| Biodegradable polymers have been developed into the design and preparation of temporaryimplants that substitute or fulfill certain functions as long as required to support healing processessafely, reliably, economically and physiologically compatibly as well as could be gradually degradedwithout toxic products in human beings. Among the three types of biodegradable polymers from nature,microorganism production and chemical synthesis, synthesized biodegradable polymers enjoy the mostpopularity with their controllable structures and favorable properties. Polyurethanes （PUs）, usuallyprepared through step polymerization of diisocyanate and polyols, possess a microstructure ofalternating soft and hard segments which facilitate themselves with excellent mechanical properties andfavorable biocompatibility. In a wide variety of applications as biomaterials, polyurethanes areespecially utilized in implantable devices such as drug administration carriers, artificial blood vessels,heart valves, nucleus prosthesis, and other tissue engineering materials.In order to explore polyurethanes of defined cleavable s ites and controllable degradability,disulfide groups were introduced into the synthesis of polyurethanes as chain extender or soft segment.When in the presence of reduced glutathione （GSH）, the disulfide groups in polyurethanes could becleaved into thiols through reversible thiol-disulfide exchange reaction, which fulfilled thepolyurethanes with responsive biodegradability. The structures of polyurethanes were confirmed byFourier Transform Infrared （FTIR） and NMR Spectroscopy. Molecular weight was performed by GelPermeation Chromatography （GPC）. Thermal properties were examined by Differential ScanningCalorimeter （DSC） and Thermogravimetric Analysis （TGA）. Surface morphology was studied with aField Emission Scanning Electron Microscope （SEM）.（1） Synthesis and properties study of biodegradable poly（ether-urethane）s.Firstly, poly（ether-urethane）s were synthesized from poly（ethylene glycol）（PEG） as soft segmentand1,6-hexamelthylene diisocyanate （HDI） incorporating cystine dimethyl ester chain extender as hardsegment, and the influence of soft segment to the properties of polyurethanes was investigated. PEGwith higher molecular weight led to increased phase separation, more hydrophilicity and higher critical temperature of polyurethanes. In the presence of glutathione, polyurethanes were degraded due to thecleavage of disulfide groups into thiols, which was confirmed by1H-NMR. The solubility of degradedfragments depended on the hydrophilicity of polyurethanes, for the polyurethanes with PEG2000and3000could entirely dissolve into the aqueous medium. The molecular weight of degraded fragmentsdepended on the content of disulfide groups, for it reduced almost76%of the polyurethane withPEG600after30days.（2） Synthesis and properties study of biodegradable poly（ester-urethane）s.Secondly, poly（ester-urethane）s were synthesized from poly（-caprolactone） diols （PCL） as softsegment and1,6-hexamelthylene diisocyanate incorporating cystine dimethyl ester chain extender ashard segment, and the influence of soft segment to the properties of polyurethanes was investigated.With the molecular weight of PCL increased, phase separation of polyurethanes were raised, tensilestrength and elongation at break of polyurethanes were improved as well, with the range of factor tan δbecoming wider. The degradation behavior in the presence of glutathione media showed that thedegradability of polyurethanes deceased with the increasing of PCL molecular weight, in which thepolyurethane with PCL550presented the best degradability for only remaining approximately53%ofthe original molecular weight. All the polyurethanes were testified to be non-cytotoxic andbiocompatible with WST-1.（3） Degradation kinetics of cystine-based biodegradable polyurethanes.Following the previous two chapters, in order to systemically study the degradation behavior andkinetics of cystine-based poly（ether-urethane）s and poly（ester-urethane）s, the polyurethanes weredesigned and synthesized with poly（ethylene glycol）（PEG） or poly（-caprolactone） diols （PCL） as softsegment and1,6-hexamelthylene diisocyanate （HDI） incorporating cystine dimethyl ester and lysinemethyl ester chain extender as hard segment, and the influence of the proportion of cystine dimethylester/lysine methyl ester in chain extender was investigated. The degradation experiment in thepresence of glutathione indicated that the degradability of polyurethanes increased with the content ofdisulfide groups increasing. The degradation velocity constants and degradation kinetic equations wereestablished through the relationship between degradation reaction velocity and polymerization degreeversus degradation time. When the concentration of glutathione was0.1mol·L-1, the degradationreaction velocity constants were213×10-3and0.33×10-3day-1for PEG-and PCL-containedpolyurethanes respectively.（4） Synthesis and properties study of biodegradable polyurethanes containing dithiol-polyols.Finally,11,11’-dithiodiundecanol （DSU） was synthesized and introduced to be mixed withpoly（ethylene glycol）（PEG） as soft segment and1,6-hexamethylene diisocyanate incorporating1,4-butanediol as hard segment to synthesize polyurethanes, and the influence of the proportion of DSU/PEG to the properties of polyurethanes was investigated. In the presence of glutathione (0.1mol·L-1), the degradability of DSU-based polyurethanes increased with the proportion of DSU/PEG increasing,for the polyurethane with100%DSU soft segment was entirely degraded after30days. Thedegradation kinetics of DSU-contained polyurethanes was further studied, for the degradation reactionvelocity constants were120×10-3day-1and the degradation kinetic equations were established as well. |
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