39
tosensitive groups into the gel network, such as azobenzenes, photoionization and
decomposition of monomers (such as triphenylmethane derived hydrazine) or other
photosensitive molecules (such as cinnamic acid) (Fig. 3.5). In this sense, the photosensitive groups in the gel network are crosslinked, photoisomerized or photodissociated, due to ultraviolet (UV) radiation, which leads to an increase in local temperature
or a configuration change in the polymer chains. The gel is then swollen, thus exhibiting the shape memory phenomenon (Lendlein et al. 2005; Song et al. 2009; Sun
et al. 2009; Tomatsu et al. 2005). Light-induced SMGs as compared to thermo-sensitive SMGs, the light stimulation signal could not only be transmitted immediately
to the detection site, but also the amount of signal can be well controlled. Azobenzene
is a typical compound with photoisomerization property.
Peng et al. (2010) reacted functionalized dextran (Dex) with functionalized azobenzene (FA) and cyclodextrin (CD) to produce two products, which were named
FA-Dex and CD-Dex, thus obtaining a light-induced SMG via a ‘click’ chemistry
reaction. Thereby, azobenzene in the molecular chain would change from trans
structure to cis structure under UV irradiation. From the macroscopic view, the
hydrogel changed from a gel state to a sol state in order to release the drug.
Yamaguchi et al. (2012) also reported a photocontrolled hydrogel based on PAAm
chemically crosslinked with azobenzene, which was able to be assembled or disassembled using CD under UV irradiation conditions.
3.4.1.3 Ultrasound-Induced SMGs
Ultrasound is another important stimulant for SMPs, as high intensity ultrasound
can facilitate the shape recovery of SMPs through special energy converters. Li
et al. (2015) prepared PVA/melamine-based SMGs by means of the freeze-thaw
method, which could respond to the ultrasound produced by the therapeutic ultrasound system. The introduction of melamine hardened the resulting PVA hydrogel,
due to hydrogen (H)-bonding interactions between melamine and PVA, thus providing a stable polymer skeleton for the shape memory performance of the hydrogel system. This type of hydrogels could be implanted in the blood vessels, being
controlled by ultrasound harmless to humans, as well as they could also be used for
aerospace applications.
3.4.1.4 Redox-Induced SMGs
Redox reactions can also be used as an external driving force to obtain SMGs.
Redox-induced SMGs show excellent cyclical shape memory behavior under ambient conditions. In addition, a triggering stimulus could induce multi-responsive
SMGs, since reversible interactions are multi-response (Le et al. 2016; Xiao et al.
2013, 2016).
As for driving force, host-guest interactions are vital for constructing redox
induced SMGs. With this in mind, Miyamae et al. (2015) used two different types
3 Smart and Shape Memory Polymers
tosensitive groups into the gel network, such as azobenzenes, photoionization and
decomposition of monomers (such as triphenylmethane derived hydrazine) or other
photosensitive molecules (such as cinnamic acid) (Fig. 3.5). In this sense, the photosensitive groups in the gel network are crosslinked, photoisomerized or photodissociated, due to ultraviolet (UV) radiation, which leads to an increase in local temperature
or a configuration change in the polymer chains. The gel is then swollen, thus exhibiting the shape memory phenomenon (Lendlein et al. 2005; Song et al. 2009; Sun
et al. 2009; Tomatsu et al. 2005). Light-induced SMGs as compared to thermo-sensitive SMGs, the light stimulation signal could not only be transmitted immediately
to the detection site, but also the amount of signal can be well controlled. Azobenzene
is a typical compound with photoisomerization property.
Peng et al. (2010) reacted functionalized dextran (Dex) with functionalized azobenzene (FA) and cyclodextrin (CD) to produce two products, which were named
FA-Dex and CD-Dex, thus obtaining a light-induced SMG via a ‘click’ chemistry
reaction. Thereby, azobenzene in the molecular chain would change from trans
structure to cis structure under UV irradiation. From the macroscopic view, the
hydrogel changed from a gel state to a sol state in order to release the drug.
Yamaguchi et al. (2012) also reported a photocontrolled hydrogel based on PAAm
chemically crosslinked with azobenzene, which was able to be assembled or disassembled using CD under UV irradiation conditions.
3.4.1.3 Ultrasound-Induced SMGs
Ultrasound is another important stimulant for SMPs, as high intensity ultrasound
can facilitate the shape recovery of SMPs through special energy converters. Li
et al. (2015) prepared PVA/melamine-based SMGs by means of the freeze-thaw
method, which could respond to the ultrasound produced by the therapeutic ultrasound system. The introduction of melamine hardened the resulting PVA hydrogel,
due to hydrogen (H)-bonding interactions between melamine and PVA, thus providing a stable polymer skeleton for the shape memory performance of the hydrogel system. This type of hydrogels could be implanted in the blood vessels, being
controlled by ultrasound harmless to humans, as well as they could also be used for
aerospace applications.
3.4.1.4 Redox-Induced SMGs
Redox reactions can also be used as an external driving force to obtain SMGs.
Redox-induced SMGs show excellent cyclical shape memory behavior under ambient conditions. In addition, a triggering stimulus could induce multi-responsive
SMGs, since reversible interactions are multi-response (Le et al. 2016; Xiao et al.
2013, 2016).
As for driving force, host-guest interactions are vital for constructing redox
induced SMGs. With this in mind, Miyamae et al. (2015) used two different types
3 Smart and Shape Memory Polymers