Processing of bioresorbable and other polymers for medical applications - Durability and Reliability of Medical Polymers - page 72

∑ emulsion,
∑ suspension, and
∑ dispersion.
The emulsion process allows the production of particles with a diameter in
the nanometer range (10-10 4 nm). The monomer is dispersed in water with
a water-soluble initiator and a surfactant which forms uniform micelles.
Polymerization occurs inside micelles, in which the monomer is able to diffuse,
because the initiator is not miscible in the dispersed monomer phase.
The dispersion process produces particles in the range of 0.5-10 µm.
Monomer, initiator and stabilizer are dissolved in an organic medium.
The initiator is soluble in monomer droplets stabilized by the surfactant.
Polymerization takes place at this point. Polymer droplets are not soluble
in the organic medium and therefore form a precipitate. Aggregation is
avoided by the stabilizer. Suspension polymerization produces micrometer-
sized particles (50-500 µm). The monomer is dispersed in water with a
stabilizer. The initiator is soluble in the monomer phase where polymerization

occurs.
Microspheres can also be prepared using linear polymers with techniques
such as spray drying or the solvent evaporation method. Microspheres can be
obtained by the evaporation of an organic solvent from dispersed oil droplets
containing both polymer and drug, thus also achieving drug loading. The drug
is first dissolved in water. This aqueous phase is then dispersed in an organic
solvent (such as dichloromethane) which contains the polymer, forming a
water/oil emulsion. This is dispersed in a stabilized aqueous medium to
obtain the final oil/water emulsion. Organic solvent evaporation is realized,
forming the polymer microspheres, which encapsulate the drug.

3.11 Conclusions
It can be seen that injection molding and melt extrusion are suitable methods
for biomaterial processing. Injection molding increases the crystallinity of the
material. This improves device performance because degradation is reduced
and mechanical strength increases. Solvent casting and melt spinning are also
suitable methods for the production of biomedical devices, provided that the
residual solvent amount is below the minimum value. Electrospinning allows
the production of fibers with diameters in the range of nanometers as long as
the solvent is removed. Porous structures can be obtained both using foaming
process or particle leaching technique. This last technique allows control of
pore diameter in tissue scaffold materials. Polymeric microparticles, which
are of growing importance in drug delivery, can also be synthesized in ways
that avoid thermal degradation or residual solvent traces.

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