Supplementary Information

Polyurethane Foam Coated with Nanocomposite of Multi-Walled Carbon Nanotubes and Polyaniline for a Skin-Like Stretchable Array of Multi-Functional Sensors

Soo Yeong Hong, Ju Hyun Oh, Heun Park, Jun Yeong Yun, Sang Woo Jin, Lianfang Sun, Goangseup Zi, and Jeong Sook Ha*

S. Y. Hong, J. H. Oh, H. Park, J. Y. Yun, Prof. J. S. Ha

Department of Chemical and Biological Engineering, Korea University, Seoul 02453, Republic of Korea

E-mail: jeongsha@korea.ac.kr

S. W. Jin, Prof. J. S. Ha

KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea

L. Sun, Prof. G. Zi

Department of Civil, Environmental and Architectural Engineering, Korea University, Seoul 02453, Republic of Korea

Keywords: Stretchable sensors, multi-functional sensors, liquid metal interconnections, carbon nanotube-polyaniline-polyurethane sponges, electronic skins

*Corresponding-Author: Prof. J. S. Ha, e-mail: jeongsha@korea.ac.kr

Supplementary Note S1: Experimental procedure

Functionalization of negatively charged MWCNTs: MWCNTs (Aldrich, >90% carbon basis,

length 5–9 μm, outer diameter 110–170 nm) were refluxed in concentrated sulfuric acid and

nitric acid (3/1 v/v, Sigma-Aldrich) at 70 °C for 3 h. The functionalized MWCNTs were

rinsed with DI water several times using a cellulose ester membrane filter (ADVANTEC

MFS, Inc.; pore size 0.2 μm, diameter 47 mm). After osmosis filtration using a tube cellulose

membrane (Sigma; average flat width 33 mm, average diameter 21 mm), MWCNT-COOH

was obtained.

Synthesis of MWCNT-PANI nanocomposite: The MWCNT-PANI nanocomposite was

synthesized during the chemical polymerization of aniline monomer. 100 mg of MWCNT-

COOH in 60 mL of 3 M hydrochloric acid was sonicated for 5 min to obtain a uniform

suspension. A mixture of 25 mL of ethanol, 0.25 g of aniline monomer, and the suspension

were stirred for 40 min. Then, 0.38 g of aniline ammonium persulfate (APS, Sigma-Aldrich)

was dissolved in 20 mL of 1 M hydrochloric acid and slowly dropped into the suspension to

initiate the polymerization. The resulting reaction mixture was sonicated for 20 min and

allowed to react for 18 h at -14 °C with stirring so that aniline could be fully polymerized.

After polymerization, the products were rinsed with DI water and ethanol, and then dried in

ambient atmosphere. The colors of the bare MWCNT, PANI, and MWCNT-PANI

nanocomposite solutions are shown in Figure S2. The SEM images of bare MWCNT and

PANI are shown in Figure S3.

Preparation of multi-functional sensing foam (MFSF): The MWCNT-PANI nanocomposite

film was mixed with DI water to make up a concentration of 1 mg/mL. Polyurethane (PU)

foam (thickness: 700 μm, Elongation: >300%, young’s modulus: 0.3 MPa,) was pre-treated

with (3-aminopropyl) triethoxylane (APTES) self-assembled monolayer (SAM). Next, the

pre-treated PU foam was dipped into the solution and stored in a desiccator for 10 min. After

annealing of the PU foam in an oven at 65 °C for 20 min, the same process was repeated 20

times to obtain MFSF (resistance: 1.6 MΩ).

Liquid metal patterning: Lines of Galinstan (68.5% Ga, 21.5% In, and 10% Sn; Rotometals)

were directly printed using a stationary blunt syringe needle and a syringe pump (NE-300,

NEWERA) coupled with a motorized XYZ printer (TinyCNC-SE, TINYROBO) and an

optical imaging CCD camera. Galinstan was ejected from the syringe needle at a constant

flow rate by means of a syringe pump. The height of the syringe needle from the substrate

was controlled using a z-stage on the XYZ printer for height adjustment and a CCD camera

for visually detection of the needle in contact with the substrate (≈ ±5 μm). To initiate direct

printing, the stage was moved to the starting position, in order to make contact between the

liquid meniscus and the substrate. Galinstan from the syringe pump began to flow

immediately at the preset rate after the start of the stage movement. After final movement of

the syringe needle, the syringe pump was stopped. Detachment of Galinstan within the

syringe needle from the pattern was achieved by simultaneously increasing the height. A

mixture of Silbione (Silbione RT Gel 4717 A&B, Bluestar Silicones, USA) and PDMS (Dow

coming, Sylgard 184A) in 9:1 ratio was spin-coated on glass at 1000 rpm for 1 min. Then, it

was cured in a dry oven at 65 °C for 10 min, and the liquid metal was patterned by this

method to obtain “Layer 4” (thickness: 150 μm).

Characterization: Surface morphology and cross-section of the fabricated sensors were

investigated from the SEM (Hitachi S-4800) images. The electrical performance of the

sensors was measured using HP 4140B under ambient conditions. The MWCNT and

MWCNT-PANI nanocomposite were analyzed by micro-Raman spectroscopy using a diode

pumped solid-state laser (Omicron) at a wavelength of 532 nm under back-scattering

geometry, where a beam spot size was approximately 3 μm on the sample. Photographic

images were obtained using a Canon Eos-7D camera and a cellphone.

Figure S1. Optical images of MWCNT (left), PANI (center), and MWCNT-PANI (right)

suspensions.

Figure S2. SEM images of (a) Bare MWCNT, (b) PANI (inset shows a high-resolution

image), and (c) silver nanowire (Ag NW) (d) Cross-sectional SEM image of the Ag NW

sticker.

Figure S3. SEM images of the MWCNT-PANI nanocomposite.

Figure S4. Raman spectra of MWCNT-PANI nanocomposite (blue) and MWCNT (black).

Figure S5. (a) Schematic of the fabrication process for stretchable multi-functional sensor

array. (b) Assembly of prepared layers and liquid metal interconnection.

Figure S6. (a–c) The width and resistance of the printed liquid metal with different syringe

needles: (a) 21G, (b) 25G, and (c) 30G. (d) A syringe pump dispenses liquid metal while an

x-y-z motorized stage moves relative to the syringe. A computer synchronizes the movement

and a camera captures images of the resulting structures. (e) Normalized resistance (R/R0) of

the liquid metal patterning with varying strain (0–50%). (f) Normalized resistance (R/R0) as a

function of number of stretching cycles under a strain of 50%. R0 and R are the resistances

before and after stretching, respectively.

Figure S7. (a) Current response of the pressure sensor to various pressures for MWCNT

(black circles) and MWCNT-PANI composite (blue circles). (b) Output voltage responses of

the temperature sensors to temperature gradient for PANI (green circles) and MWCNT-PANI

composite (blue circles). (c) Schematic of applying both temperature and pressure, and the

corresponding changes in current and output voltage.

Figure S8. Optical images while measuring the temperature of (a) Fingertip and (b) Wrist

using an infrared radiation thermometer.

Figure S9 . Bare MWCNT based gas sensor: (a) Response time and recovery time as a

function of NH3 concentration. (b) Sensitivity vs. NH3 concentration.

Table S1. Comparison of gas sensor performance in this work with that in previous reports.

a)S = sensor sensitivity, b)Tres = response time (Tres), and c)Trec = recovery time (Trec)

Figure S10. Schematic representation of ammonia adsorption and the corresponding electron

transfer in the MWCNT-PANI nanocomposite (left), and the corresponding band diagram

(right).[6]

Figure S11. (a) Response of a sensor to 25 ppm NH3 during six cycles of adsorption and

desorption. (b) Response and recovery of a gas sensor exposed to 25 ppm NH3 at room

temperature.

Figure S12. Dimensions of the stretchable MF sensor array with embedded Galinstan

interconnections.

Figure S13. Photographs of Silbione/PDMS adhesion on hand skin with repeated

attachment/detachment: after the first time (left), after 10 times (center), and after washing

with soap and water (right)

Figure S14. (a) Photograph of wearable MF sensor array on the back of the hand, in contact

with a small ice cube (left). Temperature (center) and pressure (right) distribution. The

corresponding mapping of (b, c) the temperature and pressure distribution via measurement

of the normalized current and output voltage with wrist attachment. Here, I0 and I indicate the

current before and after the attachment of the sensor to the wrist.

Figure S15. Optical image and schematic illustration of the MF sensor array of

temperature/pressure, and an ammonia gas sensor, integrated using directly printed liquid

metal interconnections.

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