Environment–Stimuli Response of Chitosan-Based Hydrogels - Chitosan Based Hydrogels: Functions and Applications - page 250

potential applications. However, some problems remain to be addressed, such as the
possibility to prepare graft-copolymers with precise MW and MW distribution in the
absence of chitosan-g-PNIPAAm. And some research found that acrylamide-based poly-
mers, in general, are not suitable for biomedical applications due to cell toxicity. The
presence of unreacted monomeric residues with acrylamide-based polymers may be
responsible for their toxicity. The observation that acrylamide-based polymers activate
platelets when in contact with blood, along with the poorly understood metabolism
of PNIPAAm and its nondegradability, makes it difficult to win FDA approval [69].
Therefore, it can be expected that in the next few years an improvement of graft copoly-
merization of chitosan leads to the preparation of smart polymers based on chitosan
with excellent biocompatibility.

5.3.2.3 Chitosan/PNVCL Hydrogels

Poly( N -vinylcaprolactam) (PNVCL) is one of several nonionic water-soluble polymers with
thermosensitivity. Compared with PNIPAAm, PNVCL is nontoxic because it does not pro-
duce small amide derivatives upon hydrolysis. Aqueous PNVCL gels undergo two heat-
induced transitions: a low-temperature transition at 31.5°C, attributed to microsegregation
of hydrophobic domains, and a higher-temperature transition around 37.5°C, correspond-
ing to the gel volume collapse itself [70]. Gong and coworkers [71] grafted carboxyl-
terminated poly( N -vinylcaprolactam) (PNVCL-COOH) chains onto a chitosan backbone
and found that its aqueous solution shows a temperature-dependent transmittance change
due to the introduction of thermosensitive PNVCL graft chains. And the LCST value of
chitosan-g-PNVCL is ca. 32°C, which is the same as that for pure PNVCL-COOH ( cf.
Figure 5.20). Above this temperature, disruption of hydrogen bonding with water and
increasing hydrophobic interaction among caprolactam groups [72]. In addition, the ther-
mosensitivity is influenced by the pH of the medium, and the phase transmittance in acidic
medium is slightly higher than that in neutral or basic medium.

100

PNVCL-COOH

Chitosan-g-PNVCL

80

60

40

20

0

20 25 30 35

Te mperature (°C)

40 45 50

Figure 5.20
Optical transmittance of pure PNVCL-COOH (3% w/v) and chitosan-g-PNVCL copolymer (3% w/v) at pH 7.4.