Biomedical Engineering Reference
In-Depth Information
the contrary, a very compressed conformation of Ciba-MPEO appears on its
air interface. As illustrated in Fig. 3a, this “lying-down” pose of Ciba-MPEO
is shaped due to the incompatibility between SMA bodies and the ambient
air. Interestingly, being different from either the compatible standing-up con-
formation or the incompatible lying-down conformation, a combined con-
formationisbuiltupbyMSPEOonitswaterinterface.Bytheinfluenceof
ambient water, the hydrophilic PEG spacers tend to stretch out and head to
the water phase, simultaneously the hydrophobic C18 endgroups are excluded
by the water phase and draw back towards the material bulk. These combined
tendencies finally result in a compromised “bending-loop” conformation of
MSPEO as demonstrated in Fig. 3d. This conformation is also verified by
quantitative XPS analysis and demonstrated in Fig. 4. It indicates that for
MSPEO-modified PEU surfaces, when the circumstance is altered from the
air to water, the allocation of MSPEO C18 groups observably retreats by ex-
panding the distribution from the original narrow superficial area (depth
<
5 nm) back to a much broader zone with the depth of over 10 nanome-
ters [149, 150]. This variation clearly suggests a bending-back action of the
C18 endgroups. All these findings are also supported by surface spectro-
scopic evidences (ART-FT-IR) and surface wettability measurements (contact
angle) [76, 77, 94-96, 151-154].
Given the fact that biomaterial surface architecture mainly functions on
the water interface, whereas many non-hydrophilic ligands (like C18 groups)
are engaged as SMA functional endgroups being introduced onto material
surfaces, the compromised surface conformations like the bending-loops (as
shown in Fig. 3d) are extensively formed in various SMA-based surface-
modifying models. In order to counteract the bending-back tendency of the
hydrophobic endgroups and hence assure the exposure of these functional
ligands on the water interface, larger-sized hydrophilic spacers (like PEO,
Mn 12k Da) are applied. As indicated in Fig. 4, compared to the trials with
short-chained spacers (Mn 2300 Da), the attempts with long-chained spac-
ers induce a significant increase of C18 content ranging from 50%to150%at
different depths of the surface layer. Especially at the most superficial level,
the C18 enrichment becomes equivalent to that of the MSPEO standing-up
conformation. This result suggests that the conformational defect, like that
of the bending-loops, can be made up by altering functional fractions in
SMA compositions, such as the increase of the hydrophilic PEG moieties
in MSPEO. Nevertheless, unilateral enlargement of the spacer size has both
pros and cons. The dual dilemmas are that, from the thermodynamics angle,
the greater fraction of hydrophilic spacer moieties may reinforce the physi-
cal lift for hydrophobic endgroups on the water interface, but it also carries
the risk of diluting the functionality by a too heavy amount of spacers; and
from the kinetics angle, the longer spacers are capable of offering higher
mobility to the hydrophobic endgroups on the water interface, but simul-
taneously it may also result in a greater kinetic hindrance (repulsion) to the
