Flexible and Stretchable Devices

Wearable electronics not only enrich daily lives by providing enhanced smart functions, but also provide health information by monitoring body conditions. For example, compliant wearable sensor systems have the potential to better interface with human skin, thus improving the sensitivity of detection of health indicators. Conferring wearability to electronics can also dramatically enhance their applications for emerging Internet of Things (IoT) applications such that people, processes, data, and devices are integrated and connected to improve the quality of life. Moreover, advances in wireless technologies, low-power electronics, and in the domain of digital health are driving innovations in wearable devices at a tremendous pace. Due to these advantages, it is predicted that the wearable device market will reach US 20. 6 billion in 2018. To facilitate the growth of this emerging field, it is critical that flexible and stretchable devices that can accommodate strain and maintain high performance under deformation are developed. The crucial aspects toward the advancement of flexible and stretchable devices include the development of novel mechanically durable materials, flexible and stretchable substrates, deformable electrodes and circuits, novel processing methods, and system integration. Some strategies to achieve this include the use of naturally flexible and stretchable polymeric substrates in combination with emerging molecular and nanostructured materials. Based on these substrates, various types of flexible and stretchable devices can be further developed through the integration of novel materials and processing technologies. For example, the unique mechanical, electrical, and optical properties of graphene make it an attractive candidate for incorporation in flexible and stretchable substrates to produce optoelectronic and energy-harvesting devices. The combination of flexible and stretchable sensors with low-power silicon-based electronics can also lead to the new development of wearable electronics. To showcase recent advances this emerging field, we present this special issue of Advanced Materials on the topic of “Flexible and Stretchable Devices”. The issue brings together contributions from leading experts and covers the following five critical aspects of flexible and stretchable devices: Mechanically durable and highly stretchable materials are fundamentally important to the development of flexible and stretchable devices. Regarding this topic, Darren Lipomi discusses the stretchable figures of merit in deformable electronics, and highlights the importance of mechanically durable materials. Yang Yang and co-workers report recent progress regarding different inorganic, organic, and hybrid semiconductors with printing capabilities for future wearable sensing technologies. Stéphanie Lacour and co-workers report their work on an intrinsically stretchable biphasic thin metal formed by physical vapor deposition of gallium onto an alloying metal film, which enables extremely robust, multilayer and soft circuits, sensors, and actuators. For alternative materials that can also confer flexibility and stretchability to devices, Xuanhe Zhao and co-workers report their work on stretchable, robust, and biocompatible hydrogel devices by integrating various electronic components and drug-delivery channels and reservoirs into compliant hydrogel matrix. Magnus Berggren, Xavier Crispin, and co-workers show the latest progress on the use of electronically conducting polymers for printed electronics and flexible electronics. Hong-Koo Baik, Unyong Jeong, and co-workers report their work on using a conducting polymer dough for deformable electronics. Finally, Xiaodong Chen and co-workers review the utilization of silk materials, which offers exquisite mechanical, optical, and electrical properties, as both passive and active components in the development of next-generation biocompatible electronic devices. Besides the mechanical properties of the substrates, the intrinsic mechanical properties of active electronic materials are also important toward fabrication of flexible and stretchable electronics. Paul Heremans, Gerwin Gelinck, and co-workers discuss the mechanical and electronic properties of thin-film transistors used in active-matrix displays and the effect of mechanical stress in the electrical performance of semiconductor materials. Huanli Dong, Wenping Hu, and Yifan Yao summarize new insights on charge transport in organic and polymeric semiconductors under certain strain effects, which is important to the development of high-performance flexible and stretchable organic field-effect transistors (OFETs). Zhenan Bao and co-workers present their recent work on the development of all-carbon-based materials for mechanically durable and highly stretchable transistors. Lastly, Jong-Hyun Ahn and co-workers summarize the progress on using graphene to fabricate flexible and stretchable electronics. Novel processing methods can be designed to allow robust device operation after repeated deformation. To achieve this goal, printing technologies, which offer large-area, high-throughput production capabilities onto mechanically flexible substrates, are drawing much attention. For example, solution-processed carbon nanotubes have proven to be a promising candidate for such printing processes, offering stable devices with high performance. On this topic, Ali Javey and co-workers discuss recent progress that has been made in carbon nanotubes for printed electronics. These capabilities have enabled low-cost disposable electronic devices for health monitoring, as well as extremely large format electronic displays, interactive wallpapers, and sensing arrays. Meanwhile, Daniel Cohn, Shlomo Magdassi, and co-workers report the use of 3D-printed shape-memory polymers to fabricate flexible electronic devices. Alternatively, Chong-an Di, Daoben Zhu, and co-workers summarize the recent efforts of engineering organic field-effect transistors (OFETs) for sensitive and specific flexible sensors with enhanced signal transduction and signal amplification by the modulation of the active-layer thickness, functional receptor implantation, and device geometry optimization. Muhammad Hussain and co-workers also discuss the development of complementary metal oxide semiconductor (CMOS) technology for flexible and stretchable electronics, with a particular focus on bulk monocrystalline silicon (100). New applications in wearable and high-technology optoelectronic devices are responsible for generating great interest in flexible or/and stretchable devices, particularly in new nanomaterials that confer flexibility or stretchability based on their dimensionality. To address this interest, Hyoyoung Lee and co-workers provide a brief introduction on the use of silver nanowires and graphene for flexible and stretchable optoelectronic devices, and place great emphasis on the unique functions that originate from the intrinsic properties of the nanomaterials. Esma Ismailova, George Malliaras, and co-workers demonstrate a stretchable keyboard based on capacitance sensors made of conductive polymer electrodes patterned on a knitted textile, which is able to read tactile input from the finger. This potentially paves the way for the facile, large-area fabrication of wearable input devices. Meanwhile, for optoelectronics that involve light-emitting diodes, metallic conductors are required for the injection of electric current. This renders electroluminescent devices stiff and brittle, and therefore unable to satisfy the fast-growing demands for mobile and wearable applications. Zhigang Suo and co-workers address this challenge by realizing highly stretchable electroluminescent devices through the use of transparent and stretchable ionic conductors to light up phosphors. Pooi See Lee and co-workers also report using ionic conductors to fabricate extremely stretchable electroluminescent devices. Such development provides new opportunities in stretchable lighting and displays, inter-active readout systems, and other unprecedented applications. In order for wearable electronics to be portable, it is crucial that an energy source is integrated to power the electronics. Thus, there is great research significance in rendering energy sources flexible and stretchable. An example of such energy sources is flexible nanogenerators, which efficiently convert mechanical energy into electrical energy. Zhong Lin Wang and co-workers review the progress in flexible piezoelectric nanogenerators and flexible triboelectric nanogenerators for harvesting of mechanical energy. Another type of portable energy source is flexible electrochemical-energy-storage devices, which possess great potential for deriving emerging personal electronic devices. On this topic, Feng Li, Hui-Ming Cheng, and Lei Wen summarize the use of carbon nanotubes and graphene for flexible electrochemical energy storage. Wei Weng, Huisheng Peng, and co-workers also discuss the latest progress in wearable fiber-shaped Li-ion batteries. The last type of portable energy source is flexible and light-weight solar cells, which are important not only for powering wearable and portable devices, but also for reducing the transportation and installation cost of solar panels. For this, Jianpu Wang, Feng Gao, Wei Huang, and co-workers briefly introduce the merits of organometal halide perovskites as promising candidates for the development of high-efficiency, flexible and light-weight solar cells. Flexible and stretchable devices configured in soft, water-resistant formats uniquely address seminal challenges in biomedicine. John Rogers and co-workers report an epidermal stimulation and sensing platform for sensorimotor prosthetic control, management of lower back exertion, and electrical muscle activation. Dae-Hyeong Kim and co-workers report recent advances in soft bioelectronics for advanced medical diagnostics and therapies based on the integrated development of soft materials and devices. Meanwhile, various flexible peripheral nerve interfaces have been developed in the last few decades and transferred into neuroscientific research or clinical applications. Toward this end, Chengkuo Lee and co-workers present a novel flexible neural ribbon electrode with a self-adaptive feature for signal acquisition from different sciatic nerves. Flexible medical devices with the capability of monitoring human vital signs, such as body temperature, heart rate, and blood pressure, show potential applications in both fitness monitoring and medical diagnostics. Flexible and stretchable sensors, which have unique characteristics such as ultrathinness, low modulus, light weight, high flexibility, and stretchability, can be conformably attached on the surface of organs or skin, thus providing an enhanced performance for monitoring of human activity and personal healthcare. Ana Arias and co-workers outline and discuss the essential components required for these vital sensors, including the reported sensor systems, sensing mechanisms, sensor fabrication, power, and data-processing requirements. Nae-Eung Lee and Tran Quang Trung describe the latest developments in flexible and stretchable physical sensors for detection of temperature, pressure, and strain. They also provide a detailed overview on the recent progress regarding flexible and stretchable sensor-integrated wearable platforms, as well as self-powered wearable-sensor platform technologies. It is worth noting that the development in this emerging field benefits greatly from the cross-fertilization between the materials science, electrical engineering, biomedical engineering, chemistry, physics, and energy research communities. We hope that this issue provides stimulating highlights of this interdisciplinary research frontier, which has demonstrated promising applications such as electronic skin, healthcare monitoring, and energy storage and conversion, as well as biomedical devices. Novel mechanically durable materials and structures, responsive and active soft materials and devices, stretchable electrodes and circuits, device–bio communicative interfaces, and system-level integration will be several key areas to be further addressed. In addition, new developments in processing, printing, and device engineering would be needed to promote and ignite the exploration on device fabrication and engineering. To close, we are most appreciative for the kind support from the editorial team of Advanced Materials, in particular Dr. Esther Levy and Dr. Lorna Stimson. We would also like to express our gratitude to the colleagues who have shared their insight to this exciting and explorative volume of flexible and stretchable devices. Zhenan Bao is a Professor of Chemical Engineering and by courtesy Professor of Chemistry and Professor of Material Science and Engineering at Stanford University. Prior to joining Stanford in 2004, she was a distinguished member of technical staff in Bell Labs, Lucent Technologies, from 1995 to 2004. She pioneered a number of design concepts for organic electronic materials. Her work has enabled flexible electronic circuits and displays. In her recent work, she has developed skin-inspired organic electronic materials, which have resulted in unprecedented performance or functions in medical devices, energy storage and environmental applications. Xiaodong Chen is an Associate Professor at Nanyang Technological University, Singapore. He received his B. S. degree (Honors) in chemistry from Fuzhou University (China) in 1999, M. S. degree (Honors) in physical chemistry from the Chinese Academy of Sciences in 2002, and Ph. D. degree (Summa Cum Laude) in biochemistry from University of Muenster (Germany) in 2006. After working as a postdoctoral fellow at Northwestern University (USA), he started his independent research career as a Singapore National Research Foundation Fellow and Nanyang Assistant Professor at Nanyang Technological University in 2009. He was promoted to Associate Professor with tenure in September of 2013. His research interests include programmable materials for energy conversion and integrated nano–bio interfaces.