Chronic neural recording electrodes are essential for neurological research and the development of neural prostheses. Although advances in recording technology have been made, probe structures and implantation methods have improved little over time. Insertion of a flexible neural probe in the brain at a precise target location for chronic recording or stimulation without damaging the brain is a challenge for research scientists. Current standard compressive insertion methods require probes to be constructed from hard, stiff materials of a large diameter shank to penetrate the brain, especially to reach deeper structures.
Commonly-used stiff neural probes made of rigid wire or silicon substrate suffer from mechanical mismatch between stiff probe and soft brain tissue. The stress caused by these probes results in electrically-isolating glial scarring, leaving them unable to measure neural spikes of interest. By reducing the diameter of probes and, thereby, the area of material exposed to tissue, flexible thin wire electrodes displace a smaller tissue volume causing much less compression of surrounding tissue compared to rigid electrodes. As a result, the induced immune response is reduced. However, manual insertion is not reasonable for flexible electrodes. An insertion system has been developed (top figure) to prevent buckling of the flexible electrode by utilizing rotational energy to stabilize the probe during insertion. The rotating motion of the electrode is sufficient to surpass the adhesive force at the surface of the electrode-tissue interface while simultaneously preventing the electrode from deviating from its intended path.
The present automated prototype device consists of four main parts: an insertion depth controller, a variable spinner, a stacked pipette electrode guide (bottom left figure), and a robotic micro-wire cutter. The CID has used the developing system to implant 1 cm long micro-wires with diameters as little as 25 micro-meters (bottom right figure) with improving accuracy. Once inserted, the ultra-thin electrodes may be used to measure the electrical activity of single neurons when attached to a signal acquisition system.