Flexible Strain Sensors Fabricated by Meniscus-Guided Printing of

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Flexible Strain Sensors Fabricated by Meniscus-Guided Printing of Carbon Nanotube−Polymer Composites Muhammad Wajahat,†,‡,∥ Sanghyeon Lee,†,§,∥ Jung Hyun Kim,†,‡ Won Suk Chang,†,§ Jaeyeon Pyo,† Sung Ho Cho,*,§ and Seung Kwon Seol*,†,‡ †

Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon-si, Gyeongsangnam-do 51543, Republic of Korea ‡ Electro-Functionality Materials Engineering, Korea University of Science and Technology (UST), Changwon-si, Gyeongsangnam-do 51543, Republic of Korea § Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Republic of Korea S Supporting Information *

ABSTRACT: Printed strain sensors have promising potential as a human−machine interface (HMI) for health-monitoring systems, human-friendly wearable interactive systems, and smart robotics. Herein, flexible strain sensors based on carbon nanotube (CNT)−polymer composites were fabricated by meniscus-guided printing using a CNT ink formulated from multiwall nanotubes (MWNTs) and polyvinylpyrrolidone (PVP); the ink was suitable for micropatterning on nonflat (or curved) substrates and even three-dimensional structures. The printed strain sensors exhibit a reproducible response to applied tensile and compressive strains, having gauge factors of 13.07 under tensile strain and 12.87 under compressive strain; they also exhibit high stability during ∼1500 bending cycles. Applied strains induce a contact rearrangement of the MWNTs and a change in the tunneling distance between them, resulting in a change in the resistance (ΔR/R0) of the sensor. Printed MWNT−PVP sensors were used in gloves for finger movement detection; these can be applied to human motion detection and remote control of robotic equipment. Our results demonstrate that meniscus-guided printing using CNT inks can produce highly flexible, sensitive, and inexpensive HMI devices. KEYWORDS: printed electronics, flexible strain sensor, piezoresistivity, MWNT−PVP composites, meniscus-guided printing, CNT inks

1. INTRODUCTION Recent developments in printed electronics pave the way for the production of commercially viable, flexible, low-cost, and portable devices, including antennae, touchpads, microfluidic devices, displays, sound sources, printed circuit boards, and sensors.1−11 Techniques such as screen, gravure, and inkjet printing offer rapid and inexpensive methods of fabricating electrical circuits on flexible substrates.12−15 Printed strain sensors, for which electrical resistance is dependent on applied strain, hold tremendous promise for use in a diverse range of wearable applications, including electronic skins, healthmonitoring devices, human-friendly wearable interactive systems, and smart robotics.16−19 Printed strain sensors offer distinct advantages over conventional metal-foil/semiconductor-slab strain sensors, including flexibility, smaller dimensions, and higher sensitivity.17,20 A number of advanced materials for printing highly flexible strain sensors have been previously reported, including metallic nanoparticles,21 carbon nanotubes (CNTs),22−25 graphene,26 poly(3,4-ethylenedioxythiophene) polystyrene sulfonate,27 and electrically conductive composites.17,28 © XXXX American Chemical Society

CNTs are a candidate for use in highly sensitive strainsensing materials because of their unique mechanical and electronic properties. Individual CNT-based piezoresistive strain sensors have achieved high performances with gauge factors (GFs) greater than 3000, compared to silicon-based sensors, with GFs of ∼200; this results from the superior intrinsic piezoresistivity of the CNTs, which arises from their deformation.29,30 However, the use of individual CNTs in strain sensor applications faces difficulties in the measurement and positioning of homogeneous CNTs with desired orientations and locations. To date, a diverse range of methods have been proposed to overcome such difficulties. Significant advances have been made in using CNT−polymer composite films and fibers as transducers in strain sensors.31−41 The strain-sensing properties of CNT−polymer composites are mainly due to the piezoresistive properties of the conductive network of CNTs Received: March 12, 2018 Accepted: May 16, 2018

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DOI: 10.1021/acsami.8b04073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Instrument). The CNT ink was introduced at the rear of the micronozzle and drawn to the front tip by capillary action. The micronozzle position and pulling rate were controlled using three-axis stepping motors with 250 nm positioning precision. The printing process was observed in situ using a high-resolution monitoring system consisting of an optical lens (200×) and a charge-coupled device camera (Spot RT Xplore). Polyimide (PI) films, Si wafers, and polymethyl methacrylate (PMMA) dome structures (curvature radius = 3.85 cm) were used as substrates for the CNT patterns. The microscopic characteristics of the printed patterns were analyzed using field-emission scanning electron microscopy (FE-SEM) (S4800 Hitachi), optical microscopy (ECLIPSE LV 150N NIKON), and atomic force microscopy (AFM) (XE100 Park Systems). 2.3. Printing and Characterization of Printed Strain Sensors. Flexible CNT strain sensors, which were produced by printing MWNT−PVP microlines (60 μm width and 5 mm length) on a PI substrate, were estimated at room temperature. Copper wires attached to both ends of printed lines by silver paste were used to obtain an electrical connection with the measurement system. To evaluate the sensitivity of the flexible strain sensors, we carried out a bending test by changing the chord length of the strain sensor. A Keithley 2612A two-probe instrument was used to measure the room-temperature electrical resistance of the printed strain sensors as a function of applied strain, under unstrained, tensile, and compressive conditions. Applied strain can be expressed as: ε = ±h/2r, where h is the thickness and r is the bending radius of the sensor. Bending radius (r) has a relation of chord length between bending radius (r) as c = 2r × sin(l/ 2r), where l is the arc length of the sensor under bending state. The tensile (outward direction) and compressive (inward direction) strains were applied to the sensor by adjusting the bending direction. To demonstrate an HMI application of the printed strain sensors, a sensor was attached on a finger of a glove (Microflex Ansell) and the electrical signals registered on moving the finger were used to control the motion of a robotic arm (ROB0142 DFRobot).

in the polymer matrices; changes in the CNT network induced by mechanical deformation of the composite result in an increased resistivity. This resistivity change is caused by changes in the contacts and tunneling distances between the CNTs.31−41 To prepare CNT−polymer composite-based strain sensors with the desired properties, a diverse range of fabrication techniques have been developed, including contact film transfer, spray coating, inkjet printing, and screen printing.25,35−38 Although the contact film transfer technique is able to fabricate CNT-based strain sensors with good sensitivity, transferring the film to the substrate is timeconsuming and complex, often requiring surface treatment of the substrate and multiple steps; this increases the overall cost and limits the scalability of this technique. Conventional spray deposition processes often use an airbrush with a large spray area to fabricate strain sensors, but they are not able to deliver CNT−polymer composites to specified locations; thus, they have a high material cost. Inkjet and screen printing, which are convenient and low-cost patterning techniques, can deliver CNT−polymer composites to specified locations;37−39 however, both techniques are limited to the patterning of CNT− polymer composite structures on flat substrates only, but the patterning of nonflat (or curved) substrates is essential for advanced strain-sensing applications. In this study, we fabricated flexible, printed strain sensors by meniscus-guided printing of a CNT−polymer composite, enabling micropatterning on nonflat (or curved) substrates. Our approach is based on the precise deposition of a CNT ink formulated from multiwall nanotubes (MWNTs) and polyvinylpyrrolidone (PVP). The printed strain sensors have GFs of 13.07 (tensile strain) and 12.87 (compressive strain) and exhibit changes in electrical resistance caused by contact rearrangements and changes in the tunneling distance between the MWNTs; these sensors exhibit rapid responses to both tensile and compressive bending strains and high reproducibilities (∼1500 bending cycles). We successfully developed a gesture-based human−machine interface (HMI) by exploiting the resistance change of glove-mounted sensors while bending a finger. This method can be generalized for producing strain sensors on flexible, nonflat (or curved), and even threedimensional structures.

3. RESULTS AND DISCUSSION Figure 1a illustrates a schematic diagram for the printing of MWNT−PVP zigzag patterns by meniscus-guided printing.42−50 The pattern (FE-SEM image inset in Figure 1a) was printed by horizontally pulling a nozzle (Dos = 20 μm), filled with the CNT ink, without applying pressure (Movie S1 and S2, Supporting Information). When the micronozzle was brought into contact with and then moved ca. 3 μm above the substrate, a CNT ink meniscus with a height of ∼3 μm was formed at its opening. Sufficient stress was applied by pulling the micronozzle to enable continuous extrusion of the CNT ink. As the nozzle was horizontally pulled, at a pulling rate (υh) of 50 μm·s−1, the printed lines solidified because of the rapid evaporation of the water solvent. The CNT ink was formulated from A-MWNTs and PVP. The addition of PVP to the A-MWNT suspension improved the printability of the CNT ink. PVP is a hydrophilic watersoluble polymer which was noncovalently grafted onto the CNT surfaces by wrapping: PVP improves the steric stabilization of the CNTs by providing hydrophilicity.48,51 The printable CNT ink exhibits a viscosity of ∼60 mPa·s (at a shear rate of 10 s−1) (see Figure 1b); this is an order of magnitude higher than that of water, indicating shear-thinning behavior. Different MWNT−PVP micropatterns were successfully printed on a curved substrate, as well as a flat one, by pulling the micronozzle (Dos = 15 μm) horizontally (Figure 1c,d). During the printing process, the CNT ink was supplied continuously without the nozzle clogging. Figure 1c shows several patterns printed on a flat Si wafer (clockwise from bottom left: a car, the word “Carbon”, and a portrait of a

2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of the CNT Inks. The MWNT and PVP [molecular weight (Mw) = 10 000] used in this study were purchased from Iljin Nanotech (Korea) and Aldrich (USA), respectively. The MWNTs were acid-treated by suspending in a mixture of condensed H2SO4 and HNO3 (3:1 volume ratio) and refluxing at 70 °C for 24 h. The acid-treated MWNTs (A-MWNTs) were washed several times with deionized water, until the washings were neutral, and then dried in a vacuum oven at 100 °C for 24 h. The CNT inks were prepared by mixing A-MWNT (7 wt %) and PVP (17 wt %) in water. The viscosity of the inks was measured by performing a strain sweep and varying the shear rate from 10 to 100 s−1; a rheometer (MCR102, Anton Paar) equipped with cone-and-plate geometry was used for these experiments. The PVP mass loss was estimated via thermogravimetric analysis (TGA) (TA Instruments, Q600, USA). The samples were heated to 250 °C in alumina crucibles at 10 °C min−1 and then maintained at the temperature for 1 h. 2.2. Printing of MWNT−PVP Patterns. MWNT−PVP patterns were printed at the meniscus formed at the tip of a micronozzle filled with an aqueous ink formulated from A-MWNT (7 wt %) and PVP (17 wt %). Glass micronozzles with opening sizes (Dos) of 15, 20 and 25 μm were prepared using a micropipette puller (P-97, Sutter B

DOI: 10.1021/acsami.8b04073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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traditional Korean official). Moreover, the MWNT−PVP line pattern was printed easily on a PMMA substrate with a curvature radius of 3.85 cm, simply by adjusting the micronozzle along the substrate contour (red dotted arrow in Figure 1d and Movie S3, Supporting Information). We investigated the current−voltage (I−V) characteristics of the printed MWNT−PVP line before and after thermal treatment (Figure 2a). A line with a width (WL) of 60 μm and length (LL) of 5 mm was printed on a PI substrate. Because the surface PVP groups inhibit contact between the MWNTs, the as-printed pattern exhibited an electrical resistance in the gigaohm range, which is not suitable for use as a transducer in a strain sensor. A resistance of transducers in the region of hundreds of kiloohm is required for sensing applications.25 The printed line was thermally treated in air at 250 °C for 1 h to thermally decompose PVP for decreasing the resistance. After thermal treatment, the resistance decreased from 1.3 GΩ to 261 kΩ; this was due to the partial removal of PVP, resulting in increased contact and decreased tunneling distance between the MWNTs. In Figure 2b, the yellow arrows indicate MWNTs exposed after the thermal treatment; the line width remained at 60 μm, despite the thermal treatment. The thickness of the line (measured using AFM) decreased from 1.93 to 1.69 μm after the thermal treatment. As a result, the printed line experienced a volumetric shrinkage of ∼12.43% because of the thermal treatment. To understand the thermally induced mass changes in the CNT ink, TGA was performed under conditions similar to those of the thermal treatment (Figure 2c). The ink was heated from room temperature to 250 °C at 10 °C min−1, and the temperature was maintained at 250 °C for 1 h. The weight loss at around 100 °C is associated with the evaporation of water, accounting for 76.1 wt % of the CNT ink. Thermal decomposition of PVP begins generally at 250 °C.52 After 1 h at a constant temperature of 250 °C, the ink mass reduced from 23.9 to 21.8% of its starting value because of the partial decomposition of PVP.

Figure 1. (a) Schematic diagram of the meniscus-guided printing process. An MWNT−PVP zig-zag pattern is printed by horizontally pulling the micronozzle (inset: FE-SEM image); (b) viscosity as a function of shear rate for the CNT ink (7 wt % A-MWNT with 17 wt % PVP); (c) optical images of various patterns printed with a Dos value of 15 μm. Clockwise from bottom left: a car, the letters “carbon”, and a portrait of a traditional Korean character; and (d) optical image of the printing process for an MWNT−PVP line on a curved PMMA substrate (curvature radius = 3.85 cm). The red dotted arrow indicates the printing direction of the line.

Figure 2. (a) I−V characteristics of the printed line before and after thermal treatment at 250 °C for 1 h. The applied voltage was varied from −1.0 to +1.0 V in 20 mV steps; (b) FE-SEM (top) and AFM (bottom) images of the printed line before (left) and after (right) the thermal treatment. The yellow arrows in the enlarged FE-SEM image indicate MWNTs exposed as result of the partial removal of PVP by the thermal treatment; and (c) TGA curves for the CNT ink (7 wt % A-MWNT and 17 wt % PVP). Inset: Magnification of the gray region. C

DOI: 10.1021/acsami.8b04073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Response, ΔR/R0, of the sensor as a function of εt between 0 and 2.0%. Linear regression gives the GF of the sensor as 13.07 with an Rsquare value of 0.98. Inset: Photos of the sensor at selected strains; the red dotted lines indicate the bending. (b) FE-SEM images showing the morphology of the MWNT−PVP line as initial state, loading strain state (εt = 2.0%), and unloading strain state (εt = 0.0%). The yellow dotted lines in the enlarged image indicate reconnection of microcracks at unloading strain state. (scale bar: 200 nm) (c) Response, ΔR/R0, of the sensor at selected values of εt: 0.8, 1.3, 1.5, and 2.0%. (d) Durability test of a sensor subjected to 1500 cycles of strain (εt = 2.0%). (e) Response, ΔR/R0, of the sensor as a function of εc between 0 and 2.0%. The sensor has a GF value of 12.87 and an R-square value of 0.99. (f) FE-SEM image showing the morphology of the MWNT−PVP line at εc ≈ 2.0%. The red arrows indicate MWNTs overlapped along the crack (yellow dotted lines).

deformation of the CNT network induced by crack formation generates the change of ΔR/R0 value. Microcracks increase the distance between the MWNTs, decreasing the number of contacts and increasing the tunneling distance between the MWNTs; this leads to increased resistance. As shown in Figure 3c, dynamic strain-sensing responses were measured with cyclic loading−unloading of strain at 0.8, 1.3, 1.5, and 2.0%. For all applied strain levels, the responses, ΔR/R0, remained consistent after repeated cycling of strain, suggesting that the printed strain sensor has high durability. The derived response rate of the strain sensor was about 400 ms in cyclic loading−unloading conditions of εt = 2.0% (Figure S2, Supporting Information). The ΔR/R0 values measured over ∼1500 bending cycles had standard deviations of 0.88 and 0.32, at εt ≈ 2.0% strain and when relaxed, respectively, confirming the durability of the printed strain sensor. We also studied the response of the sensor as a function of the applied compressive strain (εc). Linear regression gives the GF of the sensor as 12.87 with an R-square value of 0.99 (Figure 3e). Overlapping of MWNTs otherwise segregated by microcracks occurs when compressive strain is applied, leading

We evaluated the piezoresistivity of a thermally treated MWNT−PVP line as a function of applied strain. The flexible CNT sensor was assembled by printing a line of WL = 60 μm and LL = 5 mm on a PI substrate, and tensile strain was applied by bending the outer surface using a home-made bending equipment; the change in resistance (ΔR/R0) was measured by 10 tests as a function of strain (Figure 3a, inset). An MWNT− PVP line with highly concentrated MWNT printed by the meniscus-guided printing contributed us to reduce the sensing variability of sensors (Figure S1, Supporting Information). The strain applied to the sensor can be expressed as ε = ±h/2r, where h and r are the thickness (∼240 μm) and bending radius of the sensor, respectively. As shown in Figure 3a, ΔR/R0 of the sensor exhibited a linear response under applied tensile strains (εt) of 0 to 2.0%. The GF of the sensor was 13.07, and the coefficient of determination (R-square) was 0.98. Figure 3b shows the morphological changes of the printed line under tensile strain. As the applied strain increases, the number of microcracks generated increases gradually; several microcracks can clearly be observed at εt ∼2.0%. The cracks affect ΔR/R0 of the line during the bending test. The D

DOI: 10.1021/acsami.8b04073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Demonstration of human-motion detection using the sensor under applied tensile strain. When ΔR/R0 is within a specified range, LEDs with different colors are illuminated. (b) HMI for remote control of a robotic finger: the robotic finger bends/unbends when the forefinger of the operator has a bending degree (θf) of 130° or 0°. The response, ΔR/R0, changes stably throughout repeated variation of θf.

In the strain range from 0 to 2.0%, the printed strain sensors have GFs of 13.07 and 12.87 under tensile and compressive strains, respectively. Printed strain sensors in this work exhibit higher GFs (and larger strain range) than strain sensors fabricated by other printing methods with other nanomaterials (Table S1, Supporting Information). The response, ΔR/R0, of the sensors is attributed to the contact rearrangement of the MWNTs and the variation of the tunneling distance between the MWNTs caused by applied strain. The sensor has good long-term stability and reproducibility during multiple bending cycles (∼1500 cycles) at 2.0% strain. The printed MWNT− PVP sensors are applied to human-motion detection and realtime control of a robot through finger motion. The applications presented in this work are only a few examples of the huge variety of devices achievable through the described approach. We show that the meniscus-guided printing approach and MWNT−PVP inks can be applied to fabricate flexible CNT strain sensors for emerging HMI applications.

to a decrease in the resistance of the strain sensor. The red arrows in Figure 3f indicate overlapping MWNTs along cracks (yellow dotted lines) at εc ≈ 2.0%. We further investigated the potential of the flexible strain sensor for use in HMI, where it is attractive for applications involving smart robotics and wearable electronic devices. To test under both tensile and compressive strains, we attached the sensor onto the forefinger of a glove (Figure 4). In Figure 4a, the measured ΔR/R0 of the sensor is divided into four different ranges, from 0 to 25% with 6% steps, for human-motion detection. Each range is assigned a different light-emitting diode (LED) color (yellow, red, blue, and white). Upon bending the forefinger, ΔR/R0 of the sensor increases because of tensile strain; as the response value of the sensor moves within the specified range, the appropriate LED is illuminated (Movie S4, Supporting Information). Figure 4b shows HMI by application of compressive strain to the sensor attached to the glove. The linear response of the sensor is wellmatched with the full range of forefinger bending (θf), from 0° to 130°. The robotic finger bends and unbends repeatedly, according to the forefinger motion of the operator (Movie S5, Supporting Information). The sensing response was recorded for repeated cycles of bending/unbending; the results indicate that the printed strain sensors are able to monitor human movements and are suitable for wearable devices and HMI devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04073. Response of the 10 sensors and its standard deviation as a function of εt between 0 and 2.0%; response of the printed strain sensor in cyclic loading−unloading conditions; and summary of performance results of reported flexible printed strain sensors (PDF) Printing of MWNT−PVP lines (AVI) Printing of MWNT−PVP zigzag pattern (AVI) Printing of MWNT−PVP line on the curved substrate (AVI) Human motion detection using flexible strain sensor under tensile strain (AVI) Remote control of a robotic finger using flexible strain sensor under compressive strain (AVI)

4. CONCLUSIONS We report a simple and convenient method for the fabrication of flexible strain sensors via meniscus-guided printing of a CNT fluid ink, formulated from MWNTs and PVP; partial removal of PVP via thermal treatment is then required. In our approach, the CNT ink is extruded continuously by moving a micronozzle, without requiring application of pressure. We have presented a few examples of MWNT−PVP micropatterns printed on a flexible substrate and on a curved substrate with a curvature radius of 3.85 cm. E

DOI: 10.1021/acsami.8b04073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.H.C.). *E-mail: [email protected]. (S.K.S.). ORCID

Seung Kwon Seol: 0000-0002-8733-4374 Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Korea Electrotechnology Research Institute (KERI) Primary research program through the National Research Council of Science & Technology (NST) funded by the Ministry of Science and ICT (no. 18-12N0101-17).

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ABBREVIATIONS AFM, atomic force microscopy; A-MWNT, acid-treated MWNT; CNT, carbon nanotube; FE-SEM, field-emission scanning electron microscopy; GF, gauge factor; HMI, human−machine interface; MWNT, multiwall nanotube; PI, polyimide; PMMA, polymethyl methacrylate; PVP, polyvinylpyrrolidone; TGA, thermogravimetric analysis

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REFERENCES

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DOI: 10.1021/acsami.8b04073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b04073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX