Long-term intracranial electrode implants have become a crucial tool for measuring and manipulating neural activity in patients that suffer from neurological disorders ranging from Parkinson's disease to seizure disorders and spinal cord injuries. Traditional implants, however, require invasive surgical approaches, such as open craniotomies, which can cause brain trauma, chronic inflammation and disruptions to the blood–brain barrier. Additionally, these invasive approaches limit the placement of electrode arrays to superficial areas of the brain. To circumvent these obstacles, a team led by Thomas Oxley at Melbourne University (Australia) has developed an endovascular stent-electrode array (the 'stentrode') that enables stable long-term recordings in the brain using a minimally invasive surgical approach (Nat. Biotechnol. 34, 320–327; 2016). With this new stentrode, which they implanted in freely moving sheep, the research team reports successful recordings of cells in motor cortex for 190 days.

Oxley et al. developed their endovascular electrode array using stent technology that is currently in clinical use. These self-expanding stents served as a scaffold to which 750-μm platinum electrode discs were attached as recording contacts. To ensure that the stents maintained their superelastic properties, which allow them to conform to various blood vessel sizes and shapes, the platinum discs were attached at repeating stent strut cross-links and separated by 2.5 mm. The self-expanding stents were compressed for minimally invasive delivery through a catheter and guided into the superior sagittal sinus immediately adjacent to the motor cortex. The researchers had to overcome several difficulties inherent to catheterization of the cerebral vascular system, including maneuvering the stentrode through valves, chordae and arachnoid granulations. They confirmed placement of the stentrode near the motor cortex using MRI and post-implantation computerized tomography.

Long-term stability of stents depends on their incorporation into the walls of blood vessels, so Oxley et al. monitored incorporation of the stentrode using x-ray imaging and impedance measurements from the platinum electrode discs. This monitoring suggested that stents had indeed been successfully incorporated within vessel walls. The researchers assessed the electrical recording properties of successfully implanted stentrodes using cortically generated evoked potentials and by measuring baseline electrical activity under awake and anesthetized conditions. They also found significant improvements in the number of electrode discs with measureable electrical responses over the course of a one-month testing period. This improvement in recording quality over time, combined with the minimally invasive catheterization procedure for implantation, makes the stentrode an exciting new technology for long-term intracranial recordings with important potential clinical applications.