Detection and Description of Tissue Disease: Advances in the Use of Nanomedicine for Medical Imaging - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 253

The shell can be made of protein, galactose, lipid, or polymers
(see Ref. [27]). Following early studies with a range of water-based
solutions, encouraging results were obtained with microbubbles
consisting of air trapped in a denatured albumin shell made by
sonication of an albumin solution. This was an important development
because it provided the first useful US contrast agent, but the method
of manufacture produced a wide range of particle sizes. This is a
significant issue in UCA development because diff erent sized bubbles
have diff erent echogenicities. Also, as noted previously, larger bubbles
are trapped in small blood vessels, especially in the lungs, hindering
transition from the venous injection to arterial distribution, and
reducing the circulation time of a significant fraction of the particles.
On the other hand, smaller microbubbles also have poor persistence,
because they can be damaged by ultrasound radiation.
Lipid-based shells have a number of desirable properties. They are
smaller (<5 μm), it is easier to control their size, they are mechanically
much more robust than microbubbles, and the composition of the
shell can be altered to optimize their properties. Alkan-Onyuksel
and colleagues [28] systematically investigated the echogenicity
of liposomes produced from combinations of phosphatidylcholine
(PC), phosphatidylglycerol (PG), phosphatidylethanolamine (PE)
and cholesterol, selected because they are the main constituent of
cell membranes (PC), confer a negative charge (PG), provide options
for conjugation (PE), and increase rigidity (cholesterol). They found
that high concentrations of PE (e.g., 4%), low concentrations of PG
(1%), and constant cholesterol (30%) gave highest echogenicity.
These findings were confirmed by in vitro and in vivo studies.
It was also found that echogenicity was related to the structure
of the liposomes. More echogenic liposomes tended to have
multilamellar structures, while the less echogenic liposomes had
thick, unseparated, unilamellar structures. It was suggested that
the formulation of the liposomes caused these diff erences, with,
for example, the negative charge of PG preventing aggregation and
the small head of PE aff ecting the formation of the multiwalled
structures. In later work by the same group [29], it was found that
entrapment of air in the liposomes during the lyophilization step
in their manufacture was a requirement for their echogenicity. The
inclusion of mannitol in the formulation as a cryoprotectant resulted
in the exposure of hydrophobic regions to air during lyophilization
and then its subsequent retention in the rehydrated form.