Page 186 - Biodegradable Polyesters

164 7 Electrospun Scaffolds of Biodegradable Polyesters: Manufacturing and Biomedical Application

acrylic acid in plasma. These complex processes involve reactions between
species in the gas phase, reactions between surface species, and reactions
between gas-phase species and surface species.
3) Etchingand surfacecleaning: Undesirable materials at the polymer surface
or atmospheric contaminants are removed from the surface by chemical
reactions and physical etching at the surface to form volatile products.
Organic contaminants presented at the surface are usually removed by
oxygen-containing plasmas. Etching differs from cleaning only in the
amounts of materials that are removed from the surface.

Depending on the plasma conditions, the plasmas are generally divided into
two main categories: near-equilibrium or nonequilibrium plasmas, which are
defined as hot plasmas or cold plasmas, respectively. Very high temperatures
of electrons and heavy particles, charged and neutral, are typically found in hot
plasmas, which also present maximal degrees of ionization (close to 100%). Cold
plasmas, on the other hand, are composed of low-temperature particles (charged
and neutral molecular and atomic species) and relatively high-temperature elec-
trons. They are characterized with very low or much lower degrees of ionization
than hot plasmas. Electrical arcs, plasma jets of rocket engines, thermonuclear
reaction generated plasmas, and so on, are examples of hot plasmas. Cold plasmas
are formed in low-pressure DC and RF discharges, discharges from fluorescent
illuminating tubes, and in corona discharges [63].
In contrast to the previously described wet chemical modification techniques,
plasma treatment represents an efficient methodology to incorporate functional
groups on the surface of biodegradable polyesters without changing the benefi-
cial bulk properties [66]. In addition, plasma treatment is a solvent-free technique
and the use of hazardous solvents is avoided [67]. Moreover, it can be employed
to uniformly treat the surface of complex shaped scaffolds. For example, Shen
and colleagues [68] used oxygen plasma pretreatment to introduce hydrophilic
oxygen-containing groups onto the PLGA surface with a simultaneous increase in
roughness of the surface. Cationized gelatin was then anchored onto the surface
of the oxygen plasma which had been pretreated with PLGA. The authors found
an optimum condition for oxygen plasma pretreatment of 10 min under 50 W of
power and 20 Pa of oxygen pressure. After these plasma treatments, cationized
gelatin was efficiently anchored onto plasma pretreated PLGA surface. PLGA with
cationized gelatin grafted on its surface had better hydrophilicity, higher surface
energy, more N-containing groups and better combining stability than gelatin-
anchored PLGA without plasma pretreatment. The surface modification method
combining oxygen plasma treatment with anchorage of cationized gelatin demon-
strated an effective way of enhancing cell affinity of polylactone-type biodegrad-
able polymers.
Plasma treatment was also employed to immobilize Arg-Gly-Glu-Ser peptides
(RGDS) on poly-L-lactic acid (PLLA) scaffolds to improve their cell affinity [66],
which was evaluated by culture of osteoblast-like cells with the prepared scaffolds.