Chemistry Reference
In-Depth Information
was to synthesize artificial red blood cells for subcutaneous implantation where the
semipermeable membrane would deliver hemoglobin to the patient at a controlled
rate. To generate the capsules, Chang combined hemoglobin with an alkaline
aqueous dispersed phase of 1,6-hexamethylenediamine. Outside the emulsion
droplets, the continuous phase was composed of cyclohexane and chloroform, with
sebacoyl chloride acting as the second condensation monomer. The capsule size in
this system was dependent on the stir rate and surfactant concentration. Chang
found that increasing the stir rate and surfactant concentration caused the average
microcapsule diameter to vary from about 80 to 20 mm.
It is also possible to generate microcapsules through interfacial polymerization
using only one monomer to form the shell. In this class of encapsulations, polymer-
ization must be performed with a surface-active catalyst, a temperature increase, or
some other surface chemistry. Herbert Scher of Zeneca Ag Products (formerly
Stauffer Chemical Company) developed an excellent example of the latter class of
shell formation (Scher 1981; Scher et al. 1998). He used monomers featuring isocya-
nate groups, like poly(methylene)-poly(phenylisocyanate) (PMPPI), where the
isocyanate reacts with water to reveal a free primary amine. Dissolved in the oil-
dispersed phase of an oil-in-water emulsion, this monomer contacts water only at
the phase boundary. The primary amine can then react with isocyanates to form a
polyurea shell. Scher used this technique to encapsulate pesticides, which in their
free state would be too volatile or toxic, and to control the rate of pesticide release.
Because the size of the emulsion droplets dictates the diameter of the resulting
capsules, it is possible to use miniemulsions to make nanocapsules. To cite a
recent example, Carlos Co and his group developed relatively monodisperse
200-nm capsules by interfacial free-radical polymerization (Scott et al. 2005).
Dibutyl maleate in hexadecane was dispersed in a miniemulsion of poly(ethylene
glycol)-1000 (PEG-1000) divinyl ether in an aqueous phase. They generated
the miniemulsion by sonication and used an interfacially active initiator, 2,2
0
-
azobis(N-octyl-2-methyl-propionamidine) dihydrochloride, to initiate the reaction,
coupled with UV irradiation.
A new variation of interfacial polymerization was developed by Russell and
Emrick in which functionalized nanoparticles or premade oligomers self-assemble
at the interface of droplets, stabilizing them against coalescence. The functional
groups are then crosslinked, forming permanent capsule shells around the droplets
to make water-in-oil (Lin et al. 2003; Skaff et al. 2005) and oil-in-water
(Breitenkamp and Emrick 2003; Glogowski et al. 2007) microcapsules with elastic
membranes.
8.2.3. Colloidosomes
Colloidosomes are a recent class of microcapsules and thus far have only been
applied to catalysis and drug delivery in a few cases. The term colloidosomes was
coined by Anthony Dinsmore and colleagues in 2002 to refer to capsules where
the shells are composed of close-packed layers of monodisperse colloidal particles
(usually micron-sized polymer beads) that have been linked together by sintering,
