Biomedical Engineering Reference
In-Depth Information
The electrochemical behavior of microelectrode arrays varies strongly with center-to-
center spacing; the larger the spacing the more sigmoidal are the recorded results, which
translates to better sensitivity. Also, the temporal response improves with increased cen-
ter-to-center spacing, with current-time transients reaching a steady-state response more
quickly using loosely packed arrays [30]. However, decreased density compromises the
spatial resolution and also results in higher area requirement.
10.9
Evaluation of Microstructure Interaction with Skin
It is evident that the design of the bioengineered interfaces involves optimization of var-
ied performance parameters. This process is not an easy task. The physiological and struc-
tural complexity of the skin hinders accurate biophysical or biochemical modeling. Thus,
simulation alone cannot arrive at the finalized design. Testing the interface mechanism has
to be included within almost each design iteration. This increases the criticality of testing
procedure, and care must be given to precise calibration of test equipment.
10.9.1
Microneedle Testing
Testing methods are being evaluated at various research facilities. For example, at the
University of South Florida, DRIE-based SiO
2
microneedle arrays have been employed for
experimental testing to examine the significance of geometry on transdermal testing.
Reliability testing based on measurement of buckling force and skin penetration force is per-
formed before confirming use of the needles for applications like drug delivery and fluid
extraction. Measurements are carried out via testing module utilizing an accurate load trans-
ducer and real-time viewing capability. Preliminary characterization is established on skin-
like polymer before real-time experimentation on excised split-skin samples. Mechanical
tests are also performed on the polymer to correlate its properties with skin. Even though
the polymer does not behave like skin since its properties are isotropic and elastic unlike
skin, this material can be used for preliminary characterization of microneedles before real-
time testing on cadaver skin. An important part of skin-interface testing is fracture and pen-
etration testing. It is necessary to validate the ability of the microneedles to penetrate skin
without fracturing. This is performed by quantifying the penetration force in real-time
microneedle insertion tests done on split-thickness skin and isolated SC (Figure 10.9).
90
µ
m deep marks
FIGURE 10.9
Optical microscope image (20
magnification)
illustrating split thickness. Skin section with 90-
m-deep, 20-
m-wide needle marks into epi-
dermis.
