Biomedical Engineering Reference
In-Depth Information
36 CHAPTER 3.
IN VITRO
TISSUE ENGINEERING
acid-based materials, and reconstituted tissue matrices. Each material has strengths and weaknesses
to its use, and results can vary depending on the application.
Alginate is a polysaccharide extracted from algae and can be used to encapsulate cells in a
three-dimensional matrix. Encapsulation maintains a chondrocyte's rounded morphology, which has
been shown to induce re-differentiation of monolayer expanded cells [
318
]. This approach can also
be applied when differentiating stem cells along the chondrocytic lineage. Besides encapsulation,
one of the main advantages of alginate is its proven biocompatibility [
319
]. For sterile applications,
alginate can be purified by filtration, precipitation, or extraction. However, alginate is not an ideal
material for many tissue engineering applications. The material does not degrade rapidly
in vivo
,
which can interfere with new tissue growth. Long-term implants encounter problems with alginate
since the scaffold loses its integrity within a year [
319
].
Agarose is a polysaccharide derived from seaweed that exhibits temperature-sensitive solubility
in water, an attribute convenient for encapsulating cells [
319
]. Similar to alginate, agarose provides
a biocompatible, three-dimensional environment for culturing chondrocytes. Unfortunately, the
degradation properties of agarose are similar to alginate and cannot be easily altered to tailor the
life of the scaffold. In addition, it is unclear whether agarose is eventually degraded or removed as
the cells make matrix. Despite these deficiencies, many
in vitro
studies use agarose as a scaffold
material when investigating the effects of mechanical stimuli on chondrocytes [
320
,
321
]. Since
it is a continuous, hydrogel matrix, applied mechanical forces are transmitted to the embedded
chondrocytes, stimulating them to produce extracellular matrix proteins [
322
].
Another common scaffold material used for cartilage tissue engineering is collagen, specifically
collagen type I or II. Collagen is the major component of extracellular matrix in connective tissues. As
with most other natural materials, collagen has to be processed before use to decrease its antigenicity.
Collagen type I scaffolds have facilitated cartilaginous tissue formation in studies investigating direct
compression [
323
] and cross-linked proteoglycans [
324
,
325
]. However, this material alone can also
result in dedifferentiation of chondrocytes [
319
], likely due to the fact that type II collagen, not
type I, is the predominant collagen in native articular cartilage. Cells seeded onto type II collagen
scaffolds show a retention of the chondrocytic phenotype [
326
]. Unfortunately, fabricating collagen
type II scaffolds is a difficult and expensive process compared to other natural materials due to its
limited availability.
Chitin is a semi-crystalline polymer derived from the exoskeleton of crustaceans. After
deacetylation, chitin is termed chitosan and is a natural biomaterial possessing a high degree of
biocompatibility
in vivo
[
319
]. The molecular structure of chitosan is similar to many glycosamino-
glycans, allowing it to interact with growth factors and adhesion proteins [
319
]. The degradation
of chitosan is controlled by the degree of deacetylation within the polymer, which can be altered
during processing of the original chitin material. Unlike the natural materials described previously,
chitosan scaffolds can degrade rapidly
in vivo
to allow space for the formation of new tissue [
319
].
The porosity of the biomaterial can also be controlled during processing, effectively modulating
the overall strength and elasticity of the scaffold [
327
]. Oftentimes, chitosan is combined with
