Studies On Photoresponsive And Photocrosslinked Hydrogels Based On Polysaccharides

In recent years, environment-sensitive hydrogels have been widely investigated. Among them, light as an external stimulus to control structure and volume of the hydrogels has received increasing attention. Since the light is readily available, inexpensive, safe, clean and easily manipulated, light-sensitive hydrogels may possess special advantages over others and have been used in diverse applications, such as chemical valves, shape memory switches, drug delivery, immobilization of enzymes, and tissue engineering.Polysaccharides are an important class of natural polymers with good biocompatibility, biodegradability and low toxicity. As a result, they are widely used as functional materials in medical and pharmaceutical applications. In this work, photoresponsive and photocrosslinked hydrogels based on polysaccharides have been designed and synthesized. Moreover, their properties such as swelling, mechanical strength, photoresponse and drug release were investigated.The innovative points in this work are as follows. (1) Photo- and pH-responsive hydrogels based on azobenzene-containing dextran derivatives were prepared as a new carrier for controlled drug release. The release rate of drug could be modulated by 365 nm UV irradiation. (2) Azobenzene-modified amphiphilic amylopectin derivatives were synthesized and could self-aggregate into the nanogels by the hydrophobic association interactions of incorporated azobenzene groups for controlled drug release. (3) A novel water-soluble chitosan derivative having a nitrocinnamate moiety was synthesized and characterized. Different from the photocrosslinkable polysaccharide derivatives reported previously in the literature, this modified polysaccharide has an unique photogelation ability in the absence of potentially toxic photoinitiator or catalyst, which makes it to be suitable particularly for the preparation of photocrosslinked hydrogel biomaterials. (4) It has been firstly revealed that a novel amylopectin derivative with nitrocinnamate as pendant groups is suitable for the photoencapsulation and controlled release of lysozyme in the absence of photoinitiators. The secondary structures of the encapsulated lysozyme could be maintained and its biological activities with more than 83 % could be kept.The main contents and conclusions in this dissertation are as follows:(1) Studies on photo- and pH-responsive hydrogels based on dextranThe photo- and pH-responsive hydrogels were obtained from radical copolymerization of methacrylated dextran with 4-[(4′-acryloyloxy)phenylazo)]benzoic acid in dimethyl sulfoxide with 2,2′-azobisisobutyronitrile as an initiator. The FT-IR and 1H-NMR analyses were used to characterize the chemical structure of modified dextran, azobenzene monomers and their hydrogels, and elemental analysis was used to determine the azobenzene amount in the hydrogels. The obtained hydrogels showed good pH- and photo-responsive properties due to the ionization of carboxyl groups and the cis–trans isomerization of azobenzene, respectively. By changing the azobenzene content, the properties of hydrogels could be modulated. The swelling degree of the hydrogels increased with the decrease of azobenzene content and the increase of pH of the medium. A relatively high azobenzene content could make the hydrogel have a denser network structure, as confirmed by the scanning electron microscopy observation. In addition, the drug release behavior of resultant hydrogels was investigated in pH 2.2 and 7.4 phosphate buffer solution (PBS) before and after 365 nm UV photoirradiation by using vitamin B12 as a model drug. The result showed that the release rate of the encapsulated vitamin B12 increased with the decrease of azobenzene content and the increase of pH of the medium. Moreover, the release rate of vitamin B12 had an increase after 365 nm UV photoirradiation due to trans to cis photoisomerization of azobenzene groups in the hydrogel.(2) Studies on photoresponsive polymeric nanogels base on amylopectinAzobenzene-modified amphiphilic amylopectin derivatives (Amy-Azo) were synthesized and could self-aggregate into the nanogels by the hydrophobic association interactions of incorporated azobenzene groups, as confirmed by fluorescence measurements, particle size analyses and transmission electron microscopy observation. Meanwhile, the content of incorporated azobenzene groups and UV photoirradiation were found to have a great influence on the property of resultant nanogels. A higher azobenzene content resulted in the decreases of the critical aggregation concentration and mean size of the nanogels. Moreover, the mean size of the nanogels had an increase after 365 nm UV photoirradiation due to trans to cis photoisomerization of incorporated azobenzene groups. Furthermore, the trans-to-cis photoisomerization rate and the isomerization degree of Amy-Azo in H2O/DMSO media were investigated by UV-vis spectroscopy. It was found that the photoisomerization rate and the isomerization degree at the photostationary state decreased with the increase of the water content in H2O/DMSO media. In addition, the drug release behavior of resultant nanogels was investigated in PBS solution before and after 365 nm UV photoirradiation by using indomethacin as a model drug. The result showed that a relatively high azobenzene content could make the nanogels have a slower drug release rate. The release behavior of encapsulated indomethacin from these nanogels could be regulated by (trans–cis) photoisomerization of the azobenzene moiety present in the crosslinker.(3) Studies on photocrosslinked hydrogels based on chitosanA novel water-soluble chitosan derivative having a nitrocinnamate moiety (CMCS-NC) was synthesized and characterized. The UV-vis spectra showed that the gelation phenomenon of aqueous CMCS-NC solution upon exposure to 365 nm irradiation in the absence of photoinitiator may be caused by the intermolecular cycloaddition formation of the incorporated nitrocinnamate groups under the photoirradiation. By changing the photoirradiation time and incorporated nitrocinnamate content, its photogelation property could be modulated. For the hydrogels incorporated with various nitrocinnamate contents, their properties such as swelling, viscoelasticity, in-vitro biodegradation and drug release were investigated. It was found that a relatively high nitrocinnamate content could make the photocrosslinked hydrogel have a greater mechanical strength, a slower swelling degree, biodegradation and drug release rate. Furthermore, when the drug-loading CMCS-NC hydrogel was placed in PBS solution containing 4 mg/mL lysozyme, both the released rate and amount of the drug were higher than the hydrogel placed in PBS solution without lysozyme.(4) Studies on photocrosslinked hydrogels based on amylopectinA water-soluble amylopectin derivative (Amy-NC) with photoreactivity was prepared by the conjugation of amylopectin with 4-nitrocinnamate. Upon 365 nm UV irradiation, Amy-NC PBS solution underwent an intermolecular crosslinking by means of photodimerization of cinnamoyl groups, resulting in the hydrogel formation. The UV-vis spectra showed that the crosslinking degree of cinnamoyl groups was 38% after UV irradiation for 1800 s. By changing the photoirradiation time and Amy-NC concentration, its photogelation properties such as swelling, viscoelasticity, in-vitro biodegradation and drug release were investigated. In the cases of a longer irradiation time and a higher Amy-NC concentration, the photocrosslinked hydrogels have a denser network structure, a greater mechanical strength, a slower biodegradation and drug release rate. Based on rheological method and the scaling model, the fractal characteristics were confirmed for the hydrogel systems. Moreover, the in-vitro drug release behavior of the photocrosslinked hydrogels in the absence of photoinitiator was investigated in PBS solution by using lysozyme as a model protein drug. It was found that there was a sustained release behavior in all cases, and the release mechanism was confirmed to follow the Fickian diffusion. The circular dichroism spectroscopy showed that the secondary structure of encapsulated lysozyme could be maintained. The biological activity of released lysozyme could be kept to be more than 83% after 48 h.