Dielectric Spectroscopy: Choosing the Right Approach - Pharmaceutical Technology

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Dielectric Spectroscopy: Choosing the Right Approach
This tutorial paper is meant to aid in dielectric-sensor selection


Pharmaceutical Technology
Volume 9, Issue 32, pp. 8293

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).


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