Biomedical Engineering Reference
In-Depth Information
3.1.2
Polymer Characterization of PEO-PCL
As PEO has been well characterized [28-30] as a polymer and in
biological systems, the development of new PCL cores often warrants
the copolymers characterization. PCL is a crystalline polymer, known
to act as a rubber at room temperature since its glass transition
temperature ( T g ) is -60°C and its melting point ( T m ) is 58.5°C when
it is 21 kDa [31]. Thus its crystallinity is less aff ected through the
addition of solutes into the PCL core of micelles compared with
polylactic acid. Both the T g and T m are usually measured with DSC
(diff erential scanning calorimetry). X-ray diff raction is also used to
decipher the amorphorous or crystalline state of copolymers with or
without drugs [32-34]. A variety of PEO-PCL copolymers have been
developed for drug delivery: from conventional diblocks and triblocks
to more exotic micelles with multi-arm stars, and substituted PCL
blocks. Therefore, the following section will review the physical
characterization of PEO-PCL micelles in aqueous solution.
3.1.3
Characterization of PEO-PCL Micelles
In the context of micelle-based drug delivery, encapsulation efficiency
and release are irrevocably related to the equilibrium between
unimers and micelles in aqueous solution. This equilibrium can be
inferred indirectly from CMC measurements. The aggregation number
( N agg ) determined from small angle neutron scattering (SANS) or
fluorescent dye quenching techniques [35] provides a number average
of unimers required to form a micelle. Self-diff usion coefficients of
free PEO in solution obtained through NMR provide a relative scale to
assess the presence of unimers in solution. This knowledge combined
with the conformation and shape of the micelles provides a physical
model of amphiphilic copolymer association on a thermodynamic and
molecular basis. Understanding the discrete association of PEO-PCL
copolymers may allow future block copolymers to be designed that
do not require harsh conditions (e.g., organic solvents, sonication, or
heating) to form micelles in aqueous solution. Jérôme and colleagues
correlated SANS and small-angle x-ray scattering (SAXS) data with
statistical mechanics models to more accurately describe the self
association of PEO-PCL polymers in aqueous solution [36]. In order
to dissolve the polymers directly in aqueous solution with heating
a series of PEO-PCL polymers were synthesized with a constant
 
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