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| EDITORIAL |
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| Year : 2018 | Volume
: 4
| Issue : 4 | Page : 145-147 |
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Electronic skin sensor arrays: A high potential application to medical devices
Debao Zhou
Professor, Department of Mechanical Engineering, University of Minnesota, Duluth, MN, USA
| Date of Web Publication | 28-Dec-2018 |
Correspondence Address:
Debao Zhou
Department of Mechanical Engineering, University of Minnesota, Duluth, MN
USA
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/digm.digm_30_18
How to cite this article:
Zhou D. Electronic skin sensor arrays: A high potential application to medical devices. Digit Med 2018;4:145-7 |
Artificial skin sensor arrays or electronic skin refers to flexible, stretchable, and scalable electronics that are able to mimic functionalities of human or animal skin.[1] The broad class of such systems often contains sensing abilities that are intended to reproduce the capabilities of human skin to respond to environmental factors such as changes in heat and pressure.
Considerable progress has been made in the development of high-performance artificial skin sensor arrays because they have shown great potential in biomedical devices, especially for health monitoring equipment. The demand for detecting contact pressure on curved surfaces has led to the emergence of highly stretchable skin-like sensor arrays, which can be integrated on the medical devices with curvilinear-shaped surfaces.[2]
The main components of an artificial skin sensor array consist of sensing elements and stretchable conductive interconnections. The sensing elements convert physical phenomena, such as pressure, to electrical signals, such as voltage, for computer processing. The stretchable conductive interconnections send out the electronic signals to enable computer measurement.
There are mainly two well-accepted approaches to fabricate stretchable conductive interconnections: utilizing conventional metallic thin films to build flexible structures and applying intrinsically stretchable conductive materials to build interconnections. For building mechanically conformable structures using conventional electronic materials, networks of two-dimensional materials such as nanoporous metal films are usually applied.[3] By tailoring the geometries of the metallic wires to embed them into elastomer materials, stretchable and foldable devices can be fabricated without exceeding local elastic limits. For example, the structures utilize layers of horseshoe-shaped gold/copper, meander-shaped nickel-gold, or ion-implanted elastomers. Another type of the conformable structure is called a “device island serpentine bridge” structure, where narrow deformable interconnections are used to connect small, localized islands which can be patterned by conventional fabrication approaches.[4] These out-of-plane bridge structures accommodate nearly all of the strain when the thin films are stretched, so the stiff islands are effectively protected from fracturing. This strategy could allow the device to be stretched up to 300%. For the fabrication method to utilize intrinsically stretchable properties, materials made of carbon nanotubes are popular. Stretchable organic light-emitting diodes can also be used where electrodes can be reversibly stretched with little change in sheet resistance.
To measure the pressure in pressure-sensitive electronic skin sensors, there are several methods, including utilizing piezoresistive, piezoelectric, capacitive, or optical sensing principles. Among these, materials with piezoresistive properties have been widely used because of the ease of signal processing and definite signal correspondence. Inorganic materials utilized in electronic skin sensor design for sensing elements include mainly semiconductor nanoparticles and nanowires. Their stress-induced piezoelectric properties make them attractive candidates for pressure sensing.[5] Carbon nanotubes, carbon black, graphene, zinc oxide, gallium arsenide, silicon, or germanium has demonstrated good functionality in pressure detection applications. Quite a few pressure-sensitive sensors with ultra-high sensitivity within a certain pressure range have been developed with these materials.
To provide sufficient stretchability for a thin-film sensor, the conducting structures, as well as the pressure sensing elements, must be able to sustain multiaxial stretches. With the goal of realizing pressure detection inside a colon, a high-performance, three-layer skin-like tactile sensor array has been fabricated using piezoresistive elements and stretchable interconnections made of silver nanowires (AgNWs). This sensor array is capable of measuring the pressure on a tube-shaped medical device where the sensor array in thin-film style is mounted on the tubular surface under dynamic stretching, as shown in [Figure 1]. | Figure 1: (a) Optical images of an e-skin (an 8 × 8 sensor array); (b) e-skin under stretching load; (c) e-skin under twisting load; (d) e-skin under bending load; scale bars, 1 cm
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In this application, AgNWs have been chosen as the main materials to provide conductivity to the interconnections because they eclipse the performance of indium tin oxide, carbon nanotubes, and graphene films, and they are also capable of withstanding large flexible stretching and enduring repeated bends. AgNWs' network distribution can also help maintain low sheet resistance and high optical transparency under diverse types of embedment, which makes it a perfect component for flexible tactile electrodes. For the substrate design, stretchability is valued more in the application to endoscope because such a sensor array is expected to provide ultra-high conformability and endure repeated multi-direction stretching. To meet these needs, Dragon Skin silicone rubber is used in this work to build the substrates of the sensor array. Proven to be biocompatible, nontoxic, chemically inert, and skin safe, Dragon Skin silicon rubber has a high tensile limit and can provide strong anti-tear ability. The sensing elements of this sensor array are made of a graphene-based piezoresistive material. This sensor array is with the spatial resolution at 9 mm2 per sensing element and its pressure resolution changes at different pressure levels. With the tests on an endoscope, the pressures generated by the bending deformation have been experimentally quantified. Visualized pressure intensity display has also been achieved with high accuracy.[2] The sensor array has been fabricated using a routine which does not depending on sophisticated processes or tools, and it is a simple and economical way to fulfill the application goal, i.e., to be applicable on medical devices. The system has the following unique applications: (1) excessive force location identification for perforation avoidance and full insertion realization for cancer detection, (2) unnecessary contact pressure avoidance for more comfortable colonoscopy, and (3) minimizing complications due to endoscope trauma. It can also be used to train new gastroenterologists to decrease training time. It is hope that the data obtained from this sensor array can be used to upgrade the strategies of colonoscopy for safer operations and provide new routines to optimize tactile sensor design for other medical applications.
Such an electronic sensor array system can also be used on catheters or digestible devices, for example, digestible capsules. The market value of digestible capsules with sensing functions can be more than 10 million US dollars.[6] Since contact happens everywhere, it is reasonable to predict that electronic skin sensor arrays may have high potential applications, especially in the field of medical devices.
| References | |  |
| 1. | |
| 2. | Sun Y, Zhou D, Bai J, Hauser E, Wang S. Test of a tubular shaped artificial skin sensor array for colonoscopy. IEEE Sens J 2018;18:2291-8.  |
| 3. | Araki T, Nogi M, Suganuma K, Kogure M, Kirihara O. Printable and stretchable conductive wirings comprising silver flakes and elastomers. IEEE Electron Device Lett 2011;32:1424-6. |
| 4. | Rogers JA. Epidermal electronics. Abstr Pap Am Chem Soc 2012;243:1. |
| 5. | Keil P, Baraki R, Novak N, Rodel J, Fromling T. Gauge factors for piezotronic stress sensor in polycrystalline ZnO. J Phys D Appl Phys 2017;50:1-5. |
| 6. | Kalantar-Zadeh K, Ha N, Ou JZ, Berean KJ. Ingestible sensors. ACS Sens 2017;2:468-83. |
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