Design Of The Multi-Responsive Shape Memory Hydrogels And Their Applications In Biomedicine

Shape memory hydrogels（SMHs）are a class of functional polymers that can fix a temporary shape under external environmental stimuli,and can return to their original shape when external stimuli change.Because SMHs have similar water content to biological tissues,adjustable chemical properties and excellent biocompatibility,they are considered to be one of the most promising smart materials.How to improve the mechanical strength and environmental sensitivity of SMHs to make them durable under a variety of external environmental stimuli,deformability and controllability is still a huge challenge.Based on the proposal of the dual-network hydrogel,the introduction of a multi-network structure with reversible interactions into the hydrogel has become the most commonly used method to improve the mechanical properties of the gel.The introduction of multiple reversible interactions also makes it possible to construct multiple stimulus-responsive hydrogels.How to construct a multi-network interpenetrating structure without affecting the basic properties of the gel according to the assumption,and give the gel more functionality at the same time,has become a hot and difficult research topic.In response to the above problems,this thesis has done the following work based on the construction of multiple stimulus response shape memory hydrogels:（1）To construct multiple stimulus responses and multiple shape memory hydrogels.Current SMHs possess fundamental limitations in terms of their responsive performance and mechanical properties,which severely limits their practical applications.Herein,a triple-network composite hydrogel with Fe³⁺,NIR light,and thermal responsive abilities is fabricated and developed as multi-SMHs,named as PDA/CMC-Fe³⁺/PVA hydrogel.In this hydrogel system,the interpenetrating triple network are formed by chemically-physically co-crosslinked polyvinyl alcohol（PVA）networks,and physically crosslinked carboxymethyl cellulose（CMC）-Fe³⁺network.In addition,polydopamine（PDA）,which is introduced into the hydrogel system by in-situ polymerization way,could serve as an energy converter to endow the hydrogel with NIR-based responsive ability.Remarkably,the as-prepared PDA/CMC-Fe³⁺/PVA hydrogels with the optimized compositions exhibit relatively excellent mechanical properties due to the synergistic effect of the triple-network and PDA doping（tensile strength=163.48±10.42 k Pa,elastic modulus=45.42±3.96 k Pa,toughness=165.02±7.01 k J/m³）.Moreover,the composition of the hydrogel is optimized to realize a stable temporary shape and rapid recovery to the permanent shape,which is controlled by Fe³⁺,NIR irradiation,and thermal stimulus.The resultant hydrogels exhibit not only multi-stimulus responsive abilities,but also programmable multi-shape memory properties.（2）To construct a multi-stimulus response fluorescent visual shape memory hydrogel.Carbon quantum dots（CDs）containing boric acid groups,carboxyl groups and other functional groups on the surface were prepared by hydrothermal synthesis.It is added to the prepolymerized solution of CMC and PVA by doping and blending.In this gel system,the reversible metal coordination formed by-COO^-and Fe³⁺,and the boronic ester bond formed by boronic acid groups on the surface of CDs and PVA are used as temporary crosslinking points to fix the temporary shape.Under the stimulation of EDTA,acidic solution and glucose solution,the temporary crosslinking are destroyed,and the gel return to its original shape which realizes the shape memory function.At the same time,CDs undergoes electron transfer under Fe³⁺stimulation to quench the fluorescence;and during the process of p H change,the CDs surface groups undergo deprotonation and protonation processes,which cause the fluorescence color to change.Under the action of different stimuli,the fluorescence of the gel shows different phenomenon,the functions of naked eye visibility and stimulus source recognition are realized.In addition,the gel solvent is replaced with dimethyl sulfoxide（DMSO）/water binary solvent.The addition of the organic phase improves the mechanical properties of the gel while taking into account the anti-freezing and antibacterial properties.Tests show that the tensile strength of the composite organohydrogel is increased to 1.23±0.04Mpa,and the elastic modulus is increased to 0.68±0.19Mpa,which is better than other reported shape memory gels.The content of DMSO,CDs and composition of the hydrogel are optimized to achieve shape fixation and shape recovery,which is controlled by Fe³⁺,p H,and glucose stimulus.The shape fixation rate can reach more than 95%,and the shape recovery rate can reach 75%.Notably,the shape recovery of the gel showed synergistic consistency with the fluorescence changes.