Biomedical Engineering Reference
In-Depth Information
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
4
5
6
7
8
9
Number of cycles
FIGURE 21.8
Compressive stress as function of freezing-thawing cycles: (
■
) 25% PVA, (
●
) 25% PVA 3%
PVP, (
◆
) 25% PVA 3% succinic acid, and (
▼
) 25% PVA 3% gluconic acid.
observed by Watase et al. [39], but they differ in shape. PVA cryogels prepared using succinic acid
are characterized by the ribbon-like and ordered structure than the honeycomb-like structure. After
introducing multifunctional bioactive molecules of carboxylic acid (e.g., gluconic acid) (Figure
21.7b) as a cross-linking agent, long and parallel fi brils, bridged through thin lamella, form porous
structure with a pore size less than 1 µm. In this case, pores of honeycomb-like structure are formed
during subsequent freezing-thawing cycles.
High internal porosity strongly infl uences the mechanical properties of cryogels as illustrated in
Figure 21.8. Materials with less ordered, mesh-like morphology obtained in a low number of freezing-
thawing cycles and difunctional molecules exhibit lower compressive stress as compared to samples
with multifunctional molecules subjected to higher number of freezing-thawing cycles. It has also
been found that prepared blends were characterized by elastic modulus of
E
c
=
7.2
-
14.1 MPa at 60%
strain, which is very well comparable to the properties of natural cartilage (
E
c
=
1.9
-
14.4 MPa by
30% of strain) [40].
Improvement in the characteristics of hydrogel could also be achieved by the addition of bio-
logical macromolecules. For this purpose, biopolymers such as collagen, found in the extracellular
matrix, can be employed [40].
Another type of emerging polymeric biomaterial used for cartilage repair is poly(ethylene
glycol) (PEG) hydrogel. PEG is widely used in many biomedical applications because of its com-
bination of outstanding physicochemical and biological properties such as hydrophilicity, lack of
toxicity, antigenocity, and immunogenocity [41]. PEG copolymers of ethylene oxide and propylene
oxide can be used as injectable matrices for chondrocytes transplantation [41].
Microporous poly(2-hydroxyethyl methacrylate) (poly(HEMA)) gels were also investigated for
use as replacement of AC [42]. The tensile strength of this material was almost 20 times lower than
that for AC, which reduced the possibility of its use as artifi cial cartilage.
Another material that has potential in cartilage replacement is a semi-interpenetrating polymer
(SIPN), which is synthesized from
N
-vinyl pyrrolidone-methyl methacrylate (copolymer) and rein-
forced with cellulose acetate butyrate (polymer) [43]. The tensile strength (10 MPa) and modulus
(90 MPa) of this SIPN correspond with the mechanical properties of natural cartilage. Additionally,
the elongation to break (120%) is greater than that of cartilage (80%). However, biocompatibility of
such a polymer is still not fully tested (tests are still in progress).
