© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3817 wileyonlinelibrary.com COMMUNICATION A Flexible, Stretchable and Shape-Adaptive Approach for Versatile Energy Conversion and Self-Powered Biomedical Monitoring Po-Kang Yang, Long Lin, Fang Yi, Xiuhan Li, Ken C. Pradel, Yunlong Zi, Chih-I Wu, Jr-Hau He,* Yue Zhang, and Zhong Lin Wang* P. K. Yang, L. Lin, F. Yi, Dr. X. Li, K. C. Pradel, Dr. Y. Zi, Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology Atlanta, GA 30332–0245, USA E-mail: zlwang@gatech.edu P. K. Yang, Prof. C. I. Wu Institute of Photonics and Optoelectronics & Department of Electrical Engineering National Taiwan University Taipei 10617, Taiwan, ROC P. K. Yang, Prof. J. H. He Computer, Electrical and Mathematical Sciences and Engineering (CEMSE) Division King Abdullah University of Science & Technology (KAUST) Thuwal 23955–6900, Saudi Arabia E-mail: jrhau.he@kaust.edu.sa F. Yi, Prof. Y. Zhang State Key Laboratory for Advanced Metals and Materials School of Materials Science and Engineering University of Science and Technology Beijing Beijing 100083, China Prof. Z. L. Wang Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing, China DOI: 10.1002/adma.201500652 power was successfully demonstrated, [12] with numerous applications such as self-powered electronics [13,14] and active sensors. [15] In this work, we developed a new type of flexible TENG (FTENG), which was fabricated by assembling serpentine-pat- terned electrodes and a wavy-structured Kapton film. Owing to the unique structural design, the FTENG could be operated at both compressive and stretching mode. At the traditional com- pressive mode, a high-output power density of 5 W m −2 was delivered at a load resistance of 44 MΩ. At the stretching mode, the FTENG was capable of withstanding a tensile strain of up to 22% and its output performance was up to 70 times larger than that of the planar TENG in the control experiment. Moreover, the FTENG was able to provide reliable output performance on curved surfaces (with curvatures of up to 36 cm −1 ). On the basis of this superior feature, the FTENG could be conformably attached onto human skin for monitoring the gentle motions of joints, muscles, or even the Adam’s apple. This research pre- sents an unprecedented advancement in energy harvesting and self-powered sensors, and paves the way for the next-generation stretchable electronics and bio-integrated systems. The device structure and fabrication process flow of the FTENGs are schematically illustrated in Figure 1a. The FTENG is composed of a wavy-structured Kapton thin film sandwiched by two layers of serpentine copper electrodes deposited on stretchable polydimethylsiloxane (PDMS) substrates. First, the super high stretchability of the PDMS substrates was achieved by mixing the elastomer base and curing agent at a 30:1 ratio, [16] by which an optimized stability of the deposited electrode was demonstrated (Figure S1a, Supporting Information). Then, ser- pentine copper electrodes were deposited on the PDMS mem- branes by a two-step sputtering approach (see Experimental Section for details). It has been well recognized that the serpen- tine patterns can provide extraordinary stability to metal elec- trodes even under tensile strain. [17–20] In this work, the unique advantage of the serpentine electrode was successfully verified by a controlled experiment, in which the conductivities of both linear and serpentine electrodes were monitored upon a series of tensile strains, and the resistance of the serpentine elec- trode was much lower and more stable than that of the linear electrode, especially under high tensile strains. Finally, a wavy- structured Kapton film was fabricated by a high-temperature annealing process (Experimental Section) and inserted between the two layers of electrodes. It can serve as both the triboelec- tric material and the spacer layer based on its reversible elastic deformation upon compressing or stretching. Nanowire struc- tures were introduced on both surfaces of the wavy-structured Flexible and stretchable electronics have attracted long-lasting attentions for their promising applications in next-generation functional devices, including flexible circuitries, [1] stretchable displays, [2] stretchable sensors, [3] epidermal electronics, [4] and implantable devices. [5] This new class of electronics allows devices to be deformed into complex shapes while maintaining the device performance and reliability. However, a sustainable power source is highly desired to drive those electronic devices, and the implementation of traditional power supply remains a challenge due to inconvenient operations and indispensable wire connections. In this regard, a more efficient way is to integrate a power generator to scavenge the ambient energy, especially for the mechanical energy from stretching motions. Attempts have been made to develop flexible and stretchable power generators, [6,7] but their output performances still need further enhancement for practical applications. Recently, tribo- electric nanogenerators (TENGs) have been invented based on triboelectrification and electrostatic induction, [8–10] which are demonstrated to be a cost-effective and high-efficient approach for harvesting ambient mechanical energy. [11] Various working modes were developed to accommodate energy conversion from different types of mechanical motions, and high-output Adv. Mater. 2015, 27, 3817–3824 www.advmat.de www.MaterialsViews.com