The effect of residual endotoxin contamination on the neuroinflammatory response to sterilized intracortical microelectrodes

Abstract

A major limitation to the use of microelectrode technologies in both research and clinical applications is our inability to consistently record high quality neural signals. There is increasing evidence that recording instability is linked, in part, to neuroinflammation. A number of factors including extravasated blood products and macrophage released soluble factors are believed to mediate neuroinflammation and the resulting recording instability. However, the roles of other inflammatory stimuli, such as residual endotoxin contamination, are poorly understood. Therefore, to determine the effect of endotoxin contamination we examined the brain tissue response of C57/BL6 mice to non-functional microelectrodes with a range of endotoxin levels. Endotoxin contamination on the sterilized microelectrodes was measured using a limulus amebocyte lysate test following FDA guidelines. Microelectrodes sterilized by autoclave, dry heat, or ethylene oxide gas, resulted in variable levels of residual endotoxins of 0.55 EU/mL, 0.22 EU/mL, and 0.11 EU/mL, respectively. Histological evaluation at two weeks showed a direct correlation between microglia/macrophage activation and endotoxin levels. Interestingly, astrogliosis, neuronal loss, and blood brain barrier dysfunction demonstrated a threshold-dependent response to bacterial endotoxins. However, at sixteen weeks, no histological differences were detected, regardless of initial endotoxin levels. Therefore, our results demonstrate that endotoxin contamination, within the range examined, contributes to initial but not chronic microelectrode associated neuroinflammation. Our results suggest that minimizing residual endotoxins may impact early recording quality. To this end, endotoxins should be considered as a potent stimulant to the neuroinflammatory response to implanted intracortical microelectrodes.

Mechanically-compliant intracortical implants reduce the neuroinflammatory response

“…All implants were ethylene oxide sterilized with the exception of sixteen week implants, which were UV sterilized before implantation. Previous work has established that sterilization method does not affect the inflammatory response at chronic time points (15). …”

The roles of blood-derived macrophages and resident microglia in the neuroinflammatory response to implanted Intracortical microelectrodes

“…To assess propagation of neuroinflammatory events following microelectrode implantation, a sterile, single-shank, non-functional ‘Michigan-style’ silicon microelectrode (2 mm × 123 μm × 15 μm) was implanted in wildtype and chimera mice for two, four, eight or sixteen weeks according to previously published protocols [29,30]. All surgeries in this study were performed in a class II sterile hood by the same surgeon to minimize infection and variability, respectively.…”

“…At two, four, eight and sixteen weeks post implantation, mice were perfused and brain tissue was cryoprotected according to methods previously described [29,30]. Briefly, mice were anesthetized with an intraperitoneal (IP) injection of rodent cocktail (150 μl 100 mg/ml Ketamine HCl, 150 μl 20 mg/ml Xylazine HCl, 50 μl 10 mg/ml Acepromazine).…”

“…Immunohistological (IHC) analysis of neuroinflammatory associated cellular markers (microglia/macrophages, activated microglia/macrophages and astrocytes) as well as blood protein Immunoglobulin G (IgG) was performed in wildtype and chimera mice to quantify the extent of the neuroinflammatory response at the tissue–electrode interface, as previously described [29,30]. Additionally, IHC analysis of CFP+ cells was also performed in chimera mice in order to quantify the populations of infiltrating blood-derived cells over time surrounding the microelectrode.…”

“…Neuronal densities were assessed using diaminobenzidine (DAB) histochemistry following the methods previously described [29,30]. Tissue sections were equilibrated to RT and permeated as described in Section 2.7.1.…”