This article presents three-dimensional printed and highly sensitive polydimethylsiloxane/multi-walled carbon nanotube sensors for compressive strain and pressure measurements. An electrically conductive polydimethylsiloxane/multi-walled carbon nanotube nanocomposite is developed to three-dimensional print compression sensors in a freestanding and layer-by-layer manner. The dispersion of multi-walled carbon nanotubes in polydimethylsiloxane allows the uncured nanocomposite to stand freely without any support throughout the printing process. The cross section of the compression sensors is examined under scanning electron microscope to identify the microstructure of nanocomposites, revealing good dispersion of multi-walled carbon nanotubes within the polydimethylsiloxane matrix. The sensor’s sensitivity was characterized under cyclic compression loading at various max strains, showing an especially high sensitivity at lower strains. The sensing capability of the three-dimensional printed nanocomposites shows minimum variation at various applied strain rates, indicating its versatile potential in a wide range of applications. Cyclic tests under compressive loading for over 8 h demonstrate that the long-term sensing performance is consistent. Finally, in situ micromechanical compressive tests under scanning electron microscope validated the sensor’s piezoresistive mechanism, showing the rearrangement, reorientation, and bending of the multi-walled carbon nanotubes under compressive loads, were the main reasons that lead to the piezoresistive sensing capabilities in the three-dimensional printed nanocomposites.

Topological lattices have recently generated a great deal of interest based on the unique mechanical properties rooted in their topological polarization, including the ability to support localized modes at certain floppy edges. The study of these systems has been predominantly restricted to the realm of in-plane mechanics, to which many topological effects are germane. In this study, we stretch this paradigm by exploring the possibility to export certain topological attributes to the flexural wave behavior of thin lattice sheets. To couple the topological modes to the out-of-plane response, we assemble a bilayer lattice by stacking a thick topological kagome layer onto a thin twisted kagome lattice. The band diagram reveals the existence of modes whose out-of-plane character is controlled by the edge modes of the topological layer, a behavior elucidated via simulations and confirmed via laser vibrometer experiments on a bilayer prototype specimen. These results open an alternative direction for topological mechanics whereby flexural waves are controlled by the in-plane topology, leading to potential applications for flexural wave devices with engineered polarized response.

Periodic networks on the verge of mechanical instability, called Maxwell lattices, are known to exhibit zero-frequency modes localized to their boundaries. Topologically polarized Maxwell lattices, in particular, focus these zero modes to one of their boundaries in a manner that is protected against disorder by the reciprocal-space topology of the lattice’s band structure. Here, we introduce a class of mechanical bilayers as a model system for designing topologically protected edge modes that couple in-plane dilational and shearing modes to out-of-plane flexural modes, a paradigm that we refer to as “omnimodal polarization.” While these structures exhibit a high-dimensional design space that makes it difficult to predict the topological polarization of generic geometries, we are able to identify a family of mirror-symmetric bilayers that inherit the in-plane modal localization of their constitutive monolayers, whose topological polarization can be determined analytically. Importantly, the coupling between the layers results in the emergence of omnimodal polarization, whereby in-plane and out-of-plane edge modes localize on the same edge. We demonstrate these theoretical results by fabricating a mirror-symmetric, topologically polarized kagome bilayer consisting of a network of elastic beams via additive manufacturing and confirm this finite-frequency polarization via finite element analysis and laser-vibrometry experiments.