Drug Delivery Systems That Eradicate and/or Prevent Biofilm Formation - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 398

2 Vesicular Systems
Vesicular systems are highly ordered assemblies of one or several concentric
bilayers that are formed when certain amphiphilic building blocks are dispersed
in water. A wide variety of lipids and surfactants can be used to prepare vesicular
carriers (Crommelin and Schreier 1994 ; Mullertz et al. 2010 ). The composition of
the vesicles influences their physicochemical characteristics such as size, charge,
thermodynamic phase, lamellarity, and bilayer elasticity. These physicochemical
characteristics have a significant effect on the behavior of the vesicles and hence on
their effectiveness as a drug delivery system. Vesicular systems have been able to
address the problems of drug insolubility, instability, and rapid degradation. These
systems delay elimination of rapidly metabolizable drugs and function as sustained
release systems. Encapsulation of a drug in vesicular structures can be predicted to
prolong the existence of the drug in systemic circulation, and reduce the toxicity if
selective uptake can be achieved (Todd et al. 1982 ). Vesicular systems can incor-
porate both hydrophilic and lipophilic drugs. Hydrophilic drugs can be entrapped
into the internal aqueous compartment, whereas amphiphilic, lipophilic, and
charged hydrophilic drugs can be associated with the vesicle bilayer by hydropho-
bic and/or electrostatic interactions (Martin and Lloyd 1992 ).
The applications of vesicles in drug delivery are based on physicochemical and
colloidal characterization such as composition, size, and loading efficiency and the
stability of the carrier, as well as their biological interactions with the cells. A major
interaction is lipid exchange whereby liposomal lipids are exchanged with the lipids
of various cell membranes. This depends on the mechanical stability of the bilayer
and can be reduced by the addition of cholesterol (which gives rise to greatly
improve mechanical properties, such as increased stretching elastic modulus,
resulting in stronger membranes and reduced permeability). The second major
interaction is adsorption into cells, which occurs when the attractive forces (elec-
trostatic, electrodynamic, van der walls, hydrophobic interaction, hydrogen bond-
ing, etc.) exceed repulsive forces (electrostatic, steric, hydration, undulation,
protrusion, etc.) (Lasic 1993 ; Lipowsky 1995 ; Israelachvili 1991 ).
2.1 Liposome-Based Drug Delivery Systems
Liposomes are globular lipid vesicles with a bilayer membrane consisting of
amphiphilic lipid molecules (Zhang and Granick 2006 ). The liposome or lipid
vesicles are self-forming, enclosed lipid bilayers upon hydration; liposomal drug
delivery systems have played a considerable role in the transport of potent drugs to
improve therapeutics. Liposomal drug delivery system can be made of either
natural or synthetic lipids. One of the most frequently used lipid moieties in
liposome preparations is phosphotidylcholine, which is an electrically neutral
phospholipid that contains fatty acyl chains of varying degrees of saturation and