ABSTRACT
According to the release behavior, controlled release systems (CRS) can be
classified into passively preprogrammed, actively preprogrammed, and ac-
tively self-programmed systems [1]. Although in the first category (passively
preprogrammed system) the release rate is predetermined and it is irrespon-
sive to external biological stimuli, in the second category (the actively pre-
programmed system), the release rate can be controlled by a source external
to the body as in the case of insulin and urea deliveries [1,2]. It is clear that the
last category represents the new generation of delivery systems (regardless of
the administration route) as self-programmed delivery systems will be able to
autonomously regulate release kinetics in response to external stimuli such as
the concentration of a fixed analyte [2,3]. While 15 years ago the majority of
CRS fell into the first category, at present the importance of the last two
categories has considerably increased [2]. Indeed, only the last two categories
can fully accomplish the modern concept of therapeutic treatment the aim of
which is to increase drug effectiveness and patient compliance, two variables
strictly related to the administration frequency, and side effects connected to
dosage. In this framework, both the ‘‘designing step’’ and the use of new
materials are becoming more and more significant for the realization of CRS.
Consequently, CRS requires knowledge of engineering, chemistry, pharmacy,
and medicine merging in an attempt to realize more and more effective and
reliable delivery systems [4]. Accordingly, mathematical models, a typical
engineering tool, turn out to be very useful in predicting CRS behavior or in
measuring some important related parameters, such as the drug diffusion
coefficient. In addition, as the mathematical model is nothing more than a
‘‘mathematical metaphor of some aspects of reality’’ [5] (that, in this case,
identifies with drug release), this approach requires a clear knowledge of the
ensemble of phenomena ruling release kinetics.
