sensors Article

Stretchable Complementary Split Ring Resonator (CSRR)-Based Radio Frequency (RF) Sensor for Strain Direction and Level Detection Seunghyun Eom and Sungjoon Lim * School of Electrical and Electronics Engineering, College of Engineering, Chung-Ang University, 221 Heukseok-dong, Dongjak-gu, Seoul 156-756, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-820-5827 Academic Editors: Ferran Martín and Jordi Naqui Received: 5 August 2016; Accepted: 8 October 2016; Published: 11 October 2016

Abstract: In this paper, we proposed a stretchable radio frequency (RF) sensor to detect strain direction and level. The stretchable sensor is composed of two complementary split ring resonators (CSRR) with microfluidic channels. In order to achieve stretchability, liquid metal (eutectic gallium-indium, EGaIn) and Ecoflex substrate are used. Microfluidic channels are built by Ecoflex elastomer and microfluidic channel frames. A three-dimensional (3D) printer is used for fabrication of microfluidic channel frames. Two CSRR resonators are designed to resonate 2.03 GHz and 3.68 GHz. When the proposed sensor is stretched from 0 to 8 mm along the +x direction, the resonant frequency is shifted from 3.68 GHz to 3.13 GHz. When the proposed sensor is stretched from 0 to 8 mm along the −x direction, the resonant frequency is shifted from 2.03 GHz to 1.78 GHz. Therefore, we can detect stretched length and direction from independent variation of two resonant frequencies. Keywords: stretchable sensor; 3D printing; microfluidic channel; CSRR; EGaIn; Ecoflex

1. Introduction In order to realize stretchable features, depositing thin-film metals on elastomers [1–7], filling microfluidic channels with conducting materials [8,9] and Deep Reactive Ion Etching (DRIE) of silicon [10] are typically used. A common way to fabricate microfluidic channels is sealing a substrate (e.g., glass or PDMS (polydimethylsiloxane)) [11–14]. This is realized by special methods such as thermal evaporation, electro- deposition, e-beam evaporation, sputtering, patterning the metal (lithography), spin coating and ultraviolet exposure. Recently, microfluidic channels have been built by laser ablation [15,16] and a three-dimensional (3D) printer [17,18]. A complementary split ring resonator (CSRR) is a well-known structure. CSRR is the complementary structure of the split ring resonator (SRR). Because CSRR is usually etched on the ground plane of a substrate, it does not require additional area. Therefore, it has the benefit of size reduction. Ecoflex 00-30 is skin-safe and has high tear strength (=38 pli) and high elongation at break (=900%), while PDMS-184 has tear strength (=15 pli) and elongation at break (=160%). Therefore, Ecoflex is suitable for stretchable applications. Figure 1 shows that an Ecoflex substrate with 1 mm thickness has higher stretchability than a PDMS substrate with 1 mm thickness. Recently, there have been many studies for stretchable applications using Ecoflex [19–21]. In case of flexible and stretchable applications, the filling of microfluidic channels with conductive material, liquid metal (eutectic gallium-indium, EGaIn, Ga 75.5% and In 24.5%) has been widely used because it is non-toxic and easy to inject into a microfluidic channel [22–24].

Sensors 2016, 16, 1667; doi:10.3390/s16101667

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Figure 1. AAcomparison comparison between Ecoflex and polydimethylsiloxane substrate: Ecoflex Figure 1. between Ecoflex and polydimethylsiloxane (PDMS)(PDMS) substrate: Ecoflex substrate substrate at (a) non-stretched state and (b) stretched state. PDMS substrate at (c) non-stretched at (a) non-stretched state and (b) stretched state. PDMS substrate at (c) non-stretched statestate and and (d) stretched state. (d) stretched state.

In this paper, a radio frequency (RF) complementary split ring resonator (CSRR) stretchable In this paper, a radio frequency (RF) complementary split ring resonator (CSRR) stretchable sensor sensor is proposed in order to detect strain level. The proposed stretchable sensor can detect strain is proposed in order to detect strain level. The proposed stretchable sensor can detect strain level from level from change of the resonant frequencies. Due to the existence of two different CSRRs, strain change of the resonant frequencies. Due to the existence of two different CSRRs, strain level along level along different directions can be independently detected. In order to simplify the fabrication different directions can be independently detected. In order to simplify the fabrication process, a 3D process, a 3D printer is used for fabrication of microfluidic channels. High stretchable Ecoflex printer is used for fabrication of microfluidic channels. High stretchable Ecoflex elastomer is used as elastomer is used as dielectric material. Eutectic gallium-indium (EGaIn) liquid metal is used for dielectric material. Eutectic gallium-indium (EGaIn) liquid metal is used for conductive patterns. conductive patterns. 2. Operating Principle and Design 2. Operating Principle and Design As illustrated in Figure 2a, the proposed RF stretchable sensor has two different complementary As illustrated in Figure 2a, the proposed RF stretchable sensor has two different complementary split ring resonators (CSRRs) using coplanar waveguide (CPW) technology. In order to realize split ring resonators (CSRRs) using coplanar waveguide (CPW) technology. In order to realize stretchable conductive patterns, EGaIn is injected into the microfluidic channels. The thickness stretchable conductive patterns, EGaIn is injected into the microfluidic channels. The thickness of of conductive patterns is 0.5 mm. The boundary of EGaIn is assigned as finite conductivity with conductive patterns is 0.5 mm. The boundary of EGaIn is assigned as finite conductivity with 3.4 × 106 Siemens/m. Microfluidic channels are built from two Ecoflex 00-30 (Smooth-On Inc., 3.4 × 106 Siemens/m. Microfluidic channels are built from two Ecoflex 00-30 (Smooth-On Inc., Easton, Easton, PA, USA) materials. Top and bottom Ecoflex layers have 1.5 mm and 2 mm thickness, PA, USA) materials. Top and bottom Ecoflex layers have 1.5 mm and 2 mm thickness, respectively. respectively. The relative permittivity and dielectric loss of Ecoflex are 2.8 and 0.02, respectively. The relative permittivity and dielectric loss of Ecoflex are 2.8 and 0.02, respectively. The bottom layer The bottom layer (thickness = 2 mm) has CPW (coplanar waveguide) channel (thickness = 0.5 mm) (thickness = 2 mm) has CPW (coplanar waveguide) channel (thickness = 0.5 mm) and two and two complementary split ring resonators (CSRRs) are different sizes. complementary split ring resonators (CSRRs) are different sizes. Two CSRRs are designed to resonate at 2.03 GHz and 3.63 GHz. Figure 3 shows the Two CSRRs are designed to resonate at 2.03 GHz and 3.63 GHz. Figure 3 shows the equivalent equivalent circuit model of the CSRR. The CSRR structure behaves as a parallel LC resonant circuit. circuit model of the CSRR. The CSRR structure behaves as a parallel LC resonant circuit. The The capacitance Cc is the etched pattern of the CSRR ellipse. The inductance Lc is the inner ellipse capacitance Cc is the etched pattern of the CSRR ellipse. The inductance Lc is the inner ellipse of the of the conductive pattern to the ground [25–28]. Therefore, the resonant frequency of the CSRR is conductive pattern to the ground [25–28]. Therefore, the resonant frequency of the CSRR is determined by determined by 1 √ f = , (1) 2π 1 Lc Cc f  , (1) 2 L C c

c

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11 CSRRs is stretched without deforming the other CSRR, the electrical3 oflength When one of two is stretched withoutin deforming other CSRR, theresonant electricalfrequency length of of of the stretched CSRR is CSRRs increased which results higher Lthe the c . Therefore, When one of two CSRRs is which stretched without deforming the otherthe CSRR, the electrical length of the stretched CSRR is increased results in higher L c . Therefore, resonant frequency of the the stretched CSRR is decreased. In the meantime, the resonant frequency of the other CSRR is not the stretched CSRR is increased which results in higher L c. Therefore, the resonant frequency of the stretched CSRRitsiselectrical decreased.length In theismeantime, the Therefore, resonant frequency of the other CSRR is not changed because unchanged. strain level from two CSRR can be stretchedbecause CSRR isitsdecreased. In the is meantime, theTherefore, resonant frequency offrom the other CSRRcan is not changed electrical length unchanged. strain level two CSRR be independently detected from variation of resonant frequencies of two different CSRRs. changed because its electrical length is unchanged. Therefore, strain level from two CSRR can be independently detected from variation of resonant frequencies of two different CSRRs. In order to design the proposed RF sensor, finite-element-method (FEM)-based ANSYS independently from variationRF of sensor, resonantfinite-element-method frequencies of two different CSRRs.ANSYS highhigh In order todetected design the proposed (FEM)-based frequencyInstructure simulator (HFSS) is used. First of all, a 50-ohm CPW transmission line is designed order to design the proposed sensor, finite-element-method (FEM)-based high frequency structure simulator (HFSS) is RF used. First of all, a 50-ohm CPW transmission lineANSYS is designed as the feeding line. The width of the signal is 4 mm and the gap between signal and ground (GND) frequency structure simulator (HFSS) is used. First of all, a 50-ohm CPW transmission line is designed as the feeding line. The width of the signal is 4 mm and the gap between signal and ground (GND) as feeding line. The width of the signal is 4 mm and the gap between signal and ground (GND) lineslines is the 0.8 mm. Next, two CSRR are designed on the GND plane of the CPW. The CSRR #1 is realized is 0.8 mm. Next, two CSRR are designed on the GND plane of the CPW. The CSRR #1 is realized lines 0.8GND mm. Next, twothe CSRR are designed on the plane of the CPW. The CSRR #1 isfrequencies realized on the left GND plane while CSRR #2 onGND theright right GND plane. The resonant on theisleft plane while the CSRR #2isisrealized realized on the GND plane. The resonant frequencies on the left GND plane while the CSRR #2 is realized on the right GND plane. The resonant frequencies of the #1 and #2 #2 areare 2.03 GHz GHz(f(f2),2 ),respectively. respectively. of CSRR the CSRR #1 and 2.03 GHz(f(f1 1))and and 3.63 GHz

of the CSRR #1 and #2 are 2.03 GHz (f1) and 3.63 GHz (f2), respectively.

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(c) (c) Figure 2. (a) Layout of the complementary split ring resonators (CSRRs); (b) Bird’s eye view of the Figure 2. (a)2.Layout of the complementary split ring (CSRRs);(b) (b) Bird’s eye view of the Figure (a) Layout of the complementary ring resonators resonators (CSRRs); proposed stretchable sensor; (c) Side view of split the proposed stretchable sensor. Bird’s eye view of the proposed stretchable sensor; (c) Side view of the proposed stretchable sensor. proposed stretchable sensor; (c) Side view of the proposed stretchable sensor.

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(c) (c) Figure 3. (a) Layout and (b) equivalent circuit model of the CSRR; (c) Equivalent circuit model of the Figure 3. sensor (a) Layout and (b) #1 equivalent circuit model of the CSRR; (c) Equivalent circuit model of the proposed and #2.circuit Figure 3. (a) Layoutwith and CSRR (b) equivalent model of the CSRR; (c) Equivalent circuit model of the proposed sensor with CSRR #1 and #2.

proposed sensor with CSRR #1 and #2.

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Figure shows equivalent circuit model of the proposed RF sensor CSRRs. Its Figure 3c3c shows thethe equivalent circuit model of the proposed RF sensor withwith twotwo CSRRs. Its circuit circuit parameters C1, LC 2, and C2) are extracted from the S-parameters. The equivalent inductance parameters (L1 , C1 , (L L21,, and 2 ) are extracted from the S-parameters. The equivalent inductance (L1 ) (L 1) and capacitance (C1) of the un-stretched CSRR #1 are 2.38 nH and 2.69 pF, respectively. The and capacitance (C1 ) of the un-stretched CSRR #1 are 2.38 nH and 2.69 pF, respectively. The equivalent equivalent(Linductance (L2) and capacitance (C2) of the un-stretched CSRR #2 are 1.06 nH and 1.85 pF, inductance 2 ) and capacitance (C2 ) of the un-stretched CSRR #2 are 1.06 nH and 1.85 pF, respectively. respectively. The simulated transmission coefficients of the proposed RF sensor are shown in Figure 4. When the The simulated transmission coefficients of the proposed RF sensor are shown in Figure 4. When RF sensor is not stretched, the CSRRs #1 and #2 resonate at 2.03 GHz (f 1 ) and 3.63 GHz (f 2 ), respectively, the RF sensor is not stretched, the CSRRs #1 and #2 resonate at 2.03 GHz (f1) and 3.63 GHz (f2), as shown in Figure 4a. respectively, as shown in Figure 4a. Figure 4b shows the simulated transmission coefficient when the proposed sensor is stretched Figure 4b shows the simulated transmission coefficient when the proposed sensor is stretched by L = 4 mm along −x direction and along +x direction is not stretched. The resonant frequency by L = 4 mm along −x direction and along +x direction is not stretched. The resonant frequency of f1 of decreases f 1 decreases 1.92while GHzf2 is while f 2 is notFigure changed. Figure 4c shows transmission the simulated fromfrom 2.03 to2.03 1.92to GHz not changed. 4c shows the simulated transmission coefficient when the proposed sensorbyisLstretched by L = mm along direction coefficient when the proposed sensor is stretched = 4 mm along +x4direction and+x along −x and along − x direction is not stretched. The resonant frequency of f decreases from 3.63 to 3.43 GHz direction is not stretched. The resonant frequency of f2 decreases from2 3.63 to 3.43 GHz while f1 is not while f is not changed. Finally, when the proposed sensor is stretched by L = 4 mm along +x direction 1 changed. Finally, when the proposed sensor is stretched by L = 4 mm along +x direction and L = 4 mm and L = −x 4 mm along Figure −x direction, Figure shows that f1 and f2 2.03 decrease from 2.03 1.92 3.63 GHztoand along direction, 4d shows that 4d f1 and f2 decrease from to 1.92 GHz andtofrom from to respectively. 3.43 GHz, respectively. 3.433.63 GHz,

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Figure 4. Simulated transmission coefficient results of the proposed stretchable sensor (a) when the Figure 4. Simulated transmission coefficient results of the proposed stretchable sensor (a) when the sensor is not stretched; (b) when the proposed sensor is stretched by 4 mm along −x direction; (c) sensor is not stretched; (b) when the proposed sensor is stretched by 4 mm along −x direction; (c) when when the proposed sensor is stretched by 4 mm along +x direction and (d) when the proposed sensor the proposed sensor is stretched by 4 mm along +x direction and (d) when the proposed sensor is is stretched by L = 4 mm along +x direction and L = 4 mm along −x direction. stretched by L = 4 mm along +x direction and L = 4 mm along −x direction.

Figure 5 shows the magnitudes of electric field distribution when the proposed sensor is not Figure 5The shows the magnitudes electricresponse field distribution whenfrequencies. the proposed is not stretched. CSRR provides strongofelectric at the resonant At sensor 2.03 GHz, stretched. The CSRRoccurs provides strong electric response the GHz, resonant frequencies. 2.03 GHz,from electric electric resonance from slot of the CSRR #1. Atat3.63 electric resonant At is generated the slot of the CSRR resonance occurs from#2. slot of the CSRR #1. At 3.63 GHz, electric resonant is generated from the slot of the CSRR #2.

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(c) Figure5.5. Simulated Simulated magnitude of of thethe proposed stretchable sensor at (a)at Figure magnitudeofofelectric electricfield fielddistribution distribution proposed stretchable sensor 2.03 GHz and (b) 3.63 GHz; (c) Simulated vector of excited electric field on the wave port. (a) 2.03 GHz and (b) 3.63 GHz; (c) Simulated vector of excited electric field on the wave port.

3. Fabrications and Measurement Results 3. Fabrications and Measurement Results Figure 6 shows the fabrication process. The microfluidic channel frame is first built using a 3D Figure 6 shows the fabrication process. The microfluidic channel frame is first built using a 3D printer (Ultimaker2, Ultimaker BV, Geldermalsen, Netherlands). After considering stretched liquid printer (Ultimaker2, Ultimaker BV, Geldermalsen, Netherlands). After considering stretched liquid metal and printing resolution, the channel depth is 0.5 mm which becomes the thickness of liquid metal and printing resolution, the channel depth is 0.5 mm which becomes the thickness of liquid metal. Next, the substrate is realized using the Ecoflex. Ecoflex silicone elastomer base and curing metal. Next, the substrate is realized using the Ecoflex. Ecoflex silicone elastomer base and curing agent is mixed at a ratio of 1:1. After mixing, a vacuum chamber is used to remove air bubbles of agent mixed atAlthough a ratio ofthe 1:1. Aftersolution mixing,can a vacuum used to remove airhours, bubbles mixedissolution. mixed be curedchamber at a roomistemperature for four it isof mixed solution. Although the mixed solution can be cured at a room temperature for four hours, it is cured at 100 °C for one hour in order to reduce a fabrication time. Before curing, we poured uncured

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◦ C1667 Sensorsat2016, 6 of 11 cured 10016, for one hour in order to reduce a fabrication time. Before curing, we poured uncured solution to the microfluidic channel frame. A copper tape and SMA connector are attached solution to the microfluidic channel frame. copper tape and SMA connector are attached to to the the solution to the microfluidic channel frame. A copper tape and SMA connector are attached to the cured cured substrate. substrate. Each Each layer layer is is bonded bonded by by the the uncured uncured Ecoflex. Ecoflex. Finally, Finally, in in order order to to realize realizeconductive conductive cured substrate. Each layer is bonded by the uncured Ecoflex. Finally, in order to realize conductive patterns, patterns,EGaIn EGaInisisinjected injectedinto intothe themicrofluidic microfluidicchannels channelsusing usingthe thesyringe. syringe. patterns, EGaIn is injected into the microfluidic channels using the syringe.

Figure 6. 6. Fabrication Fabrication sensor. Figure process of the proposed stretchable sensor. Figure 6. Fabricationprocess processof ofthe theproposed proposedstretchable stretchable sensor.

Figure 7a shows the picture of the fabricated RF sensor prototype. The transmission Figure shows picture the fabricated RFprototype. sensor prototype. The transmission Figure 7a7a the the picture of sensor theoffabricated RF sensor transmission coefficients ofshows the fabricated RF are measured using Anritsu The MS2038C vector coefficients network coefficients of the fabricated RF sensorusing are measured using Anritsu MS2038C vector In network of the fabricated RF sensor are measured Anritsu MS2038C vector network analyzer. order analyzer. In order to stretch the fabricated RF sensor, the clamping fixture is used as shown in analyzer. In order to stretch the fabricated RF sensor, the clamping fixture is used as shown in toFigure stretch7b. theInfabricated RF sensor, the clamping fixture is used as shown in Figure 7b. In Figure Figure 8, the simulated and measured transmission coefficients of the proposed RF 8, Figure 7b. In Figure 8, the simulated and measured transmission coefficients of the proposed RF the simulated and measured transmission coefficients of the proposed sensor areare compared sensor are compared when it is not stretched. The measured resonantRF frequencies 2.03 GHzwhen and it sensor are compared when it isresonant not stretched. The measured resonant frequencies are the 2.03simulated GHz and is not stretched. The measured frequencies are 2.03 GHz and 3.68 GHz while 3.68 GHz while the simulated resonant frequencies are 2.03 GHz and 3.63 GHz. In EM simulation, 3.68 GHzfrequencies while the simulated resonant frequencies are 2.03 GHz and 3.63 GHz. constant In EM simulation, resonant areof2.03 GHz 3.63 GHz. In simulation, dielectric of Ecoflex the dielectric constant Ecoflex isand characterized at EM a low frequencythe around 2 GHz. Because the the dielectric constant of Ecoflex is characterized at a low frequency around 2 GHz. Because isdielectric characterized at a low frequency around 2 GHz. Because the dielectric constant of Ecoflex is slightly constant of Ecoflex is slightly decreased with a higher frequency, a slight difference isthe dielectric constant of Ecoflex is aslightly decreased with a higher frequency, a slight is decreased a higher frequency, slight difference at the resonant frequency of difference the CSRR #2. occurredwith at the resonant frequency of the CSRR #2.is occurred occurred at the resonant frequency of the CSRR #2.

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(a)fabricated RF sensor and (b) the test setup. The fabricated (b) Figure7.7.Pictures Picturesof of (a) (a) sensor is fixed to a to Figure fabricated RF sensor and (b) the test setup. The fabricated sensor is fixed clamping fixture. The stretching length (L) can be manually varied from 0 mm to 16 mm (= 8 mm aFigure clamping fixture.ofThe stretching RF length (L) and can (b) be manually varied 0 mm to 16 mm (=8 mm 7. Pictures (a) fabricated sensor the test setup. Thefrom fabricated sensor is fixed to a along +x direction +8 mm along −x direction). along +x direction along −length x direction). clamping fixture. +8 Themm stretching (L) can be manually varied from 0 mm to 16 mm (= 8 mm along +x direction +8 mm along −x direction).

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Transmission coefficients coefficients of of the the proposed proposed RF RF sensor from circuit simulation, EM simulation Figure 8. Transmission measurement. and measurement.

Figure 99shows measured transmission coefficients at different stretching length (L). Because Figure showsthethe measured transmission coefficients at different stretching length (L). the clamping fixture has a step of 0.2 mm, the minimum stretchable range is 0.57% stain. Due to Figure 8. Transmission coefficients of the proposed RF sensor from circuit simulation, EM simulation Because the clamping fixture has a step of 0.2 mm, the minimum stretchable range is 0.57% stain. and measurement. limitation of the measurement setup, lower than 0.57% strain cannot be measured. Because of the Due to limitation of the measurement setup, lower than 0.57% strain cannot be measured. Because of mechanical strength of the Ecoflex material, the maximum detectable range isis22.86% of and the mechanical strength ofthe the Ecoflex material, the maximum detectable range 22.86% of strain strain and Figure 9 shows measured transmission coefficients at different stretching length (L). Because critical frequencies are 1.78 GHz and 3.13 GHz which are the minimum and maximum frequencies, critical frequencies 1.78has GHz and GHz are stretchable the minimum maximum frequencies, the clampingare fixture a step of 3.13 0.2 mm, thewhich minimum range and is 0.57% stain. Due to respectively. When the fabricatedsensor sensor stretched byLLstrain mm along the−−x direction and mm limitation ofthe the fabricated measurement setup,isis lower than 0.57% cannot be the measured. Because and of the respectively. When stretched by ==44mm along x direction LL== 00 mm strength the Ecoflex material, the9a maximum is 22.86% of strain andfrom along the themechanical +x direction, it is isofobserved from Figure 9a that the thedetectable resonantrange frequency from 2.03 2.03 along +x direction, it observed from Figure that resonant frequency ff11 decreases decreases critical frequencies are 1.78 GHz and 3.13 GHz which are the minimum and maximum frequencies, to 1.97 GHz while f 2 is not changed. When the fabricated sensor is stretched by L = 4 mm along +x to 1.97 GHz while f 2 is not changed. When the fabricated sensor is stretched by L = 4 mm along +x respectively. When the fabricated sensor is stretched by L = 4 mm along the −x direction and L = 0 mm direction and and L==00mm mmalong along−−x direction,ititisisobserved observedfrom fromFigure Figure9b9b that the resonant frequency direction x direction, that resonant f2 along L the +x direction, it is observed from Figure 9a that the resonant frequency f1 the decreases fromfrequency 2.03 fdecreases 2 decreases from 3.68 to 3.52 GHz while f1 is not changed. Figure 9c shows the measured transmission 3.68while to 3.52 while f 1When is not shows the transmission tofrom 1.97 GHz f2 isGHz not changed. thechanged. fabricatedFigure sensor is9cstretched by Lmeasured = 4 mm along +x coefficients when the fabricated sensor is stretched stretched by LL mm along along +x resonant direction and L L == 44 mm mm direction and L =fabricated 0 mm alongsensor −x direction, it is observed from 9b that +x the frequency coefficients when the is by == 44Figure mm direction and f 2 decreases from 3.68 to 3.52 GHz while f 1 is not changed. Figure 9c shows the measured transmission along − −xx direction. along direction. ff1 1and andf2f 2decrease decreasefrom from2.03 2.03toto1.97 1.97GHz GHzand andfrom from3.68 3.68to to3.52 3.52GHz, GHz, respectively. respectively. coefficients when the fabricatedthe sensor is stretched by L = 4 mm along +x direction and L4.8333 = 4 mmand 4.58, When the sensor is not stretched, measured Q-factors of CSRR #1 and #2 are When the sensor is not stretched, the measured Q-factors of CSRR #1 and #2 are 4.8333 and 4.58, along −x direction. f1 and f2 decrease from 2.03 to 1.97 GHz and from 3.68 to 3.52 GHz, respectively. respectively. When When the the stretching stretchinglength length(L) (L)isissimultaneously simultaneouslyvaried variedalong alongthe the+x +xand and − −xx directions, respectively. directions, When the sensor is not stretched, the measured Q-factors of CSRR #1 and #2 are 4.8333 and 4.58, the transmission transmission coefficients are measured as shown in Figure 9d. L is varied from 0 to 16 mm mm in in the coefficients are measured as shown in Figure 9d. L is varied from 0 to 16 respectively. When the stretching length (L) is simultaneously varied along the +x and −x directions, increments of 4 mm (L of +x direction is varied from 0 to 8 mm in increments of 2 mm and L of −x the transmission coefficients are measured as shown in Figure 9d. L is varied from 0 to 16 mm in increments of 4 mm (L of +x direction is varied from 0 to 8 mm in increments of 2 mm and L of increments offrom 4 mm0(L is varied from to 8 mm in increments offrequencies 2 mm and L of −x listed in direction is varied 8 +x mm inin increments ofof202mm). resonant are − x direction is varied fromto 0ofto 8 direction mm increments mm).Their Their resonant frequencies are listed in direction is varied from 0 to 8 mm in increments of 2 mm). Their resonant frequencies are listed in Table 1. Although the stretched physical lengths along +x and −x directions are same, the frequency Table 1. Although the stretched physical lengths along +x and −x directions are same, the frequency Table 1. Although the stretched physical lengths along +x and −x directions are same, the frequency shift is is different different because because the the electrical length length at the the resonant resonant frequencies frequencies of the the CSRR CSRR #1 #1 and and #2 #2 are are shift shift is different becauseelectrical the electrical lengthat at the resonant frequencies of theofCSRR #1 and #2 are differentdifferent each other. other. different each each other.

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Figure 9. Cont.

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9. Measured transmission coefficients of the fabricated radio frequency (RF) sensor when (a) Figure 9.Figure Measured transmission coefficients of the fabricated radio frequency (RF) sensor when (a) the the fabricated sensor is stretched by L = 4 mm along −x direction; (b) the fabricated sensor is stretched fabricated sensor is stretched by L = 4 mm along −x direction; (b) the fabricated sensor is stretched by by L = 4 mm along +x direction; (c) the fabricated sensor is stretched by L = 4 mm along +x direction L = 4 mm along (c) the and fabricated sensor is stretched L = 4 mm along +x direction and and L = 4 +x mmdirection; along −x direction (d) L is varied from 0 mm to 16by mm. L = 4 mm along −x direction and (d) L is varied from 0 mm to 16 mm. Table 1. Relationship between the resonant frequency and strain level of L.

Table 1. Relationship the resonant frequency and of #2 L. (GHz) Strainbetween (%) CSRR #1 strain (GHz) level CSRR 0 11.42 = 5.71 (along +xStrain direction) (%) + 5.71 (along −x direction) 22.84 = 11.42 (along +x direction) + 11.42 (along −x direction) 0 34.28 = 17.14 (along +x direction) + 17.14 (along −x direction) 11.4245.72 = 5.71 (along +x direction) + 5.71 (along −x−xdirection) = 22.86 (along +x direction) + 22.86 (along direction)

2.03

CSRR 1.97 #1 (GHz) 1.92

2.03 1.86 1.97 1.78 1.92 1.86 1.78

22.84 = 11.42 (along +x direction) + 11.42 (along −x direction) 34.28 = 17.14 (along +x direction) + 17.14 (along −x direction) The frequency change (Δf) is defined as 45.72 = 22.86 (along +x direction) + 22.86 (along −x direction)

f  f non  stretched  f stretched ,

3.68 3.52#2 (GHz) CSRR 3.37 3.68 3.25 3.52 3.13

3.37 3.25 3.13

(1)

Thewhere frequency change (∆f ) is defined as fnon-stretched and fstretched are the measured resonant frequencies in the non-stretched and stretched states, respectively. Let us now define relative resonant frequency change as

∆ f = | f non−stretched − f stretched | , Relative frequency change =

f f0

(2) (2)

where fnon-stretched and fstretched are the measured resonant frequencies in the non-stretched and stretched reference frequency non-stretched state.frequency change as where f0 is theLet states, respectively. us now defineatrelative resonant In this work, the sensitivity (SL) is defined as

  f f0 change Relative Sfrequency  mm-1 = L

l





∆f f0

(3)

(3)

is stretching length. where f 0where is theLreference frequency at non-stretched state. Figure 10a shows the measured resonant frequencies when the fabricated sensor is stretched In this work, the sensitivity (SL ) is defined as

along +x and −x direction from −22.86% to +22.86%. When L is increased from 0% to 22.86% along +x direction, the resonant frequency is decreased from 2.03 to 1.78  GHz. When L is increased from 0% δ (∆ f / f 0 )  to 22.86% along −x direction, the resonant mm−1 from 3.63 to 3.13 GHz. Figure 10b,c (4) S L = frequency is decreased δl show the frequency change (Δf) versus stretching length for CSRR #1 and #2, respectively. The is linear and their fitted curves are y = 11.25x − 7.175 (MHz/%) and y = 24x + 1.36 (MHz/%), where L relationship is stretching length. respectively. Figure 10d shows the relative frequency change (Δf/f0) versus strain level from 5.72% to Figure 10aforshows the measured resonant frequencies when0.0192 the fabricated sensor is stretched 22.86% each CSRR. The sensitivity is also indicated as maximum mm−1.

along +x and −x direction from −22.86% to +22.86%. When L is increased from 0% to 22.86% along +x direction, the resonant frequency is decreased from 2.03 to 1.78 GHz. When L is increased from 0% to 22.86% along −x direction, the resonant frequency is decreased from 3.63 to 3.13 GHz. Figure 10b,c show the frequency change (∆f ) versus stretching length for CSRR #1 and #2, respectively. The relationship is linear and their fitted curves are y = 11.25x − 7.175 (MHz/%) and y = 24x + 1.36 (MHz/%), respectively. Figure 10d shows the relative frequency change (∆f /f 0 ) versus strain level from 5.72% to 22.86% for each CSRR. The sensitivity is also indicated as maximum 0.0192 mm−1 .

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(a)

(b)

(c)

(d)

Figure 10. Experimental characterization of the proposed sensor under tension: (a) resonant frequency

Figure 10. Experimental characterization of the proposed sensor under tension: (a) resonant frequency versus stretching length along +x and −x direction; (b) frequency change (Δf) when only CSRR #1 is versus stretching length along +x and −x direction; (b) frequency change (∆f ) when only CSRR #1 is stretched from 2.86% to 22.86%; and (c) frequency change (Δf) when only CSRR #2 is stretched from stretched from 2.86% to 22.86%; and (c) frequency change (∆f ) when only CSRR #2 is stretched from 2.86% to 22.86%; and (d) relative frequency change (Δf/f0) of CSRR #1 and #2. 2.86% to 22.86%; and (d) relative frequency change (∆f /f 0 ) of CSRR #1 and #2.

In Table 2, the proposed sensor is compared with other RF strain sensors in [29–33]. Because the In Table sensor 2, the proposed compared with other RF strain in [29–33]. Because proposed is built onsensor liquid is metal and Ecoflex substrate, widersensors frequency tuning range andthe proposed sensor is built on liquid metal and substrate, wider frequency tuning range and higher strain level are achieved compared withEcoflex other sensors. In addition, rapid prototyping with 3D printing provides theachieved flexibilitycompared required to make thissensors. crucial trial and errorrapid process possible for higher strain level are with other In addition, prototyping with withrequired RF sensors low lossy substrates in [32,33], the Q-factor of thefor 3D physical printingproducts. providesCompared the flexibility towith make this crucial trial and error process possible proposed sensor Compared is relativelywith low due to dielectric stretchable materials. physical products. RF sensors withloss lowoflossy substrates in [32,33], the Q-factor of the

proposed sensor is relatively low due to dielectric loss of stretchable materials.

Table 2. Comparison Table of Proposed Sensor and Other RF Strain Sensors.

Table 2. Comparison Table Sensor and Other This Work [29]of Proposed [30] [31] RF Strain Sensors. [32] [33] Substrate Ecoflex Kapton tape Si PDMS RT/Duroid 5880 Kapton Conductive This Work [29] [30] [31] [32] [33] EgaIn Au Au Au Cu Cu/Al Material RT/Duroid Kapton Substrate Ecoflex Kapton tape Si PDMS Spin coating, Lithography, 5880 Fabrication Lithography, Lithography, 3D printing Evaporation, Lithography Manual Conductive EgaIn Au Au Au Cu Cu/Al Method Material Evaporation Evaporation Photolithography assembly Spin coating, Lithography, Lithography, Lithography, Maximum Strain Fabrication Method 3D printing Evaporation, Lithography Manual 45.72 N/A N/A 9.78 0.2 1 Evaporation Evaporation Level (%) Photolithography assembly Tuning Ratio (%) 12.31, 14.95 5.69 0.21 5.14 0.14 2.35 Maximum Strain Level (%) 45.72 N/A N/A 9.78 0.2 1 Number Of 2 1 1 2 1 2 Tuning Ratio (%) 12.31, 14.95 5.69 0.21 5.14 0.14 2.35 Detect Direction Number Of Detect Resonant 2.03, 3.682 Direction Frequency (GHz) Resonant Frequency (GHz) 4.58~4.83 2.03, 3.68 Q factor Resonator Metamaterial Q factor 4.58~4.83 Resonator

12.31

1 0.4742

7002

† 12.3 12–18 Metamaterial 12–18 †

0.4742 N/A Metamaterial N/A

† 3~4700 Metamaterial 3~4 †

It is estimated from frequency response figures. Metamaterial Metamaterial Metamaterial Metamaterial †



It is estimated from frequency response figures.

51

2 3.62

5 † 40~49 † Patch40~49 Antenna

3.62† 40~50 † Open loop 40~50

Patch Antenna

Open loop

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4. Conclusions In conclusion, we proposed a stretchable RF sensor. The proposed sensor consists of the Ecoflex layers with microfluidic channel. Each layer is bonded by uncured Ecoflex. 3D-printing technology is adopted to realize the conductive CSRR patterns with liquid metal on stretchable Ecoflex substrate. The microfluidic channel frames with Ecoflex elastomer is used for the stretchable substrate. The proposed stretchable sensor can detect the resonant frequency change. When the fabricated sensor is stretched along the +x and −x direction from −22.86% to +22.86%, the resonant frequencies of CSRR#1 and CSRR #2 are changed from 2.03 GHz to 1.78 GHz and from 3.68 GHz to 3.13 GHz, respectively. The relationship of frequency change (∆f ) versus stretching length for CSRR #1 and #2 is linear. The fitted curves of CSRR #1 and CSRR #2 are y = 11.25x − 7.175 and y = 24x + 1.36. When we stretched the proposed stretchable sensor, the resonant frequency is changed because the electrical length of resonator is changed. Because each resonator is working independently, we can detect direction and stretched length depending on strain level from independent variation of two resonant frequencies. The Q-factor of the proposed sensor can be further improved by using low lossy stretchable materials. Acknowledgments: This research was supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2016-H8501-16-1007) supervised by the IITP (Institute for Information & communications Technology Promotion) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A1A11050010). Author Contributions: Seunghyun Eom designed, analyzed, fabricated and measured the sample. Sungjoon Lim conceived the idea and contributed to revising of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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Stretchable Complementary Split Ring Resonator (CSRR)-Based Radio Frequency (RF) Sensor for Strain Direction and Level Detection.

In this paper, we proposed a stretchable radio frequency (RF) sensor to detect strain direction and level. The stretchable sensor is composed of two c...
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