A wide range of materials and material combinations, from hard and brittle to soft and elastic, is now available for the design of ultraflexible organic electronic circuits. Potential applications range from large-area active-matrix sensor arrays to displays, usable in next-generation smart appliances for mobile health, sports, and well-being. Weight and flexibility dominate the mechanical response and perception of such electronic skins, and have been developed into key figures of merit in circuit design. We review the design of thin (0.3–3 µm), ultralight (0.7–6 g/m2) large-area “imperceptible” electronic foils employing low-cost fabrication techniques compatible with mass production.

Electronics are evolving from “brittle” to “flexible” and are expected to advance to “stretchable.” Development of stretchable semiconductor materials is a key step for the realization of highly deformable transistors. This article introduces the technological strategies for achieving stretchable polymer semiconductors, including a geometrical approach, formation of a nanofibril network, microcrack formation, and synthesis of new polymers. We conclude with perspectives on further development of stretchable polymer semiconductors.

Stretchable and ultraflexible electronic devices have a broad range of potential uses, from robust devices for energy storage and conversion to biomedical devices that make conformal interfaces with the skin and internal organs. Organics have long been associated with mechanical compliance, which enables inexpensive manufacturing via roll-to-roll printing. This article provides an overview of the use of organic electronic materials, including π-conjugated polymers and small molecules, in highly deformable devices. It begins with a comparison of devices based on organic devices to those based on inorganic composites. The thin-film nature of organic semiconductor devices has also led to the development of several techniques for metrology that can be applied specifically to brittle organic thin films. The article concludes with a brief discussion of the applications of stretchable and ultraflexible organic electronic devices and a prescriptive outlook for successful collaborative work in this exciting, interdisciplinary field.

Compliant robots, a class of so-called soft robots, made from elastomeric materials, require flexible or stretchable sensors for functional sophistication beyond that of open-loop controls and actuations. These robots have expanded the scope of research in robotics from fast, strong, and precise industrial manufacturing toward new needs of adaptation and safety—the realm of human–robot interactions (HRIs). HRIs include circumstances ranging from existing tasks such as vacuum cleaning to the far-reaching goal of direct contact with the heart for ventricular assist devices, and wearable robots as an intermediate task for force-augmenting exoskeletons. Toward these goals, many efforts are being made to impart sensation for feedback control via flexible or stretchable sensors that can be integrated with the soft bodies of these robots without hindering their motion or reducing their safety. This article briefly reviews the key techniques and tradeoffs for designing and fabricating these sensors. We describe the sensors that our research group uses for fluidically powered soft robots. We conclude with some perspectives about future directions of sensing integration for improved autonomy and interaction with humans in close proximity.

In the development of high-performance organic electronics, there has been significant effort in establishing relationships between microstructure and electronic properties, which has provided a deeper understanding of device operation and has guided performance improvements. When considering flexible and stretchable organic electronics, the mechanical behavior of the active layers becomes a critical attribute alongside electronic functionality. Thus, there is a need to establish the role of film morphology on both electronic properties and thermomechanical behavior, and the relationship between mechanical and electronic properties. In this article, we highlight recent advances in establishing these important relationships and the approaches employed to manage film morphology to optimize both mechanical behavior and device performance. Additionally, in stretchable applications, the film morphology may not be static, and capturing the microstructure changes under deformation is necessary to establish structure–property relationships over the expected physical operating space. Thus, also discuss film morphology change under large deformation for various stretchable film approaches.

The mechanical properties of organic electronic materials and interfaces play a central role in determining the manufacturability and reliability of flexible and stretchable organic electronic devices. The synergistic effects of mechanical stress and deformation, together with other operating parameters such as temperature and temperature cycling, and exposure to solar radiation, moisture, and other environmental species are particularly important for longer-term device stability. We review recent studies of basic mechanical properties such as adhesion and cohesion, stiffness, yield behavior, and ductility of organic semiconducting materials, and their connection to underlying molecular structure. We highlight thin-film metrologies to probe the mechanical behavior, including when subjected to simulated operational conditions. We also report on strategies for improving reliability through interface engineering and tailoring material chemistry and molecular structure. These studies provide insights into how these metrologies and metrics inform the development of materials and devices for improved reliability.

Stretchable devices, with the capability of retaining their functionalities under stretching, are drawing much attention as a promising solution to address the mechanical mismatch between traditional stiff electronics and soft curvilinear biological systems. Intensive efforts have been made toward the advancement of stretchable devices, such as the development of novel mechanically durable materials, deformable conductors and circuits, novel processing methods, and elastic matrixes for stretchable substrates and system integration. Among these, the elastic substrate constitutes the component that bears the applied strain and thus endows the device with stretchability, rendering its properties crucial to the overall performance of stretchable devices. This article provides a summary of the elastic materials commonly employed as stretchable substrates, as well as reveals fundamental insights into the properties requirements in the selection of stretchable substrates. Important challenges and strategies in the development of elastic matrices for stretchable devices are also discussed.