Biomaterials for MEMS

MEMS in the Nervous System 71

cords, peripheral nerves, invertebrate ganglia and other electrically active tissues.

Microwire arrays and glass pipette electrodes are still widely used for electrophys-

iological recording and stimulation. Groups have performed long term studies

in monkeys using microwire arrays, and they perform well several months after

implantation.

25

With the refinement of CMOS and MEMs processes and the development of

new techniques and equipment, silicon electrodes have become less expensive to

manufacture and more popular than before; while many groups do create their

own custom electrodes for stimulation and recording, the best known silicon

devices used in mammalian brains are the Utah Array and the Michigan Probe

electrodes.

4.2.1 The Utah Electrode Array

The original Utah array is composed of 100 pillars arranged in a 1 × 1mmsquare

base (see Fig. 4.3). It was first fabricated in the lab of Richard Normann at the

University of Utah in the late 1970s. The electrodes are electrically isolated from

one another, and signals can be individually measured from each one. Electrodes

are approximately 90 micrometers wide and 1000 to 1500 micrometers long.

26,27

These arrays have been used in primate studies and have been implanted into

humans.

28

Arrays with differing numbers of pillars can also be created; for a study

of the cat sciatic nerve, a Utah array of 25 electrodes (5 by 5 configuration) was

manufactured instead of the typical 100.

29

This particular procedure is described in Campbell et al.,

27

though alternative

methods have been explored since. The device is fabricated from a 1.7 mm thick n-

type <100> silicon wafer. Aluminum is evaporated onto the surface and patterned

into a 10 by 10 grid of 325 μm by 325 μm squares. The wafer is then heated on

Figure 4.3. The Utah Electrode Array.

26

Reprinted from Journal of Neuroscience Methods,

Rousche and Normann, 1998, Vol 82, Issue 1, pp 1–15, Copyright 1998, with permission

from Elsevier.

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