© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 3027–3031 3027 www.advmat.de www.MaterialsViews.com COMMUNICATION By Yingying Zhang,* Chris J. Sheehan, Junyi Zhai, Guifu Zou, Hongmei Luo, Jie Xiong, Y. T. Zhu, and Q. X. Jia* Polymer-Embedded Carbon Nanotube Ribbons for Stretchable Conductors The creation of stretchable electronics is emerging as one of the most interesting research topics in materials science and technology. [1,2] Devices that are stretchable, foldable, and deformable into complex curvilinear shapes can enable many new applications that would be impossible to achieve by con- ventional rigid electronics. Examples of such applications range from flexible displays, electronic eyeball cameras to stretch- able electronic implants and conformable skin sensors. [3–6] One of the major challenges towards stretchable electronics is the development of stretchable electrical wiring that is both conductive and stretchable. To fabricate stretchable conductors, one strategy which has been developed in recent years is to fabricate wavy or net-shaped conductive structures by releasing pre-strained elastic substrate with conductive materials lying on it. [7] Different stretchable conductors, such as metal-coated net films, wavy one-dimensional metal ribbons or two-dimensional metal membranes have been demonstrated based on the above strategy. [8–11] Another strategy is to utilize conductive materials with large aspect ratio or in liquid state, [12,13] which can bridge cracked regions to maintain them conductive under tensile strains. Carbon nanotubes (CNTs) have high aspect ratio, good conductivity, as well as high thermal stability and mechan- ical robustness, [14] which make them very promising mate- rials for stretchable conductors. Although CNT films have been extensively studied for making flexible and transparent electrodes, [15–18] their stretchability has been rarely explored. Very recently, composite films made from CNTs/ionic liquid/ fluorinated copolymer with punched net-shaped holes were reported as elastic conductors, which showed deteriorated conductivity when stretched. [2] This rubber-like composite was black in color. Transparent CNT films with randomly distributed CNTs were also reported as elastic conductors, where the films remained conductive under linear strains up to 700%. However, their conductivity decreased superlinearly with strains. [19] Here, we report a transparent stretchable conductors that can maintain stable conductivity under repetitive stretching, in which well-aligned CNT ribbons are embedded in poly(dimethylsiloxane) (PDMS) (hereafter defined as CNT/ PDMS film for convenience). Since the demonstration of CNT ribbons being directly drawn from CNT forests, [20] exciting applications of CNT ribbons such as high-performance CNT fibers, [21] flexible loudspeakers [22] and binder-free anodes [23] have been explored. CNT ribbons represent a unique CNT assembly in which aligned millimeter-long CNTs form bun- dles with neighbor CNTs along their axial direction, [24] keeping the ribbons continuous up to meters. This implies that CNT ribbons may be stretched and remain continuous if the CNTs can slide in the axial direction against each other under uniaxial strains. To investigate the stretchability of this unique CNT assembly, we measured the dependence of resistance on the tensile strain. Compared with the deteriorated conductivity of the reported CNT films under strain, [2,19] the conductivity of our CNT/PDMS film remains stable under linear tensile strain after the first several cycles of stretching/releasing. The results show that CNT ribbons can be used as stretchable conductors with stable conductivity under tensile strain up to 100%. The CNT ribbons used in this study were directly drawn from well-aligned CNT (multi-walled carbon nanotube, MWNT) forests. Figure 1a shows a side view of scanning electron micro- scope (SEM) image of the CNT forest, in which the CNTs are straight and parallel to each other. The CNT forest has a height around 0.6 mm, and the CNTs in it are continuous from the bottom to the top. As reported in our previous work, [24] only this kind of well-aligned CNT forest enables the drawing of contin- uous CNT ribbons. An edge of the forest, where CNT ribbons are drawn out, is shown in the right side of Figure 1a as well as in the left side of Figure 1b. Figure 1b shows the top view of the CNT ribbons drawn from the CNT forest. The ribbons are uni- form and aligned along the drawing direction (shown by arrow). As shown in a magnified SEM image (Figure 1c), curved CNTs exist in the ribbons although they are aligned macroscopically. The CNT ribbons were further studied by transmission elec- tron microscope (TEM). Figure 1d shows a macroscopic TEM image of the ribbons, where aligned CNTs dominate the whole structure. A magnified TEM image is shown in Figure 1e, which shows clearly the existence of CNT bundles and the DOI: 10.1002/adma.200904426 [*] Dr. Y. Y. Zhang, Dr. J. Y. Zhai, Dr. G. F. Zou, Dr. H. L. Luo, [+] Dr. J. Xiong, Dr. Q. X. Jia Center for Integrated Nanotechnologies Los Alamos National Laboratory Los Alamos, NM 87545 (USA) E-mail: yyzhang@lanl.gov; qxjia@lanl.gov C. J. Sheehan Superconductivity Technology Center Los Alamos National Laboratory Los Alamos, NM 87545 (USA) Dr. Y. T. Zhu Department of Material Science and Engineering North Carolina State University Raleigh, NC 27695 (USA) [ +] Present address: Department of Chemical Engineering, New Mexico State University, Las Cruces, NM 88003, (USA)