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A flexible two dimensional force sensor using PDMS nanocomposite
Microelectronic Engineering 174 (2017) 64–69
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
A flexible two dimensional force sensor using PDMS nanocomposite Tallis H. da Costa a, Jin-Woo Choi a,b,⁎ a b
School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, USA Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, LA 70803, USA
a r t i c l e
i n f o
Article history: Received 28 October 2016 Received in revised form 18 January 2017 Accepted 2 February 2017 Available online 6 February 2017 Keywords: Carbon nanotubes Pattern transfer Flexible force sensor PDMS
a b s t r a c t This paper presents a two dimensional force sensor fabricated on PDMS nanocomposite, with patterned carbon nanotubes (CNTs) acting as a force sensing unit. A novel fabrication method is composed of inkjet printing of CNTs onto polyethylene terephthalate (PET) and subsequent transfer of the CNT patterns to PDMS, resulting in a CNT-elastomer nanocomposite that is flexible and conductive. This approach allows patterning of a largearea conductive carbon nanotube pattern on PDMS. The achieved sheet resistance of the transferred patterns on PDMS was 1.2 kΩ/□ when printed 35 times, using an office inkjet printer. The fabricated sensor changes its resistance when force is applied perpendicularly to the sensor. A two dimensional force sensor, working on the principle of compression-induced deformation was fabricated and characterized with achieved resolution of four sensing cells per cm2. Additionally, we demonstrate a two dimensional flexible force sensor capable of creating a pressure map of the applied force. Together with inkjet printing, this pattern transfer process represents a highly effective patterning technique for embedding carbon nanotubes in PDMS. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Flexible sensors and electronics are a promising technology for use in applications including human-like touch sensing [1,2], wearable electronics [3], flexible strain gauges, etc. Flexibility can be achieved using elastomer as the structural material, since they allow a very high degree of stretching and bending, and conductivity can be achieved by embedding conductive nanomaterials in the polymer matrix. For the structural material, elastomer like poly(dimethylsiloxane) or PDMS provides conformal attachment to the substrate surface, allowing it to be employed on non-flat surfaces such as a fingertip or a foot sole. Among the most versatile elastomers is PDMS, due to its easy fabrication, biocompatibility and possibility of stretching more than 40% of its original length [4]. PDMS has been used in a variety of applications providing both structural and functional uses. However, the PDMS polymer matrix itself is considered non-conductive, so that it is necessary to embed a conductive material, thus creating a polymer-conductive nanocomposite. Carbon nanotubes (CNTs) have been used as the conductive nanomaterial, since by different deposition methods it is possible to form a thin film inside the polymer matrix, allowing electric current to pass depending on the nanotube network density. Compared to other nanostructures, carbon nanotubes have high aspect ratio (length over
⁎ Corresponding author at: School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, USA. E-mail address: [email protected] (J.-W. Choi).
http://dx.doi.org/10.1016/j.mee.2017.02.001 0167-9317/© 2017 Elsevier B.V. All rights reserved.
width), which results in lower percolation threshold. In addition, they have been shown to provide high electrical conductivity [5]. Many approaches have been developed for depositing carbon nanotubes in polymer matrices, such as in situ polymerization [6], solution processing [7–9], transfer printing [10,11], screen printing [12], and others [13,14]. Our group has developed a method of patterning conductive PDMS nanocomposites by combining microcontact printing and cast molding techniques [15]. Printing [16], spraying [17,18] and aerosol-based [19] fall into the solution processing category, that is, carbon nanotubes are dispersed in a solvent for posterior deposition. Loh et al. [20] have fabricated thin films of single-walled carbon nanotubes using the layer-by-layer (LbL) assembly method, in which they sequentially immersed the sample in a polycation and a polyanion, thus assembling a film with desired number of layers. The films formed were characterized in terms of SWCNT concentration, poly(sodium 4-styrene-sulfonate) (PSS) concentration and number of layers, to obtain the sensor performance for a RFID-type strain sensor that can be embedded into structural materials for remotely monitoring structural strain. Pinto et al. [21] studied the properties of MWCNT thin film deposited by spray deposition. In this process, they used MWCNT as the sensing material, PSS and NMP for dispersion of carbon nanotubes in solution. They characterized the monotonic and cyclic flexural loading both under tension and compression, in order to compare the suitability of carbon nanotubes and carbon fibers for strain monitoring. The results indicated a higher sensitivity for carbon fibers when applied to flexural loadings. The airbrush deposition has been employed to form a MWCNT-latex film for structural health monitoring [22]. The MWCNT
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were dispersed with PSS solution and latex was added for improved adhesion on PET. In [23], the layer-by-layer method was used to fabricate carbon nanotube films on polyimide substrate for structural health monitoring. In that study, the optical lithography was used to pattern the films, which is an expensive method. In one of the earliest studies [24], MWCNT carbon nanotubes were inkjet-printed onto transparency foil and paper. In that study, the carbon nanotubes were functionalized with acid treatment and achieved sheet resistivity was 40 kΩ/□ with multiple prints. Methods such as drop casting, spin coating, vacuum filtration, and Meyer rod deposition have also been employed for the deposition of carbon nanotubes [25]. In this work, we present a practical deposition process because it allows patterning of carbon nanotube films through the inkjet printing step, without the need of mask, stencil, or photolithography, coupled with the transfer of the patterns to PDMS providing a simple and costeffective method for depositing carbon nanotubes onto PDMS. Inkjet printing has shown to be an efficient deposition method due to the simplicity of the process, potential for large scale manufacturing and the advantageous patterning capability for CNT films. However, inkjet printing of large area films of CNTs directly on PDMS is difficult due to irregular wetting and evaporation of the ink solvent. Therefore, in the proposed method herein the CNT dispersion is first inkjet-printed onto PET sheet and then transferred to PDMS. This process is classified under the transfer printing class [26]. However, the film patterning occurs during printing, as opposed to at later stages [27]. This approach allows uniform printing of CNTs onto PET and subsequently the PDMS to fill the micro pores of the coating material. The resulting carbon nanotubes are embedded into the polymer matrix, thus forming a shallow layer of conductive film. While other methods use the surface adhesion of CNTs to a stamp in order to perform the transfer step, our method employs the uncured PDMS to mold around the CNTs actively embedding the CNTs inside the polymer. When the PDMS is cured and peeled off, then the CNTs are actually transferred from PET to PDMS. Moreover, no stamps or masks are necessary. Here we report on a new method of patterning carbon nanotubes on PDMS, which consists of inkjet printing CNTs on poly(ethylene terephthalate) (PET) film sheets and later pattern-transfer to PDMS. The inkjet printing of CNTs allows for a low cost method of patterning fine structures of CNTs as well as simple and rapid development by the use of a CNT solution (ink), while the PDMS provides flexibility and conformal adjustment to non-planar surfaces. As a demonstration of the technology proposed, an all-polymer two dimensional force sensor that is flexible and stretchable was fabricated and characterized. Applications for the force sensor include human-like touch sensors and conformal sensor array for feet pressure mapping, among others.
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2.2. Transfer printing process One of the challenges of using water-based ink is to find a suitable substrate that absorbs the ink instantly, in order to have a sharp pattern characteristic without blur and to avoid the coffee stain effect [16]. To minimize this issue, we employed the inkjet printing of CNTs onto a PET film and sequentially transferring the CNT patterns to PDMS. To this end, a PET film designed for inkjet printing (Inkpress ITF851150) was used. The CNT ink was printed several times in order to obtain a good uniformity of the CNT film, as shown in Fig. 1(a). This is necessary because the printing head ejects micro droplets of ink onto the film, which are sparse from one another. As more prints are performed, the coverage area increases and the network starts to be conductive at 4 to 5 prints. For the PDMS preparation, PDMS base was thoroughly mixed with curing agent (weight ratio 10:1) and placed in vacuum to remove air bubbles. Then, PDMS was cast over the surface of the pattern (Fig. 1(b)) and the sample was spun at a speed of 300 RPM (which results in a PDMS thickness of ~300 μm). After this step, the sample was again vacuumed to remove remaining bubbles and finally cured in oven at 90 °C for 2 h.
2. Experimental details 2.1. Ink solution preparation Ink solution was prepared with multi-walled carbon nanotubes (MWCNT) from Cheap Tubes Inc. (Brattleboro, Vermont), used without modification and sodium n-dodecyl sulfate (SDS) from Alfa Aesar (Ward Hill, Massachusetts). In this work, MWCNTs were chosen due to their low cost and broad availability. First an ink solution was prepared containing 1 wt% of MWCNT, 0.6 wt% of surfactant SDS and 5 ml deionized water. MWCNTs and SDS were mixed in a vial and then sonicated for 30 min. Following, the dispersion was centrifuged with a rotating speed of 12,000 rpm for 5 min. Subsequently, the supernatant was recovered with a syringe, leaving behind aggregates and bundles of carbon nanotubes. The resultant dispersion of CNTs was directly injected into a printer cartridge for later inkjet printing. The inkjet printer used was an HP Envy 4501 with a corresponding cartridge for the printer. The cartridge was opened and thoroughly cleaned before addition of the carbon nanotube ink.
Fig. 1. The transfer printing process: (a) inkjet printing of carbon nanotube ink on a PET film; (b) casting PDMS over the printed carbon nanotube pattern; (c) PDMS peeled off after curing, and (d) bonding top and bottom layers of PDMS for device fabrication.
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Fig. 2. Schematic showing the experimental setup and device operation: a) picture showing the setup; b) 3D model of the characterization setup highlighting the load cell, which provides force feedback information, and c) schematic of the compression-induced deformation of the CNT film on PDMS matrix. A ball-shaped tip applies force onto the device, making the CNTPDMS composite to deform. The deformation generates a resistance change (piezoresistivity).
After fully cured, the PDMS was peeled off from the PET film, bearing a thin carbon nanotube film which was previously embedded into PDMS during the curing time, as shown in Fig. 1(c). However, a portion of the CNT network remained on PET film because they were strongly attached onto the PET film. Lastly, another layer of corona-treated PDMS was cast on top of the device, which served as protective coating and passivation layer (Fig. 1(d)). The bonding was performed using corona discharge treatment as described elsewhere [28], employing the same procedure and conditions. In this treatment, the instrument creates an electric field which created silanol groups at the surface of PDMS. When two surfaces were brought into contact, the surface groups bonded and exhibited a similar bonding strength obtained using oxygen plasma treatment [28]. Using the transfer printing process described above, two types of sensors were fabricated: (i) a single carbon nanotube line that works as a force sensing resistor and (ii) a two-dimensional force sensor composed of multiple lines of carbon nanotubes, having the capability to map force application in two dimensions, shown in Fig. 1(d). The single line sensor required only two layers of PDMS, whereas the two dimensional sensor required a third layer. Therefore, after bonding the middle
layer, its top surface was also corona-treated and the third layer was assembled, forming a three-layer device. For sheet resistance measurements, patterns with dimensions of 10 mm × 10 mm were printed on PET sheet from 5 to 35 times, and subsequently transferred to PDMS. The sheet resistance was obtained with four-point probe method. 2.3. Force sensing resistor fabrication and measurement A single carbon nanotube line was obtained with the transfer printing process described above, where the carbon nanotubes were embedded in the PDMS substrate (300 μm). A corona-treated PDMS layer (300 μm) was bonded on top for passivation. For characterization, additional PDMS layers were placed below the sensor, giving total substrate thickness of 4.8 mm, shown in Fig. 2(a). For testing the change in resistance upon compression force, a device was built having a 10 × 0.5 mm2 strip of CNT and tested in a universal testing machine (GeoJac Automated Load Actuator). The linear actuator speed was set to 0.1 in./min. The force applied was measured by a load cell (CNC Pacific Weighing), as depicted in the 3D model of Fig. 2(b), and
Fig. 3. Schematic of the two dimensional force sensor: (a) carbon nanotube lines in the x and y directions provide a map of the applied force; (b) picture of the printed CNT film showing well defined and uniform patterns; and (c) picture of the actual device, with snap-in connectors. The layers are bonded with corona treatment (detailed in Section 2.2).
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the resistance was measured with a Keithley 2100 DMM. For characterization of the sensor, the ball indentation experiment was employed [29]. In this experiment, a steel ball of diameter 12.7 mm (0.5 in.) descended with constant speed at normal direction to the sensor, reaching the sample and creating an indentation into the sensor, which caused a strain on the carbon nanotube network, schematically depicted in Fig. 2(c). The ball stayed at maximum indentation for 5 s and then retreated to its initial position. Silicone oil was painted on the ball to prevent probe-sample adhesion. 2.4. Two dimensional force sensor Fabrication of the two dimensional force sensor comprised the transfer of carbon nanotubes to two separate layers of PDMS, depicted in Fig. 3(a). One layer contained lines patterned in the x direction and another layer had lines patterned in the y direction. The middle layer of PDMS was corona-treated and assembled to separate the conductive carbon nanotube lines. All three layers of PDMS have a thickness of 300 μm. The final two dimensional force sensor is illustrated in Fig. 3(c).
Fig. 5. SEM image of the CNT film on PDMS showing good uniformity of the CNT network.
The sheet resistance of the CNT film depends on the number of prints of CNT ink. As the number of prints increases, a thicker layer of CNT forms on the surface of the substrate, thus lowering the sheet resistance of the film. Other factors that influence the resistivity are the CNT and SDS concentrations [30], since the weight percentage of CNTs is directly related to the percolation of the CNT network (i.e., the ability to form a conductive path), that is, a solution containing higher CNT weight percentage will deposit more CNTs; and finally the SDS, which bonds to the CNT surface in order to prevent bundling of CNTs, with the downside of lowering the ability of making good contacts with other carbon nanotubes. The sheet resistance was measured both on PET film and on PDMS when varying the number of prints (Fig. 4). When printed at least 20 prints, the error bars show small variations between the samples measured (n = 5), which confirms the reproducibility of the process. The achieved sheet resistance on PDMS was 1.2 kΩ/□ with 35 prints. It is clear that the sheet resistance decreases a great extent on the first prints, but reaches a bulk value when printed enough times. Fig. 4 provides the sheet resistance of CNT film on both PET and PDMS substrate, with respect to the number of prints. The number of prints is the controllable variable for the inkjet printing deposition process, such that one layer of CNT is added to the film at each print. In that case, the desired sheet resistance of the CNT feature can be directly related to the necessary number of prints. Additionally, the ratio between
the sheet resistance on PDMS and the sheet resistance on PET provides a measure of how much the resistance increased after the carbon nanotubes have been transferred to PDMS. Fig. 5 shows the SEM micrograph of transferred CNT film on PDMS. The uniformity of the deposited CNTs was maintained after the pattern transfer process, and the CNT network allowed resistive measurement. The sensor exhibits force sensing capability, because when a force is applied perpendicularly to the sensor, the PDMS substrate deforms and generates a deformation-induced strain on the carbon nanotube network, as shown schematically in Fig. 2(c). The network thus changes its resistance according to the strain that resulted from deformation. To illustrate the change in resistance of the sensor when force was applied perpendicularly to the sensor, a ball indentation experiment was performed. In Fig. 6, the applied force went to 0.5 N, staying at that point for 5 s and then returning to 0 N. It was observed a 17% change in resistance of the sensor when the load reached 0.5 N. The PDMS nanocomposite exhibits viscoelastic behavior, where the initial application of force produces a more pronounced resistance change compared to the subsequent forces. To minimize the effect of viscoelastic behavior, the sensor was pre-loaded with a higher force prior to the measurement. As shown in Fig. 6, the response stabilizes at subsequent cycles. In Fig. 6, it was observed that the resistance level does not return to original value, which could be ascribed to stress relaxation caused by the viscoelastic effect in PDMS. Such behavior has been further explored previous publications [31,32]. Although the resistance does not return to original value, when a force is being applied, the difference between the peak and the base value of the resistance is representative of the magnitude of the force.
Fig. 4. The sheet resistance changes with the number of prints. By increasing the number of prints, the CNT film becomes thicker and the sheet resistance decreases. The inset shows that the sheet resistance achieved on PDMS was 1.2 kΩ/□ when printed 35 times.
Fig. 6. Graph showing the force applied on the sensor and the corresponding change in resistance. There is an increase in resistance when the sensor is first subjected to force, then the response stabilizes at later cycles.
3. Results and discussion 3.1. Characterization of the device
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It was found that a minimum number of prints was necessary so the instrument could reliably measure the resistance of the sensor. At the same time, the sheet resistance of the sensor decreased. However, after a high number of prints, the sheet resistance decreased only by a few percent, which indicates that further prints did not improve significantly the sheet resistance. In that case, it was found that 35 prints provided a balance between the overall resistance and sheet resistance.
3.2. Two dimensional force sensor
Fig. 7. Change in resistance of the sensor when load is applied.
Fig. 7 shows the linear region of the ball indentation experiment. The change in resistance reaches 21% when the load is 0.5 N. Due to the small thickness of the PDMS substrate (900 μm), the sensor is expected to operate in the low-pressure regime.
A two dimensional force sensor was fabricated (the device shown in Fig. 3(c)). The sensor contains lines of carbon nanotubes in the x and y directions. Connections to carbon nanotubes were established with the use of snap-in connectors. The initial resistance of each CNT line was measured to be 300 kΩ, excluding the ground portion. To demonstrate force mapping, we applied force on different locations and observed the resistance change of the corresponding carbon nanotube lines, giving a two dimensional map of the force, as shown in Fig. 8. The mechanism of force-sensitive resistance change is based on the electron path inside the CNT network. A PDMS/CNT nanocomposite is
Fig. 8. Two dimensional force sensor: application of force on different locations results in the force map.
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formed when PDMS is spin coated over the printed CNTs and allowed to mold around the CNTs, thus forming a conductive network of carbon nanotubes embedded in PDMS. When a force is applied perpendicularly to the sensor, the CNT network is deformed along with the PDMS matrix, which causes the electron path to break a portion of its connections [31] represented by adjacent carbon nanotubes. The measured results clearly demonstrate that the resistivity of the film increases as the deformation becomes more pronounced and is restored when the load is removed. 4. Conclusions We have developed a method for fabricating conductive nanocomposites of CNTs on PDMS, which allows the patterning of large area CNT films onto PDMS matrix. This transfer printing process only utilizes aqueous solution of carbon nanotubes with surfactant and is extremely simple, requiring only a few steps. First the ink is printed on PET sheet and PDMS is spin coated over the CNTs, which actively embeds the CNT film into PDMS. Subsequently, the PDMS is peeled off and the patterns are transferred to PDMS with high uniformity. The resulting composite is flexible and conforms to the surface. Through resistivity measurements, we verified the electrical path of the CNT network, and SEM pictures showed that the pattern transfer maintains the CNT network after the transferring step. As a demonstration of the technology, an all-polymer and flexible two dimensional force sensor was fabricated and characterized, showing force mapping capability. In addition, the flexibility of the sensor allowed it to conform to a variety of non-flat surfaces, enabling the device to be used as a wearable sensor for pressure and force monitoring. The inkjet printing is a simple, yet efficient way of patterning CNTpolymer composites. The method proposed here allows patterning of large area networks of carbon nanotubes on PDMS, which has been difficult due to incompatibilities between PDMS and aqueous solutions. The demonstrated method avoids the use of masks or stamps, and is well suited for low cost BioMEMS devices and wearable sensors. Acknowledgments This work was funded in part by CAPES Foundation, Ministry of Education of Brazil, Process 18705/12-0. References [1] D.J. Lipomi, M. Vosgueritchian, B.C.-K. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. Bao, Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes, Nat. Nanotechnol. 6 (2011) 788–792, http://dx.doi.org/10.1038/nnano. 2011.184. [2] L. Viry, A. Levi, M. Totaro, A. Mondini, V. Mattoli, B. Mazzolai, L. Beccai, Flexible three-axial force sensor for soft and highly sensitive artificial touch, Adv. Mater. 26 (2014) 2659–2664, http://dx.doi.org/10.1002/adma.201305064. [3] W. Zeng, L. Shu, Q. Li, S. Chen, F. Wang, X.-M. Tao, Fiber-based wearable electronics: a review of materials, fabrication, devices, and applications, Adv. Mater. 26 (2014) 5310–5336, http://dx.doi.org/10.1002/adma.201400633. [4] I.D. Johnston, D.K. McCluskey, C.K.L. Tan, M.C. Tracey, Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering, J. Micromech. Microeng. 24 (2014) 35017, http://dx.doi.org/10.1088/0960-1317/24/3/035017. [5] M.T. Byrne, Y.K. Gun'ko, Recent advances in research on carbon nanotube-polymer composites, Adv. Mater. 22 (2010) 1672–1688, http://dx.doi.org/10.1002/adma. 200901545. [6] C. Park, Z. Ounaies, K.A. Watson, R.E. Crooks, J. Smith Jr., S.E. Lowther, J.W. Connell, E.J. Siochi, J.S. Harrison, T.L.S. Clair, Dispersion of single wall carbon nanotubes by in situ polymerization under sonication, Chem. Phys. Lett. 364 (2002) 303–308, http://dx.doi.org/10.1016/S0009-2614(02)01326-X. [7] J. Hwang, J. Jang, K. Hong, K.N. Kim, J.H. Han, K. Shin, C.E. Park, Poly(3hexylthiophene) wrapped carbon nanotube/poly(dimethylsiloxane) composites for use in finger-sensing piezoresistive pressure sensors, Carbon 49 (2011) 106–110, http://dx.doi.org/10.1016/j.carbon.2010.08.048.
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
A flexible two dimensional force sensor using PDMS nanocomposite Tallis H. da Costa a, Jin-Woo Choi a,b,⁎ a b
School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, USA Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, LA 70803, USA
a r t i c l e
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Article history: Received 28 October 2016 Received in revised form 18 January 2017 Accepted 2 February 2017 Available online 6 February 2017 Keywords: Carbon nanotubes Pattern transfer Flexible force sensor PDMS
a b s t r a c t This paper presents a two dimensional force sensor fabricated on PDMS nanocomposite, with patterned carbon nanotubes (CNTs) acting as a force sensing unit. A novel fabrication method is composed of inkjet printing of CNTs onto polyethylene terephthalate (PET) and subsequent transfer of the CNT patterns to PDMS, resulting in a CNT-elastomer nanocomposite that is flexible and conductive. This approach allows patterning of a largearea conductive carbon nanotube pattern on PDMS. The achieved sheet resistance of the transferred patterns on PDMS was 1.2 kΩ/□ when printed 35 times, using an office inkjet printer. The fabricated sensor changes its resistance when force is applied perpendicularly to the sensor. A two dimensional force sensor, working on the principle of compression-induced deformation was fabricated and characterized with achieved resolution of four sensing cells per cm2. Additionally, we demonstrate a two dimensional flexible force sensor capable of creating a pressure map of the applied force. Together with inkjet printing, this pattern transfer process represents a highly effective patterning technique for embedding carbon nanotubes in PDMS. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Flexible sensors and electronics are a promising technology for use in applications including human-like touch sensing [1,2], wearable electronics [3], flexible strain gauges, etc. Flexibility can be achieved using elastomer as the structural material, since they allow a very high degree of stretching and bending, and conductivity can be achieved by embedding conductive nanomaterials in the polymer matrix. For the structural material, elastomer like poly(dimethylsiloxane) or PDMS provides conformal attachment to the substrate surface, allowing it to be employed on non-flat surfaces such as a fingertip or a foot sole. Among the most versatile elastomers is PDMS, due to its easy fabrication, biocompatibility and possibility of stretching more than 40% of its original length [4]. PDMS has been used in a variety of applications providing both structural and functional uses. However, the PDMS polymer matrix itself is considered non-conductive, so that it is necessary to embed a conductive material, thus creating a polymer-conductive nanocomposite. Carbon nanotubes (CNTs) have been used as the conductive nanomaterial, since by different deposition methods it is possible to form a thin film inside the polymer matrix, allowing electric current to pass depending on the nanotube network density. Compared to other nanostructures, carbon nanotubes have high aspect ratio (length over
⁎ Corresponding author at: School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, USA. E-mail address: [email protected] (J.-W. Choi).
http://dx.doi.org/10.1016/j.mee.2017.02.001 0167-9317/© 2017 Elsevier B.V. All rights reserved.
width), which results in lower percolation threshold. In addition, they have been shown to provide high electrical conductivity [5]. Many approaches have been developed for depositing carbon nanotubes in polymer matrices, such as in situ polymerization [6], solution processing [7–9], transfer printing [10,11], screen printing [12], and others [13,14]. Our group has developed a method of patterning conductive PDMS nanocomposites by combining microcontact printing and cast molding techniques [15]. Printing [16], spraying [17,18] and aerosol-based [19] fall into the solution processing category, that is, carbon nanotubes are dispersed in a solvent for posterior deposition. Loh et al. [20] have fabricated thin films of single-walled carbon nanotubes using the layer-by-layer (LbL) assembly method, in which they sequentially immersed the sample in a polycation and a polyanion, thus assembling a film with desired number of layers. The films formed were characterized in terms of SWCNT concentration, poly(sodium 4-styrene-sulfonate) (PSS) concentration and number of layers, to obtain the sensor performance for a RFID-type strain sensor that can be embedded into structural materials for remotely monitoring structural strain. Pinto et al. [21] studied the properties of MWCNT thin film deposited by spray deposition. In this process, they used MWCNT as the sensing material, PSS and NMP for dispersion of carbon nanotubes in solution. They characterized the monotonic and cyclic flexural loading both under tension and compression, in order to compare the suitability of carbon nanotubes and carbon fibers for strain monitoring. The results indicated a higher sensitivity for carbon fibers when applied to flexural loadings. The airbrush deposition has been employed to form a MWCNT-latex film for structural health monitoring [22]. The MWCNT
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were dispersed with PSS solution and latex was added for improved adhesion on PET. In [23], the layer-by-layer method was used to fabricate carbon nanotube films on polyimide substrate for structural health monitoring. In that study, the optical lithography was used to pattern the films, which is an expensive method. In one of the earliest studies [24], MWCNT carbon nanotubes were inkjet-printed onto transparency foil and paper. In that study, the carbon nanotubes were functionalized with acid treatment and achieved sheet resistivity was 40 kΩ/□ with multiple prints. Methods such as drop casting, spin coating, vacuum filtration, and Meyer rod deposition have also been employed for the deposition of carbon nanotubes [25]. In this work, we present a practical deposition process because it allows patterning of carbon nanotube films through the inkjet printing step, without the need of mask, stencil, or photolithography, coupled with the transfer of the patterns to PDMS providing a simple and costeffective method for depositing carbon nanotubes onto PDMS. Inkjet printing has shown to be an efficient deposition method due to the simplicity of the process, potential for large scale manufacturing and the advantageous patterning capability for CNT films. However, inkjet printing of large area films of CNTs directly on PDMS is difficult due to irregular wetting and evaporation of the ink solvent. Therefore, in the proposed method herein the CNT dispersion is first inkjet-printed onto PET sheet and then transferred to PDMS. This process is classified under the transfer printing class [26]. However, the film patterning occurs during printing, as opposed to at later stages [27]. This approach allows uniform printing of CNTs onto PET and subsequently the PDMS to fill the micro pores of the coating material. The resulting carbon nanotubes are embedded into the polymer matrix, thus forming a shallow layer of conductive film. While other methods use the surface adhesion of CNTs to a stamp in order to perform the transfer step, our method employs the uncured PDMS to mold around the CNTs actively embedding the CNTs inside the polymer. When the PDMS is cured and peeled off, then the CNTs are actually transferred from PET to PDMS. Moreover, no stamps or masks are necessary. Here we report on a new method of patterning carbon nanotubes on PDMS, which consists of inkjet printing CNTs on poly(ethylene terephthalate) (PET) film sheets and later pattern-transfer to PDMS. The inkjet printing of CNTs allows for a low cost method of patterning fine structures of CNTs as well as simple and rapid development by the use of a CNT solution (ink), while the PDMS provides flexibility and conformal adjustment to non-planar surfaces. As a demonstration of the technology proposed, an all-polymer two dimensional force sensor that is flexible and stretchable was fabricated and characterized. Applications for the force sensor include human-like touch sensors and conformal sensor array for feet pressure mapping, among others.
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2.2. Transfer printing process One of the challenges of using water-based ink is to find a suitable substrate that absorbs the ink instantly, in order to have a sharp pattern characteristic without blur and to avoid the coffee stain effect [16]. To minimize this issue, we employed the inkjet printing of CNTs onto a PET film and sequentially transferring the CNT patterns to PDMS. To this end, a PET film designed for inkjet printing (Inkpress ITF851150) was used. The CNT ink was printed several times in order to obtain a good uniformity of the CNT film, as shown in Fig. 1(a). This is necessary because the printing head ejects micro droplets of ink onto the film, which are sparse from one another. As more prints are performed, the coverage area increases and the network starts to be conductive at 4 to 5 prints. For the PDMS preparation, PDMS base was thoroughly mixed with curing agent (weight ratio 10:1) and placed in vacuum to remove air bubbles. Then, PDMS was cast over the surface of the pattern (Fig. 1(b)) and the sample was spun at a speed of 300 RPM (which results in a PDMS thickness of ~300 μm). After this step, the sample was again vacuumed to remove remaining bubbles and finally cured in oven at 90 °C for 2 h.
2. Experimental details 2.1. Ink solution preparation Ink solution was prepared with multi-walled carbon nanotubes (MWCNT) from Cheap Tubes Inc. (Brattleboro, Vermont), used without modification and sodium n-dodecyl sulfate (SDS) from Alfa Aesar (Ward Hill, Massachusetts). In this work, MWCNTs were chosen due to their low cost and broad availability. First an ink solution was prepared containing 1 wt% of MWCNT, 0.6 wt% of surfactant SDS and 5 ml deionized water. MWCNTs and SDS were mixed in a vial and then sonicated for 30 min. Following, the dispersion was centrifuged with a rotating speed of 12,000 rpm for 5 min. Subsequently, the supernatant was recovered with a syringe, leaving behind aggregates and bundles of carbon nanotubes. The resultant dispersion of CNTs was directly injected into a printer cartridge for later inkjet printing. The inkjet printer used was an HP Envy 4501 with a corresponding cartridge for the printer. The cartridge was opened and thoroughly cleaned before addition of the carbon nanotube ink.
Fig. 1. The transfer printing process: (a) inkjet printing of carbon nanotube ink on a PET film; (b) casting PDMS over the printed carbon nanotube pattern; (c) PDMS peeled off after curing, and (d) bonding top and bottom layers of PDMS for device fabrication.
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Fig. 2. Schematic showing the experimental setup and device operation: a) picture showing the setup; b) 3D model of the characterization setup highlighting the load cell, which provides force feedback information, and c) schematic of the compression-induced deformation of the CNT film on PDMS matrix. A ball-shaped tip applies force onto the device, making the CNTPDMS composite to deform. The deformation generates a resistance change (piezoresistivity).
After fully cured, the PDMS was peeled off from the PET film, bearing a thin carbon nanotube film which was previously embedded into PDMS during the curing time, as shown in Fig. 1(c). However, a portion of the CNT network remained on PET film because they were strongly attached onto the PET film. Lastly, another layer of corona-treated PDMS was cast on top of the device, which served as protective coating and passivation layer (Fig. 1(d)). The bonding was performed using corona discharge treatment as described elsewhere [28], employing the same procedure and conditions. In this treatment, the instrument creates an electric field which created silanol groups at the surface of PDMS. When two surfaces were brought into contact, the surface groups bonded and exhibited a similar bonding strength obtained using oxygen plasma treatment [28]. Using the transfer printing process described above, two types of sensors were fabricated: (i) a single carbon nanotube line that works as a force sensing resistor and (ii) a two-dimensional force sensor composed of multiple lines of carbon nanotubes, having the capability to map force application in two dimensions, shown in Fig. 1(d). The single line sensor required only two layers of PDMS, whereas the two dimensional sensor required a third layer. Therefore, after bonding the middle
layer, its top surface was also corona-treated and the third layer was assembled, forming a three-layer device. For sheet resistance measurements, patterns with dimensions of 10 mm × 10 mm were printed on PET sheet from 5 to 35 times, and subsequently transferred to PDMS. The sheet resistance was obtained with four-point probe method. 2.3. Force sensing resistor fabrication and measurement A single carbon nanotube line was obtained with the transfer printing process described above, where the carbon nanotubes were embedded in the PDMS substrate (300 μm). A corona-treated PDMS layer (300 μm) was bonded on top for passivation. For characterization, additional PDMS layers were placed below the sensor, giving total substrate thickness of 4.8 mm, shown in Fig. 2(a). For testing the change in resistance upon compression force, a device was built having a 10 × 0.5 mm2 strip of CNT and tested in a universal testing machine (GeoJac Automated Load Actuator). The linear actuator speed was set to 0.1 in./min. The force applied was measured by a load cell (CNC Pacific Weighing), as depicted in the 3D model of Fig. 2(b), and
Fig. 3. Schematic of the two dimensional force sensor: (a) carbon nanotube lines in the x and y directions provide a map of the applied force; (b) picture of the printed CNT film showing well defined and uniform patterns; and (c) picture of the actual device, with snap-in connectors. The layers are bonded with corona treatment (detailed in Section 2.2).
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the resistance was measured with a Keithley 2100 DMM. For characterization of the sensor, the ball indentation experiment was employed [29]. In this experiment, a steel ball of diameter 12.7 mm (0.5 in.) descended with constant speed at normal direction to the sensor, reaching the sample and creating an indentation into the sensor, which caused a strain on the carbon nanotube network, schematically depicted in Fig. 2(c). The ball stayed at maximum indentation for 5 s and then retreated to its initial position. Silicone oil was painted on the ball to prevent probe-sample adhesion. 2.4. Two dimensional force sensor Fabrication of the two dimensional force sensor comprised the transfer of carbon nanotubes to two separate layers of PDMS, depicted in Fig. 3(a). One layer contained lines patterned in the x direction and another layer had lines patterned in the y direction. The middle layer of PDMS was corona-treated and assembled to separate the conductive carbon nanotube lines. All three layers of PDMS have a thickness of 300 μm. The final two dimensional force sensor is illustrated in Fig. 3(c).
Fig. 5. SEM image of the CNT film on PDMS showing good uniformity of the CNT network.
The sheet resistance of the CNT film depends on the number of prints of CNT ink. As the number of prints increases, a thicker layer of CNT forms on the surface of the substrate, thus lowering the sheet resistance of the film. Other factors that influence the resistivity are the CNT and SDS concentrations [30], since the weight percentage of CNTs is directly related to the percolation of the CNT network (i.e., the ability to form a conductive path), that is, a solution containing higher CNT weight percentage will deposit more CNTs; and finally the SDS, which bonds to the CNT surface in order to prevent bundling of CNTs, with the downside of lowering the ability of making good contacts with other carbon nanotubes. The sheet resistance was measured both on PET film and on PDMS when varying the number of prints (Fig. 4). When printed at least 20 prints, the error bars show small variations between the samples measured (n = 5), which confirms the reproducibility of the process. The achieved sheet resistance on PDMS was 1.2 kΩ/□ with 35 prints. It is clear that the sheet resistance decreases a great extent on the first prints, but reaches a bulk value when printed enough times. Fig. 4 provides the sheet resistance of CNT film on both PET and PDMS substrate, with respect to the number of prints. The number of prints is the controllable variable for the inkjet printing deposition process, such that one layer of CNT is added to the film at each print. In that case, the desired sheet resistance of the CNT feature can be directly related to the necessary number of prints. Additionally, the ratio between
the sheet resistance on PDMS and the sheet resistance on PET provides a measure of how much the resistance increased after the carbon nanotubes have been transferred to PDMS. Fig. 5 shows the SEM micrograph of transferred CNT film on PDMS. The uniformity of the deposited CNTs was maintained after the pattern transfer process, and the CNT network allowed resistive measurement. The sensor exhibits force sensing capability, because when a force is applied perpendicularly to the sensor, the PDMS substrate deforms and generates a deformation-induced strain on the carbon nanotube network, as shown schematically in Fig. 2(c). The network thus changes its resistance according to the strain that resulted from deformation. To illustrate the change in resistance of the sensor when force was applied perpendicularly to the sensor, a ball indentation experiment was performed. In Fig. 6, the applied force went to 0.5 N, staying at that point for 5 s and then returning to 0 N. It was observed a 17% change in resistance of the sensor when the load reached 0.5 N. The PDMS nanocomposite exhibits viscoelastic behavior, where the initial application of force produces a more pronounced resistance change compared to the subsequent forces. To minimize the effect of viscoelastic behavior, the sensor was pre-loaded with a higher force prior to the measurement. As shown in Fig. 6, the response stabilizes at subsequent cycles. In Fig. 6, it was observed that the resistance level does not return to original value, which could be ascribed to stress relaxation caused by the viscoelastic effect in PDMS. Such behavior has been further explored previous publications [31,32]. Although the resistance does not return to original value, when a force is being applied, the difference between the peak and the base value of the resistance is representative of the magnitude of the force.
Fig. 4. The sheet resistance changes with the number of prints. By increasing the number of prints, the CNT film becomes thicker and the sheet resistance decreases. The inset shows that the sheet resistance achieved on PDMS was 1.2 kΩ/□ when printed 35 times.
Fig. 6. Graph showing the force applied on the sensor and the corresponding change in resistance. There is an increase in resistance when the sensor is first subjected to force, then the response stabilizes at later cycles.
3. Results and discussion 3.1. Characterization of the device
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It was found that a minimum number of prints was necessary so the instrument could reliably measure the resistance of the sensor. At the same time, the sheet resistance of the sensor decreased. However, after a high number of prints, the sheet resistance decreased only by a few percent, which indicates that further prints did not improve significantly the sheet resistance. In that case, it was found that 35 prints provided a balance between the overall resistance and sheet resistance.
3.2. Two dimensional force sensor
Fig. 7. Change in resistance of the sensor when load is applied.
Fig. 7 shows the linear region of the ball indentation experiment. The change in resistance reaches 21% when the load is 0.5 N. Due to the small thickness of the PDMS substrate (900 μm), the sensor is expected to operate in the low-pressure regime.
A two dimensional force sensor was fabricated (the device shown in Fig. 3(c)). The sensor contains lines of carbon nanotubes in the x and y directions. Connections to carbon nanotubes were established with the use of snap-in connectors. The initial resistance of each CNT line was measured to be 300 kΩ, excluding the ground portion. To demonstrate force mapping, we applied force on different locations and observed the resistance change of the corresponding carbon nanotube lines, giving a two dimensional map of the force, as shown in Fig. 8. The mechanism of force-sensitive resistance change is based on the electron path inside the CNT network. A PDMS/CNT nanocomposite is
Fig. 8. Two dimensional force sensor: application of force on different locations results in the force map.
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formed when PDMS is spin coated over the printed CNTs and allowed to mold around the CNTs, thus forming a conductive network of carbon nanotubes embedded in PDMS. When a force is applied perpendicularly to the sensor, the CNT network is deformed along with the PDMS matrix, which causes the electron path to break a portion of its connections [31] represented by adjacent carbon nanotubes. The measured results clearly demonstrate that the resistivity of the film increases as the deformation becomes more pronounced and is restored when the load is removed. 4. Conclusions We have developed a method for fabricating conductive nanocomposites of CNTs on PDMS, which allows the patterning of large area CNT films onto PDMS matrix. This transfer printing process only utilizes aqueous solution of carbon nanotubes with surfactant and is extremely simple, requiring only a few steps. First the ink is printed on PET sheet and PDMS is spin coated over the CNTs, which actively embeds the CNT film into PDMS. Subsequently, the PDMS is peeled off and the patterns are transferred to PDMS with high uniformity. The resulting composite is flexible and conforms to the surface. Through resistivity measurements, we verified the electrical path of the CNT network, and SEM pictures showed that the pattern transfer maintains the CNT network after the transferring step. As a demonstration of the technology, an all-polymer and flexible two dimensional force sensor was fabricated and characterized, showing force mapping capability. In addition, the flexibility of the sensor allowed it to conform to a variety of non-flat surfaces, enabling the device to be used as a wearable sensor for pressure and force monitoring. The inkjet printing is a simple, yet efficient way of patterning CNTpolymer composites. The method proposed here allows patterning of large area networks of carbon nanotubes on PDMS, which has been difficult due to incompatibilities between PDMS and aqueous solutions. The demonstrated method avoids the use of masks or stamps, and is well suited for low cost BioMEMS devices and wearable sensors. Acknowledgments This work was funded in part by CAPES Foundation, Ministry of Education of Brazil, Process 18705/12-0. References [1] D.J. Lipomi, M. Vosgueritchian, B.C.-K. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. Bao, Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes, Nat. Nanotechnol. 6 (2011) 788–792, http://dx.doi.org/10.1038/nnano. 2011.184. [2] L. Viry, A. Levi, M. Totaro, A. Mondini, V. Mattoli, B. Mazzolai, L. Beccai, Flexible three-axial force sensor for soft and highly sensitive artificial touch, Adv. Mater. 26 (2014) 2659–2664, http://dx.doi.org/10.1002/adma.201305064. [3] W. Zeng, L. Shu, Q. Li, S. Chen, F. Wang, X.-M. Tao, Fiber-based wearable electronics: a review of materials, fabrication, devices, and applications, Adv. Mater. 26 (2014) 5310–5336, http://dx.doi.org/10.1002/adma.201400633. [4] I.D. Johnston, D.K. McCluskey, C.K.L. Tan, M.C. Tracey, Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering, J. Micromech. Microeng. 24 (2014) 35017, http://dx.doi.org/10.1088/0960-1317/24/3/035017. [5] M.T. Byrne, Y.K. Gun'ko, Recent advances in research on carbon nanotube-polymer composites, Adv. Mater. 22 (2010) 1672–1688, http://dx.doi.org/10.1002/adma. 200901545. [6] C. Park, Z. Ounaies, K.A. Watson, R.E. Crooks, J. Smith Jr., S.E. Lowther, J.W. Connell, E.J. Siochi, J.S. Harrison, T.L.S. Clair, Dispersion of single wall carbon nanotubes by in situ polymerization under sonication, Chem. Phys. Lett. 364 (2002) 303–308, http://dx.doi.org/10.1016/S0009-2614(02)01326-X. [7] J. Hwang, J. Jang, K. Hong, K.N. Kim, J.H. Han, K. Shin, C.E. Park, Poly(3hexylthiophene) wrapped carbon nanotube/poly(dimethylsiloxane) composites for use in finger-sensing piezoresistive pressure sensors, Carbon 49 (2011) 106–110, http://dx.doi.org/10.1016/j.carbon.2010.08.048.
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