8/3/2019 adfm_201002508_sm_suppl

1/11

CopyrightWILEYVCHVerlagGmbH&Co.KGaA,69469Weinheim,Germany,2010.

SupportingInformation

forAdv.Funct.Mater.,DOI:10.1002/adfm.201002508AMicrofluidic,ReversiblyStretchable,LargeAreaWirelessStrainSensor

Shi Cheng* and Zhigang Wu*

8/3/2019 adfm_201002508_sm_suppl

2/11

Submitted to

1 1

Electronic Supplementary Information

Microfluidic Reversibly Stretchable Large-Area Wireless Strain

Sensor

Shi Cheng1,

and Zhigang Wu2,

1Advanced Technology, Laird Technologies, Box 1146, SE 164 22, Kista, Sweden

2Microsystems Technology, Department of Engineering Sciences, Uppsala University, Box

534, The Angstrom Laboratory, SE 751 21, Uppsala, Sweden

To whom correspondence should be addressed (Shi.Cheng@ieee.org and Zhigang.Wu@angstrom.uu.se)

8/3/2019 adfm_201002508_sm_suppl

3/11

Submitted to

2 2

1. Principles and Implementation1.1. RF Transmitter and Receiver Sub-Modules

RF

GND GND

EN

VCC

VOUT

DCpowersupply

PRL1

C1

C2

C1

Power

de

tector

(L55

34)

DCpowersupply

C =1nF,1 C =100pF,C =0.1 F,C =1.5pF,L =19nH2 3 4 1

NC

VCC

GO1555

GND

GND GND

O/PGND

VCTR

Tomicrovoltdigitalmultimeter(DMM)

C3

PR

Controlvoltage

C =1nF,1 NC=Noconnection

C4

b)

a)PT

Figure S1. Circuit schematics of a) the RF transmitter sub-module in the integrated strain

sensor (coupling capacitor: C1), and b) the RF power detection unit in the PC-assisted receiver

(decoupling capacitors: C2 and C3, coupling capacitor: C1, matching capacitor: C4, matching

inductor: L1).

The simplified RF transmitter in the integrated wireless sensor device is composed of a

voltage controlled oscillator (VCO) (Gennum, GO1555), and a coupling capacitor (C1), on a

flexible laminate of 13 mm 10 mm in size, cf. Figure S1(a). The choice of the VCO is based

on its proper oscillation frequency range, miniaturized package size, relatively low power

8/3/2019 adfm_201002508_sm_suppl

4/11

Submitted to

3 3

consumption, as well as sufficient RF output power. The chosen VCO generates stable

continuous wave (CW) RF signals around 1.5 GHz when a supply voltage in the range of

2.25-2.75V and a control voltage ranging from 1.0V to 1.5V are applied. Moreover, it only

draws a maximum supply current of 8 mA. This factor implies that this RF transmitter

circuitry can be effectively powered by two serially connected commercially available AA or

other types of standard portable batteries.

Figure S1(b) shows the circuit schematic of the RF power detection sub-module in the PC-

assisted receiver. In addition to an RF power detector (Linear Technology, LT 5534), this unit

also contains two decoupling capacitors (C2 and C3), one coupling capacitor (C1), one

matching capacitor (C4), and one matching inductor (L1), all soldered on a small piece of

FPCB with a size of 10 mm 20 mm. The integrated RF power detection circuitry is capable

of precisely converting the injected RF signals, in a decibel scale ranging from -60 dBm to 0

dBm, into DC voltage in a linear scale around 1.5 GHz.

8/3/2019 adfm_201002508_sm_suppl

5/11

Submitted to

4 4

1.2. Strain Sensor Contained Liquid Metal Antenna

c)

d)

e)

f)

g)

Blanksheet

Sealinlet3withadhesivetape

Injectupperfluidmetallayer,encapsureoutletsintopreplicawithuncuredPDMSdroplets&sealinlets1&3

Flipsample&injectlowerfluidmetallayer,encapsulateinlet2andoutletsinbottomreplicawithuncuredPDMSdroplets

Removeadhesivetapeoninlets1&3

PDMS Upperfluidmetallayer Adhesivetapefortemporarilysealinginlets

Bottomfluidmetallayer

Topreplica&blanksheetPunchoutaholeforinlet3&sealtobottomreplica

Bottomreplica

Topreplica&blanksheet&bottomreplica

Topreplica&blanksheet&bottomreplica

Bottomreplica&blanksheet&topreplica

Topreplica&blanksheet&bottomreplica

Inlet3

b)

a)

TopreplicabondedtoablankPDMSsheet

Topreplica

To p a nd b ot t om P DM S e la st o me r patterned by replica molding, withpunchedholesforinlets1&2,andoutletsBottomreplica Outlets

Inlet1

Inlet2

Topreplica

Figure S2. Fabrication process steps of the multi-layer microfluidic stretchable patch antenna

integrated in the presented wireless strain sensor.

Minor modifications have been made to adapt the previously reported fabrication processes to

the multi-layer FSRFEs based wireless strain sensor presented in this paper.[1-2]

The

implementation of the integrated sensor device can be briefly summarized as the following

steps: fluidic patch antenna fabrication, active circuit assembly, and hybrid device integration.

The latter two resemble the manufacturing and integration processes presented in the previous

work. Concerning the antenna fabrication, one more PDMS as well as liquid metal alloy

8/3/2019 adfm_201002508_sm_suppl

6/11

Submitted to

5 5

layers have been added to implement multi-layer FSRFEs. More detailed process steps are

illustrated in Figure S2 and described as below:

First of all, the upper (antenna patch) and lower (ground plane) microfluidic channels were

respectively constructed in the top and the bottom PDMS slabs, with a standard soft

lithography. And then a few inlets and outlets were punched as depicted in Figure S2. In

addition, the blank middle PDMS sheet with a thickness of 1.5 mm was also fabricated. Later,

the micro-structured top PDMS layer was bonded to the blank PDMS slab using corona

discharging activation, and the inlet 3, cf. Figure S2, was punched on the bonded PDMS sheet.

Subsequently, the bottom PDMS slab was bonded to the upper PDMS layers with plasma

bonding. After sealing the inlet 3 with a small piece of Scotch

tape, the upper microfluidic

channels were filled with galinstan fluid, and the ventilation outlets in the top PDMS slab

were encapsulated using PDMS prepolymer afterwards. Whereafter, both the inlets 1 and 3

were sealed with Scotch

tape, and galinstan alloy was injected into the lower microfluidic

channels from the bottom side. All remaining ventilation outlets together with the inlet 2 were

encapsulated, and the inlets 1 and 3 were reserved for connecting the RF transmitter circuitry

in the hybrid sensor device.

8/3/2019 adfm_201002508_sm_suppl

7/11

Submitted to

6 6

PDMSUpperfluidmetallayer

CrossView hupper

d

h

f

z

y

x

xy

z

q

TopView

-+

Lowerfluidmetallayer

-+

h lowerhmiddle

Wms

L inset

Wpatch

Lpatch

Lground

Wground

Two openings for connecting RFfeedcableinthestandaloneantennacharacterizations or RF transmittersub-moduleintheintegrateddevice

Figure S3. Geometric configuration of the elastic fluid metal patch antenna. Dimensions are:

Lpatch = 56.0 mm, Wpatch = 50.0 mm,Lground = 110.0 mm, Wground= 80.0 mm,Linset= 16.0 mm,

Wms = 3.7 mm, h = 2.5 mm, hupper= 75.0 m, hmiddle = 1.5 mm, and hlower= 75.0 m.

Assuming that the PDMS elastomer had the relative permittivity of 3.0 and the dissipation

factor of 0.01 around 1.5 GHz, the conductivity of the liquid alloy, galinstan, was about 3.46

106

S/m, and the spacing (hmiddle) and the width (Wpatch) of the patch antenna were 1.5 mm

and 50.0 mm, an initial value of 57.3 mm for Lpatch was found, according to the transmission

line model of the rectangular microstrip patch antennas.[3]

Of course, electrical performance

of the patch antenna based on the proposed geometrical configuration must be verified in

more accurate numerical simulations, and some minor adjustments are probably needed for

8/3/2019 adfm_201002508_sm_suppl

8/11

Submitted to

7 7

tuning its resonant frequency. Since the thickness of the PDMS membrane on top of the

antenna patch is only approximately 425 m, 1/500 of the operational wavelengths at 1.5 GHz,

its influence on the antenna electrical performance are negligible.

1.3. Integrated Wireless Strain Sensor

C

GO1555

b)

a)

c)

d)

e)

f)

C

GO1555GO1555

C

GO1555

PDMSUpperfluidmetallayer

Lowerfluidmetallayer

Kaptonfoil VCOchip

MetallicpinCopper Capacitor Anode Cathode

C

Solder

Figure S4. Schematic drawings of the integration procedure for the demonstrated

microfluidic stretchable wireless strain sensor.

In brief, two tin-plated contact pins resembling cantilevers were first soldered to the RF

output and the ground pads on the FPCB of the RF transmitter, respectively. And then a semi-

spherical solder ball was mounted to the bottom surface of each pin at the other end.

Whereafter, three thin wires were respectively soldered to the supply voltage, the control

voltage, and the ground pads of the RF transmitter circuitry for external power supply

connection, as observed in Figure 1. Subsequently, this RF transmitter sub-module was

attached to the top surface of the previously fabricated liquid metal patch antenna, and each

solder ball at the end of the contact pin was directly immersed in the fluid metal enclosed in

8/3/2019 adfm_201002508_sm_suppl

9/11

Submitted to

8 8

the micro-structured elastomeric channels, via the two opening in the upper PDMS membrane,

cf. Figures 1, S3 and S4. In the end, the complete RF transmitter including the two contact

pins were locally encapsulated in a PDMS LSC for protecting from stretching.

8/3/2019 adfm_201002508_sm_suppl

10/11

Submitted to

9 9

1.4. System Demonstration

RemoteMonitor

PR

IntegratedWirelessStrainSensor

Hornantenna

Voltage-controlledosillator(VCO)

DCpowersupply

Microfluidicelasticantenna

Flexiblecircuitry

PT

5m

RFpowerdetector

MicrovoltDMM

DCpowersupply

PC

Tunableattenuator

DCpowersupply

RFamplifier

Figure S5. Schematic illustration of the system demonstration setup for remotely sensing

mechanical strains in real-time, using the integrated sensor device.

Besides the self-contained sensor device, a custom-designed PC-assisted RF receiver has also

been implemented for remote monitoring in the system demonstration, as illustrated in Figure

S5. Of course, similar receiving function can also be realized using commercially available

RF measurement instruments, for instance, spectrum analyzers or RF power meters, but the

intention here was to remove all costly and bulky RF test equipment, and replace them with

cost-effective modules. Figure S5 displays the schematic illustration of the system

demonstration setup for remotely sensing high tensile mechanical strains in real-time, of

which the receiver part consists of a standard gain horn (Flann Microwave, 08240-10), a

coaxial amplifier (Mini-circuits, ZKL-2R5), a tunable HP attenuator (HP, 8495A), a Keithley

197A autoranging microvolt digital multimeter, a laptop, a DC power supply, and an RF

power detection unit powered by four serially connected AA rechargeable batteries with a DC

supply voltage of 5.23V.

8/3/2019 adfm_201002508_sm_suppl

11/11

Submitted to

10 10

2. Results and Discussion2.1. RF Power Detection Unit

-70 -60 -50 -40 -30 -20 -10 00

0.5

1

1.5

2

2.5

InputRFPower(dBm)

OutputDC

Voltag

e(V)

Figure S6. Measured output DC voltages of the RF power detection sub-module in the

custom-designed RF receiver versus varying input RF power around 1.5 GHz.

[1]

S. Cheng, Z.G. Wu,Lab Chip.

2010

,10

, 3227.

[2] S. Cheng, Z.G. Wu, P. Hallbjorner, K. Hjort, A. Rydberg,IEEE T. Antenn. Propag.2009,57, 3756.

[3] C. A. Balanis,Antenna Theory, 2nd ed, John Wiley & Sons, New York, 1997.