Stimuli-Responsive Lipid -Based Self-Assembled Systems - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 257

as in the case of peptide hormones such as insulin, would greatly improve
patient quality of life and compliance.
• Imaging guided therapy: There is much research being undertaken into
novel imaging contrast agents, and there is potential to use the same
carrier to carry active compounds so that imaging reveals the ideal loca-
tion for remote activation of the release mechanism.
• Chronically administered compounds with high risk of complications
from administration: For example, the necessity for frequent intravitreal
injection of steroids or anti-vascular endothelial growth factor (VEGF)
compounds for macular degeneration is associated with a high risk of
infection and retinal detachment.
• Symptom - responsive drug release: The ability to rapidly administer a
dose of drug on the development of symptoms without going through an
administration procedure, via external activation of a prior injected dose
form, for example, where a patient loses consciousness as a result of a
disease condition, making regular drug administration diffi cult.

It is proposed that self-assembled systems have an unrealized potential in
providing external control over drug delivery, through the manipulation of
self-assembled structure and hence optimization of drug release, disposition,
and bioavailability. The thermodynamic stability of self-assembled systems is
a distinct advantage over, for example, polymer-based systems, because in
theory the self-assembly mode may be reversibly changed from one structure
to another to modify drug release—in effect acting as an inherent on-off
“switch.” This chapter will provide a view of the landscape of stimuli-responsive
self-assembled systems with a focus on surfactants and lipids, particularly for
drug delivery applications.

9.3 STIMULI-RESPONSIVE SYSTEMS

For the purpose of this chapter, we propose that there are two main categories
of stimuli-responsive systems for drug delivery applications, namely endoge-
nous and external stimuli. The specifi c stimuli-responsive mechanisms in each
category are summarized in Table 9.1. This chapter, therefore, discusses the

TABLE 9.1 Categories of Stimuli - Responsive Systems in Self - Assembled
Lipid - Based Drug Delivery Vehicles

Endogenous Stimuli External Stimuli

• Ionic, osmotic, and pH changes
• Increased expression and activity of proteins

• Temperature
• Light
• Electromagnetic fi eld
• Ultrasound