Biomedical Engineering Reference
In-Depth Information
defect shape, have predetermined mechanical proper-
ties, and induce poor vascularization. Therefore, inject-
able biomaterials such as collagen, fibrin, gelatin, alginate,
HAc, chitosan, and PEG gels emerged as candidates for
scaffolds in the replacement of large-size tissue defects.
Injectable hydrogel materials are advantageous because
they require a minimally invasive procedure and conform
to irregular shapes. However, they are mechanically
weaker than soft tissues and many cells perform poorly
when suspended within a bioinert hydrogel.
Fascia
Dura mater
7.2.4.3 Soft, elastic scaffolds
Pericardium
For soft tissue engineering, synthetic elastomeric mate-
rials with tunable degradation properties would be
preferable. For instance, for cardiovascular applications,
elastomeric behavior with low modulus would allow the
transmission of stresses to seeded or infiltrating cells
early in the implant or culture period. Such mechanical
training has been shown to be important in developing
mechanically robust tissues with appropriate cellular
orientation. The PGA, PLA, and their copolymers
(PLGA) are relatively stiff and nonelastic and are not
ideally suited for engineering of soft tissues under a me-
chanically demanding environment such as cardiovascular,
urological, and gastrointestinal tissue, unless specifically
processed. Mechanical signals are thought to be necessary
for developing the cell alignment that leads to tissue
structure with correct biomechanical properties and
functions. Elastic scaffolds may be essential for the en-
gineering of any tissue under conditions of cyclic me-
chanical strain. This is the reason for synthesis of
absorbable and pliable PUs.
Fig. 7.2-14 Orientation of collagen fibrils in connective tissues.
7.2.4.1.2 Fibrin gel
The advantage of fibrin gels over the other gels is that it
can be obtained autologous. Furthermore, cells entrapped
in the fibrin gels were reported to produce more collagen
and elastin than those entrapped in collagen gel. Fibrin
degrades within several days by cell-associated enzymatic
activities when no degradation inhibitors are used. In
tissue engineering applications using cells encapsulated in
fibrin gel, degradation inhibitors are often used to pre-
serve the scaffold function of the fibrin. The effect of the
inhibitor concentration on collagen formation in the gels
differs among studies.
7.2.4.1.3 Matrigel
Matrigel is commercially available from BD Biosciences
(San Jose, CA, USA). This gel is a basement membrane
preparation extracted from Engelbreth-Holm-Swarm
mouse sarcoma and solubilized in Dulbecco's modified
Eagle's medium (DMEM). This contains several com-
ponents of basement membranes enriched with LN.
Matrigel is liquid at 2-8
C and sets to a gel rapidly at
22-35
C.
7.2.4.4 Inorganic scaffolds
The most abundant bioceramic HAp can be used as
a scaffold for bone regeneration because HAp will be
integrated in the regenerated bone so far the shape and
size is acceptable, although the absorption rate is quite
low unless osteoblast frequently attacks. However, HAp
scaffolds whose pores are not interconnected but inde-
pendent are not adequate for tissue engineering. Con-
tinuous pore interconnection is a prerequisite as scaffold.
Modification of marine coral into HAp possessing porous
structure suitable for tissue engineering has been applied
for scaffold fabrication, but the use of coral-derived HAp
should be refrained if the coral harvest violates the
Washington Treaty. In contrast to HAp, b-tricalcium
phosphate (b-TCP) exhibits rapid degradation and hence
is a good candidate for inorganic scaffolds. To serve as the
scaffold in which bone ingrowth and vascularization take
place, b-TCP should be highly porous, but this inorganic
7.2.4.1.4 Marine natural scaffold
Some natural biomaterials including coral, sea urchins,
and marine sponges are much less expensive than the
natural ECMs and appear to provide affordable, readily
available scaffolds with a number of unique and suitable
properties such as open interconnected channels.
7.2.4.2 Injectable scaffolds
Large-scale, prefabricated scaffolds require invasive sur-
gery for implantation, are difficult to contour to the
