Biomedical Engineering Reference
In-Depth Information
(4) to have mechanical properties appropriate to withstand any
in vivo
stress. It is important to bear in
mind that the last requisite is difficult to combine with the high porosity of the material. That is why
it is necessary to use polymeric matrices with special or reinforced properties, especially if the poly-
mer is a hydrogel.
The design of polymeric scaffolds depends on the types of applications, but in any case it must
achieve structures with the aforementioned characteristics, which are necessary to their correct func-
tion. Success in doing that is conditional upon two factors which are the materials to be used includ-
ing both the porogen and the reticulate polymer (which is infiltrated in the porogen to become a
scaffold) and the internal and external architectures as shown by the scaffold porosity, (high sur-
face area/volume ratio), geometry, and pore size. Different processing techniques have been devel-
oped to design and fabricate three-dimensional scaffolds for tissue engineering implants
[4]
. These
techniques include phase separation, gas foaming, fiber bonding, photolithography, solid free form
(SFF), and solvent casting in combination with particle leaching. However, none of these techniques
have achieved a suitable model of three-dimensional architecture so that the scaffolds can fulfill
their desired aims. Using phase separation, a porous structure can be easily obtained by adjusting
the thermodynamic and kinetic parameters. However, because of the complexity of the processing
variables involved in the phase-separation technique, the pore structure cannot be easily controlled.
It is also difficult to obtain large pores and these may exhibit lack of interconnectivity. Gas foaming
has the advantage of room temperature processing but produces a highly nonporous outer layer and
a mixture of open and closed pores within the center, leaving incomplete interconnectivity. The main
disadvantage of the gas foaming method is that it often results in a nonconnected cellular structure
within the scaffold. Fiber bonding provides a large surface area for cell attachment and a rapid dif-
fusion of nutrients in favor of cell survival and growth. However, these scaffolds, as the ones used
to construct a network of bonded polyglycolic acid (PGA), lack the structural stability necessary
for
in vivo
use. In addition, the technique does not lend itself to easy and independent control of
porosity and pore size. Photolithography has also been employed for patterning, obtaining structures
with high resolution, although this resolution may be unnecessary for many applications of pattern-
ing in cell biology. In any case, the disadvantage of this technique is the high cost of the equipment
need, resulting in limited applicability. SFF scaffold manufacturing methods provide excellent con-
trol over scaffold external shape, and internal pore interconnectivity and geometry, but offer lim-
ited microscale resolution. Moreover, the minimum size of global pores is 100 μm. Additionally,
SFF requires complex correction of scaffold design for anisotropic shrinkage during fabrication.
Moreover, it needs high cost equipment. Finally, solvent casting in combination with particulate
leaching method, which involves the casting of a mixture of monomers and initiator solution and a
porogen in a mold, polymerization, followed by leaching-out of the porogen with the proper solvent
to generate the pores, is inexpensive but still has to overcome some disadvantages in order to find
engineering applications, namely the problem of residual porogen remains, irregular shaped pores,
and insufficient interconnectivity.
The proposed scaffolds may find applications as structures that facilitate either tissue regeneration
or repair during reconstructive operations. So far, three-dimensional materials with porous structures
have been designed for the cell scaffold from glycolide-lactide copolymer nonwoven fabrics, col-
lagen sponges, calcium phosphate ceramics, and PEG-based hydrogels
[5]
. Among them, hydroxy-
apatite (HAp) and β-tricalcium phosphate (β-TCP) have been intensively investigated as the scaffold
