Supporting Information: Washable, Breathable, and Stretchable e-Textiles Wirelessly Powered by Omniphobic Silk-based Coils

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Supporting Information
Washable, Breathable, and Stretchable e-Textiles
Wirelessly Powered by Omniphobic Silk-based Coils
Marina Sala de Medeiros
a
, Debkalpa Goswami
a,b
, Daniela Chanci
a
, Carolina Moreno
a
,
and Ramses V. Martinez
a,c,*
a
School of Industrial Engineering, Purdue University,
315 N. Grant Street, West Lafayette, IN 47907, USA.
b
Institute for Medical Engineering and Science, Massachusetts Institute of Technology,
45 Carleton Street, Cambridge, MA 02142, USA.
c
Weldon School of Biomedical Engineering, Purdue University
206 S. Martin Jischke Drive, West Lafayette, IN 47907, USA.
*
E-mail: rmartinez@purdue.edu

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1. Step-by-step guide to prepare OSCs
1.1. Degumming Silk
The silk from B. mori cocoons has two main protein components: silk fibroin (SF), responsible for
the remarkable mechanical performance of silk and sericin, a glycoprotein that glues SF fibers.
OSCs are fabricated using only SF so, in order to remove the sericin proteins from the cocoons
(process commonly known as “degumming”), we cut the B. mori cocoons in small pieces and
added 10g of the Mori-cocoons into a 3L boiling 0.02M solution of Na
2
CO
3
(pH≈10.5). After
boiling for 45 min, the degummed SF was rinsing thoroughly with water to remove the sericin.
The remaining SF proteins were left to dry in a desiccator overnight at room temperature.
1.2. Generation of SF/CaCl
2
Solid Samples
SF, prior to its incorporation to the SF/MWCNT/ChC mixture, was combined with CaCl
2
, which
served as a plasticizer, by dissolving 10g of SF in 70 mL of a 2 wt.% aqueous solution of CaCl
2
(C1016; Sigma Aldrich). After 10 rinsing cycles with DI water, the SF was allowed to dry at
room temperature. The resulting dialyzed SF solution was frozen at -20°C and lyophilized at
- 80°C (Labconco Lyophilizer Freeze Dryer 75035), generating solid sponge-like samples.
1.3. Synthesis of Chitin Carbon
Chitin from shrimp shells (C7170; Sigma-Aldrich Inc.) were converted into powder using a
planetary ball mill instrument. Chitin carbon (ChC) was obtained through the hydrothermal
processing (220°C for 10h) of a 20 wt.% aqueous solution of chitin powder in a sealed pressure
vessel and drying for 2h at 120℃ in a vacuum oven at 36 Torr. After drying, an N
2
atmosphere

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was created in the oven and the temperature was incremented to 860℃ at a rate of 5 ℃ min
-1
.
After reaching ~860℃, the temperature was kept constant for 2h, leading to the generation of
chitin-derived carbon (ChC).
1.4. Synthesis of Multiwall Carbon Nanotubes
Multiwall carbon nanotubes (MWCNTs, average diameter 10
–
60nm nm, aspect ratio >100) were
produced by Chemical Vapor Deposition (CVD) in a quartz tube inserted in a multizone
furnace.[1] The CVD process consisted of the decomposition of a 50% mixture of xylene and
ferrocene gases at 750 °C, which cause the nucleation of iron nanoparticles that served as seeds
for the growth of MWCNTs on the surface of the quartz tube. The MWCNT were functionalized
with COOH groups by immersing 1g of MWCNT in 160mL of H
2
SO
4
/HNO
3
(3:1, v/v) at 70 °C
and sonicated during 1h. After three washing cycles, the COOH modified MWCNTs were dried
at 70 °C for 10h in a N
2
atmosphere.
1.5. Electrospinning SF/MWCNTs/ChC Solutions
To prepare the electrospinning solution, we dissolved CaCl
2
(0.8 wt.%) and NaDDBS (0.08 wt.%)
in formic acid (all from Sigma Aldrich) and incorporated to this solution SF/MWCNTs/ChC
(90/7/3 wt.%) keeping a total solid concentration of 20 wt.%. MWCNTs were added first to the
formic acid solution and dispersed using sonication (12W, 55kHz) for 5h. ChC nanoflakes were
added later and dispersed by sonication during 3h. After the complete suspension of the carbon
nanomaterials, we incorporated a 90 wt.% SF to the solution by mechanical stirring (1h) and
sonication (20min). Finally, this SF/MWCNTs/ChC solution was electrospun through a 20-gauge

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stainless-steel needle at a 12mL/h rate, 25°C, and less than 30% relative humidity. The
electrospinning needle formed a 45° spinning angle with respect to the center of rotation of an
aluminum rotating cylinder (diameter: 100 mm, width: 500mm), which was kept at a 100 mm
distance from the needle. During the electrospinning process, the flow rate of the
SF/MWCNTs/ChC solution was 12mL/h, the applied voltage was 17 kV, and the rotating speed
of the grounded collector was 900 rpm.
1.6. Fabrication of Thread using SF/MWCNTs/ChC Microfibers and Sewing of OSCs
The aligned fibers collected from the collector were twisted into yarns as illustrated in Fig. 1b.
This threading process, based on the method described by Yang et al. [2], started with the
generation of 3 yarns of SF/MWCNTs/ChC microfibers under the same electrospinning conditions
(17kV DC, 10 cm needle-mandrel distance and 900 rpm mandrel rotation). These yarns were
twisted into a ~350µ m-thick thread that was used during the sewing of the OSCs (see Fig. 1c).
Since both MWCNTs, ChC, and SF are biodegradable materials [3–5], we expect the fabricated
SF/MWCNT/ChC conductive threads to contribute to the development of future biodegradable e-
textiles. We designed and sewed the OSCs in a serpentine pattern (Fig. S1) to enhance their
performance upon stretching [6–9]. We chose the curvature angles of the serpentine pattern (α
1
=
90º and α
2
= 135º), to ensure that the transmission efficiency of the OSC increases (rather than
decrease) when stretched up to strains of ~10% (Fig. 4d; for reference, the maximum shear strain
that the human skin can sustain is ~11%; [10]). Moreover, the choice of this serpentine pattern
allows the transmission efficiency to remain within 70% of its maximum possible value up to

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strains of 50% (for reference, the maximum longitudinal strain that the human skin can sustain is
~30%; [11]); see Fig. 4c, d.
Fig. S1. Serpentine design used in the fabrication of OSCs.
OSCs reported in this manuscript have a resistance in the range 80–90 Ω/m (at the optimized
composition of 90 wt.% SF with 7 wt.% MWCNT and 3 wt.% ChC; see Fig. 2c), can withstand
strains up to 500% (see Fig. 2a), and maintain an apparent static contact angle of ~158° even after
50 cycles of machine washing (Fig. 6 and Fig. S13). As a comparison, commercial silver-coated
yarns have resistances ranging 50–150 Ω/m (dry) and limited stretchability (strain at failure ε
f
<20%). Additionally, the resistance of these yarns increases ~100−300% over 50 wash cycles, as

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silver oxidizes in the presence of moisture [12]. Commercially available carbon fibers exhibit
resistances of ~1K Ω/m and, while they can be very tough, their stretchability is limited (~8%)
[13]. Recently reported yarns based on carbon (reduced graphene oxide) exhibit resistances as low
as 12 Ω/m, but cannot endure stretching >30%, as the graphene oxide flakes cannot provide a
tensile mechanical reinforcement as efficient as MWCNTs due to their aspect ratio [14,15]. The
static contact angle of these graphene-based nanocomposites depends on their polymeric
matrix resistances and oscillates between 60-110° [16].
We used a commercial sewing machine (EverSewn Hero; EverSewn Inc.) equipped with a USB
port to import custom sewing designs with sizes up to 110 × 170 mm. Note that if the sewing
machine patterns the OSC using a high sewing tension (tight stitches), this will cause the
SF/MWCNT/ChC thread to experience near 90° bending on each stitch, which could
compromise conductivity, particularly upon stretching. Since we aimed to fabricate highly
flexible and stretchable e-textiles (stretchable up to ε=100%), we maintained the sewing machine
tension low (4N), which resulted in bending angles of ~36° (Fig. S2).After sewing the OSCs on
the textile, we interface the ends of the coil with a miniaturize wearable impedance matching
circuit using conductive epoxy (8331D, MG Chemicals Inc.). We further secured the electrical
connection by coating them with a flexible epoxy adhesive (FlexEpox; TotalBoat Inc.).

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Fig. S2 a) Image of the thread tension mechanism used to keep the
SF/MWCNT/ChC thread at ~4 N tension during sewing. b) Image of the thread as it travels
through a stitch, showing how the low sewing tension reduces the bending experienced by the
conductive thread as it is sewn on the textile.
After silanizing the surface of the e-textile by spraying it with a 4.76% v/v solution of trichloro
(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-henicosafluorododecyl)silane (CF
3
(CF
2
)
9
CH
2
-
CH
2
SiCl
3
; “C
12F
”) in isopropanol, the silane and the sub-10µm size of the
fluorinated clusters attached to the e-textile (Fig. S3) prevent water (and virtually any fluid with a
surface tension ≥27.05 mN/m) [17] from contacting the surface of the SF/MWCNT/ChC fibers or

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the underlying textile. This robust silanization does not interfere with the gas transport through the
textile (air permeability ~90.5 mm/s, Fig. 1e, Movie S2) or limit its flexibility [18]. Additionally,
the neglectable increase of radius and coefficient of friction of silanized threads makes OSCs
compatible with current textile mass production technologies. Additionally, the neglectable
increase of radius and coefficient of friction of silanized threads makes OSCs compatible with
current textile mass production technologies.
Fig. S3. High-resolution SEM images of the fibers of a cotton textile rendered omniphobic
by silanization with C
12F
Fluorinated polymeric clusters—generated by the reaction between
the sprayed fluoroalkylated silane and the surface-bound water—can be found distributed over
the cotton fibers, without significantly reducing the porosity or the gas permeability of the
textile. The apparent static contact angle of a 10µL water droplet on omniphobic cotton is
2
H O
app
159
θ
= °
.

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Fig. S4. Covalent modifications of OSC and e-textile fibers [19].

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Fig. S5. Gas permeability of an omniphobic OSC-powered e-textile (smart glove shown in
Fig. 1d, 1e, 5c, 5d, and Movie S2). We deposited a 10-µL drop of liquid pH indicator over the
top surface of the omniphobic wool glove (
2
H O
app
158
θ
= °
), which rests on a plastic film (gas
barrier) that separates the glove from an open scintillation vial containing NH
4
OH (panel I). At
t=0, the plastic film is removed, allowing the NH
3
gas in the solution to contact the glove, travel
through its highly porous structure, and reach the pH indicator at the other side (panel II).
The color of the pH indicator immediately begins to change from light blue (pH = 5) to dark
blue (pH =10), a process that takes ~15s, as the NH
3
gas permeates through the omniphobic
glove (panels II – IV).

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2. Structural Characterization of OSCs and Omniphobic E-textiles
2.1. Scanning Electron Microscopy
We used a scanning electron microscope (SEM; FEI Nova NanoSEM 200), operating at a working
distance of 3–6 mm, to characterize the structure of the textile substrates and the conductive silk
thread (Fig. 1c) used in the fabrication of OSC-powered e-textiles. To facilitate charge dissipation
during imaging, the textile substrates were mounted on an orbital rotation stage and coated with a
~20-nm-thick layer of platinum using a sputter coater (208HR, Cressington Scientific Instruments,
UK) operating at a 40 mA filament current during 60s.
2.2. Fourier-Transform Infrared Spectroscopy
Fourier-Transform Infrared Spectroscopy (FTIR) spectra were collected using a 550 Nicolet
Magna-IR Spectrometer. Fiber samples were placed over a 2-mm-thick ZnSe optical window and
kept under a nitrogen atmosphere prior to the collection of the spectra. Each collected spectrum
corresponded to the average of 180 scans in transmittance mode.

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Fig. S6. Comparison between the FTIR spectra of SF/MWCNTs/ChC microfibers and pure
silk microfibers. The peaks identified in the graph indicate the presence of secondary structures
(α-helix, β-sheet, and random-coil) in the SF.
3. Mechanical Characterization of OSCs
We characterized the mechanical performance of the SF/MWCNTs/ChC threads used to sew OSCs
according to ASTM D2256 specifications using a universal testing machine (MTS Insight 10; MTS
Systems Corp.), with a 1 kN load cell (model 661.18.F01), while applying tensile loads at a
crosshead speed of 5 mm min
−1
. We used at least eight samples with a length of 10cm to
characterize the dependence of the properties of the conductive threads on stretching.

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Fig. S7. Stress-strain curves generated during the tensile testing of eight different
100-mm-long OSC threads.
Fig. S8. Dependence of the resistance of the SF/MWCNTs/ChC thread on the concentration
of carbonaceous nanostructures (70/30 wt.% mixture of MWCNTs and ChC).

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Fig. S9. Stretchability and bendability of omniphobic OSC-based e-textiles. a) OSC-powered
e-textile shown in Fig. 3b withstanding a strain e=70% in multiple directions, followed by 180°
twisting without inducing damage to the sewn circuitry or suffering any noticeable degradation in
performance. b) Pictures of the front and the back of the textile showing the electronic
components, which weigh ~0.3g, and occupy an area of ~18 mm
2
. c) Schematic diagram and actual
picture of the wearable MRC circuit and the SMD LEDs (Z
L
) comprising the e-textile, where
C
2
= 940 nF, C
s2
= 80 nF, C
p2
= 75 nF, C
r
= 4700 nF, and D
r
being a high frequency Schottky diode.

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Note that we use the sub index 2 to identify that this MRC circuit is applied in the wearable device
(secondary coupling stage feed by the primary coil). We use the sub index “s” or “p” to identify
elements connected in series or parallel with the OSC, respectively. The sub index “r” is used to
identify the elements (a diode D
r
, and a capacitor C
r
) responsible for the rectification of the altern
current induced by the primary coil on the OSC.
4. Electrical Characterization of OSCs
We connected the primary coil powering the e-textiles to a vector network analyzer (E5071B ENA,
Agilent Technologies) and characterized the OSCs passively by bringing them in the vicinity of
the primary coil. We first measured the real and imaginary components of the impedance,
Z(f) = R(f) + jX(f); |Z| = (R
2
+ X
2
)
1/2
, where the real part is the resistance (R) and the imaginary part
is the reactance (X), as a function of the operating frequency (f), and then calculated the inductance
L = X/2
π
f, phase
θ
= tan
-1
(X/R), and quality factor Q = X/R, see Fig. S11.
The vector network analyzer records the fraction of power reflected by the OSC back to the
primary coil (|S
11
|
2
) via a one-port measurement, from which the transmission is calculated as
T = (1 - |S
11
|
2
)×100%. We then performed impedance-matching to optimize the magnetic resonant
coupling between the external coil and the OSC by introducing matching capacitances in series
(C
s1
= 280pF, C
s2
= 80nF) and parallel (C
p1
= 6nF, C
p2
= 75nF) on their respective MRC circuits (Fig
3a). The value of these capacitances was determined using the bandpass filter theory described by
Chappell et al.[20] As a result of this impedance matching, the transmission efficiency increases
from ~1.5% to ~37% at 5.2 MHz (Fig. 4b).

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Fig. S10. Schematics of the experimental set-up used to maintain the OSC axis collinear with the
primary coil during the stretching characterization experiments.

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Fig. S11. Frequency-dependent electrical characteristics of the wirelessly powered OSC-based e-
textile (acquired passively from the primary coil side) shown in Fig. 3b: a) Resistance (R),
b) Reactance (X), c) Impedance (Z), d) Inductance (L), e) Phase (
θ
), and f) Quality Factor (Q).

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Fig. S12. Image of the primary coil used for MRC WPT with OSCs presented in this work. The
coil is 5cm in diameter and made of polymer enamel coated 27 AWG copper wire forming 6 turns.
5. Safety of Use of OSC-based e-textiles with respect to Human Exposure to Electromagnetic
Fields
We followed the IEEE Standard for safety levels with respect to human exposure to
electromagnetic fields to guarantee that the wireless power transfer to the OSC-powered wearable
devices respects the safety limits established in the USA [21]. The maximum power per unit area
allowed for wireless devices operating on a frequency range from 1.0 MHz to 30 MHz is given by
Max. Power Density (W/m
2
) = 9000/f
M2
, where f
M
is the frequency of operation in MHz. Since all
the OSC-powered e-textiles presented in this study operate at a frequency of 5.2MHz, the
maximum power density that allows them to operate safely under the maximum exposure reference
level (ERL) is 33.2 mW/cm
2
. The table below demonstrates that all the wearable, OSC-powered,
devices discussed in the main manuscript respect this ERL safety limit.

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Table S1. Power and power density levels of OSC-based e-textiles, and maximum allowed
safe power density level according to IEEE standard for safety with respect to human
exposure to electromagnetic fields (1–30MHz).
Device Seen in Figure Total Power
required (mW)
Power density
received by the
OSC (mW/cm
2
)
OSC-based display
with a “snail”
design.
Fig. 3b
120
9.55
OSC-based display
with a
“bike” design.
Fig. 3c 120 9.55
OSC-based PPG
sensor
Fig. 5 155 12.33
OSC-powered Non-
contact voltage
detector
Fig. 6 60 4.77
Maximum allowed
safe power density
level according to
IEEE standard for
safety with respect
to human exposure
to
electromagnetic
fields (1–30
MHz)[21].
— 33.2
6. OSC-powered wearable non-contact voltage detector
The non-contact voltage detection circuit mounted on the glove is composed by two
bipolar junction transistors (BC547) that acts as a current amplifier system that can amplify the

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small current captured by the conductive embroidered glove fingerprint, and by another transistor
which acts as a switch for the OSC that is powering the LED.
The current received by the NPN transistor connected to the glove fingerprint is amplified with a
gain of β
1
= 200, and it is further amplified by the next transistor with a gain of β
2
= 200. This
configuration is known as Darlington configuration and gives a much higher current gain
than using a single transistor. The gain of the Darlington (β
darlington
) configuration of the non-
contact voltage detector is given by the product of the individual gain of each transistor
(β
darlington
= β
1*
β
2
).
We placed different OSC-powered e-textiles inside a commercial laundering machine
(TR3000WN, Whirlpool) using 2kg of conventional textiles as ballast and followed the laundering
test specifications dictated by the AATCC Test Method 135–2014. Each washing cycle consisted
of 120L of water at 22°C (wash and rinse cycles) under an 8-min delicate laundering program. The
spinning process reached ~120 rpm during washing and rinsing, followed by an additional 3 min
of spinning under ~450 rpm. After cleaning, the textiles were hung and allowed to dry at room
conditions.
To illustrate the laundry resistance of OSC-based wearable devices we characterized the
non-contact voltage detection glove shown in Fig. 6 after 50 washing cycles.
The omniphobic character of OSC-powered e-textiles allow them to operate even when fully
immerged in water. Fig. S13 shows the fibrous structure of the wool as purchased,
after silanization, and after 50 washing cycles. While the amount of fluorinated polymeric clusters
coating the wool fibers reduces as a consequence of the shear action of the washing cycles (see
Fig. S13 b and c), this degradation is not enough to modify the apparent static contact angle of

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water on the surface of the silanized wool ( ; see Fig. S13 b and d). Similarly, Fig. S14
shows how the electrical detection performance of the circuitry in this OSC-based e-
textile remains unchanged after 50 washing cycles. The non-contact voltage detection
capabilities of this glove—wirelessly powered through its OSC (Fig. 6a)—depend both on the
magnitude of the voltage connected applied to the cable and the distance separating the electrical
cable from the fingertip of the glove (covered with an embroidered SF/MWCNTs/ChC conductive
pad), see Fig. S14a.

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Fig. S13. Laundry resistance of the wearable non-contact voltage detection glove. High
resolution SEM image of the fibrous structure of the wool glove as purchased (a), after
being rendered omniphobic by silanization with C
12F
(b), and after 50 washing cycles (c). d) The
apparent static contact angle of a 10µL water droplet on this OSC-powered wearable device
remains unchanged after 50 washing cycles.

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Fig. S14. Dependence of voltage detected with the non-contact voltage detection glove shown
in Fig. 6 and Fig. S13 on its separation from the cable before (a) and after (b) 50 cleaning
cycles.
7. Wireless, Battery-free, PPG Wristband
Photoplethysmography (PPG) is a low-cost optical technique that can detect changes in the volume
of blood flowing through blood vessels due to the rhythmic activity of the heart [22].
This blood volume change is measured by illuminating the skin of the user with a green SMD LED
and collecting the reflected light using a SMD photosensor (see Fig. 5b). The amount
of reflected light is proportional to the volume of blood circulating under the skin of the user.
Therefore, by continuously collecting the reflected light from the skin of the user it is possible to
identify different cardiac phases and associated parameters [23]. Fig. S15 shows
a representative PPG waveform collected from a single cardiac cycle. This waveform is composed
of the following sequence of peaks and valleys: the systolic peak, dicrotic notch, and diastolic
peak. By identifying these peaks, other cardiac parameters can be calculated: pulse interval (PI),

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inflection point area ratio (IPAR), ΔT, crest time (CT), see Fig. S15. These cardiac
parameters have been commonly used to monitor cardiac output during exercise [24] and arterial
stiffness due to aging [25].
Fig. S15. Information extractable from the PPG waveforms collected with the wearable
OSC-based PPG sensor (single cardiac cycle). The waveform is composed of a systolic peak
(I), a dicrotic notch (II), and a diastolic peak (III). The period of this PPG signal corresponds to
the pulse interval (PI). Additionally, other relevant cardiac parameters can be extracted from this
waveform, such as ΔT, the crest time (CT), and the inflection point area ratio (IPAR;
ratio between the area under the S
1
and the S
2
regions) [23].
Fig. S16 shows a high-resolution SEM image of the C
12F
-silanized wristband, where a PPG sensor,
a low-power microcontroller and Bluetooth chip is integrated into a textile wristband. We used
conductive epoxy (8331D, MG Chemicals Inc.) to connect the terminals of the OSC to the flexible
miniaturized electronics embedded in the textile (Fig. S9). The electronic components weigh
~12 g, and occupy an area of ~540 mm
2
, minimally affecting their somatosensory perception.

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Fig. S16. OSC-powered PPG wristband. a) High-resolution SEM images of C
12F
-silanized
wristband. b) Image of the PPG sensor embedded in the omniphobic wristband. Inset shows the
copper coil used for the external coupling circuit.
8. Fabrication Costs
Using the non-contact voltage detection glove (Fig.6, S13) and the PPG wristband (Fig.5, S16),
we found that the averaged fabrication cost of OSC-based e-textiles could be less than $0.28 per
coil (Table S2). This calculation does not consider either the cost of the textile or the associated
labor or capital expenses. Moreover, the ultimate functionality of OSC-based e-textiles relies on
the textile-mounted electronics (SMD electronics, microcontroller with Bluetooth connectivity...).

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Silk from B. mori cocoons $ 0.090
MWCNTs $ 0.055
ChC $ 0.042
Silane $ 0.040
Solvents and Reagents $ 0.049
Total cost $ 0.276
Table S2. Itemized cost of the materials needed to fabricate OSCs. These prices
correspond to small quantities and could be further reduced by volume discounts.
9. Supporting Information Movies
Movie S1. Omniphobic OSC-powered e-textile.
Movie S2. Breathability of OSC-powered e-textiles.
Movie S3. Stretchable wirelessly powered textiles.
Movie S4. Wirelessly powered washable textiles.
Movie S5. Wirelessly powered textiles.
Movie S6. Wireless non-contact voltage detection glove.
Movie S7. Powering OSC-based e-textiles underwater.

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