Elastic substrates for stretchable devices
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Introduction Biological tissues are soft, curvilinear, and elastic, while traditional devices tend to be stiff and rigid, resulting in a mechanical mismatch that limits the functionality of these devices for biological and everyday applications. For example, the human wrist can be easily bent and the human skin stretched, but everyday devices such as the cell phone cannot be deformed in order to conform to the bent wrist. With the current explosive advancements in technology, there is an increasing demand for applications that are impossible to realize with traditional hard and planar integrated circuits, such as artificial skin for robotics, sensing devices for human health monitoring, and wearable communications devices.1–6 To overcome these challenges, it is crucial to develop stretchable devices that can accommodate stretching while retaining high performance. These devices would require electrical and electronic circuits that can be elastically or inelastically stretched by more than a few percent while retaining their functionalities. Such elastic technology would surmount the mismatch in mechanics between biology and devices and provide a vast improvement to our daily lives.7–9 An ideal strategy to achieve fully stretchable devices is to enable the electronic components themselves to be intrinsically stretchable; all materials used for the electronic circuits
should possess the ability to maintain good electrical function under stretching. In reality, however, typical materials used for electrical functions (e.g., copper and silicon) are stiff and easily form cracks when a strain is applied, due to their low elastic limits (≤5%), as shown in Figure 1.10 In contrast, the average elastic limit of human skin is 75%, which is several times larger than that of traditional electronics.11 To surmount this problem, researchers have integrated the active electrical components with elastic substrates such as poly(dimethylsiloxane) (PDMS) and Ecoflex (a type of platinum-catalyzed silicone), the elastic limits of which exceed 200%, in order to achieve stretchable devices.12 Two strategies have been widely used in the fabrication of stretchable devices: (1) embedding of nanomaterials into elastic matrixes;13 and (2) transfer of active materials (thin films or ribbons) onto the surface of elastic substrates.14 In such configurations, the role of the stretchable substrate for stretchable devices is just like that of foundation for a skyscraper. The substrate not only determines the stretchability of stretchable devices, but also influences the functional stability of the active components integrated on it. Therefore, the selection of the stretchable substrate material plays an important role in the overall performance of a stretchable device. PDMS, Ecoflex, and polyurethane are common candidates
Dianpeng Qi, Innovative Center for Flexible Devices, School of Materials Science and Engineering, Nanyang Technological University, Singapore; [email protected] Zhiyuan Liu, Innovative Center for Flexible Devices, School of Materials Sci
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