Biomedical Engineering Reference
In-Depth Information
a
b
Fig. 20 (a) Chemical structures of poly[Glc-Asn( N -isopropyl)]. (b) Photographs showing
temperature-responsiveness of poly[Glc-Asn( N -isopropyl)] 5% aqueous solution at below
( upper ) and above ( lower ) the LCST. [ 304 ]
Iwasaki and coworkers have synthesized temperature-responsive polyphosphoesters
by ROP of two cyclic phosphoester monomers, ethyl ethylene phosphate (EP) and
isopropyl ethylene phosphate (IPP) [ 40 ]. The obtained copolymers, poly(IPP- co -EP)s,
showed reversible LCST-type temperature-responsiveness and the LCST linearly
increased with an increase in the composition of IPP, indicating that the LCST of poly
(IPP- co -EP)s can be controlled to physiological temperatures. Polyphosphoesters
have been known to be degraded through enzymatic digestion of phosphate linkages
under physiological conditions [ 305 ]. Thus, these properties indicate that poly(IPP-
co -EP)s have potential as biodegradable smart biomedical materials.
8.2 Biodegradable Injectable Polymers
The temperature-responsive biodegradable polymers mentioned above are
homopolymers or random copolymers exhibiting LCST-type phase transitions.
Other types of temperature-responsive biodegradable polymers having amphiphilic
block- or graft-type structures have been also reported [ 111 , 127 , 137 , 306 - 321 ].
Some of these are reported to show temperature-responsive sol-gel type transitions,
not soluble-insoluble-type transitions, and are called thermo-gelling polymers.
They form a physically crosslinked hydrogel through noncovalent interactions,
such as hydrophobic interactions, in aqueous solution triggered by a temperature
change. Biodegradable thermo-gelling polymers with a sol-gel transition point
between room temperature and body temperature are useful for injectable polymer
(IP) systems in biomedical applications [ 127 ] because the polymer solution is in a
sol state in a syringe at room temperature but then becomes a hydrogel in situ after
injection into the body. Furthermore, they can be degraded into metabolizable
monomers and/or low molecular weight water-soluble polymers, which can be
excreted through the kidney.
IPs are very useful for DDS because IP systems allow easy entrapment of
pharmaceuticals and bioactive agents and can be used as a depot for their sustained
release after a simple injection with a syringe at the target site in a human
body. Such a system can minimize the requirement for surgical operations for
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