Foldable and Stretchable Liquid Metal

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Article in IEEE Transactions on Antennas and Propagation · January 2010

Impact Factor: 2.18 · DOI: 10.1109/TAP.2009.2024560 · Source: IEEE Xplore

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Foldable and Stretchable Liquid Metal

Planar Inverted Cone Antenna

Shi Cheng ,Student Member,IEEE ,Zhigang Wu,Paul Hallbj?rner,Klas Hjort ,Member,IEEE ,and

Anders Rydberg ,Member,IEEE

Abstract—A mechanically ?exible planar inverted cone antenna (PICA)for ultrawideband (UWB)applications is presented.It can be both folded and stretched signi?cantly without permanent damage or loss of electrical functionality.The antenna is manufac-tured with a process in which conductors are realized by injecting room temperature liquid metal alloy into micro-structured channels in an elastic dielectric material.The elastic dielectric material together with the liquid metal enables bending with a very small radius,twisting,and stretching along any direction.Port impedance and radiation characteristics of the non-stretched and stretched antenna are studied in simulations and experiments.The presented antenna has a return loss better than 10dB within 3-11GHz and a radiation ef?ciency

of 70%over 3-10GHz,also when stretched.Tests verify that stretching up to 40%is possible with maintained electrical performance.The presented antenna is useful for example for body-worn antennas and in applications in harsh environments where mechanical ?exibility helps improve durability.

Index Terms—Liquid alloy,planar inverted cone antenna (PICA),polydimethylsiloxane (PDMS),stretchable electronics,ultrawideband (UWB).

I.I NTRODUCTION

A

NTENNAS that can be stretched,folded,and twisted see a growing demand in more and more applications in areas such as body area networks,implanted medical devices,inter-active gaming,and conformal array antennas for aeronautic re-mote sensing [1]–[5].In devices that are in contact with the skin of the user,deformable antennas may signi?cantly enhance the comfort of the user.In any application with a curved mechanical interface,?exible antennas remove the need for antenna designs with an exact shape.Instead,?at antennas can be built and at in-stallation be shaped to conform to any surface.Furthermore,in applications in harsh environments such as heavy industry,an antenna can better tolerate severe mechanical shock by ?exing instead of breaking.

Manuscript received February 12,2009;revised April 22,2009.First pub-lished June 10,2009;current version published December 01,2009.

S.Cheng and A.Rydberg are with the Department of Engineering Sciences,Microwave Engineering,Uppsala University,Uppsala 75121,Sweden (e-mail:shi.cheng@angstrom.uu.se).

Z.G.Wu and K.Hjort are with the Department of Engineering Sciences,Materials Science,Uppsala University,Uppsala 75121,Sweden (e-mail:zhigang.wu@angstrom.uu.se).

P.Hallbj?rner is with the Department of Engineering Sciences,Microwave Engineering,Uppsala University,Uppsala 75121,Sweden and also with the SP Technical Research Institute of Sweden,50115Bor?s,Sweden (e-mail:paul.hallbjorner@sp.se).

Color versions of one or more of the ?gures in this paper are available online at https://www.360docs.net/doc/cf6338094.html,.

Digital Object Identi?er 10.1109/TAP.2009.2024560

Conventional materials for antenna manufacturing include a variety of metals for conductors and dielectric materials for me-chanical support.Most of them are however rigid.Flexible di-electric foils are sometimes used,but their ?exibility is mainly due to a small foil thickness and not the material in itself.A small thickness,in turn,makes them useful only for certain an-tenna types and frequencies.Furthermore,because the mate-rial is not very ?exible,?exible foils can only be bent and not stretched.Metal conductors on the ?exible foil are usually made of standard metals such as copper or gold.This also limits the ability to stretch.

New materials and processes are needed to achieve full me-chanical ?exibility including stretchability.Various approaches to implement stretchable electronics have been introduced by several research groups [6]–[17].Stretchable and foldable sil-icon integrated circuits (IC)have been demonstrated by Rogers et al.where active ICs were fabricated on thin “wavy”silicon ?lms embedded in elastic substrates [6]–[8].Wagner et al.pre-sented stretchable interconnects and thin ?lm transistor (TFT)circuits on pre-stretched elastic substrates [9]–[11].Using me-andered line con?guration,stretchable interconnects at different frequency ranges,with a longitudinal elongation up to 100%,were shown [12],[13].However,stretchability is still restricted by the solid metals used in these approaches because of the mechanical mismatch between the solid metals and elastic materials.A highly stretchable self-assembled nanocomposite material,Metal Rubber,has been demonstrated by NanoSonic Inc.[14].Nevertheless,the relatively high sheet resistance of this material may result in high losses,especially at high frequencies.Liquid metal ?lled elastomeric micro-structured channels have recently been used for stretchable interconnects,with which a high multiaxial stretchability as well as very low resistance can be obtained [15]–[17].However,these studies deal with either low-frequency ICs or relatively simple inter-connects.The practical usefulness of these stretchable materials in antennas is not reported so far.

II.A NTENNA D ESIGN

This paper presents an antenna design using liquid metal for conductors and a stretchable substrate material.A planar in-verted cone antenna (PICA)for the ultrawideband (UWB)fre-quency range 3.1-10.6GHz is designed [18],[19].The PICA is a highly broadband antenna similar to the volcano antenna [20]and the circular disk antenna [21].Its uniplanar structure makes it a suitable antenna type for a design that should be bend-able.Fig.1shows the design schematically,with a leaf-shaped radiator and a large ground plane.The dimensions of the ra-diator (and )are critical for the electrical performance.

0018-926X/$26.00?2009IEEE

Fig.1.Schematical view of PICA antenna made of two sheets of solid metal. The ground area should be“in?nite”,which in practice means that it should be signi?cantly larger than the radiator.In the 3.1-10.6GHz band,the antenna features diverse modes at dif-ferent frequencies.The PICA acts as a monopole at the lower end of the frequency range.The antenna radiator is20mm in

height which corresponds to a quarter free-space wave-

length at3.75GHz.The upper end of the frequency range is

mainly determined by the

gap between radiator and ground.

In the presented design,the gap is

300m.The omnidirectional

radiation of the PICA makes it suitable for mobile terminals.

Similar antennas have been used for portable devices or body

area networks[22],[23].

When realizing this design with liquid metal,the conducting

surfaces are replaced with a grid of liquid metal?lled channels

inside the substrate.Fig.2depicts the PICA design in more de-

tail,with dimensions.The antenna is fabricated with a process

that incorporates room temperature liquid metal alloy into

micro-structured channels in a sheet of polydimethylsiloxane

(PDMS).Elasticity,lightness,and low manufacturing cost,as

well as favorable electrical properties

(

,

@100kHz),makes this material a suitable choice for substrate

material.A large number of PDMS posts are aligned to space

the top and bottom PDMS membranes in the areas of the

radiator and ground plane,as shown in Fig.2.

In brief,the process steps can be summarized as follows:

master fabrication,molding,liquid-alloy injection,and encap-

sulation,cf.Fig.3.

The antenna design is transferred to a

100thick SU-8

100(MicroChem,Newton,MA)layer on a silicon wafer with a

standard soft lithography[24].The design pattern is developed

and thermally stabilized

at for30min for complete

adherence.The PDMS prepolymer and cross linker(Elastosil

RT601A and B,Wacker Chemie,Munich,Germany)are thor-

oughly mixed at a ratio of9:1(wt:wt)and poured onto the

SU-8master.After degassing,it is cured

at in an oven for

30min.The thin PDMS replica is peeled off and a couple of

holes are punched out for liquid alloy injection.Meanwhile a

thin blank PDMS lid is cast on a polished blank silicon wafer.

The PDMS replica and blank lid are bonded,using corona dis-

charging(ETP,Chicago,IL)activation.Through the

punched

Fig.2.Geometric con?guration of the2-D stretchable PICA.Dimensions are:

R=10mm,R=47:5mm,L=25mm,W=40mm,W=

1:25mm,W=1:75mm,L=1:5mm,W=1:5mm,D=

2mm,G=300 m,h=1mm,and

h=100 m

.

Fig.3.Fabrication process steps.

holes in the PDMS,the liquid alloy(Galinstan,68.5%Ga,

21.5%In,10%

Sn,)is manually injected

into the channels.This alloy remains in liquid state

from

to

1300.The ventilation outlets are encapsulated

with uncured PDMS mixture as mentioned above.

The proposed concept is a cost-effective solution for imple-

menting high-performance stretchable electronics.The material

cost of the PDMS and liquid alloy for the presented antenna is

around0.4and0.6Euros,respectively,which can be further re-

duced in mass production.No costly facility or clean-room envi-

ronment is required in the entire fabrication process.Hence,the

processing cost will be negligible compared to the material cost

CHENG et al.:FOLDABLE AND STRETCHABLE LIQUID METAL PLANAR INVERTED CONE ANTENNA

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Fig.4.Sketch of feed cable connection to the

antenna.

Fig.5.Photographs of the stretchable PICA:(a)and (c)non-stretched antenna,(b)stretched antenna with 40%x -axis elongation,(d)stretched antenna with 40%y -axis elongation,(e)folded antenna,and (f)twisted antenna.The corre-sponding coordinate system is shown in Fig.2.

in large volume production once the automatic injection process is established.

The inlet openings are preserved for RF coaxial cable connec-tion.Fig.4illustrates a semirigid coaxial cable connection to the antenna.The two pins connecting to the antenna are the center conductor of the cable and a pin which is soldered to the outer conductor of the cable.The gap between cable end and substrate represents an unwanted inductance.With a gap smaller than 1mm the effect of this inductance is negligible at the frequencies of operation.As depicted,the cable is perpendicular to the sub-strate out to approximately 10mm from the substrate,at which point it bends to run parallel to the ground plane,for minimal interference during measurements.

III.M ECHANICAL T ESTS

Mechanical tests of stretching,folding,and twisting the an-tenna are carried out.No mechanical damage is found as a re-sult of these deformations.As shown in Fig.5(b)and (d),a rel-ative stretching of 40%

along -

and -axis is veri?ed in the tests.A stretchability of up to 100%along any direction can be reached,were it not for the openings in the membranes,which limit the achievable stretching.Furthermore,the rigid feed cable can easily damage the antenna when stretched more than 50%.Folding with a radius of a few mm,as well as severe twisting,[depicted in Fig.5(e)and (f)]causes no damage to the antenna.

An antenna is frozen

to

,i.e.,a few degrees below the freezing temperature of the liquid metal,without any visible damage.No electrical tests were carried out during freezing,

though.

Fig.6.Simulated and measured re?ection coef?cient of the non-stretched

antenna.

Fig.7.Measured re?ection coef?cient of the stretched antenna.

IV .E LECTRICAL C HARACTERIZATION

Port impedance and radiation characteristics of the non-stretched and stretched antenna are studied experimentally.In the measurements,the PICA is fed by a thin RF coaxial cable via the two openings in the top PDMS membrane.Prior to all the experiments,full-wave simulations on the non-stretched antenna are performed using Ansoft HFSS.For the stretched antennas,comprehensive analysis including both mechanical and electrical aspects is complicated,which is why no numer-ical results on the stretched antennas are reported in this paper.A.Port Impedance

Re?ection

coef?cient measurements are carried out using a network analyzer (Agilent Technologies E8364B,PNA series).Simulated and measured re?ection coef?cients are presented in Figs.6and 7.The non-stretched antenna shows

impedance match,de?ned

as

,within 3-11GHz.When the antenna is stretched along

the -axis,the overall height of the radiator increases,resulting in a lower ?rst resonance frequency,cf.Fig.7.Port impedance is somewhat sensitive to the geometry,and consequently the antenna features slightly varying impedance matching while stretched.Good impedance match remains at frequencies higher than 3.4GHz even if the antenna is stretched to 40%.

3768IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION,VOL.57,NO.12,DECEMBER

2009

Fig.8.Measured xz -plane (according to the coordinate system in Fig.2)radi-ation patterns at 2.5GHz of the non-stretched and stretched antenna with 40%x -axis

elongation.

Fig.9.Measured yz -plane (according to the coordinate system in Fig.2)radi-ation patterns at 2.5GHz of the non-stretched and stretched antenna with 40%x -axis elongation.

B.Radiation Performance

Figs.8–11show measured radiation patterns of the non-stretched and stretched antenna at 2.5GHz.Similar to conventional fat monopole antennas,the non-stretched antenna features broad coverage,especially in the yz -plane where almost perfect omnidirectionality is seen.The maximum antenna gain at 2.5GHz is around 2.2dBi.The cross-polariza-tion discrimination is very good,something which also veri?es that disturbances from the setup (e.g.,feed cable)are low.Nu-merical simulations are in good agreement with the presented experimental measurements.Stretching the antenna along

the

Fig.10.Measured xz -plane (according to the coordinate system in Fig.2)radi-ation patterns at 2.5GHz of the non-stretched and stretched antenna with 40%y -axis

elongation.

Fig.11.Measured yz -plane (according to the coordinate system in Fig.2)radi-ation patterns at 2.5GHz of the non-stretched and stretched antenna with 40%y -axis

elongation.

-axis

and -axis up to 40%,slight variations in the measured radiation patterns at 2.5GHz are noticed.No signi?cant gain reduction is however observed.

Similarly,measured radiation patterns of the non-stretched and stretched antenna at 5GHz are presented in Figs.12–15.At this frequency,ripples and slight asymmetry occur in the patterns,something which is believed to be caused by distur-bance from the feed cable.As a result of the presence of higher order modes at 5GHz together with the cable in?uence,the level of cross-polarization increases to approximately 3dB below https://www.360docs.net/doc/cf6338094.html,pared to the experimental results at 2.5GHz,larger variations on the measured radiation patterns at

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Fig.12.Measured xz -plane (according to the coordinate system in Fig.2)ra-diation patterns at 5GHz of the non-stretched and stretched antenna with 40%x -axis

elongation.Fig.13.Measured yz -plane (according to the coordinate system in Fig.2)ra-diation patterns at 5GHz of the non-stretched and stretched antenna with 40%x -axis elongation.5GHz are seen while the antenna is stretched along

the -axis

and -axis,especially in the yz -plane.

Thanks to relatively high conductivity of the liquid metal alloy and large cross section of the micro-structured channels,as well as low loss factor of the substrate material,a high radiation ef?ciency of the antenna is expected.The radiation ef?ciency of the antenna is measured in a reverberation chamber [25].Fig.16shows the result within the frequency range 3-10GHz.Although the radiation ef?ciency at the lower end of the frequency range decreases when the antenna is stretched,it is still above 70%.The shift of the ?rst resonance frequency due to stretching ex-plains this decrease in radiation ef?ciency at the lower end of

the

Fig.14.Measured xz -plane (according to the coordinate system in Fig.2)ra-diation patterns at 5GHz of the non-stretched and stretched antenna with 40%y -axis

elongation.

Fig.15.Measured yz -plane (according to the coordinate system in Fig.2)ra-diation patterns at 5GHz of the non-stretched and stretched antenna with 40%y -axis elongation.

frequency range.The results imply that electrical connections in the micro-structured channels are not interrupted by stretching.This is due to excellent wetting between the liquid alloy and the surface of the microstructured elastic channels.

Electrical characteristics of the antenna prototype when folded or twisted in three dimensions are not measured due to dif?culties in connecting the RF coaxial cable.

V .C ONCLUSIONS

A highly foldable and stretchable PICA with good electrical performance has been presented.The demonstrated PICA ful?lls the requirements of UW

B applications operating in the

3770IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION,VOL.57,NO.12,DECEMBER

2009

Fig.16.Measured radiation ef?ciency of the non-stretched and stretched antennas.

3.1-10.6GHz band.The antenna allows stretching up to40% along any axis,as well as folding and twisting,all without mechanical damage.Measurements show that the antenna maintains wide impedance bandwidth even when stretched 40%.Moreover,both the non-stretched and stretched antenna features favorable radiation characteristics with a radiation ef?ciency greater than70%within3-10GHz.

A wide range of applications can be foreseen in various areas such as wearable computing,healthcare&?tness wireless sen-sors,and radio frequency identi?cation(RFID)systems.

VI.F UTURE W ORK

The presented design represents a fully functional antenna prototype.However,much work remains before the design can be implemented in https://www.360docs.net/doc/cf6338094.html,plete environmental durability tests,e.g.,vibration,temperature cycling,and aging,are needed, since it is indeed a new way of manufacturing antennas,with new materials.Electrical characteristics of the antenna under extreme temperature condition,e.g.,below the freezing point of the liquid alloy,will be measured when a special antenna measurement setup is established.Development of the antenna radio interface is also necessary.

Active devices as well as complex routing can be integrated in the area of the ground plane.Studies of how to optimize such design features would also be a topic for future work.

Owing to the small mechanical mismatch between the liquid metal alloy and PDMS,the prototype presented in this paper exhibits much higher quasi-isotropic stretchability than other stretchable electronics using solid metals.This feature gives cause for optimism when it comes to robust stretchable elec-tronics with long life time.Even higher stretchability should be expected if an optimal channel con?guration,e.g.,mesh or me-andered line,is employed.

Development of fully integrated stretchable wireless elec-tronic systems comprising?exible thin embedded active chips, stretchable interconnects at different frequencies,and highly ef?cient stretchable antennas are the end objective,with a need for much research and tests.

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Shi Cheng (S’07)was born in Jiangsu,China,in 1980.He received the B.E.degree in radio engi-neering from Southeast University,Nanjing,China,in 2003and the M.Sc.degree in engineering physics from Uppsala University,Uppsala,Sweden,in 2006,where he is currently working toward the Ph.D.degree.

His research interests include integrated antennas for various wireless sensor applications,RF micro-electromechanical systems (MEMS)based recon?g-urable circuits and antennas,and millimeter-wave an-tennas.He has published about 30scienti?c papers in several journals and in-ternational

conferences.

Zhigang Wu was born in Hubei,China.He received the B.Sc.degree in mechanical engineering and sci-ence from Huazhong University of Science and Tech-nology (HUST),China,in 2001and the Ph.D.degree in micro?uidics from Nanyang technological Univer-sity (NTU),Singapore,in 2005.

From 2005to 2006,he was a Research Fellow working on micro?uidics and integrated bio-pho-tonic chip at the School of Electrical and Electronic Engineering of NTU.From 2006to 2007,he was a Postdoctoral Fellow at Uppsala University,Uppsala,

Sweden,working on polymer based lab-on-a-chip systems for microbiomics.Currently,he works as a Senior Researcher in the same group.His research interests include:polymeric lab-on-a-chip devices and systems for clinical applications,home healthcare and biomics,rapid microfabrication techniques for lab-on-a-chip applications,BioMEMS components and their integration,transport effects in microscale and their applications in life-sciences and chem-istry,and stretchable radio frequency electronics in point-of-care healthcare and motion monitoring.In recent years,he has focused on high throughput micro?uidic biological particle separation and stretchable radio frequency electronics in point-of-care healthcare.He has authored or coauthored more than 35scienti?c papers and a number of book

chapters.

Paul Hallbj?rner was born in Uppsala,Sweden,in 1966.He received the B.Sc.,M.Sc.,and Ph.D.degrees from Chalmers University of Technology,G?teborg,Sweden,in 1988,1995,and 2005,respec-tively,all in electrical engineering,

From 1989to 2006,he worked in the telecom industry where he was employed by Ericsson,Saab,and Allgon.He is currently with the SP Technical Research Institute of Sweden as well as Uppsala University,where he was appointed Associate Pro-fessor in 2008.He has worked with mobile terminal

antenna design,recon?gurable and steerable antennas,antenna measurement techniques,wave propagation,passive microwave circuits,dielectrometry,microwave building practice,and millimeter-wave technology.He has more than 50scienti?c publications and is the inventor to several

patents.

Klas Hjort (M’96)received the M.Sc.degree in en-gineering physics and the Ph.D.degree in materials science from Uppsala University,Uppsala,Sweden,in 1988and 1993,respectively.

In 2008,he was appointed Professor of Micro Systems Technology at Uppsala University.He is experienced in advanced micro-and nano-engineering,which is often developed in close collaboration with application oriented research groups and industry.Recently he has focused on microsystems made in circuitboard technology for

Bio-MEMS and wireless sensor node applications.He is a world leader in ion track lithography in printed circuitboard foils and laminates,demonstrating integrated microwave and sensor components.Also,by the use of ?exible printed circuitboard technology and novel actuators micro?uidic systems are made like a world-strongest sub-cm micromechanic pump.He has published more than 200scienti?c papers and is the inventor of several patents.

Prof.Hjort is an Associate Editor for the Journal of Micromechanics and Mi-croengineering .He is a member of the Key Technology Group of the European Platform on Smart Systems (EPoSS).He is also a member of two scienti?c ad-visory boards and is often used as an expert for examinations,proposals and promotions.As a member of MEMSWA VE he was awarded as one of ten ?nal-ists for the Descartes Prize 2002of the Commission of the European

Union.

Anders Rydberg (M’89)received the M.Sc.degree from Lund Institute of Technology,Lund,Sweden and the Ph.D.degree from Chalmers University of Technology,G?teborg,Sweden,in 1976and 1988,respectively.

From 1977to 1983,he worked at the National Defence Research Establishment,ELLEMTEL De-velopment Co.and the Onsala Space Observatory.In 1991,he was appointed Docent (Associate Professor)at Chalmers University of Technology,G?teborg,Sweden.Between 1990to 1991,he worked as a

Senior Research Engineer at Farran Technology Ltd.,Ireland.In 1992,he was employed as an Associate Professor and,in 2001,as Professor in Applied Microwave and Millimeterwave Technology at Uppsala University,where he heads the Microwave Group,Department of Engineering Science.Since 2007,he is also joint-owner of Integrated Antennas AB,Uppsala,Sweden.

Prof.Rydberg is a member of the editorial board for the IEEE T RANSACTIONS ON M ICROW A VE T HEORY AND T ECHNIQUES .He is an adjunct member of Sec-tions B and D of the Swedish Member Committee of URSI (SNRV)and Chairman for the Swedish IEEE MTT/AP Chapter.