The field of flexible electronics has rapidly expanded over the last decades, pioneering novel applications, such as wearable and textile integrated devices, seamless and embedded patch-like systems, soft electronic skins, as well as imperceptible and transient implants. The possibility to revolutionize our daily life with such disruptive appliances has fueled the quest for electronic devices which yield good electrical and mechanical performance and are at the same time light-weight, transparent, conformable, stretchable, and even biodegradable. Flexible metal oxide semiconductor thin-film transistors (TFTs) can fulfill all these requirements and are therefore considered the most promising technology for tomorrow's electronics. This review reflects the establishment of flexible metal oxide semiconductor TFTs, from the development of single devices, large-area circuits, up to entirely integrated systems. First, an introduction on metal oxide semiconductor TFTs is given, where the history of the field is revisited, the TFT configurations and operating principles are presented, and the main issues and technological challenges faced in the area are analyzed. Then, the recent advances achieved for flexible n-type metal oxide semiconductor TFTs manufactured by physical vapor deposition methods and solution-processing techniques are summarized. In particular, the ability of flexible metal oxide semiconductor TFTs to combine low temperature fabrication, high carrier mobility, large frequency operation, extreme mechanical bendability, together with transparency, conformability, stretchability, and water dissolubility is shown. Afterward, a detailed analysis of the most promising metal oxide semiconducting materials developed to realize the state-of-the-art flexible p-type TFTs is given. Next, the recent progresses obtained for flexible metal oxide semiconductor-based electronic circuits, realized with both unipolar and complementary technology, are reported. In particular, the realization of large-area digital circuitry like flexible near field communication tags and analog integrated circuits such as bendable operational amplifiers is presented. The last topic of this review is devoted for emerging flexible electronic systems, from foldable displays, power transmission elements to integrated systems for large-area sensing and data storage and transmission. Finally, the conclusions are drawn and an outlook over the field with a prediction for the future is provided.

In the last years, the development of new materials as well as advanced fabrication techniques have enabled the transformation of electronics from bulky rigid structures into unobtrusive soft systems. This gave rise to new thin-film devices realized on previously incompatible and unconventional substrates, such as temperature-sensitive polymers, rough organic materials or fabrics. Consequently, it is now possible to realize thin-film structures on active substrates which provide additional functionality. Examples include stiffness gradients to match mechanical properties, mechanical actuation to realize smart grippers and soft robots, or microfluidic channels for lab-on-chip applications. Composite or microstructured substrates can be designed to have bespoke electrical, mechanical, biological and chemical features making the substrate an active part of a system. Here, the latest developments of smart structures carrying thin-film electronics are reviewed. Whereby the focus lies on soft and flexible systems, designed to fulfill tasks, not achievable by electronics or the substrate alone. After a brief introduction and definition of the requirements and topic areas, the materials for substrates and thin-film devices are covered with an emphasis on their intrinsic properties. Next, the technologies for electronics and substrates fabrication are summarized. Then, the desired properties and design strategies of various active substrate are discussed and benchmarked against the current state-of-the-art. Finally, available demonstrations, and use cases are presented. The review concludes by mapping the available technologies to innovative applications, identifying promising underdeveloped fields of research and potential future progress.

We present a gas sensitive thin-film transistor (TFT) based on an amorphous Indium–Gallium–Zinc–Oxide (a-IGZO) semiconductor as the sensing layer, which is fabricated on a free-standing flexible polyimide foil. The photo-induced sensor response to NO₂ gas at room temperature and the cross-sensitivity to humidity are investigated. We combine the advantages of a transistor based sensor with flexible electronics technology to demonstrate the first flexible a-IGZO based gas sensitive TFT. Since flexible plastic substrates prohibit the use of high operating temperatures, the charge generation is promoted with the help of UV-light absorption, which ultimately triggers the reversible chemical reaction with the trace gas. Furthermore, the device fabrication process flow can be directly implemented in standard TFT technology, allowing for the parallel integration of the sensor and analog or logical circuits.