Biomedical Engineering Reference
In-Depth Information
Hydrophilic
headgroup
Phospholipid bilayer
Hydrophobic
alkyl chain
MLV
LUV
SUV
OLV
FIGURE 1.9
Types of liposomes based on microscopic morphology. Liposomes bilayers (lamella) are made of phospholipid
molecules each having a cylindrical geometry. MLV
5
multilamellar liposome/vesicle; LUV
5
large unilamellar
liposome/vesicle; SUV
small unilamellar liposome/vesicle; OLV
oligolamellar liposome/vesicle.
5
5
Liposomes are promising drug delivery carriers owing to their safety, biocompatibility, and bio-
degradability. However, liposomes are unstable in aqueous dispersions and most of the methods
used to prepare liposomes are unsuitable for large-scale production. This review has come across a
range of technologies which may be applied to scale up the production of stable liposomes. These
include freeze drying (lyophilization) to produce powdered liposome formulations or proliposome
technologies to produce liposome precursor formulations. Various types of liposomes have been
manufactured with biological functionality. These are summarized below.
The biological functionality of liposomes is determined by liposome size and bilayer composi-
tion. Accordingly, liposomes are classified into conventional liposomes, cationic liposomes, ther-
mosensitive liposomes, pH-sensitive liposomes, long-circulating (sterically stabilized) liposomes,
and ultradeformable liposomes. Some liposome formulations may however fall under more than
one category. For instance, inclusion of certain copolymers within pH-sensitive liposomes may
enhance their escaping tendency from the blood phagocytes and hence such liposomes can be
classified as both pH sensitive and long circulating.
Conventional liposomes are multilamellar vesicles (MLVs) made of lipids having neutral or
negative charge. These liposomes are large, and because of their surface characteristics they are
readily cleared from blood circulation by reticuloendothelial system (RES) cells and hence they
have short biological half-life. Conventional liposomes are most commonly used in research to
investigate the entrapment of compounds and their release profiles. They are commonly studied as
model biological membranes.
Delivery of gene to diseased cells may repair the cause of the disease. This approach of delivery
is commonly called gene therapy. Because DNA molecules are very large, their ability to penetrate
the target cell and be expressed may be poor. This necessitates the presence of safe carriers, such
