Direct Electrical Heating and Multi-Cycle Stretching Method for Micro Wire Straightening

Wire straightening is essential to produce straight wires from a coil for micro-needle-assisted implantation of flexible neural electrodes in brain-computer interfaces. This article introduces a novel direct electrical heating and multi-cycle stretching (DEHMS) method, offering advancements over the current state of the art. For the first time, it has been possible to straighten tungsten wires with diameters smaller than 40 μm effectively, significantly minimizing deflection through multi-cycle stretching and direct current heating. The straightening device was housed in an inert gas chamber to prevent surface oxidation. A force sensor monitored the tension in real-time during the process, providing feedback to the motion module to accurately adjust the wire's stretching and releasing. Our investigation into tension variation, stretch-release cycles, and applied voltage showed that achieving optimal straightness (≈1 μm/1000 μm) minimizes positioning errors caused by the non-straightness of tungsten wires for cortical electrode implantation to below a neuronal cell's length. Optimal conditions were found to be 1.6 N tension, 8 V, and 14 stretch-release cycles in an argon atmosphere.