Biomedical Engineering Reference
In-Depth Information
After the transplantation, a tissue-engineered construct cell and tissue remodeling are important for
achieving stable biomechanical conditions and vascularization at the host site. Hence, the 3-D scaffold/
tissue construct should maintain suffi cient structural integrity during the
in vitro
or
in vivo
growth
and remodeling process. The degree of remodeling depends on the tissue itself (e.g., skin 4-6 weeks,
bone 4-6 months), and its host anatomy and physiology. Scaffold architecture has to allow for initial
cell attachment and subsequent migration into and through the matrix, mass transfer of nutrients and
metabolites, and provision for suffi cient space for development and later remodeling of organized tis-
sues. The degradation and resorption kinetics of the scaffold needs to be designed based on the relation-
ships among mechanical properties, molecular weight (
M
w
/
M
n
), mass loss, and tissue development.
In addition to these essentials of mechanics and geometry, a suitable construct will possess
surface properties, which are optimized for the attachment and migration of cell types of interest
(depending on the targeted tissue). The external size and shape of the construct must also be con-
sidered, especially if the construct is customized for an individual patient [3]. Furthermore, con-
siderations of scaffold performance based on a holistic tissue engineering strategy and practical
considerations of manufacture arise. From a clinical point of view, it must be possible to manufac-
ture scaffolds under Good Manufacturing Practice (GMP) conditions in a reproducible and quality-
controlled fashion at an economic cost and speed. To move the current tissue engineering practices
to the next frontier, some manufacturing processes will accommodate the incorporation of cells and
the growth factors during the scaffold fabrication process.
What follows in this chapter is a description of one of the main fabrication methods that have
been used over the last 5 years for creating nanoscale features for scaffolds—electrospinning.
5.2 ELECTROSPINNING
5.2.1 I
NTRODUCTION
Nanofi brous scaffolds are being exploited in tissue engineering due to their inherently high porosities
and surface area-to-volume ratios as well as a wide variety of topographical features to encourage
cellular adhesion, migration, and proliferation. Furthermore, the physical properties can be easily
altered by altering the fi ber size. Thus, there has been an exponential increase in tissue engineer-
ing scaffold materials research using the electrospinning technique (Figure 5.1). There are several
different methods that have been employed to produce nanofi brous scaffolds for tissue engineer-
ing applications. These methods include self-assembly [4] and thermally induced phase separation
(TIPS) each with its own benefi ts and drawbacks in replicating features of the natural ECM. Pep-
tides are used to produce self-assembling scaffolds; however, this method requires highly specifi c
physical and chemical interactions, which can place restrictions on the chemistry and mechanical
properties of the scaffold. Phase separation can produce fi bers of the same size range as the ECM,
and 3-D porous networks can be generated within the scaffold, but it is often diffi cult to control
fi ber alignment and diameters. Furthermore a lengthy multistep process encumbers this procedure.
Electrospinning provides a convenient approach to fabricate fi bers that are within the size range of
the ECM. This method allows for the rapid production and manipulation of unique sets of spatial
and surface structures on the nano- and microscale and can also be used across a broad range of syn-
thetic and natural polymers and their combinations. These aspects form the focus of this chapter.
5.2.1.1 ElectrospinningPrinciples
Electrospinning, which so far has only been performed with synthetic polymers is now performed
using either solutions with synthetic or natural polymers or a polymer melt. Electrospinning com-
monly results in a nonwoven mat of fi bers (Figure 5.2), although other collection techniques are
expanding the morphological nature of the scaffolds, particularly in fabricating oriented fi bers.
Typically, a high (positive or negative) voltage is applied to a polymer solution or melt that is pumped
