Studies On Preparation And Properties Of Micelles-Incorporated Microcapsules

Due to their unique hollow structure, microcapsules can be used for encapsulation of fragile substances to minimize the influence of environmental factors such as light, oxygen and pH condition as well as to avoid the undesired reaction between encapsulated substances and to manipulate the site and time of the substance release. Hence, they have been widely used in various fields such as food, medicine and cosmetics. Recently, Mohwald and co-workers reported the preparation of a new generation of microcapsules by sequential assembly of polyelectrolytes onto sacrificial colloidal particles followed by template removal. The so-called Layer-by-Layer (LbL) technique could provide accurate control over the composition and structure of the capsules obtained and the possibility to impart the capsules smart properties. Thus, it has received tremendous attention in the past decade. Our research interests in this field focus on the loading of small, uncharged hydrophobic substances into the capsules, the possibility to impart the capsules glucose-sensitivity at the physiological pH condition and the improvement of capsule preparation towards higher efficiency.First, poly (styrene-b-acrylic acid) (PS-b-PAA) micelles were employed for the loading of small, uncharged hydrophobic substances and incorporated into the LbL microcapsules by either assembling them into the capsule wall or encapsulating them in the capsule interior. In the former case, poly(allylammine) (PAH) was used together with PS-b-PAA micelles to fabricate LbL multilayer films and hollow capsules. The planar multilayer growth proceeded smoothly in an exponential way because of the surface roughness increase during the sequential adsorption. Hollow microcapsules were obtained with intact hydrophobic regions of the micelles in the wall. These capsules showed extraordinary stability against extreme pH conditions because of the existence of the micelle network mediated by the positively charged PAH molecules. Furthermore, additional assembly of poly (styrene sulfonate) (PSS)/PAH multilayer films before template removal could impart these capsule semi-permeability. In the latter case, the micelles were loaded inside the capsules by using micelles-doped CaCO3 particles as templates followed by the PSS/PAH multilayer film formation and further template removal. These shells showed better resistance towards the external osmotic pressure increase and the micelles loaded could be partly released when exposed to alkaline conditions.To prepare smart microcapsules responsive to carbohydrates at the physiological pH range, Concanavalin A (Con A) and glycogen were used to fabricate microcapsules in a Layer-by-Layer fashion through the specific interaction between the glucose binding sites in Con A and glucose units in glycogen. It was found that at least four binding sites in one Con A molecule were required for the successful sequential assembly. Intact capsules can be obtained after the assembly of one bi-layer and the size and diameter of the as-prepared capsules varied with assembly cycles. Although stabilized mainly by multiple hydrogen bonds, these capsules could remain stable between pH 6-9 for at least 1h. The assemblies showed specific responses to mannose, fructose, glucose and dextran (Mw 20 kDa) caused by competitive binding towards the Con A molecules, resulting in film mass loss and densification through molecular re-arrangements. When triggered with dextran (Mw 40 kDa), the assemblies responsed by film densification without any apparent mass loss. The responses were found to be dependent on both the molecular weight as well as the concentration of the carbohydrates and existed between pH 6-9 as shown with glucose as an example. The corresponding hollow capsules showed responses to these carbohydrates in the form of shell shrinkage, distortion or destruction. Further assembly on PS-b-PAA micelles-doped CaCO3 microparticles resulted in micelles-loaded capsules and the micelles can be partly released when triggered by glucose and dextran-20k.To improve the capsule fabrication efficiency, the concept of controlled precipitation (CP) was expanded by employing bovine serum albumin (BSA), chitosan and poly (lactic-co-glycolic acid) (PLGA) as the capsule components. Hollow BSA capsules can be obtained by the dropwise addition of ethanol into BSA aqueous solution and further complex particle adsorption on MnCO3 particles followed by chemical cross-linking. Using glutaraldehyde (GA) as the cross-linker, the intactness and thickness of the capsules can be manipulated. During a proper range, the thickness of the as-prepared capsules could vary between～10 nm to～90 nm by adjusting the initial BSA concentration, the amount of the poor solvent and the CP repetition times. With dithiobis(succinimidyl propionate) (DSP) as the cross-linking agent, intact capsules could be obtained while the destruction of the capsules under reducing conditions by the cleavage of disulfide bonds could only be realized with the aid of ultrasonication. Similar single-component microcapsules could be obtained using chitosan and PLGA as the shell component, demonstrating the versatility of the CP technique to some extent.Finally, intact Con A/glycogen microcapsules were prepared by a one-step CP process in which the complex particles formed by mixing of Con A and glycogen solutions adsorbed on the CaCO3 particle surfaces to form the capsule wall. The poly(ethylenimine) (PEI) modification of the template particles was found to be critical for capsule fabrication and the structure of the as-prepared capsules varied with different amount ratio of the two components. These capsules showed responses to glucose and dextran (Mw 20 kDa and 40 kDa) caused by competitive binding in the form of shell shrinkage and distortion. Micelles-incorporated hybrid capsules were prepared through a Con A/glycogen CP process onto micelles-doped CaCO3 particles and micelles-adsorbed CaCO3 particles. The as-parepared shells showed different responses towards glucose treatments because of the micelle adsorption onto the capsule walls. However, the exact mechanisms still required further investigations.