Transparent and Flexible Triboelectric Sensing Array for Touch Security Applications Zuqing Yuan,†,§,⊥ Tao Zhou,†,⊥ Yingying Yin,†,§ Ran Cao,†,§ Congju Li,*,† and Zhong Lin Wang*,†,‡ †
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National Center for Nanoscience and Technology (NCNST), Beijing 100083 China ‡ School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 United States § University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Tactile sensors with large-scale array and high sensitivity is essential for human−machine interaction, smart wearable devices, and mobile networks. Here, a transparent and flexible triboelectric sensing array (TSA) with fingertip-sized pixels is demonstrated by integrating ITO electrodes, FEP film, and signal transmission circuits on an undivided palm-sized polyethylene terephthalate substrate. The sensing pixels can be triggered by the corresponding external contact to induce the electrostatic potential in the transparent electrodes without power consumption, which is individually recognized by the sensor. By testing the response of the pixels, the electrical characterization is systematically investigated. The proposed TSA exhibits excellent durability, independence, and synchronicity, which is able to realize real-time touch sensing, spatial mapping, and motion monitoring. The integrated TSA has great potential for an active tactile system, human−machine interface, wearable electronics, private communication, and advanced security identification. KEYWORDS: transparent and flexible, triboelectric nanogenerator, tactile sensor, spatial mapping, motion monitoring ing,31−34 microfluidic assays,35,36 and transparent interfaces37,38 due to its low cost and simple mechanism to convert mechanical energy to electric signals without power consumption. However, the present findings are unable to completely meet the practical requirement, especially on the handiness, fashion, independence, and convenience for touch screens or security applications. This paper demonstrates a transparent and flexible triboelectric sensing array (TSA) for spatial mapping and trajectory monitoring with concise structure, high pressure sensitivity, and excellent durability, which has potential in large-scale production. The self-powered TSA consists of a transparent polyethylene terephthalate (PET) substrate, an elastic adhering polydimethylsiloxane (PDMS) layer and an electrification layer of FEP film, which is based on the single-electrode triboelectric nanogenerator.39,40 A 5 × 5 TSA with each ITO pixel of 1 × 1 cm2 was fabricated to spatially map multiple touch stimuli. The ITO layers were connected by the conductive holes, which served as induced electrodes and transmission circuits at the same time and established a compact multichannel sensing system on a transparent and flexible film. Owing to the elastic PDMS layer and surface functionalization of the FEP film, the
R
apid development of mobile networks and smart terminals has inspired the exploration of intelligent sensor technology for the increasing demand for human−machine interaction, artificial intelligence, and wearable electronics.1−3 In the field of smart devices, great concern has been focused on the research of tactile sensors with high performance, such as cell phones, intelligent watches, and health monitors.4−10 At present, significant breakthrough has been made for large-scale touch sensing arrays with high sensitivity and high resolution, based on various physical mechanisms, including piezoelectricity,11−14 piezoresistivity,15,16 and capacitance.17−20 However, among the special characteristics, the wide-range sensitivity, response time, portable convenience, and conformance and multifunction have become hot topics to evaluate the practicability of a sensor.21−23 It is easy to see that manufacturing an electrical sensing array on an intact transparent substrate with high sensitivity and flexibility is still urgently needed. Recently, the triboelectric nanogenerator (TENG) has been developed for available solutions to the difficulty of the traditional sensor technique and applications.24,25 The triboelectric charges can induce electrostatic potential or current driven by usual mechanical relative motion, which serves as the analogue signals of the sensors. Hence, TENG is ideally suited for self-powered sensing systems,26,27 which have been successfully applied to biomedical systems,28−30 track monitor© 2017 American Chemical Society
Received: May 25, 2017 Accepted: July 24, 2017 Published: July 24, 2017 8364
DOI: 10.1021/acsnano.7b03680 ACS Nano 2017, 11, 8364−8369
Article
www.acsnano.org
Article
ACS Nano
electrodes and the conductive circuits, leaving regular sensing arrays on the top side of the film, which reduces the malfunction induced by the electrostatic effect and the disoperation of contacting transmission routes. In order to enhance the output performance of the TSA, the FEP film was treated by ICP plasma to create vertically aligned polymer nanowires on the surface, shown in inset 3. Detailed fabrication process of the TSA is elaborated in the Methods section. Figure 1b is the photograph of a TSA with 5 × 5 touch pixels array, where the area of each touch pixel is 1 × 1 cm2. The working mechanisms of a single sensing pixel in the TSA is based on a single-electrode triboelectric nanogenerator that could work under both contact-separation mode and sliding mode, which are shown in Figure 2a,b, respectively. When the high electronegative FEP film contacts with an external object, charges will transfer between the FEP and object due to the contact-electrification effect, leaving the FEP film with negative charges and the object with positive charges. The charges are able to remain on their surface for a period of time.41,42 Under contact-separation mode, the object is driven by an external force to contact and separate the FEP film periodically, which will induce a reciprocating current between the ground and the ITO electrodes due to electrostatic induction effect, as shown in Figure 2a. Considering that the bottom ITO electrode connected to the ground, the open-circuit voltage of the device can be qualitatively investigated by the following equation:43
self-powered sensor is able to achieve a sensitivity of 2.79 mV/ Pa in the low-pressure region, a limit of detection of
