Biomedical Engineering Reference
In-Depth Information
High-speed stirrer
Chitosan solution
Polyanionic solution
Chitosan particles
Figure 7.5
Schematic representation of the preparation of chitosan particulate systems by the ionic gelation method.
much attention because the process is very simple and mild [8,9]. In addition, reversible
physical cross-linking by electrostatic interaction, instead of chemical cross-linking, has
been applied to avoid possible toxicity of reagents and other undesirable effects. TPP is
a polyanion, which can interact with the cationic CS by electrostatic forces [10,11].
However, TPP/CS microparticles have poor mechanical strength, thus limiting their
usage in drug delivery.
7.2.1.6 Reverse Micellar Method
Reverse micelles are thermodynamically stable liquid mixtures of water, oil, and surfac-
tant. Macroscopically, they are homogeneous and isotropic, structured on a microscopic
scale into aqueous and oil microdomains separated by surfactant-rich films. One of the
most important aspects of reverse micelle-hosted systems is their dynamic behavior.
Nanoparticles prepared by conventional emulsion polymerization methods are not only
large (N200 nm), but also broad in size range. The preparation of ultrafine polymeric nano-
particles with a narrow size distribution can be achieved by using a reverse micellar
medium [12]. The aqueous core of the reverse micellar droplets can be used as a nanoreac-
tor to prepare such particles. Since the size of the reverse micellar droplets usually lies
between 1 and 10 nm [13] and these droplets are highly monodispersed, preparation of
drug-loaded nanoparticles in reverse micelles will produce extremely fine particles with a
narrow size distribution. Since micellar droplets are in Brownian motion, they undergo
continuous coalescence followed by reseparation on a timescale that varies between mil-
lisecond and microsecond [14]. The size, polydispersity, and thermodynamic stability of
these droplets are maintained in the system by a rapid dynamic equilibrium. In this
method, the surfactant is dissolved in an organic solvent to prepare reverse micelles. To
this, aqueous solutions of CS and drug are added with constant vortexing in order to avoid
any turbidity. The aqueous phase is regulated in such a way as to keep the entire mixture
in an optically transparent microemulsion phase. An additional amount of water may be
added to obtain nanoparticles of larger size. To this transparent solution, a cross-linking
agent is added with constant stirring, and cross-linking is achieved by stirring overnight.
The maximum amount of drug that can be dissolved in reverse micelles varies from drug
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