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Supplementary Information

Energy harvesting textiles for a rainy day: woven piezoelectrics

based on melt-spun PVDF microfibres with a conducting core

Anja Lund1,2, Karin Rundqvist2, Erik Nilsson3, Liyang Yu1, Bengt Hagström3,4

and Christian Müller1

1 Department of Chemistry and Chemical Engineering, Chalmers University of Technology,

412 96 Göteborg, Sweden

2 The Swedish School of Textiles, University of Borås, 501 90 Borås, Sweden

3 Department of Materials, Swerea IVF, Box 104, 431 22 Mölndal, Sweden

4 Department of Industrial and Materials Science, Chalmers University of Technology,

412 96 Göteborg, Sweden

Correspondence: anja.lund@chalmers.se, christian.muller@chalmers.se

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Fig. S1 Photograph and WAXS results for the melt spun yarns and its components.

(a) Wide-angle X-ray scattering (WAXS) image for the bicomponent fibres. Using the

(110/200) reflection for β-phase PVDF, Herman’s orientation factor fx was calculated as

1 3

/

/

(S1)

where δ is the angle from the position of maximum intensity and I(δ) is the intensity at δ.

(b) Diffractograms obtained by integration of WAXS images along the azimuthal axis for ()

pristine PVDF (granules), () bicomponent fibres and () the core compound of 10%

carbon black in polyethylene. After fibre spinning, the peak positions for PVDF are largely

shifted from the positions representative for α phase to the positions representative for β

phase. (c) A roll of melt spun PVDF bicomponent yarn.

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Fig. S2 Photograph and schematic illustration of the contact poling procedure. (a)

Schematic illustration of a piezoelectric fibre and its electrical connections during poling and

electromechanical characterization. The axis denoted 1 is the mechanical axis and the axis

denoted 3 is the electrical or the polarization axis. The piezoyarn has 24 fibres electrically

connected in parallel with a common outer silver paste-electrode. As the electric field during

poling is between the perimeters of the fibres and their centre, it follows that for a high

piezoelectric effect the applied deformation is ideally a symmetrical radial compression. This

translates to tensile strain. (b) Photo of a piezoyarn in the setup for contact poling. During

poling, the core electrode is connected to ground and the outer electrode is connected to the

positive node of a high voltage supply. The general principle for poling is as follows: after

melt spinning and cold drawing, dipoles constituted by polar crystals are (c) randomly

oriented in the amorphous phase of the semi-crystalline PVDF. By applying a high electric

field the dipoles are (d) aligned resulting in macroscopic polarization. In a coaxial fibre, the

dipole orientation (e) is expected to follow a rotational symmetry about the core. In this

schematic cross-section of a polar PVDF crystallite (e bottom part), dipoles are stabilized by

trapped charges. The piezoelectricity in PVDF is due to the presence of a net dipole in the

crystal unit cell, possibly combined with some effect from trapped charges1,2.

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Fig. S3 Modelled frequency behaviour for a single piezoyarn (L = 10 mm) as a function

of frequency and load. (a) Peak voltage Vout over a load equivalent to the (b) oscilloscope,

for frequencies of 0.1 Hz to 1 kHz. (c) Generated power for different values of Rload (no Cload)

and as a function of frequency up to 10 Hz. Rload was () 10 MΩ, () 100 MΩ and () 468

MΩ. The real (or active) power output P was calculated as:

P = 0.5VpIpcosΘ (= VrmsIrmscosΘ) (S2)

where Vp is the peak voltage, Ip is the peak current and Θ is the phase angle.

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Fig. S4 Schematic illustration and photographs of the corona poling procedure.

(a) Schematic illustration of corona poling of bicomponent fibres. Corona poling can be used

for continuous poling of piezoyarns3, in-line with or separate from melt spinning. Here,

corona poling was used for the woven textiles, as the method is less sensitive (compared to

contact poling) to the macroscale inhomogeneities of the textile geometry. (b) A setup for

corona poling was devised as an open box with two needle boards, facing the top and bottom

of the piezoelectric textile. Each needle board had 45 stainless steel needles evenly distributed

over an area of 30 mm x 100 mm. The textile was suspended in air between the needle boards,

with the cores of the piezoyarns and the needles connected to the respective nodes of a high

voltage supply. The conducting yarn constituting the outer electrode, is expected to distribute

the electric field through all parts of the fabric, resulting in a radial polarization similar to that

in contact poling (Fig. S2). (c) Close-up of the needle tips during corona poling, showing

local high voltage discharge.

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Fig. S5 Schematic illustrations and photographs of woven bands.

(a) In the woven bands, the piezoyarns’ core electrodes were accessed by cutting the fibre

ends with a razor blade and subsequently melt pressing the fibre ends sandwiched between

two films of a 10 wt% carbon black/polyethylene composite at 135°C. For characterisation,

the black film accessing the core electrodes was connected to electrical ground, and the

conducting yarn constituting the outer electrode was connected by a hook probe. (b) In a weft

rib weave construction the weft yarn floats over two warp yarns. It results in a construction

where the warp yarns run relatively straight, without crimp, through the weave. Both surfaces

are dominated by the weft yarn. (c) Twill construction; the back side of the fabric is

dominated by the weft yarn. (d) Photo of three of the woven bands, with the three different

weft yarns. The width of the bands is on average 25 mm.

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Fig. S6 Tensile tests of woven bands. Tensile test results for the bands of (a) plain weave

and (b) twill construction, carried out on 3 specimen of each textile at a pre-load of 0.5 N. (c)

When a yarn is woven into a textile, its effective length L will decrease to a crimped length x.

When stretched, the initial deformation is related to removal of the crimp and requires little

force. Once the crimp is removed, the stiffness of the textile increases. Tensile tests of the

bands were carried out on an Instron 5966 with a gauge length of 100 mm, with a pre-load of

0.5 N. The strain was εmax = 5% with a strain rate of 360 mm/min.

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Fig. S7 Piezoelectric voltage generated by the woven bands of weft rib construction.

Electromechanical characterization was carried out at εmax = 0.25% and f = 4 Hz, for 3

samples each with (a) PAsilver and (b) PAcarbon black based weft yarns. The pre-load was 30 N.

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Fig. S8 Piezoelectric performance of several samples of the woven bands. Piezoelectric

voltage generated by woven bands at ε = 0.25% and f = 4 Hz, for 3 samples each of the (a)

twill/PAsteel, (b) plain weave/PAsteel, (c) twill/PAsilver, (d) plain weave/PAsilver, (e) twill/PAcarbon

black and 2 samples of (f) plain weave/PAcarbon black. The pre-load was 30 N.

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Fig. S9 Frequency characteristics of woven bands. Generated voltage from woven bands of

twill construction with PAsteel and PAsilver based weft yarns, at ε = 0.1% and f = 0.1 – 10 Hz.

The pre-load was 30 N.

Fig. S10 Open-circuit voltage of model circuits. Modelled generated voltage Vout from

equivalent circuits (see Fig. 4a, b) for woven bands of twill construction with (a) PAsteel, (b)

PAsilver and (c) PAsilver + water based weft yarns, at f = 2 Hz and a load equivalent to a

measurement device (NI-DAQ: Rinput = 100 GΩ, Cinput = 100 pF).

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Fig. S11 Schematic illustrations of the electrical contact points in the piezoelectric textile

in the case of (a) dry state, (b) with the addition of a liquid, (c) schematic illustration of

the plain-weave fabric and (d) close-up of a cross-section depicting all 24 fibres of the

piezoyarn and with an added liquid. Consider the simplified case of fibres as rigid rods

with one-on-one contact points as in (a). Here, the piezoelectric fibre constitutes a cylindrical

capacitor with one of its electrodes limited to the small point of contact with the conducting

yarn. If the liquid added to this system (b) is a conductor, it will act as an expanded outer

electrode for the cylindrical capacitor, in the water volume where there is overlap of the

piezoelectric and conducting yarn. The increase in contact area will result in an increase of the

circuit capacitance (cf eq. 3). Moreover, the liquid will increase the contact area between the

conducting yarns (grey fibres in (c)) thereby lowering the contact resistance and contributing

to a decrease in the resistance of the outer electrode. If instead, the liquid is a dielectric, the

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result will be the formation of a double-layer capacitor in the overlapping volume. This does

not affect the circuit resistance, but contributes to a decrease in capacitance Ctot according to:

1/Ctot = 1/C1 + 1/C2 (S3)

Note that this case also results in an increase in the electrode area for the cylindrical

capacitors.

Considering that tap water can be regarded as a leaky dielectric4, and further considering the

complex geometry of the piezoyarns in the textile (d), we expect that both of the described

cases are in play to some extent. Based on our observation that the capacitance of the textile

system increases considerably in the wet state, we conclude that the most important role that

the water plays is that it increases the contact surface area between the yarns of the textile.

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Fig. S12 Model and photograph of the energy harvesting circuit.

(a) Screen-shot from the simulation software LTspice, modelling the piezoelectric shoulder

strap connected to an energy harvesting circuit (EHC) LTC3588-1 and its related components,

as supplied on a (b) demo-board from Linear Technology. Notably, in the schematic, C1 is

the 22µF capacitor for energy storage and Rload is the 1MΩ-resistor connected to the output

of the EHC. The piezoelectric circuit was modelled according to the measured impedance of

the twill-woven band with the PAsilver + water based outer electrode, and its inherent voltage

Vi was set to an amplitude of 8 V and a frequency of 3 Hz, to emulate the piezoelectric

voltage generated by a handheld case as recorded (c) by an oscilloscope.

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Fig. S13 Piezoelectric voltage generated by the modified shoulder strap. Stair-walking

with the case over the shoulder generated a peak voltage > 4 V (a) for the dry strap with the

twill/PAsilver band. (b) The voltage amplitude doubled when water was added to the surface of

the strap. When stair-walking with the dry strap (c) the voltage of the storage capacitor

reached 0.9 V after 1 minute, equivalent to a stored energy W = 10 µJ. The energy W stored in

a capacitor is calculated as

W = 0.5CV2 (S4)

where C is the capacitance and V is the voltage across the capacitor.

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Table S1. Electrical properties of piezoyarns and woven bands.

Capacitance C (nF) Resistance R (kΩ)

Piezoyarn, L = 10 mm 0.068 14 000

Wov

en b

and

s

Plain weave, PAcarbon black 3.63 569

Twill, PAcarbon black 3 584

Plain weave, PAsteel 0.969 1 130

Twill, PAsteel 0.256 4 935

Plain weave, PAsilver 6.25 115

Twill, PAsilver 3.5 100

Twill, PAsilver + water 220 41

The capacitance and resistance were measured using a Keysight U17331C LCR-meter set to series-coupled RC-circuit and f = 100 Hz.

We have previously carried out capacitance measurements in the frequency range 20 - 10 000 Hz on the polarized bicomponent yarns, and found that the capacitance varies very little at frequencies below 100 Hz 5.

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Table S2. Voltage and electric field during corona poling of the textile bands.

Voltage (kV) Electrical field E

(MV/m) Sample Needle tips

VN

Fabric surface

VF

Plain weave PAsilver 7.4 0.7 38.9

Plain weave PAsteel 7.7 0.8 44.4

Plain weave PAcarbon black 7.7 0.6 33.3

Plain weave

insulating weft

7.6 1.6 88.9

Weft rib PAsilver 7.7 0.4 22.2

During corona poling, the electrical voltage on the tips of the needles and on the surface of the

textile bands was measured using a high voltage probe, FLUKE 80K-40 HV. The electrical

field E over the piezoyarns was calculated as E = VF/t where t is the thickness of the PVDF-

layer in the yarns (18·10-6 m). A control sample woven with pure PVDF yarns as (insulating)

weft was also included.

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Table S3. Density and cover factor of the weft yarns for the different constructions.

Construction Weft yarn Weft density

(No. of threads per cm)

Weft cover factor

(%)

Plain weave PAsilver 11 27

Plain weave PAsteel 11 22

Plain weave PAcarbon black 11 31

Twill PAsilver 10 25

Twill PAsteel 11 22

Twill PAcarbon black 11 31

Weft rib PAsilver 21 52

Weft rib PAcarbon black 19 54

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Supplementary References

1 Naegele, D. & Yoon, D. Y. Orientation of crystalline dipoles in poly(vinylidene

fluoride) films under electric field. Appl. Phys. Lett. 33, 132-134 (1978).

2 Rollik, D., Bauer, S. & Gerhard-Multhaupt, R. Separate contributions to the

pyroelectricity in poly(vinylidene fluoride) from the amorphous and crystalline

phases, as well as from their interface. J. Appl. Phys. 85, 3282-3289 (1999).

3 Hagström, B., Lund, A. & Nilsson, E. Method of producing a piezoelectric and

pyroelectric fiber. WO 2014/161920 A1 (2014).

4 Wang, D.-W., Du, A., Taran, E., Lu, G. Q. & Gentle, I. R. A water-dielectric capacitor

using hydrated graphene oxide film. J. Mater. Chem. 22 (2012).

5 Nilsson, E., Lund, A., Jonasson, C., Johansson, C. & Hagström, B. Poling and

characterization of piezoelectric polymer fibers for use in textile sensors. Sens. Act. A

Phys. 201, 477-486 (2013).