FULL PAPER © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2900 wileyonlinelibrary.com such as transistor, [5] triboelectric, [6] capaci- tive, [7,8] piezoelectric, [9–11] and piezoresis- tive [12–35] properties. Piezoresistive pressure sensors, which transform an input force into an electrical signal caused by the change in the resist- ance, have attracted considerable atten- tions by virtue of its simplicity and low cost in design and implementation. Most flexible piezoresistive sensors are pre- pared by loading conductive nanomate- rials (e.g., carbon nanotubes (CNTs), [12–28] graphene, [29–32] nanowires, [33–35] nan- oparticles) onto flexible substrates (e.g., fibers, [12,13] films, [14–17] open- cell foams [29] ) via a number of pro- cessing methods, such as blending, [19,20] coating, [21,29] and printing. [17] Among the different conductive nanomaterials, carbon nanotubes have attracted a con- siderable amount of attention due to their remarkably high piezoresistive sensitivity. [36,37] In addition to the nano- materials, which are the active sensing elements, the properties of the substrates also play a key role in determining the overall sensor per- formance. [27,28] Most studies on the effects of the substrates focus on the modulus, and it has been suggested that porous substrates with reduced elastic modulus result in increased sensing properties. [19] Yet from the classical mechanics point of view, the other most fundamental property that dictates the elastic properties is the Poisson ratio, which is defined as the ratio of the lateral contractile strain to the longitudinal tensile strain for a material undergoing tension in the longitudinal direction. Collectively, they define the elastic properties and deformation characteristics of the materials in a 3D space. Conceivably, the Poisson ratio would impact the sensing per- formance of piezoresistive sensors; however, this effect has not been studied. Classical mechanics predicts that for isotropic materials, the Poisson ratio lies between –1 and 0.5, a fairly small range. [38] With a few exceptions such as α-cristobalite, [39] certain cubic metal, [40] and few biological tissues, [41] the range of Poisson ratio of almost all natural or synthetic materials is even smaller, typically 0.3–0.5. [42] Research on fabrication of auxetic mate- rials or materials with negative Poisson ratios has progressed steadily since the initial report by Lakes [43] on the possibility of such materials. [44,45] Poisson Ratio and Piezoresistive Sensing: A New Route to High-Performance 3D Flexible and Stretchable Sensors of Multimodal Sensing Capability Yan Li, Sida Luo, Ming-Chia Yang, Richard Liang,* and Changchun Zeng* The performance of flexible and stretchable sensors relies on the optimization of both the flexible substrate and the sensing element, and their synergistic interactions. Herein, a novel strategy is reported for cost-effective and scalable manufacturing of a new class of porous materials as 3D flexible and stretchable piezoresistive sensors, by assembling carbon nanotubes onto porous substrates of tunable Poisson ratios. It is shown that the piezoresistive sensitivity of the sensors increases as the substrate’s Poisson’s ratio decreases. Substrates with negative Poisson ratios (auxetic foams) exhibit significantly higher piezoresistive sensitivity, resulting from the coherent mode of deformation of the auxetic foam and enhanced changes of tunneling resistance of the carbon nanotube networks. Compared with conventional foam sensors, the auxetic foam sensor (AFS) with a Poisson’s ratio of –0.5 demonstrates a 300% improvement in piezoresistive sensitivity and the gauge factor increases as much as 500%. The AFS has high sensing capability, is extremely robust, and capable of multimodal sensing, such as large deformation sensing, pressure sensing, shear/torsion sensing, and underwater sensing. AFS shows great potential for a broad range of wearable and portable devices applications, which are described by reporting on a series of demonstrations. DOI: 10.1002/adfm.201505070 Dr. Y. Li, Dr. S. Luo, M.-C. Yang, Prof. R. Liang, Prof. C. Zeng High Performance Materials Institute Florida State University Tallahassee, FL 32310, USA E-mail: zliang@fsu.edu; czeng@fsu.edu Dr. Y. Li, Dr. S. Luo, M.-C. Yang, Prof. R. Liang, Prof. C. Zeng Department of Industrial and Manufacturing Engineering FAMU-FSU College of Engineering Tallahassee, FL 32310, USA 1. Introduction Flexible, stretchable, highly sensitive, and low-cost pressure sensors are key elements in advancing wearable or implant- able measuring devices. [1–4] Since the last decade, [1,2] the pur- suit of such sensors has become a rapidly expanded area of research that covers electronics, chemistry, physics, mechanics, and materials science, and has enabled a wide variety of new ideas in sensor design based on different sensing mechanisms, Adv. Funct. Mater. 2016, 26, 2900–2908 www.afm-journal.de www.MaterialsViews.com