Biomedical Engineering Reference
In-Depth Information
modulated-release systems. h e drug release is controlled by external stim-
uli such as temperature, pH, ionic strength, electric and electromagnetic
i eld, light, etc. h erefore, polymers that respond to these external stimuli
can be used as controlled-release devices.
Delivery applications of smart polymers have already generated an
overwhelming collection of articles. h e micelles are the most trivial
structures used for the entrapment and subsequent release of small hydro-
phobic molecules. However, the use of classic micellar structures is lim-
ited to the encapsulation of hydrophobic drugs in the core. A number of
other polymeric nanostructures have shown great potential in drug deliv-
ery, including smart surfaces, dendrimers, and in situ forming nanogels.
Stimuli-responsive polymers can mimic biological systems in response to
small changes in environmental stimuli, such as, temperature, pH, ionic
strength, etc. h is also includes a combination of several stimuli at the
same time. Stimuli-responsive polymers are also called “smart, “intelligent”
or “environmentally sensitive” polymers [9]. One important feature of this
type of material is reversibility, i.e. the ability of the polymer to return to
its initial state upon application of a counter-trigger. h ese unique charac-
teristics are of great interest in drug delivery, cell encapsulation and tissue
engineering [10]. h e most important systems from a biomedical point of
view are those sensitive to temperature and/or pH of the surroundings.
h ey present a i ne hydrophobic-hydrophilic balance in their structure.
pH-responsive polymers are important due to the human body exhibits
variations of pH along the gastrointestinal tract, and also in some spe-
cii c areas like certain tissues (and tumoral areas). In the case of the tem-
perature-responsive polymers, a small change around a critical solution
temperature make the polymeric chains collapse or extend, responding
to adjustments of the hydrophilic and hydrophobic interactions between
them and the aqueous medium [11, 12].
2.3.1 Temperature-responsive Polymers
Temperature is the most widely utilized triggering signal for a variety of
drug delivery systems. h is is due to the fact that the human body temper-
ature ot en deviates from the physiological value (37
C) in the presence of
pathogens or pyrogens. h is deviation can be a useful stimulus to activate
release of therapeutic agents from various temperature-responsive drug
delivery systems for diseases accompanied by fever. Drug-delivery sys-
tems responsive to temperature utilize various polymer properties, includ-
ing the thermally reversible transition of polymer molecules and swelling
change of networks [13].
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