Structural Designs and Implantation Strategies of Penetrating Flexible Neural Electrodes: From Planar Shanks to Volumetric Neural Interfaces

This article reviews recent advances in the structural design and implantation strategies of penetrating flexible neural electrodes for intracortical recording. Traditional rigid electrodes suffer from limited long-term stability due to mechanical mismatch with brain tissue. While flexible electrodes improve biocompatibility, they face significant implantation challenges. The article systematically analyzes penetrating-capable structures, including tapered shank electrodes, thread-like electrodes, mesh and bioinspired electrodes, and hollow electrodes, and discusses corresponding implantation strategies such as biodegradable stiffening coatings, shuttle-assisted delivery, syringe injection, as well as stylet or fluid pressure based insertion. It further introduces three-dimensional, high-density electrode arrays enabled by rolling- and assembly-based fabrication techniques, which combine tissue compliance with high electrode density and functionality. Finally, we highlight persistent challenges and future opportunities in scalable fabrication, device reliability, and implantation mechanics, emphasizing the need for integrated approaches to enable next-generation flexible neural interfaces.