Recent Developments in Lyotropic Liquid Crystals as Drug Delivery Vehicles - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 218

Phyt

HO

OH OH

GMO

O

O

HO

OH

Figure 8.2 Chemical structures for phytantriol (Phyt) and glyceryl monooleate
(GMO) (Rizwan et al., 2009 ).

8.1.1 Monoolein and Phytantriol: Main Building Blocks of
Lipid Mesophases

Only a few synthetic amphiphiles can mimic the behavior of biological lipids
and form inverted mesophases. The unsaturated monoglycerides (monoolein
and monolinolein) belong to this category. Glycerol monooleate (GMO,
monoolein) is a polar lipid commonly used as a food emulsifi er. Presently,
GMO is the preferred amphiphile for formulating LC phases for scientifi c
research and drug delivery. This is a nontoxic, biocompatible, and biodegrad-
able lipid, which possesses low water solubility but swells and forms several
LC phases in excess water (Drummond and Fong, 2000).
Another lipid, phytantriol, was recently shown to form cubic LC. The
advantage of this lipid, compared to GMO, is that it is not susceptible to
esterase-catalyzed hydrolysis and therefore provides additional stability to the
mesophases. Phytantriol is commonly used as an ingredient in the cosmetics
industry for improving moisture retention. Its phase behavior as a function of
water concentration and temperature is very similar to that of GMO, although
structurally they are very different (Fig. 8.2) (Rizwan et al., 2009).

8.1.2 Cubic Phases

The bicontinuous cubic phase is the most complicated among all the LLCs.
Here the lipids are located in a complex, optically isotropic three-dimensional
(3D) lattice. Cubic structures can be either normal (type I, O/W) or inverse
(type II, W/O), and their topology is differentiated as bicontinuous or micellar,
resulting in seven cubic space groups. Only three space groups are both inverse
and bicontinuous structures (Fig. 8.3): the gyroid (G) type (Ia3d, denoted Q 230 ),
the diamond (D) type (primitive lattice Pn3m, denoted Q 229 ), and the primitive
(P) type (body-centered lattice Im3m, denoted Q 224 ) (Gruner et al., 1988). The
aqueous channels of G surface consist of two separate, left-handed and right-
handed helical channels. The aqueous channels can extend through the matrix,