Biomedical Engineering Reference
In-Depth Information
The hydrogels are good candidates for tissue applications when their elastic moduli
are close to that of natural tissue components. For instance, articular cartilage
contains
70% water and bears loads up to 100 MPa, but most hydrogels, either
synthetic or natural, can be easily broken indicating that they are much weaker
than native cartilage tissue. The degradable elastic polyurethane hydrogels have
elastic moduli ranging from 16.8
∼
3.9 MPa, which are very close to
the properties of native cartilage showing promise for soft- and hard-tissue regen-
eration [17] .
For engineering of soft tissue, elastic hydrogel scaffolds are desirable since they
are amenable to mechanical conditioning regimens that might be desirable during
tissue development. Elasticity values of most of the single component hydrogels
were lower than 10 kPa, while higher percentage of multicomponent hydrogels
exhibited high elastic mechanical property up to 100 kPa [3]. The compressive
modulus of hard tissue such as articular cartilage is in the range of 0.53- 1.82 MPa
[27]. In order to promote cartilage regeneration, a hydrogel scaffold must be able
to exhibit mechanical integrity in the face of loading from the body, while at the
same time guide appropriate cartilaginous tissue growth. A biodegradable hydro-
gel scaffold with elastic properties could be useful for application in cartilage
treatment.
±
3.3 to 26.6
±
9.2
Synthesis of Elastic Hydrogels
9.2.1
Chemical Elastic Hydrogels
Chemical hydrogels are those that have covalently crosslinked networks. Thus,
chemical hydrogels will not dissolve in water or other organic solvents unless
covalent crosslinks are cleaved. There are generally two different methods to
prepare chemical elastic hydrogels. Chemical elastic hydrogels can be prepared by
polymerization of water-soluble monomers in the presence of bi- or multifunc-
tional crosslinking agents. Chemical hydrogels can also be prepared by crosslink-
ing water-soluble polymers using chemical reactions that involve functional
groups of the polymer. Due to the high strength of the covalent linkages, the
three-dimensional networks of hydrogels are permanent and the formation of
crosslinks is usually irreversible.
9.2.1.1
Polymerization of Water-Soluble Monomers in the Presence
of Crosslinking Agents
Polymerization of water-soluble monomers in the presence of crosslinking
agents results in the formation of chemical hydrogels. Typical water- soluble mono-
mers for the preparation of chemical elastic hydrogels include acrylic acid, AM,
hydroxyethyl methacrylate, and so on. The crosslinking agents for the synthesis
of elastic hydrogels are not only low-molecular-weight agents such as
N
,
N
′
methylenebisacrylamide but also inorganic agents such as hectorite clay.
