Biomedical Engineering Reference
In-Depth Information
the carboxylic acid groups on polymer
19
through an amide linkage. The rab-
bit mesenchymal stem cells (rMSCs) on the conjugate hydrogels were shown to
express markers for all stages towards osteogenesis, indicating the beginning of
the maturation process. Both significantly high mineralization level for calcium
contents and high expression of collagen type I were detected after 4th week. Thus
the authors suggested that the poly(organophosphazene)-RGD conjugate holds a
promise for cell delivery material to induce osteogenic differentiation for bone tis-
sue engineering.
4 Dual Crosslinkable Biodegradable
Poly(Organophosphazenes) Thermogels
One of the important aspects of poly(organophosphazenes) over other polymers
lies on the ease of control of final properties by either introducing additional
functionalities during macromolecular substitution reactions or secondary
modification of side groups on the backbone. Thus, properties can be finely
tuned with respect to different applications. Examples of biodegradable ther-
mogelling poly(organophosphazenes) with additional functionality are shown
in Table
4
.
Chemically crosslinkable thermogelling poly(organophosphazenes) containing
multiple thiol groups (
20
) shown in Fig.
6
were synthesized as injectable mate-
rials for biomedical applications [
134
]. In addition to the thermogelling behav-
iour of the polymer aqueous solution at body temperature, the gel strength can
be further improved by the crosslinking of thiol groups with crosslinkers, such as
divinyl sulfone and PEG divinyl sulfone under physiological conditions to form
dual crosslinked hydrogels. The chemically crosslinked thermogel exhibited both
enhanced storage modulus and decreased swelling ratio due to the increased
crosslinking density. In the in vivo degradation study, the physically crosslinked
gel degraded faster, compared to the dual crosslinked gels. Also, the gel strength
can be controlled through the crosslinking degree. As a result, this chemically
crosslinkable thermogel system has a unique advantage on degradability adjust-
ment, tunable gelation properties, and controllable chemical and physical hydrogel
network. Another dual crosslinkable poly(organophosphazene) hydrogel, contain-
ing acrylate groups (
21
) with improved structural properties was developed by the
use of star-shaped multi-thiol crosslinkers, such as eight arm PEG thiol, compared
with linear dithiol, PEG dithiol [
139
]. From FE-SEM, dual crosslinked hydrogel
showed a sponge-like microporous structure with a relatively non-uniform pore
size distribution in the matrix. The gels obtained by the physical and chemical
crosslinking with star-shaped eight-arm thiols showed superior gel properties com-
pared to gels with linear thiols. Through the nature of the crosslinking molecule
and crosslinking density, the mechanical properties, swelling behaviors, three-
dimensional inner networks and degradation rates can be controlled. Based on
the similar concept, self-crosslinkable polymer blending system between polymer
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