Biomedical Engineering Reference
In-Depth Information
system prior to any
in vitro
or
in vivo
experimentation. Alternatively, electrospinning of polymers
from their molten state can be used to bypass the toxicity issues related to solution spinning.
5.2.3.2 Melt Electrospinning
A very recent aspect of electrospinning is the fabrication of electrospun fi bers scaffolds from poly-
mer melts. Melt electrospinning has some technical advantages over polymer solutions for certain
tissue engineering applications. It is possible to melt electrospin directly onto cells
in vitro
with-
out issues of solvent toxicity or suitable cross-linking while cell vitality remains unaffected by
the process. Figure 5.9a shows the setup of a simple melt electrospinning system and an electron
micrograph of a melt-spun PCL-PEG block copolymer. Fibers obtained were slightly larger than
solvent electrospinning but uniformity and morphology were comparable. Figure 5.9b shows results
of a direct
in vitro
melt electrospinning experiment onto live porcine bone marrow cells previously
seeded on a PCL/Col nanofi ber mesh. Cells remained viable after 1 week post-spinning. Electron
micrograph of the construct revealed that cells adhered to both PCL/Col nanofi ber substrate and the
melt-spun PCL-PEG polymer. These results indicate nontoxicity and cyto-compatibility of the melt
electrospinning process. This is important for fabricating layered tissue-engineering constructs,
where cells and electrospun scaffolds are ordered into lamellar structures, and for directly electro-
spinning onto tissue for
in vivo
applications.
With polymer melts, there is additionally no solvent evaporation at the spinneret compared with
the formation of a polymer skin when using volatile solvents. Such defects, which can affect the
morphology of the collected electrospun fi bers with large-sized structures deposited on top of the
smaller fi bers, are commonly encountered when electrospinning from polymer solutions is con-
ducted over long time periods. Melt electrospinning has the potential, therefore, to produce more
uniform fi bers over longer electrospinning times than fi bers electrospun from polymer solutions.
5.3 PHYSICAL CHARACTERIZATION OF ELECTROSPUN SCAFFOLDS
The morphology and surface properties of scaffolds strongly infl uence their interaction with cells.
The surface properties of the biomaterials can be categorized into geometric, topographical, and
surface chemical properties. Geometric and topographic properties include the roughness of two-
dimensional (2-D) polymer surfaces [26-28] as well as the porosity, pore size, and pore size distri-
bution, intrafi ber surface roughness and the specifi c surface area for porous and nonwoven polymer
membranes. Surface chemical properties affect the water wettability or hydrophilicity of scaffolds
and their energies and can often be manipulated by surface modifi cations.
5.3.1 M
EASURING
P
OROSITY
, S
URFACE
R
OUGHNESS
,
AND
S
PECIFIC
S
URFACE
E
NERGY
OF
S
CAFFOLDS
A traditional method for characterizing the porosity and pore size distribution is mercury porosim-
etry (Figure 5.10). This method is a liquid intrusion method; it assumes that the pores are cylindrical
and pore size is expressed in terms of the diameter of the opening. Pores with diameters smaller
than 2 nm, between 2 and 50 nm, and larger than 50 nm
are referred to as micropores, mesopores,
and macropores, respectively. Mercury has a high surface tension (485.5 dynes/cm at 25°C) and
forms large contact angles with most other materials (
130°) [29] and does not penetrate pores by
capillary action. A positive pressure must be applied to force mercury into the pores. The pore size
can be calculated from the pressure applied by the following equation:
≅
=
-
4
γ
cos
θ
_________
D
P
where
D
is the diameter of the pore,
γ
is the surface tension of mercury,
θ
is the contact angle between
mercury and the solid, and
P
is the applied pressure. The direct data acquired is the accumulated
