Tissue micromotion

The brain is not static: it pulsates with the cardiac cycle, shifts with respiration, and moves with posture and acceleration, producing relative displacement on the order of micrometres to tens of micrometres between an implanted device and the surrounding tissue. When a stiff probe is rigidly tethered to the skull, this micromotion repeatedly strains the tissue at the probe tip, driving sustained mechanical injury and inflammation that compound the foreign-body response.

Design responses are to make the device compliant so it moves with the tissue rather than shearing against it, and to untether it mechanically — floating arrays, flexible leads, and free-floating meshes and threads that follow local motion. Micromotion is often the reason a probe that records well acutely loses units chronically even without gross scarring, and it is closely coupled to, but distinct from, static modulus mismatch.