MEMS in the Nervous System 81
is divided into acute and chronic phases; during the acute phase, microglia become
activated and respond to the foreign electrode.
It is suspected that immune cells
circulating through the blood may also enter the brain,
possibly via breached
blood vessels. Microglia can be seen around an electrode as soon as 24 hours after
The acute phase lasts for approximately 1–3 weeks after insertion,
after which the chronic phase begins.
One of the attributes of the chronic phase of inﬂammation is an astrocytic
sheath formed around the implant (Fig. 4.8).
Resting astrocytes form the blood
brain barrier, buffer excess potassium and neurotransmitters, and may supply
neurons with nutrients.
When astrocytes become reactive, they can secrete com-
pounds inhibitory to neural growth, such as chondroitin sulfate proteoglycans
6 weeks and then becomes thinner and denser.
Microglia are located on the
inside of this astroglial sheath;
activated microglia have been shown to release
inﬂammatory cytokines and reactive oxygen species (see Block and Hong for
), and this constant release of compounds from activated microglia can
It has been shown that a region devoid of neurons appears next to
an implanted electrode.
Thus, the neurons from which the probe is most likely
to record signals may be killed because of this chronic foreign body response. The
dense glial encapsulation may also interfere with electrical recording by forming
a high-impedance layer around the electrode,
so several strategies have been
employed to lessen the severity of the glial response. Groups have experimented
with modifying electrode cross section, geometry, and tip shape, but none of these
parameters appears to mitigate the chronic glial response.
have been developed to enhance neuronal attachment,
Figure 4.8. The response of rat cortical tissue 8 weeks after insertion. The green stain
marks glial ﬁbrillary acidic protein (GFAP), a marker for astrocytes, and the red NeuN stain
marks neuron nuclei. The scale bar is 100 micrometers. Notice the astrocytes apparent
at the electrode interface, and that the neurons appear further away. Image courtesy of
George McConnell, Bellamkonda Lab, Georgia Institute of Technology. For color reference,
see page 260.
SO13997_text.indd 89SO13997_text.indd 89 26/01/2011 3:50 PM26/01/2011 3:50 PM