Ultra-sensitive measurement of brain penetration mechanics and blood vessel rupture with microscale probes
Microscale electrodes, on the order of 10-100 m, are rapidly becoming critical tools for neuroscience and brain-machine interfaces (BMIs) for their high channel counts and spatial resolution, yet the mechanical details of how probes at this scale insert into brain tissue are largely unknown. Here, we performed quantitative measurements of the force and compression mechanics together with real-time microscopy for in vivo insertion of a systematic series of microelectrode probes as a function of diameter (7.5-100 m and rectangular Neuropixels) and tip geometry (flat, angled, and electrochemically sharpened). Results elucidated the role of tip geometry, surface forces, and mechanical scaling with diameter. Surprisingly, the insertion force post-pia penetration was constant with distance and did not depend on tip shape. Real-time microscopy revealed that at small enough lengthscales (<25 m), blood vessel rupture and bleeding during implantation could be entirely avoided. This appears to occur via vessel displacement, avoiding capture on the probe surface which led to elongation and tearing for larger probes. We propose a new, three-zone model to account for the probe size dependence of bleeding, and provide mechanistic guidance for probe design. Significance StatementMicroscale neural probes are central to next-generation brain-machine interfaces, yet how they physically penetrate living brain remains poorly quantified. Using a high-sensitivity force sensor integrated with real-time microscopy, we measured in vivo force-displacement and visualized vascular responses for microwires (7.5-100 m) and Neuropixels. We find that once the brains protective pia membrane is breached, insertion force remains essentially constant with depth, while pia puncture force and pre-penetration compression scale linearly with probe diameter. Real-time imaging reveals a sub-25 m regime in which blood vessels are displaced rather than ruptured. These results motivate a three-zone model of vessel capture versus displacement and provide actionable mechanical design rules for low-trauma, high-density neural interfaces.