Fabrication And Performance Of Structure-Controllable Polysaccharide Microcapsules Via Microfluidic Droplet Interface Reaction

As a kind of natural high molecular polymers,polysaccharide has abundant sources,low processing cost,good biocompatibility,biodegradability,non-toxicity,hydrophilicity and multiple biological activities.These advantages make it have broad application prospects as biological materials.Polysaccharide microcapsules have shown great application potential in the fields of biomedicine,environmental protection,biocatalysis,sensors,etc.,and have received extensive attention from researchers.The application performances of the microcapsules are closely related to their structures.For microcapsules with a single compartment,the loading capacity and release behavior mainly depend on the compartment size and the shell thickness.Therefore,in practical applications,it is necessary to precisely regulate and control the structural parameters of microcapsules,such as size and shell thickness.Meanwhile,introducing functional polymers or nanoparticles into the shells can endow microcapsules with unique functions,which is of great significance in a myriad of applications.Chitosan microcapsules can be fabricated by various approaches such as complex coacervation,spry-drying,template coating,and electrostatic ion gelation methods.However,chitosan microcapsules prepared by these traditional methods always have unavoidable problems,such as poor monodispersity,uncontrollable shell thickness,and weak mechanical strength.Besides,these fabrication methods sometimes need tedious operating procedures that are complex and laborious,and the residual organic solvent in the fabrication process often limits their applications in biological and medical fields.Due to the precise manipulation of the fluid on a microscale,microfluidic techniques can flexibly fabricate multiple emulsion droplets with controllable structures.Chitosan microcapsules with uniform shell thickness and good monodispersity can be fabricated using multiple emulsions as templates by microfluidics.However,controlling the structure of multiple emulsion templates requires elaborate design and construction of microfluidic devices,as well as accurate control of the flow rate of each fluid phase,which limits their large-scale mass production.Especially for the fabrication of ultrathin-wall chitosan microcapsules with membrane thickness less than 100 nm,these methods are very limited.Therefore,a more flexible and efficient strategy for the fabrication of stable chitosan microcapsules with controllable structures and good monodispersity still remains challenging and significant.Currently,alginate microcapsules are mostly used for the immobilization of enzymes.Microcapsules with sub-micron membrane thickness can be prepared through layer-by-layer self-assembly and Ca²⁺ gelation.However,the mechanical strength and chemical stability usually poor.By modifying with protamine or inorganic salt in the later stage or during the fabrication process,mechanical strength and stability can be improved,but making the preparation process complicated and cumbersome.Therefore,it still has difficulties to prepare thin-walled alginate microcapsules with good monodispersity and stable structure by a simple method,which has important research significance.This thesis uses microfluidic technology to design and prepare monodisperse emulsion templates to establish the oil-water phase interface.Based on the physical or chemical cross-linking reaction of chitosan,alginate and corresponding cross-linking agents at the emulsion template interface,by adjusting the reaction parameters,different polysaccharide microcapsules with different structures were prepared.This thesis mainly includes three parts of research works as follows.In chapter 2,a simple microfluidic approach for the fabrication of chitosan microcapsules with controllable structures based on the interfacial cross-linking reaction of W/O emulsion templates is developed.In the study,a coaxial capillary microfluidic device was used to prepare a monodisperse W/O single emulsion with a chitosan aqueous solution as the dispersed phase and an oil solution containing surfactant as the continuous phase.After the emulsion templates are formed,terephthalaldehyde（TPA）in the outer oil phase slowly diffuses from the oil phase into the aqueous droplet templates unidirectionally due to its concentration difference and slight water solubility.Then the capsule membrane is formed on the interface of W/O emulsion templates.In this process,by adjusting the preparation parameters such as cross-linking time,cross-linking agent concentration,and regulating the interfacial cross-linking reaction of TPA and chitosan,chitosan microcapsules with different membrane thickness and structures can be controllably fabricated.On this basis,a poly（dimethylsiloxane）（PDMS）microfluidic chip with arrayed uniform droplet-capture traps is introduced to study this interfacial cross-linking process in real time.By immobilizing the emulsion templates with the droplet-capture traps,the cross-linking reaction can be clearly observed online and controlled by regulating the solutions injected into the PDMS chip.The chitosan microcapsules with controllable structures are fabricated in both batchwise and continuous conditions.And a simple method for preparing chitosan microcapsules with controllable structure in batches using W/O emulsion as a template is proposed.In addition,by adding Fe₃O₄ nanoparticles in the dispersed phase,the microcapsules can be given magnetic responsiveness.The effect of the interfacial cross-linking reaction process of W/O droplets on the structure of chitosan microcapsules is studied.This work provides important guidance and reference for precisely controlling the droplet interface crosslinking reaction and the one-step fabrication of functionalized chitosan microcapsules with controllable structure using the single emulsion as a template.Based on chapter 2,a simple microfluidic method is proposed for controllable fabrication of chitosan microcapsules with membrane thickness less than 10 nm for drug control release in chapter 3,The membrane thickness of the microcapsules can be precisely controlled at the nanometer level by adjusting the preparation parameters,such as adjusting the cross-linking time,the concentration of reactants and the viscosity of the aqueous solution.In this research,the thickness of the microcapsules can be adjusted from 2.8 nm to 8 nm.Such nano-membranes of chitosan microcapsules are quite stable,and the structures of the microcapsules can keep intact for more than 150 days.Compared with traditional microcapsules,the prepared microcapsules with nano-membrane have larger internal cavity and higher load capacity.And drugs can quickly penetrate and diffuse through microcapsules membranes,due to the nanothickness membranes,showing a faster mass transfer rate.In addition,since no surfactant is used in the whole preparation process,and the prepared chitosan microcapsules present acceptable biocompatibility.The microfluidic fabrication strategy presented here provides valuable guidance for the design and fabrication of polymeric microcapsules with ultrathin walls to enhance their performances in biological and medical applications.In Chapter 4,combining the research work of Chapter 2 and Chapter 3,a simple microfluidic method for one-step fabrication alginate/chitosan composite microcapsules with double-layer membrane via water-in-oil-in-water（W/O/W）emulsion templates.The sodium alginate aqueous solution is used as the inner phase,the chitosan aqueous solution is used as the outer phase,and the oil solution containing the crosslinking agent calcium iodide（Ca I2）and TPA is used as the middle phase.W/O/W emulsion templates are generated by a secondary capillary microfluidic device.With these templates,alginate/chitosan composite microcapsules with double-layer membrane are formed through the reactions of interfacial gelation of alginate and Ca²⁺,interfacial crosslinking of chitosan and TPA,and complex coacervation of alginate and chitosan.And the membrane thickness of the microcapsule is in the sub-micron level.Compared with simple calcium alginate microcapsules,the prepared microcapsules have better mechanical strength and good molecular selective permeability.The double-layer composite membrane of the microcapsule can selectively permeate substances of different molecular weights.Small molecules can diffuse through the membrane freely,while the macromolecular substances will be intercepted outside of the microcapsule by the membrane.For hydrophilic active substances,the composite microcapsules have high-efficiency encapsulation performance.When used for the immobilization of peroxidase,the microcapsule can realize the immobilization and protection of the enzyme,and their thin-walled composite membranes provide the possibility of rapid mass transfer for the enzyme-catalyzed reaction.Thus,alginate/chitosan composite microcapsules enabling the immobilized enzyme to achieve rapid catalytic reaction,and exhibits good reusable performance.In addition,by adding Fe₃O₄@PDA nanoparticles in the internal phase,microcapsules with magnetic responsiveness and photothermal conversion effect can be prepared by the above method.The composite microcapsule can be used as an encapsulation carrier for enzymes and catalysts,and provides a new idea for designing fast catalytic systems with good catalytic performance,stability and repeatability.In summary,this research has constructed different oil-water interfaces through microfluidic methods,based on the interfacial cross-linking reaction of chitosan and TPA,the interfacial gelation reaction of alginate and Ca²⁺,and the interfacial complex coacervation reaction chitosan and alginate,three simple methods for controllable fabrication of polysaccharide microcapsules with different structures are proposed.The influence of preparation parameters on their structure is systematically studied.In the first two parts of the work,for microcapsules with different structures,the drug release performances are systematically studied,and their potential applications in the field of controlled drug release are demonstrated.And the fourth chapter shows the application prospects of the prepared composite microcapsules in the field of biocatalysis through enzyme immobilization research.The research works in this thesis provide new ideas and important theoretical guidance for the controllable fabrication of polysaccharide microcapsules with different structures.