The chronic biocompatibility limit

The hardest wall for invasive BCI is time. The foreign-body response, comprising glial encapsulation, chronic inflammation, neuronal die-back around the implant, and material degradation, degrades signal yield over months to years, and no material has yet demonstrated indefinitely stable single-unit recording in humans. Flexible, tissue-matched, and bioresorbable approaches attack the mechanical and immune drivers, but decade-scale stability at thousands of channels in humans remains unproven.

This limit gates everything downstream: a decoder cannot outlast its electrodes, a memory prosthesis or augmentation must justify chronic risk against benefit, and clinical adoption depends on lifetimes measured against the patient's remaining years. Whether the frontier is solved by better materials, by tolerating and modeling drift, or by minimally-invasive endovascular and non-penetrating routes, is one of the defining engineering bets of the field.