Stimuli-Responsive Lipid -Based Self-Assembled Systems - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 255

such as the inverse hexagonal (H 2 ) and inverse bicontinuous cubic (v 2 , also
often denoted as Q 2 ) phases analogous to the lamellar phase described above.
The lipids that typically form these nonlamellar phase structures have been
recently reviewed (Kaasgaard and Drummond, 2006). Thus lipid and surfac-
tant self-assembly not only provides access to a range of thermodynamically
stable structures but also provides systems with very different classes of behav-
ior that can be translated into different applications.
The unique nanostructures formed by these lipids dictate their use, whether
it be to control the rate of release of actives from within (Lee et al., 2009a),
matrix-guided synthesis (Hegmann et al., 2007), or more classical surfactant
applications such as wetting and detergency applications. In addition to their
thermodynamically stable long-range order, they also possess excellent bio-
compatibility due to their composition—lipid and water.
The self-assembled structures formed by these lipids are determined by
the specifi c local packing of the amphiphiles in the matrix. In addition to
molecular geometry, their packing is dictated by the presence of additives,
solution conditions, for example, pH, and environmental conditions such as
temperature and pressure. The addition of known amounts of specifi c additives
can result in quantifi able modifi cations in phase behavior—for example, cho-
lesterol in liposomes affects bilayer fl uidity (Bisby et al., 1999b), oleic acid
(OA) in glycerol monooleate (GMO) (Aota-Nakano et al., 1999; Borne et al.,
2001) and vitamin E acetate in phytantriol (Dong et al., 2006) cause the reduc-
tion of the v 2 to H 2 to L 2 phase transition temperatures, allowing for the ability
to fi ne tune the desired properties of lipid-based systems. These parameters,
and hence lipid packing, can be manipulated providing specifi c stimuli. For
example, in the case of liquid crystalline structures, an increase in temperature
will tend to induce a higher degree of chain splay, therefore increasing the
spontaneous mean curvature of the monolayer and forcing the structure to
switch to a more energetically favorable phase with potential to thereby
manipulate drug release.

9.1.2 Transitions between Self-Assembled Structures

Transitions between different self-assembled geometries for lipids in aqueous
environments are ubiquitous in nature and have received a huge amount of
attention from biophysical chemistry researchers (Brink-van der Laan et al.,
2004; Conn et al., 2006). The formation and structure of cellular membranes,
membrane fusion, and some cellular uptake mechanisms all involve manipula-
tion of lipid self-assembly at the molecular and mesolength scales. The rich
diversity of structures that lipids may adopt lends itself as the basis of complex
biological functions. There is potential to utilize the reversible phase transi-
tions in lipid systems to impart responsiveness into materials. With the advent
of advanced synchrotron and neutron scattering facilities, we are only now
coming to grips with the mechanisms by which transformations between lipid
self-assembled structures take place (Kriechbaum and Laggner, 1996; Rappolt