Bioelectronics Meets the Brain: Establishing Biostability of Multi-Layered Polyimide-Based Intracortical Implants

Flexible neural implants are extremely favored, as the most successful strategy to promote probe-tissue integration and avoid severe gliosis relies on reducing the mechanical mismatch between probe and brain tissue. But what is a reliastic requirement for achieving chronic recording stability? Which are the critical dimensions and main factors determining device integration? To answer these questions, two types of hair-sized polyimide-based flexible intracortical (PIXI) arrays were fabricated, differing only in cross-section. Chronic tissue reaction to both types was evaluated in rats, in different implantation setups. Interfacial stresses were found to play a critical role in long-term implant tissue integration. Still, all the devices provided high quality chronic recordings of single units and expression of the inflammatory genes was not significantly upregulated for the larger device. Our study points out that the most relevant factor in eliciting FBR is played by probe-tissue mechanical interactions and not by device size alone.