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Low-costultra-stretchablestrainsensorsformonitoringhumanmotionandbio-signals

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DOI:10.1016/j.sna.2018.01.028

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Sensors and Actuators A 271 (2018) 182–191

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

j ourna l ho me page: www.elsev ier .com/ locate /sna

ow-cost ultra-stretchable strain sensors for monitoring humanotion and bio-signals

eyed Reza Larimia, Hojatollah Rezaei Nejadb, Michael Oyatsi a, Allen O’Briena,ina Hoorfara, Homayoun Najjarana,∗

School of Engineering, The University of British Columbia, Kelowna, BC, V1V 1V7, CanadaElectrical and Computer Engineering Department Tufts University, 200 Boston Ave. Suite 2700, Medford, MA 02155, USA

r t i c l e i n f o

rticle history:eceived 8 October 2017eceived in revised form 9 December 2017ccepted 13 January 2018vailable online 22 January 2018

eywords:train sensorsraphene

a b s t r a c t

The emerging need for interactive wearable devices has been a driving force for the development of flex-ible electronics. Due to their ability to conform to the complex nature of the human physique, stretchablestrain sensors have been extensively used to measure bio-signals and monitor human motion. Here, anew fabrication method of a piezo-resistive strain sensor is introduced, and the ability of the sensor tomeasure the human heartbeat and track a wide range of human motion is demonstrated. The sensoris fabricated by infusing graphene nano-flakes into a rubber-like adhesive pad. The fabricated sensor ishighly stretchable and can withstand strain up to 350%. Even after 10,000 cycles of stretching and relax-ing a robust and stable electrical response is maintained. Furthermore, the gauge factor of the sensor

dhesive padsuman motion monitoringio-signals

ranges from 2 to upto 160; which in turn allows the sensor to monitor a great variety of human motions.Hence, three wearable devices are developed using the fabricated graphene-based strain sensor to mea-sure human knee movement, finger movement and heartbeat through the radial artery. The sensor isalso used in a robotic haptic application to control a robotic finger. These experiments demonstrate theapplicability of the sensor for real-time monitoring, specifically in wearable human interactive devices.

© 2018 Elsevier B.V. All rights reserved.

. Introduction

Recent advances in wearable devices have shown substan-ial promise for applications in health monitoring and biomedicalelds. On the other hand, miniaturizing electronic technology hasnabled researchers to introduce ad hoc wearable devices for cer-ain applications. However, realizing the full potential of wearableevices for commercial purposes still requires advancements inensitivity, stretchability and flexibility [1–5] of the sensing ele-ents to easily conform to broad varieties of human physique

nd respond to large and small joint movements, such as bend-ng and rotating. Consequently, flexible electronics are accepted asn excellent candidate to be used in wearable devices causing lessiscomfort while measuring human bio-signals and physiologicalctivities [6–9].

Stretchable strain sensors are a specific type of flexible elec-ronics that allow precise measurement of deformation in wearableevices. There have been many endeavours to make highly stretch-

∗ Corresponding author.E-mail address: homayoun.najjaran@ubc.ca (H. Najjaran).

ttps://doi.org/10.1016/j.sna.2018.01.028924-4247/© 2018 Elsevier B.V. All rights reserved.

able and flexible strain sensors that are more compliant withhuman body specifications and have reasonable size, sensitivity,performance, production cost, and robustness in various envi-ronments [10–14]. In that regard, stretchable and flexible strainsensors should be sensitive enough to respond accordingly to dif-ferent and complex body motions that may range from infinitesimalmovements and bio-signals (e.g., heartbeat) to large stroke musclemovements (e.g., walking and running) [1,15,16].

Strain sensors are generally categorized into three groups:piezo-voltage, piezo-capacitive, and piezo-resistive. Piezo-voltagestrain sensors are effective at detecting small strains due totheir large gauge factors (GFs). However, they are not inadequatefor human motion monitoring because they are not sufficientlystretchable and cannot withstand large strains (<30%). [5,17,18]Piezo-capacitive strain sensors have a more linear response anda lower hysteresis but have a lower gauge factor (GF < 1). [1,19,20]Among the three major strain sensor groups, piezo-resistive strainsensors have received the most attention due to both their sim-plicity and their tunable mechanical and electrical properties.

However, the low gauge factor (GF < 2) [21–23], low range oftolerable strain (� < 5%) [15,24], and high brittleness (Young’s mod-ulus > 1 × 1010 Pa) [25] of the previously reported piezo-resistive

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train sensors hindering their full functionality in wearable devices.o better facilitate the interaction of piezo-resistive strain sen-or with the human body, their sensing ability and mechanicalompliance has been improved through recent use of various mate-ials and fabrication techniques. These include implementation ofanoscale conductive films on flexible substrates, developmentf metallic nanocomposite structures [26–28], and most recently,he use of carbon nanotubes [23,29–31], nanowires [21,32–34],nd graphene nano-powder on flexible substrates [35–39]. Despitehese recent advancements, stretchable strain sensors require fur-her development to meet the necessary criteria for conformingo complex human motions. As previously mentioned, this crite-ia includes reasonable sensitivity and stretchability, robustnessn various environments and durability as well as easy fabricationrocess and low production cost [11,40–42].

The aim of this study is to address the current lack of ade-uate flexible electronics technology for versatile wearable devicesy fabricating a new stretchable strain sensor and applying ito interactive human-machine interface applications. This wasccomplished by developing an easy fabrication method for a lowost, highly stretchable piezo-resistive strain sensor (capable ofetecting a wide range of strain) and utilizing it for both humanotion monitoring and haptic implementation. The sensor was fab-

icated by infusing graphene nano-flakes (GNFs) into an adhesivead and exhibits superior mechanical properties that can withstandore than 350% strain. With our particular fabrication method weade a graphene-based strain sensor with an initial resistance as

ow as 8 k� and a gauge factor as high as 161. The sensor has ainear response below 40% strain and exhibits very low hystere-is up to 100% strain. The fabricated sensor is also omnidirectionalnd is sensitive to different types of deformation including bend-ng, twisting, compressing and stretching. To utilize the sensor foruman motion monitoring, we embedded the sensor in an Ecoflex

®

ubstrate (a widely used skin compatible polymer). The sensor isapable of detecting strains as small as a human pulse and as larges human knee movement; since the sensor also has a fast recoveryime, this enabled us to measure various human physical activitiese.g. walking and running).

. Results and discussion

A fabrication method for development of a new stretchablend flexible strain sensor has been achieved by infusing GNFs intoano-pores of a commercially available rubber-like adhesive padRe-POP

TMClear Adhesive Pad) (Fig. 1). Initially, the pad was placed

nto an acetone bath; this resulted in the pad to undergo volu-etric expansion and increased to over twice its initial size. This

ncreased the pore size of the pad and facilitated the infusion ofNFs into the substrate. Next, a solution of GNFs, methanol, andater (GMW) was prepared; the solution was mixed in a sonica-

ion bath to ensure uniform dispersion of GNFs. To infuse GNFs intohe swollen adhesive pad (AP) the pad was placed into the GMWolution. The solution was left in a sealed container at room tem-erature for 6 h without any agitation. The pad infused with GNFsGNF-Pad) was removed from the solution and dried overnight atoom temperature. The GNF-Pad was washed to remove residualraphene particles and expose the strain sensor’s smooth, black,nd bright surface.

To characterize the quality of the sensor’s specifications, eachtep in the aforementioned fabrication process was analysedFig. 1). The rubber-like adhesive pad swells when submerged in

ifferent organic solvents, including acetone and chloroform. Ace-one was selected for the solvent in the fabrication process sincet is less hazardous than chloroform. Various soaking times in ace-one were examined (Fig. 2a) to characterize the pad’s swelling

ators A 271 (2018) 182–191 183

behaviour; we found that the pad swells to its maximum size (2.3times its initial volume) after 5 h. Furthermore, soaking in acetone(upto 48 h) did not affect the mechanical properties of the AP, bothin terms of the Young’s modulus and ultimate tensile strain afterit dried (Fig. 2b and c). Interestingly, the adhesive property of thepad was unaffected by soaking in acetone, and the pad recoveredits adhesive property when removed from the bath and dried. Thepad’s adhesive property restricts the infused GNFs movement; theflakes stick to the polymer substrate and cannot freely slide withinthe medium. Thus, they are moved apart increasing the resistancewhen the polymer substrate stretches. Consequently, this strainsensor is very durable and does not require any additional elementsto create adhesion between GNFs and the substrate.

During the second step of the fabrication process the swollen APwas submerged in the GMW solution. Due to differences in osmoticpressure, the acetone exited the pad and GNF particles enteredthrough the expanded pores. During this phase the pad shrunk to itsoriginal size. To ensure smooth deposition and linear shrinking ofthe pad, the GMW solution composition was optimized. We exam-ined the swollen AP’s deformation during shrinking when placed inGMW solution with different ratios of water to methanol (100%:0%,80%:20%, 75%:25%, and 50%:50% water to methanol ratios, respec-tively) (Fig. S1). A ratio of 75%:25% water to methanol displayedlinear shrinking of AP without any bending; this ratio was selectedto create the GMW solution as it guaranteed uniform deposition ofthe GNFs on the surface of the AP. Although optimizing the GMWis important to ensure uniformity of the sensor, the final qual-ity largely depends on the AP soaking time in the GMW solution.As such, the effect of soaking time was measured and the maxi-mum depth of infusion was found to be approximately 13 �m, andachieved after 6 h soaking (Fig. 2d). The initial resistance R0(definedas resistance without any applied strain) of the GNF-Pad for differ-ent soaking times was also assessed (Fig. 2e). The R0 is directlyrelated to the soaking time; resistance of the GNF-Pad soaked for30 min was 150 k�/cm but dropped to 8 k�/cm when soaked for6 h. Since longer soaking times did not significantly change theresistance of the pad (Fig. 2e), a 6-h soaking time was used tofabricate sensors for further study.

To achieve a uniform and robust GNF layer on the pad, theGNF-Pad was removed from the GMW solution and dried at roomtemperature overnight. The water content of the GMW solutionin the soaked pad, lowers the evaporation rate and provides moretime for the GNFs to orient and uniformly deposit on the substratewhile drying. This creates a bright and smooth layer of GNFs onthe substrate. The sensor was washed with deionized water afterdrying to remove residual GNFs on the substrate.

A stress-strain curve was conducted to assess the effect ofthe fabrication process on the mechanical properties of the APand the final sensor (Fig. 2f). Both samples displayed very sim-ilar stress-strain curves suggesting the process had a minimaleffect on the mechanical properties. Scanning electron microscope(SEM) images taken from cross-sections of the GNF-Pad revealeda uniform infusion of the GNF (Fig. 2g–i). The GNF also were inclose proximity with one another and had continuous distribution(Fig. 2i). Collectively these characteristics give the sensor a lowinitial resistance.

The response of the fabricated strain sensor was examined forvarious levels of strain (Fig. 3a). The sensor was capable of with-standing 350% strains without any observable cracking or othersigns of failure. The sensor’s response up to 40% strain was lin-ear and the resistance increased to 3 times its initial significant,this may not be a major concern in the application of the pro-

posed sensor in wearablvalue of 8 k� (�R/R0 = (R − R0)/R0 = 3 at40% strain�R/R0 = (R − R0)/R0; R0 is the initial resistance of sensorwith no strain, Fig. 2e). Further increase in strain caused an expo-nential increase in the relative resistance (�R/R0): at 350% strain

184 S.R. Larimi et al. / Sensors and Actuators A 271 (2018) 182–191

Fig. 1. Schematic diagram of the fabrication process of GNF-Pad and its sensitivity in different types of deformation. (a) Preparation of GNF-Pad by infusing the graphenenano-powder to clear adhesive pad. (b) Piezo-resistive response of GNF-Pad regarding the applied strain by stretching. (c) resistance changing of GNF-Pad during twistingf ◦ ◦ ◦ nse ofi

ti

rom 0 to 180 . (d) Sensitivity of GNF-Pad dealing with bending (0 –90 ). (e) Responput in pressing and releasing of the sensor are more pronounced.

he sensors relative resistance was almost 600 times higher thants initial value (Fig. 3a). The hysteresis of the fabricated sensor was

the GNF-Pad under pressuring. The higher order response (overshoot) for the step

also monitored and almost no hysteresis was observed for both 60%and 100% strain cycles (Fig. 3b)

S.R. Larimi et al. / Sensors and Actuators A 271 (2018) 182–191 185

Fig. 2. Characterization of GNF-Pad and the fabrication process. (a) swelling ratio of the adhesive pad (AP) in the acetone as a function of soaking time. (b–c) the effect soakingtime on the mechanical properties of the pad, showing the mechanical properties of AP is not affected by acetone. (d) infusion depth of GNF into the Pad versus soakingt GMWf ages

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ime. (e) initial resistance of the sensor (no applied strain) versus the soaking time inabricated GNF-Pad. (g–i) section cut of the GNF-Pad and the corresponding SEM im

The strain sensor’s gauge factor (GF) represents the sensitivityf the sensor at different applied strain and was calculated basedn Eq. (1) (Fig. 3c),

F = �RR0ε

(1)

here � is the strain on the sensor and �R/R0 is the relative changen the resistance of the sensor at different strain. The gauge factor ofhe GNF-Pad ranges from 2 to 161 and exhibits different sensitivityt low (0%–5%), medium (5%–100%) and high strain (100%–350%);his allows the sensor to detect and distinguish between differentypes of applied strain (Fig. 3c). When the strain was changed from% to 100%, the GF increased linearly from 4 to 15.2. For strainsigher than 100% the value of the GF grew exponentially (approx-

mately to the power of two) and reached 161.23 at 350% strainFig. 3c)

To study the mechanism responsible for the sensor’s behaviour,EM images of the sensor’s surface were obtained under differenttrain conditions (Fig. 3d–h). When no strain was applied there wasniform distribution of the GNFs on the surface (Fig. 3d). As thetrain increased to 60%, random disconnections between the flakes

ere observed (Fig. 3e); these disconnections are attributed to the

light changes in the sensor’s resistance (Fig. 3a). At 100% strain,he grain boundaries between the GNFs became increasingly per-endicular to the applied strain direction (Fig. 3f). At 200% strain,

solution (used to fabricate the sensor). (f) stress-strain curves of AP substrate andafter drying and washing.

cracks started to appear between the flakes (Fig. 3g) and at 300%strain deep grooves were visible perpendicular to the applied straindirection (Fig. 3h); these changes induced disconnections betweenGNFs infused deeper within the substrate and were responsible forthe sensor’s exponential increase in resistance (Fig. 3a). These dis-connection under higher strains (e.g., 300%) may further exasperatesensors creep behaviour and the hysteresis curve over time.

The sensor was installed on a test bed (Fig. S2) to observe thesensor’s response to different strain cycles (5%, 10%, 15%, 30%, and60% strain cycles of stretching and releasing at 1 Hz). The sensordemonstrated a repeatable response for each strain cycle (Fig. 3i).When the GNF-Pad was tested for its durability, there was lessthan 1% change in its response to strain after exposed to 10,000cycles (Fig. 3j). The strain sensor underwent 35% strain cycles(stretching and releasing) 10,000 times at a frequency of 1 Hz. Theresponse of the sensor was very stable with almost no observ-able difference (less than 1%) for 90–100 cycles as compared to9,990–10,000 cycles; this demonstrated the high durability of ourGNF-Pad (Fig. 3j). The physical properties of the GNF-Pad remainedunaffected during the 10,000 dynamic cycles at relatively largeapplied strain (35% strain cycles). This was due to two reasons:

first, the high elasticity of the sensor substrate (Fig. 2b and c), andsecond, the substrate’s adhesive property. This adhesiveness bothprevents the infused GNFs from deforming and moving within thesubstrate and prevents the GNFs delaminating from the surface.

186 S.R. Larimi et al. / Sensors and Actuators A 271 (2018) 182–191

Fig. 3. (a) GNF-Pad response to tensile strain, (b) hysteresis plot of the sensor at 60% and 100% strain cycle, showing the sensor has no hysteresis for those cycles. (c) gaugefactor GNF-Pad as a function applied strain, showing sensitivity of the sensor at different strain. (d–h) SEM images of sensor surface at different applied strain. (i) plot ofr test ofa or at d

Tt

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elative resistance as a function of the strain for different strain cycles. (j) durability

t a frequency of 1 Hz. (k) Histograph of the sensor, where the response of the sens

hus, the conductivity and robustness of the sensor, despite its lowhickness, is still exceptionally high.

Both the substrate (AP) and the GNFs contribute to the robustesponse and superior mechanical properties of the final strain sen-or. The AP has excellent elastic properties which allows the sensoro handle a large applied strain, whereas the GNFs have solid lubri-ant properties and the graphene layers can readily slide whilender strain. As a result, the strain sensor easily recovers to its initialondition even after 350% strain.

The transient response of the sensor, which occurs specificallyuring the initial period of the sensor’s response, was also captured.

he sensor response was tested at different strain cycles (his-ogram), including 1, 5, 15, 30, 100 and 10,000 cycles. After 15 cycleshe sensor response became nearly stable. The peak value (35%train) at the 15th cycle was less than 2% different than the peak

the strain sensor under 35% strain cycles. The test was performed for 10,000 cyclesifferent cycles were compared to each other.

value at the 10,000th cycle. After 30 cycles the response becamesteadier and remained almost unchanged (with less than 1% differ-ence between the peak values at the 30th cycle and 10,000th cycle)(Fig. 3k).

Various experiments were performed to examine the mechan-ical performance (e.g., stretchability) and electrical performance(e.g. sensitivity) of the fabricated GNF-Pad in real applications(Fig. 4). Even though the GNF-Pad poses little risk of skin irritationto human users, the sensor was encased in a skin-friendly highlystretchable substrate (Ecoflex

®00-50).

Initially, we evaluated the GNF-Pad as a bio-signal sensor. The

sensor was embedded in a wristband and placed on the radialartery to monitor human heartbeat (Fig. 4a). The acquired sig-nals clearly illustrated the time and number of heartbeats. Theseresults showed that the GNF-Pad has high sensitivity which is suit-

S.R. Larimi et al. / Sensors and Actuators A 271 (2018) 182–191 187

Fig. 4. Applicability of the sensor for measuring wide range of strain including ultra-low, medium and high strain range. (a) heartbeat monitoring using GNF-Pad implantedi ® a fing ®

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n Ecoflex wristband. (b) finger pose reading. Three GNF-Pad were assembled on

ngle of each finger joints during bending the finger. (c) wireless knee band to traitting down and standing up.

ble for monitoring delicate movements and bio-signals (Fig. 4a).ince the fabricated strain sensor has a broad GF range, in addi-ion to monitoring the very small movements, the GNF-Pad waslso capable for monitoring medium range movements (such asuman finger motions, Fig. 4b). Three GNF-Pad were assembledn a finger band that fits on three joints of the human finger; theesponse was recorded for each position (Fig. 4b). The signal cap-ured from the group of sensors clearly detected different positionf the human finger (Fig. 4b), expressing its capability to be used inaptic applications [43].

The sensor was further tested for applications involving largemounts of strain; we installed the sensor on the knee of auman user and monitored the sensors response to different activ-

ties including walking, running and siting and standing (Fig. 4c).lthough all these activities required human knee movement, theNF-Pad sensor clearly distinguished between each activity. In

er pad made out Ecoflex and used to measure the strain caused by change in thee knee angle changing during different activities, including, walking, running and

other words, due to differences in both signal shape and frequencyrecorded, the type of activities performed were detectable (Fig. 4c).

We also demonstrated the applicability of the GNF-Pad to haptictechnology. For this purpose, we designed and utilized an in-house3◦ of freedom (DOF) robotic finger (Fig. 5). The robot’s gestures werecontrolled in the real-time using sensors assembled on a humanfinger (Fig. 5). The robotic finger was made by assembling 3 servomotors on a 3D printed body made out of ABS plastic [43]. Thecontroller setup consisted of a finger pad with three GNF-Pad tomeasure the three angles of the finger joints. The finger pad wasconnected to a microcontroller that communicated with a trainedadaptive network-based fuzzy inference system (ANFIS) in MATLAB(R2014a). The ANFIS translated the signals recorded from the finger

pad to the corresponding angles of each joint [43]. Then it sendsthe calculated angles to servo motors in the robotic finger throughthe microcontroller (Fig. 5a). Using the fabricated strain sensors,we have shown that it is possible to control the movements and

188 S.R. Larimi et al. / Sensors and Actuators A 271 (2018) 182–191

F ) showfi orrespa

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ig. 5. Demonstrating real-time application of the sensor for haptic technology, (anger with human finger gesture. (b) different human finger gesture and real-time cssembled on the human finger for different gestures.

estures of the robotic finger (Fig. 5b and c) and can be applied tohe field of human-machine interfacing.

To avoid any effect from the humidity on the proposed strainensor in the manufactured wearable devices, all the sensors areealed by skin-friendly highly stretchable materials like Ecoflexnd medical tape. This sealing method makes the wearable device

aterproof and provides an overall increase in robustness andurability. Although the impact of extreme temperatures can beignificant, this may not be a major concern in the application ofhe proposed sensor in wearable devices.

ing the sensors assembly and schematic of the system used to control the roboticonding gesture of the robotic finger. (c) real-time recorded response of each sensor

4. Experimental section

2.1. Synthesis of graphene/methanol/water (GMW) solution

Graphene Nano-flake (Graphene Supermarket, Carbon: 97%,Hydrogen 1%, Oxygen 2%) with the average flake thickness

approximately 1.6 nm (less than 3 monolayers) was used inGraphene/Methanol /Deionized Water (MEDICA-R 7/15) solution.In order to make a high concentration (5 mg/mL) and uniformly dis-persed GNF in M/W solvent (GMW solution), the solution was put in

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onication bath (Branson B3510MTH) for 2.5 h. The optimum ratiof methanol and water to make GMW solution was found to (25:75)espectively. The GNF after dispersion did not sediment during theabrication time.

.2. Fabrication of the strain sensor

To infuse the GNF into the adhesive pad (Re-POPTM

Clear Adhe-ive Pad), the pad was soaked in acetone for 6 h. After swellinghe pad to nearly two times its initial length, the swelled pad wasoaked in GMW solution for different soaking times. The final con-uctivity of each sensor was mainly related to its soaking time. Afteroaking, the sensor was dried in room temperature and washedith deionized water.

.3. Electromechanical Testing: (device characterization)

SEM images were carried out using TESCAN-MIRA3 XM SEMith the resolution of 1.0 nm at 30 kV. The resistance of all sen-

ors for characterization were read by Digital Mustimeter (NI DMM072) and transferred to National Instruments PXI (NI PXI-1042Q)or analysing.

Due to negligibility of the resistance between sensor and probesf DMM (∼0.02 �) in comparison with the sensor resistance8 K� or higher), all experiment measurements were made with

probes. A high precision tension device was made from twoigh-performance low-profile ball bearing linear stages (Model:ewport 423) on an optical table (Newport M-SA2-12-D) to moni-

or the response of the GNF-Pad in various strain load. Applying thetrain to the GNF-Pad was done with a stepper motor controlled by

microcontroller (ATmega328P, Arduino-UNO). The sensor sam-les which are used in tensile testbed mostly have an initial lengthnd width of 15 mm and 3 mm respectively. The efficient lengthfter fixing the sensor on the tensile testbed decreased to 8 mm.

.4. Fabrication of wristband

To monitor the heartbeat one GNF-Pad was implanted inside theristband. The two sides of the strain sensor were connected to theicrocontroller on the ATmega328P (Arduino-UNO) using copper

ape and magnetic wires (Gauge 32). To measure the change inhe resistance of the fabricated GNF-Pad during strain cycles, theNF-Pad was serried by a known and constant resistor (10 K�). Tox the sensor inside the band and also prevent the direct contactetween human skin and GNF-Pad, we covered the GNF-Pad withcoflex

®(00-50). The Ecoflex

®was molded and then cured in room

emperature for 3 h. The wristband was fixed onto the wrist byelcro tape.

.5. Fabrication of finger band

To track the finger motion, fabricated strain sensors were assem-led on a band made out of Ecoflex. For this purpose, threeNF-Pads were attached on top of the flexible band and attached to

he substrate by curing Ecoflex on each end of the sensor. To readhe resistance of each sensor, the same way explained in the wrist-and fabrication a 10 K� resistance was serried with each sensor.our Velcro tapes were used to fix the band on the finger segments.

Fabrication of Knee Band:Monitoring the knee angle in different situations requires more

lasticity and flexibility. Therefore, a GNF-Pad was inserted in

coflex00-50 uncured mixture and then substrate was cured atoom temperature for 3 h. The change in the resistance of thembedded sensor was performed by ATmega328 microcontrollerith16 MHz clock speed and acquired data were transferred wire-

ators A 271 (2018) 182–191 189

lessly with Bluetooth Low Energy (CC2540, BT 4.0) to the computerfor further analysis.

3. Conclusions

In summary, we have introduced an easy and highly repeat-able fabrication method to create a piezo-resistive strain sensorwith outstanding mechanical and electrical properties at a verylow cost. The sensor was made by infusing GNFs into a commer-cially available adhesive substrate. The fabricated sensor has aninitial resistance of 1.6 k�/cm and can be stretched up to 350% ofits initial length without any sign of failure or damage. The sen-sor has a gauge factor between 2–160, which varies depending onthe applied strain, suggesting that the sensor can be used to mea-sure a wide range of applied strains. The fabricated GNF-Pad exhibitzero hysteresis even when exposed to excessive strain cycles aslarge as 100%. The sensor was assembled into different wearablepieces such as a wristband, a finger band (glove) and a knee band.The wristband was used to monitor heartbeat by sensing the pulseof the artery. In an entirely different range of motion, the fingerand knee bands were used to monitor a variety of human bodymotions including finger gestures and larger scale muscle move-ments during walking, running, sitting down and standing up. Wehave further demonstrated the efficacy of GNF-Pad as a haptictechnology in real-time applications by precisely replicating thehuman finger gestures using a three-joint robotic finger. In con-clusion, the GNF-Pad’s high elasticity, sensitivity (to wide range ofstrains), selectivity (to different type of deformation), and dura-bility makes it an ideal sensing element to be implemented intowearable devices.

Acknowledgements

We would like to acknowledge Natural Sciences and Engineer-ing Research Council (NSERC) Canada under the Discovery Grant(DG) program and Canada Foundation Innovation (CFI) under theLeaders Opportunity Fund to provide the financial support for thisresearch.

Appendix A. Supplementary data

Supplementary material related to this article can be found,in the online version, at doi:https://doi.org/10.1016/j.sna.2018.01.028.

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Biographies

S. Reza Larimi received the B.Sc. and M.Sc. degree in mechanical engineering fromK.N.Toosi University of Technology, Tehran, Iran, in 2010 and 2012. He is currentlyworking toward the Ph.D. degree in mechanical engineering in the Advanced Con-trol and Intelligent System Laboratory (ACIS Lab), The University of British Columbia,Okanagan Campus, Canada. His current research interests include wearable technol-ogy, sensor fabrication, control and haptic interfaces.

Dr. Hojatollah Rezaei Nejad, received his Ph.D. in 2016 fromUniversity of BritishColumbia (UBC), where hestudied physics of particles and cells in microfluidic sys-tems. His work at UBC has led to discovery of several new physics that describebehaviour of particles in microscale and resulted in development of novel par-ticles focused/isolation/separation techniquesfor Lab-on-a-chip application. Dr.Nejad’smain expertise is in fluid mechanics with a focus on fluidbehavior andfluid-particle interaction in micro- and nano-scale. He is also a known specialistin Micro/Nano Fabrication and bio-fabrication technologies. His research involvesdevelopment of advanced Micro/NanoTAS systems for point-of-care devices andtissue engineering applications. Dr. Nejad joined Nano lab at Tufts University as aPost-doctoral research fellow in 2017 to developinnovative lab-on-chip diagnosticdevices. He was also a Pre- and Post-doctoral research fellow at Harvard-MIT Divi-sion of Health Sciences and Technology (HST) from 2015 to 2017. At HST, he hasdevelopedbio-fabrication technologies based-on microfluidics that promote angio-genesis, and facilitate fabrication of mm-sized vascularized tissue constructs. He is a

winner of several prestigious awards including Friedman Foundation Scholarshipsin Health Sciences. He has received his B.Sc. (ShahidChamran University, Ahwaz,Iran)and M.Sc. (K.N.Toosi University, Tehran, Iran) in Mechanical Engineering, wherehe studied nucleation of nano-bubble and nan-droplet and their characteristics.His work has proved the validity of Young-Laplace equation in nanoscale and have

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hown the fundamental properties of fluids such as surface tension is affected byhe size of nano-droplet and -bubble. Dr. Nejad is from Shiraz, Iran.

ichael Oyatsi is an undergraduate student pursuing a mechanical engineeringegree at the University of British Columbia Okanagan. A strong interest in roboticsnd its accompanying research has led him to work with the Advanced Control andntelligent Systems laboratory for the past year. Here, he has fostered his interestn the research field, as well as learnt engineering approaches to design and formal

riting.

llen O’Brien is the lab and project manager in the Advanced Thermo-Fluidic Lab-ratory (ATFL) at the University of British Columbia (UBC). He studied immunologynd infectious disease at the University of Alberta and obtained a M.Sc. in control ofnfectious disease from the London School of Hygiene and Tropical Medicine, special-zing in program design, epidemiology, and statistical methods. At ATFL his primaryesponsibilities are to support high quality research and oversee the developmentf microfluidic lab-on-chip devices and gas sensor prototypes with applications in

uisance sewer gas detection, natural gas leakage, and breath-analysis.

r. Mina Hoorfar is a mechanical engineer specializing in microflow in miniaturizedevices. She has related experience in the design and development of innovativeicrofluidic systems for a variety of applications ranging from online monitor-

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ing sensors for water quality assessment, electrochemical-based gas sensors fordetection of diabetes from exhaled breath, and DNA purification using microfluidicdevices for forensic applications. She has been active in the design of microflu-idic platforms capable of manipulation, pre-concentration, separation, capture, anddetection of target bioparticles. Another aspect of her research endeavors involvesthe design and fabrication of gas sensors to achieve low detection limits requiredfor environmental leak detection applications.

Dr. HomayounNajjaran received his Ph.D. from the Department of Mechanical andIndustrial Engineering at the University of Toronto in 2002. From 1999 to 2002,he worked as a Research Assistant in the Robotics and Automation Laboratory atthe University of Toronto, and a Senior Consultant for Engineering Services Inc.,Toronto, Canada, where his work mainly focused on the development of hardwareand software of robotic and automation systems. From 2003 to 2006, he workedas a Research Officer at the National Research Council Canada where his researchfocused on the development of robotic systems and sensor technologies. He joined

the Okanagan School of Engineering at the University of British Columbia (UBC)in May 2006. Dr. Najjaran is the founder of the Advanced Control and IntelligentSystems (ACIS) Laboratory at the School of Engineering. His research focuses on theanalysis and design of advanced control systems in a variety of applications rangingfrom service and humanoid robots to digital microfluidic systems.