Electrical Signal Guided Assembly Of Chitosan Multilayer Hydrogel:Preparation And Property

Chitin/chitosan is the second most abundant natural resource in the nature.Based on chitosan's excellent physical,chemical and biological properties,it is usually been considered as the most promising hydrogel material and plays significant role in many fields such as,environmental protection,biomedicine and tissue engineering.The hydrogel with organized structure(eg.multilayer)always shows unique advantages in cell culturing and drug controlled release system.Electrophoresis deposition is a new approach for rapid fabrication of chitosan hydrogel with controlled morphology.Until now,the mechanism of electrophoretic deposition of chitosan and molecular-level details of chitosan's assembly on the electrode surface are yet poorly understood.This thesis has studied the electrophoretic deposition of chitosan in the microfluidic channel and characterized the chitosan hydrogel by using qPLM,Brillouin microscopy,fluorescent microscope which reveal the correlation between transfer charge(Q=?idt)and deposited hydrogel.At the same time,we have studied the influence of saltconcentrations on the preparation of chitosan hydrogel and recognized the main process of electrophoretic deposition.We have fabricated a chitosan hydrogel with multilayer internal structure by tuning electric field,as well as the further investigations of the mechanism of interface formation and its properties and potential applications.The main research results are as follows:(1)Study on chitosan chains' electrophoretic behavior and the microstructure of chitosan hydrogel prepared under various salt concentrations.In order to characterize the process of chitosan's electrical induced assembly in the microfluidic channel device(1×1 mm),advanced optical techniques have been used such as fluorescent microscope,quantitative polarizing microscope and Brillouin microscope.This result indicates that the microstructure and property of fabricated chitosan hydrogel could be controlled by electric field and salt condition,which could provide theoretical information for the preparation of chitosan hydrogel with controllable microstructure and property.(2)A unique "On-Off" electrical signal is used for the preparation of chitosan hydrogel with multilayer microstructure on the cathode.Specially,a moving front model has been built to describe and quantify the chitosan self-assembly process base on the Faraday law and the result shows good linear relationship which is only 17%lower than we expected.The "on" signal could induce gel growth,while the "off" signal creates an interface between the layers.The number of layers could be controlled by the cycles of "off" steps.(3)Study on the mechanism of electrical signal interruption induced interface formation and its properties.In the initial research of the interruption of gelation process,we have studied the effects of the pH gradient and electrical field.During this period,there is no further consumption of protons and thus the steep pH gradient established at the gel-solution interface expects to relax.The current density across the electrode-solution interface is set to zero but there is a resting potential of about-0.5 V still applied to the electrode.Thus there remains an electric field that may be capable of providing a force on positively charged chitosan chains.Various kinds of interfaces were fabricated by changing the function time of "off" steps(eg.2s,5s,10s).Thickness and mechanical properties of each interface within chitosan matrix could be programmablly controlled by using a complex electrical signal input sequence.(4)Study on how the interfaces influence the diffusion rate of model molecules within hydrogel.According to the former study of chitosan multilayer hydrogel formation,it could be found that the formed boundary within chitosan hydrogel shows higher density.Importantly,the thickness of each boundary could be controlled by the off signal.Specially,we used electrochemical,fluorescent labeling and spectrophotometry methods to test how the boundary influences to the diffusion rate of model molecules within chitosan multilayer hydrogel.From the results,we could observe that boundaries can slow down the diffusion rate of model molecules.(5)Based on the strong reaction between the chitosan chains and metal ions,a chitosan/silver nanoparticles composite hydrogel has been fabricated.The silver ions were electrochemical reduction to silver nanoparticles by using multilayer chitosan hydrogel as a template.From the SEM,TEM,EDS measurements,the formed silver nanoparticles were well distributed in the hydrogel matrix with well sphere shape.The results of dissolution and antibacterial experiments indicate that the high-density interfaces could slow down hydrogel dissolution rate in weak acid environment.The fabricated composite hydrogel also shows good antibacterial property,which could be used as antibacterial material.(6)Study on the pH responsive of chitosan chains in agar hydrogel and the formation of multilayered chitosan/agar composite hydrogel.We prepare the medium from a blend of two stimuli-responsive self-assembling polysaccharides agar(thermal-responsive)and chitosan(pH-responsive).Upon cooling the blend,agar forms the hydrogel medium while the embedded chitosan chains can be induced to self-assemble in response to imposed pH cues.By using inserted electrode and imposed pulse electric potential,the self-assembly of chitosan induces the formation of multilayered structure.The multilayered structure can be erased by acid and regenerated in the composite hydrogel.