Biomedical Engineering Reference
In-Depth Information
[46]. Interestingly, the embedded cells appeared to ingest the nanofi bers.
No attempts have been made to control or quantify the degradation rate of
PA scaffolds in vivo , an important parameter of any TE scaffolding system.
Few studies have investigated the potential of PA scaffolds to support
bone growth in vivo , although they appear to be quite promising as carti-
lage tissue scaffolds [47]. One recent study found ectopic bone formation
in rats when injected subcutaneously with PAs and free BMP-2, indicating
that PAs have some potential as bone TE scaffolds [48], or at a minimum as
BMP delivery agents. Much more work remains—including determining
PA scaffold effi cacy in a realistic fracture-healing model, before any con-
clusions on their utility can be made as bone tissue engineering scaffolds.
More recently the focus has shifted to covalently immobilizing growth
factors or other drugs on the PA molecules, with the goal of spatially con-
trolling growth factor distribution and preventing diffusion of the bioactive
molecules out of the targeted wound site. Human mesenchymal stem cells
showed increased SMAD expression, a component of the BMP activated
signaling cascade, on PA sheets incorporating the active domain of BMP-2
when cultured in vitro [49]. PAs functionalized with a fragment of vascular
endothelial growth factor (VEGF) were shown to be an effective treatment
of ischemia in a mouse model [50]. Non-protein bioactive molecules can also
be bound to peptide amphiphiles, such as the steroid dexamethasone [51].
This is particularly interesting for bone TE applications as dexamethasone
could be used to simultaneously reduce wound site infl ammation as well
as promote osteogenic differentiation of invading mesenchymal stem cells.
4.2.3 Conclusions
Given the limitations in controlling scaffold morphology, and their hydrogel-
like structure, current self-assembling systems should be seen more as
easily tailorable, injectable drug or growth factor delivery systems at the
moment, rather than as bone scaffolding materials per se . In spite of the
diffi culties in fabricating a suitable scaffold structure, PAs have demon-
strated the capability to support mineralization and MSC differentiation
in vitro. More studies are needed, especially in realistic injury models
where the mechanical drawbacks of PA-based scaffolds are minimized,
such as in spinal fusion procedures.
4.3 Electrospun Scaffolds
4.3.1
Fabrication and Physical Properties
The technique of electrospinning was developed nearly 100 years ago, but
until recently was used primarily for the production of particulate fi lters
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