Page 183 - Biodegradable Polyesters

7.3 Improving the Bioactivity of Electrospun Polyesters 161

30 nm. PC, with a self-cleaning TiO layer, exhibitedbetterhardness andscratch
2
resistance as well as good photocatalytic and mechanical properties.
Chromic acid and potassium permanganate in sulfuric acid have been used
to introduce reactive oxygen-containing moieties to poly(ethylene) (PE) and
poly(propylene) (PP) [56, 57]. PP, like PE and other polyolefins, has become
an increasingly important material. However, these polyolefins are generally
hydrophobic materials and in many practical instances, improved adhesion, wet-
tability, printability, or biocompatibility is desired. As a result, there is continuing
and widespread interest in new chemical processes capable of modifying these
polymer surfaces. For example, PP was initially oxidized by etching to produce a
modified surface [57]. A series of repetitive grafting experiments using a diamine
derivative of poly(tert-butyl acrylate) were then used to produce surfaces con-
taining significant amounts of poly(acrylic acid) obtaining water contact angles of
∘
approximately 20–30 . On the one hand, treatment of the modified PP surfaces
by chemical etching with alkali produced a more hydrophilic carboxylate surface.
On the other hand, treatment of these surfaces first with ethyl chloroformate
followed by pentadecylfluorooctylamine produced a hydrophobic fluorinated
surface. Modifying a pristine polyolefin surface allows for further processing of
the polymer, an increase in the application range, and impact of the material in
the market.
The chemical surface treatment method was also used because of its simplicity
in modifying the surface of biodegradables polyesters [58]. For instance, while
biodegradable, biocompatible polyesters such as PLGA may have mechanical
and degradative properties ideal for the manufacture of tissue engineering
scaffolds, their surface properties are not ideal for cell growth [59]. Their natural
hydrophobicity does not favor cell growth and the surface is unable to interact
with specific cells. In addition, PLGA surfaces do not possess any functional
groups for the attachment of biologically active molecules [60]. Although a
number of approaches to modify the chemical properties of PLGA surfaces
have been reported, their applicability for soft tissue scaffolds, which combine
large volumes, complex shapes, and extremely fine structures, is questionable.
In this sense, a promising approach appears to be the surface modification
of PLGA scaffolds after preparation, which gives useful surface properties
to the polymer, maintaining the properties of the bulk unchanged. Croll and
colleagues [60] have reported on the use of controlled hydrolysis in aqueous
∘
sodium hydroxide at 20 C for grafting carboxylic acid functional groups and
aminolysis to produce primary and secondary amine groups on the surface of
thin PLGA films in a highly controlled manner. Contact angle data showed that
the level of modification quickly reached a limiting value, independent of time
and concentration of modifying species, as expected, given the chain-scission
mechanism of hydrolysis and aminolysis. In addition, covalent binding of a
model amine-functional macromolecule chitosan to the newly formed functional
groups was characterized. An important finding of these authors was that for
direct tissue growth within a tissue engineering scaffold, it is not sufficient to
simply provide a surface on which cells grow well. A phenotype of the tissue