Fabrication And Properties Of Microcapsules Templated On Removable Cores And Based On New Interaction Forces

Nowadays,the applications of microcapsules to various fields such as micro-reactors,drug carriers,protective shells for cells or enzymes,and transfection vectors in gene therapy,have been extensively explored.To meet the requirements for different applications,design of new-structured microcapsules is especially important. This work puts emphases on the studies of the fabrication and the properties of new-structured microcapsules.Microcapsules with high modulus and high stabilities were fabricated through layer-by-layer covalent assembly.Aminosilanized SiO₂ microparticles were used as templates.Poly(glycidyl methylacrylate)(PGMA)and poly(allylamine hydrochloride) (PAH)which acted as the building blocks were alternately immobilized onto the particle surfaces through a coupling reaction between the epoxides and the amines. Thus,a highly cross-linked structure was produced in this process.When the desired layer number was reached,the particles were subjected to a HF solution to obtain the hollow micrcapsules.Morphologies of the microcapsules were characterized with atomic force microscope(AFM),transmission electron microscope(TEM),and confocal laser scanning microscope(CLSM).To investigate the mechanism of the covalent assembly,the assembly process was simulated on a fiat substrate.Linear increase of the film thickness as a function of layer number demonstrated the layer-by-layer formation mechanism.The microcapsules can survive through extreme pHs and elevated temperature.AFM images did not show any difference between the microcapsules before and after acid-,base-,or thermo-treatments.Using a method of osmotic-induced invagination,mechanical properties of the as-prepared and acid- or base-treated microcapsules were quantified.The modulus of the microcapsules without any treatments was found to be as high as 910MPa.The acid- and base-treatment cannot decrease the modulus of the microcapsules.Spontaneous deposition of fluorescence-labeled dextran with various molecular weights in the microcapsules was studied.Owing to the residues of some positively charged species in the microcapsules,negatively charged FITC-labeled dextran could spontaneously deposit into the microcapsules.Moreover,the deposition behaviour is molecular-weight-dependent,e.g.dextran with a high molecular weight could deposit into the microcapsules more easily than those with low molecular weight.Similar phenomena were observed when PSS-doped calcium carbonate particles were used as templates.The technique of covalent assembly was extended to fabricate biodegradable microcapsules.For example,highly-reactive polydichlorophosphazene and hexamethylenediamine were chosen as the building blocks.These new-structured microcapsules were characterized with AFM,TEM and CLSM.The chemical structure of the microcapsules was determined using FTIR.The in vitro biodegradation experiment which was carried out in phosphate buffer at 37℃showed that the microcapsules could be destroyed by hydrolysis.On the basis of the host-guest interactions betweenβ-cyclodextrin-grafted PAH and ferrocene-grafled PAH,the hollow microcapsules were fabricated.The mechanism for the formation of the microcapsules was determined through the same assembly on a flat substrate.Owing to the large amount of positive charges on the weak polyelectrolyte chains,the size of the microcapsules was sensitive to the fluctuations of pH values and ionic strength.The competition between free cyclodextrin in solution and immobilized cyclodextrin in microcapsules could also affect the diameter of the microcapsules.Finally,sensitivity of the microcapsules to pH values was utilized to load and deliver substances.Template polymerization was applied to fabricating the microcapsules as well. First,electrostatic-driven template polymerization was adopted to fabricate polyelectrolyte microcapsules.Polymerization of 4-vinylpyridine hydrochloride(the positive charged monomer)in the presence of poly(sodium 4-styrenesuifonate)(the negative charged template polymer)and silica particles was carried out.After a core-removal process,hollow polyelectrolyte microcapsules were obtained.The chemical structures and compositions of the microcapsules were determined by FTIR and elemental analysis.Similarly,hydrogen-bonded microcapsules were produced through the template polymerization.The monomer and template polymer were acrylic acid and poly(vinylpyrrolidone),respectively.The resulting microcapsules were composed of PAA/PVP complexes.The microcapsules were characterized and confirmed by TEM, SEM,AFM,and CLSM.The chemical compositions of the microcapsules and hydrogen bonding in the shells were determined by elemental analysis and FTIR spectroscopy,respectively.The microcapsules fabricated without additional cross-linkers were composed of PVP and PAA with an equal molar ratio of AA to VP. The microcapsules could increase their size along with the pH increase,following a sigmoidal regime.The capsule swelling-shrinking process was completely reversible at high and low pH values.To functionalize the PAA/PVP microcapsules,the template polymerization was carried out in a gold colloidal solution.The presence of the gold nano-particles in the capsule walls was confirmed by electronic differaction.The distribution of the gold nano-particles in the shells of the microcapsules was visualized through TEM.