Page 188 - Biodegradable Polyesters

166 7 Electrospun Scaffolds of Biodegradable Polyesters: Manufacturing and Biomedical Application

Treatment in aqueous ammonia solution introduced amide- or amino-groups
onto the PS surface, together with hydroxyl and carbonic acid groups. Treatment
in pure water introduced only hydroxyl and carbonyl groups onto the surface.
Ozone can be dissolved in water by aeration of the gas and if it is excited by
UV light ( < 254 nm), the excited ozone decomposes in water and produces
• •
very reactive oxygen species, such as hydroxyl (OH ), peroxide (OOH ), and
•−
superoxide radicals (O ), according to the following scheme [87]:
2
∗
O + ℎυ → O + O
3 2
− •
O + H O → 2OH
2
•− •
O + OH → HO 2 + O 2
3
• •−
HO 2 ≡ O 2
Many different polymers have been studied over the years with UV/O treat-
3
ment, including biodegradable polymers. A number of different strategies have
been used for the production of ozone and its application in the modification of
scaffold polyesters [88–95]. In addition to the traditional UV/O treatment, sev-
3
eral research groups have begun using other techniques, such as dielectric barrier
discharge in air for surface modification of polyester substrates [92, 93].
The use of UV/O treatment in the functionalization of scaffolds is of particular
3
interest because in vitro evaluation tests commonly reveal that the cells are only
able to survive close to the surface to within a critical depth, which also depends
on the cell type [96]. In order to support the growth of a large volume of tissue
(typically more than 1 mm), it is necessary to promote cell growth within the scaf-
fold. This can only be achieved when nutrients are delivered to the cells and waste
products removed. Vascularization of the scaffold is a key component to the suc-
cess of this strategy. The use of highly porous scaffolds with suitable surface and
interior properties can allow the growth of large organized cell communities and
permit spatially uniform tissue regeneration.
Plasma methods are effective techniques for biomaterial surface modification,
where the reactive species in the reactive gaseous phase, such as oxygen-
containing radicals, impinge and react to incorporate functional groups onto the
biomaterial surfaces. However, the functionalization depth of plasma treatments
for the processing of 3D porous scaffold materials is restricted by the surface
region of the modification, where, depending on the material, only a few top
monolayers are modified. Through plasma treatment, the reactive species are
unable to reach the deep region of scaffolds. In this sense, ozone has potential
ability for biomaterial surface modification and in addition, the bulk region
can also be modified by diffusion of ozone into the scaffold. Liu and colleagues
[96], have recently investigated the surface processing of cross-linked collagen
scaffolds by the ozone perfusion processing technique. They have achieved an
improved surface wettability both for exterior and interior surfaces of porous
3D collagen scaffolds. It was demonstrated that ozone perfusion processing
protocol is capable of effectively modifying both the superficial and deep region