Biomedical Engineering Reference
In-Depth Information
Fig. 8
Electron micrographs of the surface of
a
a nonmineralized PLG film (molar ratio
lactide : glycolide = 85 : 15), or
b
a PLG film with deposited bonelike minerals.
c
Higher
magnification image of the mineral film (Biomaterials [136], with permission of Elsevier)
hydrogels as injectable VEGF delivery systems, however, is dependent on the
fluid properties of the pre-gelled solution, and the mechanical properties of
the post-gel. Specifically, the gel has to be fluid-like during the injection pro-
cedure (e.g., flow through a needle, or endoscope), and then transform into
a mechanically stable gel upon injection into the body [138]. Alginate hydro-
gels exhibiting a bimodal MW distribution may represent attractive systems
fulfilling these prerequisites, as their fluid and mechanical properties can be
decoupled from the total polymer concentration [139]. This finding may be
attributed to the fact that low MW alginate components contribute only negli-
gibly to the viscosity of a pre-gelled solution, while they significantly enhance
the stiffness of the formed gel; adding a small fraction of high MW alginate
to the binary hydrogels additionally maintains a high strain at failure [139].
Consequently, alginate gels with adjusted MW distribution may be readily
used as injectable delivery systems for VEGF, and offer favorable properties
in mechanically dynamic environments.
4
Applications for VEGF Delivery Systems
VEGF driven tissue regeneration is highly dynamic and involves an elabo-
rate spatiotemporal interplay between cells, the ECM, mechanical signals, and
other soluble factors (e.g., growth factors and cytokines). Although these base
elements are present in all tissues, the specific characteristics of these compo-
nents and the kinetics of their interactions may vary according to the precise
defect. Traditional polymeric drug carriers may not appropriately mimic the
natural complexity of VEGF signaling, whereas bioinspired multifactor ap-
proaches have the potential to recapitulate the specific interactions elicited
during normal tissue repair (Fig. 9). The diversity of polymeric delivery sys-
tems required for various applications will be described below in the context
of three specific applications: therapeutic angiogenesis, bone regeneration,
and nerve regeneration.
