Nanoscale Analysis of Surface Bending Strain in Film Substrates for Preventing Fracture in Flexible Electronic Devices

Abstract

be circumvented by exploiting an elementary mechanics that surface bending strain decreases linearly with a thickness of a substrate. For example, flexible substrates with a thicknesses of 10 µm experience peak surface strain of only 0.1% upon bending to the radius of curvature of 5 mm, and this strain remains well below the fracture limits of semiconductors (≈1%), metals (1-2%), and hard coatings (1-3%); indeed, the use of a substrate within a range below tens of micrometers enables comparable growth, resulting in high-performance durable flexible devices for epidermal, implantable, and wearable applications (Figure 1b). [10-20] On the other hand, the development of foldable electronics devices that have thicknesses of hundreds of micrometers is an emerging challenge. This highlights the ever-growing importance of the substrates strategically designed to reduce surface strain without thinning (Figure 1a right and 1b). However, it is not yet possible to measure nanoscale surface bending strain in real time: none of the currently available advanced methods fulfill all of the necessary spatial-temporal resolution, accuracy, precision, and a wide range of measurable materials. Electrical strain sensors including strain gauges exemplify the most common strain analytical method. [21-23] Although they provide surface bending strains of materials targeted in real time, their With the rapid development of flexible electronics and soft robotics, there is an emerging topic of preventing fracture in materials and devices integrated on largely bending film substrates of >100 µm thickness. The high demand for strategically reducing strain in bending materials requires a facile method that enables one to accurately and precisely analyze the surface bending strain in a wide variety of materials. This study proposes the surface-labeled grating method that is the fundamental and efficient technique for measuring surface bending strains merely by labeling a thin, soft grating onto various film substrates composed of flexible polymeric and rigid inorganic materials. The surface strain with a single-nanoscale (<1.0 nm) can be quantified in real time with no need of material information such as Poisson's ratio, Young's modulus, and film thickness. The fracture limit of a hard coating overlying flexible substrates is successfully determined by the accurate and precise quantification of surface bending strains. Furthermore, a multilayer film substrate with surface bending strain reduced by 50% prevents fractures of hard coatings and organic thin film transistors (OTFTs) since the strains remain below the fracture limit under large bending.

Novel Bending Sensor Based on a Solution-Processed Cu2O Film with High Resolution Covering a Wide Curvature Range

“…The value of ε can be calculated using the formula ε = (d × κ)/2, where d is the thickness of the PET substrate and κ is the applied curvature. 36 In the present study, d is 0.125 mm. As shown in Figure 3d, the curve of the resistance variation in the perpendicular bending versus curvature can be divided into two linear parts, region I (0 < κ < 0.05 mm −1 ) and region II (0.05 < κ < 0.20 mm −1 ) with coefficients of determination of 0.994 and 0.995, respectively.…”

“…We analyzed the result of Figure d by evaluating the sensor sensitivity using GF, which is defined as GF = (Δ R / R 0 )/ε, where ε is the surface bending strain. The value of ε can be calculated using the formula ε = ( d × κ)/2, where d is the thickness of the PET substrate and κ is the applied curvature . In the present study, d is 0.125 mm.…”

Light-Energy-Harvested Flexible Wireless Temperature-Sensing Patch for Food Cold Storage

“…Flexible electronics with high adaptability and easy deployment has been widely applied. − The flexible electronics usually consists of a flexible substrate, the circuits assembled on a flexible substrate, and the electronic components or modules. The flexible substrate could be films such as polyimide (PI), poly­(ethylene terephthalate) (PET), and poly­(ethylene naphthalate) (PEN). − The circuits could be various metals such as gold (Au), silver (Ag), and copper (Cu) . The electronic components or modules could be silicon-based or organic thin film-based small electronic components or the miniature electronic modules made by conventional print circuit boards (PCBs) .…”

Neutral Mechanical Plane Shifting in Bending Elastomer Film Revealed by Quantification of Internal Strain

“…[22][23][24] Other methods using light diffraction or digital image correlation, etc., have been limited to the strain analyses on surfaces or cross-section, in principle. [25][26][27][28][29][30] Therefore, a flexible strain sensor, which can be used to facilely measure the internal strain in bending materials, could play a key role in identifying the NMP position.…”