Biology Reference
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use of micelles as therapeutic nanocarriers due to the variety of molecules that can
be incorporated into the core. In fact, the scientific community has long been study-
ing the application of polymers as carrier of drugs (e.g., polymer-drug conjugates)
[ 40- 42 ] as well as forming micelles [ 43- 46 ] with the core as a separated
nanocontainer.
Micellization through self-assembly is often the result of a delicate balance
between intermolecular forces [ 3 ]. As stated previously, the polymeric micelles
have the hydrophobic interactions as the main driving force for the micellization
[ 47 ] when they are composed of amphiphilic block copolymers and assembled in
aqueous media. In this case, the core segregation from aqueous milieu directs the
formation of micelles and is induced by the hydrophobic interactions [ 2, 47, 48 ] and
proceeds through the combination of other intermolecular forces which include
metal complexation [ 49, 50 ] , organic/inorganic interactions [ 51- 54 ] , and hydrogen
bonding [ 30, 55 ] .
8.4.1
Formation of Polyion Complex Micelles
The variation of the intermolecular force of core-forming segments enables the
regulation of micelle formation and its stability. In fact, the concept of micellization
of core-shell structures by the formation of hydrophilic shell surrounding a segre-
gated hydrophobic core can be extended for the micellization using the electrostatic
interactions as the main driving force [ 3, 6, 28 ]. New types of block copolymer
micelles formed through electrostatic interaction between oppositely charged mol-
ecules in aqueous medium are named polyion complex (PIC) micelles. The oppo-
sitely charged molecules include, among others, synthetic polymers of various
architectures and biopolymers (proteins, enzymes, and oligonucleotides).
In this sense, the homopolymer as the polycation and negatively charged oligo-
nucleotides precipitate to form water-insoluble organic salts. However, in order to
provide dispersibility to the aggregates, additional segments to the homopolymer
are necessary for the assembly into micelles. Hence, the design of block copolymers
comprising a block of neutral hydrophilic polymer such as poly(ethylene glycol)
(PEG) and a block of polyion is often used to the micellization process to achieve
colloidal stability. The hydrophilic segments will form a shell surrounding the poly-
ionic core and provide the dispersibility through the stealth property.
The micellization in this case arises from the association of a pair of polyelectro-
lytes with opposite charge which induces the release of the counterions from each
polyelectrolyte as free ions to increase the translational entropy, while the electro-
static attraction of opposite charges occurs due to Coulomb force [ 28, 56- 58 ] . With
the increase in residual energy at the interface between PIC and an aqueous phase,
the size of the complexes should consequently increase to decrease the relative sur-
face area, allowing a reduction of the interfacial free energy [ 47 ] . This balance
between interfacial energy and conformational entropy of the polymer strands
determines the thermodynamically stable size of the PIC micelles [ 3 ] .
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