DEVELOPMENT OF FULLY-PRINTED STRETCHABLE DEVICES

By

Yiheng Zhang

A THESIS

Submitted to

Michigan State University

in partial fulfillment of the requirements

for the degree of

Electrical Engineering – Master of Science

2018

ABSTRACT

DEVELOPMENT OF FULLY-PRINTED STRETCHABLE DEVICES

By

Yiheng Zhang

In recent years, stretchable flexible electronics has become a research hotspot in the electron-

ics industry and academia due to its outstanding expansibility, adaptability, and portability.

With the improvement of science and technology, materials science, mechanics, and manu-

facturing process have all achieved rapid development. Advanced technologies have made

possible the manufacture of stretchable flexible electronic devices with excellent performance.

In the first half (chapter 2) of this thesis, we developed an inkjet printing method to

pattern silver nanowires (AgNWs) with length of up to ˜ 40 m on various substrates. Well-

defined and uniform AgNWs features could be obtained by optimizing the printing conditions

including nozzle size, ink formulation, surface energy, substrate temperature, and printing

speed. Such printed AgNWs have been used to demonstrate bi-axially stretchable conductors

that could maintain stable conductance under an areal strain of up to 156% (256% of its

original area). By printing the AgNWs electrodes on a composite composed of PDMS and

electroluminescent (EL) phosphors, I have demonstrated a printed stretchable EL display. In

chapter 3, I developed a silver nanoparticles (AgNPs) based stretchable sensor for pressure

sensing through directly printing AgNPs on Polyurethane Acrylate (PUA) substrate. High

sensitivity is indicated while I test it within a low-pressure load.

Its resistance increases

100% when a 5.5 kPa pressure is applied on it.

I explored the applications of heartbeat

detection and wind sensing in this work.

ACKNOWLEDGMENTS

I would like to extend my most sincere gratitude to all who have given me care and help

during my master’s studies.

First of all, I feel very honored that Dr. Chuan Wang has accepted me into his team so

that I get to work with incredible colleagues. His optimism, help, and encouragement has

helped me discover the joy of scientific research. I would also like to thank my committee

members: Dr. Wen Li and Dr. Nelson Sep´ulveda for their valuable suggestions and kindly

responding to my questions and doubts. Also I want to thank Dr. Tim Hogan for his advice

on class selection when I just entered MSU and felt lost.

At the same time, I would like to thank the seniors of my group for their care and

guidance. They are my guides on the academic road. When I first entered the laboratory,

Its Dr. Le Cai who passed on to me a lot of valuable experience and instrument operation

methods. Thanks to Dr. Jinshui Miao, Dr. Suoming Zhang, Dr. Lei Zhang and Dr. Min

Yu for their help and companionship. With them, I don’t have to dine alone and conduct

experiments alone. Thanks to Dr. Suoming for working out with me. Thanking Zhihao

for letting me have tofu dumplings in the United States. I would like to thank Hongyang

Shi as well. When I almost gave up after a dozen failures in making pressure sensors, he

kept encouraging me and helping me. It is HongYang who made me understand that I must

be patient with scientific research. Thanks Shuo Zhang for the help on the modification of

format on this dissertation.

In the end, I want to thank my parents for giving me unconditional trust and support.

Without their love I wouldnt have made it here.

iii

TABLE OF CONTENTS

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 Flexible and Stretchable Electronic Devices
. . . . . . . . . . . . . .
1.1.2 Printing Method Applied in Device Fabrication . . . . . . . . . . . .
1.1.2.1 Printing Technology . . . . . . . . . . . . . . . . . . . . . .
1.1.2.2 Printing Materials
. . . . . . . . . . . . . . . . . . . . . . .
Inkjet Printing . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.2.3
1.2 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 2 DIRECT PRINTING FOR ADDITIVE PATTERNING OF SILVER

NANOWIRES FOR DISPLAY APPLICATION . . . . . . . . . . . .
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Design and Fabrication Method . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Test
2.3.1 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 Experimental Results and Analysis . . . . . . . . . . . . . . . . . . .
2.4 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 3

INKJET-PRINTED STRAIN SENSOR FOR PRESSURE SENSING .
3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Design and Fabrication Method . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Experimental Results and Analysis
. . . . . . . . . . . . . . . . . . . . . . .
3.4 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 Heart Rate Detection . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2 Wind Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 4 SUMMARY AND FUTURE WORK . . . . . . . . . . . . . . . . . .
4.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

v

vi

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10
10
13
15
16
17

19
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21
22
22
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28
31

32
32
34
35
41
42
44
45

46
46
46

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48

iv

LIST OF TABLES

Table 1.1: Comparison of various fabrication technology . . . . . . . . . . . . . . . .

11

v

LIST OF FIGURES

Figure 1.1: schematic of the cross-section of an organic transistor circuit technology

based on flexible printed wiring boards.[12] . . . . . . . . . . . . . . . . .

Figure 1.2: (a) Schematic of the island-bridge structure with serpentine geometry;

(b) optical image of the as fabricated serpentine structures.[16] . . . . . .

Figure 1.3: The dynamically tunable electronic eye camera from the team of John

A. Rogers. [39]

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 1.4: Paper smart phone and flexible display that HP exhibited in MobileBeat

conference.[48] [49]

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 1.5: Tattoo-based platform for noninvasive glucose sensing. (A)Schematic of
the printable iontophoretic-sensing system displaying the tattoo-based
paper (purple), Ag/AgCl electrodes (silver), Prussian Blue electrodes
(black), transparent insulating layer (green), and hydrogel layer (blue).
(B) Photograph of a glucose iontophoretic-sensing tattoo device applied
to a human subject. (C) Schematic of the time frame of a typical on-
body study and the different processes involved in each phase.[50]

. . . .

Figure 1.6: Electrical performance of nanomesh conductors on skin and their sen-
sor applications. a, b, Conductance of nanomesh conductors attached
to a nger when the hand is opened and closed. c, Depiction of an on-
skin wireless sensor system. Electrical signals are transmitted from the
ngertip to a wireless module through on-s kin nanomesh conductors,
anisotropic lms and conductive threads. The electrode array acts as a
touch sensor for conductive materials. It can also be used as a ther-
mal and/or pressure sensor when polymer PTC and pressure-sensitive
rubbers are attached, respectively. df, Electrical performance of touch
(d), temperature (e) and pressure (f) se nsors. The inset shown in (e)
shows the thermal sensor laminated onto a silicone replica substrate
with nanomesh conductors. Scale bars, 3 mm.[51]

. . . . . . . . . . . . .

2

4

6

7

8

9

Figure 2.1: schematic diagram illustrating the printing process.[118]

. . . . . . . . .

23

Figure 2.2: (a) MSU shaped AgNW patterns printed on PDMS. Scale bar: 1 cm.
(b) Straight line patterns of AgNWs printed on silicon wafer, glass slide,
polyimide thin film and VHB tape. Scale bars: 2 cm.[118]

. . . . . . . .

24

Figure 2.3: Optical micrograph (top) and SEM image (bottom) of the AgNW net-

work printed on Si wafer. Scale bars: 50 m and 1 m, respectively.[118] . .

25

vi

Figure 2.4: Electrical and electromechanical characterization of the printed AgNW
features with different nanowire lengths. (a) SEM images of the AgNW
network with average nanowire length of ˜ 4.4 m, ˜ 15.6 m and ˜ 38.5 m
(from top to bottom). Scale bar: 5 m. (b) Nanowire length distribution
of the three different AgNW inks. (c) Feature resistance as functions
of number of printing runs for AgNWs with different nanowire lengths.
Inset, same data presented in log scale. (d) Resistance as a function of
printed feature length for all three kinds of AgNW inks. (e) Relative
change in resistance as a function of tensile strain for features with
average nanowire lengths of ˜ 38.5 m and ˜ 4.4 m. (f) Cyclic stretching
test results for the feature with an average nanowire length of ˜ 38.5 m.
Inset, measurement setup. Scale bar: 1 cm.[118] . . . . . . . . . . . . . .

26

Figure 2.5: Bi-axially stretchable conductors based on the printed AgNWs. (a) Fea-
ture resistance as functions of areal tensile strain (top) and stretching
cycles (bottom) for a AgNW feature printed on a bi-axially pre-stretched
PDMS substrate. Insets: the sample at relaxed state (left) and being
stretched to an areal strain of 125% (right). (b) The current-voltage (I-
V) curves of the LED-driving circuit using the printed bi-axially stretch-
able AgNW conductor as interconnect with the areal strain varying from
0% to 156%. Insets: photographs of the circuit while the interconnect
is under 0 (left) and 125% (right) areal tensile strain. Scale bar: 2 cm.[118] 27

Figure 2.6: Stretchable electroluminescent (EL) displays using the printed AgNWs
as electrodes. (a) Schematic of the device structure. (b) EL spectra
of the device made of green phosphors under different stimulation con-
ditions. (c, d) EL intensity of the device made of green phosphors as
functions of frequency (c, measured at 1 kV) and magnitude (d, mea-
sured at 500 Hz) of the AC voltage. (e) Normalized EL intensity plotted
as a function of tensile strain when measured under AC voltage with
different magnitudes and a frequency of 1 kHz. Inset: photographs of a
device made of green phosphors under 0 and 20% tensile strains. Scale
bars: 5 mm. (f) Normalized EL intensity versus stretching cycles mea-
sured under AC voltage with a magnitude of 1.5 kV and a frequency
of 1 kHz.
(g) Photographs of a 66 EL display array made of green
phosphors (left), an MSU display pattern using orange, green and blue
phosphors for M, S and U, respectively (middle), and the MSU-shaped
display under 0 and 20% tensile strains (right). Scale bars: 1 cm.[118] . .

Figure 3.1: Capacitive soft pressure sensor array based on PEDOT:PSS (scale bar,
20 mm). (a) Device structure. (b) Capacitance change in response to
pressure.[151]

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

34

Figure 3.2: Schematic diagram of printing process and device pattern.

. . . . . . . .

35

vii

Figure 3.3: A sample of printed pressure sensor.

. . . . . . . . . . . . . . . . . . . .

36

Figure 3.4: Pressure loading platform for measurement.

. . . . . . . . . . . . . . . .

37

Figure 3.5: Force-to-voltage response curve of the FSR.[151] . . . . . . . . . . . . . .

37

Figure 3.6: Pressure sensor based on printed AgNPs.

sistance and pressure as a function of time.
resistance as a function of pressure.

(a) Relative change in re-
(b) Relative change in

. . . . . . . . . . . . . . . . . . . . .

39

Figure 3.7: (a) Optical image of the device before test (b) Optical image of the

device after test.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40

Figure 3.8: The relative change in AgNPs patterns resistance under different pres-

sure loading frequency.

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

41

Figure 3.9: Heart rate sensing test. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42

Figure 3.10: Heart rate sensing: resistance of sensor as a function of time. . . . . . . .

43

Figure 3.11: Windspeed sensing test. (a) Measurement setup. (b) Fixture for sample
in wind tunnel. (c) Relative change in sensor resistance as a function of
windspeed.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44

viii

CHAPTER 1

INTRODUCTION

Flexible electronics [1] represents a new kind of electronic technology that has been widely

used in many fields. Evolving thin-film materials and devices have brought great progress

in various applications to flexible electrons. The large-area, low-cost manufacturing process

has endow flexible electronics with unique flexibility and extensibility. It shows us broad

application prospects in almost all fields including information, energy, medical, and national

defense. Moreover, many devices have been developed such as flexible transistors, which have

already been commercialized [2, 3, 4] large area print pressure sensors [5], RFID tags [6],

which have been put into wide application, soft solar cell [7], and a light emitting diode

(LED) application, etc. The flexible electronics applications have been covered from the

biomedical field energy to space science and art [8, 9].

1.1 Background

1.1.1 Flexible and Stretchable Electronic Devices

Current general-purpose conventional electronic devices are generally fabricated on a rigid

substrate like a silicon wafer, and their hard and brittle nature makes it difficult for the elec-

tronic devices to bend or extend. Once there is a large deformation applied, the structure of

the electronic devices will be destroyed or the performance will be compromised. Therefore,

it is difficult for conventional electronic devices to adapt to the high requirements of the next

generation electronic products in terms of convenience and flexibility, and it is urgent to find

a method that can realize the flexibility of electronic devices. In 1998, the ordered buckling

of metal thin films on an elastic substrate was first proposed by N. Bowden et al.[10]. The

study of the flexibility of electronic devices began with the study of organic electronics [11].

People hoped to use organic semiconductors to replace traditional silicon materials. Scien-

1

tists at Bell Labs [12] combined thin-film transistors with flexible substrates to implement

electronic circuits at a clock frequency of 1 kHz. Figure 1.1 shows the schematic of the cross-

section of an organic transistor circuit technology based on flexible printed wiring boards

used in that study.

Figure 1.1 schematic of the cross-section of an organic transistor circuit technology based
on flexible printed wiring boards.[12]

In 2004, Professor S. R. Forrest [13] of Princeton University reviewed the research sta-

tus and development direction of organic electronics, proposed a conceptual design and

manufacturing method, and demonstrated a pen-shaped flexible display. However, organic

semiconductors have quite different physical and electrical properties compared with silicon-

based semiconductors. This severely restricts the practical and industrialization of organic

electronics and makes it difficult for the study of organic electronics to move forward.

In recent years, with the continuous development of mechanics, materials science, and

manufacturing process levels and by improving the traditional manufacturing processes and

methods of silicon electronic devices, it is now feasible to manufacture stretchable flexible

electronic devices, which has good portability and adaptability, as well as the ability to

maintain good performance under tension, compression, bending and twisting. Instead of

replacing the current silicon chip technology, current flexible electronic technology is not

an improvement on the silicon substrate structure. Mostly it is based on the principle of

2

integrating micro-structures on a flexible substrate. Usually, the silicon substrate is placed

on a flexible substrate, for example, Polydimethylsiloxane (PDMS). This method can balance

out the disadvantages of non-flexible silicon-based chips that are thick and brittle, making

the electronic devices flexible and light and thin and anti-knock. From the initial ”wave”

structure [14, 15] to the current more stretchable buckling in serpentine microstructure, not

only the max strain reaches 170%, the occupancy of the active device has also reached 81%

[16]. Figure 1.2 gives the serpentine structures used in Yihui’s study. At the same time,

the silicon substrate is replaced by a large amount of inexpensive plastic substrates, the cost

is greatly reduced. The development of flexible electronic technology will inevitably create

new application areas for the design electronic products which has various shapes, fitting

the human body and is easy to use, such as flexible sensors, flexible electronic eyes, wearable

electronic clothing, flexible electronic paper, flexible circuit boards, artificial muscles, flexible

heart monitoring clothing, flexible keyboards and flexible electronic displays. Compared with

traditional electronic devices, flexible electronic devices with extraordinary flexibility and

extensibility will undoubtedly have a prospect of comprehensive and practical applications

in the areas of communications and information, biomedicine, machinery manufacturing,

aerospace and defense security.

Apart from the broad prospect in development, flexible electronics also have very high

commercial value due to its advantages of being lightweight, flexible, extensible, and adapt-

able to complex and uneven surfaces. Many countries have included flexible electronics re-

search in their major national development plans. Many universities and research institutes

have scrambled to join the study of flexible electronics. Many internationally well-known

universities like Illinois University, Northwestern University, Princeton University, Harvard

University and Cambridge University have established research institutes specifically for

flexible electronic technology. Intel, IBM, Philips, Sony and other leading companies have

invested a lot of manpower for related research and development.

In 2008, Prof. Y. Huang’s research group [17, 18] analyzed the mechanical properties of

3

Figure 1.2 (a) Schematic of the island-bridge structure with serpentine geometry; (b) optical
image of the as fabricated serpentine structures.[16]

the hard film-soft substrate compliant structure, studied the controllable buckled structure

of the hard film on the flexible substrate. Meanwhile the controllable bending mechanism

of thin-film nanobelts on compliant substrate was analyzed [19], and the precise control of

corrugated structures was studied from the mechanical mechanism [20]. The joint team of

Prof. Y. Huang and Prof. J. A. Rogers [21] successfully fabricated the world’s first flexible

electronic eye camera, and introduced the structure, function and preparation process of the

flexible electronic eye. Figure 1.A. J. Baca et al. [22] introduced the method of combining

phase shift lithography with anisotropic chemical etching. This method can be used to

4

produce high-quality single crystal silicon nano bars or nano thin films. A. Carlson et al.

[23] studied the transfer technology for the materials assembly and devices assembly at

micro-nanometer. R. Saeidpourazar et al.

[24] introduced the method of micro-structure

fabrication using laser driven micro-transfer placement.

In 2012 Prof. J. A. Rogers [25]

introduced the latest research results of nanometer-scale printing. Team of Prof. J.A Rogers

[26] have done a lot of research from theoretical analysis to experimental verification and

preparation. Using the printing technology to make single crystal silicon film strips integrated

on a flexible plastic substrate, a flexible integrated circuit was successfully prepared. A

flexible broadband membrane reflector [27] was prepared on glass and it was found that

incorporation of InGaAs in silicon films of silicon solar cells can enhance cell efficiency [28],

and micro and nanopatterning techniques for flexible electronics are studied [29]. Successfully

fabricated microstructures on the device surface, Prof. S. Wagner [30] studied the bending

and stretching properties of microelectronics, analyzed the mechanical properties of bending

deformation of the structure, studied the connection of stretchable electronic circuits in

flexible electronic structures [31], and analyzed the connection problem of the corrugated

structure of the metal wire [32]. Prof. Z.G. Suo’s research team [33] conducted research

work on the thin film and interface fracture in flexible structures, analyzed the mechanical

properties of the stretchable metal film on the elastic substrate, and crack initiation of the

composite structure containing the organism from the dynamics [34]. J. Wu et al. [35, 36]

pointed out that the flexible substrate in a flexible electronic structure has the effect of

strain isolation, and the transfer printing efficiency is analyzed using a viscoelastic model

[37]. J. Xiao et al.

[38] conducted theoretical analysis and experimental research on the

buckling mechanism of the film, studied the dynamically tunable hemispherical electronic

eye camera system with adjustable zoom capability [39], and analyzed the noncoplanar design

for stretchable electronic circuits and structural mechanical properties [40]. G.X. Qin et al.

[41] studied the performance of flexible thin-film structures and the fabrication of flexible

nanodevices. The high-frequency characteristics of flexible silicon thin-film transistors on

5

plastic substrates under bending conditions were analyzed, and the flexibility on plastic

substrates was studied, as well as microwave single crystal germanium nanomembrane diode

performance and fabrication technology [42].

Figure 1.3 The dynamically tunable electronic eye camera from the team of John A. Rogers.
[39]

The team of Z.Q. Ma [43] successfully fabricated high-speed flexible electronic devices

using strained silicon thin films, fabricated large-area flexible broadband reflectors using laser

interference lithography [44], and analyzed the relevant characteristics of rippled graphene

and silicon thin films [45]. A thermoelectric wireless sensor for fuel monitoring [46] was

prepared, and semiconductor nanomembranes technology for integrating silicon photonic

devices and flexible photonic devices was introduced [47].

In 2015, Queens University of

Canada has developed a new concept mobile phonepaper mobile phone, which can make

telephone calls and play music like other smart phones. The difference is that it is lighter

and more flexible. HP has successfully developed a flexible display that is ideally suited for

future handheld devices. Figure 1.3 shows the dynamically tunable electronic eye camera

from the team of John A. Rogers. Figure 1.4 presents a possible structure of paper smart

phone and the flexible display exhibited on HP conference.

With more and more researchers joining the research work of flexible electronics, the

6

Figure 1.4 Paper smart phone and flexible display that HP exhibited in MobileBeat
conference.[48] [49]

mechanical properties and performance level of flexible electronic devices will certainly be

improved.

In the future, more advanced materials such as carbon nanotubes, graphene,

and germanium will be gradually applied to mature flexible electronic structures, and their

flexibility and other properties will be greatly enhanced. New flexible sensors made with

these new materials will adapt to more complex, uneven surfaces and expand the range of

applications for sensors. These new flexible sensors can play an important role in electronic

skin, biopharmaceuticals, wearable electronics, and aerospace, such as implantable micro-

flexible temperature sensors for detection of abnormal cells in biopharmaceuticals; wearable

electronic medical devices will continuously and dynamically monitor the physical condition

of the human body anywhere and anytime;

Figure 1.5 shows a tattoo-like sensor that can detect glucose levels without a painful

finger prick [50]. Figure 1.6 shows the study of team of Prof. Someya, they fabricated a

motion sensor, thermal sensor and temperature sensor on a finger [51]. Flexible sensors can

also be wrapped on the surface of the wing to monitor and transmit cruise speed and the

aircraft’s health information.

7

(A)Schematic of the
Figure 1.5 Tattoo-based platform for noninvasive glucose sensing.
printable iontophoretic-sensing system displaying the tattoo-based paper (purple), Ag/AgCl
electrodes (silver), Prussian Blue electrodes (black), transparent insulating layer (green),
and hydrogel layer (blue). (B) Photograph of a glucose iontophoretic-sensing tattoo device
applied to a human subject. (C) Schematic of the time frame of a typical on-body study
and the different processes involved in each phase.[50]

With the further development of flexible electronics research and the improvement of the

manufacturing process of flexible electronic devices, it is expected that in the future people

will be able to produce flexible electronic products that are more adaptable, lighter, and

more economical. Paper mobile phones, flexible electronic clothing and pen-shaped flexible

rollable displays will surely turn concepts into reality.

In the foreseeable future, relevant

flexible electronic products will be popular and greatly enriched the market. A new era, the

era of flexible electronics, will surely come, and it will revolutionize electronic technology

and have a major impact on human life.

8

Figure 1.6 Electrical performance of nanomesh conductors on skin and their sensor appli-
cations. a, b, Conductance of nanomesh conductors attached to a nger when the hand is
opened and closed. c, Depiction of an on-skin wireless sensor system. Electrical signals are
transmitted from the ngertip to a wireless module through on-s kin nanomesh conductors,
anisotropic lms and conductive threads. The electrode array acts as a touch sensor for con-
ductive materials. It can also be used as a thermal and/or pressure sensor when polymer
PTC and pressure-sensitive rubbers are attached, respectively. df, Electrical performance
of touch (d), temperature (e) and pressure (f) se nsors. The inset shown in (e) shows the
thermal sensor laminated onto a silicone replica substrate with nanomesh conductors. Scale
bars, 3 mm.[51]

9

1.1.2 Printing Method Applied in Device Fabrication

1.1.2.1 Printing Technology

Silicon-based semiconductor microelectronics technology has dominated electronics technol-

ogy for half a century. However, with the development of electronic products demanding

flexibility, the brittle materials used in silicon-based electronic products are insufficient in

flexibility, and the production line needs a huge amount of investment and increasingly com-

plex processes have stimulated the demand for flexible electronic technology. The research on

electronic technology based on plastic substrates has developed rapidly. At this stage, there

are two main manufacturing strategies for flexible electronics: One is to transfer components

onto a flexible substrate by transfer printing [52]; The other is to directly fabricate compo-

nents on the surface of the substrate by printing method [53]. The former is mainly based

on silicon-based electronic technology, which realizes flexible circuit by transfer processing.

This method still cannot resolve the complexity of the manufacturing process; the latter on

the other hand is the focus of current flexible electronics development. Currently there are

some published papers expounding the development of printed electronics technology, for

example, Singh et al. [54] and Yin et al. [55] published inkjet printing technology. Schift et

al. [56] mainly involved nano-printing technology, Carl. Son et al. [57] published a transfer

printing technique. Perl et al. [58] published an overview paper on micro contact printing,

and Sndergaard et al. [59] published a roll-to-roll (R2R) processing technology paper, Khan

et al.

[60] reviewed the contact and non-contact printing technologies. A new electronic

technology, namely printed electronics, has been created.

10

Table 1.1 Comparison of various fabrication technology

F abrication T echnology
M icro − nano processing M aking masks − P reparing substrate − P hotoresist spin

P rocess Steps

coating − U V exposure − development − Etching
P reparing template − P reparing materials−

T ransf er P rinting

Digital printing

P rinting − P ost − processing

P reparing materials − P rinting−

P ost − processing

Printed electronics, as the name suggests, is a process that combines traditional print-

ing techniques to produce electronic devices. Through a variety of printing processes, pre-

designed patterned structures are printed on functional inks on flexible (or non-rigid) sub-

strates to connect electronic components [61, 62, 63, 64, 65]. The printing systems currently

developed fall into two main types: contact and non-contact. In the contact process, the pat-

terned surface with ink is used for the direct physical contact with substrate. In non-contact

processes, the ink is moved through the openings or nozzles, while moving the substrate mate-

rial or nozzle through a pre-programmed pattern to achieve the structure printing. Contact

printing technology include gravure printing, gravure offset printing, screen printing, and

elastic printing. Non-contact printing technology includes extrusion coating printing and

inkjet printing. From Table 1.1, it can be seen that silicon-based microelectronics technol-

ogy requires complex manufacturing processes, while printed electronics, especially digital

printed electronics, greatly reduce the processing steps of circuit, and the invention of printed

electronics technology has made large-scale IC and R2R production possible.

Taking traditional inkjet printing as an example, it is a method of directly printing a

designed graphic structure to a substrate through a printer. It is an additive manufacturing

process and is an efficient solution [66, 67, 68, 69]. Compared to the lithography process [70],

digital printing electronics has unparalleled advantages. This is a crossover area integrating

the two major industries of electronics and printing. Printing flexible electronic technology

helps to greatly reduce the costs of fabricating electronic devices and sensors. Simple process

steps, minimal material waste, low processing costs, and simple graphical techniques make

11

flexible electronics very attractive [71]. Among them, the key point is that digital printing

does not require production of the template, which is also an important feature distinguish-

ing it from other printing technologies. It further shortens the product’s development and

production cycle. At present, regardless of the printing method or the printing material, it

has attracted great attention of the industry.

Flexible electronic devices require not only flexible substrates, but also to produce smaller

functional circuits and components on large-area flexible substrates, and materials and sub-

strates must have good compatibility. High-molecular organic materials [72, 73] and inor-

ganic materials [74] are two major types of materials currently used in flexible electronics.

Although high molecular organic materials can naturally combine with flexible substrate

materials as they have inherent flexibility and ductility. The performance of electronic de-

vices that are based on high molecular organic materials is not ideal. Especially its electrical

conductivity often fails to meet the requirements of electronic components. This has led

to attention about the use of inorganic materials for flexible electronic processing to sup-

plement organic materials. Conductive materials made of metal nanoparticles have been

widely praised [75, 76, 77, 78], but the use of non-flexible inorganic materials has encoun-

tered great challenges in the design and processing of stretchable and foldable devices. So

many researchers start to optimize structure design such as the use of inorganic thin film

structures, which are now widely accepted, to limit the influence that inorganic materials

used in flexible electronic devices In addition, other inorganic conductive materials, such as

carbon nanotubes, graphene, etc., have also been fabricated into conductive inks [79, 80, 81]

for the preparation of functional electronic components. The mechanical properties of flexible

electronic systems are the important parts of the research and development and application.

Major breakthroughs that have greatly advanced the development of flexible electronics have

been seen in bendability, stretchability, and cracking problem [82].

Digital Printed Electronics is an Additive Manufacturing (AM) technology. Because the

manufacturing process is simple and the selectivity of materials is relatively low, it can

12

easily explore new materials and process ways to develop new sensors and systems. For

example, a series of inorganic semiconductor materials have been developed and used for

the preparation of thin-membrane transistors and flexible electronic components, as well as

electronic skins for enveloping the body or prosthesis [83, 84, 85, 86]. Also, digital printing

technology can easily achieve industrial production of R2R, providing a convenient way

for low-cost electronics. Printing electronics technology opens up new avenues for low-cost

electronic processing with its unique advantages. Research in this field is slowly incorporating

microelectronic technology and traditional printing techniques [87].

1.1.2.2 Printing Materials

A typical flexible electronic device mainly comprises a flexible base layer and an electrically

functional material layer. The materials used for the electrically functional material layer

can be divided into conductive materials, semiconducting materials and dielectric materials.

For low cost, high throughput, light weight printed electronics, a large number of organic

materials and inorganic materials are used to prepare the above functional materials.

In

addition, an important advantage of printed electronics is that it can print any functional

material, such as some composite materials have dual properties, such as insulation prop-

erties, ferroelectric properties, piezoelectric properties and photosensitivity characteristics,

these materials are widely used in thin film printed electronic devices. In addition, hybrid

organic/inorganic materials are also widely used in printed electronic devices [88].

For ink materials, inorganic conductive materials are mainly used in deforming wires,

which acts like a blood vessel that connects all parts of the body, ensure the normal oper-

ation of the entire system. Inorganic conductive materials have always been the preferred

choice for conductive materials. To achieve digital printing, many metal nanoparticles are

used to prepare conductive printing inks, such as gold nanoparticles, silver nanoparticles,

and copper nanoparticles [89, 90]. Among metal materials, copper is relatively easily oxi-

dized, gold is expensive, and silver has good physical and electronic properties, so all kinds

13

of printed materials based on metallic silver are the main choices for most researchers [91].

Carbon nanotubes (CNTs) have many excellent physical properties, such as good conduc-

tivity, good mechanical strength, selectable semiconductor/metal characteristics, and good

field emission behavior. CNTs are divided into single-walled and multi-walled carbon nan-

otubes (SWCNTs/MWCNTs). These carbon nanotubes have a wide range of applications

in many devices [92]. Carbon nanotubes, after a certain treatment, dissolved in water with

dispersant are used to prepare printed conductive film materials [93, 94]. But often the

first layer of printing does not have a very good conductive network. When the number of

print layers increases, the conductive network gradually becomes better. Graphene has the

same electrical conductivity and uniqueness as the metal conductor in the printed electrons.

The advantages of these excellent properties include high conductivity, chemical stability,

and flexibility, among others [95]. Seekaew et al. [96] use the combination of graphene and

PEDOT:PSS to print out gas detectors. Graphene has become an emerging materials, which

gets rapid development and application in printing flexible electronics.

Utilizing different printing technologies, the high-molecular polymer flexible substrate

can provide a low-cost, high-speed, large-area production substrate material for the flexible

electronic device.

In order to replace the rigid substrate, the flexible substrate material

needs to have a stable structure size, thermal stability, low coefficient of thermal expansion

(CTE), very good resistance to dissolution, and good water and gas barrier properties.

There are three types of base materials, thin glass, metal foil, and plastic [97, 98]. Thin

glass is flexible, but its brittleness limits its use on flexible electronic devices. The metal foil

can maintain a very high temperature and can provide a deposited platform for inorganic

materials, but its surface roughness and high material cost affect its use on flexible electronic

devices. Plastic substrate materials have high bending properties, transparency, low cost and

other characteristics, so the plastic substrate material is a compromise in physical, chemical,

mechanical and optical properties.

14

1.1.2.3 Inkjet Printing

The main printing process used in this thesis is inkjet printing. Inkjet printing is a common

printing technology. Its principle is the same as our ordinary inkjet printer. It can directly

print some complex 2D or 3D structure drawings from computer software. This printing

technology has already begun being widely used in electronic devices, biological devices

and other sensors [99, 100]. Inkjet printing is the formation of text or images by spraying

conductive ink onto the substrate, which is a non-contact printing. In this way, there is no

restriction on the choice of substrate material, either on the plane or on a non-planar. When

the head is integrated into a multi-degree system for 3D printing.

Inkjet printing can be used to realize true 3D electronic printing. Pulse Electronics has

introduced the FluidANT family of products for antenna printing, including a 3D printer

called FluidWRITER that can be used to print any circuit structure on any 3D object,

including curved flexible surfaces. Ink jet printing eliminates the need for masks, allowing

direct use of CAD/CAM data for processing, greatly enhancing the flexibility of manufactur-

ing. Inkjet printing has been widely recognized in the printing of flexible electronic devices

[101, 102]. Except of a few organic materials, almost all electrical functional materials can-

not be printed directly, so printed materials need to be made with solvents. Inkjet inks or

colloidal solutions. The development of colloidal solutions has provided fundamentals for

achieving droplet ejection in targeted areas. But it still faces significant challenges in order

to guarantee the performance and acceptability of printed circuits. It is due to the evapo-

ration speed of the solvent and the directivity of the active particles that will directly affect

the quality of the circuit. At the same time, the ink jet printing technology is limited in

resolution (2050 m or more) due to the behavioral dynamics of the droplet flying and the

way to the target substrate. Therefore, when the technology is used for printing flexible

electronic devices, the physical properties of the ink, the driving method of the printer, the

selection of the substrate material, and the control of the printing process are all important

parameters. The ink viscosity, concentration, and solvent system will directly affect the

15

quality and stability of printing. How to ensure continuous ejection of ink droplets without

clogging the nozzle. The traditional low-speed inkjet printing process leads to low produc-

tivity, which makes the printing electronics industry production technology become very

challenging. In addition, the limited number and possible blocking problems of the nozzle

make inkjet printing complicated. With the application of inkjet technology in flexible elec-

tronic printing, these problems have been solved slowly. Traditional inkjet printing methods

mainly use push processing methods. Through two common methods, one is piezoelectric,

the other is hot bubble to eject ink. There are two more common ink-jet methods used to

generate ink droplets, one is a continuous jet (CIJ). This method can generate continuous

jets of uniform spacing and size through periodic disturbances. The other one is Drop On

Demand (DOD), which can generate inkjet droplets as needed. No matter using which mode

of printing, important physical characteristics are the surface tension and viscosity of the

inkjet ink, as well as the frequency and amplitude of the modulation. Because these pa-

rameters directly affect the print quality and accuracy. In general, the CIJ system requires

lower viscosity than the DOD system. At the same ink jet speed, a larger number of liquid

pumps and circulatory systems are required. The DOD method is widely used due to its

high precision, controllability, and high material utilization.

1.2 Contributions

In this thesis, we have developed a direct printing process for additively pattern Ag-

NWs with length up to ˜ 40 m on various substrates. Well-defined and uniform AgNW

features could be obtained by optimizing the printing conditions including nozzle size, ink

formulation, surface energy, substrate temperature, and printing speed. Systematic char-

acterizations were performed to investigate the electrical and electromechanical properties

of the printed features with different nanowire lengths. By printing the AgNWs on a bi-

axially pre-stretched PDMS substrate, we have realized a stretchable conductor that could

maintain stable conductance under an areal strain of up to 156% (256% of its original area).

16

Additionally, using the printed parallel AgNWs as electrodes, I have demonstrated an elec-

troluminescent display on mechanically compliant substrates, implying the great potential

of this unique additive patterning method in wearable electronics applications.

In chapter 3, I fabricated a soft, low-cost strain gauge through directly printing AgNP

ink on a PUA substrate, Which can detect pressure changes with high sensitivity. The

performance of this sensor is good and I can use it to sense very small pressure. Its resistance

increases 100% when a 5.5 kPa pressure is applied on it. I tried to apply it on heartbeat

detection and successfully get human heart rate. Also I used it in wind sensing because wind

can cause little deformation on the sensor then change its resistance.

1.3 Thesis Organization

This thesis is outlined as follows: In chapter 1, the concept and characteristics of flexible

electronics are introduced, and several methods for achieving flexibility of electronic devices

are analyzed and compared. The development status of flexible electronic technology is

reviewed. I also discussed the research progress and existing deficiencies of the flexible elec-

tronic technology. The development prospects of flexible electronic technology are analyzed

and predicted. In the second half of Background part, I reviewed the application and re-

search progress of digital printing technology based on flexible substrates, including printed

materials on flexible substrates, flexible substrate materials, digital printing technologies,

and digital printing applications on flexible electronic devices. Explained the limitations of

traditional technologies and the advantages of digital printing flexible electronics. I analyzed

the key technologies and applications of various aspects of printing flexible electronics. In-

troduced the inkjet printing which is mainly used in completing this thesis. Chapter 2 shows

a direct printing method additively pattern AgNWs with length of up to ˜ 40 m on various

substrates uniformly. I discussed the test results and introduced possible its application in

making sensors and display. In chapter 3, I developed a soft pressure sensor through printing

AgNPs on an elastic substrate, which is very sensitive to the positive pressure applied on it.

17

I did a series test and discussed the results. Its expected applications on heartbeat detection

and wind speed sensing are also discussed.

18

CHAPTER 2

DIRECT PRINTING FOR ADDITIVE PATTERNING OF SILVER

NANOWIRES FOR DISPLAY APPLICATION

A unique direct printing method has been developed to additively pattern silver nanowires

(AgNWs) with length of up to 40 m. Uniform and well-defined AgNW features have been

printed on various substrates by optimizing a series of parameters including ink composi-

tion, printing speed, nozzle size, substrate temperature and hydrophobicity of the substrate

surface. The capability of directly printing such long AgNWs is essential for stretchable elec-

tronics applications where mechanical compliance is required, as manifested by a systematic

study comparing the electrical and electromechanical performance of printed AgNW features

with different nanowire lengths. Such printed AgNWs have been used to demonstrate bi-

axially stretchable conductors and stretchable electroluminescent displays, indicating their

great potential for applications in low-cost wearable electronics. This strategy is adaptable

to other material platforms like semiconducting nanowires, which may offer a cost-effective

entry to various nanowire-based mechanically compliant sensory and optoelectronic systems.

2.1 Introduction

Materials with high electrical conductivity and excellent mechanical compliance are cru-

cial to realize stretchable electronics [103]. Researchers have explored a wide range of materi-

als, among which are carbon nanotubes [104], graphene [105], metal nanowires [106], conduc-

tive polymers [107] and metal nanoparticles dispersed in elastomers [108]. Silver nanowires

(AgNWs), in particular, are considered as one of the most promising candidates owing to

the combination of superior conductivity, high transparency, relatively low cost, and excel-

lent stretchability [109]. In recent years, AgNWs have been extensively investigated as not

only stretchable conductors but also electrodes of various flexible and stretchable functional

devices like light emitting diodes [110],solar cells [111], and electronic skins [112]. Despite

19

the significant progress in the field, patterning AgNWs by additive manufacturing methods

such as ink-jet printing that can significantly reduce the fabrication cost and improve the de-

sign flexibility [113] has rarely been investigated and proven extremely challenging. In most

published studies, the AgNW networks were deposited by drop casting or rod coating that

does not possess patterning capability unless combined with specially engineered hydropho-

bic/hydrophilic micropatterns to manipulate wetting of the AgNW dispersion on substrate

surfaces [114]. There are several reports of patterning AgNW networks by stencil printing

or spray coating through a shadow mask, albeit with patterning resolution limited to mil-

limeter scale and inevitable material waste. Indeed, the use of masks greatly compromises

the adaptability and flexibility of these methods.

The reasons that the direct printing of AgNWs can be extremely challenging are mul-

tifold [115]. Firstly, the AgNWs used as stretchable conductors are usually at least tens

of micrometers long, which significantly exceeds the empirically determined printable size

of a/50 (a is the size of the printing nozzle) for regular ink-jet printers. Hence the nozzle

tends to be clogged very easily. An additional issue is the lack of effective control over the

ink bleeding and solvent evaporation because of the low volume content of AgNWs in the

ink. As a result, the printed AgNW features often possess rather rough edges and poor

uniformity. Recently, researchers have tried reducing the nanowire length by ultrasonication

scissoring [116] or tuning the ink rheology by adding polymers to facilitate direct printing

[117]. Although very sharp and uniform features can be obtained, the conductivity and

stretchability are greatly compromised because of the dramatically reduced nanowire length

or the presence of polymer additives. Herein, we report a method for directly printing Ag-

NWs with length up to

40 m on a variety of substrates including silicon wafer, polyimide,

glass, polydimethylsiloxane (PDMS) and 3M VHB tape. By optimizing a series of printing

parameters, we were able to obtain very well-defined and uniform AgNW features that can

be used as stretchable interconnections as well as electrodes for capacitive pressure sensor

arrays and stretchable electroluminescent displays. This unique additive patterning method

20

may find applications in a wide range of nanowire-based functional stretchable electronic

and optoelectronic systems.

2.2 Design and Fabrication Method

Materials: Water based silver nanowire (AgNW) dispersion was purchased from ACS

Materials, LLC. PDMS (Sylgard 184) and Ecoflex (0010) were purchased from Dow Corning

and Smooth-on, respectively. Ethylene glycol and Triton X-100 were purchased from Alfa

Aesar and Sigma Aldrich, respectively. Liquid metal (Galinstan) and PELCO colloidal silver

paste were purchased from Rotometals and Ted Pella, Inc., respectively. Orange (D611),

green (D512) and blue-green (D502) phosphors were purchased from Shanghai KPT Co.,

Ltd. Polyimide (PI-2525) was purchased from HD Microsystems. Carbon nanotube (CNT)

paste (VC101) was purchased from SouthWest NanoTechnologies.

Printing AgNWs: The water-based AgNW dispersion was first centrifuged to remove Ag

particles and nanowire aggregates. 20 vol.% ethylene glycol was then added to the dispersion

to tune the ink rheology and to avoid non-uniform features caused by severe coffee-ring

effect of pure water. The ink was subsequently printed on pristine surface of Si wafers,

glass slides or polyimide films using a Sonoplot Microplotter at a substrate temperature

of

60oC. Specifically, the glass micropipette was gradually brought into contact with the

substrate surface, which induced a meniscus to appear and to bridge the micropipette and the

substrate. The micropipette was then moved back-and-forth along a pre-designed trajectory

in a controlled manner, dragging the meniscus around and leaving some AgNWs on the

surface after each pass. Generally, sufficiently conductive features can be obtained by

40

printing runs. For printing on PDMS, the surface of PDMS was treated with O2 plasma

(30W, 15 sec), and 0.01 vol.% Triton X-100 was added to the ink to tune its surface energy.

Fabricating EL displays: PDMS and phosphors were mixed thoroughly with a weight

ratio of 2:1 and then casted to form thin films of

130 m thick. AgNW lines were then

printed onto both sides of the composite film to fabricate the display array. The EL display

21

array adopts a similar structure with the pressure sensor array. For the MSU-shaped display,

AgNWs were printed with the desired pattern on one side of the PDMS/phosphor composite

film, and CNT paste was painted onto the other side as the back electrode. For the EL

device used for luminance measurements and stretching tests, continuous AgNW networks

were blade-coated on both sides of a composite film which was then sliced into narrow strips.

Coppers wires were used as the measurement leads and all devices were fully encapsulated

in PDMS before testing.

2.3 Test

2.3.1 Measurement Setup

Electrical and stretchability measurements: All electrical characteristics including the I-V

curve, the resistance was measured using an Agilent B1500A Semiconductor Parameter Ana-

lyzer. Scanning electron micrographs and optical micrographs were captured with a Hitachi

S-4700II field emission scanning electron microscope (FESEM) and an Olympus BX51 op-

tical microscope, respectively. The stretching tests were conducted automatically using a

syringe pump (Legato 110). For EL display characterization, AC voltage was generated

from a function generator (Agilent 33220A) and then amplified by a voltage amplifier (Trek

2210). All measurements were carried out under ambient conditions.

2.3.2 Experimental Results and Analysis

The printing process is illustrated in Figure 2.1. Briefly, AgNW ink is loaded into a glass

micropipette by capillary force. The micropipette is then brought into contact with the

substrate surface. A liquid meniscus is subsequently formed if the AgNW ink wets the

substrate surface. Finally, the micropipette is moved along a predefined path back-and-

forth in a controlled manner and drags the meniscus around. AgNWs are consequently

deposited on the trajectory of the meniscus. Unlike conventional ink-jet printing, this direct

writing process does not involve delivering discrete liquid droplets onto the substrate and

22

thus has no problem of uncontrollable solvent evaporation or ink bleeding.

In fact, the

solvent dries almost immediately after the meniscus passes. Additionally, nozzle clogging can

be effectively alleviated by using micropipettes with bigger openings and expelling AgNW

aggregates regularly. It is nontrivial, however, to obtain uniform and sharp printed features.

A number of parameters including AgNW content, solvent composition and surface tension,

nozzle size, micropipette moving speed, substrate temperature as well as the hydrophobicity

of substrate surface, all need to be carefully optimized. By adjusting the above parameters,

we were able to achieve optimal conditions for successfully printing uniform AgNW patterns

onto various substrates, as shown in Figure 2.2a and Figure 2.2b. Figure 2.3 presents the

dark field optical micrograph and SEM image of the dense AgNW network printed on Si

wafer, revealing very good uniformity and well-defined sharp edges.

Figure 2.1 schematic diagram illustrating the printing process.[118]

23

Figure 2.2 (a) MSU shaped AgNW patterns printed on PDMS. Scale bar: 1 cm. (b) Straight
line patterns of AgNWs printed on silicon wafer, glass slide, polyimide thin film and VHB
tape. Scale bars: 2 cm.[118]

24

Figure 2.3 Optical micrograph (top) and SEM image (bottom) of the AgNW network printed
on Si wafer. Scale bars: 50 m and 1 m, respectively.[118]

We have systematically studied printing of AgNW inks with different average nanowire

lengths, namely 4.4 m, 15.6 m and 38.5 m. Figure 2.4a and Figure 2.4b present their SEM im-

ages and nanowire length distribution, respectively. All three kinds of AgNWs can be printed

with very well-defined and uniform features. It is impressive that the AgNWs with length

significantly exceeding the rule-of-thumb printable size (¡10 m for typical micropipettes used

here) can still be printed very easily, which is likely due to the flow-induced alignment as the

nanowires pass through the nozzle. This is manifested by the well-aligned AgNWs on the

inner wall of the micropipette. The electrical performance of the printed AgNW patterns, as

shown below, greatly depends on the nanowire length. Figure 2.4c shows the resistance ver-

25

sus number of printing passes for all three kinds of AgNWs measured from printed features of

5000 m long and 500 m wide. Regardless of nanowire lengths, the feature resistance initially

decreases drastically by more than an order in magnitude and starts to reduce with a much

lower rate after a certain number of printing passes, implying the percolating conduction of

the AgNW networks. As expected, longer nanowires require fewer printing passes to form a

conductive network and have lower electrical resistance compared with patterns with shorter

nanowires for a given number of passes. Figure 2.4d presents the resistance as a function of

printed feature length (feature width is

500 m). All three kinds of AgNWs show excellent

linearity, indicating the AgNWs are uniformly distributed throughout the entire pattern.

Figure 2.4 Electrical and electromechanical characterization of the printed AgNW features
with different nanowire lengths.
(a) SEM images of the AgNW network with average
nanowire length of ˜ 4.4 m, ˜ 15.6 m and ˜ 38.5 m (from top to bottom). Scale bar: 5
m. (b) Nanowire length distribution of the three different AgNW inks. (c) Feature resis-
tance as functions of number of printing runs for AgNWs with different nanowire lengths.
Inset, same data presented in log scale.
(d) Resistance as a function of printed feature
length for all three kinds of AgNW inks. (e) Relative change in resistance as a function of
tensile strain for features with average nanowire lengths of ˜ 38.5 m and ˜ 4.4 m. (f) Cyclic
stretching test results for the feature with an average nanowire length of ˜ 38.5 m. Inset,
measurement setup. Scale bar: 1 cm.[118]

26

As mentioned above, AgNW is a promising candidate for stretchable conductors and we

have found that the stretchability greatly depends on the nanowire length. Fig. 2.4e shows

the stretchability of patterns printed using AgNWs with different lengths. When stretched

to 20% tensile strain, the resistance of printed features with 4.4-m-long AgNWs increases

drastically to nearly 10 times of its pristine value. In contrast, the resistance of features with

38.5-m-long AgNWs changes by only 90%. This difference can be interpreted by the fact

that longer nanowires tend to have greater curvature and better capability to retain good

inter-wire contacts upon stretching. The reliability of printed patterns with 38.5-m-long

AgNWs upon cyclic stretching tests is shown in Figure 2.4f. The patterns could maintain

excellent conduction at both relaxed and stretched states throughout the course of up to

2,000 stretching cycles with a maximum tensile strain of 20%.

Figure 2.5 Bi-axially stretchable conductors based on the printed AgNWs.
(a) Feature
resistance as functions of areal tensile strain (top) and stretching cycles (bottom) for a AgNW
feature printed on a bi-axially pre-stretched PDMS substrate. Insets: the sample at relaxed
state (left) and being stretched to an areal strain of 125% (right). (b) The current-voltage (I-
V) curves of the LED-driving circuit using the printed bi-axially stretchable AgNW conductor
as interconnect with the areal strain varying from 0% to 156%. Insets: photographs of the
circuit while the interconnect is under 0 (left) and 125% (right) areal tensile strain. Scale
bar: 2 cm.[118]

In order to further improve the stretchability of the printed AgNW features, we pre-

stretched the PDMS substrate to 30% strain along both parallel and perpendicular directions

27

before printing. After printing the AgNWs onto the pre-stretched PDMS substrate, the

pre-applied strain was released and bi-axial stretching tests were conducted on the AgNW

patterns. As shown in the top panel of Figure 2.5a, the stretchability was greatly improved

by pre-stretching the substrate during the printing process. The feature resistance only

increases very slightly from 145 to 200 when stretched to 125% areal strain (50% strain

along both horizontal and vertical directions). We have also investigated the durability of the

AgNW patterns printed on the pre-stretched PDMS substrates. As illustrated in the bottom

panel of Figure 2.5a, the feature resistance at 0 and 125% areal strains only shows very slight

change throughout the 500 stretching cycles. The application of the printed AgNW pattern

as stretchable interconnection was subsequently demonstrated by incorporating it into the

driving circuit of a light-emitting diode (LED). As shown in Figure 2.5b, the I-V curves

of the LED essentially overlap with each other during the process of stretching the AgNW

interconnect to an areal strain of 125%. Hence, the LED shows no noticeable variation in

luminance, as presented in the two insets of Figure 2.5b. When the areal strain is further

increased to 156%, the I-V curve shows slight downshift, but is still sufficient to drive the

LED. In principle, the stretchability could potentially be improved even further by using

greater pre-strains during printing. To the best of our knowledge, this is the first report of

printed, bi-axially stretchable conductors based on AgNWs.

2.4 Application

The capability of direct printing offers a viable approach of additively pattern AgNWs

for display applications. By sandwiching a composite composed of PDMS and electrolu-

minescent (EL) phosphors between two AgNWs electrodes, I have demonstrated a printed

stretchable EL display. It is well known that EL phosphors, like ZnS doped with transition

metals, can emit light under AC voltage stimulation, which was speculated to be caused

by electric field induced tunnelling through embedded junctions and subsequent radiative

recombination [119]. The light intensity of the EL display increases with the magnitude and

28

frequency of the AC voltage. By changing the type and concentration of dopants, poly-

chrome light emission can be realized. While EL phosphors have been widely used in the

form of powder EL cells for back lighting of liquid crystal displays, they are very brittle

and thus not suitable for flexible or stretchable display applications. An extensively used

strategy to make stretchable materials from brittle inorganic micro-/nano- particles is to

disperse them in a polymer matrix like silicone such that the desired electrical properties

of the inorganic particles and the elasticity of silicone are preserved simultaneously [120].

The same strategy has proven to work well for phosphors [121]. In recent years, a couple

of groups have reported stretchable EL display/lighting devices based on composites com-

posed of silicone and phosphors. Here, I demonstrated silicone/phosphor composite-based

stretchable EL displays in various configurations using our printed AgNWs as electrodes. As

shown in Figure 2.6a, the EL device adopts a capacitance structure, the layer in between is a

PDMS/phosphor composite. The light emission performance of the EL devices was studied

systematically. Figure 2.6b presents the light emission spectra of a green EL device under

various stimulation conditions. The peaks of the EL spectra center at

510 nm and show

no noticeable changes when the excitation frequency changes from 0.5 kHz to 1kHz and

magnitude changes from 1 kV to 2 kV. The luminance is plotted as functions of excitation

frequency and magnitude in Figure 2.6c and d, respectively. As expected, the luminance

increases dramatically as the excitation frequency is increased from 10 Hz to 1 kHz or as

the voltage is increased from 300 V to 2 kV. The relation between luminance and excitation

voltage can be fit very well using the established equation. The devices were then subjected

to stretching tests and the results are shown in Figure 2.6e and f.

29

Figure 2.6 Stretchable electroluminescent (EL) displays using the printed AgNWs as elec-
trodes. (a) Schematic of the device structure. (b) EL spectra of the device made of green
phosphors under different stimulation conditions. (c, d) EL intensity of the device made of
green phosphors as functions of frequency (c, measured at 1 kV) and magnitude (d, measured
at 500 Hz) of the AC voltage. (e) Normalized EL intensity plotted as a function of tensile
strain when measured under AC voltage with different magnitudes and a frequency of 1 kHz.
Inset: photographs of a device made of green phosphors under 0 and 20% tensile strains.
Scale bars: 5 mm. (f) Normalized EL intensity versus stretching cycles measured under AC
voltage with a magnitude of 1.5 kV and a frequency of 1 kHz. (g) Photographs of a 66 EL
display array made of green phosphors (left), an MSU display pattern using orange, green
and blue phosphors for M, S and U, respectively (middle), and the MSU-shaped display
under 0 and 20% tensile strains (right). Scale bars: 1 cm.[118]

30

The devices could withstand tensile strains up to 20% for more than 1000 cycles, albeit

with noticeable degradation in luminance at 20% strain. The degradation in luminance could

be attributed to the gradual loss in conductance of the AgNW electrodes or the electrical

contacts between the electrodes and measurement leads as they get stretched. The stability

in light emission performance under tensile strain can be further improved by using longer

nanowires or using more robust conductive glue as the electrical contacts during the electrical

measurements. Figure 2.6g presents EL display devices in different configurations that were

enabled by the direct printing of AgNWs. The demonstrations include a 66 green display

array (left) and an MSU shaped polychrome (orange, green and blue) display with different

types of phosphors patterned in one composite film. The polychrome display worked very

well under a tensile strain of 20%, albeit with some degradation in light intensity.

2.5 Conclusion

In summary, we have developed a direct printing process for additively pattern Ag-

NWs with length up to

40 m on various substrates. Well-defined and uniform AgNW

features could be obtained by optimizing the printing conditions including nozzle size, ink

formulation, surface energy, substrate temperature, and printing speed. Systematic char-

acterizations were performed to investigate the electrical and electromechanical properties

of the printed features with different nanowire lengths. By printing the AgNWs on a bi-

axially pre-stretched PDMS substrate, we have realized a stretchable conductor that could

maintain stable conductance under an areal strain of up to 156% (256% of its original area).

Additionally, using the printed parallel AgNWs as electrodes, we have demonstrated an elec-

troluminescent display on mechanically compliant substrates, implying the great potential

of this unique additive patterning method in wearable electronics applications. Further-

more, the same strategy can be adapted to other material platforms like semiconducting

nanowires, which may offer a cost-effective entry to various nanowire-based mechanically

compliant sensory and optoelectronic systems.

31

CHAPTER 3

INKJET-PRINTED STRAIN SENSOR FOR PRESSURE SENSING

In this chapter, I studied an inkjet-printed flexible pressure sensor.

I printed the silver

nanoparticle ink directly on an elastic substrate. When it is subjected to longitudinal pres-

sure, cracks will occur and the insufficient stretchability of the silver nanoparticles will in-

fluence the conductivity of wires. Thus, the change in pressure is sensed by measuring the

change in resistance. The flexible substrate ensures that the device that receives pressure

can be restored to its original shape after the pressure has been withdrawn. Since cracks are

generated during longitudinal compression, the resistance after recovery almost has no shifts,

which ensures the repeatability and stability of the sensor. From the test results, it can be

seen that this type of pressure sensor has a very high sensitivity in the low-pressure section,

and the sensitivity can reach around 20%/kPa. Finally, I also explored possible applications

of this device in heart rate detection and wind detection.

3.1

Introduction

Among various types of sensors, pressure sensors possess the advantages of small size,

light weight, high sensitivity, stability, reliability, low cost, and ease of integration. They can

be widely used in measurement and control of pressure, height, acceleration, liquid flow, flow

rate and liquid level. While soft flexible pressure sensors, as one of the vital component units

in the next generation of wearable electronics for electronic skin [122, 123, 124, 125, 126,

127, 128], touch-on flexible displays [129, 130], soft robotics[131, 132] and energy harvesting

[133, ?, 134], have drawn increasing attention. Existing soft pressure sensors are mainly based

on force-induced changes in capacitance [123, 129, 135, 136], piezoelectricity [137, 138, 139],

triboelectricity [128, 140] and resistivity [141, 142].

Recently, various nanomaterials, including nanowires [125, 143, 144], carbon nanotubes

[129, 135, 141], polymer nanofibers [137, 139, 142, 145], metal nanoparticles [146, 147, 148]

32

and graphene have been used for the design of novel flexible pressure and strain sensors.

The majority of these nanomaterials-based pressure sensors are based on capacitance or

piezoelectricity except for a few recent reports [141, 142], where resistivity was used for

designing strain gauge sensors. The advantage of resistive pressure sensors lies at simplicity

in device fabrication as well as relatively low energy consumption in operation. Despite the

promise, to date, only limited examples have reported success on resistivity-based flexible

pressure sensors [141, 142, 149]. Metal materials that are typically used are gold, gold

nanoparticles (AuNPs) and gold nanowires (AuNWs). An AuNWs based pressure sensor

with low response time (less than 0.05s) has been reported [149].

AgNPs [150] has excellent electrical conductivity, as an attractive novel electronic mate-

rial for flexible device fabrication. Compared with gold materials, AgNPs costs much less on

material and fabrication process. The AuNWs and AuNPs based pressure sensors mentioned

above are all directly depositing or coating Au NWs or AuNPs solutions on patterned sub-

strate, most materials are wasted during this process. Herein, I developed an inkjet-printed

pressure sensor fabricated by AgNPs. The device shows high sensitivity on pressure sensing

and repeatability, following by two potential applications- to detect human heart rate or

wind speed.

33

Figure 3.1 Capacitive soft pressure sensor array based on PEDOT:PSS (scale bar, 20 mm).
(a) Device structure. (b) Capacitance change in response to pressure.[151]

Some capacitive soft pressure sensors have been studied. Figure 3.1a is a sandwich struc-

ture which is mostly followed [151]. A soft substrate works as dielectric layer. When pressure

is applied, it reduces the thickness of the dielectric layer thereby increasing the capacitance.

But the sensitive region is limited and the sensitivity is basically around 1%/kPa-5%/kPa.

Figure 3.1b shows the sensitivity of the capacitive pressure sensor developed by H. Yang

[151]. The pressure sensor studied in this thesis is resistive sensing. Pressure will partly

break the connection between nanoparticles, cause cracks and result in a larger resistance.

The AgNPs path is like a 2D structure thus the breaks along vertical direction will strongly

influence the device resistance. So a higher sensitivity is obtained.

3.2 Design and Fabrication Method

Substrate preparation: The PUA substrate was made by the mixture of Siliconized ure-

thane acrylate oligomer (CN990, Sartomer), ethoxylated bisphenol A dimethacrylate (SR540,

Sartomer) and 2,2-Dimethoxy-2- phenylacetophenone (photoinitiator, Sigma-Aldrich) with

the ratio of 10:1:0.01, followed by 20-mins UV light curing. The CN990 was chosen in order

to improve the stretchability of the substrate, the SR540 was chosen to adjust the stiness of

34

the substrate and the photoinitiator serves as catalyzer.

Device fabrication: The silver nanoparticles ink (PG-007AA from Paru Corporation,

South Korea) was printed onto PUA substrate (pre-treated by the oxygen plasma, 60W for

10s) using a GIX Microplotter (Sonoplot Inc.) with the layout pre-designed by the software,

and then the whole device was put on top of a hotplate to be annealed for 1 hour under 120

C. Silver Conductive Epoxy Adhesive A and B (MG Chemicals Ltd) and copper wire was put

on the two ends of the devices serving as the connection for measurement, and then another

layer of PUA solution was poured and cured by the UV light to form the encapsulation.

3.3 Experimental Results and Analysis

The resistance was measured using an Agilent B1500A Semiconductor Parameter Ana-

lyzer. Optical micrographs were captured with an Olympus BX51 optical microscope. The

pressure was automatically applied using a converted syringe pump (Legato 110). All mea-

surements were carried out under ambient conditions.

Figure 3.2 Schematic diagram of printing process and device pattern.

Figure 3.2 presents the schematic diagram of the sensor. The silver nanoparticle ink (PG-

35

007AA from Paru Corporation, South Korea), loaded into the ink dispenser, was printed

onto a polyurethane acrylate (PUA) substrate using a GIX Microplotter (Sonoplot Inc.). To

make the device stretchable, the serpentine structure was pre-designed using the SonoDraw

(Sonoplot Inc.). And then the Ag pattern together with the PUA substrate was placed on a

hotplate under 120 C for 1 hour to improve their conductivity. For the measurement, I have

also made lead wires which are connected to the electrodes of sensors. Silver Conductive

Epoxy Adhesive A and B (MG Chemicals Ltd) were combined with the ratio of 1:1. Put

that combination and copper wires on the electrodes area of the sensor, waiting 30 min for

drying. Then I poured another layer of PUA solution and cured it by the UV light to form

the encapsulation. A finished device is presented on Figure 3.3.

Figure 3.3 A sample of printed pressure sensor.

36

Figure 3.4 Pressure loading platform for measurement.

Figure 3.5 Force-to-voltage response curve of the FSR.[151]

37

I used a syringe pump (Legato 110, KD Scientific, Inc.) to apply the loading, through

3D-printed PLA (Polylactic acid) parts.

In particular, the loading assembly consisted of

three parts, a base, a loading rod, and a top plate (Figure 3.4). The loading rod was

mounted on the top plate, while the base and the top plate were respectively mounted on

the fixed block and pusher block of the syringe pump. To apply vertical loading, the syringe

pump was placed on its side. I controlled the loading pressure by setting the displacement

of the tapered loading rod. Besides, the syringe pump provided the Withdraw and Infuse

mode, which enabled cyclic loading. To measure the actual pressure between the loading

rod and the soft capacitive sensor, a force sensing resistor (FSR) sensor (FSR 402, Interlink

Electronics, Inc.) was placed on the loading platform. I characterized the FSR device first

and obtained the force-to-voltage response curve (Figure 3.5). For simplified calculation of

intensity of pressure, I needed to define a constant contact area. So the FSR was attached on

a glass slide (96.5 mm2), which was then placed between the loading rod and the capacitive

sensor. The active area of the FSR sensor was 14.68 mm, while the loading area of the loading

rod was 10 mm. When the loading rod was pressed against the soft capacitive sensor, the

loading surface of the loading rod was fully covered by the FSRs active area, and the force

measurement from the FSR was used to calculate the pressure applied to the sensor.

38

Figure 3.6 Pressure sensor based on printed AgNPs. (a) Relative change in resistance and
pressure as a function of time. (b) Relative change in resistance as a function of pressure.

39

At first, I measured the pressure and the relative change of AgNPs pattern resistance,

and the real-time record is shown in Figure 3.6a. Originally the data from FSR were voltage

data, and I made a real-time conversion from voltage to intensity of pressure through Matlab.

I setup the pressure loading period as 8s. From Figure 3.6a we can see that the resistance of

AgNPs was changing following the pressure with an acceptable response time. The relative

change in resistance can reach 19%/kPa. Also we can find difference on negative edges of the

two curves, which can be found in Figure 3.6b as well. When pressure was withdrawn, the

changes of resistance cannot follow the path through when pressure was applied. This is due

to the recovery time of PUA substrate. The depression on the negative edge of blue curve

in Figure 3.6a demonstrates that resistance cannot go back to its original value because the

PUA substrate still needs about 4 more seconds to recover from deformation while at that

time the pressure has already evacuated.

Figure 3.7 (a) Optical image of the device before test (b) Optical image of the device after
test.

Since pressure was applied to the device from the longitudinal direction, it does not pull

it laterally. When the pressure was removed, cracks created when the pressure is applied

can be almost completely repaired as long as the substrate is given enough time. Figure 3.7

shows a sample before and after a round of test. The pattern shows almost no visible cracks

under a 50x microscope. The stability and repeatability of this sensor can also be derived

40

from the fact that the resistance eventually returns to R0.

Figure 3.8 The relative change in AgNPs patterns resistance under different pressure loading
frequency.

I also measured the relative change in AgNPs patterns resistance under different pressure

loading frequency, as shown in Figure 3.8. Due to the defect of syringe pump, the loading

rod was almost impossible go back to a constant height after all the test rounds. So the

max pressure loaded in each test round varies from one to another. From Figure 3.8, we can

find that the device can well adapt to pressure changes at different frequencies while high

sensitivity is ensured.

3.4 Application

Flexible pressure sensors play an important role in the fields of electronic skin, artificial

limbs, and medical monitoring and diagnosis. I tried to use the pressure sensor developed in

41

this thesis to detect heart rate and wind speed respectively.

3.4.1 Heart Rate Detection

Today’s contactless heart rate sensors are mostly photoelectric sensors that use infrared

or visible light for feedback. Problem is that they still have to be attached to slightly

cumbersome carriers such as watches and wristbands. There are also applications that

use a cell phone flash for photodetection to calculate the heart rate. However, real-time

monitoring cannot be conducted in this way. Contact heart rate detectors mostly adhere

to the principle of piezo-electricity, and rigid devices that are in close contact with human

skin are accompanied by a user-unfriendly experience. The flexible pressure sensor in this

chapter can effectuate real-time heart rate detection, and its small, soft features can better

fit the appeal of comfort, having good application prospects.

Figure 3.9 Heart rate sensing test.

Figure 3.9 shows the measurement setup for heart rate detection. The pressure sensor is

simply fastened to the wrist by a cleanroom tape. In the future, other viscous materials can

be used instead of tape to attach the sensor to the wrist. Pulse-induced slight deformation

of the skin can be detected by the pressure sensor and fed back by changes in resistance. I

42

use the Agilent B1500A Semiconductor Parameter Analyzer to collect data on the resistance

of the sensor as a function time.

Figure 3.10 Heart rate sensing: resistance of sensor as a function of time.

Test result is presented in Figure 3.10, since the balance point of this curve kept shifting,

it is hard to define where the original resistance value is. Motions of hand may cause the

volatility of mean pressure. Despite the fact that original resistance kept changing, we can

still get the information of heart rate as we wish. There peaks were indicated in 2.5s, so the

heart rate was around 72 bpm in that test.

43

3.4.2 Wind Sensing

Figure 3.11 Windspeed sensing test. (a) Measurement setup. (b) Fixture for sample in wind
tunnel. (c) Relative change in sensor resistance as a function of windspeed.

Due to high sensitivity of this pressure sensor, I came up with the idea of applying it to wind

speed sensing. In Figure 3.11a, the sample was held in a wind tunnel by a steel frame and

placed perpendicular to the wind direction. When wind blows from front of the sensor it

bends the sensor, which triggers a deformation that affects the resistance. Figure 3.11c shows

the result of wind sensing test. With the relative change obviously in resistance increasing

by the wind speed, we are confident to see the successful application of this sensor in wind

sensing.

44

3.5 Conclusion

In summary, I have successfully developed an inkjet-printed stretch sensor for pressure

sensing by printing AgNPs ink on an elastic PUA substrate. The sensor has very high

sensitivity (up to 20%/kPa) and good stability. Its flexibility allows it to fit some irregular

surface like human skin, thus making happen a variety of applications.

In this study I

developed it for pressure sensing, heart rate detection and wind sensing. It shows consistently

good performance during every test. It can be expected that this sensor will be beneficial to

the research of motion detection, medical monitoring and wearable electronics.

45

CHAPTER 4

SUMMARY AND FUTURE WORK

4.1 Summary

Looking back at this thesis, we have developed a direct printing process for additively

pattern AgNWs with length up to

40 m on various substrates. Well-defined and uniform

AgNWs features could be obtained by optimizing the printing conditions including nozzle

size, ink formulation, surface energy, substrate temperature, and printing speed. Using

printing technology, we have brought to fruition a stretchable conductor that could maintain

stable conductance under an areal strain of up to 156% (256% of its original area) and an

electroluminescent display on mechanically compliant substrates. By printing AgNPs on

PUA substrate, I have demonstrated a stretchable pressure sensor with very high sensitivity

which can be used for heart rate and windspeed sensing.

However, there are still some improvement should be done to the strain sensor. First, as

the test platform for the pressure sensor is remolded from a syringe pump, the movement

speed of the loading rod is not constant (In Figure 3.5a, the pressure curve cannot run

smoothly) and it cannot memorize the original position in each test round (the max pressure

is not constant in every test). Thus I missed some performance data of the sensor such as

the response time and the linearity in sensitivity, which will be the priority of our subsequent

work.

4.2 Future work

Whether stretchable conductor, flexible display or stretchable pressure sensor, what I

made are all theoretical samples and they are far away from practical application. For

flexible display, the pixels on our sample can only be lit at the same time or turned off at the

same time. I cannot selectively light a certain pixel. In the future, I will add more printed

46

devices such as flexible resistors and flexible transistors on the samples to form circuits,

and then really fulfill its function as a screen. For stretchable pressure sensor, instead of

heart rate detection, our goal at the beginning of this study was to improve the existing

cumbersome procedures of blood pressure measuring and achieve real-time blood pressure

monitoring. The reason why I shifted our focus is because I lack support methods to fit the

skin and the recovery speed of the PUA substrate is not fast enough for us to find information

about blood pressure response from the experimental results. In follow-up studies, I may

need to re-select a substrate material that has a faster recovery speed and applicable for

printing technology, so that I can finally effectuate blood pressure monitoring by improving

design of the device and supporting methods.

47

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