Biomedical Engineering Reference
In-Depth Information
Self-Assembly
The strategy of self-assembly is widely used for the development of complex and multifunc-
tional nanostructures. Many unique performances of natural materials emerge from a precise
hierarchical self-assembled organization of the molecular structures. In native tissues, collagen,
elastin, and fibrin fibers provide examples of self-assembled structures [22]. In this process, the
atomic and molecular components spontaneously arrange themselves through weak noncova-
lent interactions such as hydrogen bond, ionic bonds, Van der Waals forces, and hydrophobic
interactions into stable and greater complex structures under equilibrium conditions [22].
Nanoscaled supramolecular structures (Figure 8.2D) like nanofibers can be self assembled
using natural or synthetic macromolecules [23]. Berndt
et al.
mimicked human ECM through
synthesizing a thermally stable protein-like architecture based on a mimetic synthetic peptide
amphiphile (PA) system, which consisted of a hydrophobic dialkyl chain moiety (tail group)
attached to a hydrophilic N-α amino group peptide chain (head group). The peptide head
chain was derived from the ECM collagen ligand sequence [19, 22, 24]. The dialkyl chains of
the PA were replaced with monoalkyl chains for thermal stability improvement. Both the dial-
kyl and monoalkyl chain-based PAs readily self-assembled to form a stabilized three-dimensional
triple-helical conformation in an aqueous solvent at the liquid-air interface and it appeared
similar to the natural ECM [19, 22]. The scope of amino-acid selection and alkyl-tail modifi-
cation in the peptide-amphiphile molecules was investigated by Hartgerink
et al.,
who found
that nanofibers varied in morphology, surface chemistry, and potential bioactivity. In addition,
they described three different modes for self-assembly of nanofibers, including pH-controlled
self-assembly, drying on surface-induced self-assembly, and divalent-ion-induced self-assembly.
They also demonstrated that PAs can be self-assembled reversibly into nanofiber networks,
which result in the formation of aqueous gels through pH changes. Furthermore, the PA fibers
can be reversibly polymerized to enhance their stability. These two switchable events controlling
the formation of the supramolecular structure and polymerization produce a remarkably
versatile material [25]. It is also possible to fabricate a bioactive self-assembled PA by incorpo-
ration of bioactive sequences within the PA to improve adhesion, spreading, and proliferation
of cells on such scaffolds [26]. Triple helical PAs can be applied as surface coatings to modify
and improve the biocompatibility of material surfaces and promote cellular response [26, 27].
In addition to peptides, synthetic polymer nanofibers have been prepared by self-assembly of
diblock polymers ((
A
)
n
(
B
)
m
) when the two blocks segregate from one another in bulk owing to
their incompatibility. The volume fraction of
A
and
B
can be controlled to obtain cylinder-
shaped
B
domains, with nanoscaled diameter, embedded in a continuous matrix of
A
[23, 28].
Although, self-assembly presents a complex and extremely elaborate technique with low pro-
ductivity, in comparison with electrospinning, it can still produce much finer nanofibers [23].
Electrospinning and Electrospraying
Electrospinning offers the capability of designing nanoscale to microscale fibers in the form
of nonwoven structures that can meet the demands of scaffold-based TE applications [29].
In this electrostatic technique, a high electric field is applied between a polymer solution or
melt contained in a syringe with a capillary tip and an electrical conductive collector. A
droplet of the polymer solution held at the tip by surface tension is deformed into a conical
shape, known as the Taylor cone. When the electric field overcomes the opposing forces,
such as the surface tension of the deformed droplet and the gravitational force, a liquid jet is
ejected from the tip of the Taylor cone. The electrically charged jet travels straightly for some
distance and beyond a certain distance, the path becomes more complicated and undergoes
a series of bending instabilities during its passage. As the polymer jets cool or lose the sol-
vent, they are drawn, solidified, and collected as an interconnected web of fine fibers on a
grounded rotating drum or other specially shaped grounded targets (Figure 8.3A) [9, 30-32].
