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Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Stretchable and Transparent Kirigami Conductor of Nanowire Percolation Network for Electronic Skin Applications Phillip Won,†,# Jung Jae Park,†,# Taemin Lee,† Inho Ha,† Seonggeun Han,† Mansoo Choi,† Jinhwan Lee,‡ Sukjoon Hong,§ Kyu-Jin Cho,† and Seung Hwan Ko*,†,∥,⊥ †
Department of Mechanical Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea Mechatronics R&D Center, Samsung Electronics, 1-1 Samsungjeonja-ro, Hwaseong, Gyeonggi-do, Korea § Department of Mechanical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Korea ∥ Institute of Advanced Machines and Design, Seoul National University, Seoul 08826, Korea ⊥ Institute of Engineering Research, Seoul National University, Seoul 08826, Korea
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‡
S Supporting Information *
ABSTRACT: Recent research progress of relieving discomfort between electronics and human body involves serpentine designs, ultrathin films, and extraordinary properties of nanomaterials. However, these strategies addressed thus far each face own limitation for achieving desired form of electronic-skin applications. Evenly matched mechanical properties anywhere on the body and imperceptibility of electronics are two essentially required characteristics for future electronic-skin (E-skin) devices. Yet accomplishing these two main properties simultaneously is still very challenging. Hence, we propose a novel fabrication method to introduce kirigami approach to pattern a highly conductive and transparent electrode into diverse shapes of stretchable electronics with multivariable configurability for E-skin applications. These kirigami engineered patterns impart tunable elasticity to the electrodes, which can be designed to intentionally limit strain or grant ultrastretchability depending on applications over the range of 0 to over 400% tensile strain with strain-invariant electrical property and show excellent strain reversibility even after 10 000 cycles stretching while exhibiting high optical transparency (>80%). The versatility of this work is demonstrated by ultrastretchable transparent kirigami heater for personal thermal management and conformal transparent kirigami electrophysiology sensor for continuous health monitoring of human body conditions. Finally, by integrating E-skin sensors with quadrotor drones, we have successfully demonstrated human-machineinterface using our stretchable transparent kirigami electrodes. KEYWORDS: Stretchable transparent conductor, metal nanowire percolation network, kirigami engineering, electronic skin, electrophysiology sensor, human-machine interface
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ultrathin substrates.11,21−23 Although investigations of structural design to grant stretchability in flexible electronics have been widely demonstrated by serpentine or horseshoe structure,24 making the appropriate structure to avoid the generation of large or irreversible cracks in a certain strain region (usually falls within 20∼50%) is only achieved.9,25−27 Nevertheless, transparency and electrical conductivity is limited. Thus, with increasing demands of multifunctional wearable devices including display, energy generator/storage devices, various sensors, and acutators,6,8,20,28−33 E-skin devices must be composed of characteristics that are mechanically stable under large tensile strain (>50%) and
lectronic-skin (E-Skin) that can detect and monitor biopotentials while having conformal integration onto the skin is a desired form of delivering mechanical/electrical stimulus into quantitative electrical signals,1 and its investigations are extensively ongoing for human machine interface (HMI),2 soft robotics,3,4 and continuous personal healthcare monitoring system in the field of ICT/IoT.2,5−7 Two essential characteristics, conformability and imperceptibility (or transparency), are crucial when developing E-Skin to provide noninvasive precise monitoring of biosignals directly from the surface of skin8 and to look imperceptible to human-naked eyes in forms of a tattoo,9 a bandage10 and a skinreplacement.11−15 However, achieving stretchability and transparency simultaneously in high tensile strain (>50%) without degradation of electrical and mechanical properties is currently very challenging even with extraordinary properties of nanocomposites,16−19 stretchable serpentine structure,20 and © XXXX American Chemical Society
Received: May 16, 2019 Revised: July 21, 2019
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DOI: 10.1021/acs.nanolett.9b02014 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. Kirigami patterning of NWs transparent conductor in various shapes. (a) Schematic illustration of digital kirigami cutting process on ultrathin/flexible NWs/cPI nanocomposite to fabricate various shapes of stretchable transparent conductors. (b) SEM images of various kirigami structures and their 2D designs in the insets: (i) uniaxial, (ii) biaxial, and (iii) square spiral (scale bars are 200 μm). (c) UV−vis spectra of colorless polyimide (cPI), AgNWs/cPI nanocomposite transparent conductor, and the transparent conductor with kirigami patterned in uniaxial pattern. Inset graph shows optical transmittance versus electrical sheet resistance of AgNWs/cPI, (d) OM image of AgNWs/cPI in biaxial kirigami pattern (on the left) and (e) SEM images of AgNWs in the magnified blue box (on the right) (scale bars are 200 and 50 μm, respectively).
reversibility even after 10 000 cycles of stretching while exhibiting optical transparency of >80%. Moreover, this microkirigami pattern adheres along skin morphology with minimal out-of-plane buckling and higher van der Waals adhesive force when the structure tuned to skin-modulus,42,43 so it validates the advantage of monitoring vital signs using the kirigami-structured transparent electrode. Demonstration of a stretchable and transparent heater that elongates up to high strain of 200% strain without electrical instability and leakage shows great possible capabilities of personal thermal treatment,16,25,26 wearable thermal haptic,44 and wound-healing monitoring20 that usually require large-dynamic mechanical deformations.31 Characteristics of soft and thin but conformal properties of TKE enable one to capture various electrophysiology (EP) signals on curvilinear and irregular surfaces of the human body (for example, the chest, forehead, temple, forearm, and so forth).9,21,22,45,46 Long-term monitoring of EP signals using the electrode is realized by coating nontoxic, oxidization resistant material, in this case gold (Au), on the exposed surface of AgNWs.4 Enhanced electrical stability under environmental/sweating conditions after the galvanic coating process of Au47 also assists the stable long-term monitoring of the electronics.6 Finally, the versatility of our Eskin sensor is demonstrated by skin surface electromyogram (sEMG) on both forearms that are measured in real time to use the human gestures for controlling a quadrotor drone in an
transparent at the same time to realize vast range of applications.18,19,34−36 Here, we show a simple strategy that is learned from the ancient art of repetitive paper cutting called kirigami29,37−40 to achieve a transparent and stretchable electrode. In this research, the electrodes are fabricated rapidly with an aid of laser ablation technique, an advanced manufacturing process that uses pulsed UV irradiations in nanosecond intervals.41 By varying the cutting parameter on a digital computer-aided design (CAD) program,41 we are able to modulate the strain elongation of transparent conductive nanocomposite materials without generation of patterning masks and fabrication steps in conventional photolithography process.41 The electrode used in this research generally consists of silver nanowires (AgNWs) and an ultrathin colorless-polyimide (cPI) layer (
