Page 184 - Biodegradable Polyesters

162 7 Electrospun Scaffolds of Biodegradable Polyesters: Manufacturing and Biomedical Application

should be retained and the scaffold surface should ideally resemble as closely as
possible the natural ECM of the desired tissue.
A drawback of these wet chemical methods is the nonspecific effect which
introduces a range of oxygen-containing functional groups. Chain surface orien-
tation, as in Polymethyl methacrylate (PMMA) ester modification, for example,
creates additional complexity in the treatment processes and the degree of surface
functionalization may therefore not be repeatable between polymers of different
molecular weight, crystallinity, or tactility. These wet chemical methods can also
generate hazardous chemical waste and can lead to irregular surface etching [61].
Many of these techniques also require extended treatment in concentrated cor-
rosive solutions. For these reasons, while useful in the laboratory environment,
these wet chemical processes may not be suitable for larger scale applications.

7.3.1.2 Plasma
Broadly speaking, plasma can be defined as a gas containing neutral and charged
species, including electrons, positive and negative ions, radicals, atoms, and
molecules in ground and excited states and electromagnetic radiation (mainly
UV and Vacuum-UV (VUV)). In a plasma, the average electron temperature
12
9
−3
ranges between 1 and 10 eV, the electron density varies from 10 to 10 cm ,and
thedegreeofionizationcan be between10 −6 and 0.3 [62]. There are a wide range of
scientific published reviews [62–65], books (see review [63]), and references con-
cerning plasma treatment of polymers and only a brief account will be given here.
The techniques for the treatment of plasma in their different forms have moved
out of the academic fold to become important industrial processes for modifying
the surface properties of polymers. The main advantages of plasma modification
techniques can be summarized in the following:
1) Bulk properties of the polymer remain intact after treatment because the
surface modification can be confined to the surface layer, typically less than
∼100 nm.
2) By choosing the gas in the plasma, it is possible to graft particular chemical
functionalities at the polymer surface.
3) Common problems encountered in wet chemical techniques (Section 7.3.2.1),
such as residual solvent on the surface and swelling of the substrate, can be
avoided with the use of a gas plasma.
4) Excited species in gas plasma can modify the surfaces of all polymers, regard-
less of their structures and chemical reactivity and this modification is fairly
uniform over the treated surface.
The disadvantages of the plasma processes are as follows:
1) A vacuum system is required in traditional plasma treatments, increasing the
operation cost.
2) Optimal parameters developed for one system usually cannot be adopted for
another system. Because the plasma process is extremely complex, it is nec-
essary to have very good control of the plasma parameters, such as radio fre-
quency (RF), power level, gas flow rate, gas composition, gas pressure, sample