Nano Res.

Electronic Supplementary Material

A low-cost, printable, and stretchable strain sensor basedon highly conductive elastic composites with tunablesensitivity for human motion monitoring

Yougen Hu1, Tao Zhao1, Pengli Zhu1 (), Yuan Zhang1,2, Xianwen Liang1, Rong Sun1 (), Ching-Ping Wong3,4

1 Guangdong Provincial Key Laboratory of Materials for High Density Electronic Packaging, Shenzhen Institutes of Advanced Technology,

Chinese Academy of Sciences, Shenzhen 518055, China 2 Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China 3 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA 4 Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong 999077, China

Supporting information to https://doi.org/10.1007/s12274-017-1811-0

Table S1 The relationship between mass fraction and volume fraction of PS@Ag fillers in the PS@Ag/PDMS composites.

PS@Ag in the PS@Ag/PDMS composite Silver in the PS@Ag/PDMS composite

mass fraction (wt%) volume fraction (vol%) mass fraction (wt%) volume fraction (vol%)

30 15.92 18.36 2.11

40 22.76 24.48 3.02

44 25.77 26.93 3.50

45 26.55 27.54 3.53

46 27.35 28.15 3.64

47 28.15 28.76 3.75

47.5 28.56 29.07 3.81

48 28.97 29.38 3.86

49 29.80 29.99 3.97

50 30.65 30.60 4.08

52.5 32.81 32.13 4.38

55 35.07 33.66 4.68

60 39.86 36.72 5.33

65 45.07 39.78 6.03

70 50.77 42.84 6.81

Address correspondence to Pengli Zhu, pl.zhu@siat.ac.cn; Rong Sun, rong.sun@siat.ac.cn

| www.editorialmanager.com/nare/default.asp

Nano Res.

The calculation of volume fraction was obtained according to equation (1):

@

@ @

@

@@

@

PS Ag

PS Ag PS Ag

PS Ag

PS AgPS Ag PDMS PDMS

PS Ag PDMS

m

V

mV V m

(1)

where VPS@Ag and VPDMS are the volumes of the PS@Ag fillers and PDMS polymer matrix, respectively; mPS@Ag

and mPDMS are the mass contents, and ρPS@Ag and ρPDMS are the densities of PS@Ag filler (2.263 g/cm3) and PDMS

matrix (1.0 g/cm3). Moreover, the corresponding mass fraction and volume fraction of Ag can be calculated by

equation (2) and (3):

@

@

@

61.2% 61.2%PS Ag

Ag PS Ag

PS Ag PDMS

m

m m

(2)

@ g

Ag

@ @

61.2%

61.2% (1 61.2%)

Ag PS A

Ag Ag

Ag

Ag PS Ag PS AgAg PS PDMS PS PDMS PDMS

Ag PS PDMS Ag PS PDMS

m m

V

m m mV V V m m m

(3)

where 61.2% is the mass fraction of Ag in PS@Ag hybrid particles and ρPS is the density of PS (1.05 g/cm3).

Table S2 Comparison of the percolation threshold, conductivity and sensing performance of electrically conductive composites.

Filler specifications Polymer matrix Percolation threshold

Conductivity (S m-1) Strain range Gauge factor Stability Reference

Ag powder, 1-3 μm PDMS ~75 wt% 2.3 × 104 at 82 wt% of Ag 0-30% Not shown fail after a fewhundred tensile

cycles [S1]

Ag platelet, 1.2-2.2 μm PDMS 83 wt% 3 × 104 at 86wt% of Ag 0-80% Not shown Not shown [S2]

Ag powder, 2 μm PDMS ~68 wt% 8.3 × 103 at 73wt% of Ag 0-57% Not shown Not shown [S3]

Ag coated Cu flakes, 10% Ag PDMS 75 wt% 6.7 × 104 at 83wt%

of Cu@Ag Not shown Not shown 120 min at 80 °C [S4]

Ag particles, 2-3.5 μm PDMS 55.2 wt% 2.65 × 105 at 85wt% of Ag 0-40% Not shown Not shown [S5]

Ag powder, 2-3.5 μm PDMS ~60 wt% 600 at 79.3wt% of Ag 0-165% Not shown Not shown [S6]

Ag particles, 100 nm Epoxy Not shown 2.74 × 102 at 56 wt% of AgNon-

stretchabilityNo Not shown [S7]

Ni micro-particles with nanoscale surface features,

2-3.5 μm

Supramolecular hydrogen bonding network

55.7 wt% 40 at 75 wt% of Ni 0-100% Not shown Not shown [S8]

Ag fractal dendrite Epoxy 8 wt% 9.26 × 106 at 70 wt% of

Ag FD Non-

stretchabilityNo

1500 h at 85 °C& 85%RH

[9]

Ag coated polystyrene nanospheres (PS@Ag), ~600 nm, 90 wt% Ag

P(St-BA) latex Not shown 6.96±1.19× 104 at 60 wt%

of PS@Ag (bottom surface)0-60%

5.68 (extract from figure)

Not shown [S10]

Ag coated polystyrene microspheres (PS@Ag),

2-11 μm PDMS

26.9 wt% of Ag

4.12 × 104 at 42.8 wt% of Ag

0-80% 17.5 (0%-10%),

6.0 (10%-60%) and 78.6 (60%-80%)

1750 h at 85 °C& 85%RH

This work

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

Nano Res.

Figure S1 Measurement of the Poisson’s ratio of the PS@Ag/PDMS films. (a) Schematic diagram of the measurement procedure; (b) Photographic image of the measurement equipment.

The Poisson’s ratio (ν) of the PS@Ag/PDMS film is calculated from the length and width change under

uniaxial tensile load, i.e.

-y

x

d

d

where εx is axial strain, εy is transverse strain perpendicularly stretching direction.

Figure S2 Cyclic stress-strain curves of the PS@Ag/PDMS conductive elastic films.

| www.editorialmanager.com/nare/default.asp

Nano Res.

Table S3 The GF values and the corresponding correlation coefficient R2 of the linear fitting of PS@Ag/PDMS strain sensors.

Region 1 Region 2 Region 3 samples

GF1 R12 GF2 R2

2 GF3 R32

0%-5% strain 5%-50% strain 50%-fracture strain PS@Ag/PDMS-70

6.38 0.973 3.8 0.947 10.6 0.993

0%-5% strain 5%-50% strain 50%-fracture strain PS@Ag/PDMS-65

10.3 0.971 4.7 0.952 15.2 0.988

0%-10% strain 10%-60% strain 60%-fracture strain PS@Ag/PDMS-60

17.5 0.992 6.0 0.905 78.58 0.932

0%-60% strain 60%-75% strain 75%-fracture strain PS@Ag/PDMS-55

27.6 0.909 255.3 0.954 1567.7 0.971

0%-37% strain 37%-45% - PS@Ag/PDMS-50

1139 0.852 1264290 0.819 - -

Beside the correlation coefficient of PDMS-PS@Ag-50 sensor shows a low level, other sensors with higher

filler loading than PS@Ag/PDMS-50 exhibit high correlation coefficient larger than 0.9, which indicate that these

strain sensors except PS@Ag/PDMS-50 express good linearity between the normalized resistances and tensile

strains.

Strain dependence of resistance for conductivity invariant materials

Consider an unstrained sample of length, l0 and across sectional area, s0. After straining to ε, the length is l and

across sectional area, s. Assume the conductivity, σ, is independent of strain. The unstrained resistance is,

00

0

1 lR

s

while the strained resistance is

1 lR

s

Then,

0 0

0 0 0

1R R sR l

R R s l

For an incompressible material, the volume is constant, i.e. l0s0=ls, so

2

2

0 0

1 1 -1 2R l

R l

（ ）

In which the strain is defined as 0

0

l l

l

At low strains, due to ε<<2, the above equation can be reduced to 0

2R

R

This shows that 0/

2R R

GF

for an incompressible material with strain independent conductivity.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

Nano Res.

Figure S3 Experimental (symbols) and theoretical (solid lines) data of strain-relative resistance dependence for conductive elastic composites of (a) PS@Ag/PDMS-70; (b) PS@Ag/PDMS-65; (c) PS@Ag/PDMS-60; (d) PS@Ag/PDMS-55 and (e) PS@Ag/PDMS-50, respectively.

Figure S4 Magnified images of the changes in relative resistance of the sandwich structured PS@Ag/PDMS-60 sensor during various human motions monitoring.

| www.editorialmanager.com/nare/default.asp

Nano Res.

Figure S5 Illustration of the fabrication process of the sandwich structured PS@Ag/PDMS strain sensors.

Supporting Video Captions

Video S1. The change of electrically conductive performance of PS@Ag/PDMS-65 sensor during stretching

process until rupture by lighting a LED—demonstrating a potential application in stretchable conductor.

Video S2. The change of electrically conductive performance of PS@Ag/PDMS-50 sensor during stretching/

releasing cycles of 70% strain by lighting a LED—demonstrating a potential application in insulator-conductor

switch.

Video S3. Demonstration of the printed interdigital electrode arrays of PS@Ag/PDMS-60 as an “on-off” switch

under different working states.

Video S4. Wearable sensor of sandwich structured PS@Ag/PDMS-60 in real-time monitoring of human motion

of finger bending/releasing cycles. The response signal pattern of the motion was extracted and demonstrated

in Figure 8a.

Video S5. Wearable sensor of sandwich structured PS@Ag/PDMS-60 in real-time monitoring of human motion

of knee squatting cycles. The response signal pattern of the motion was extracted and demonstrated in Figure 8d.

References

[S1] Ruhhammer, J.; Zens, M.; Goldschmidtboeing, F.; Seifert, A.; Woias, P. Highly Elastic Conductive Polymeric MEMS. Sci.

Technol. Adv. Mater. 2015, 16, 015003.

[S2] Niu, X. Z.; Peng, S. L.; Liu, L. Y.; Wen, W. J.; Sheng, P. Characterizing and Patterning of PDMS-Based Conducting Composites.

Adv. Mater. 2007, 19, 268.

[S3] Cong, H. L.; Pan, T. R. Photopatternable Conductive PDMS Materials for Microfabrication. Adv. Funct. Mater. 2008, 18,

1912-1921.

[S4] Chuang, H.-S.; Wereley, S. Design, Fabrication and Characterization of a Conducting PDMS for Microheaters and Temperature

Sensors. J. Micromech. Microeng. 2009, 19, 045010.

[S5] Yu, C. J.; Zhang, Y. H.; Cheng, D. K.; Li, X. T.; Huang, Y. G.; Rogers, J. A. All-Elastomeric, Strain-Responsive Thermochromic

Color Indicators. Small 2014, 10, 1266-1721.

[S6] Larmagnac, A.; Eggenberger, S.; Janossy, H.; Voros, J. Stretchable Electronics Based on Ag-PDMS Composites. Sci. Rep. 2014,

4, 7254.

[S7] Wu, H. P.; Liu, J. F.; Wu, X. J.; Ge, M. Y.; Wang, Y. W.; Zhang, G. Q.; Jiang, J. Z. High Conductivity of Isotropic Conductive

Adhesives Filled with Silver Nanowires. Int. J. Adhes. Adhes. 2006, 26, 617-621.

[S8] Tee, B. C-K.; Wan, C.; Allen, R.; Bao, Z. N. An Electrically and Mechanically Self-Healing Composite with Pressure- and

Flexion-Sensitive Properties for Electronic Skin Applications. Nat. Nanotechnol. 2012, 7, 825-832.

[S9] Yang, C.; Cui, X. Y.; Zhang, Z. X.; Chiang, S. W.; Lin, W.; Duan, H.; Li, J.; Kang, F. Y.; Wong, C.-P. Fractal Dendrite-Based

Electrically Conductive Composites for Laser-Scribed Flexible Circuits. Nat. Commun. 2015, 6, 8150.

[S10] Chen, H. Y.; Shen, H. P.; Wu, H. C.; Wang, M. S.; Lee, C. F.; Chiu W. Y.; Chen, W. C. Synthesis of Monodispersed Polystyrene-

Silver Core-Shell Particles and Their Application in the Fabrication of Stretchable Large-Scale Anisotropic Conductive Films. J.

Mater. Chem. C 2015, 3, 3318-3328.