The vascular response during cortical microelectrode insertion was measured with amplitude decorrelation-based quantitative optical coherence angiography (OCA). Four different shank-style microelectrode configurations were inserted in murine motor cortex beneath a surgically implanted window in discrete steps while OCA images were collected and processed for angiography and flowmetry. Quantitative measurements included tissue displacement (measured by optical flow), perfused capillary density, and capillary flow velocity. The primary effect of insertion was mechanical perturbation, the effects of which included tissue displacement, arteriolar rupture, and compression of a branch of the anterior cerebral artery causing a global decrease in flow. Other effects observed included local flow drop-out in the region immediately surrounding the microelectrode. The mean basal capillary network velocity for all animals was 0.23 (±0.05 SD) and 0.18 (±0.07 SD) mm/s for capillaries from 100 to 300 μm and 300 to 500 μm, respectively. Upon insertion, the 2-shank electrode arrays caused a decrease in capillary flow density and velocity, while the results from other configurations were not different from controls. The proximity to large vessels appears to play a larger role than the array configuration. These results can guide neurosurgeons and electrode designers to minimize trauma and ischemia during microelectrode insertion.
Despite advances in functional neural imaging, penetrating microelectrodes provide the most direct interface for the
extraction of neural signals from the nervous system and are a critical component of many high degree-of-freedom braincomputer
interface devices. Electrode insertion is a traumatic event that elicits a complex neuroinflammatory response.
In this investigation we applied optical coherence microscopy (OCM), particularly optical coherence angiography
(OCA), to characterize the immediate tissue response during microelectrode insertion. Microelectrodes of varying
dimension and footprint (one-, two-, and four-shank) were inserted into mouse motor cortex beneath a window after
craniotomy surgery. The microelectrodes were inserted in 3-4 steps at 15-20°, with approximately 250 μm linear
insertion distance for each step. Before insertion and between each step, OCM datasets were collected, including for
quantitative capillary velocimetry. A cohort of control animals without microelectrode insertion was also imaged over a
similar time period (2-3 hours). Mechanical tissue deformation was observed in all the experimental animals. The
quantitative angiography results varied across animals, and were not correlated with device dimensions. In some cases,
localized flow drop-out was observed in a small region surrounding the electrode, while in other instances a global
disruption in flow occurred, perhaps as a result of large vessel compression caused by mechanical pressure. OCM is a
tool that can be used in various neurophotonics applications, including quantification of the neuroinflammatory response
to penetrating electrode insertion.
Optical coherence microscopy (OCM) provides real-time, in-vivo, three-dimensional, isotropic micron-resolution
structural and functional characterization of tissue, cells, and other biological targets. Optical coherence angiography
(OCA) also provides visualization and quantification of vascular flow via speckle-based or phase-resolved techniques.
Performance assessment of neuroprosthetic systems, which allow direct thought control of limb prostheses, may be aided
by OCA. In particular, there is a need to examine the underlying mechanisms of chronic functional degradation of
implanted electrodes. Angiogenesis, capillary network remodeling, and changes in flow velocity are potential indicators
of tissue changes that may be associated with waning electrode performance. The overall goal of this investigation is to
quantify longitudinal changes in vascular morphology and capillary flow around neural electrodes chronically implanted
in mice. We built a 1315-nm OCM system to image vessels in neocortical tissue in a cohort of mice. An optical window
was implanted on the skull over the primary motor cortex above a penetrating shank-style microelectrode array. The
mice were imaged bi-weekly to generate vascular maps of the region surrounding the implanted microelectrode array.
Acute effects of window and electrode implantation included vessel dilation and profusion of vessels in the superficial
layer of the cortex (0-200 μm). In deeper layers surrounding the electrode, no qualitative differences were seen in this
early phase. These measurements establish a baseline vascular tissue response from the cortical window preparation and
lay the ground work for future longitudinal studies to test the hypothesis that vascular changes will be associated with
chronic electrode degradation.