concise encyclopedia of high performance silicones

275

Atul Tiwari and Mark D. Soucek (eds.) Concise Encyclopedia of High Performance Silicones, (275–292) 2014 © Scrivener Publishing LLC

18Silicone-Based Soft Electronics

Shi Cheng1,* and Mikael Östling2

1Ericsson AB, Stockholm, Sweden, and KTH Royal Institute of Technology, Stockholm, Sweden2KTH Royal Institute of Technology, Stockholm, Sweden

AbstractLately, the marriage of elastic silicone rubbers and electronics has given birth to a radically new class of electronic devices and systems,

so-called silicone-based soft electronics. Based on this new concept, electronics can nowadays be built in soft , rubbery, biological for-

mats, for enabling numerous new applications, in which conventional rigid electronics are impossible to use.

Th is chapter briefl y reviews recent research advances in the emerging fi eld of soft electronics. Various strategies for realizing elastic

electronics are addressed, and the focus of this chapter lies in microfl uidic approaches. Recently reported elastomeric soft electronic

devices using microfl uidic techniques, e.g., elastic passive antennas, a soft RF radiation sensor, as well as a reversibly stretchable,

body-worn wireless strain sensor, are presented and analyzed in detail. Lastly, future perspectives and outlook for silicone-based soft

electronics are discussed.

Keywords: Antennas, elastomers, galinstan, integrated circuits (ICs), liquid alloy, microfl uidics, planar inverted cone antenna

(PICA), polydimethylsiloxane (PDMS), radiofrequency (RF), sensors, silicones, stretchable electronics, ultrawideband (UWB),

voltage-controlled oscillators (VCOs)

18.1 Introduction

Silicones, a category of polymers, are widely used in sealants,

adhesives, medical applications, and insulation, and can oft en

be made in “soft ” formats with fl exibility, foldability, and stretch-

ability. On the contrary, electronics represent a totally diff erent

fi eld, where devices are usually rigid, and retain static shapes

once fabricated. Th e overlapping between these two “soft ” and

“stiff ” worlds has not yet been seen until a few years ago.

Th e simple motivation to “soft en” conventional electronic

devices is that we as human beings are in soft , biological formats,

and wish to turn electronics into similar manner as our bodies.

Not only for enhanced user experience, but also for enabling a

broad spectrum of new applications. Reversible deformability,

transparency, and lightweight are desired features for future

electronics. Devices based on this new technology will be fold-

able, twistable, and stretchable into almost arbitrary curvilinear

shapes. Application examples may range from ultrathin, con-

formable health monitoring tapes that seamlessly attach to the

skin, “electronic/second skin,” to elastomeric medical implants

that are truly biocompatible to the tissues.

Exciting research on silicone-based soft electronics that

can withstand severe folding, twisting, and stressing, but

with maintained electronic functions, have been recently

pursued. Whitesides investigated mechanical characteristics

of gold thin fi lms on silicone rubber, polydimethylsiloxane

(PDMS) surfaces for the fi rst time in 1998 [1]. Wrinkled

electrodes on pre-strained elastomeric substrates were

subsequently employed to realize electroactive polymer

actuators [2]. Later, Wagner’s and Suo’s groups at Princeton

studied wrinkled gold electrodes on PDMS substrates [3, 4].

In-depth investigations on micromechanics as well as fur-

ther development of fabrication processes using relaxed and

pre-stretched PDMS substrates were also conducted. Rogers’

group at the University of Illinois, Urbana-Champaign,

has made the most impressive contribution to this emerg-

ing fi eld. Th eir core technology of foldable and stretchable

silicon integrated circuits (ICs) on “wavy” silicon ribbons

encased in elastomers opened up the possibilities of realiz-

ing soft electronic devices with a variety of new integrated

features and functions [5, 6]. A series of appealing devices

based on this concept, including hemispherical electronic

*Corresponding author: [email protected]

276 Concise Encyclopedia of High Performance Silicones

eye camera [7], stretchable solar modules [8], smart gloves/

skins [9], instrumented balloon catheters [10], and epi-

dermal electronic systems [11], and stretchable microscale

inorganic light-emitting diodes [12], were shown soon

aft erwards. An alternative approach utilizing anisotropic

etching of bulk wafers was also introduced by Rogers [13,

14]. Lately, meandered stretchable interconnects embedded

in thin silicone sheets for diff erent frequency ranges were

demonstrated [15–17]. New electrical nanocomposite mate-

rials consisting of sliver nanoparticles or carbon nanotubes/

grapheme exhibited promising electrical characteristics and

mechanical deformability as well [18, 19].

Initial research on silicone-based soft electronics involv-

ing microfl uidic methods was fi rst carried out by Whitesides

[20, 21]. Th is concept was subsequently extended to imple-

ment elastomeric direct current (DC) circuits, by injecting

eutectic gallium and indium alloy (EGaIn) into microfl uidic

channels in thin silicone rubber sheets [22–24]. All the pre-

viously mentioned studies, however, deal with either low

frequency ICs or relatively simple interconnects. Soft elec-

tronics operating at radio frequencies remained an unex-

ploited fi eld until early 2009, at which time Cheng reported

the fi rst stretchable fl uidic antennas for enabling wireless

communication and remote sensing [25, 26]. Similar work

using EGaIn alloy as conductors for the antennas instead

of Galinstan was presented soon aft er [27]. Signifi cantly

improved elasticity of the resulting antennas could be

achieved by introducing a diff erent type of siloxane [28], and

mechanically reconfi gurable antennas could also be realized

[29]. In 2010, Cheng proposed a heterogeneous integra-

tion strategy for the fi rst demonstration of active silicone-

based RF electronics, a fully integrated RF radiation sensor,

cf. Figure 18.1, [30]. A year later, further development on

this concept has been made to implement multilayer con-

fi gurations, and an elastomeric integrated large-area wireless

strain sensor was published, as seen in Figure 18.1 [31].

More details on emerging silicone-based soft eletronics

using microfl uidic approaches, are presented in the following

sections. Fabrication processes, intergration stragetries, as well

as application examples are addressed and discussed. Th e entire

chapter ends with concluding remarks and future perspectives.

18.2 Silicone-Based Passive Soft Electronics

Soft lithography, a well-known fabrication technique in the

microfl uidic society for constructing microstructured chan-

nels, is adapted to build soft electronics [32]. Microfl uidic

channels in soft electronic devices are fi lled with liquid metal

alloys other than chemical solutions or biological fl uids as in

conventional lab-on-chip microfl uidic devices.

18.2.1 Fabrication of Single-Layer Passive Elastomeric Electronics

Figure 18.2 illustrates the manufacturing process steps for

realizing silicone-based single-layer soft electronics. Master

construction, molding/casting, plasma/thermal bonding,

channel fi lling, as well as encapsulation are essential steps.

Using soft lithography techniques, all design patterns

are fi rst transferred to a SU-8 layer on top of a silicon wafer

from either plastic or glass masks. Th e thickness of the

SU-8 layer may vary from a few tenths of microns to sev-

eral hundred microns, depending on the design of electronic

devices. Development and thermal stabilization in order to

enhance the adhesion between SU-8 layer and silicon sub-

strate come as the second step. In the case of 100 μm thick

Figure 18.1 Silicone-based soft electronic devices: a) A reversibly

stretchable, body-worn wireless strain sensor. b) A microfl uidic,

soft RF radiation sensor.

Figure 18.2 Fabrication process of single-layer, silicone-rubber-

based passive soft electronics.

Silicone-Based Soft Electronics 277

SU-8 100 (MicroChem Newton, MA) layer, baking at 150°C

for 30 min is recommended. Subsequently, the mixture of

the PDMS prepolymer and crosslinker is poured onto the

structured SU-8 master, and then cured either at 70°C in an

oven for 30 min or at room temperature for 24 h. Th e fi n-

ished silicone rubber samples cured at the latter condition

oft en feature higher elasticity and fl exibility. A thin blank sil-

icone lid without the need for SU-8 masters can be prepared

in a similar way. Optional bonding techniques are plasma,

thermal, and uncured PDMS bonding. Liquid metal alloy,

e.g., Galinstan (68.5% Ga, 21.5% In, 10% Sn, σ=3.46⋅106

S/m) or EGaIn (75.5% Ga, 24.5% In, σ=3.40⋅106 S/m) can

be employed to fi ll microfl uidic channels inside the bonded

PDMS replica and blank lid. Th e entire fabrication process

ends with encapsulation of injection inlets and ventilation

outlets using uncured PDMS mixture. No special tempera-

ture requirements are needed for liquid metal alloy handling

since they remain at a liquid state at room temperature as

stated in Table 18.1.

Liquid alloys like galinstan exhibit excellent wettability on

almost any surface. Th is poses one of the biggest challenges

in manipulating this category of liquid metals. Solutions like

ethanol or isopropy alcohol can be used to clean up silcone

rubber surfaces contaminated by galinstan. Timely cleaning

is of great importance, as it is very diffi cult to remove con-

taminations aft er a couple of days.

18.2.2 Silicone-Based Elastic, Liquid Metal, Unbalanced Loop Antenna

Making use of the manufacturing method described in the

previous section, a single-layer, liquid metal, stretchable,

unbalanced loop antenna, has been successfully imple-

mented, as depicted in Figure 18.3 [25]. Th e resulting soft

antenna prototype comprises a radiating element with its one

end galvanically connected to a semicircular ground plane.

Five cylindrical reservoirs are periodically aligned along the

upper semiannular tube of the antenna to enable good elec-

trical connectivity while folding, fl exing, or stressing. Th e

presence of these reservoirs increases the electrical length of

the unbalanced loop antenna so that there is a slight decrease

in the fundamental resonance frequency of the antenna. Th e

lower semicircular ground plane is designed in the meshed

confi guration. Dozens of square posts are introduced to

space the top and bottom silicone rubber membranes in this

area. Negligible infl uence on electrical characteristics of the

antenna is seen compared with a solid ground. Th is meshed

region is also reserved for integration of active electronics,

discrete components, power supplies and so on.

Prior to measurements on electrical performance, simple

tests on mechanical properties of the elastic antenna proto-

type were fi rst performed. Superior bendability, fl exibility as

well as stretchability can be observed in Figure 18.4. High

degrees of folding and twisting in the experiments did not

cause any mechanical failures. Extreme levels of strains of

up to 40% did not result in any damage either. Uneven sub-

strate thickness and heterogeneous pattern of the antenna

introduce slight mechanical inhomogeneity, which can be

seen when stressing the antenna along diff erent orienta-

tions, cf. Figure 18.4.b and c. In theory, elasticity as high as

100% is in reach. Nevertheless, openings for accessing feed

cables for characterizations are weak spots that set a limit for

mechanical deformability, reliability and robustness, since

the antenna can be easily torn from these points. Also, rigid

feed cables oft en cause failures as they might pierce the sili-

cone substrates while being attached to the soft antennas.

Electrical properties of the relaxed antenna such as port

impedance, antenna effi ciency, and radiation patterns were

studied in numerical simulations, using a full-wave elec-

tromagnetic simulator, Ansoft High Frequency Structural

Simulator (HFSS). Experimental verifi cation on both relaxed

and fl exed antennas were performed aft erwards. Simulated

Table 18.1 Comparison of physical properties of galinstan and EgaIn.

Galinstan EGaIn

Melting point (°C) −19** 15.5

Boiling point (°C) > 1300 2000

Density (kg/m3) 6440 6280

Electrical conductivity (S/m) 3.46×106 3.4×106

Viscosity (Pa·s) 2.4×10–3 2.0×10–3

Surface tension (N/m) 0.718 0.624

Figure 18.3 Schematic drawing of an elastomeric unbalanced

loop antenna. Dimensions are: R=18.1 mm, WS=400 μm, h=1.0

mm, and hmetal

=100 μm.


and measured refl ection coeffi cients (S11

) are presented in

Figure 18.5. Th e non-strained antenna features good imped-

ance matching around 2.44 GHz, with an input impedance

of approximately 75+15j Ω. Strains applied on the antenna

lead to an increase in the length of the upper radiating loop

and result in a decreased resonance frequency, as seen in

Figure 18.5.b. Stressing the antenna along y-axis decreases its

input resistance due to the increased coupling between the

upper radiation arm and the lower ground plane, whereas

the resistance goes up when strains are applied along x-axis.

Numerical and experimental results on the antenna

radiation patterns at 2.44 GHz are shown in Figure 18.6.

Th e antenna in its relaxed state exhibits similar broad beam

coverage as conventional unbalanced loops, especially in

Figure 18.4 Photographs of the foldable, stretchable, fl uidic unbalanced loop antenna enclosed in a silicone substrate: a) in its relaxed state,

b) w/ 40% vertical strain, and c) horizontal elongation, d) in its folded state, e) w/ severe twisting, and f) biaxial stretching.

(a) (b)

Figure 18.5 Simulated and measured refl ection coeffi cients, S11

, of a silicone-based elastic unbalanced loop antenna. a) Simulated and

measured S11

(in logarithm scale) of the relaxed and fl exed antenna; b) Measured S11

of the fl exed antenna w/ 20% x-axis (solid), 40% x-axis

(dashed) and 20% y-axis (dotted), 40% y-axis (dashdot) strain. Th e S11

at 2.44 GHz is marked by a cross symbol on each curve.


the yz-plane, where nearly perfect omnidirectionality can

be observed in the measurements. Th is is favored in many

portable electronic devices. Th e maximum antenna gain is

about 2.7 dBi, and the measured cross-polarization (Gφ in

the xz-plane and Gθ in the yz-plane) is approximately 15 dB

lower than the corresponding co-polarization. Ripples can

be seen in the radiation patterns of the stressed antenna,

cf. Figure 18.6b, which degrade the omnidirectionality

in the yz-plane to some extent. Th is degradation is more

obvious while strains are applied along the y-axis. Greater

gain variations within the angle of 45°–135° on the mea-

sured xz-plane radiation pattern can be explained by the

increased cable infl uence resulted from the y-axis elonga-

tion of the antenna. Distortions from the feed cable also

cause the small increase of the cross-polarization, as seen

in Figure 18.6.

Low conductive losses and negligible eff ect from the

silicone rubber substrate on antenna electrical perfor-

mance ensure that excellent radiation effi ciency can be

achieved at all states, as shown in Table 18.2. High con-

ductivity of galinstan and large cross sections of microfl u-

idic channels are the main reasons for the low conductive

losses. Despite the signifi cant decrease on resonance fre-

quencies due to stretching, antenna radiation effi ciency

remains greater than 80%. Th is fact implies that galvanic

connections in microstructured elastomeric channels are

not interrupted by stressing. Robust electrical continu-

ity of the liquid metal alloy is attributed to its excellent

wetting property on silicone rubber surfaces. Th ough the

(a) (b)

Figure 18.6 Simulated and measured radiation patterns of the antenna a) w/o and b) w/ strain at 2.44 GHz. Th e corresponding coordinate

system is presented in Figure 18.3.

Table 18.2 Measured radiation effi ciency at 2.44 GHz and resonance frequency of the relaxed and fl exed antenna.

Relative Strain Radiation

Effi ciency

Resonance

Frequency

0% 98% 2.43 GHz

20% x-axis 92% 2.22 GHz

40% x-axis 88% 1.97 GHz

20% y-axis 96% 2.32 GHz

40% y-axis 84% 2.17 GHz

presented silicone-based elastic unbalanced loop antenna

achieves good radiation characteristics, the resonance

frequency detuning introduced by straining moderately

degrades its total effi ciency around 2.44 GHz. Alternative

antenna concepts with more robust port impedance and

radiation characteristics in response to severe deforma-

tion need to be exploited.

18.2.3 Elastomeric, Bendable, Stretchable, Fluidic UWB Antenna

Th e concept of silicone-based soft electronics is then

extended to implement a planar inverted cone antenna

(PICA), operating at the ultrawideband (UWB) frequency


range of 3.1–10.6 GHz [26]. Th e choice for such a wideband

antenna is that its electrical performance is expected to be

insensitive to mechanical deformation. Furthermore, its

ultrathin uniplanar confi guration makes it capable of toler-

ating extreme degrees of bending and stretching. Schematics

of the elastic PICA are depicted in Figure 18.7. Th e height

of the leaf-shaped radiator roughly corresponds to a quar-

ter wavelengths at the lowest resonance frequency of the

antenna, and the size of the ground plane has a minor eff ect

on electrical characteristics as long as it is suffi ciently large.

Th e gap between the radiating element and the ground

plane plays an essential role in antenna performance, and

determines the upper range of operational frequencies. Th e

smaller gaps lead to the higher frequencies. Th e two sym-

metrical tapered apertures formed by the inverted cone and

the ground plane act as a broadband impedance transformer

that gradually transfers the 50 Ω port impedance to the free

space impedance of approximately 377 Ω.

Resembling the soft unbalanced loop proposed previ-

ously, the soft PICA prototype features excellent revers-

ible foldability, fl exibility, and stretchability, as shown in

Figure 18.8. Extreme levels of straining of up to 40% along

either x- or y-axis do not cause any mechanical failures, cf.

Figure 18.7 Geometrical schematics of a 2D stretchable fl uid

metal PICA encapsulated in a silicone rubber substrate: R=10 mm,

Lg=25 mm, W

g=40 mm, G=300 μm, h=1 mm, and h

metal=100 μm.

Figure 18.8 Photographs of the resulting silicone-based highly fl exible, stretchable PICA prototype: a) in its relaxed state; w/ 40% strain

along its b) x- and c) y-axis; and d) w/ severe twisting. Th e corresponding coordinate system is shown in Figure 18.7.


Figure 18.8b,c. Severe twisting and bending do not result in

any damages either. Aft er the removal of applied mechanical

forces, the antenna returns to its original state without any

visible deformation.

Figures 18.9 and 18.10 present simulated and measured

refl ection coeffi cients of the microfl uidic soft PICA in vari-

ous states. Th e relaxed antenna achieves good impedance

match (S11

< -10 dB), within 3–11 GHz, both in simulations

and experiments.

As a result of the increased height of the radiator intro-

duced by the x-axis strains, the fi rst resonance frequency of

the antenna decreases, cf. Figure 18.10. Port impedance of

the antenna is somewhat sensitive to its geometry, and con-

sequently the antenna exhibits slightly varying impedance

matching while being stressed. Nevertheless, good imped-

ance match is seen at the entire UWB frequency band even

if the PICA is signifi cantly strained up to 40%.

Radiation pattern measurements at 2.5 GHz of the relaxed

and fl exed antennas are displayed in Figure 18.11. Like

conventional monopoles, the non-strained antenna shows

broad beam coverage, especially in the yz-plane. Th e maxi-

mum antenna gain at 2.5 GHz is found to be 2.2 dBi accord-

ing to the experimental data. Excellent cross- polarization

discrimination is obtained. Numerical results are in line

with the corresponding experimental data. Stretching the

antenna along either x- or y-axis to as high as 40%, results in

slight gain variations in its radiation patterns, but does not

cause any signifi cant gain degradation.

Figure 18.9 Simulated and measured S11

of the silicone rubber antenna in its original state.

Figure 18.10 Measured refl ection coeffi cients of the strained antenna.


As shown in Figure 18.12, similar simulations and exper-

iments are also performed at 5 GHz, where stronger ripples

and minor asymmetry due to greater disturbance from feed

cable compared to that at lower frequencies, occur in the

radiation patterns. Th e presence of higher order modes at

5 GHz in combination with the increased cable infl uence

degrades the cross-polarization discrimination. Compared

with the experimental data at 2.5 GHz, larger variations on

the measured radiation patterns at 5 GHz are observed, par-

ticularly in its yz-plane in the fl exed state.

Figure 18.13 presents measured radiation effi ciency over

the entire UWB frequency band. Although the radiation

effi ciency at the lower end of the frequency range decreases

in response to straining, it is still above 70%. Th is, together

with the measured refl ection coeffi cients, indicates that

the silicone-based soft PICA prototype achieves good total

(a) (b)

(c) (d)

Figure 18.11 Measured radiation patterns of the relaxed and fl exed silicone-based elastic antenna at 2.5 GHz. a) xz- and b) yz-plane

(according to the coordinate system depicted in Figure 18.7.


antenna effi ciency within the operational frequency band

regardless of stretching.

However, a great amount of work remains. Firstly, com-

prehensive reliability, durability, as well as environmental

impact tests, e.g., vibration, temperature cycling, and aging,

are needed, since it is indeed a new way of fabricating passive

electronic devices, with new materials. Electrical characteris-

tics of this type of soft electronics under extreme temperature

(a) (b)

(c) (d)

Figure 18.12 Measured radiation patterns of the relaxed and fl exed silicone-based elastic antenna at 5.0 GHz. a) xz- and b) yz-plane

(according to the coordinate system depicted in Figure 18.7.

conditions, e.g., below the melting point of the liquid alloy,

should be evaluated when a special experiment setup is

established. Studies on radio interfaces are also of impor-

tance. Implementation of fully integrated ultra-soft wireless

electronic systems, containing fl exible, thin embedded active

chips, stretchable interconnects, and highly effi cient stretch-

able passive components like antennas are the next big objec-

tive, with a need for considerable research eff orts.


18.3 Silicone-Based Integrated Active Soft Electronics

18.3.1 Implementation of Single-Layer Stretchable, Elastomeric Integrated Active RF Electronics

With the aid of newly invented localized stiff islands (LSIs)

concept, silicone-based soft electronics have been further

developed to the integrated active device level, as illustrated

in Figure 18.14 [30]. A microfl uidic, soft , large-area antenna

is realized in the same manner as the previous elastic anten-

nas, by incorporating liquid metal alloy into microstruc-

tured elastomeric channels. Commercially available IC chips

associated with discrete passive elements such as inductors,

capacitors, and resistors, are assembled onto small fl ex-

ible laminates. Subsequently, a few tin-plated metallic con-

tact pins resembling cantilevers are soldered to the fl exible

circuits. And then a semispherical solder ball is mounted

on the bottom surface of each contact pin on the other

end to improve galvanic connection to the liquid fl uid, cf.

Figure 18.14e. Th ereaft er, the fl exible circuits are embedded

into the elastomeric substrate, with all contact pins immers-

ing in the liquid-metal-fi lled microfl uidic channels. Finally,

uncured PDMS mixture droplets are deposited on top of the

fl ex foils to locally stiff en the silicone rubber substrate and

encapsulate the fl exible circuitries as well as fl ex-to-stretch

interfaces. Th e cured LSIs with locally enhanced stiff ness

then surrounding areas ensure nearly zero stress and dis-

placement between the rigid and soft parts inside, when the

heterogeneously integrated device is being fl exed. Compared

with standalone single-layer passive soft electronics, the

overall elasticity of the hybrid device is degraded to some

extent. But, mechanical reliability and robustness of the inte-

grated device are considerably improved. Reliable electrical

connections between stiff circuits and fl uidic metal are also

obtained regardless of stretching, due to excellent wetting of

the liquid metal alloy on tin-plated pins and solder balls.

18.3.2 Highly Deformable, Large-Area, RF Radiation Sensor

Th e proposed hybrid integration approach was utilized for

realizing a 900 MHz microfl uidic stretchable RF radiation

sensor, as presented in Figure 18.15. Th e integrated radia-

tion sensor comprises three submodules fully encapsulated

in a large-area elastomeric substrate, including an elastic

unbalanced loop antenna for capturing RF radiation from

ambient environments, an RF power detection unit for

converting received RF energy to DC voltages, and a light

emission diode (LED) for visualization. Th is sensor device

is capable of continuously monitoring the human exposure

level to electromagnetic fi elds (EMFs). Once the exposure

level to EMFs exceeds the threshold, the LED indicator will

be switched on as a warning sign. Th e recent trend towards

mobility has turned more and more conventional electronic

devices and systems into wireless. Th is shift is of course

greatly improving the quality of our daily life, but also rais-

ing considerable concerns on EMF exposure related human

health issues. Th e demonstrated ultrathin, soft RF radiation

sensor is thereby seeing increasing interest in society.

Prior to the fi nal integration, electrical properties of

the standalone soft antenna and the RF energy detection

submodule are individually characterized. Th e stretchable

fl uidic antenna exhibits similar mechanical and electrical

performance as the previously presented unbalanced loop,

but with lower operational frequencies around 900 MHz.

Th e RF power detection submodule in the integrated RF

radiation sensor involves an off -the-shelf power detector IC

chipset (Linear Technology, LT 5534), two decoupling and

Figure 18.13 Measured radiation effi ciency of the stretchable antenna in its original state and w/ diff erent strains.


(a) (b)

(c) (d)

(e)

Figure 18.14 Schematic drawing of the heterogeneous integration procedure of the silicone-based single-layer soft electronics. Reproduced

from Ref [30] with permission from Th e Royal Society of Chemistry.

one coupling capacitors, an inductor for impedance match-

ing at the RF input, and a green LED indicated, all assem-

bled on a small fl ex foil with a size of 10 mm × 18 mm, as

seen in Figure 18.15. When the RF energy captured by the

antenna exceeds the power threshold, the LED indicator will

be switched on, and vice versa. Th e entire integrated power

detection unit can be powered by four serially connected

AA rechargeable batteries with a supply voltage of approxi-

mately 5.2 V. Th e RF-DC power conversion behavior of the

integrated device is fi rst measured using a signal generator

and a digital multimeter. Figure 18.16a shows experimen-

tal data on the output DC voltages versus varying RF input

power at 900 MHz. A dynamic range of −60 dBm to 0 dBm is

achieved, and the RF power threshold for turning on/off the

LED indicator is found to be slightly higher than −30 dBm.

Th e system level demonstration is conducted with a setup

shown in Figure 18.17. Th e RF radiation source consists of

an RF signal generator and a horn antenna placed 5 m away

from the sensor in the line-of-sight.

Experiments presented in the photographs in Figure 18.18

verify that the resulting silicone-based soft sensor device

maintains its radiation sensing capabilities even if being

strained along multiple axis. Extreme levels of twisting do

not cause any sensing failures or mechanical damages either.

18.3.3 Fabrication of Multi-Layer Silicone-Based Integrated Active Soft Electronics

Taking one step further, the emerging fi eld of silicone-

based soft electronics has been advanced to build multilayer

confi gurations, with the demonstration of a microfl uidic,

reversibly stretchable, large-area wireless strain sensor [31].

Th e manufacturing process for multilayer microfl uidic soft

electronic devices resembles the single-layer fabrication, but

with a few minor modifi cations and additional process steps,

cf. Figure 18.19.

In brief, the upper and lower microfluidic channels

are respectively constructed in the top and the bottom

silicone substrates, using standard soft lithography tech-

niques. Then a number of inlets and outlets are punched.

A blank middle silicone rubber slab is also prepared. The

microstructured top elastomer layer is bonded to the

blank silicone substrate using thermal or plasma bond-

ing, and addition inlets are punched on the bonded elas-

tomer sheet. The bottom silicone elastomer is bonded

Figure 18.15 Photograph of an integrated large-area elastomeric

RF radiation sensor. Reproduced from Ref [30] with permission

from Th e Royal Society of Chemistry.


(a) (b)

(c) (d)

Figure 18.16 Numerical and experimental results of the standalone submodules prior to the fi nal integration. a) Measured output DC

voltages versus varying input power, of the RF power detector, w/ the LED ON/OFF states marked. b) Simulated and measured refl ection

coeffi cients, S11

, of the standalone silicone-rubber-based fl uidic unbalanced loop antenna in its relaxed state. c) Measured S11

of the fl exed

antenna with varying strain along its x-axis. d) Measured S11

of the fl exed antenna with varying strain along its y-axis. Th e corresponding

coordinate system is shown in Figure 18.15.

Figure 18.17 Schematic illustration of the demonstration setup for RF radiation sensing.


to the previously bonded PDMS layers afterwards. Prior

to filling the upper microfluidic channels with galinstan

fluid metal, the inlet 3 is sealed with a piece of Scotch®

tape. Later, the ventilation outlets in the top silicone

rubber sheet are encapsulated using PDMS prepolymer.

Whereafter, both the inlets 1 and 3 are taped, and galin-

stan alloy is injected into the lower microfluidic channels

from the bottom side. All remaining ventilation outlets

and the inlet 2 are then encapsulated, and the inlets 1 and

3 are reserved for connecting active circuitry in a hetero-

geneously integrated device.

Th e subsequent fabrication process steps are active cir-

cuit assembly and hybrid device integration, which are

similar to the manufacturing and integration processes

presented previously. Schematic illustrations describing the

entire assembly and integration procedure are displayed in

Figure 18.20, and no detailed discussions are presented in

this section.

18.3.4 Multilayer, Reversibly Stretchable, Large-Area Wireless Strain Sensor

Th e fi rst silicone based multilayer soft electronic device is

a self-contained wireless strain sensor, fully encapsulated

in a thin large-area silicone rubber substrate, as shown in

Figure 18.21. An elastic fl uid metal microstrip patch antenna

comprising an upper rectangular meshed patch and a lower

ground plane constructed in the same manner, takes up

the major area of the hybrid device. It is worth mention-

ing that this antenna not only serves as a radiator like any

other conventional antenna, but also acts as a sensing device

owing to its varying electrical characteristics in response to

mechanical strains. Th e overall size of the entire integrated

sensor device is slightly larger than the meshed liquid alloy

ground with a size of 100 mm × 80 mm, and roughly four

times as big as the antenna patch. Th ough the liquid metal

patch is the actual strain sensing element, the large ground

(a) (b)

(c) (d)

Figure 18.18 Photographs of the integrated elastomeric RF radiation sensor operating in ordinary offi ce environment: a) in its relaxed state,

b) w/ 15% stretching along its y-axis, c). w/ manually applied 2D strain, and d) w/ manual twisting. Th e presented sensor device was directly

illuminated by an RF radiation source placed 5 m away in the demonstration. Th e coordinate system is seen in Figure 18.15. Reproduced

from Ref [30] with permission from Th e Royal Society of Chemistry.


(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 18.19 Manufacturing process for multilayer silicone-based soft electronics.

(a)

(b)

(c) (d)

(e)

(f)

Figure 18.20 Schematic of the hybrid integration process for a multilayer silicone-based soft electronic device.


Figure 18.21 Implemented large-area, reversibly stretchable wireless strain sensor, encased in a silicone rubber substrate: a) Optical

photograph associated with a circuit schematic. Photographs of the integrated sensor device in its b) relaxed and c) fl exed states.

(a) (b)

(c)

Figure 18.22 Characterizations of the standalone elastomeric fl uid metal patch antenna. a) xz- and c) yz-plane radiation patterns (including

mismatch losses) at the original resonant frequency of the antenna in its relaxed state, and w/ 15% elongation along its x-axis. Th e antenna gain

along φ and θ orientations is defi ned as Gφ and G

θ, respectively. Th e antenna co- and cross-polarization along +z-axis are in parallel to the x- and

y-axis. c) Simulated and measured refl ection coeffi cients of the antenna in diff erent states. Th e corresponding coordinate system is presented in

Figure 18.21.


plane can also serve an eff ective sensing area, and extend the

strain sensing functionality to almost the entire integrated

device. Apart from the self-contained wireless strain sensor,

a custom-designed personal computer (PC)-assisted radio

receiver for remotely collecting, processing, and storing the

measured data wirelessly transmitted from the sensor device

is also realized. It removes the need for costly RF measure-

ment facilities, and signifi cantly reduces the cost for build-

ing up such a system.

Port impedance and radiation characteristics of the

standalone, mechanically reconfi gurable, elastomeric patch

antenna are measured both in its relaxed and fl exed states.

Excellent port impedance matching around 1.46 GHz along

with very good correlation between the simulated and mea-

sured refl ection coeffi cients of the non-stressed antenna are

seen in Figure 18.22c. Applying increasing strain of up to 15%

along its x-axis introduces persistent downshift on the antenna

resonance frequency. Th e lowest resonance frequency of

approximately 1.33 GHz is measured at the maximum x-axis

elongation of 15%. Th e measured resonance frequency of the

relaxed antenna is slightly lower than the minimum opera-

tional frequency of the integrated transmitter circuit. Th is

negative frequency off set is crucial for strain sensing, since it

ensures that the mismatch losses persistently rise at 1.46 GHz

in response to increasing strains along the x-axis of the inte-

grated device. Furthermore, this off set should be as little as

possible to avoid too high mismatch losses so that reasonably

long remote sensing ranges as well as suffi cient strain sens-

ing sensitivity can be attained. Placing the antenna original

resonance frequency above the highest operational frequency

of the transmitter is not an option, as inconsistent variation

of mismatch losses would occur if the silicone-based strain

sensor is stressed from its relaxed state to a high tensile strain.

Experimental results on radiation patterns, including

mismatch losses, at 1.46 GHz, of the relaxed and fl exed

antennas are shown in Figure 18.22a,b. Th e realized peak

gain is obtained along the +z-axis (according to the coordi-

nate system depicted in Figure 18.21a) of the mechanically

reconfi gurable antenna, and signifi cantly decreases from 2.0

dBi to −10.7 dBi while an increasing strain from 0% to 15%

is applied along its x-axis. Th e meshed ground plane directs

the radiation forward, with a front-to-back ratio of 10.0 dB,

regardless of stretching. Moreover, the cross-polarization

discrimination is very good in both relaxed and strained

cases. As seen in Figure 18.22c, the considerably large drop

in the realized peak gain at 1.46 GHz can be explained by the

worsening port impedance match resulted from the increas-

ing strains.

Mechanical properties of the standalone soft patch antenna

are evaluated aft er characterizations on its electrical perfor-

mance. Th e realized antenna prototype is more than twice as

thick as the previously demonstrated soft electronic devices,

and thus features degraded mechanical deformability. Yet

moderate twisting or folding do not cause any mechanical

failures to the fl uidic patch antenna during experiments.

Th e measured total effi ciency of the standalone stretch-

able patch antenna including mismatch losses is 36.9% in its

relaxed state at 1.46 GHz. Straining the patch to 15% along

its x-axis leads to signifi cant total effi ciency decrease of

33.8%. In the intermediate states with the strains between 5%

and 10%, the total effi ciency of 11.5% and 5.8% is achieved

according to the experiments.

Th e RF power detection submodule in the custom-

designed radio receiver is characterized. Figure 18.23 pres-

ents the measured DC voltages with respect to varying RF

input power at 900 MHz. Fairly linear behaviors are obtained

within a large dynamic range of 70 dB.

Th e system level calibration on the demonstration setup is

performed by measuring the output DC voltages of the RF

power detector when diff erent static stresses are applied to

Figure 18.23 Measured output DC voltages corresponding to varying RF power directly injected into the standalone RF energy detection

submodule.


the integrated wireless strain sensor, cf. Figure 18.24. A lin-

ear decline of the measured DC voltages versus increasing

mechanical elongation along the x-axis of the self-contained

sensor device is seen, and can be explained by decreased

resonant frequencies and increased mismatch losses of the

transmitting, elastomeric antenna because of incremental

stretching. As co-polarization components dominate in both

microfl uidic stretchable patch antenna and the receiving horn

in the custom-designed RF receiver, the measured output

voltages with respect to the cross-polarization are considerably

lower than that of the co-polarization, and also exhibit smaller

variations with diff erent strains. Th e steepest voltage decline ver-

sus increasing stretch is found in the case of the co- polarization

measured in an anechoic chamber, cf. Figure 18.25a. When it

comes to ordinary offi ce environment, the presence of refl ec-

tions and scatterings limits the range of voltage variations so as

to degrade the strain sensing sensitivity.

Th e photograph in Figure 18.25b shows the demonstration

of remote sensing of repeated body motion in ordinary offi ce

environment, using the silicone-based soft wireless strain sen-

sor prototype. Periodically repeated dynamic strains of up to

15% along its x-axis, with a period of 10s and a duty cycle

of 50%, are manually applied to the integrated, elastic, strain

sensor. Th e output DC voltages in the PC-assisted RF receiver

varying with mechanical strains applied to the integrated,

body-worn, sensor device are continuously monitored and

recorded. Th e measured data of six cycles, well correlated with

the applied varying tensile strains, are presented in the subplot

in Figure 18.25b, in which six fairly uniform, quasi-rectangular

waves with varying amplitude between 1.28V and 1.55V are

recorded in an overall period of 60s. Th is experimental data

also verifi es that the implemented wireless strain sensor can

Figure 18.24 Measured output DC voltages at the receiver in response to varying mechanical stress applied along the x-axis (according to

the coordinate system in Figure 18.21.

Figure 18.25 Demonstration of remote strain sensing, using the

silicone-based integrated wireless strain sensor: a) Wireless strain

monitoring in an anechoic chamber. b) Remote strain sensing

in ordinary offi ce environment. Th e subplot shows the real-time

recorded data at the PC-assisted RF receiver 5 m away.


rapidly return to its original state without any hysteresis, once

removing the applied stress. Th is great feature refl ects revers-

ible deformability as well as high degree of elasticity of the

multilayer soft electronic device.

18.4 Conclusion

Recent advances in the emerging fi eld of silicone-based soft

electronics that are able to be compressed, twisted, and stressed

to extreme levels, or be conformed into complex curvilinear

shapes, have been briefl y reviewed. Various techniques and

strategies for realizing fl exible, foldable, stretchable electronic

devices and system have been discussed. As the main focus

of this chapter, elastomeric electronics based on microfl uidic

approaches have been addressed in detail. Fabrication pro-

cesses, hybrid integration techniques, as well as appealing

application examples involving single- and multilayer inte-

grated soft electronic devices have also been presented.

Microfl uidics-based elastic electronics together with

other members in the new class of soft electronics are intro-

ducing a revolution in the world of electronics, and shap-

ing the future for electronics so as to change our daily life

of tomorrow and contribute to our networked society. It is

anticipated that more than 50 billion devices will be wire-

lessly connected by 2020, which would involve units as

intelligent as smart-phones/tablets, and as soft as our newly

invented elastic electronic devices.

Acknowledgements

Th e author currently holds a Young Research Fellow posi-

tion fi nanced by the Swedish Research Council (VR) under

the contract (Avtals-ID: C0486501).

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concise encyclopedia of high performance silicones

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