Printing semiconductor-based devices and circuits for flexible electronic skin

Electronic skin (e-skin), capable of sensing a physical or chemical stimulus and triggering a suitable response, is critical in applications such as healthcare, wearables, robotics, and more. With a substantial number and types of sensors over a large area, the low-cost fabrication is desirable for e-skin. In this regard, printing electronics attract the attention as it allow efficient use of materials, “maskless” fabrication, and low-temperature deposition. Additionally, the use of e-skin in real-time applications calls for faster computation and communication. However, due to limitations of widely used materials (e.g., low mobility) and the printing tools (e.g., poor print resolution), the use of printed electronics has been restricted to passive devices for low-end applications until recent years. Such limitations are now being addressed through high-mobility materials and highlighted in this review article, using e-skin as a vehicle. This paper discusses techniques that allow printing of high-quality electronic layers using inorganic nanostructures, and their further processing to obtain sensors, energy harvesters, and transistors. Specifically, the contact printing, transfer printing, and direct roll printing are discussed along with working mechanisms and the influence of print dynamics. For the sake of completeness, a few examples of organic semiconductor-based devices are also included. E-skin presents a good case for 3D integration of flexible electronics, and therefore, the use of high-resolution printing to connect various devices on a substrate or 3D stack is also discussed. Finally, major challenges hindering the scalability of printing methods and their commercial uptake are discussed along with potential solutions.