Topology-engineered piezoresistive lattices with programmable strain sensing, auxeticity, and failure modes

This study investigates the programmable strain sensing capability, auxetic behaviour, and failure modes of 3D-printed, self-monitoring lattices made from in-house-engineered polyetheretherketone (PEEK) reinforced with multi-walled carbon nanotubes (MWCNTs). A skeletally parametrized geometric modelling framework, combining Voronoi tessellation with 2D wallpaper symmetries, is used to systematically explore a vast range of non-traditional, non-predetermined topologies. A representative set of these architectures is realized via fused filament fabrication, and multiscale characterization-including macroscale tensile testing and microstructural analysis-demonstrates tuneable multifunctional performance as a function of MWCNT content and unit cell topology. Real-time electrical resistance measurements track deformation, damage initiation, and progression, with the sensitivity factor increasing from below 1 in the elastic regime (strain sensitivity) to as high as 80 for PEEK/MWCNT at 6 wt% under inelastic deformation (damage sensitivity). Architecture-topology tailoring further allows fine-tuning of mechanical properties, achieving stiffness values ranging from 9 MPa to 63 MPa and negative Poisson's ratios between -0.63 and -0.17 at ∼3 wt% MWCNT and 25% relative density. Furthermore, a novel piezoresistive finite element model, implemented in Abaqus via a user-defined subroutine, accurately captures stress-induced intrinsic piezoresistivity, geometry-driven deformation, and damage evolution up to the onset of ligament failure. Together, the experimental results and predictive modelling enable "design for strain-sensitivity" and "design for failure", demonstrating how architecture-topology tuning can be leveraged to tailor strain sensitivity, auxeticity, and failure modes-ultimately guiding the development of multifunctional piezoresistive architected composites for applications such as smart orthopaedic implants, aerospace skins, and impact-tolerant systems.