Design of Engineered Elastomeric Substrate for Stretchable Active Devices and Sensors

Abstract

In the field of flexible electronics, emerging applications require biocompatible and unobtrusive devices, which can withstand different modes of mechanical deformation and achieve low complexity in the fabrication process. Here, the fabrication of a mesa-shaped elastomeric substrate, supporting thin-film transistors (TFTs) and logic circuits (inverters), is reported. High-relief structures are designed to minimize the strain experienced by the electronics, which are fabricated directly on the pillars' surface. In this design configuration, devices based on amorphous indium-gallium-zinc-oxide can withstand different modes of deformation. Bending, stretching, and twisting experiments up to 6 mm radius, 20% uniaxial strain, and 180° global twisting, respectively, are performed to show stable electrical performance of the TFTs. Similarly, a fully integrated digital inverter is tested while stretched up to 20% elongation. As a proof of the versatility of mesa-shaped geometry, a biocompatible and stretchable sensor for temperature mapping is also realized. Using pectin, which is a temperature-sensitive material present in plant cells, the response of the sensor shows current modulation from 13 to 28 °C and functionality up to 15% strain. These results demonstrate the performance of highly flexible electronics for a broad variety of applications, including smart skin and health monitoring.

Flexible Sensors—From Materials to Applications

“…Metals are some of the most commonly employed materials as conductors in both flexible and stretchable sensors. Copper (Cu) [7,8], gold (Au) [9][10][11][12][13], molybdenum (Mo) [14,15], silver (Ag) [16][17][18][19][20], platinum (Pt) [21,22], chromium (Cr) [23,24], aluminium (Al) [25][26][27][28], nickel (Ni) [11,29] and magnesium (Mg) [30,31] have all been widely used due to their intrinsic electrical conductivity and high mechanical stability under bending stress down to micrometer scale [32]. Au has been extensively used in the form of thin-film metallic contacts due to its resistance to oxidation.…”

“…Also, Mg has recently been employed as a contact due to its biocompatibility (Figure 2a). In addition, metal thin films can be easily deposited on flexible substrates through conventional techniques such as electroplating [37][38][39], sputtering [15,23], thermal/e-beam evaporation [9,22,40], and solution methods [19,41,42]. Nevertheless, although these materials are adequate for flexible applications, they are not easily employable in stretchable scenarios in the form of thin films, unless delicate substrate modifications are applied [32,43,44].…”

“…In addition, process compatible melting/glass transition temperatures and outgassing rates, low surface roughness, chemical stability and large area compatibility are all desirable properties for flexible substrates. Common materials used for this end are PI [13][14][15][23][24][25]34,36], PET [17,18,40,70,73,109,169], PEN [9,20,26,204,311] and PDMS [16,21,22,89,168,[312][313][314][315]. However, there are several other examples of flexible substrates suitable for sensor applications reported in literature, including PU [96,316,317], PLA [293], polysulfone (PSU) [318], polyetheretherketone (PEEK) [319], polycarbonate (PC) [79], parylene [47], polyvinyl alcohol (PVA) [12], polyarylate (PAR) [239], Ecoflex™ [320,321], Dragon Skin™ [101,[321][322]...…”

“…Finite Element Analysis (FEA) has been the most common numerical method used for simulations, specially using commercial software such as Abaqus [136,451,[568][569][570] or Comsol [571][572][573][574]. These tools have been used to better understand the mechanical behaviour of the sensors prior to the fabrication [568], perform parametric analysis of geometries under adverse mechanical loadings [22,136,[569][570][571], understand electrical behaviour of the components [569,[573][574][575], and to analyze the electrical response under a deformed state [569,573]. Besides, numerical simulations have been proved to be a practical tool in the analysis of changes in capacitance with the effect of strain [576] and for the analysis of electrical performance and mechanical stability of flexible circuits with different configurations under bending loads [572].…”

Smart Sensing Systems Using Wearable Optoelectronics

“…In addition, the ability to provide thermally applicable processing at higher temperatures than any glass transition or melting temperatures, along with large‐area compatibility, has garnered tremendous interest for application in wearable optoelectronic sensors. Several conventional candidates are available, such as polyimide (PI), polyethylene terephthalate (PET) as plastic materials, and silicone rubber matrices like Ecoflex and poly‐dimethylsiloxane (PDMS) with a low Young's modulus. In addition to choosing soft materials with a low Young's modulus, another active area is in atomic scale or ultrathin underlying substrates, such as Cu or Al foils and glasses .…”