Biomedical Engineering Reference
In-Depth Information
Polymerization
Free-radical polymerization is widely used to develop scaffolding microstruc-
tures. Materials for free-radical polymerization are composed of monomers
and initiators. As the building blocks for microstructures, monomers have one
or more crosslinking groups, which can be activated by free radicals and forms
covalent binding between the monomers. The monomers can be either syn-
thesized materials or derivatives from the aforementioned natural materials.
For example, gelatin and hyaluronan were modified by methacrylate groups
and became crosslinkable gels [68, 69]. On the other hand, initiators generate
the free radicals in response to specific stimulations, such as pH value, heat,
and irradiation. For example, azobisisobutyronitrile (AIBN) decomposes and
produces free radical 2-cyanoprop-2-yl at elevated temperature (above 50°C)
[70]. As well, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone (Irgacure
2959), which is a commonly used photoinitiator for tissue engineering, pro-
duces free radicals upon irradiation with ultraviolet light [71]. Among the vari-
ous initiators, photoinitiators are mostly used for making three-dimensional
tissue-engineering scaffolds because of the convenience and non-invasive
process by irradiation [72].
Enzyme-catalyzed polymerization of proteins is another method for creat-
ing solid microstructures from biomaterials. In the mechanism to form a blood
clot, for example, fibrinogen molecules crosslink and become solid fibrin gel in
the present of thrombin [73]. This type of gelation has been used for cell-culture
study and was applied to clinical application as “tissue glue” [74, 75]. Protein
polymerization also takes place in the present of synthetic catalysts. For exam-
ple, in a light-activated state, ruthenium trisbipyridyl chloride [RuII(bpy
3
)
2+
]
catalyzes the formation of dityrosine bonds between tyrosine-abundant pro-
teins, such as fibrinogen, and converts the peptides into a solid gel [76, 77].
Charged bio-polymers, such as chondroitin sulfate, alginate, and hyaluronan,
can be electrostatically crosslinked by oppositely charged ions [78]; to crosslink
alginate, for example, calcium ions are used. Ionic crosslinking can also become
photo-controllable. Photoliable chelators for calcium, such as DM-nitrophen
(a brand name for 1- (2-nitro-4,5-dimethoxyphenyl)- N,N,N',N'-tetrakis
[(oxycarbonyl) methyl]-1,2-ethanediamine), were used as “calcium cages”
to temporarily bind to calcium ions and neutralize the ions' charges; upon
irradiation, however, these chelators degrade and release the calcium ions,
which then crosslink the charged polymers [79].
Microfabrication Platforms
Scanning Laser Stereolithography (SLS)
SLS by Continuous-Wave (CW) Laser
The main components of a SLS microfabrication platform include a colli-
mated laser and an optical lens; the lens focuses the laser beam in a bath of
photocrosslinkable monomer; at the focal point the monomer solidifies and
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