Biomedical Engineering Reference
In-Depth Information
to control the differentiation of human ES (hES) cells into the musculoskele-
tal lineage [72]. However, these advantages are overshadowed by the fact
that their acidic degradation products provoke an immunoresponse from the
body [73].
Another class of synthetic biomaterial that has been extensively explored
for biomedical applications such as controlled release of drugs include
poly(anhydrides) [74, 75]. These materials are useful for drug delivery be-
cause of their inherent surface erosion mediated degradation, unlike their
polyester counterparts which undergo bulk degradation [76]. The anyhydride
bonds present in the polymer backbone can be hydrolyzed by the aqueous
milieu of the body when it is implanted in vivo. Poly(anhydride) has also
been approved by the FDA (Gliadel) for the delivery of carmustine for treating
brain cancer [76, 77]. Poly-Aspirin is another novel poly(anhydride) which
can be degraded into salicylic acid in order to reduce inflammation and pain
locally [78].
The strong mechanical properties and degradation kinetics of poly(an-
hydrides) have attracted many researchers to investigate their potential as
a bone tissue engineering scaffold. According to Ibim et al. poly(anhydride-
co-imide) polymers are comparable to PLGA in terms of their biocompatibil-
ity and they are also capable of supporting critical bone regeneration [79].
Recently, Anseth et al. have developed photocrosslinkable poly(anhydrides)
for bone regeneration where polymerization can be achieved in situ, which
makes them suitable for minimally invasive applications [80]. Although
poly(anhydrides) offer some advantages, their applicability in delivering cells
to the defect site is limited because of their highly crosslinked (dense) poly-
mer network structure which reduces cell viability [81].
Scaffold matrices may possess a “quasi” two dimensional (2D) geometry,
where the cells are seeded
onto
the scaffolds, or a three dimensional (3D)
geometry, where the cells are seeded
within
the scaffold. Recent findings have
demonstrated that porous three-dimensional scaffolds are superior to their
two-dimensional counterparts in providing a proper physiological environ-
ment to the cells [82, 83]. When cells are seeded within three-dimensional
scaffolds, the ECM proteins produced by the cells are deposited uniformly
within the scaffold, which then remodel to yield the desired cytoarchitecture.
Porous 3D polymeric networks with interconnected pores to allow cell
growth, vascularization and diffusion of nutrients are created by various
methods. Freeze drying has been utilized to create porous collagen-GAG
composite matrices for skin regeneration by Chen et al. [84], whereas Sas-
try et al. have used hydrocarbon templating to create porous scaffolds [85].
A gas-foaming process using carbon dioxide (CO
2
) as the foaming agent has
also been used to fabricate highly porous polymer structures [86-88]. Salt
leaching is another approach for producing structures similar to foam or
sponge where the size of the pore is controlled by the size of the salt crys-
tals [89]. Both woven and unwoven fibers have also been commonly used to
