Biomedical Engineering Reference
In-Depth Information
migrate, they always move bound with their anchoring PEU segments. Con-
sequently, when the micro-phase separation occurs, the migration of SMA is
inevitably encumbered with the connected huge PEU chains, which sterically
prevents the SMA migration from aggregating into micelles within the matrix
bulk, but just takes a surface-ward orientation [76, 77].
2.1.4
SMA Surface Conformation and Optimization
SMA-based surface architecture fundamentally influences the events and re-
sponses of biological substances that approach from the ambient to bioma-
terial surfaces, thereby directly dominates the overall performance of the
surface modifying efforts.
Once SMA is physically enriched on the material surface and exposed to
the ambient, the SMA-based surface architecture is mainly determined by
the variables of SMA hydrophilicity and the ambient polarity, since these
parameters remain the only thermodynamic factors that may alter the inter-
face energy and thus substantially affect the SMA conformation. Accordingly,
to explicate the surface topology of the model systems, factorial conditions
are manipulated with various hydrophilic/hydrophobic combinations of SMA
components versus the ambient. The distinction of ambient polarity has been
well acknowledged. Aqueous mediums are undoubtedly of polar circum-
stances; while the vacuum or air atmosphere is recognized as a non-polar
environment with hydrophobic characteristics. For the model SMA, MPEO
derivatives, the role of the spacer is always undertaken by PEG chains that
possess diverse compatibilities with multiple categories of mediums; in con-
trast, the candidate selections for SMA endgroups are much more flexible.
For example, with the same purpose of pursuing albumin-selective binding
on MPEO-decorated PEU surfaces, hydrophilic compounds of Cibacron Blue
F3G-A (a triazine dye, “Ciba”) and hydrophobic groups of 18-Carbon fatty
alkyl (stearyl or “C18”) are respectively conjugated to MPEO templates as
functional endgroups so as to make use of their specific albumin affinity.
The achieved penta-blocked MPEO-derivatives, MSPEO and Ciba-MPEO, are
schematically illustrated in Fig. 3. The manipulated SMA surface conforma-
tions are also demonstrated in Fig. 3.
As described in Sect. 2.1.3, the tendency towards the lowest surface energy
directly dominates all the spontaneous procedures occurring on the SMA-
ambient interface, including the construction of SMA surface conformations.
On the air interface, MSPEO-decorated PEU surfaces are covered by vast
numbers of hydrophobic C18 groups. As revealed by angle-dependent X-ray
photoelectron spectroscopy [XPS] (Fig. 4), almost all of these C18 groups are
concentrated in the most superficial zone of the surface layer with a depth
less than 5 nm. This quantitative information suggests a “standing-up” con-
formation for the short-chained (Mn 2300 Da)MSPEOontheairinterface
