All-in-one piezoresistive-sensing patch integrated with

micro-supercapacitor

Yu Songa, Haotian Chena,b, Xuexian Chena,b, Hanxiang Wua, Hang Guoa,b, Xiaoliang

Chenga, Bo Menga, Haixia Zhanga,b,*

aNational Key Laboratory of Science and Technology on Micro/Nano Fabrication,

Peking University, Beijing 100871, China.

bAcademy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871,

China.

*E-mail: zhang-alice@pku.edu.cn

Figure S1. a-c) SEM images of various types of sugar particles used as templates for

porous structure. d-f) SEM images of morphologies of corresponding porous CNT-

PDMS elastomer after sugar dissolving with different pore sizes.

Figure S2. Initial resistance of CNT-PDMS elastomer with different CNT contents,

showing the percolation value of the elastomer is 2.5 wt%.

Figure S3. a) Ideal honeycomb cell model of the porous structure for further

calculation and analysis. b) Honeycomb deformation by elastic buckling: i) the

original honeycomb, ii) the buckling mode in uniaxial loading and iii) the associated

forces, moments, displacement and rotation. c) Piezoresistance measurements of the

PRS with the general push-pull gauge. d) Resistance response variation with different

CNT contents under different compressive strain. e) Stability of the electrical

resistance of the PRS under repeated compressing-releasing cycles. f) Instant response

of the PRS with the response time of 22 ms (rise time) and 80 ms (release time),

respectively.

A unit cell of a hexagonal honeycomb is described in Figure S3a, where a and b

are two kinds of length of the honeycomb, t is the thickness of the cell, h is the depth

and θ is the cell angle, respectively. For our device with an elastomeric honeycomb,

the compressive stress-strain curve has the broadly similar shape, where a linear-

elastic regime is followed by a plateau of roughly constant stress, leading into a final

regime of steeply rising stress. In detail, on first loading, the cell walls bend, giving

linear elasticity. When a critical stress is reached, the cells begin to collapse by the

elastic buckling of the cell walls. Eventually, at high strains, the cells collapse

sufficiently where opposing cell walls touch and further deformation compresses the

cell wall material itself, causing the final, steeply rising portion of the stress-strain

curve.

Figure S3b shows the honeycomb model of the elastomeric material during the

compressing processes. We wish to analyze the response to loads applied in the X1- X2

plane, especially in the plateau of stress-strain curve caused by elastic buckling.

During the compression, the cell walls most nearly parallel to the loading direction

behave like an end-loaded column. Such column buckles when the load exceeds the

Euler buckling load:[S1]

(1)

Figure S3b ii) shows the buckling mode observed when honeycombs are

compressed in the X2 direction (loads parallel to X1 simply causing bending). The load

per column is related to the remote stress, by:

(2)

Elastic collapse occurs when Pcrit=P, giving the elastic collapse stress, E1, as:

(3)

The end constraint factor, n, describes the rotational stiffness of the node where

three cell walls meet. It depends on the degree of constraint to rotation at the node B

caused by the walls AB and BC. According to previous researches, n=0.69 for regular

hexagons. I is the second moment of inertia of the cell wall (for a wall of uniform

thickness t):

(4)

We can now evaluate the collapse stress for regular hexagons, where θ=30°, a=b,

and t is constant,

(5)

Besides, the Poisson’s ratios (ν) are calculated by the strains normal to, and

parallel to, the loading direction. We find for loading in the X1 direction:

(6)

, where ν12 means the strain along X1 axis. For regular hexagons, ν12= 1. As loading in

the X2 direction:

(7)

,again giving ν21 = 1 for regular hexagons. When θ is less than zero (that is, the cells

are inverted), Poisson’s ratio is negative, implying that a compressive stress in one

direction produces a contraction in the normal in-plane direction, rather than the usual

expansion.

REFERENCES

[S1] L. J. Gibson, and M. F. Ashby, Cellular solids: structure and properties,

Cambridge university press, 1999.

Figure S4. The relationship of resistance response under different compressive stress

with the 7% CNT content. Inset: Linear relationship between the resistance response

and compressive stress at low stress region with the sensitivity of 0.51 kPa-1.

Figure S5. Fabrication process of the MSC with cross-section and overlook view. a)

Laser patterning PMMA with designed interdigital structure, b) filling CNT-PDMS-

sugar mixture into the grooves, c) spray-coating electrolyte on the surfaces, d)

transferring the CNT-PDMS-sugar mixture by the electrolyte, e) dissolving sugar to

form porous structure, and f) penetrating electrolyte to obtain freestanding MSC.

For the fabrication of MSC, we combine the laser patterning and electrolyte

transferring process shown in the Figure S5. Based on the solution-evaporation

method, we prepare solidified CNT-PDSM-sugar mixture for the following

fabrication. Firstly, PMMA mold is patterned with designed interdigital structure

through laser-cutting process. Then the CNT-PDMS-sugar mixture is filled into the

grooves and gel electrolyte is spray coated among the PMMA surface. As for the

solid-state electrolyte, it is developed by adding polyvinyl alcohol (PVA) powder (6

g) into H3PO4 aqueous solution (6 g H3PO4 into 60 ml deionized water). The whole

mixture is heated to 85°C under vigorous stirring until the solution becomes clear.

After the mold is dried in an oven at 40°C for 12h to vaporize the excess water, the

above layers could be easily peeled off from the PMMA substrate. Thus, the CNT-

PDMS-sugar mixture is successfully transferred to the dried electrolyte film without

further substrate. To promote the contact area and ion exchange, the film is soaked in

the warm water to dissolve the sugar completely. Finally, the electrolyte is coated

among the film again to penetrate into the porous structure and the whole MSC device

could be successfully produced.

Figure S6. a) Interdigital structure and b) detailed parameters of designed MSC unit.

Figure S7. Ragone plots of our micro-supercapacitor compared with other related

works.

As for energy storage devices, how much charges could be stored and how

quickly the charges could be stored and released are very important for their

applications. Therefore, energy density and power density (Ragone plots) are other

key metrics for evaluating the different types of energy storage devices. The

volumetric energy and power density of all MSCs are calculated from CV curves at a

voltage scan rate of 10 to 200 mV/s, and shown in Figure S7. Both of the energy and

power density of these MSCs at initial state could be achieved by the following

Equations (8)-(10):

CV =QV∙∆V

= 1k∙V∙∆V ∫

V 1

V 2

I(V)dV (8)

E= 12×3600

CV (∆V)2 (9)

P= E∆t

×3600 (10)

, where V is the volume of the MSC with the thickness of 50 μm, CV is the volumetric

capacitance of the MSC which can be achieved through Equation (8), Δt is the

discharging time, E is the energy density and P is the power density, respectively.

Compared to other energy storage devices, our device can obtain a maximum power

density (2.09 mW/cm3) at the scan rate of 200 mV/s. As the scan rate changes to 10

mV/s, it can continue to maintain energy density (13.8 μWh/cm3). Definitely, both of

them vary slightly with the increase of scan rates. The results reveal that these MSCs

possess good capability in delivering energy.

REFERENCES

[S2] S. Pyo, J. Lee, M. Kim, T. Chung, Y. Oh, S. Lim, J. Park, J. Kim, J. Micromech.

Microeng. 24 (2014) 075012.

[S3] S. Pyo, J. Lee, M. Kim, T. Chung, Y. Oh, S. Lim, J. Park, J. Kim, J. Micromech.

Microeng. 24 (2014) 075012.

[S4] S. Pyo, J. Lee, M. Kim, T. Chung, Y. Oh, S. Lim, J. Park, J. Kim, J. Micromech.

Microeng. 24 (2014) 075012.

Figure S8. Discharging profile of the micro-supercapacitor for driving

piezoresistance sensor circuit.

Figure S9. Data flow chart of the human-machine interface in a) user identification

and b) safety communication.

Figure S10. a) Optical photograph of the sensing patch matrix loaded by “PKU”

character-shape acrylic plates. b) Reconstructed map with column heights

corresponding to the resistance response for pressure sensing.

Table S1. Comparison between morphology and conductivity of different CNT-PDMS

elastomers.

Sample Our work Our work [S5]

CNT diameter 10-20 nm 20-30 nm 10-15 nm

CNT length 10-30 μm 10-30 μm 20 μm

Purity > 98% > 98% > 95%

Conductivity Good Bad Good

Percolation threshold 2.5% --- ---

REFERENCES

[S5] S. Pyo, J. Lee, M. Kim, T. Chung, Y. Oh, S. Lim, J. Park, J. Kim, J. Micromech.

Microeng. 24 (2014) 075012.

Table S2. Summary of the performance of the sponge-like pressure sensors.

Materials Sensitivity Detection Range Reference

Graphene-polyurethane 0.26 kPa-1 0-2 kPa [S6]

Granphene and CNTs 0.19 kPa-1 0-2.5 kPa [S7]

Carbon black -

polyurethane0.068 kPa-1 0-2.3 kPa [S8]

Gold-PU sponge 0.059 kPa-1 0-4.7 kPa [S9]

CNT Sponge 0.13 kPa-1 0-10 kPa [S10]

CNT-PDMS Sponge 0.033 kPa-1 0-15 kPa [S11]

CNT-PDMS elastomer 0.51 kPa-1 0-2 kPa Our work

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[S8] X. Wu, Y. Han, X. Zhang, Z. Zhou, C. Lu, Adv. Funct. Mater. 26 (2016) 6246.

[S9] Y. Wu, H. Liu, S. Chen, X. Dong, P. Wang, S. Liu, Y. Lin, Y. Wei, L. Liu, ACS

Appl. Mater. Interfaces 9 (2017) 20098.

[S10] Q. Chen, P. Cao, R. Advincula, Adv. Funct. Mater. 28 (2018) 1800631.

[S11] Y. Song, H. Chen, Z. Su, X. Chen, L. Miao, J. Zhang, X. Cheng, H. Zhang,

Small 13 (2017) 1702091.