Biomedical Engineering Reference
In-Depth Information
and degradation. Both non-degradable and materials that degrade through either
hydrolytic or enzymatic mechanisms have been synthesized [
94
]. Additionally, the
processing of synthetic materials into desired structures may be much simpler than
with natural materials. However, potential limitations in the use of synthetic
materials include toxicity and a limited repertoire of cellular interactions, unless
they
are
modified
with
adhesion
peptides or
designed
to
release
biological
molecules.
Due to their biocompatibility and use in medicine, poly(a-hydroxyesters) have
been used extensively in the field of tissue engineering and for stem cell culti-
vation. Among the most popular are scaffolds made from poly(glycolic acid)
(PGA), poly(lactic-acid) (PLA) and their co-polymer, poly(lactic-co-glycolic acid)
(PLGA) [
36
,
95
,
96
]. For example, a scaffold made of a blend of PLGA/PLLA was
seeded with hESCs and differentiation was induced through incorporation of the
appropriate growth factors in culture media [
98
].
Poly(ethylene glycol) (PEG) hydrogels are popular matrices for encapsulating
stem cells [
96
,
98
-
102
]. PEG hydrogels are elastic, biocompatible and can be
tailored to possess mechanical properties similar to various natural tissue types.
But these nonionic and covalently cross-linked networks are very different from
the self-assembled polyelectrolytes that comprise the bulk of natural ECMs. The
PEG hydrogels are inherently hydrophilic, so protein adsorption and cell inter-
actions are minimal. Thus, PEG hydrogels are often modified with tethered groups,
such as adhesion peptides, to enable cellular interactions [
102
-
105
].
In addition to the variation existing in chemistry of the synthetic ECM, mate-
rials have been processed in different formats and porosity; the most common are:
• Macro-porous scaffolds—prepared by cross-linking (chemical or physical) of a
biomaterial solution into the desired shape, with a subsequent solidification step
and/or drying/freeze-drying. The porous form with its interconnected pores
provides efficient mass transport, cell permeation and interstitial fluid flow. Such
porous structures allow tissue growth in vitro or in-growth in situ [
33
,
70
,
72
,
74
,
76
,
77
,
81
,
84
,
88
,
105
-
109
].
• Hydrogels—3D networks of hydrophilic polymers that absorb a large quantity
of water as well as biological fluids, prepared by physical or covalent cross-
linking of the polymer. Due to their aqueous nature, hydrogels simulate the
hydrated structural aspect of native ECMs. 3D cell encapsulation can be
achieved through in-situ formation of materials around stem cells [
82
,
83
,
86
,
104
,
110
,
111
].
• Nanofibrous scaffolds—prepared by self-assembly of amphiphilic peptides or
by electro-spinning. The nanofibrous structure mimics the cell environment,
comprised of complex network of ECM molecules with nano-micro scale
dimensions [
112
-
115
].
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