Biomedical Engineering Reference
In-Depth Information
FIGURE 10.7
Hemispherical diffusion in microelectrodes as opposed to
planar transport in macroelectrodes [14]. (From Imisides,
M. D., John, R., and Wallace, G. G., Microsensors based on
conducting polymers . Chemtech , 1996, 26 (5): 19-25.)
Microelectrode
Macroelectrode
in pulmonary ventilation and gastric emptying. Furthermore, they are mini-
mally intrusive and thus can be deployed to make measurement in sensitive
areas that cannot afford deep probing (e.g., brain).
Although the benefits of microelectrodes do remove the disadvantages of macroelec-
trodes, one has to bear in mind that microelectrodes can tend to be too focused. For exam-
ple, in the case of recording electrical activity of the brain, while taking single neuron
measurements using microelectrodes, neuronal interactions may be neglected. One may
not be able to differentiate between different neurological conditions or behavioral states.
In such cases, microelectrode measurements supplement macroelectrode analysis.
Optimum diagnosis is obtained by resolving the “system level” data along with “compo-
nent level” data.
10.7.2
Benefits from Microneedles
In typical clinical and biomedical applications, drug injection or biofluid sampling is done
using hypodermic needles. Though utilitarian, this method causes undesirable pain and
excessive tissue trauma. This is particularly troublesome when frequent administration of
drugs is necessary (such as administration of insulin). Furthermore, control of drug deliv-
ery is highly approximate using macroneedles.
A painless alternative to macroneedles, which has lately started to become popular, is
the transdermal patch. It is simply a piece of plastic with an adhesive on it with the drug
being suspended in the adhesive. The patch is stuck on the skin and the drug passively
diffuses slowly to lower layers. However, this method of drug delivery poses certain chal-
lenges. The effectiveness of the adhesive is directly affected by the concentration of the
drug within the adhesive. Thus, if a larger dose of the drug is required, either it has to be
reapplied more frequently or a larger patch has to be used. Increasing the concentration of
the drug does not help if the adhesive bond with the skin is weak [17]. Furthermore, it is
a slow method and not very precise, and is thus not suitable to many applications. The
barrier properties of SC pose unique obstacles to drug application by providing a very low
topical bioavailability [18]. The “route” taken by the externally applied drug is tortuous—
around the dead, dense corneocytes. The permeant diffuses in the intercellular channels,
which contain structured lipid bilayers. Thus, the problem is compounded, as molecules
have to cross, sequentially, lipophilic and hydrophilic domains.
Microneedles are increasingly being sought out as an alternative to the macroneedle and
the transdermal patch as it combines their advantages while eliminating the disadvantages
of both. In recent times, numerous types of microneedles have been fabricated and used for
transdermal drug delivery [19-23], vaccine delivery [24], fluid analysis and sampling [25],
dialysis [26], and cellular DNA delivery [27], among other applications. Figure 10.8 shows
an array silicon dioxide microneedles fabricated at the University of South Florida.
Vital technological advantages obtained by using microneedles are:
Search WWH ::




Custom Search