Biomedical Engineering Reference
In-Depth Information
fragile mechanically. Consequently most electrospinning applications are
restricted to where a highly aligned fl at surface is desired, as the case of
many neural TE applications, or where a thin, strong tube is required, as
for vascular TE or guided tissue membrane applications.
4.4
Thermally Induced Phase Separation (TIPS)
Scaffolds
4.4.1
Fabrication and Physical Properties
Phase separation is a phenomenon in which a homogeneous multicom-
ponent system separates into two or more phases. A decrease in solubility
of components of the mixture leads to distinct phases separating out of a
single solution in order to reduce free energy in the system, such as result-
ing from temperature reduction [92]. This behavior can be used to sepa-
rate a polymer solution (i.e., a solvent-polymer mixture) into regions of
solvent-rich and polymer-rich composition. The solvent is then removed,
and spaces formerly occupied by the solvent phase become empty pores.
For tissue engineering purposes, phase separation can be utilized to pro-
duce highly porous open-celled foams with micro- or nanoscale structures
whose properties can be tailored through varying the composition and
processing parameters of the polymer solution.
The structure of the polymer foam is governed both by the polymer
type and solubility in the chosen solvent as well as process parameters
such as rate of cooling and fi nal temperature [93]. While the relation-
ships between these factors are complex, most effects can be thought of in
terms of affecting the rate of the liquid-liquid phase separation, the size
of subsequently generated polymer-rich liquid regions, and the ability
of a given polymer chemistry to self assemble into a nanoscale fi brous
architecture [30].
Our lab has pioneered a technique for creating highly porous (90%
)
nanofi brous matrices from polymers, such as poly(L-lactic acid) (PLLA),
though a thermally induced phase separation process (Figure 4.3) [30].
A variety of solvents and freezing temperatures have been investigated
and material surface topographies range from nearly fl at to nanofi brous,
depending on polymer and processing conditions. By incorporating a
second polymer in the polymer solution, for example poly(D,L-lactide)
(PDLLA), a material with a partially nanofi brous, partially solid architec-
ture can be produced [93]. Biological polymers such as gelatin can also be
utilized to create surfaces with nanofi brous architecture [94]. Copolymers
of PLA have been shown to produce a nanofi brous surface architecture as
well, opening the possibility of further customization of chemical proper-
ties and degradation rates [95].
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