Biomaterials for MEMS

MEMS in the Nervous System 73

has undergone design iterations over time, shifting from single shank to multi-

shank, and utilizing different electrode configurations.

35−38

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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