Biomedical Engineering Reference
In-Depth Information
matrix as well as decreased cell attachment and
proliferation. As a result, current methodolo-
gies allow for the ECM to be processed in a way
in which crosslinking is unnecessary and the
scaffolds have clinically viable degradability
properties
[66]
.
These natural scaffolds represent an interest-
ing class of bioscaffolds that will become more
useful as processing methods are further refined,
fostering a new generation of 3D bioscaffolds
that could potentially aid in more efficient
regeneration of human tissue.
7.2.3.2 Injectable Scaffolds
Injectable scaffolds
are attractive options for
tissue engineering applications because they
are delivered in a minimally invasive manner,
have the potential to set
in situ
, and can poten-
tially conform to complex, intricate tissue
defects. These scaffolds avoid the invasive sur-
geries required to implant grafts and preformed
scaffolds. In addition, injectable scaffolds afford
the ability to seed and deliver cells more effi-
ciently, avoiding the inability to seed cells deep
into preformed scaffolds. Moreover, injectable
scaffolds shorten operating times, minimize
post-operative pain and scar tissue, and reduce
cost
[68, 124-131]
.
Figure 7.3
shows a schematic
of
in situ
cross-linked poly(ethylene glycol)
(PEG)-hyaluronic acid (HA) injectable hydro-
gel. A blend of PEG, HA, the photoinitiator, and
eosin Y are injected transdermally and then
photocrosslinked using a light-emitting diode
(LED)
[131]
.
These bioscaffolds can be formed from a vari-
ety of materials, depending on the tissue being
regenerated. Calcium-phosphorous ceramic
injectable bioscaffolds have been created for
bone tissue engineering applications. Once
these scaffolds were mixed with cells encapsu-
lated in alginate hydrogel beads and fibers, they
exhibited suitable biological and mechanical
properties
[68]
. For soft-tissue defects, several
injectable hydrogels have been used. Natural
polymer systems, which use collagen and
FIGURE 7.3
Schematic showing a transdermally injected,
photoactivated soft-tissue bioscaffold. An
in situ
crosslinked
injectable hydrogel was obtained by combining PEG, hyalu-
ronic acid (HA) and photoinitiator, eosin Y. A mixture of these
components was injected transdermally (
a
) and could be
formed into a desired shape (
b
). The material was then
crosslinked by exposing the bioscaffold to an array of LEDs
emitting light (
c
). This light was shown to penetrate tissue
depths of up to 4 mm with a 2 min. exposure time, sufficient
to activate the eosin Y and photocrosslink the hydrogel scaf-
fold. Reprinted with permission from Ref.
131
; copyright 2012
American Association for the Advancement of Sciences.
glycosaminoglycans (GAGs) reinforced with
synthetic polymers for mechanical integrity and
decreased gelation time, have shown promise
to aid in soft-tissue regeneration
[69]
.
Although numerous engineering approaches
have been developed to create injectable scaf-
folds, new classes of peptide-based
in situ
gel-
ling scaffold materials have garnered significant
interest
[124-127]
. Rationally designed peptides
are rapidly assembled into hydrogel scaffolds in
physiological condition by physical or chemical
cross-linking mechanisms providing encapsu-
lated cells with an artificial ECM environment.
Peptides with environmental stimuli respon-
siveness are physically assembled by controlling
the temperature, pH, or ionic strength while
