Metallic Flexible NiTi Wire Microcrack Transducer for Label-Free Impedimetric Sensing of E. coli

Flexible biosensors offer rapid and low-cost diagnostics but are often limited by the mechanical and electrochemical instability of polymer-based designs in biological media. Here, we introduce a metallic flexible sensing platform that exploits the intrinsic deformability of superelastic nickel–titanium (NiTi) for label-free impedimetric detection. Mechanical bending of NiTi wires spontaneously generates martensitic-phase microcracks whose metal–gap–metal geometry forms the active transduction sites, where functional interfacial layers and captured analytes modulate the local dielectric environment and governing the impedance response. Functionalization with thiolated monolayers and Escherichia coli-specific antibodies enables these microdomains to modulate interfacial charge transfer in response to analyte binding, creating a direct coupling between mechanical deformation and resulting impedance signal. The γ-bent NiTi sensors achieved stable and quantitative detection of E. coli ATCC 25922 in sterile human urine, with a detection limit of 53 CFU mL⁻¹ within 45 minutes, without redox mediators, external labels, or amplification steps. This work establishes the first use of self-healing martensitic microcracks in a superelastic alloy as functional transduction elements, defining a new class of metallic flexible biosensors that integrate mechanical robustness, analytical reliability, and scalability for point-of-care biosensing.