Biomedical Engineering Reference
In-Depth Information
The other class of rapid prototyping methods,
particle-bonding techniques, imparts advanta-
geous features to the resulting bioscaffold. In gen-
eral, particle-bonding techniques involve the
selective bonding of particles into a thin 2D layer.
These 2D layers are then bonded one by one to
produce the desired 3D structure. Various sol-
vents have been used to bind the layers, including
water with complex sugars [56] and organic sol-
vents with synthetic polymers [56] . The technique
gives rise to macroporous and microporous struc-
tures, but the extent of the micropores is limited
by the size of polymer granules in the powder.
The primary advantage of particle-bond methods
over melt deposition methods is that the powder
causes the scaffold to have a rough exterior, which
has been shown to promote cell differentiation,
growth, and proliferation [56] . Particle-bonding
techniques also do not use heat, which allows for
various material types to be used.
The final rapid prototyping methods are
indirect methods. They take advantage of a
mold to produce a 3D porous scaffold. Indirect
methods involve casting material into a mold
that will create desired external and internal
structures. These methods are useful because
they require less raw material and a variety of
material blends can be accommodated. The
original material properties are maintained
throughout processing of the 3D scaffold. Ade-
quate microporous and macroporous structures
are produced [56, 83, 86] .
Hydrogel matrices are formed from a variety
of synthetic and natural hydrophilic polymers as
well as polymeric blends. The most common syn-
thetic polymers used include polyvinyl alcohol
(PVA), polyethylene glycol (PEG)/polyethylene
oxide, and poly-2-hydroxyethyl methacrylate.
Some natural polymers used to create hydrogels
are chitosan, alginate, hyaluronic acid, and
collagen [87-90] .
The chosen material can significantly
improve the function of the scaffold. To prevent
dissolution of the hydrophilic polymer,
crosslinking methods are needed. Some poly-
mers used for hydrogels are charged, which
serves to create a physical crosslink between
oppositely charged macromolecules. Chemical
methods involve covalently attaching a linker
molecule to polymer chains and then reacting
the chain-molecule complex with other macro-
molecular chains. Hydrogels can also be formed
by polymerizing monomers in the presence of
water [58, 87-90] .
The mechanical properties of hydrogels are
enhanced through the use of ceramics and
other blends. To make the scaffold more ame-
nable to cell adhesion, given its smooth topog-
raphy, a variety of peptide moieties may be
attached to the polymer. The RGD (arginine,
glutamic acid, aspartic acid) amino-acid
sequence is among the oligopeptides used to
mimic adhesion proteins that interact with cell
surface receptors. Control over hydrogel poros-
ity influences diffusion of material in and out
of the scaffolds and dictates the ability of cells
to infiltrate the scaffold [59] . For better control
over hydrogel porosity, 3D printing techniques
involving liquid-liquid bonding have been
used [91, 92] .
7.2.1.5 Hydrogels
Hydrogels represent the final class of 3D bioscaf-
folds discussed in this survey. Hydrogels are
ubiquitous in tissue engineering because they
closely approximate the ECM. These bioscaf-
folds can be synthesized from a variety of mate-
rials, are highly biocompatible owing to their
large water content (ranging between 70% and
90%), are processed under mild conditions, and
can be augmented to achieve desired mechani-
cal properties.
7.2.2 Nanofibrous Scaffolds
Once used almost exclusively for textiles,
nanofibers, , designated by the National Science
Foundation as fibers with diameters of 100 nm
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