Biomedical Engineering Reference
In-Depth Information
restriction of zero DC currents. Pure noble metals are too soft for practical use, and thus
alloys with other noble metals are often used to improve mechanical properties. Iridium
alloys are quite common, as they also provide catalytic properties in some cases. Other
metals are also used for various reasons. Stainless steel is used due to its high strength,
noncorrosive nature, and low price. They are however unsuitable for low-noise, small-sig-
nal measurements. Tin and lead alloys are used for low-noise properties. Also they have
low melting points, which translates to fabrication advantages in that they can be easily
formed or molded. Nickel is used for its flexibility. However, it may give allergic skin reac-
tions. In applications such as electrotherapy or skin iontophoresis, the pharmaceutical or
bactericidal properties of the metal are of prime importance; thus, iron, aluminum, or zinc
might be considered [13].
Nonmetallic materials have other advantages. Glass microelectrodes are best suited for
recording DC or slowly varying DC potentials. Silicon dioxide microneedles are popular
due to their ease of fabrication and good biocompatibility characteristics. However,
strength and reliability may not be sufficiently high for certain applications. Composite
material microneedles, such as those made of oxide-nitride-oxide stack might be used
instead. Another nonmetallic material being investigated for microelectrodes is carbon
because it is X-ray translucent but is still an electronic conductor. Combinations of carbon
and rubberlike materials may be used because they are soft and thus highly adaptable to
anatomical variations. Polymers such as polyaniline, polypyrrole, polythiophenes, and
their derivatives [28] are becoming highly popular because of their multiple advantages.
Since they are biodegradable, microneedles can be used without danger of infection due
to needle tips breaking off. Conductive polymers are often used for microelectrodes due
to their selective interaction properties with proteins, enzymes, and antibodies, thus mak-
ing them ideal for certain biochemical sensing applications. Furthermore, polymers are
highly flexible too.
10.8.2.2 Geometrical Considerations
Geometry of the microprobe affects most of the electrical, mechanical, or other character-
istics highlighted earlier. Common geometrical considerations are probe length, probe
width, area/perimeter of cross-section, shape, thickness of sidewalls (for microneedles),
tip geometry, and taper angle.
Each geometrical variable can affect a number of parameters, and thus the optimum
choice of geometry is often a trade-off between efficiencies. For example, sharper needle
tips can be expected to require less force for insertion, but the reduced penetration force
comes at the expense of reduced strength near the tip [29].
Probe length determination is particularly critical as the probe must be sufficiently deep
to allow reliable test results, and for drug delivery or sample extraction, they have to reach
the blood capillaries. The possibility of probe fracture and buckling increase as probe
length increases. Also, to be painless, the probes cannot be so long as to touch the nerve
endings. Increasing cross-sectional area increases strength but may compromise certain
electrical characteristics [30]. For microneedles, constriction of needle lumen size is limited
by the size of species being transferred. Finally, biofouling properties always impose crit-
ical constraints, it being less intensive for less invasive geometries.
10.8.2.3 Array Layout
Microprobes are most often used in an array formation. Thus, device functionality
depends not only on their individual, but also on their “team” efficiency. Array size, den-
sity, and configuration are most important considerations.
