Biomedical Engineering Reference
In-Depth Information
intestinal tissue engineering [90]. A number of options have been suggested for modifying the bio-
materials used to fabricate the scaffold to increase neovascularization. These include biodegradable
polymers that provide localized and sustained growth factor release [91,92].
Incorporation of angiogenic growth factor proteins into polymeric devices is one way of stimu-
lating neovascularization in the tissue-engineered construct that also enables the duration of growth
factor release at the target site to be controlled. The duration of delivery can be infl uenced by protein
loading, the type of polymer used to fabricate the device, and the processing conditions. Therefore,
the type of growth factor, dosage, release kinetics, and duration of delivery are all parameters that
require optimization. Growth factors can be incorporated into the scaffold either during or after
scaffold fabrication. When biodegradable scaffolds are used, the growth factor is released as the
scaffolds degrade, stimulating vascularization and tissue growth to replace the lost scaffold. The
growth factors are released either via diffusion mechanisms, which are controlled by the porosity of
the scaffold, or by erosion mechanisms.
A steady supply of growth factors, such as vascular endothelial growth factor (VEGF) and
fi broblast growth factor-2 (FGF2) that show combined effects on angiogenesis and maturation of
blood vessels is required to provide tissues with adequate exposure to the angiogenic stimulus. At
the same time, because of the potent stimulatory effect of VEGF and other growth factors, systemic
exposure is not desirable since it may enhance angiogenesis associated with pathological conditions,
such as neoplasia or retinopathy. Therefore, scaffolds that provide sustained localized delivery of
the growth factor are sought.
The most straightforward way to incorporate growth factors into tissue engineering scaffolds is
to either soak the fabricated scaffold in a solution of the growth factor and rely on adsorption of the
protein to the scaffold material or mix growth factors with the suspension of cells during the cell
seeding process. Although both approaches are relatively easy to perform, the controlled release
and duration of growth factor activity are usually not optimal.
A different approach relies on incorporating functional groups into the scaffold that will bind the
growth factor and exert some control over the release of the protein. This has recently been demon-
strated using acellular collagen-heparin scaffolds [93]. Porous collagen scaffolds were prepared by
freeze-drying a solution of collagen dissolved in acetic acid. The scaffolds were cross-linked using
1-ethyl-3-dimethyl aminopropyl carbodiimide and N -hydroxysuccinimide in 2-morpholinoethane
sulfonic acid in the presence of ethanol and heparin. After washing, the scaffolds were incubated
in phosphate buffer saline containing either VEGF or bFGF. The covalently bound heparin in the
collagen scaffold allowed coupling of the heparin-binding growth factors bFGF and VEGF. When
implanted subcutaneously in an animal model, the dual growth factor-loaded scaffold led to the
earlier establishment of a high-density vasculature that was well-developed when compared with
the other acellular scaffolds tested [93].
Alternatively, an additional device, such as microspheres, capable of delivering growth factors
can be incorporated into the scaffold to provide sustained and controlled localized delivery. Scaf-
folds containing polymer microspheres have been fabricated using a gas foaming or particulate
leaching process that enables controlled release of VEGF [94]. The scaffolds were fabricated from
either poly(lactic- co -glycolic acid) microspheres and VEGF or a mixture of poly(lactic- co -glycolic)
microspheres, VEGF, and poly(lactic- co -glycolic) particulates ground to an average diameter of
125 µm. Poly(lactic- co -glycolic acid) microspheres were prepared using a double emulsion (water-
in-oil-in-water) process incorporating VEGF. The fi rst emulsion was prepared by dissolving the
polymer in ethyl acetate into which an aqueous solution of the growth factor was mixed. Poly(vinyl
alcohol) in ethyl acetate was mixed with the fi rst emulsion and stirred into a solution of ethyl ace-
tate, poly(vinyl alcohol) and water. The ethyl acetate was evaporated from the solution, allowing the
microspheres to harden, which were then collected by fi ltration and freeze-dried. The polymer micro-
spheres (with or without particles) were combined with NaCl particles (250-425 µm). Alginate was
added to the mixture to increase incorporation and stabilization of the VEGF protein. The polymer
or growth factor mixture was freeze-dried and pressed into a pellet using a Carver press. The pellets
 
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