Research Article www.acsami.org
Paper-Based Bimodal Sensor for Electronic Skin Applications Minhyun Jung,† Kyungkwan Kim,† Bumjin Kim,† Haena Cheong,‡ Kwanwoo Shin,‡ Oh-Sun Kwon,‡ Jong-Jin Park,*,§ and Sanghun Jeon*,† †
Department of Display and Semiconductor Physics, Korea University, Sejong 30019, Republic of Korea Department of Chemistry and Institute of Biological Interfaces, Sogang University, Seoul 04107, Republic of Korea § School of Polymer Science and Engineering, Chonnam National University, Gwangju 61186, Republic of Korea ‡
S Supporting Information *
ABSTRACT: We present the development of a flexible bimodal sensor using a paper platform and inkjet printing method, which are suited for low-cost fabrication processes and realization of flexible devices. In this study, we employed a vertically stacked bimodal device architecture in which a temperature sensor is stacked on top of a pressure sensor and operated on different principles, allowing the minimization of interference effects. For the temperature sensor placed in the top layer, we used the thermoelectric effect and formed a closedloop thermocouple composed of two different printable inks (conductive PEDOT:PSS and silver nanoparticles on a flexible paper platform) and obtained temperature-sensing capability over a wide range (150 °C). For the pressure sensor positioned in the bottom layer, we used microdimensional pyramidstructured poly(dimethylsiloxane) coated with multiwall carbon nanotube conducting ink. Our pressure sensor exhibits a high-pressure sensitivity over a wide range (100 Pa to 5 kPa) and high-endurance characteristics of 105. Our 5 × 5 bimodal sensor array demonstrates negligible interference, high-speed responsivity, and robust sensing characteristics. We believe that the material, process, two-terminal device, and integration scheme developed in this study have a great value that can be widely applied to electronic skin. KEYWORDS: e-skin, flexible device, paper electronics, inkjet printing, wearable device
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devices.21 Beyond these well-known advantages, paper has physical characteristics that endow it with other potential functionalities.22 Cellulose fiber is the main component of the paper and can be made thin, lightweight, and low-cost by pulp processing,23 making it suitable for various applications because liquids can penetrate into the hydrophilic fiber assembly without any additional active pumping process or external source.24 Besides, paper’s properties such as reactivity, permeability, and hydrophilicity can be changed by artificial means.25 In addition to these benefits, paper has flexibility that is necessary for wearable devices. Many researchers have studied paper-based photodiodes,26 thin-film transistors (TFT),27 displays,28 and radio frequency identification devices (RFID).29 These developments are focused on the technological advancement to produce new types of devices.30 User interface and input signal are required for new types of electronic devices that interact with humans.31−34 To detect an input signal from users, tactile-based or touch-sensing flexible electronics can be used. These devices detect resistance change35 or capacitance.36 Definitely, technologies that are being commercially developed for flexible touch-based systems for display or control equipment could be adopted to various
INTRODUCTION Recently, there has been a significant interest in the development of body-attachable electronic skin (e-skin) that is significant for the realization of artificial intelligence requiring direct contact with humans. Tactile,1 temperature,2 and vibration3 sensing as well as flexibility4 and a low-cost fabrication process are key requirements in advanced artificial intelligence applications.5 To fabricate e-skins that mimic the sensing capability of human skin,6 diverse approaches have been suggested for different transduction modes7 including those employing resistive, 8 capacitive, 9 piezoelectric, 10 strain,11,12 and triboelectric sensors.13 In particular, to detect various physical and chemical stimuli, e-skins with multiple functionalities based on the integration of each sensor on flexible substrates14−16 have been demonstrated. However, most of the previous studies focused either on the detection of only one type of mechanical stimuli or developed e-skins that were not capable of discriminating multiple mechanical stimuli and their directions.17 To realize multifunctional sensors that can be used in applications such as wearable electronics18 and healthcare-monitoring systems,19 various sensing capabilities should be manifested. Integrating individual temperature- and pressure-sensing systems into a single pixel is a convenient approach for realizing a bimodal detection device.20 Paper is a common, ecofriendly, low-cost, and easy-to-make material and is of significant interest as a substrate for electronic © XXXX American Chemical Society
Received: April 24, 2017 Accepted: July 19, 2017 Published: July 19, 2017 A
DOI: 10.1021/acsami.7b05672 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustration of inkjet printing method and material analysis of thermoelectric ink. (a) Fabrication process of sensor with inkjet printing technique. (blue leg, p-type thermoelectric material; red leg, n-type thermoelectric material). SEM images of (b) multiwall carbon nanotube (MWCNT), (c) composite of PEDOT:PSS doped with DMSO, and (d) silver nanoparticles (AgNPs) on paper substrate. Scale bars are 1 μm. Raman spectra of (e) MWCNT, (f) PEDOT:PSS doped with DMSO, and (g) AgNP.
developments in disposable and paper-based electronics.37 In addition, with the development of inkjet printing technology, the most suitable substrate for a solution-based process is paper, which enables a low-temperature process that is different from conventional semiconductor fabrication processes requiring high-temperature thin-film deposition and annealing. The inherent characteristics of paper that allow for liquid transfer and its availability with other materials are the major benefits of paper-based sensors.21 Here, we demonstrate a flexible bimodal sensor that utilizes a paper platform and an inkjet printing method, which are suitable for a low-cost fabrication process and the realization of flexible devices. We employed a vertically stacked bimodal device architecture in which a temperature sensor was stacked on top of the pressure sensor and the sensors are operated on the basis of different principles (piezoresistive effect and thermoelectric effect). This approach involved transducing an external signal into separate electric signals and minimizing interference effects without decoupling the data while different physical stimuli were applied at the same time. For the temperature sensor, we utilized the thermoelectric effect (TE) and formed a closed-loop thermocouple composed of two different printable inks such as a conductive polymer PEDOT:PSSand silver nanoparticles (AgNPs) on a flexible paper platform and obtained good temperature-sensing performance over a wide range (∼150 K). For the pressure
sensor, we used microdimensional pyramid-structured poly(dimethylsiloxane) (PDMS) coated with multiwall carbon nanotube (MWCNT) conducting ink, which demonstrated a high-pressure sensitivity over a wide range under 100 Pa to 5 kPa with high-endurance characteristics of 105. Our 5 × 5 bimodal sensor array demonstrates negligible interference, high-speed responsivity, and robust sensing characteristics. Since the core materials used in this study (including organic materials, nanoparticles, thermoelectric materials, silicon rubber, and paper platforms) are inherently flexible and inexpensive even for a large area, these materials can be widely used in flexible sensor and electronic sensor applications.
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EXPERIMENTS
Figure 1a illustrates the schematic of the manufacturing process using inkjet printing technology. We used different conducting inks as the core ingredients in the bimodal sensor. For creating conductive inks, we prepared a blended conductive polymer solution. Before polymer blending, the PEDOT:PSS solution was mixed with DMSO and FC5120 (3 M fluorosurfactant) (94:5:1 (w/w/w)). DMSO and fluorosurfactant were reported to enhance the conductivity of PEDOT:PSS. The PEDOT:PSS was stirred at a high speed (>500 rpm) while blending and adding different amounts of DMSO mixed with FC-5120. All polymer mixtures were filtered through a syringe filter (0.5 μm pore size) after mixing. As another conductive solution, MWCNT powder (TCI America) and dispersing solution were mixed by using a magnetic stirring system with deionized water (0.4:13:86.6 (w/w/w)) and mixed at 80 °C for 5 h. The dispersing solution was B
DOI: 10.1021/acsami.7b05672 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. Overview of the flexible bimodal sensor using printing technique. Fabrication process of (a) temperature sensor using the printing method (left column), pressure sensor (right column), and measurement setup (bottom of Figure 1a), (b) Principle of the thermoelectric temperature sensor and (c) sensing mechanism of micropyramid-structured pressure sensor. (d) Equivalent circuit diagram of the bimodal sensor matrix in a single pixel. dissolved in DI water with 1-naphthylacetic acid, poly(ethylene glycol), and monomethyl ether (2:2:1) mixed at a slow speed < 50 rpm for 5 h at 60 °C. The long MWCNT powder-bundled solution blocks the inkjet printing nozzle easily. To solve this problem, we employed a more effective mechanical ball-milling process. During the ball-milling process, the heterogeneous solution was mixed at a frequency of 0.3 cycles/s for 3 days with alumina beads of 3 mm diameter. This results in the homogenization of the agglomerated and cross-coupled MWCNT to micrometer size. After the solution was centrifugally separated at 3000 rpm for 30 min in a centrifuge, the nondispersed MWCNT could be separated and finally the MWCNT ink for inkjet printing was obtained. These solutions, including the additionally purchased AgNP ink, were printed on a PDMS substrate and a paper substrate. The fabrication of pressure sensor that works based on relative conductance change was started with the PDMS elastomer and its hardener (mixed in a 10:1 weight ratio). After that, air bubbles were eliminated in a vacuum jar. The compound was poured onto a KOH-etched silicon pyramid-patterned mold. After a thermal curing process at 80 °C for 1 h, the pyramid-array-patterned PDMS was peeled off from the mold. The MWCNT solution was then printed onto the PDMS with an inkjet printer (Dimatix DMP-2831), and the excess solution was drained away by annealing it on a hot plate at 70 °C for 30 min. Next, the top electrode of the pressure sensor was printed at the back side of the paper. Finally, we printed a thermocouple that works as a temperature sensor on the front side of the paper.
metal materials with different properties. Therefore, we used an inkjet printing method that enabled large-area, low-cost processes using easy-to-print AgNP, MWCNT, and PEDOT: PSS. From literature, it was reported that the work-function values of the conductive ink of MWCNT, AgNP, and PEDOT:PSS are 4.5, 4.3, and 5 eV, respectively. Here, we were able to maximize the thermoelectric coefficient by employing the heterojunction of AgNP and PEDOT:PSS with high work-function difference. The resistance change of the pressure sensor depends on the structural deformation of elastic micropyramid under the external force rather than the initial material properties of the conductive material. However, when the initial resistance of conductive ink is high, then, the pressure sensor requires high operation voltage. Thus, we used low- resistivity MWCNT ink for the pressure sensor among three inks. In this study, we performed the material analysis of inkjet printed conductive materials on paper. As shown in Figure 1b−d, a nanoscale morphology of the thermoelectric materials was obtained using a scanning electron microscope (SEM). The SEM images of inkjet printed films such as MWCNT, PEDOT:PSS, and AgNP were recorded for a paper substrate. As shown in Figure 1e−g, we measured the Raman spectra of the printed thermoelectric materials using a laser with a wavelength of 532 nm. Typically, MWCNT exhibits Raman peaks in the G and D bands, which are similar to silver nanoparticles.38 The Raman spectra (Figure 1e) exhibit a peak at 1593 cm−1 (G band, high-frequency E2g first-order mode), and a peak at 1359 cm−1 (D band, disorder-induced mode) of the MWCNT. 39 We confirmed the Raman peaks of
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RESULTS AND DISCUSSION The bimodal sensor has a thermoelectric temperature sensor and a pressure sensor printed on a laminated micropyramidstructured substrate. In thermoelectric temperature sensors, the junctions must be formed by bonding two semiconductors or C
DOI: 10.1021/acsami.7b05672 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. Output characteristics of the temperature sensor. (a) Output Seebeck voltage and (b) Seebeck coefficient of thermocouple formed with AgNP and PEDOT:PSS on various substrates. Inset shows zoomed data. (c) Output Seebeck voltage and (d) Seebeck coefficient of various thermocouples on paper substrate. (e and f) Number of working devices as a function of (e) bending cycles and (f) bending diameter with various thermocouples: TC1, thermocouple formed with AgNP and PEDOT:PSS; TC2, composite of (AgNP and MWCNT) with PEDOT:PSS; TC3, PEDOT:PSS and MWCNT; TC4, AgNP and MWCNT. Output Seebeck voltage of thermoelectric temperature sensor (g) without PDMS passivation and (h) with passivation as a function of bending times. (i) Characteristics of thermoelectric-based temperature sensors for various passivation materials (polyimide, PDMS, and CYTOP).
into electric energy. It is difficult to make printable inks with most of the thermoelectric materials that are currently being researched such as Bi2Te3, Sb2Te3, SiGe, and BiSb. Therefore, in our study, we used a conductive polymerPEDOT:PSS and AgNPs as the printable ink for a flexible substrate, where the junction of both materials was used for a thermoelectric temperature sensor. For sensing pressure, we used a microsized pyramid structure with PDMS and coated it with MWCNT to fabricate highly flexible sensor devices. These commercially available materials and device fabrication processes are suitable for solution-based technology, which enables them to become an essential constituent for inkjet printed devices.42−44 The back side of the thermoelectric temperature sensor printed with MWCNT forms the counter electrode for the pressure sensor. As pressure is applied, the contact area between the micropyramid structure coated with conductive ink and the counter electrode is changed, as shown in Figure 2c. To elucidate the resistance changes in the pressure sensor with conductive micropyramid, a simple circuit model is proposed, as shown in Supporting Information Figure S2. The resultant
PEDOT:PSS and Ag nanoparticles. Figure 1f shows the Raman spectra of conductive polymer PEDOT:PSS doped with dimethyl sulfoxide (DMSO). The Raman peaks of PEDOT and PSS were reported in a previous article.40 The peaks of the vibration mode of PEDOT are located at 1259 cm−1 (inter-ring stretching vibrations), 1369 cm−1 (stretching), 1434 cm−1 (symmetrical), and 1511 cm−1 (asymmetrical).41 The Raman spectra of AgNP with the G and D bands are shown in Figure 1g. Figure 2 shows the fabrication method and the pixel structure of our bimodal sensor. We fabricated a bimodal sensor with a vertical structure in which the temperature sensor is placed on top of a pressure sensor. As shown in Figure 2a, the upper layer of the bimodal sensor uses the thermoelectric-based self-powered temperature sensor, and the lower part is composed of a pressure sensor that uses piezoresistance characteristics (change in the resistance with applied pressure). Each step is presented in Figure S1 (Supporting Information). Figure 2b illustrates the principle of thermoelectricity used in the temperature sensor. According to the Seebeck effect, the thermocouple (Figure 2b) has the capability to convert heat D
DOI: 10.1021/acsami.7b05672 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. Evaluation of the pressure sensor characteristics with relative change in current. (a) Schematic illustration of the pressure sensor measurement. (b) Changes in current due to loading and unloading. (c) Response curve of pressure sensor under loading−unloading cycles at a frequency of 6 Hz. (d) Response curve of pressure sensor with applied voltage of 10 mV under 100 Pa. Inset shows the response time. Electrical characteristics of pressure sensor: (e) I−V curves under different pressure-loading conditions (100 Pa to 5 kPa). (f) Relative current change (ΔI/I0) as a function of applied pressure. (g) Endurance test of pressure sensor with a repetition of 100,000 loading−unloading cycles at a frequency of 1 Hz under an applied pressure of 2 kPa.
LCI is the length of the contact area between the upper electrode and the lower electrode. As shown in Supporting Information Figure S2, the change in conductance (Go − G = ΔG) of the sensor under pressure is given by eq 3.
conductance (G) of the pyramid comprises the conductances of the counter electrode (GCE), contact interface (GCI), and piezoresistive electrode (GPE) as shown in eq 1. G=
GCE + GCI + G PE GCEGCIG PE
(1)
⎧ L L PE ⎫ ⎬ ΔG = 1/⎨ΔρCI CI + ΔρPE A CI DPEWPE ⎭ ⎩
Considering that the highly conductive counter electrode [GCE ≫ (GCI + GPE)] and that the other factors are influenced by geometry, eq 1 can be revised as follows: G=
A CIDPEWPE ρCI LCIDPEWPE + A CIρCI LCI
(3)
Equation 3 is the mathematical formula for the conductance change in the pressure sensor with respect to the geometrical change due to the contact interface and the printed electrode on contact with the counter electrode when the pressure is applied.45 Figure 2d shows the array circuitry of the bimodal sensor, and the right inset shows its single-pixel circuit. To verify the thermoelectric characteristic of the transducer
(2)
where AC is the contact area, DPE is the contact interface, WPE is the perimeter, ρCI is the resistivity of the counter electrode, and E
DOI: 10.1021/acsami.7b05672 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
cycles. However, after the sensor was encapsulated with a PDMS passivation layer (as shown in Figure 3h), the device retained its initial characteristics even after more than 1000 bending cycles (at a bending diameter of 10 mm). As seen in Figure 3i, even with various passivation materials (polyimide (PI), PDMS, and CYTOP), the performance of the temperature sensor is equivalent to that of a pristine device without passivation, indicating that the passivation does not adversely affect the temperature-sensing performance. The measurement system and the electrical properties of thermoelectric materials with bending stress are presented in Supporting Information Figures S4 and S5. Next, we evaluated the pressure-sensing performance of the device. To measure the pressure response time, the custombuilt pressure-applying system (shown in Figure 4a) was employed. This device can apply a wide range of pressures and repeated stress. Figure 4b presents the current change characteristics of the sensor with the applied pressure. As shown in Figure 4c, the delay time between the dynamic force measurement system (blue line) and the sensor response (black line) was measured as approximately 25 ms. Figure 4d shows the relative change in the current upon the application of a pressure of 100 Pa under an extremely low operating voltage (∼10 mV). The inset shows a fast response time (
