Environmentally Responsive Chitosan Hydrogel With Controllable Mechanical Properties

Biology has the ability to adapt their sizes,appearance and patterns to the environment by external stimulating: pine cone can be stimulated by environmental humidity and change its shape;There are gradient structures in both human body and marine organism for realizing firm connection between two parts with significant mechanical difference,reducing shear stress at the interfaces and improving the living condition of cells at the interfaces.Hydrogels with ECM-like hydrophilic network have the ability to response to environment and the biomimetic properties facilitate hydrogels' applications in tissue engineering,intelligent control system,drug-controlled release system and etc.However,artificial hydrogels still have many problems such as low and unmatched mechanical properties compared with tissues.In this thesis we adopt up-to-down approach to design and control the crosslinking of environmental response chitosan hydrogel by 3D printing and implement the reversibly control of the mechanical properties of chitosan hydrogels.Below is the specific research work:First,electrodepositon was carried out to fabricate chitosan hydrogel(EChit)on Ti foils by producing a basic environment near the cathode.Introducing anionic micelles into EChit film will markedly affect the mechanical properties of the films.Characterization of EChit films treated with anionic surfactants with different alkyl length shows the size and Zeta-potential of micelles will markedly affect the interaction between hydrogel network and anionic micelles.Large-size micelles will destroy the macromolecular network and small-size micelles cannot crosslink the network effectively.Mechanical tests show sodium dodecyl sulfate(SDS)incorporated EChit(SDS-EChit)has the largest Yong's modulus,with 6.1 MPa for SDS-EChit compared with 0.7 MPa for EChit.Further characterization was carry out to research the physicochemical properties of EChit and SDS-EChit.EChit was crosslinked by crystalline domain and showed elastic properties and occur brittle fracture in high stress situation,but EChit will re-dissolved in water when chitosan chains are protonated.SDS-EChit was crosslinked by electrostatic interaction and this interaction can be transferred from one domain to another domain under stretch,so SDS-EChit showed viscoelasticity,flexibility and self-healing properties.pH-induced reversible switching between EChit and SDS-EChit was achieved from two aspects: chemical composition and mechanical properties.We employ top-down fabrication approaches to provide the external program required to spatially organize these crosslinking mechanisms in our hydrogel network.Electrodeposition provides the initial cues that trigger chitosan's self-assembly through a cathodic neutralization mechanism that generates the crystalline network junctions.Printing technologies are used to pattern the hydrogel film with SDS micellar crosslinks.Importantly,both operations can be described by simple operational models: film growth during electrodeposition is described by a moving front model,and the anisotropic properties conferred by SDS patterning is described by models of composites.The success of these simple models suggests that they capture the essential features of the underlying physics,which involve spatial variations in the subtle balance between electrostatics,hydrogen bonding,and hydrophobic interactions.Further,these models provide a basis to design more complex patterns to generate hydrogels with more sophisticated capabilities.Also important is that the use of physical cross-linking interactions allows films to be programed and reconfigured simply,rapidly,and without the need for reagents or complex chemistries.Printing of SDS micelles onto a neutral chitosan film allows the cross-linking mechanisms to be spatially programed to confer mechanically controllable properties.The reversibility of these cross-linking mechanisms allows the patterned films to be erased and reprogramed with reconfigured mechanical properties.We envision that the reversible programmability of polymer cross-linking and the ability to reconfigure mechanical properties will provide new opportunities to create the dynamic material systems required for applications that range from regenerative and personalized medicine to soft robotics.