Biomedical Engineering Reference
In-Depth Information
degradation products should produce chronic inflammatory responses when
implanted. The degradation rate of the scaffold should correspond to the rate of
new extracellular matrix production [80]. In the following section of this
chapter, based on these criteria, nanostructured biomaterial scaffolds and
associated nanotechniques have been explored to fabricate the next generation of
improved cartilage materials.
9.3.2 Nanostructured biomaterial scaffolds
Synthetic and naturally derived hydrogels
The cartilage extracellular matrix has higher water content, no ceramic com-
ponent and consequently lower stiffness compared with bone. Therefore, instead
of using metals or ceramics, polymers are currently being considered with high
water content, i.e. hydrogels, and are one of the most popular materials for
regenerating the cartilage extracellular matrix. Hydrogels are a network of
polymer chains, sometimes found as a colloidal gel in which water is the
dispersion medium. Due to their high water content, hydrogels possess a degree
of flexibility very similar to natural tissue. In addition to hydrogels, PCL
(polycaprolactone) and PLGA have been widely investigated as cartilage
forming materials. PLGA degrades via hydrolysis in which ester bonds are
broken in the long polymer chains, gradually degrading in the physiological
environment, and eventually cleared by human body (Fig. 9.10) [81, 82]. Poly-
mers that degrade via hydrolysis have a close relationship with molecular weight
and degree of polymerization (DP). Generally speaking, lower molecular
weight, higher surface area and greater surface energy allow nanostructured
hydrogels to have higher hydrolysis rates than conventional hydrogels [82].
Another group of hydrogels tested for cartilage applications belongs to the
polysaccharide family, just like glycosaminoglycans (one of the major
components in cartilage), alginate, hyaluronic acid and agarose. They do not
have ester bonds which undergo hydrolysis and usually require a longer time to
degrade.
Just like in orthopedic applications, cartilage applications can benefit from
the low-inflammatory response of nanomaterials (especially if blood is exposed
in the degraded underlying bone) [14, 83] and the excellent select protein
adsorption capability important for mediating chondrocyte adhesion and
functions [12]. Moreover, nanofiber hydrogels have been receiving more and
9.10 Mechanisms of the hydrolysis of ester bonds during the degradation of
polymers.
