Recent Developments in Lyotropic Liquid Crystals as Drug Delivery Vehicles - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 219

Pn 3 m Im 3 m

Ia 3 d

a

a a

Figure 8.3 Schematic illustrations of the inverse bicontinuous cubic phases Ia3d
(gyroid type), Pn3m (diamond type), and Im3m (primitive type) (Seddon et al., 2000).

but the centers of the water channels never intersect. Hence they are con-
nected and the structure adopts a helical arrangement. The structure D is
characterized by a bilayer, which separates two interpenetrating aqueous
channel systems forming a diamond lattice. In this confi guration four aqueous
channels of the D surface meet at a tetrahedral angle of 109.5°. Structure P
contains two aqueous channel systems that are separated by a bilayer. The unit
cell possesses three mutually perpendicular aqueous channels, connected to
contiguous unit cells, taking shape of a cubic array.

8.1.3 Reverse Hexagonal Mesophase as Delivery Vehicles

The reverse hexagonal mesophase is of a primitive type (P6mm) and charac-
terized by one cylinder per unit cell corner. These densely packed, straight,
water - fi lled cylinders exhibit two-dimensional ordering. Each cylinder is sur-
rounded by a layer of surfactant molecules that are perpendicular to the cyl-
inder interface such that their hydrophobic moieties point outward from the
water rods (Fig. 8.4). There is a growing indication that inverse hexagonal
mesophases play structural and dynamic roles in biological systems (Gradziel-
ski, 2004; Seddon, 1990; Yang et al., 2003). These systems are assumed to be
active as transient intermediates in biological phenomena that require topo-
logical rearrangements of lipid bilayers, such as membrane fusion/fi ssion and
the transbilayer transport of lipids and polar solutes (Gradzielski, 2004;
Seddon, 1990; Yang et al., 2003). H II mesophases have recently been considered
promising drug delivery vehicles, mainly owing to their unique structural fea-
tures (Boyd et al., 2006, 2007; Lopes et al., 2007).
Another LC structure, based on the close packing of inverse micelles, is a
3D hexagonal inverse micellar phase, of the space group P63/mmc, recently
discovered by Shearman et al. (2006). Ternary lipid mixtures comprising two
lipids, dioleoylphosphatidylcholine (DOPC), dioleoylglycerol (DOG), and
cholesterol, of molar ratios 1 : 2 : 1 and 1 : 2 : 2, in the excess water - induced for-
mation of 3D hexagonal mesophases over a temperature range of 16-52°C