Force Measurements to Advance the Design and Implantation of CMOS-Based Neural Probes

Objective: Tissue penetrating active neural probes provide large and densely packed microelectrode arrays for the fine-grained investigation of brain circuits and for advancing brain-machine interfaces (BMIs). To improve the electrical interfacing performances of such stiff silicon devices, which typically elicit a vigorous foreign body reaction (FBR), here we perform insertion force measurements and derive probe layout and implantation procedure optimizations. Methods: We performed in-vivo insertion force measurements to evaluate the impact of probe design and implantation speed on mechanically induced trauma and iatrogenic injury. Because acute damage constitutes the initial trigger of FBR, these experiments allow to characterize and minimize device invasiveness. Results: Probe sharpness outweighs cross-sectional dimensions during the dimpling stage of the implantation, when the device compresses the brain before penetration. Insertion speed does not display a major effect on dimpling magnitude. A slow speed, however, significantly increases dimpling duration. Conclusion: It is crucial to use sharp devices to reduce mechanical and ischemic damage. Although slow insertion speeds typically improve the quality of acute electrophysiological recordings, we show that slow speeds should only be used upon penetration in the brain parenchyma and not during the dimpling stage. A closed-loop implantation procedure can be used to set the appropriate speed in the different insertion stages. Significance: We provide new evidence on the impact of probe layout and insertion speed on insertion force, with implications on the design and implantation procedure for minimally invasive CMOS neural probes. A novel closed-loop methodology to optimize device implantation and reduce FBR is proposed.