Poisson Ratio and Piezoresistive Sensing: A New Route to High‐Performance 3D Flexible and Stretchable Sensors of Multimodal Sensing Capability

Abstract

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...

Lattice Metamaterials with Mechanically Tunable Poisson’s Ratio for Vibration Control

“…Between the various architectures it is worth to note dimpled and perforated elastic sheets [47], origami/Kirigami-based metamaterials [18,48,49], hierarchical metamaterials with fractal cuts [50] and foams [51][52][53][54][55]. Auxetic materials and structures are intrinsically multifunctional because of the coupling originated between their unusual deformation mechanisms and their multiphysics behavior.…”

“…Auxetic materials and structures are intrinsically multifunctional because of the coupling originated between their unusual deformation mechanisms and their multiphysics behavior. For example, piezoresistive sensors with an auxetic substrate demonstrate a 300% improvement in piezoresistive sensitivity, making them capable of multimodal sensing [55].…”

Nanomaterial‐Enabled Wearable Sensors for Healthcare

“…[144] Figure 7a highlights a resistive sensor based on CNT coated foams with negative Poisson's ratio (auxetic foams) for the detection of pressure, stretching, torsion and ultrasound acoustic wave. [48] The auxetic foams were able to shrink/stretch concurrently in three dimensions, and therefore enhanced the pressure sensitivity by 300% and tensile strain sensitivity by 500%.…”

“…[191][192][193] Reproduced with permission. [48] b) Schematic illustration of the stretchable capacitive multifunctional sensors based on CNT yarns. Reproduced with permission.…”

“…[18,[33][34][35] To fabricate the wearable sensors, conventional lithographic processes can be extended to pattern nanomaterials. [23,[36][37][38] Solution based processing methods such as spray coating, [25,39,40] drop casting, [41][42][43][44][45] spin coating, [46,47] dip coating, [48] vacuum filtration, [49][50][51] and layer-by-layer assembly [52] were commonly used for fabrication of nanomaterial-based sensors. Direct spinning of CNTs onto a substrate or into yarns [53][54][55] and electrospinning of nanofibers or NWs were reported to produce fiber-like nanomaterials.…”