# Metallic Flexible NiTi Wire Microcrack Transducer for Label-Free Impedimetric Sensing of Escherichia coli

## TL;DR

A new flexible metallic biosensor uses microcracks in NiTi wire to detect E. coli in urine without labels or amplification, offering fast and reliable results.

## Contribution

Introduces a metallic microcrack transducer using superelastic NiTi for label-free impedimetric biosensing in the MHz frequency range.

## Key findings

- NiTi microcracks enable stable detection of E. coli in human urine with a limit of 64 CFU/mL.
- The sensor operates label-free and without redox mediators or amplification in 45 minutes.
- Martensitic microcrack networks in NiTi offer a new class of mechanically robust, scalable biosensors.

## Abstract

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 microcrack transducer 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 govern the impedance response. Our approach imparts a novel dielectric character to the alloy, enabling its unexplored application in the megahertz (MHz) frequency domain (0.01–10 MHz) where native NiTi is merely conductive. Functionalization with Escherichia coli (E. coli)-specific antibodies renders these microdomains biologically active. This effectively transforms the mechanically induced microcracks into tunable impedance elements driven by analyte binding. The γ-bent NiTi sensors achieved stable and quantitative detection of E. coli ATCC 25922 in sterile human urine, with a detection limit of 64 colony forming units (CFU) mL−1 within 45 min, without redox mediators, external labels, or amplification steps. This work pioneers the use of martensitic microcrack networks, mimicking self-healing behavior 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.