A Flexible Multifunctional Tactile Sensor Using Interlocked ZnO Nanorod Arrays for Artificial Electronic Skin Procedia Engineering 168 ( 2016 ) 1044 – 1047 1877 7058 © 2016 Published by Elsevier Ltd T[.]
Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 168 (2016) 1044 – 1047 30th Eurosensors Conference, EUROSENSORS 2016 A Flexible Multifunctional Tactile Sensor Using interlocked ZnO Nanorod Arrays for Artificial Electronic Skin Min-Sheng Suen, Yi-Cheng Lin, Rongshun Chen * Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Abstract This study presents a novel multifunctional tactile sensor to mimetic the human skin with high sensitive, flexible, and temperature measurable performances The substrate is flexible due to the material property of the Polydimethylsiloxane (PDMS) The top electrode and bottom electrode layers are interlocked by the zinc oxide (ZnO) nanorods, which is vertically growing on the top of PDMS, providing high sensitivity for the measurement of the contact force and environment temperature In this work, we successfully fabricated the tactile sensor array with multiple functions mentioned above by the experiments, which is expected to be employed in the artificial electronic skin (e-skin) In the future, the proposed tactile sensor can be utilized in the wearable device, flexible interface, and bionic robot skin in the industry © by Elsevier Ltd This is an openLtd access article under the CC BY-NC-ND license © 2016 2016Published The Authors Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: tactile sensor; flexible; ZnO nanorod; artificial electronic skin Introduction Tactile sensors are widely applied in the artificial e-skin to mimetic the human skin However, in the past the tactile sensors had been fabricated based on the silicon and glass substrates, causing the applicable restrictions In the recent years, flexible tactile sensors have attracted the attention by researchers due to their low-cost, bendable, and portable And great majority of artificial electronic skins were usually developed on force measurement by the changes of capacitance [1], piezoelectricity [2], and resistivity [3,4] However, human skin is actually an accurate * Corresponding author Tel.: +886-3-5742596; fax: +886-3-5722840 E-mail address: rchen@pme.nthu.edu.tw 1877-7058 © 2016 Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi:10.1016/j.proeng.2016.11.336 Min-Sheng Suen et al / Procedia Engineering 168 (2016) 1044 – 1047 1045 sensory system that composed of dermis, epidermis and sensory receptors to detect the mechanical stimuli of environment Between dermis and epidermis, microstructures are filled with various sensory nerve that can sense static touch, dynamic touch, and temperature sensory In this study, we provide an artificial electronic skin to mimic the human skin for detecting force and temperature, as shown Fig Fig Schematic of the flexible tactile sensor using inter-locked of ZnO nanorods structure Fabrications The fabrication processes of the tactile sensor are follows PDMS substrate is spin-coated with 150 μm onto the silicon wafer and patterned the sensor area (1 × 1.5 cm2) by photolithography ZnO/AZO seed layers are deposited with 100 nm onto the PDMS substrate by RF magnetron sputtering system The growth solution with hydrothermal method are mixed by dissolving with concentration of 0.05 M zinc nitrated [Zn(NO 3)2 ί6H2O] and 0.05 M Hexamethylenetetramine (HMTA) in the deionized water with 100 mL respectively The PDMS substrate is soaked in the growth solution and conducted for 10 hours in the oven to grow the ZnO nanorods Sputtered Pt with 70 nm on the sample improves the conductivity of the ZnO nanorods Lift-off the PR in the second time to employ the electrode Finally, bounding the two sheet samples manufacture the tactile sensor device Experimental results Regarding the growth process, the hydrothermal temperature and reaction time are significant conditions to grow the ZnO nanorods on the film, as listed Table The growth situation of ZnO nanorods are imaged by the scanning electron microscope (SEM) in the Fig Utilizing the developed optimal parameters, the growth height of ZnO nanorod is ~4.246 μm on ZnO seed layer, and the growth height of ZnO nanorod is ~3.141 μm on AZO seed layer Table Important parameters for the growth ZnO nanorods ZnO seed layer AZO seed layer Temperature 95oC 90oC Zn(NO3)2ί6H2O and C6H12N4 0.05M 0.05M Reaction time 10hr 10hr Aspect ratio of ZnO nanorod 15.328 15.185 1046 Min-Sheng Suen et al / Procedia Engineering 168 (2016) 1044 – 1047 Fig 2: The growth situation of the ZnO nanorods of (a) on the ZnO seed layer, length: ~4.246μm (b) on the AZO seed layer, length: ~3.141μm To investigate the crystal structure of ZnO nanorods, the sample is analysed by X-ray direction (XRD) analysis The result clearly proves the higher (002) diffraction peaks of ZnO nanorods along the c-axis direction, as shown in Fig Fig XRD spectra on the crystal structure of the ZnO nanorods To compare the sensing properties of ZnO/AZO tactile sensors, we record the changes in resistance with applied forces and various temperatures In the pressure-sensing aspect, the applied force range is increasing from Pa to 13.33 kPa to measure the sensitivity of the tactile sensor in contact area The flexible artificial e-skin enables the pressure-resistance (R’=Rloading/Runloading) variation due to the change of contact resistance The huge decrease in resistance is observed with high sensitivity (ZnO: ~-0.272 kPa-1, AZO: ~-0.123 kPa-1) at pressure below kPa in Fig Fig Change in resistance as a function of normal pressure for difference ZnO/AzO seed layers The relationship between the applied force and resistance are nonlinear, which can be fitted by the power-law Min-Sheng Suen et al / Procedia Engineering 168 (2016) 1044 – 1047 1047 function y = ax-b Furthermore, to detect the response of the various temperature, the sample is heated by a hot plate in the range from 26 oC to 150 oC The resistance of the device is reduced when the temperature stimulus is increased The variation resistance of ZnO can be illustrated by typical band conduction, and a change in temperature alters the resistance due to the charge of the surface species (O 2, O2-, O- or O2-) as well as the coverage is altered in this process [5] Similarly, Fig denotes the measured resistance changes (red dots) of ZnO and AZO (black dots) and the curve fitting for the experimental results It shows that the tactile sensor of ZnO owns higher sensitivity (~-0.66 %/oC) , among the external environment temperature change Fig Change in resistance of various temperature for difference ZnO/AzO seed layers Conclusions In summary, we demonstrate a flexible, highly sensitive, and temperature measureable tactile sensor The artificial electronic skin is developed by constructing the inter-locked geometric structure of ZnO nanorod array that can induce an obvious change in the contact area to promote the sensitivity in force measurement The experimental results illustrate that the performance of proposed tactile sensor has high pressure sensitivity (ZnO: ~-0.272 kPa-1, AZO: ~-0.123 kPa-1) and high temperature sensitivity (~-0.66 %/oC) Furthermore, the fabrication process of the tactile sensor is low cost, without the need of the complex and expensive equipment In further research, it is anticipated that the flexible tactile sensor can be applied in robotic skin, prosthetic limbs, and wearable devices to monitor the contact pressure and environmental temperature Acknowledgements This work was partially supported by the Ministry of Science and Technology (NSC102-2221-E-007-034-MY3) The authors appreciate the use of facilities at the Center for Nanotechnology, Material Science, and Microsystems (CNMM) of National Tsing Hua University, which is partly supported by the Ministry of Science and Technology References [1] H.-K Lee, C S.-I Chang, and E Yoon, A Flexible Polymer Tactile Sensor, Fabrication and Modular Expandability for Large Area Deployment, Journal of Microelectromechanical system, 15 (2006), 1681-1686 [2] C T Pan, Y C Chen, C C Hsieh, C H Lin, C Y Su, and C K Yen, Ultrasonic sensing device with ZnO piezoelectric nanorods by selectively electrospraying method, Sensors and Actuators A: Physical, 216 (2014), 318-327 [3] C Pang, G Lee, T Kim, S Kim, H Kim, S Ahn, and K Suh, A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibers Nature materials, 11 (2012), 12-14 [4] J Park, Y Lee, J Hong, Y Lee, M Ha, and Y Jung, Tactile-Direction-Sensitive and Stretchable Electronic Skins Based on Human-SkinInspired Interlocked Microstructures, ACS Nano, (2015), 12020-12029 [5] R Srivastave, Investigation on Temperature Sensing of Nanostructured Zinc Oxide Synthesized via Oxalate Route Journal of Sensor Technology, (2012), 8-12 ... detecting force and temperature, as shown Fig Fig Schematic of the flexible tactile sensor using inter-locked of ZnO nanorods structure Fabrications The fabrication processes of the tactile sensor are... ZnO nanorod array that can induce an obvious change in the contact area to promote the sensitivity in force measurement The experimental results illustrate that the performance of proposed tactile. .. Chang, and E Yoon, A Flexible Polymer Tactile Sensor, Fabrication and Modular Expandability for Large Area Deployment, Journal of Microelectromechanical system, 15 (2006), 1681-1686 [2] C T Pan,