illustration showing smart clothes for energy management and its performance. Inset (i) shows the output Isc density of a smart gridline

pattern printed on the underarm sleeve of a shirt generated by an arm moving. Inset (ii) is the rectifying circuit diagram of the power system. Inset (iii)

shows the rectified output Isc density of the smart pattern.

(B) Charging curves of a capacitor (3.3 mF) charged using the smart pattern displaced at different speeds.

(C) Charging curves of different capacitances charged using the smart pattern with a displacement speed of 13 cm/s.

(D) Photographs showing LEDs and an electrical watch driven by the power generated by the 3D-printed E-textile.

SF ink and directly printed coaxial CNTs@SF fiber-based patterns on textile. The uni-

formity and rheological properties of both inks were investigated to meet the re-

quirements for direct ink writing based a 3D printing technique. The good dispersion

of the CNTs in aqueous solution ensured a smooth flow for making the continuous

and conductive core fiber. SF microfibrils in the SF solution, prepared by partially

dissolving natural silk fibers using formic acid/CaCl2 solution, can retain the second-

ary structure of the protein molecule in natural silk fibers, thus enabling the good

mechanical properties and water resistance of the printed structures. In our demon-

stration, we showed that the obtained CNTs@SF fiber-based smart textile could har-

vest mechanical energy of human motion and achieve a maximal power density of

18 mW/m2. The direct printing of supercapacitors on fabrics for an energy-storage

textile, which showed a capacitance of 26.42 mF/cm2 at a current density of

0.42 mA/cm2, was also demonstrated. Apart from fabrication of energy-manage-

ment systems, this strategy may also be applied to the fabrication of other wearable

electronics such as flexible sensors, electric antennas, and other functional circuits,

merely by designing the printing-ink combinations. We hope that the direct printing

of smart patterns on textile, which is beyond the conventionally aesthetic purpose or

trademark identification of patterns, paves the way for the facile fabrication of

E-textile with various integrated electronics, holding great promise for practical

applications.

Matter 1, 1–12, July 10, 2019 9

EXPERIMENTAL PROCEDURES

Preparation of SF Ink

For the printing of the triboelectricity nanogenerator, SF ink was employed for

the dielectric sheath. Silk cocoon from Bombyx mori silkworm was cut into small

pieces and put into boiling water with 5 wt % NaHCO3 to degum for half an

hour. The degummed procedure was repeated twice. CaCl2 (0.22 g) was dissolved

in 5 mL of formic acid to form a solution, and 2.0 g of degummed silk fibers was

then mixed into the solution to form SF ink using a Speed Mixer for 10 min at

3,500 rpm.

Preparation of CNT Ink

CNT aqueous ink was used for the conductive core of the fiber. Multiwall carbon

nanotubes (MWCNTs, with diameters of 50–150 nm and lengths of 10–30 um)

were added into SDS dispersed aqueous solution at an MWCNT/SDS/H2O ratio of

300 mg:300 mg:10 mL, followed by ultrasonic treatment for 30 min. Ten milliliters

of PVA/H3PO4, which was used as a polymer dispersant and a viscosity regulation

agent, was then added. The PVA/H3PO4 was obtained according to a previously re-

ported procedure.24 In brief, 1 g of PVA was added to 10 mL of deionized water at

90�C under moderate stirring until totally dissolved, followed by cooling to room

temperature for further use, and thereafter 1 mL of H3PO4 was added and the solu-

tion was stirred for 30 min. Finally the obtained CNT solution was stirred at 90�C for

about 60 min for evaporation to obtain the CNT ink with a favorable rheological

property (with a solid content of �30%–40%).

3D Printing of Energy-Harvesting Textile

The CNTs@SF core-sheath fiber was extruded from a coaxial spinneret and printed

into designed patterns on textile using a 3D printer (Anycubic I3 MEGA). Two injec-

tion syringes, containing CNT ink and SF ink, respectively, were connected to the

inner (diameter: 260 mm) and outer (diameter: 840 mm) channels of a coaxial spin-

neret, respectively. The coaxial spinneret was then fixed onto a 3D printer. The

CNT and SF inks were synchronously injected to form CNTs@SF core-sheath

conductive fibers. The feeding rates of the inner and outer nozzle were kept at

10 mL/h and 25 mL/h. By exquisitely controlling the print path of the coaxial spin-

neret at a rate of 20 mm/s using a programmed procedure, customized patterns

composed of the CNTs@SF core-sheath conductive fibers were directly printed

onto the fabric substrate.

3D Printing of Energy-Storage Textile

For the energy-storage textile, the fabrication procedure was the same as that for the

energy-harvesting textile with the exception that the sheath of the printed fiber was

sodium carboxymethyl cellulose, which serves as a solid-state electrolyte and a flex-

ible buffering layer against mechanical deformation. The gap between two individ-

ual core-sheath fiber electrodes was then filled with PVA/H3PO4 gel to work as the

solid-state electrolyte for the supercapacitor. Lastly, polydimethylsiloxane was

used to encapsulate the printed supercapacitor.

Characterization of Materials

The dispersion of the CNTs in the CNT ink was investigated using a field-emission

transmission electron microscope (JEOL, JEM2012F) by dropping the ink on a

copper grid. The rheological properties of the CNT and SF inks were characterized

using a rheometer (Anton Paar, MCR301). Themorphology of CNTs@SF core-sheath

fiber was characterized using a scanning electron microscope (Zeiss, Merlin) and an

optical microscope (Leica, DMi8 S).

10 Matter 1, 1–12, July 10, 2019

Performance Measurement of the E-Textile

All the measurements of the E-textile were carried out in ambient conditions with a

relative humidity of 20% and a temperature of 25�C. The open-circuit voltage of the

energy-harvesting textile wearable triboelectricity nanogenerator was measured

using an electrometer (Keithley, 6517B). The short-circuit current of the wearable

triboelectricity nanogeneratorwas recordedby a low-noise current preamplifier (Stan-

fordResearch Systems, SR570). Theenergy-harvesting textilewasdrivenby amechan-

ical linear motor (Linmot, E1100) with a constant acceleration of 50 m/s2 and a

displacement distance of 10 cm. The electrochemical performance of supercapacitor

was measured by an electrochemical workstation (RST5200, China). The specific areal

capacitance (C) was calculated fromGCD curves using the equationC =it

AV, where I is

the discharge current, t is the discharge time, V is the discharge window, and A is sur-

face area of the fibers in the overlapping portion (the circumference of the cross-sec-

tion of the CNT multiplied by the length of fibers in the overlapping portion). Energy

density (E) of the supercapacitor was calculated according to E = C,V2=8, and the

average-power density (P) of the supercapacitor was obtained according to P =E=t.

SUPPLEMENTAL INFORMATION

Supplemental Information can be found online at https://doi.org/10.1016/j.matt.

2019.02.003.

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of

China (51672153), the National Key Basic Research and Development Program

(2016YFA0200103), and the National Program for Support of Top-notch Young

Professionals.

AUTHOR CONTRIBUTIONS

Y.Z. supervised the project. M. Zhang designed and performed most of the experi-

ment with the help from the other authors. M. Zhao, M.J., C.W., Z.Y., Xiaoping Liang,

H.W., K.X., and Xiao Liang participated in parts of the experiments. A.Y. and J.Z.

contributed to the measurement of energy-harvesting textile. M. Zhang and Y.Z.

co-wrote the manuscript with feedback from all authors.

DECLARATION OF INTERESTS

A China patent application (CN201810073665) has been filed, with Y.Z. and M.

Zhang as inventors, covering the coaxial spinneret-equipped 3D printing technique

for E-textiles described herein. The authors declare no competing interests.

Received: December 24, 2018

Revised: February 18, 2019

Accepted: February 25, 2019

Published: March 27, 2019

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