Biomedical Engineering Reference
In-Depth Information
enough, to impart elastomeric behavior of the hydrogels [5]. Shape-memory hydro-
gels constitute a class of elastic hydrogels that can be elastically deformed and fi xed
into a temporary shape, and have ability to recover the original, permanent shape
on exposure to an external stimulus such as heat or light [6].
For most biomedical applications, biodegradable elastic hydrogels are favored
over nondegradable hydrogels. This is because they can be removed or eliminated
by natural degradation from the applied sites in the body under relatively mild
conditions, thus eliminating the need for any surgical removal processes after the
system fulfi lls its goal. Biodegradable polymeric systems also provide fl exibility in
the design of delivery systems for large molecular weight drugs, such as peptides
and proteins, which are not suitable for diffusion-controlled release through non-
degradable polymeric systems [7]. In addition, the degradation can be utilized to
control the rate of drug release and the physicochemical properties of the hydrogel
systems, and thus to provide fl exibility in the design of biomedical devices, such
as drug-biomaterials combination products. However, proper techniques for pre-
dicting hydrogel degradation rates are critical for successful application of these
degradable systems as they facilitate the design of implants with optimal degrada-
tion profi les that result in proper rates of drug release or tissue regeneration and
hence maximize therapeutic effects.
9.1.3
History of Elastic Hydrogels as Biomaterials
Earlier works in elastic hydrogels were mainly focused on development of shape-
memory hydrogels for fabrication of devices and implant stents. The fi rst publica-
tion mentioning shape-memory effects in hydrogels was made by Osada
et al.
in
1995, who discovered a new phenomenon of a polymer hydrogel made by radical
copolymerization of acrylic acid and
n
-stearyl acrylate having elastic memory that
could be stretched to at least 1.5 times of its original length when the swollen gel
is heated above 50 ° C [8, 9] . Since then, biodegradable shape - memory polymers
have been synthesized, including network polymers formed by crosslinking
oligo(
- caprolactone) dimethylacrylate and
N
- butylacrylate [10] , a multiblock copol-
ymer of oligo(
ε
- caprolactone) and oligo(
p
- dioxanone)diol [11] , and polyesters of
poly(propylene oxide) (PPO) with polylactide or glycolide [12]. Improvement of the
stiffness and recovery force of shape-memory polymers can be achieved by the
synthesis of shape-memory composites. Zheng
et al.
synthesized polylactide and
hydroxyapatite composites which demonstrated better shape-memory effect than
pure polylactide polymer [13].
Recently, with the increasing interest in engineering various tissues for the
treatment of many types of injuries and diseases, a wide variety of biodegradable
elastic hydrogels with desirable mechanical, degradation, and cytophilic properties
have been developed. Elastic superporous hydrogel hybrids exhibiting mechanical
resilience and a rubbery property in the fully water-swollen state have been
reported by Park
et al.
These hydrogel hybrids of acrylamide (AM) and alginate
could be stretched to about 2-3 times of their original lengths and could be loaded
ε
