ARTICLE Transparent Electronic Skin Device Based on

CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 30, NUMBER 5 OCTOBER 27, 2017

ARTICLE

Transparent Electronic Skin Device Based on Microstructured SilverNanowire Electrode

Han-bai Lyua†, Xin-yu Pinga†, Rui-quan Gaoa, Liang-liang Xua∗, Li-jia Panb∗

a. Nanjing Foreign Language School, Nanjing 210018, Chinab. School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China

(Dated: Received on June 27, 2017; Accepted on August 6, 2017)

Transparent, flexible electronic skin holds a wide range of applications in robotics, human-machine interfaces, artificial intelligence, prosthetics, and health monitoring. Silver nanowireare mechanically flexible and robust, which exhibit great potential in transparent and electric-conducting thin film. Herein, we report on a silver-nanowire spray-coating and electrode-microstructure replicating strategy to construct a transparent, flexible, and sensitive elec-tronic skin device. The electronic skin device shows highly sensitive piezo-capacitanceresponse to pressure. It is found that micropatterning the surface of dielectric layerpolyurethane elastomer by replicating from microstructures of natural-existing surfaces suchas lotus leaf, silk, and frosted glass can greatly enhance the piezo-capacitance performanceof the device. The microstructured pressure sensors based on silver nanowire exhibit goodtransparency, excellent flexibility, wide pressure detection range (0−150 kPa), and high sen-sitivity (1.28 kPa−1).

Key words: Electronic skin, Pressure sensor, Transparent electrode, AgNWs, Microstruc-ture replica, Polyurethane

I. INTRODUCTION

Electronic skin is a kind of electronic device thatmimic the tactile function of the human skin. Withthe characteristics of flexible and thin, electronic skindevices can measure positive pressure and received in-creasing interest in recent years [1, 2]. As early as 1991,Jensen [3] and Kolesar et al. [4] have developed pro-totype of tactile devices of robot, but all the materialsused were hard materials. In 2004, Someya et al. re-ported the first example of flexible electronic skin withtactile function [5], the sensor array made of flexibleelectronics can change the shape of the robot, and canbe used for simultaneous measurement of temperatureand pressure [6]. Bao et al. reported a flexible ca-pacitive pressure sensor design of electronic skin withindium tin oxide film as electrode and microstructuredPDMS as the dielectric layer [7], and a capacitive pres-sure sensor based on carbon nanotube film on flexiblepolydimethylsiloxane (PDMS) substrate [8].

Improving sensitivity by microstructuring the func-tional layer of the pressure sensors has become a mainapproach in recent years. Pang et al. reported an in-terlocking microstructure of the nano fiber arrays toprepare the highly sensitive resistance type stress sen-

†These authors contributed equally to this manuscript.∗Authors to whom correspondence should be addressed. E-mail:[email protected], [email protected]

sor [9]. Ali et al. used nanolithography technology toprepare the electronic skin sensor [10]. In 2013, Yao etal. prepared the resistance stress sensor based on thegraphene fiber [11]. Zhang et al. reported carbon nan-otube electrode for resistance type stress sensor [12].

To design and produce electronic skin devices, thetrend is to use flexible and transparent materials toproduce electrode and use microstructure design to pro-mote sensing performance [13–16]. Silver nanowire (Ag-NWs) are mechanically flexible and robust, and showgreat potential in transparent and electric-conductingthin film. Herein, we report on a silver-nanowire spray-coating and electrode-microstructure replicating fab-rication strategy to construct a highly sensitive andflexible electronic skin device. The electronic skindevice shows sensitive piezo-capacitance response topressure. It is found that micropatterning the sur-face of dielectric layer polyurethane (PU) elastomer byreplicating from natural surfaces, such as lotus leaf,silk, and frosted glass, can greatly enhance the piezo-capacitance performance of the device. The microstruc-tured pressure sensors based on AgNWs exhibit goodtransparency, excellent flexibility, wide pressure detec-tion range (0−150 kPa), and high sensitivity (up to1.28 kPa−1).

DOI:10.1063/1674-0068/30/cjcp1706126 603 c⃝2017 Chinese Physical Society

604 Chin. J. Chem. Phys., Vol. 30, No. 5 Han-bai Lyu et al.

II. EXPERIMENTS

A. Materials and equipment

Silver nitrate, D+ glucose, poly(vinylpyrrolidone)and sodium chloride, PU (component AB and releaseagent), Ecoflex (group AB), Tris, hydrochloric acid,dopamine hydrochloride, and copper tape were com-mercially available and used as received.

B. Preparation of AgNWs

Ultra-long AgNWs were synthesized by a simple hy-drothermal route. In a typical synthesis procedure,silver nitrate (0.02 mol/L, 15 mL) (Sigma-Aldrich,10220), D+ glucose (0.12 g, 5 mL) (Sigma-Aldrich,G8270), poly(vinylpyrrolidone) (PVP,Mw=40000) (1 g,5 mL) (Fisher Scientific, BP431), and sodium chloride(0.04 mol/L, 15 mL) (Fisher Scientific, S93361) wereprepared in deionized water (DI) as four separate so-lutions, in a well dissolved form. PVP solution wasprepared at 65 C while the rest of the solutions wereprepared at normal room temperature. Glucose solu-tion was added to silver nitrate solution with continu-ous stirring. After 5 min to 10 min, PVP solution wasadded and stirred for 20 min until it mixed well. After-wards, sodium chloride solution was injected drop-wiseinto the above solution with continuous stirring untilit fully dissolved. This turbid hydrosol was added to a50 mL Teflon-lined stainless steel autoclave and heatedin an oven at 160 C for 22 h. After that, the autoclavewas air cooled to room temperature unaided and the fi-nal product in the form of a fluffy gray white precipitatewas collected by centrifugation at a speed of 2500 r/minfor 60 min and washed thrice with distilled water and 3to 4 times with isopropanol. The final product was dis-persed in isopropanol storage in refrigerator for futureuse.

C. Preparation of micropatterned PU substrate

To replicate the microstructure of natural availabletemplates of lotus leaf, silk, and frosted glass, the tem-plates was bonded to the bottom of culture dish and theliquid precursor of PU were cast in. The culture disheswere shaken to make PU precursor cover the bottomsurface, and flow flat. Then, the culture dish was thenplaced in an oven for 2 h at 80 C to cure PU, andthe flexible substrate with surface microstructure is ob-tained by peeling off the templates and cutting for therequired size.

D. Preparation of electrode

1 mL AgNWs suspension solution (40 mg/mL in iso-propyl alcohol) were measured and mixed with 20 mLsolution in isopropyl alcohol, and ultrasonic dispersing

for 5 min. The obtained AgNWs dispersed solutioncan be used for spray coating on the substrate. Re-quired amount of solution was calculated and loaded inthe spray gun. The microstructured PU substrate wasplaced on hot stage with a constant temperature 80 C,and the AgNWs suspension solution were sprayed onthe substrate until the resistance of the film measureis less than 100 Ω. After that, the substrate was heattreated at temperature of 200 C for 20 min. The elec-trode was connected with a thin copper foil tape at theedge of the conducting PU substrate to obtain a com-plete electrode.

E. Preparation of Ecoflex dielectric layer

To prepare the dielectric layer, 1 g Ecoflex componentA and 1 g Ecoflex component B were mixed in a smallplastic cup, and degassed by pumping for 2 min. Thesubstrate was dip coated in Ecoflex precursor, and spunat 600 r/min for 20 s and 8000 r/min for 40 s. The di-electric layers of the electrode were cured in the oven at70 C for 5 min, and sandwiched between two conduct-ing PU electrode to form the device. And the devicewas treated in the oven to complete curing (15 min).

III. RESULTS AND DISCUSSION

A. Device structure and working mechanism

The electronic skin devices were integrated by lam-inating the electrodes with the structure as shown inFIG. 1. The bottom layer is a driving electrode,the toplayer is an induction electrode, and the mid-dle layer is made of dielectric elastic material. Thepiezo-capacitance effect will be caused by changing thethickness of the dielectric layer under the interaction ofpressure, which is discussed as follows.

The initial capacitance (C) of the device is propor-tional to the device effective area (S) and the thicknessof the dielectric layer (d).

Cz0 =εS

d0(1)

When the interfacial stress is applied on the surface of anew type of pressure sensor, the distance in z-directionbetween the driving electrode and the sensing electrodeis reduced, which causes the output change of the ca-pacitance:

Cz =εS

d0 −∆d

= Cz01

1−∆d/d0(2)

Eq.(2) is expanded to

∆Cz

Cz0=

∆d

d0

[1 +

∆d

d0+

(∆d

d0

)2

+

(∆d

d0

)3

+ ...

](3)

DOI:10.1063/1674-0068/30/cjcp1706126 c⃝2017 Chinese Physical Society

Chin. J. Chem. Phys., Vol. 30, No. 5 Skin Device Based on Silver Nanowire Electrode 605

FIG. 1 (a) The preparation process of the electronic skin device and (b) schmatic illustration of spray coating process.

FIG. 2 SEM image shows the microstructure of as-synthesized AgNWs, scale bar 10 µm.

It can be known that there is no linear relationship be-tween the variation of output capacitance and the dis-tance between electrodes. In the above equation, thehigher order infinitesimal is ignored, and resulted in:

∆Cz

Cz0≈ ∆d

d0(4)

Eq.(4) shows that when the interfacial pressure is ap-plied on the sensor, the capacitance C increases withthe decreases of electrode spacing of d. Therefore, mea-suring C would be able to estimate the value of appliedpressure.

B. Device fabrication

AgNW is a kind of one dimensional (1D) materialwith excellent and unique electrical, mechanical, ther-mal, chemical, and electronic properties [17]. FIG. 2shows the scanned electric microscope (SEM) imageof the as-prepared AgNWs, which shows high aspectfactor>1000 with diameter ∼50 nm and length over50 µm. The AgNWs suspending solution in isopropylalcohol was used to fabricate electrodes on elastic PUsubstrate by spray coating technology.

It is found that the AgNWs are the most suitablematerial for the functional electrode of pressure sensoraming AgNW, single-walled carbon nanotubes (SCNT),and silver foil. We compared the performance of the

FIG. 3 Microstructures of (a) AgNWs and (b) SCNTsspray-coated on the substrate, scale bar 10 µm.

electronic skin device with spray coated electrode ofthese materials, the results showed that the AgNWselectrode has the highest positive pressure sensitivityas presented in FIG. 4, which may be resulted from itshigh conductivity and flexibility. The SEM images ofAgNWs and carbon nanotube electrode were comparedin FIG. 3, it seems that AgNWs tend to be homoge-neously dispersed on the substrate while the SCNTstends to be aggregated into bundles, which may alsoresult in higher sensitivity of the pressure sensor of Ag-NWs.

C. Optimization of AgNWs density

We found that the surface-specific density of the Ag-NWs affects the sensitivity of the device. As shownin FIG. 5(b), the sensitivity of the capacitive pressuresensor is increased with the increased density of the



FIG. 4 Effect of different electrode materials on the sensitiveresponse of electronic skin device.

AgNWs in a certain range. After the density of the Ag-NWs is over 2.2 mg/cm2, the sensitivity of the capac-itive pressure sensor is decreased. The high density ofAgNWs with dense distribution may be favorable for ahigher capacity because of higher occupied surface areaof AgNWs; while very thin AgNWs electrode may havelower substantial capacity. However, with too high den-sity of AgNWs, the flexibility of the electrode may bedecreased and thereafter resulting in reduced sensitivityof the device.

D. Replication of microstructure on substrate surface

Inspired from that insects and animals microstruc-ture of their sensory receptor increase their sensingperformance, we replicated the electrode surface withmicrostructure of natural existed objects, and foundthat micropatterning of electrodes significantly pro-moted the device performance. The PU substrate weremicropatterned by cast replicating from three kinds ofnatural materials as template, such as frosted glass, silk-cloth, and lotus leaf, as shown as the SEM images inFIG. 6. After that, the AgNWs were deposited on themicropatterned PU substrate.

The microstructured electrodes presents greatly en-hanced sensitivity against pressure, as shown in FIG. 7.This may be because that the surface microstructuremay show a smaller effective Young modulus, and thesurface microstructure can be deformed sensichangedeven with a small pressure. Our E-skin pressure mea-suring method has high reliability, the relative ex-panded uncertainty in the pressure measuring was cal-culated to be ∼0.44% (supplementary materials).

E. Optimization of electrode substrate

During the spray coating process of the conductiveelectrode, we found that AgNWs has low adhesionstrength on PDMS surface, most likely due to the non-

FIG. 5 (a) Transparent E-skin device with electrode of1.6 mg/cm2 AgNWs. (b) Effect of different surface specificdensity of AgNWs on the response of pressure sensors.

polar nature of PDMS, which is the most popular sub-strate material of E-skin [18]. Alternatively, we foundthat PU can be an excellent substrate, which presentboth good mechanical properties and strong adhesionwith AgNWs.

PU are elastic rubber with strong carbamate polar,oil resistance, aging resistance, and good adhesion. Fora thin film of 200 µm thickness, PU can be stretched asthe original 2 times longer without fracture. The tensilestrength, elongation at break, tear of PU is far strongerthan ordinary rubber, shows its strong wear resistanceand excellent mechanical properties.

PU and AgNWs have a strong adhesion effect, whichis in accordance with the strong adhesion of PU andsilver nanoparticles as reported in Ref.[19]. We foundthat AgNWs spray coated on PU has higher adhesionstrength, in contrast, AgNWs on PDMS is easy to beexfoliated. This is because the surface of PU contains alarge number of primary amine (-NH2) and secondaryamino (-NH-), the lone electron pair of N atom inter-acting with the empty atom obit of silver atom sofsilverand resulted in firmly attached AgNWs on the surfaceof PU.

IV. CONCLUSION

By using a high aspect AgNWs layer as the con-ductive electrode, we constructed flexible, transparent,highly sensitive electronic skin device. The high qual-ity AgNWs with high aspect ratio forms a conductive


Chin. J. Chem. Phys., Vol. 30, No. 5 Skin Device Based on Silver Nanowire Electrode 607

FIG. 6 Substrate surface microstructure replicate from: (a, b, c) frosted glass, (d, e, f) silk, (g, h, i) lotus leaf surface.

FIG. 7 Effect of substrate with different surface microstruc-ture and without microstucture on the response of pressuresensors.

network with high conductivity, which exhibited to bethe most optimal electrode material among AgNWs,SWCNT, and silver foil. We found that PU can be

an excellent flexible substrate with strong adhesion toAgNWs. The PU dielectric layer were microstructuredby replicating from natural surface of frosted glass, silk,and lotus leaf, which is proven to be a low cost micro-fabrication strategy and effectively improve the sensi-tivity of the E-skin devices. Such a kind of transparentand flexible electronic skin with low fabrication cost haswide range of potentials in robotics, human-machine in-terfaces, artificial intelligence, prosthetics, and healthmonitoring.

Supplementary materials: The calculation of therelative expanded uncertainty in the E-skin pressuremeasuring.

V. ACKNOWLEDGEMENTS

This work was supported by the National Natural Sci-ence Foundation of China (No.61674078) and Dongrun-Yau Science Silver Award (Chemistry).



[1] A. Chortos, J. Liu, and Z. Bao, Nature Mater. 15, 937(2016).

[2] T. Someya, Z. Bao, and G. G. Malliaras, Nature 540,379 (2016).

[3] T. R. Jensen, R. G. Radwin, and J. G. Webster, J.Biomech. 24, 851 (1991).

[4] E. S. Kolesar, R. R. Reston, D. G. Ford, and R. C.Fitch, J. Robot. Sys. 9, 37 (1992).

[5] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi,and T. Sakzzrai, Proc. Natl. Acad. Sci. USA 101, 9966(2004).

[6] T. Someya, Y. Kato, T. Sekitani, S. Iba, Y. Noguchi,Y. Murase, H. Kawaguchi, and T. Sakurai, Proc. Natl.Acad. Sci. USA 102, 12321 (2005).

[7] S. C. B. Mannsfeld, B. C. K. Tee, R. M. Stoltenberg, C.V. H. Chen, S. Barman, B. V. O. Muir, A. N. Sokolov,C. Reese, and Z. Bao, Nature Mater. 9, 859 (2010).

[8] D. J. Lipomi, M. Vosgueritchian, B. C. K. Tee, S. L.Hellstrom, J. A. Lee, C. H. Fox, and Z. Bao, NatureNanotech. 6, 788 (2011).

[9] C. Pang, S. H. Ahn, T. I. Kim, and K. Y. Suh, NatureMater. 11, 795 (2012).

[10] Z. Y. Fan, J. C. Ho, J. T. Takahashi, R. Yerushalmi,K. Takei, A. C. Ford, Y. L. Chueh, and A. Javey, Adv.Mater. 21, 3730 (2009).

[11] H. B. Yao, J. Ge, C. F. Wang, and S. H. Yu, Adv. Mater25, 6692 (2013).

[12] X. W. Wang, Y. Gu, Z. Cui, and T. Zhang, Adv. Mater.26, 1336 (2014).

[13] K. Ikeda, H. Kuwayama, T. Kobayashi, T. Watanabe,T. Nishikawa, T. Yoshida, and K. Harada, Sensors Ac-tuators 23, 1007 (1990).

[14] L. J. Pan, A. Chortos, G. H. Yu, Y. Q. Wang, S. Isaac-son, R. Allen,Y. Shi, R. Dauskardt, and Z. Bao, NatureComm. 5, 3002 (2014).

[15] C. Giacomozzi and V. Vlacellari, IEEE Trans. Rehabil.Eng. 5, 322 (1997).

[16] M. Yavuz, G. Botek, and B. L. Davis, J. Biomech. 40,3045 (2007).

[17] W. W. He and C. H. Ye, J. Mater. Sci. Techn 31, 581(2015).

[18] T. Akter and W. S. Kim, ACS Appl. Mater. Interface4, 1855 (2012).

[19] P. Jain and T. Pradeep, Biotech. Bioeng. 90, 59 (2005).


Documents

ARTICLE Transparent Electronic Skin Device Based on