MEMS in the Nervous System 73
has undergone design iterations over time, shifting from single shank to multi-
shank, and utilizing different electrode configurations.
3538
It has since been
commercialized and adapted by other investigators.
The details of the basic fabrication process presented here can be found in
Kipke et al.,
39
but certain applications may merit a deviation from this procedure.
The process starts with a boron diffusion through silicon to create a p
++
etch stop.
Silicon dioxide and silicon nitride are then deposited as insulation layers, and con-
ductive polysilicon is patterned on top. The insulation layers are again deposited
on top of the polysilicon and etching reveals the interconnect material in specific
places. Irridium electrodes and gold bond pads are patterned and, following a
dry etch, the structures are released from the bulk silicon. A flexible silicon cable
attached to the electrode carries signals to a commercial microconnector.
40
This
connector transfers the neuronal signals to the outside world.
Performance of the Michigan electrode is encouraging. Studies of implants
into rat barrel cortex have shown recordings for 28 weeks and one rodent had
recordings for more than 1 year.
39
Three-dimensional arrangements of the classic
two-dimensional probe have also been fabricated,
41
presumably for use in appli-
cations similar to the Utah Electrode Array.
4.2.3 Custom Electrodes and Combination Devices
While the Utah and Michigan arrays are probably some of the best known micro-
electrodes, they are not the only ones. Many labs have fabricated custom cortical
electrodes. Polyamide and parylene are both used as substrate materials, and both
are more flexible than silicon, which makes them desirable for some applications.
One method to create polyamide electrodes uses a three layer process; first,
polyamide is deposited on a pretreated metal surface, titanium and platinum are
deposited and patterned using reactive ion etching, these steps are repeated and
finally a polyamide layer covers the top.
42
Some interesting new devices have emerged which combine microelectrode
technology with other types of sensors. One such device is a “state” detector that
has been tested on rats. The detector is essentially a microelectrode array combined
with accelerometers. The microelectrode records the electroencephalogram (EEG),
while the accelerometers record movements of the rodent’s head. With this
information, one can determine the state of wakefulness or sleep an animal is in
Ref. 43.
Another important modification of the cortical probe is the inclusion of mi-
crofluidic channels. These channels are capable of delivering drugs to tissue
next to the electrode and others can sample the local electrode environment for
neurotransmitters and other factors. Microelectrodes with channels are typically
fabricated similar to standard polymer electrodes, but a few extra steps are
involved. The channel can be formed by several methods, two of which are
discussed here. One method entails depositing parylene, depositing a placeholder
sacrificial photoresist layer where the channel will be, and then depositing more
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