Biomedical Engineering Reference
In-Depth Information
3.1.4
Gene Transfer
A biologically inspired, but not biomimetic approach for delivery of growth
factors, utilizes polymeric delivery of plasmid DNA encoding the desired
growth factors (localized gene therapy). Sustained protein delivery is typ-
ically controlled by the release properties of the system, but gene transfer
strategies additionally rely on the cellular production and secretion of the en-
coded protein [4]. The key steps in this approach are (A) the release of DNA
from its delivery vehicle and cellular uptake of the plasmid and (B) in situ
cellular production and secretion of the gene product. The staged process re-
sults in a delay in growth factor availability, relative to direct protein delivery.
Gene transfer approaches may overcome some of the limitations (e.g., insta-
bility within polymers over extended time) associated with direct delivery of
proteins [69]. One limitation of this approach is delivery of naked DNA results
in low efficiency of transfection, low levels of growth factor production, and
factor production for a short time [70]. Condensing DNA with polycationic
substances such as poly (ethyleneimine) (PEI) or poly (lysine) to form small
positively charged particles prior to delivery can lead to prolonged expression
of factors both in vitro and in vivo (Fig. 5) [71-75].
Fig. 5
Release of PEI-condensed plasmid DNA from a porous polymeric scaffold results
in prolonged expression of the marker gene beta-galactosidase (
d
)ascomparedtoblank
scaffold (
a
), scaffold incorporating plasmid DNA (
b
), and bolus injection of condensed
DNA (
c
) [75]. This was shown by macroscopic analysis of X-gal stained scaffolds 15 weeks
after subcutaneous implantation (size bars are 3 mm) and by luminescence measure-
ment, which quantitatively assesses gene expression levels (Hum Gene Ther [75], with
permission of Mary Ann Liebert, Inc, Publishers)
3.2
Physicochemical Design Variables
Bioinspired VEGF delivery may be implemented by physicochemical strate-
gies that enable temporally and spatially controlled growth factor availability
