Biomedical Engineering Reference
In-Depth Information
genomics, proteomics, pharmacogenomics, etc., have all come into existence due to the
capabilities provided by these microcomponents.
Tissue interaction is usually done using macroscale components such as electrodes or
hypodermic needles (henceforth referred to as macroelectrodes or macroneedles). Such
components offer immense utilitarian advantages in their being easy to fabricate and
handle. They are irreplaceable in appropriate problem identification and drug-delivery
applications, which warrant the use of large-sized interface tools. However with the
advent new technologies and discoveries, these conventional tools are being fast replaced
with their micromachined counterparts.
10.7.1
Benefits from Microelectrodes
Traditional noninvasive techniques used in ECG and EKG record the fluctuating bioelec-
trical and biophysical properties of the epidermis. Usually macroelectrodes are pasted
onto the skin in the vicinity of the organ. Unfortunately this method does not provide
cellular-level information or interaction, and thus endows limited knowledge of the
biosystem. They average the activity in many neurons and cannot establish precise activ-
ity in a particular region.
Furthermore, the skin/sensor interface is not intimate enough due to
the inherent noise component introduced by the high resistivity of
SC
.
Therefore, such methods
are extremely crude at best and only provide an unspecific diagnosis of the relevant organ.
Transdermal measurement techniques using microelectrodes have recently come to the
limelight due to the inherent benefits that accompany their small size. They are increas-
ingly being used for well-established as well as experimental electrode applications, such
as ECG, electroencephalogram, electro impedance tomography [15], and blood flow
measurement [16].
Some of the technological advantages obtained from microelectrodes are:
1.
High spatial resolution and precision
—The small size translates to closer probes
and thus higher spatial sensitivity. This provides the opportunity to monitor
localized effects, as opposed to the broad “system level only” macroelectrode
monitoring.
2.
High temporal resolution
—The small area of microelectrodes leads to lower diffu-
sion capacitance and thus a smaller RC time constant. This translates to a faster
response and higher refresh rate (in range of microseconds).
3.
Enhanced mass transport properties
—When performing electrochemical analysis,
the rates of mass transport to and from the electrode surface are much higher for
microelectrodes. Furthermore, time-independent/steady-state mass transport
rates are much easier to establish [14]. This is because of the different shapes of
diffusion layer formed around a microelectrode and a macroelectrode. The diffu-
sion layer is hemispherical for microelectrodes and planar for macroelectrodes. To
maintain the rate of reaction, the diffusion layer does not need to extend as far
into solution in the case of microelectrodes, due to the hemispherical shape. Thus,
the population of electroactive species affected by a microelectrode exceeds that
of conventional electrodes. This translates to better mass transport properties.
Figure 10.7 illustrates this phenomenon.
4.
Reduced iR effects
—Lower area of the electrodes means low current requirement.
This allows for measurement to be done in high-resistivity media. Highly resis-
tive media can cause severe 'iR' distortions in macroelectrodes, due to the high
current in its case.
5.
Increased deployment options
—Beyond surface-level skin testing, microelectrodes
can by deployed in small or hard to reach areas within the human body such as
