wileyonlinelibrary.comsuch as transistor, [ 5 ] triboelectric, [ 6 ] capacitive, [ 7,8 ] piezoelectric, [9][10][11] and piezoresistive properties.Piezoresistive pressure sensors, which transform an input force into an electrical signal caused by the change in the resistance, have attracted considerable attentions by virtue of its simplicity and low cost in design and implementation. Most fl exible piezoresistive sensors are prepared by loading conductive nanomaterials (e.g., carbon nanotubes (CNTs), [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] graphene, [29][30][31][32] nanowires, [33][34][35] nanoparticles) onto fl exible substrates (e.g., fi bers, [ 12,13 ] fi lms, [14][15][16][17] opencell foams [ 29 ] ) via a number of processing methods, such as blending, [ 19,20 ] coating, [ 21,29 ] and printing. [ 17 ] Among the different conductive nanomaterials, carbon nanotubes have attracted a considerable amount of attention due to their remarkably high piezoresistive sensitivity. [ 36,37 ] In addition to the nanomaterials, which are the active sensing elements, the properties of the substrates also play a key role in determining the overall sensor performance. [ 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 defi ned as the ratio of the lateral contractile strain to the longitudinal tensile strain for a material undergoing tension in the longitudinal direction. Collectively, they defi ne the elastic properties and deformation characteristics of the materials in a 3D space. Conceivably, the Poisson ratio would impact the sensing performance 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 materials or materials with negative Poisson ratios has progressed steadily since the initial report by Lakes [ 43 ] on the possibility of such materials. [ 44,45 ] The performance of fl exible and stretchable sensors relies on the optimization of both the fl exible 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 fl exible 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 decrease...