Stiffness-tunable neurotentacles for minimally invasive implantation and long-term neural activity recordings

Flexible implantable microelectrodes have been demonstrated to exhibit excellent biocompatibility for chronic neural activity recordings. However, the low bending strength of the commonly employed flexible materials presents a significant challenge for probe insertion into the brain. Traditional implantation methods for flexible electrodes generally require additional auxiliary materials or tools, which tend to have a much larger footprint than the probes themselves, greatly increasing the damage to neurons during insertion. Here we have proposed a stiffness-tunable polyimide probe for deep brain implantation, referred to as Neurotentacle, enabled by embedded microchannels in which the liquid pressure is controllable (from 0.1MPa to more than 2.0MPa). During the insertion phase into the brain, the neurotentacle can pose a high stiffness under elevated internal pressure to penetrate the brain tissues without the use of any additional materials or tools. Once the device has been successfully inserted, it can regain its flexibility by reducing the internal pressure. Importantly, the novel multilayer microfabrication process keeps the structural dimensions of the neurotentacle similar to those of a regular flexible probe. Therefore, the neurotentacle can produce an extremely low level of damage to brain tissue during its insertion phase, while extending its long-term biocompatibility and stability, which has been experimentally verified in histological evaluations conducted on both acute and chronic animal specimens. In addition, the chronically implanted neurotentacles enabled stable neural activity recordings in mice with an average spike yield of 96% and an average signal-to-noise ratio of 15.2. The proposed neurotentacle does not necessitate the use of complex devices and its insertion process is straightforward and highly controllable, thus rendering it an appealing technique for minimally invasive implantation and long-term neural recording of flexible electrodes.