Chemistry Reference
In-Depth Information
Along with phosphatidylcholines, cholesterol is often included in the vesicle
bilayers in as high as a 1:1 ratio because it reduces permeability of the bilayer to
aqueous solutes.
The study of liposomes as drug-delivery agents has been ongoing for decades.
The lipids for liposome formation are typically harvested by extraction from egg
yolks and soybeans, and a number of recipes exist for generating liposomes of
various diameters. Because the shell material in liposomes is not polymeric,
we will not discuss them in depth, limiting ourselves only to those aspects that are
pertinent to synthetic analogues like the copolymer vesicles of the next section.
Copolymer Vesicles.
Although lipids can self-assemble into useful thin-walled
vesicles, their applications can be limited. Phospholipids form membranes in
aqueous solutions, but they often fail to do so in organic solvents that synthetic che-
mists find convenient (Discher and Eisenberg 2002). If the vesicles do happen to
form in these decidedly “unbiological” environments, they lack mechanical strength,
or even basic stability (Antonietti and Forster 2003). To overcome this issue,
researchers have synthesized various amphiphiles and have even encased classical
vesicles in polymer shells (Regen et al. 1984; Fukuda et al. 1986). More recently
researchers have turned to diblock copolymers to form tougher and more stable
vesicles that can be more easily tuned for specific applications (Antonietti and
Forster 2003).
Adi Eisenberg and coworkers have had great success synthesizing and characte-
rizing simple block copolymer nanocapsule vesicles (Zhang and Eisenberg 1995;
Zhang et al. 1996; Discher and Eisenberg 2002; Soo and Eisenberg 2004). For
instance, Eisenberg and Zheng showed that diblocks of PS and poly(acrylic acid)
(PAA) synthesized by anionic polymerization can form various morphologies,
including spheres, rods, lamellae, and vesicles, depending on how much electrolyte
(NaCl, HCl, or CaCl
2
) is added (Zhang and Eisenberg 1995; Zhang et al. 1996).
The general idea is that the PS block acts as a hydrophobic portion of the linear
chain, analogous to the two greasy chain ends on a phosphatidylcholine in a lipo-
some, and the PAA block acts as the hydrophilic ionic end (like the positively
charged choline nitrogen). Thus, the relative lengths of the blocks and the overall
molecular weight can be altered to tune the characteristics of the vesicle, such as
size and overall hydrophilicity.
8.3. CATALYST ENCAPSULATION
8.3.1. General
Microcapsule properties make them attractive materials for a wide variety of practical
applications. In the area of catalysts, microcapsules provide semipermeable
membranes that are readily produced and dispersed. These properties, along with
others, have inspired systems that include synthetic or man-made encapsulated
catalysts, such as organocatalysts, metal particles, enzymes, and organometallic
