Biomedical Engineering Reference
In-Depth Information
NH(CH
2
CH
2
O)
11
CH
3
NHCH(CH(CH
3
)CH
2
CH
3
)COOEt
NH(CH
2
CH
2
O)
11
CH
3
NHCH(CH(CH
3
)
CH
2
CH
3
)COOEt
P N
P
N
P N
P
N
n
n
NHCH(CH(CH
3
)
CH
2
CH
3
)COOEt
NHCH
2
CH
2
OCOCR
=
CH
2
NHCH
2
CH
2
SH
NHCH(CH(CH
3
)CH
2
CH
3
)COOEt
(20)
(21) R = H
(22) R = CH
3
NHCH
2
CH
2
OCOC
(
CH
3
)
=
CH
2
NH(CH
2
CH
2
O
)
11
CH
3
NHCH
(
CH
(
CH
3
)
CH
2
CH
3
)
COOEt
P N
P
N
P
N
n
NHCH(CH(CH
3
)CH
2
CH
3
)COOEt
NHCH
2
COOCH(CH
3
)COOEt
NH(CH
2
CH
2
O
)
11
CH
3
(23)
Fig. 6
Functional biodegradable thermogelling poly(organophosphazenes)
20
and
21
was investigated [
135
]. Thus, the leaching problems caused by the use
of crosslinkers with low molecular weight can be avoided. However, a prolonged
crosslinking time up to 4 h was observed for the polymer blending system, com-
pared with the use of small crosslinkers (40 min).
Beside the further crosslinking with multi-thiol containing crosslinkers, the
thermosensitive hydrogel bearing acrylate or methacrylate side groups can be
crosslinked upon UV radiation under mild conditions, resulting in the formation
of compact three-dimensional networks with properties of mechanically suitable
strength and controllable biodegradation for injectable biomedical applications
[
136
]. The hydrophobic interaction developed between the hydrophobic IleOEt
and methacrylate in polymer
22
facilitated the rapid dual crosslinking accom-
plishment of the photo-crosslinking at 37 ºC, leading to a fully crosslinked gel
in 5 min of exposure. The resultant dual crosslinked hydrogels showed improved
mechanical properties with several folds from the in vivo degradation stud-
ies. The reported methacrylate-based system is more attractive compared to the
thiol-based system because of its long-term stability and no crosslinker needed.
Furthermore, the amount of photoinitiator, intensity of UV light, and time of
exposure used were much less compared to the other reported photo-crosslinking
systems. Later, another rapid photo-crosslinkable thermoresponsive injectable
poly(organophosphazene) hydrogels were investigated by the presence of acrylate
functional groups on the polymer backbone (
21
) [
138
]. Photocrosslinking was
accomplished with even shorter UV exposure time, which was 120 and 180 s for
in vitro and in vivo studies, compared with methacrylate-based system (5 min)
owning to the higher reactivity of the acrylate double bond. Thus, the rapid pho-
tocrosslinking in the acrylate containing hydrogel enabled this system to be suit-
able for developing the carrier material for biomedical applications to avoid the
possible damage of loaded cells or bioactive components from long UV expo-
sure. The in vivo degradation study showed that the degradation rate of the dual
crosslinked hydrogels was mainly affected by the type and degree substitutions of
the side groups [
137
]. The incorporation of depsipeptide and long PEG chains as
in polymer
23
accelerated the polymer degradation. The rate of polymer degra-
dation in the initial period was depended on the degree of crosslinking, and later
on the amount of depsipeptide and PEG chain lengths in the polymer networks.
These results suggested that the mechanical property and degradation rate of the
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