Hello everyone, my name is Jang-Ung Park, and our class is about free-form electronics. Today, we will learn about stretchable electronics first. Nowadays, electronics have been advanced very significantly, So, the performance of electronic devices become superior. Also, the structure of electronic devices become deformable as well. For example, low-level total displays, or affordable smartphones, and so on. So, by deforming the structure of the devices, electronic devices can be integrated everywhere. For example, car bodies, or the buildings, or even inside our bodies as well. The kinds of free-form electronics can be categorized into two different groups. The first group is stretchable electronics and the second group is printed electronics. For the stretchable electronics, by stretching those device geometries, the electronic devices can be attached onto non-planar surfaces of arbitrary objects. So, we can form the devices near the surface region of the object in this way. The second group is printed electronics. So, we can print electronic devices as well as the three-dimensional object as well. In that case, electronic devices can be embedded inside three-dimensional object. So today, we will learn about stretchable electronic devices first. In order to compose these electronic devices, we need various electronic materials, including organics and inorganics as well. But typically, most inorganic materials, such as metals or ceramics, can be fractured against the strain of just one percent. So, the inorganic materials are very weak against stretching. So, let's see how we can calculate their strain. So, first let's consider the bending motions. In that case, if we bend the electronic devices, then bending induced strain becomes applied. In the slide, you will see the blue-colored region, that's the cross-sectional image of the plastic film. If the optoelectronic devices, for example, displays are attached onto the bottom side of that blue plastics, then if we bend that devices, then strain becomes applied to the electronic devices. I mean, the bottom surface region of the blue plastics. In that case, the bending induced strain can be calculated by this equation. So here, Epsilon is the strain, and t is the total thickness, and r is the bending radius. So here, total thickness can be calculated by the sum of the thickness of the plastic filling, plus the thickness of the optoelectronic devices. Typically, all light display has thickness below just one-micrometer. But the plastic filling thickness, is much thicker than the electronic devices. So in that case, total thickness t can be estimated by just filling thickness. So here, the bending induced strain becomes higher than one percent, in that case, the inorganic materials to compose those electronic devices can be pressured, for example. So in this way, you can estimate the maximum bending radius. So let's see the stress/strain curve of the materials. This stress/strain curve in this slide shows the typical behavior of the fracture motion of the materials. As the applied strain becomes increased, the strain also linearly increased. So that's the elastic motion. So, in that case, if the applied strain becomes released to zero percent, then the applied stress becomes also released as well. So, stress becomes zero in that case. So, the materials can be recovered to the initial stages as well. So, the maximum elastic limit shows the maximum elastic strain that can be applied without fractures. If the applied strain becomes above that elastic limits, then the stress becomes saturated. So the material shows the plastic deformations. Then if the strain becomes much higher, then the material becomes cracked. So typically, in order to make that stretchable electronic devices or the foldable electronic devices, we should consider the elastic limit of each material. In the case of metals or glasses, those inorganic material shows a very narrow region of the elastic limit. So they are very weak against the deformations. But in the case the of organic materials such as plastics or elastomers, they show very wide elastic deformation region. So, their elastic limit is typically, above five percent, or for the elastomers, the elastic limit can be above 30 percent as well. But inorganic materials show very narrow elastic deformation region. So their elastic limit typically are smaller than 0.5 percent. So that's the reason why if we stretch the electronic devices, the devices can be cracked typically in organic materials. Then how can you make infinitely stretchable or how can you make multiple times foldable electronic devices using the inorganic materials? Let's consider the substrate for the foldable devices, various substrates can be considered, including plastics, and papers, and very thin metal foils as well. But each of these substrate materials have their own disadvantages, but now they're typically plastic fillings such as polyamide are used for the foldable smartphones and foldable displays as well. But in the case of the plastic filling, it's not stretchable. In order to make a stretchable electronic devices, we need elastomers. But in the case of the elastomer, when the elastomer filler becomes stretched, the inorganic materials can be cracked. So then how can you avoid that kind of cracking issues? That kind of cracking problem can be solved by embedding with it components inside to the elastomer filling. So in this slide, the blue colored region shows the cross-section images of the elastic substrate, and you will see the gray color part. That gray color apart is the rigid components embedded inside the blue elastomer filling. If we embed that kind of rigid to very tiny tires inside that elastomer fillings, and then if we fabricate electronic devices on top of that rigid and elastomer hybrid components, then we can avoid the cracking problem of the electronic devices. So in my slide, I will say that they are rigid-soft hybrid substrate. So by embedding that rigid components inside the elastomer fillings, then we can make the rigid-soft hybrid substrate. So, by making that rigid-soft hybrid substrate, we can avoid the cracking issue of the electronic devices. So in this slide, the schematic image shows the example of the rigid-soft hybrid substrate. So you will see the triangular structure of the gray pattern, thus the rigid tile embedded inside the elastomer. So in this way, if we stretch that hybrid substrate, then what we need is a soft part, I mean only the soft elastomer part can be stretched, but the rigid parts remain without deformations. So let's assume that the electronic devices are located on the gray part, then we can avoid cracking of the device components in this way. So, at the bottom side of the schematic image, you will see that all the transistors becomes located on to the gray rigid apart. In that case, if we stretch it, individual transistors remain without deformations because they are located on the rigid tile. But the only deformed region is the interconnection part to link the individual transistors, which is just the interconnection networks in this case. So, in that case, we need just one unique material, which is stretchable electrodes. So, if we have a stretchable electrodes, then we can make that kind of entirely stretchable transistor arrays. So, we don't have to develop all electronic materials to compose the transistors. The only components that we need in this case is just stretchable metals. So that kind of rigid-soft hybrid substrate enables that simplification for the fabrications. This movie shows the example of the rigid-soft hybrid substrate. So here, rigid tile becomes embedded inside of that elastomer substrate. Then the substrate example can be folded. So, in here, the folded region is just the elastomer region, and the soft elastomer region becomes folded, but the rigid part remains without deformations. So, in this way, it can form completely. So here important thing is the interface between the soft region and the rigid region of the substrate, when the substrate becomes folded, the interface should not be separated. So, we need really good binding between the soft part and the rigid part of the hybrid substrate. So, in this SEM image, you will see the interface between the soft part and the rigid part for the folding conditions, and that interface becomes very well contacted without separations because they make continuous form of binding. So that's very important for complete folding issues. Another important issue here is the standoff height between the rigid part and the soft part. That kind of rigid-soft hybrid substrate can be composed by the rigid epoxy with the PDMS elastomer. So, by making that PDMS and epoxy hybrid substrate, we can reduce the stand off height. So, in this way, we can make almost flat surface regions. So as shown in this AFM images, the height difference between the rigid part and soft part becomes below 50 millimeters. So it's almost flat. So in this way, we can do the photolithography for the device fabrications. Then by using that kind of rigid-soft hybrid substrate, we can directly fabricated electronic devices, for example, transistor arrays with the active metrics forms. So in this slide, the real sample image is shown. So the sample images can be folded in sequence. So in this way, we can do the complete folding without fracturing of the materials. The transistors for example.
