Biomedical Engineering Reference
In-Depth Information
FIGURE 21.3
Poly(vinyl alcohol) (PVA) hydrogel.
H
C
CH
2
OH
n
FIGURE 21.4
Chemical structure of poly(vinyl alcohol).
increase their weight in water by the amount varying from 10% to 20% up to 1000 times. Naturally
derived hydrogel-forming polymers include collagen, chitosan, fi brin, gelatin, and HA. Synthetic
polymers that may be used to form hydrogels are poly(ethylene oxide) (PEO), poly(acrylic acid)
(PAA), poly(propylene fumarate-
co
-ethylene glycol) P(PF-
co
-EG), poly(hydroxy butyrate) (PHB),
poly(vinyl pyrrolidone) (PVP), and PVA [15-17].
Hydrogels as materials for tissue replacement have many advantages, such as physical prop-
erties similar to natural cartilage, good transport of nutrients to the cells and products from the
cells, biocompatibility, possibility of easy modifi cation with cell adhesion ligands, etc. They can be
injected
in vivo
as a liquid that gels at body temperature. However, one signifi cant disadvantage of
hydrogels is their low mechanical strength. For instance, Bray and Merrill [18] suggested, nearly
33 years ago, that PVA (Figure 21.4) might be a suitable material for synthetic AC applications.
However, there was an 85% failure rate after 150 h from seven tests on a hip joint simulator.
One of the ways to improve the mechanical properties of PVA hydrogels is by the addition of
a large amount of cross-linking agents. For hydrogels with hydroxyl or amino groups in the main
chain, the most popular cross-linking agents are glutaraldehyde, acetaldehyde, and formaldehyde.
Their reactions with polymer occur in the presence of sulfuric acid, acetic acid, or methanol [19-
21]. For instance, the use of glutaraldehyde as cross-linking agent resulted in hydrogels of lower
water contents (50%) with improved mechanical properties [22]. However, when the cross-linking
density is altered, the swelling rates are reduced [23]. Because of the toxicity of these compounds
that strongly infl uences the cell growth, some alternatives have been developed. Hydrogels were
also obtained using gamma radiation in the presence of succinic or citric acid [24]. Other examples
of nontoxic cross-linking agents are enzymes such as transglutaminase [25].
