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Microcapsules have attracted attention as an improved drug delivery method. Coating thickness and uniformity of microspheres
directly influences the rate of dissolution in the body, making it very important to study its structural properties. Previous
studies show methods to relate the relative dielectric permittivity and thickness of the capsule wall (17). Additional information
provided by dielectric measurements includes volume fraction of microspheres in solution and the dielectric properties of
the interior core of microcapsules (14).
Dielectric spectroscopy has shown promise for the measurement of structural and interfacial properties of gels. Gels are complex
systems with a number of dielectric relaxation processes that can be related to unique physical characteristics. For example,
previous studies showed the response of aqueous gels containing cetosteryl alchohol and cetrimide show four distinct bulk
processes (18), These responses were attributed to the relaxation of water, ions in bilayers formed resulting from the presence
of cetosteryl alchohol, and migration of ions in channels between these bilayers. Therefore, dielectric spectroscopy can be
used to monitor multiple physical properties of complex structures. Dielectric spectroscopy was used to study the likelihood
of high surface contact between nasal mucosa and pharmaceutical gels by measuring conductivity as pharmaceutical drug ions
transported from the gel into the nasal mucous membrane (19).
Emulsions and microemulsions have attracted interest as an effective drug delivery method for drugs that are insoluble in
water. Dielectric spectroscopy has been used to classify the types of emulsions. For example, dielectric properties are used
to distinguish between water-in-oil and oil-in-water emulsions (14).
The remainder of this article discusses key issues in dielectric sensor selection. Measurements of tablet coating thickness
are used to illustrate methods of dielectric data processing.
Dielectric-sensing selection guide
There are two major properties of dielectric spectroscopy that are typically varied to suit the desired application: the spatial
distribution of the interrogation field and the interrogation frequency range. Changes to the field's spatial distribution
of the field used in dielectric spectroscopy enables measurements in the following modes: bulk, surface/low-penetration, multiple
penetration depth, imaging, and a combination of these. The frequency range can be optimized for a particular material of
interest and can vary from 10–6 to 1011 Hz, although a frequency range of only a few orders of magnitude is typically used.
Based on the measurement constraints for the application, the spatial distribution of the field and the frequency range are
ascertained by answering a series of questions, which are discussed in the following sections.
Figure 1 (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
Through-field or fringing-field? In dielectric spectroscopy, the attenuation and phase shift of an electric field at specific frequencies are measured as
it passes through a material of interest. Attempts are typically made to either linearize the field or to limit the penetration
of the field to a known depth into the material of interest. In general, through-field implies a "pitch and catch" configuration
in which one electrode delivers an excitation signal and another electrode measures the resulting signal after passing in
an approximately linear fashion through the material of interest such as in a parallel plate capacitor configuration (see
Figure 1a). In the case of fringing-field sensing, one electrode delivers an excitation signal, and a second electrode is
placed such that the original signal passes through the material of interest in a nonlinear fashion. The most common fringing
field configuration is one in which the excitation and sensing electrodes are mounted in a coplanar fashion on the same substrate
(see Figure 1b).
A. Mathur is a graduate student at the Sensors, Energy, and Automation Laboratory, Department of Electrical Engineering, University of Washington, Seattle.
Articles by A. Mathur
K. Sundara-Rajan is a PhD candidate at the Sensors, Energy, and Automation Laboratory, Department of Electrical Engineering, University of Washington, Seattle, tel. 206.351.8101.
Articles by K. Sundara-Rajan
G. Rowe
G. Rowe is a PhD candidate at the Sensors, Energy, and Automation Laboratory, Department of Electrical Engineering, University of Washington, Seattle
Articles by G. Rowe
A. V. Mamishev
A. V. Mamishev is an associate professor at the Sensors, Energy, and Automation Laboratory, Department of Electrical Engineering, University of Washington, Seattle.
Articles by A. V. Mamishev
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